Biomedical Research l, 189--206 (1980)

HEXOSE ANOMERS, INSULIN RELEASE, AND DIABETES MELLITUS A .

ATSUSI-II NIKI and HATSUMI NIKI Department of Internal Medicine, School of Dentistry, Aichigakuin University, Chikusa-ku, Nagoya 464, Japan

REVIEW

A most fundamental problem concerning the mechanism of glucose-stimulated insulin release is the question as to whether glucose acts via its metabolism or via binding to a receptor, leading to two opposing hypotheses, the metabolism and glucoreceptor hypo- theses. The major arguments in support of either of these hypotheses are briefly review- ed. Special emphasis is laid on findings with respect to the insulinotropic actions of hexose anomers and their metabolism in pancreatic islets. Anomeric specificity in cel- lular processes other than insulin release are also summarized. Our current view is that adult-onset-type diabetes mellitus is, at least in some respects, due to a generalized disorder of glucoreceptors in the neuron-paraneuron system.

INTRODUCTION creatic B cell recognizes glucose as a signal for insulin release. According to the metabolism Glucose is one of the most fundamental fuel substances for the majority of living cells. In hypothesis (the substrate-site hypothesis), the addition, in several cells, it serves as a signal to signal for insulin release is generated by the initiate a specific physiological response. An metabolism of glucose in the B cell in the form excellent example of this dual function of glu- of a metabolite or cofactor. In more general terms, stimulus recognition may be a function cose is its action on the pancreatic B cell where- by it acts to release insulin from the B cell and of energy, of phosphate or redox potentials, or to provide energy for the releasing process by of the intracellular pH as altered by the meta- being metabolized. Whether one can differenti- bolic process. In the glucoreceptor hypothesis ate the signal efiect of glucose from its role as (the regulator-site hypothesis), glucose is a fuel has been a major subject of investigation thought to combine with a specific receptor into the physiology and patho-physiology of molecule, possibly, but not necessarily, located pancreatic islets. We shall briefly review areas on the B-cell membrane. in which advances are being made in the under- Subtle models were even olfered to reconcile standing of the mechanism by which the pan- both hypotheses, postulating that the two creatic B cell recognizes glucose as a signal for mechanisms may function simultaneously. insulin release, with special emphasis upon As the major arguments in support of the metabolism hypothesis, Malaisse er al. adopted findings from the experiments using hexose anomers. the following (64). (i) The relative insulinotrop- ic capacity of different sugars (glucose, man- nose, fructose, galactose, sorbitol, erythrose, THE METABOLISM AND GLUCORECEP- and glyceraldehyde) correlates with their apti- TOR HYPOTHESES tude to be metabolized in the islet cells. (ii) Two major hypotheses have been proposed as The more marked insulinotropic capacity of n’- to the molecular mechanisms by which the pan- D-glucose, as distinct from ,3-D-glucose, coin- 190 A. NIKI and H. NIKI cides with a higher glycolytic rate due to the mit the message of altered substrate availability stereospecificity of the enzyme phosphoglucose and catabolism to the hormone releasing ma- isomerase for a"-D-glucose-6-phosphate. (iii) It chinery of the islet cell, since r.t-ketoisocapro- is possible to stimulate insulin release in the ate, the metabolism of which is unrelated to absence of extracellular glucose by provoking that of glucose, is as potent as glucose in stim- glycogenolysis and glycolysis from endogenous ulating insulin release (l27, 51). Such general stores of glycogen. -i metabolic parameters have been reviewed in (i) It is well established that only metaboliza— detail (4, 64). ble sugars can stimulate insulin release, while One main feature of the response of the pan- nonmetabolizable ones are poorly active (44, 4, creatic B cell to glucose is its rapidity. When 124). The dose-response curve for glucose- stimulated by a constant concentration of glu- induced insulin release is sigmoidal; the thresh- cose, the pancreas releases insulin biphasically old glucose concentration is 4-5 mM, and the (37, 64): the biphasic secretory response con- half-maximum and maximum responses to glu- sists of a rapid early insulin peak followed by cose occur at 8—l0 mM and l5—25 mM, respec- a more prolonged second phase persisting tively (5, 37, 64). Rates of glucose utilization, throughout the period of stimulation. Mat- glucose oxidation and lactate formation are re- schinsky er 01. (70) have argued against the ported to be closely related to rates of insulin metabolism hypothesis on the grounds that release with varying glucose concentrations most of the metabolites and cofactors measured (5, 64). However, limited association between in rat islets were unchanged during the first 5 metabolic and insulin releasing efficiency of min of glucose stimulation, even though insulin glucose was also reported, and methodological release could be detected within l min after problems in metabolic studies were discussed injecting glucose. Other studies, however, do (126). Results with a number of inhibitors of not support this contention. Rapid changes in sugar metabolism show that blockade of glyco- concentration of hexose phosphates (6, 52) and lytic flux is accompanied by inhibition ofinsulin in pyridine nucleotide fluorescence (92) have release. For example, D-mannoheptulose which been reported in islets exposed to high concen- inhibits glucose phosphorylation blocks in par- trations of glucose. Thus, available data, but allel the metabolism and insulin-releasing ac- not all, on kinetics of changes in metabolic par- tions of glucose (5). The eflects of other inhib- ameters in islets are consistent with the meta- itors on the release of insulin are also well cor- bolism hypothesis. related with their effects on glucose metabolism (ii) Anomeric specificity of the secretory re- (127, 4). The theoretical possibility that the in- sponse to glucose will be discussed in the fol- hibitors similar in structure to glucose might lowing section. prevent binding of glucose to the putative re- (iii) Insulin release due to glycogenolysis in ceptors, however, remains to be determined. glucose-deprived islets was demonstrated by D-Glyceraldehyde and dihydroxyacetone en- Malaisse er al. (67). When pancreatic islets are ter the glycolytic pathway at the triose phos- preincubated for 20 hr in the presence of glu- phate level, and their metabolism in islets is cose (83.3 mM) and thereafter transferred to a not inhibited by D-mannoheptulose or 2-deoxy- glucose-free medium, theophylline provokes an D-glucose but inhibited by iodoacetate. Insulin immediate and dramatic stimulation of insulin release stimulated by these trioses, with or with- release. The release of insulin is not suppressed out inhibitors, reflects their metabolism ( 127, by mannoheptulose and abolished in calcium- l24, 4). These observations are not easily ex- depleted media. The theophylline-induced in- plicable in terms of the glucoreceptor hypothe- crease in glycogenolysis coincides with a higher sis without the unattractive mosaic construct rate of both lactate output and oxidation of of a site (s) for triose (s) distinct from that for endogenous “C-labelled substrate. These data glucose. Sugden and Ashcroft (l l2) observed suggest that stimulation of glycolysis from en- parallel elfects of glucose and glyceraldehyde dogenous stores of glycogen is suflflcient to pro- on insulin release and islet phosphoenolpyru- voke insulin release even in glucose-deprived vate content. The authors suggested that phos- islets, as if the binding of extracellular glucose phoenolpyruvate could mediate effects of sug- to hypothetical plasma membrane glucorecep- ars on insulin release. tors is not an essential feature of the stimulus- However, emphasis has recently shifted away secretion coupling process. from searching for specific metabolites to more Although there is now little doubt that the general metabolic parameters that might trans- glucose-recognizing capability of the pancreatic HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES 191

