Binding of Hypoglycaemic Sulphonylureas to an Artificial Phospholipid Bilayer
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Diabetologia (1984) 26:55-59 Diabetologia @ Springer-Verlag 1984 Binding of hypoglycaemic sulphonylureas to an artificial phospholipid bilayer M. Deleers and W.J. Malaisse Laboratory of Experimental Medicine, Brussels University Medical School, Brussels, Belgium Summary. Hypoglycaemic sulphonylureas bind to multilamel- gogues. It is proposed that the insertion of hypoglycaemic lar liposomes formed of egg yolk phosphatidylcholine. In this sulphonylureas into the phospholipid domain of the B cell artificial model, both specific and non-specific components of membrane could represent a primary event in the mechanism the binding phenomenon can be characterized by the same by which these agents stimulate insulin release. criteria as those used in studies performed with natural mem- branes. The relative ability of distinct sulphonylureas to inhib- Key words: Liposomes, phospholipid, hypoglycaemic sul- it the binding of 3H-glibenclamide or 3H-gliquidone to the lip- phonylurea, glibenclamide, gliquidone, gliclazide, tolbut- osomes parallels their relative potency as insulin secreta- amide, chlorpropamide. Hypoglycaemic sulphonylureas stimulate insulin re- glibenclamide and tolbutamide were obtained from Hoechst, gliqui- lease, apparently by facilitating the inflow of Ca 2+ into done from Thomae, gliclazide from Servier Benelux, Brussels, Bel- the pancreatic B cell [1, 21. Most authors favour the view gium and chlorpropamide from Pfizer, Brussels, Belgium. Multilamellar liposomes of egg yolk phosphatidylcholine were that the stimulation of Ca 2+ inflow is secondary to a de- formed as described elsewhere [13] in a Tris-HCl buffer (50 retool/l, crease in K + conductance, leading to depolarization of pH 7.30) yielding a final concentration of 2 mg/ml. The sulphonyl- the plasma membrane and gating of voltage-sensitive ureas were added to this buffer from stock solutions prepared in a calcium channels [3-6]. The molecular mechanism re- mixture of ethanol and 0.1 N NaOH (1/1, v/v), the final concentra- tion of these solvents not exceeding 0.2% (v/v). Aliquots (0.25 ml) of sponsible for the change in K § permeability is not the liposomal suspension were placed in polyethylene microcentri- known [7]. Radioisotopic studies of sulphonylurea up- fuge tubes (Beckman, Palo Alto, California, USA), incubated for 5-60 take by isolated islets suggest that these hypoglycaemic min at 37~ and centrifuged for 30min at 4~ and 3,000 g. The su- agents are bound to the plasma membrane of pancreatic pernatant solution was then removed and the bottom of the tube con- B cells [8-10], which could be equipped with sulphonyl- taining the pellet of liposomes cut, placed in a counting vial contain- ing 6 ml of scintillation fluid (Aquasol-2; New England Nuclear, Bos- urea receptors [111. The cell boundary [12] could there- ton, Massachusetts, USA), and eventually examined for its radio- fore represent the primary site of action of hypoglycae- active content. mic sulphonylureas. The specific radioactivity of 3H-glibenclamide or 3H-gliquidone The aim of the present study was to investigate how was kept unchanged in all experiments, except for measurement of the non-specific binding in which case unlabelled sulphonylurea was far the binding of sulphonylureas to natural membranes added at a final concentration of 0.1 mmol/l. Such a non-specific could be simulated in an artificial model of phospho- binding was measured for each condition under study. Results are ex- lipid bilayer. pressed as mean + SEM values together with the numbers of individu- al determinations (n). Materials and methods Results Egg yolk phosphatidylcholine was purchased from Sigma (St. Louis, Missouri, USA). 3H-glibenclamide (31.1 Ci/mmol) was kindly pro- At a 20 nmol/1 concentration of 3H-gliquidone, the ap- vided by M.H. Mertens (Hoechst Belgium, Brussels, Belgium), and parent binding of the sulphonylurea to the liposomes 3H-gliquidone (4.8 Ci/mmol) by Dr N. Kaubisch (Boehringer, Ingel- beim, FRG) and Dr Rupprecht (Thomae, Biberach, FRG). Both sul- reached an equilibrium value within 15 min of exposure phonylureas were tritiated in the cyclohexyl ring and their purity as- to the drug (Figure 1). At the same concentration of the sessed by chromatographic analysis to be >96%. Unlabelled radioactive compound but in the presence of a much 56 M. Deleers and W. J. Malaisse: Binding of hypoglycaemic sulphonylureas higher concentration of unlabelled gliquidone (0.1 sumed to represent the non-specific binding, it should be retool/l), the radioactivity associated with the lipo- substracted from other readings in order to characterize somes also rapidly reached equilibrium. Further experi- the specific binding of 3H-sulphonylurea. In a range of ments were performed over 30 rain