B cell is closely related to the capacity of the B On the basis of these findings, Davis and cell to catabolize glucose, the glucoreceptor Lazarus have postulated a ‘cascade’ theory of hypothesis needs to be kept in reserve in order insulin release (Fig. l). Glucose initiates insulin to explain sets of data that do not fit the meta- release first by interaction with a receptor on bolism hypothesis. the plasma membrane and simultaneously en- There are now three kinds of evidence that ters the B cell to be metabolized. The meba- suggest the existence of membrane glucorecep- tolites may stimulate the protein kinase and tors of the B cell [the lecture by S. J. H. Ash- thereby initiate and augment the release of in- croft entitled ‘Insulin release: The substrate- sulin. site and regulator-site hypotheses’, at the panel Other types of support for the glucoreceptor symposium in the Xth Congress of the Inter- hypothesis, (ii) the anomeric specificity of hex- national Diabetes Federation (Vienna, Austria, ose stimulation of insulin release, and (iii) the Sept. l3, 1979) and the report of ‘Hormone Se- interactions of alloxan, ninhydrin, and hexoses cretion’ workgroup (Chairman: F. M. Matsch- on insulin release, will be discussed in the fol- insky) at the National Conference on Diabetes lowing section. (Reston, U. S. A., Oct. 9-l2, 1979)]. (i) Studies on insulin release in a cell-free reconstituted system. (ii) The anomeric specificity of hexose Outside Plasma membrane Inside stimulation of insulin release. (iii) The inter- Gmcagon Second phase actions of alloxan, ninhydrin, and hexoses on insulin release. l x ‘ (i) Davis and Lazarus (23, 24) have develop- CAMP ed an in virro secreting system utilizing plasma Phospho membranes prepared from the cod endocrine diesterase pancreas and incubating these with insulin AMP granules isolated from mouse or rabbit pan- creatic islets. Cod endocrine membranes were -\—._., ATP chosen because of the difficulties of obtaining sufficient mammalian islet membranes. Glu- -'---... Protein <1’ Phospho-protein . . cose metabolism is not demonstrable in this -J---.._._ ? Phosphatase \xf>EP system. The system shows an absolute require- --.__. ment for physiologicalamounts of Ca2+ in order - ;__ "re e - - G 6 P for insulin release to proceed. ATP, in the pre- I l sence of Ca2+, is capable of liberating insulin. l- . ., -, l D-Glucose, like ATP, is able to increase insulin Giucoselblca-i FiISi Dhase liberation above that found with Ca2+ alone. D-Glucose, in the presence of both Ca” and ATE OI‘ AMP ' PNF’ ATP, causes a tremendous increase in the iallostericl amount of insulin liberated. Other sugars which Fig. l Diagram showing the view on the sequence cause liberation of insulin are D-mannose, D- of insulin release. First phase: release of insulin fructose, and D-glyceraldehyde. L-Glucose, D- occurs when glucose binds to its receptor. Ca“ is galactose and 5’-thio-D-glucose are ineffective. an absolute requirement for this binding to be ef- Among various phosphorylated intermediates fective. ATP, by binding to the same receptor mol- of carbohydrate metabolism tested, only phos- ecule, amplifies and augments the insulin release phoenolpyruvate and glucose-6-phosphate are induced by glucose binding. Second phase: glucose enters the cell and is metabolized to glucose-6- capable of releasing insulin. phosphate (G-6-P), phosphoenolpyruvate (PEP) and Glucose-initiated insulin release is as effective ATP. The glucose intermediates stimulate a mem- in the presence of non-phosphorylating ana- brane-bound kinase and cause phosphorylation logues of ATP, as it is in the presence of ATP. of specific proteins in the membrane. This phos- The effect of ATP on glucose-initiated insulin phorylation initiates second-phase release. release is thus considered to be allosteric. Glu- Glucagon, which stimulates the cyclase, causes insulin release by the production of cyclic AMP. cose-6-phosphate causes insulin release only This compound can substitute for ATP in initiat- in the presence of ATP but not in the presence ing the first—phase release. The second-phase release of non—phosphorylating analogues of ATP, sug- is brought about by cyclic AMP stimulating the gesting that ATP is necessary as a phosphoryl same protein kinase as glucose-6-phosphate and donon phosphoenolpyruvate. From reference 23 192 A. NIKI and H. NIKI

ANOMERIC SPECIFICITY OF GLYCOLY- Rossini and Soeldner (100) observed that cr- TIC ENZYMES AND INSULIN RELEASE D-glucose produced a greater insulin release than B-D-glucose also in humans. Glucose Since greater specificity to ct"-D-glucose in stim- anomers were administered intravenously to ulating insulin release was described as the ev- human volunteers at a low dose (3.5 g) over a idence which supports both the metabolism 20-sec period or a high dose (0.5 g/Kg) over a and glucoreceptor hypotheses, this item is dis- 3-min period. At the low dose 0:-D-glucose cussed here in some detail. stimulated a significantly greater insulin release Hexoses, as pyranose or furanose rings, exist than did ,3-D-glucose. However, the higher dose in two major cyclic configurations referred to infusion test showed no differences between the as the ct and )8 anomers, which differ only in anomers. having the opposite stereochemistry in the cy- 0:-Anomeric preference in glucose-induced clic structure at carbons designated C-1 or C-2. insulin release was initially considered to be These two configurations, when placed in solu- one of the compelling arguments favouring the tion, interconvert. readily. The rates of these glucoreceptor hypothesis based on a presum- interconversions vary with the hexose structure, able glycolytic rate of the two anomers in the ring size, and whether or not the hexose is islets. The free and phosphorylated forms of phosphorylated. D-Glucose in water, as well as glucose are shown to differ strikingly in their in blood, exists as an equilibrium mixture of velocities of mutarotation (9). At 37°C, the 36 percent of the or anomer and 64 percent of mutarotational rate constants for 0.'-D-glucose- the /3 anomer. Under physiological conditions, 6-phosphate and or-D-glucose are 27.6 min"1 and each anomer mutarotates rapidly into an equi- 0.098 min“1. These rates represent physiologi- librium with a half-life of approximately 7 min cal half-lifes of 1.54 sec for a:-glucose-6-phos- (9). phate and 7.1 min for at-glucose, a difference of We have previously reported that the or an- over 270-fold. Because of the inherent rapid omer of D-glucose was more effective than the mutarotation of glucose-6-phosphate, all sub- B anomer in stimulating insulin release (83). sequent metabolic steps were believed to be Batches of rat pancreatic islets were incubated insensitive to the initial glucose anomer (9). for 5 min in media supplemented with either Thus, ct’-anomeric specificity of glucose-stimu- the ct or ,3 anomer, or with equilibrated D- lated insulin release was considered to indicate glucose, at a concentration of 2 mg/ml. In- an action of glucose independent. of intracellu- sulin release stimulated by the cr anomer was lar glucose metabolism. higher than that by the /3 anomer. In the me- The observations by Idahl er al. (55, 54) were dium with equilibrated D-glucose, the amount consistent with this view. Microdissected pan- of released insulin was intermediate between creatic islets of noninbred ob/ob-mice were in- the amouts in the media with the 0: or (3 anomer cubated for 3 to 9 min with either the ct or ,3 alone. anomer of D-|:3H]glucose, or D-PH] glucose in Similar and more precise results were obtain- anomeric equilibrium at a concentration of 6 ed in the isolated perfused rat pancreas. Grod- mM; the three glucose preparations provided a sky er al. (46, 45) observed in the dose—response linear production of 3H2O during the incuba- studies that the difference of stimulatory effects tion time, and did not diifer in their rate of of the two anomers was most pronounced at conversion to 3H2O. When islets were perifused marginally stimulating, and, to a lesser extent, with 5-6 mM 0:-D-glucose or ,3-D-glucose, the at half-maximally stimulating glucose concen- concentration of D-glucose-6-phosphate rose trations. At glucose concentrations approach- within minutes and did not differ significantly ing maximal stimulation of insulin release, both between experiments with the anomers. In the anomers produced results identical to each same perifusion experiments, only tr-D-glucose other and to equilibrated glucose. Thus, the caused a pronounced stimulation of insulin apparent Km for 01-glucose is probably reduced, release. From these data, Idahl er al. ruled out while the Vmax for both anomers is the same, all pathways that branched olf from glucose- suggesting that the glucose recognition signal 6-phosphate at the possible site of signal forma- has a higher affinity for a'- than for /3-D-glucose, tion. though both anomers activate the same total As to the transport of the anomers into pan- signal. ct-Anomeric stereospecificity was de- creatic islets, it was reported that there was no monstrable commonly in both phases of insulin significant difference between the two anomers release (46, 45). of r>-glucose in inducing a counter-transport of HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES I93

3-O-methyl-D-glucose (55) or equilibrated glu- nase. The reaction was initiated by addition cose (76). Using the tritium-labelled anomers of freshly dissolved D-glucose anomer. The of D-glucose, we directly compared the uptake conversion of glucose-6-phosphate to fructose- of the at and the B anomer by isolated islets and 6-phosphate was followed by measuring the observed preferential uptake of the /5’ anomer amount of NADH formed by a fluorometric (78), as demonstrated in other tissues (30, 10, method. When the rate of glucose-6-phosphate 31). formation was low, namely in the presence of The anomeric specificity of various glycolyt- low concentrations of glucose (ll ,uM) and ic enzymes was reviewed in detail by Benkovic hexokinase (28 mU/ml), the amount of NADH and Schray (1 1). formed was significantly higher in the presence Two hexose-phosphorylating enzymes, hexo- of a'- as distinct from /3-o-glucose. When the kinase and glucokinase, lack anomeric specific- formation of glucose-6-phosphate was consider- ity, because the locus of the enzyme action on ably increased, namely at high concentrations hexoses is distant from and does not involve of glucose (83 /.zM) and hexokinase (700 mU/ the anomeric center. That the 0.’ and ,8 anomers ml), the rate of conversion of glucose-6-phos- of D-glucose were equally effective in raising phate to fructose-6-phosphate was not different the concentration of D-glucose-6-phosphate was whether 02- or [3-o-glucose was used as the pre- observed also in rat pancreatic islets (55, 54, 65, cursor hexose. These data indicate that in is- 66). Phosphoglucose isomerase, which cata- lets, as in other tissues, the phosphoglucose lyzes the isomerization of D-glucose-6-phos- isomerase displays a preferential affinity toward phate to D-fructose-6-phosphate, displays a pre- 0.’-o-glucose-6-phosphate. ferential affinity toward cv-D-glucose-6-phos- or-Anomeric specificity of the isomerase was phate. The role of the enzyme in controlling the reflected in the level of glycolytic intermediates rate of glycolysis in pancreatic islets will be dis- in intact islets exposed to a'- or ,8-D-glucose (65, cussed. Glucose-6-phosphate dehydrogenase, 66). The level of glucose-6-phosphate was sig- responsible for the oxidation of o-glucose-6- nificantly higher in the islets exposed for 5 min phosphate to D-gluconate-6-phosphate, is B- to /3-o-glucose than to 0:-o-glucose. A mirror anomer specific. Affinity of the islet glucose-6- image was found for further glycolytic inter- phosphate dehydrogenase toward ,B-o-glucose- mediates. The integrated amount of fructose- 6-phosphate was demonstrated by Malaisse ea‘ 6-phosphate, fructose-1,6-diphosphate, and al. (65, 66); addition of ,3-rm-glucose to the islet triose phosphate after exposure to at-o-glucose homogenate incubated with yeast hexokinase, was significantly higher than that found after ATP, and NADP resulted in a greater forma- exposure to ,3-D-glucose. The same authors tion of NADPH than addition of at-1)-glucose. obtained further indications that, in the intact The enzymes, phosphofructokinase and aldola- B cell like in the islet homogenate, the rate of se, which catalyze the two steps between n-fruc- glycolysis was higher in the case of 0:-D-glucose. tose-6-phosphate and triose-phosphate, are In islets exposed for 6 min to [U-“C1 glucose known to be specific for the [3 anomers. The in anomeric equilibrium, much less radio- sorbitol pathway in islets may be stereospecific activity was recovered as “CO2 when unlabelled for ,3-D-glucose, since more sorbitol is accumu- a'- as distinct from /3-D-glucose was also pre- lated in islets in the presence of ,8-o-glucose sent in the incubation medium. This indicates than in the presence of a’-n-glucose (65, 66). that the or-anomer is better able to dilute the Because neither the phosphorylation of glu- metabolic pool from which “CO2 is eventually cose, nor its metabolism through either the derived. The output of lactate from islets in- pentose or sorbitol pathway offered a satisfac- cubated for 5 min with ct’-o-glucose was signifi- tory explanation for the more marked insulino- cantly higher than that found in islets exposed tropic action of ct-in-glucose, attention focused to ,8-D-glucose. Furthermore, the inhibitory on phosphoglucose isomerase in" islets, which, effect of glucose upon “Ca efflux and the glu- with its at-anomeric specificity, might control cose-induced increment in “Ca net uptake were the rate of glycolysis below o-fructose-6-phos- more pronounced in the case of 0:-r>-glucose. phate. Malaisse er al. (65, 66) tested the stereo- Thus, Malaisse er al. have suggested that the specificity of phosphoglucose isomerase in is- more marked insulinotropic action of cr- as dis- lets; the islet homogenate was incubated with tinct from ,8-o-glucose is associated with a high- ATP, NAD, and rabbit muscle phosphofructo- er glycolytic flux, itself attributable to the kinase, aldolase, triose-phosphate isomerase, stereospecificity of the islet phosphogluose iso- and glyceralclehyde-3-phosphate dehydroge- merase (Fig. 2). 194 A. NIKI and H. NIKI