incubation. concentrations from 0.2 to 10.0-15.0 nmol/1, such a spe- In experiments performed in the presence of a cific binding was not quite proportional to the drug 10nmol/1 concentration of 3H-sulphonylurea and a concentration (Fig. 2), the specific binding recorded at 0.1 mmol/1 concentration of the unlabelled drug, the to- the highest concentration amounting to only 63% (glib- tal binding of glibenclamide and gliquidone averaged enclamide) and 77% (gliquidone) of the theoretical val- 13.3 + 0.7 and 28.2 + 0.9 nmol/mg of phospholipid, re- ues obtained by linear extrapolation of the data collect- spectively (n = 6-10). This corresponds to approximate- ed at the four lowest concentrations of sulphonylurea ly 1.00_+ 0.05 and 2.12+ 0.07 tool sulphonylurea/100 (i. e. between 0.2 and 2.0 nmol/1). mol phospholipid. If the binding of 3H-sulphonylurea Figure 3 illustrates the effect of increasing concen- at this high concentration of the unlabelled agent is as- trations of unlabelled gliquidone and gliclazide upon the specific binding of 3H-glibenclamide (10.0 nmol/1). Within the range of concentrations under investigation (10.0 nmol/1 to 0.] mmol/1), the binding of 3H-gliben- 2O clamide was much more severely inhibited by gliqui- E done than by gliclazide. Tolbutamide also slightly de- creased 3H-glibenclamide binding (p<0.01), whereas '? 16 0 chlorpropamide failed to affect 3H-glibenclamide bind- E ing significantly (p> 0.1; data not shown). ,~,12 The inhibition of 3H-gliquidone-specific binding by unlabelled sulphonylureas is illustrated in Figure 4. The three drugs tested, namely gliclazide, tolbutamide, and "ID C chlorpropamide, all caused significant inhibition. The "~ 8 slopes of the regression lines for the inhibitory effect o "o were not vastly different from one another. However, o" 4 the concentrations of sulphonylurea required to cause 50% inhibition of 3H-gliquidone-specific binding i "r spanned a range of at least two orders of magnitude, av- o I i eraging 0.6 lxmol/1 with gliclazide, 6.4 .Ltmo1/l with tol- o 5 lJO 1~5 2 0 // ;0 butamide and 0.1 retool/1 with chlorpropamide. Crn,n) Fig.l. Time course for the total ( and non-specific ( .... binding of 3H-gliquidone (20 nmol/1) to liposomes as mea- Discussion sured in the absence and presence of unlabelled gliquidone (0.1 mmol/1), respectively. Mean _+ SEM values refer to four individu- In this study we have investigated the binding of hypo- al determinations and are expressed as cpm. 10-3/rag phospholipid. glycaemic sulphonylureas to liposomes formed of egg 0.8 0.8 /// E ////~ --~" 0.6 0.6 o E // E / O / / E + "t] .c 0.4 /// .E O4 /// "1:] //I E Fig. 2. Relationship between concentration and specif- g ic binding of 3H-glibenclamide and 3H-gliquidone to ~: 0.2 ~ 0.2 / liposomes. Mean + SEM values refer to six to ten indi- ._g vidual determinations and are expressed as pmol sul- o phonylurea bound/mg phospholipid. The dotted lines 4- illustrate relationships of proportionality with a slope ( o derived from the data collected at the four lowest con- centrations of sulphonylurea (0.2-2.0 nmol/1) O ; I'O 0 0 GL,BENCLAMIDE Gmoil0 GLIQUIDONE ~mol/b M. Deleers and W. J. Malaisse: Binding of hypoglycaemic sulphonylureas 57 100 o as the liposome-associated radioactivity represented 34%-64% of the total radioactivity present in each sam- o o o ple. Three major analogies can be found between the 80 present results and those obtained with natural mem- MJo~ branes. First, in our system just as in brain membranes [11] r "0 or intact islets [15-17], the total binding of [3H] sulpho- " 6C nylurea increased progressively, albeit not proportion- ally, as the concentration of sulphonylurea was raised in "0 a range between 0.2 nmol/1 and 0.1 mmol/1. At the lat- E "2. ter concentration, the equilibrium value for total bind- -~u 40 c ing to intact islets averaged 0.15, 0.67 and 3.15 mmol/kg 0~ .Q dry weight in the case of tolbutamide [15], glibornuride [16] and glibenclamide [17], respectively. If these values ~ 20 -1- are representative of binding to the plasma membrane, as proposed by the Ume5 group, they would corre- spond, in the case of glibenclamide, to a sulphonylurea/ 0 i i I i i phospholipid molar ratio close to unity. This estimation 10-8 10-7 10-6 10-5 10-4 is based on the knowledge that, in pancreatic islets, SULPHONYLUREA (m o,/,) the plasma membrane/dry weight ratio is close to Fig.3. Inhibition of the specific binding of 3H-glibenclamide 1700 m2/kg dry weight [18, 19] and that the content in (10 nmol/1) to liposomes by increasing concentrations of unlabelled phospholipids (averaged molecular weight taken as gliquidone (0-----0) or gliclazide (O O). Each value refers to 750) of biological membranes is close to 1.4 mg/m 2 [20, the point-moving mean derived from quintuplate measurements per- 21]. However, if the hypoglycaemic sulphonylureas formed at each concentration of sulphonylurea.