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: i‘€_;i LAC TATE CO2 6 I 1'0 I 2'0I 3'0 I Z0 Time ( minutes) Fig. 2 Schematic view on the metabolism of D- glucose anomers (a'G, ,8G) in pancreatic islets. The Fig. 3 Effect of the 0: and ,8 anomers of D-mannose thick arrows illustrate the preferential utilization and equilibrated D-mannose (0: and ,9) on insulin of substrates (G6P, glucose-6-phosphate; 6PG, 6- release from the isolated perfused rat pancreas. phosphogluconate; F6P, fructose-6-phosphate) by The pancreas was preperfused for 30 min with the different enzymes (HK, hexokinase; GK, gluco- basal medium containing 0.3 mg/ml 1)-mannose. kinase; PGI, phosphoglucose isomerase; G6Pde- Test solutions (2 mg/ml) were introduced for 5 min Hase, glucose-6-phosphate dehydrogenase). Also each (indicated by bars) in the same pancreas, sepa- depicted is the postulated link between glycolysis, rated by 10 min of perfusion with the medium used calcium handling, and insulin release. From ref- for preperfusion periods. The lower graph shows erence 65 the insulin levels in samples collected at 1-min inter- vals. Points signify mean values; SE5 are shown with vertical lines at the peaks (n=-8). Statistical The findings in the intact B cell by Malaisse differences were determined with Schefl’ée’s S meth- er al. (65, 66) seem to be incompatible with the od after the data were transformed to logarithms. previous findings by Idahl er ai. (55, 54), who The upper graph shows D-mannose concentrations failed to observe corresponding difference be- in the corresponding samples. From reference 81 tween the glucose anomers in their rate of con- version to H2O. Anomeric specificity of phosphomannose iso- Among sugars other than r>-glucose, o-man- merase was proved by experimental findings nose is the most potent stimulus for insulin re- with the enzyme of yeast (99). That the speci- lease. o-Mannose was reported to be metab- ficities of phosphohexose isomerases in pan- olized to CO2 and H20 (7, 129), and to increase creatic islets may not exceptionally differ from fructose-6-phosphate levels but to have little those of theoretical estimation or those ob- effect on glucose-6-phosphate levels in the pan- served in other tissues was demonstrated by creatic islets (7). Two hexose-phosphorylating Malaisse er al. in the case of phosphoglucose enzymes, hexokinase and glucokinase, were de- isomerase (65, 66). It is conceivable, therefore, tected in islets (69) and they phosphorylate that phosphomannose isomerase of islets is both glucose and mannose. The reactions cata- stereospecific for ,3-mannose-6-phosphate, and lyzed by these enzymes do not manifest ano- hence the ,8 anomer of mannose undergoes gly- meric specificity as described previously (ll). colysis at a higher rate than the 0: anomer. It is Although the anomeric specificity of phospho- known that, although phosphoglucose isomer- mannose isomerase in islets has not been stud- ase does not catalyze the isomerization of man- ied, it is theoretically determined to display a nose-6-phosphate to fructose-6-phosphate, the preferential affinity toward ,3-o-mannose-6- enzyme catalyzes the anomerization of o:-man- phosphate: only anomers with a cis relation nose-6-phosphate to /3-mannose-6-phosphate between the C--1 and C-2 hydroxyl groups can which can be used by phosphomannose iso- be the substrates for phosphohexose isomerases merase (99). However, this enzymatic anomeri- (99). Consequently, glucose-6-phosphate must zation may, at most, explain equal utilization be the 0: anomer and mannose-6-phosphate of a:- and /3-mannose-6-phosphate. must be the /3 anomer to be the substrates for We have compared the effects of the 04- and corresponding phosphohexose isomerases. ,3- [3-D-mannose and equilibrated D-mannose on HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES I95 insulin release from isolated perfused rat pan- of glucose and as to the magnitude of each re- i creas (81). In the 5-min perfusion experiments, sponse. The same authors (40) found that

il cAMP accumulation in isolated islets was more . the [3 anomer of o-mannose at a concentration i 1 of 2 mg/ml had little effect on insulin release, marked with the ct" anomer than with the ,8 I while the at anomer was 4 times more effective anomer of o-glucose. The results add another than the same concentration of equilibrated o- feature to the parallelism between the insulin mannose (Fig. 3). The greatest difference be- and cAMP responses on stimulation by D-glu- tween the activities of glucose anomers occurred cose. In a separate study (41) a close associa- in the lower range of stimulating concentra- tion was also found between the efficiency of tions, but the effects of the two anomers were analogues, isomers, and epimers of 13>-glucose indistinguishable at high concentrations (45); to stimulate or inhibit each of the two param- similar results were obtained in dose-response eters. On the basis of these findings, they studies on the effects of mannose anomers on suggested that insulin release induced by o-glu- insulin release. In the longer perfusion experi- cose may be mediated by cAMP in the B cell. ments, ct'- D-mannose was shown to be more po- Several findings, however, do not fit with this tent than ,3-1:)-mannose in stimulating not only idea. When the islet cAMP level was elevated the first but also the second phase of insulin re- with phosphodiesterase inhibitors such as the- ophylline or with stimulators of adenylate cy- lease. Thus, despite the opposite anomeric spec- i ificities of the isomerases for glucose-6-phos- clase such as cholera toxin without stimulating phate and mannose-6-phosphate, quantitative concentrations of glucose, insulin release re- relationships between the effects of the tr and /3 mained low or was only slightly stimulated (20, anomers of o-mannose were essentially the 49). The administration of exogenous cAMP same as those observed in the case of the ano- also failed to reproduce the effect. of glucose mers of 1)-glucose, although the dose-response (16). Thus, if cAMP is envisaged to be the only curves for insulin release induced by mannose second messenger of glucose action, one must anomers were shifted to the right in comparison postulate compartmentation of cAMP in the with those induced by glucose anomers. islets, only the glucose-stimulated compart- The findings seem to be incompatible with the ment having major access to the insulin secre- view that a signal for insulin release induced tory mechanisms. There is no aflirmative evi- by hexose anomers arises from the common dence to support this speculation (43). glycolytic pathway below the phosphohexose It is well known that, apart from its initiat- isomerase step. It is postulated that the glucose ing action on insulin release, with time glucose recognition process (es) of the pancreatic B cell also potentiates its own capacity for releasing may, at least in part, be independent of glucose the hormone from the B cell. Grill and Cerasi catabolism in the cell, suggesting the direct re- (42) have reported that the induction of the cognition of the glucose molecule by a gluco- time-dependent potentiation does not involve receptor. an increased activity of the adenylate cyclase-— cAMP system of the B cell, suggesting that glu- ANOMERIC SPECIFICITY IN ISLET-CELL cose exerts two distinct actions on insulin re- FUNCTIONS AS RELATED TO INSULIN lease: one is mediated by the cAMP system RELEASE while the other is not. Cyclic AMP (cAMP) Accumulation in Islets Initirlfion 0f Pliosp/rare Flush Although several investigators have failed to detect any effect of glucose upon the concen- Freinkel er al. (33) have reported that exposure tration of cAMP in isolated islets, more recent of isolated pancreatic islets prelabelled with studies indicate that glucose does elevate total radiophosphate to D-glucose elicits an almost islet cAMP levels (108). Whether cAMP rep- immediate, transient eflfux of [32P]orthophos- resents the key messenger in the process of phate, and called the phenomenon ‘phosphate glucose-induced insulin release is a matter of flush’. Phosphate flush is initiated slightly be- conflicting views. fore, or directly coincident with, the first phase Grill and Cerasi (39) observed that cAMP ac- of stimulated insulin release and is completed cumulation preceded the initiation of insulin shortly after inception of the second phase (33, release, and that there was a close relation be- 34). tween cAMP accumulation and release of insu- Pierce and Freinkel (94) determined anome- lin both regarding their relationship to the dose ric specificity for the phosphate flush during 196 A. NIKI and H. NIKI secretory stimulation with D-glucose. Prela- It is also known that certain hexoses, including belled pancreatic islets were perifused either 1)-glucose, protect the B cell against the dia- with freshly prepared 0:-D-glucose or equilibrat- betogenic effects of alloxan (12, 13, 105, 125). ed D-glucose solution at a concentration of 0.9 The exact mechanism by which alloxan de- mg/ml. Stimulation with glucose enriched in stroys the B cell is still obscure. Since the clas- the tr anomer produced a pulse of [32P]ortho- sical studies by Cooperstein er al. (21), accumu- phosphate release which was significantly lated findings, recently discussed in detail (22), greater than that with equilibrated D-glucose. suggest that the primary site of alloxan action The phosphate flush can be triggered by other may be the membrane of the pancreatic B cell. nutrient secretagogues such as n-mannose or L- In toadfish islets, diabetogenic concentrations leucine but not by such sugars as o-fructose, D- of alloxan markedly increase the membrane galactose or L-glucose, with a high degree of permeability to mannitol which is normally re- specificity paralleling insulin-secretory poten- stricted to the extracellular space (118). Al- tialities (33, 34). It does not require intracellu- loxan causes a rapid depolarization (26) and lar metabolism of the secretagogue, since the inhibits the univalent-cation pump in mouse nonmetabolizable analogue of leucine, 2-amino- islets (53). With freeze-fracture electron micro- bicyclo (2.2.l) heptane-2-carboxylic acid (BCH), scopy, it has been demonstrated that alloxan appears to be equally effective (34). Concen- produces a loss of intramembranous particles trations of nutrient secretagogue and magni- of islet cell plasma membranes (88). tude of phosphate flush have displayed clear In an in virro perifusion system, Tomita er al. dose-response relationships in every circum- (113) observed that a brief exposure of isolated stance examined (33, 34, 95). It is not a con- islets to alloxan inhibited the subsequent glu- sequence of the exocytotic process. The phos- cose-induced insulin release without affecting phate flush still occurs during islet exposure to the tolbutamide-induced insulin response. The nutrient secretagogues even when the stimulated concurrent presence of o-glucose, o-mannose, release of insulin is inhibited by omission of or 3-O-methyl-o-glucose during this brief inter- Ca2+ or inclusion of Ni” (33, 17). Insulin re- val almost completely protected the islets from lease can be enhanced without affecting rates this effect of alloxan. Thus, there has been a of phosphate efflux by calcium ionophore recent revival of interest in the use of alloxan A23187, which presumably translocates Ca2+ as a probe to investigate the mechanisms op- into the B cell to initiate the late steps in stim- erative in glucose-induced insulin release from ulated secretion while bypassing the triggering the B cell. This was prompted by the observa- mechanisms involved in the recognition of spe- tion that the protection of the B cell against al- cific secretagogues (17). Membrane stabiliza- loxan by glucose showed anomeric specificity. tion with D20 or local anesthetics such as tetra- Rossini er al. (102, 101) found a greater pro- caine reversibly inhibits phosphate flush (33, tection by the 0: anomer of o-glucose than by 34). Freinkel and Pedley further determined the the K3 anomer against the diabetogenic effects subcellular localization from which the substan- of alloxan, which was indicated by plasma glu- tial loss of inorganic phosphate occurs, using cose concentrations 24 hours after the adminis- an electron microscopic histochemical tech- tration of alloxan to fasted rats. They also ob- nique; inorganic phosphate accumulates adja- served the same anomeric specificity for 3-O- cent to the plasmalemma and nucleolus of the methyl-o-glucose in protecting islets against B cell and this phosphate is lost from the cell alloxan (101, 103). The ability of this nonme- during secretory stimulation of islets with glu- tabolized hexose argues against metabolism cose (32). On the basis of these findings, they having a direct influence on hexose protection. have suggested that the phosphate flush is link- Using batch-incubated rat islets, we have stud- ed to earlier components of stimulus-secretion ied the effects of alloxan on both glucose-in- coupling, i. e. to some aspect of the recognition duced insulin release and biosynthesis, and the of nutrient stimuli at the cell surface. protective effects of D-glucose anomers (82). Prior exposure of islets to alloxan alone pro- Interaction of Hexoses Wif/1 Alloxan and duced marked inhibition of subsequent glucose- induced insulin release and biosynthesis. A sig- Nz'n/2ydrz'n nificantly greater protection against these inhib- Alloxan has been widely used as a diabetogenic itory effects of alloxan by the 0: anomer of o- agent which produces destruction of the pan- glucose than by the ,8 anomer was observed. creatic B cell in several species of animals (97). McDaniel er al. (76) also studied the effect of HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES 197 i

i E i D-glucose anomers on alloxan inhibition of in- Controversial data on glucose metabolism may i i sulin release in isolated perifused islets, and re- reflect these different effects of alloxan. i ported that the ability of 0:-1)-glucose compared Alloxan has been shown to produce a mono- with B-D-glucose to protect islets against al- phasic burst of insulin release, which appears s E e loxan toxicity showed a similar dose-related similar to the first phase of insulin induced by Z § response as observed in stimulating insulin re- o-glucose (60, 90, 121). Alloxan-induced insti- i lease. The shared ct-stereospecificity for D-glu- lin release is dependent upon extracellular Ca2+ cose in protecting against alloxan and in stimu- and is associated with an increased “Ca uptake i lating insulin release in these studies suggests (121). Furthermore, o-glucose protects against a common site of interaction which may involve alloxan inhibition of insulin release in a dose- the B-cell membrane. dependent manner, similar to competitive en- In view of the central position of the alloxan zyme inhibitors (121). From these findings studies in current islet research, especially with Weaver er al. have suggested that o-glucose and respect to the mechanism of glucose recogni- alloxan may be competing for a receptor site tion, we will further outline these studies. involved in insulin release which may be inside McDaniel er al."(74) reported that alloxan the islet or on the islet cell membrane (121). did not inhibit hexose transport in pancreatic Ninhydrin is another diabetogenic agent islets. Thus, the effect of alloxan on abolishing which is similar in structure and chemical re- glucose-induced insulin release may not be the activity to, but is more stable than, alloxan. Mc- result of alteration in the transport of o-glucose Daniel er al. (77, 75) found this compound to into the B cell. On the other hand, alloxan itself mimic basically the inhibitory effect of alloxan enters the intracellular space of islets and some on glucose-induced insulin release. Both o-glu- of the protective agents (3-O-methyl-D-glucose, cose and o-mannose provided substantial pro- caffeine, and cytochalasin B) partially inhibit tection against the inhibitory effect of ninhydrin; alloxan uptake. Other agents (D-glucose and the 0' anomer of D-glucose was more effective o-mannose), however, increase the uptake of than the [3 anomer. On the other hand, 3-O- alloxan, suggesting that these agents do not methyl-o-glucose, cyctochalasin B, caffeine, and provide protection against alloxan inhibition by theophyl1ine,which protect against alloxan inhi- preventing the entry of alloxan into islets (120). bition of insulin release, were ineffective in pre- Findings as to the effects of alloxan on glucose venting ninhydrin inhibition of insulin release. metabolism are conflicting: one paper reported A factor in common with these agents is that that alloxan treatment had no effect on anaer- they either use or inhibit the hexose transport. obic glycolysis (47); another reported that while system in islets. These results suggest that the alloxan impaired the ability of islets to form protective site against alloxan inhibition by 3H2O from [5-3H]glucose by 20%, the effect these transport-related agents may be at the was small in comparison to the complete block- level of alloxan entry into the B cell, whereas ade of insulin release (128). It was also report- the inability of these agents to prevent nin- ed that an alloxan treatment severely altered hydrin inhibition of insulin release may be ex- both glucose utilization and oxidation in islets plained by their ineffectiveness in altering a non- (15); the experiments were carried out at a carrier-mediated diffusion of ninhydrin in the lower temperature (4° C), and the incubation pe- B cell due to the aromatic structure of this riod of islets with alloxan was longer (30 min), molecule. The common interaction between than in other reports. Henquin er al. (50) showed glucose, alloxan, and ninhydrin was further that the glucose recognition system in the B supported by the ability of glucose or alloxan cell is the most rapidly and severely affected pretreatment to decrease the subsequent islet by alloxan. Immediately after alloxan treat- uptake of [3H]ninhydrin. On the basis of these ment, secretory response of insulin release to findings, McDaniel er al. have proposed a hy- glucose was completely abolished, while glyc- pothesis that alloxan, ninhydrin, and the active eraldehyde, at-ketoisocaproic acid, and tolbu- hexoses interact at a common receptor to ini- tamide still induced a rapid release of insulin. tiate the first phase of insulin release. If islets were exposed to glyceraldehyde or tol- Weaver er al. (119) analyzed the molecular butamide 15 min after alloxan treatment, the structure of alloxan, ninhydrin, and the two rapid insulin release was markedly impaired. hexoses, D-glucose and o-mannose, and demons- These results suggest that alloxan has different trated the molecular architecture in common effects; one is the immediate and primary effect, among all of these four compounds, which and the other is the late and secondary effect. could account for their recognition by the puta- 198 A. NIKI and H. NIKI

to compare the effects of the two anomers on in- i / sulin biosynthesis with the conventional meth- S / od using batch-incubated islets, since the incor- l” / poration of the labelled amino acid into proin_- // 4 6 , sulin does not occur in a measurable amount within a few minutes, whereas the anomers mu- tarotate rapidly. At first we observed the pref- erence of the or anomer of o-glucose to protect pancreatic B cells from the inhibitory effects of O alloxan on glucose-induced insulin biosynthesis §\5--K.- and indirectly suggested that the ct anomer of O"\./...I D-glucose could also be more effective than the /5’ anomer in inducing insulin biosynthesis (82). This was later confirmed by experiments using \_.‘44X8 free cells prepared from rat pancreatic islets Fig. 4 The pharmacophore for recognition at a glu- (84). In this system, the glucose solution direct- coreceptor. The common properties for insulin re- ly contacts the B cells without being diluted lease are: an oxygen at position 1, either axial or equatorial; a hydroxyl at position 2, either axial or with intercellular fluid and the tr anomer of equatorial; an equatorial oxygen at position 3; and o-glucose was observed to be significantly more at position 5 an electron-rich region. Structural effective than the fl anomer in inducing insulin analysis of inactive hexoses suggests that the recep- biosynthesis. Similar results were obtained by tor has essential volume near position 6 and near Zucker and Lin (130), who preincubated the axial substituents at positions 3 and 4. From ref- isolated islets with either the ct or B anomer for erence 119 four periods of 5 min each, the last period to- gether with [3H]1eucine, and determined the tive glucoreceptor (Fig. 4): an oxygen at posi- incorporation of [3H]1eucine into proinsulin by tion 1, either axial or equatorial; a hydroxyl at the immunoprecipitation method. position 2, either axial or equatorial; an equa- Although a close parallelism seems to exist torial oxygen at position 3; and at position 5 an between the processes of insulin release and electron-rich region. biosynthesis so far as the regulator is D-glucose, These findings strongly suggest the existence the glucose recognition mechanisms of the B of a glucoreceptor for insulin release with spe- cell for the two processes may not be identical: cific steric requirements, and have been adopt- the threshold and the apparent Km value for ed as the third type of support for the receptor the stimulant action of glucose are lower for hypothesis, as described in thefprevious section. insulin biosynthesis than for insulin release (80, 68). ANOMERIC SPECIFICITY IN CELLULAR PROCESSES OTHER THAN INSULIN RE- Inliibition of Glucagon Release LEASE Glucose causes monophasic inhibition of gluca- Inrluction of Insulin Bi'0.s‘ym‘l1e.s'is gon release (37). The ct anomer of 1:)-glucose The effects of various agents or conditions on proved significantly more potent than the (3 insulin biosynthesis have been reviewed in de- anomer in inhibiting spontaneous or arginine- tail (80, 63, 4). As in the case of insulin release stimulated glucagon release from the isolated close correlations between rates of metabolism perfused rat pancreas (71, 45). A trend toward and insulin biosynthesis are generally seen, al- greater suppression of glucagon by the O.’ ano- though there are some examples of agents mer was also observed in in vivo experiments which can significantly modify glucose-induced using rats and dogs (104). These results indi- insulin synthesis with only moderate or no effect cate some similarity in the recognition sites for on the rate of glucose metabolism. The mole- glucose in the pancreatic A and B cells. How- cular mechanism by which glucose or other ever, these sites may not be identical, since the agents stimulate insulin biosynthesis and threshold, half-maximum, and maximum re- whether the signal for insulin biosynthesis is the sponses of the A cell to glucose occur at much same as for insulin release still remain to be lower concentrations than those of the B cell elucidated. (37). In the case of D-mannose, we have recent- As to the anomeric specificity, it is difficult. ly observed that the tr anomer was also more HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES I99 effective than the B anomer in suppressing GLUCOSE RECOGNIZING CELLS AND arginine-stimulated glucagon release (85). DIABETES MELLITUS The pancreatic A cell is reported to be sensi- tive to alloxan, though less than the B cell is, It is generally accepted that idiopathic or pri- suggesting that alloxan may act on a glucore- mary diabetes mellitus is not a single disease ceptor system with comparable physicochemical entity and consists of a number of heterogene- characteristics common to both the A and B ous syndromes, as has been discussed in detail cells (90, 38). (28). Clinically there are two major types of dia- Production of a Sweet Taste betes mellitusz juvenile-onset-type (insulin-de- pendent) and maturity-onset-type (insulin-inde- Anomeric specificity in physiological responses pendent) diabetes. The following review does to glucose was first determined in the gustatory not deal with the former type of diabetes. How- cell. The ct anomers of o-glucose, o-mannose ever, it should be noted that a normal insulin and o-galactose are reported to be sweeter than secretory response to glucose is found among their /3 anomers (114, 91, 111). Interesting are young nondiabetic rnonozygot.ic twins of the the following observations by Zawalich (123), patients of this type of diabetes (56). who examined the effects of alloxan on taste re- Maturity-onset-type diabetes is predominant- sponse recorded electrically. Alloxan applied ly due to genetic causes, because identical twins to the surface of the tongue inhibited the taste are nearly always concordant. Since there is response induced by glucose but had no effect no absolute deficiency of insulin as is seen in on the response to substances other than glu- juvenile-onset-type diabetes, the question that cose, such as NaCl, quinine, HCl or saccha- has been under debate is whether there is a rel- rin. If alloxan was applied with glucose, sub- ative insulin deficiency or whether the basic sequent glucose responses were protected. lesion is a tissue resistance to insulin. Among substances structurally related to al- After the introduction of the radioimmuno- loxan, only those two compounds which are logic method for determination ofinsulin, Ya- diabetogenic, alloxan and propylalloxan, were low and Berson (122) demonstrated that in the found to depress sweet response. These results majority of early maturity-onset diabetic sub- suggest that there are certain chemical similari- jects the blood insulin levels were not decreased ties between sweet receptors of the gustatory or even higher than in normal subjects. The cell and the proposed glucoreceptor of the pan- authors also observed in the diabetics that in- creatic B cell. It is generally assumed that the sulin release after oral glucose administration taste-producing molecules act on the mem- frequently occurred at a slower rate than in branes of the receptor cells or their processes nondiabetics. Seltzer er al. (106) demonstrat- (36). However, the gustatory cells respond not ed that mild diabetics responded with a sluggish only to monosaccharides but to oligosaccharides insulin release to the intravenous administra- and glucosides (114, 107), the latter group of tion of glucose, while initial insulin release to sugars being ineffective on the pancreatic B cell. oral glucose was not significantly retarded. They further indicated that the high levels of Stimulation of the Release 0fGuz‘ Glucagon- serum insulin noted in mild diabetics after the lilce lmnmnoreactlve Mare/'ials (GLI) oral glucose were in reality lower than those The gastrointestinal tract contains GLI, which found in nondiabetic subjects for a correspond- differs from pancreatic glucagon in biologic, ing level of hyperglycemia. Ceraci and Luft physicochemical, and immunologic properties. (19) have proposed the hypothesis that the se- Matsuyama and Foa (72) observed that the in- cretory deficiency of the B cell to glucose is the jection of 5% glucose into a loop of the canine inherited factor that is responsible for diabetes, ileum resulted in the release of gut GLI with- on the basis of their findings in monozygotic out the concomitant absorption of glucose, and twin pairs that the initial rise in serum insulin suggested that the glucose present in the intes- during continuous infusion of glucose was lack- tinal lumen was an actual stimulus for GLI re- ing or considerably diminished in all diabetics lease. Matsuyama er al. (73), using the same ex- including prediabetics. perimental systems, found that GLI was more On the other hand, a normal or higher than readily released into the blood stream after an normal initial response followed by a later su- intestinal oi-o-glucose load than after [3-o-glu- pernormal insulin response in mild diabetes has cose. also been reported. This finding has given rise 200 A. NIKI and H. NIKI

to the concept. of insulin resistance as the initial diabetics may result from a selective defect of lesion in maturity-onset-type diabetes. Fajans islet glucose recognition rather than a reduction er al. (29, 28) observed that in this type of dia- of islet secretory capacity. betes the magnitude of the individual insulin re- Thus, despite the conflicts described above, sponses to oral glucose load encompassed a low initial insulin response to glucose is consid- wide spectrum, and have suggested that the ered to be one of the most fundamental abnor- disease includes more than one disorder. Re- malities, at least in some cases of the maturity- aven and Olefsky (96) reported that most pa- onset-type diabetics. That the acute phase in- tients with abnormal glucose tolerance had insu- sulin release, though it may account for a smal- lin responses to oral glucose greater than nor- ler portion of the total insulin release, is an mal and none had low insulin response. important determinant of glucose tolerance has To determine which of two major derange- been clearly demonstrated in normal subjects ments, inadequate insulin release or insulin re- (62) and in diabetic animals using an artificial sistance, is the prerequisite lesion in adult-onset- endocrine pancreas (2). type diabetes, Efendié er al. (27) measured the The absolute or relative excess of pancreatic insulin responses to intravenous or oral glucose and/or gastroduodenal glucagon in diabetes load in normal subjects and chemical or mild mellitus may play an important role for the maturity-onset nonobese diabetics. The insulin development of endogenous hyperglycemia response to intravenous glucose was analyzed (116). It is not clear, however, whether or not by parameter identification in a mathematical the increased release of glucagon is secondary to model. The model assumes that a dynamic in- decreased insulin release or whether the A-cell terplay of stimulatory and inhibitory forces and B-cell derangements represent independ- controls the release of insulin. The stimulatory ent abnormalities (115). The resistance of the effect of glucose on the B cell comprises two A cell to suppression of glucagon release by processes: an immediate stimulatory action (ini- hyperglycemia is present in all types of diabe- tiating action), and a slower time-dependent po- tes, although studies on the effects of intrave- tentiation of this action (potentiating action). nous glucose load on glucagon release are limit- Insulin release is also modulated by negative ed. In maturity-onset-type diabetics, the mean feedback inhibition. Computer analysis of the percent changes in glucagon levels in response data allows the identification of a further param- to glucose infusion was significantly less than eter which defines sensitivity to endogenous in normal subjects (3). Lack of suppression of insulin. In the diabetics as a group, the initiat- glucagon by the administration of glucose was ing action of glucose was reduced, while the also observed in first-degree relatives of known potentiation of insulin release was not affected. diabetics (57) or monozygotic twins of juvenile Furthermore, in the diabetics with apparent diabetics (25). Studies in virro in the isolated glucose intolerance insulin resistance was pres- pancreas of streptozotocin diabetic rats suggest- ent, aggravating the consequences of their defi- ed that the insulin-independent mechanism of cient insulin release. On the other hand, the glucose recognition exists in the pancreatic A peripheral sensitivity to insulin was not altered cell (89). Therefore, in some forms of diabetes in subjects denoted as low insulin responders. there may be a functional defect of the A cell The insulin response to oral glucose showed due to hyposensitivity to glucose. considerable overlap between diabetics and In addition, it was recently reported that the controls. The significance of low insulin re- sensitivity of the taste-receptor system in detect- sponse in the development of diabetes is suggest- ing glucose was low in overt diabetics compared ed by the finding that definite diabetes oc- to that in normal subjects (48, 61). Lawson curred exclusively in the low insulin responder er al. (61) observed that adult-onset diabetics group (58). Low insulin response to glucose and healthy first-degree relatives of diabetics persists in diabetics during the period of im- showed significantly higher glucose thresholds provement of glucose tolerance in the normal than controls. In contrast, glucose threshold in range (59). In addition, it has been reported juvenile-onset diabetics was not different from that many, though not all, maturity-onset dia- controls. No difference in salt detection was betics with low insulin response to glucose can seen in any of the groups. still respond to insulinogogues other than glu- These findings indicate that there may be a cose, such as tolbutamide (14, 117), glucagon widespread impairment of cellular glucose re- (109), and isoproterenol (98). These findings cognition in adult-onset-type diabetics and their suggest that the impaired insulin respones in relatives. As described in the previous section, HEXOSE ANOMERS, INSULIN RELEASE AND DIABETES 201 stimulation of insulin release, suppression of hexose molecules. This may indicate a. com- glucagon release and detection of sweet taste mon mechanism of glucose recognition inde- exhibit greater specificity to the it anomer than pendent of intracellular glucose metabolism. It to the /3 anomer of glucose and mannose, sug- is of great interest in this connection that the gesting a similar, though not necessarily identi- insulin-independent diabetics reveal no differ- cal, mechanism for glucose recognition among ential insulin response between anomers. Ros- the pancreatic A and B cells and the gustatory sini and Soeldner (personal communication) ex- cell. On the basis of these findings, we have amined the differential glucose anomeric re- proposed the hypothesis that diabetes mellitus sponse in three diabetics. The patients exhibit- may well be understood as a generalized disor- ed a sluggish or no insulin response to the in- der of glucoreceptors with the 0.’ anomers being fusion of each individual glucose anomer; the the preferred stimulants (79). minimal insulin response observed in two of Asplund (8) suggested that the hypothalamus the patients to ct’-glucose was nearly identical could also be affected by this generalized disor- to that seen after ,3-glucose administration. der. An impairment of glucoreceptor cells in Second, all the cells come within the category the satiety center results in enhanced food in- of neuron and paraneuron. The term ‘para- take leading to excessive obesity in laboratory neuron’ was proposed by Fujita (35) for certain animals. Most adult-onset diabetics are obese, cell groups including members listed by Pearse and these patients are associated with insulin (93) in his APUD (Amine Precursor Uptake resistance due to a decrease in insulin receptor and Decarboxylation) series, and some other concentrations (86). Major signs in adult-onset secretory and sensory cells, which shared com- diabetes, inadequate insulin release and obesity, mon morphological, physiological, and cyto- could be explained as juxtaposed expressions chemical features with neurons. If the cells of a general impairment of glucoreceptor func- under this category originate from a common tion. Whether glucoreceptor neurons in the anlage as was suggested (35), it would not be ventromedial hypothalamus differentiate ano- surprising that all the cells were afilicted with a meric configurations of D-glucose is not known. common impairment of glucoreceptors in some However, Oomura er al. (87) observed that the genetic forms of diabetes mellitus. B anomer of methyl-D-glucose was more efi°ec- tive than the at anomer in exciting the neurons. We thank Miss F. ltoh and Miss H. Tsubouchi, Growth hormone release induced by arginine Aichigakuin University, for secretarial help. or L-dopa was reported to be suppressed by acutely induced hyperglycemia in normal sub- Received jbrpublicarion I ll/fay 1980 jects but not in insulin-dependent diabetics (l8, l). Insufiicient insulin concentrations in the REFERENCES circulation might be important in modulating l. AJLOUNI K., MARTINSON D. R. and l-IAGEN T. the effect of glucose on the hypothalamic—pitui- C. (1975) Effect of glucose on the growth tary apparatus responsible for controlling hormone response to L-dopa in normal and di- growth hormone release. However, it was also abetic subjects. Diaberes 24, 633-636 reported that in potential diabetics serum 2. ALBISSER A. M., LEIBEL B. S., EWART T. G., growth hormone levels were higher than in nor- DAVIDOVAC Z., Borz C. K. and Zmoo W. 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