A New Specific Histamine-H2-Receptor Agonist R L Mcisaac,* B J JOHNSTON, and L P FIELDING from the Academic Surgical Unit, St

Gut: first published as 10.1136/gut.22.7.529 on 1 July 1981. Downloaded from Gut, 1981. 22, 529-533

Gastric mucosal blood flow and acid secretory changes in man with impromidine: a new specific histamine-H2-receptor agonist R L McISAAC,* B J JOHNSTON, AND L P FIELDING From The Academic Surgical Unit, St. Mary's Hospital Medical School, London

SUMMARY The effect of impromidine, a new histamine-H2-receptor agonist, on gastric mucosal blood flow (neutral red clearance) and acid secretion was studied in nine volunteers. Impromidine stimulated a dose-dependent increase in neutral red clearance and acid secretion. Simultaneous cimetidine in three doses caused a parallel shift to the right for both acid output and clearance with unchanged maxima compatible with simple surmountable antagonism. There were small cardiovascular changes: an increased heart rate with a decreased diastolic pressure during infusion of impromidine. These changes were antagonised by cimetidine.

The involvement of histamine in the control of was approved by the local ethical committee and gastric acid secretion was supported by the informed written consent was obtained from each discovery of the specific inhibitors of H2-receptors volunteer in accordance with the Declaration of on the parietal cell.' Studies in animals have also Helsinki. Gastric mucosal blood flow was shown that the gastric vasculature is sensitive to measured by the neutral red clearance technique.8 http://gut.bmj.com/ histamine through a combination of Hl- and H2- receptors although with different relative contri- GASTRIC FUNCTION TESTS AND butions.2 3In man histamine stimulates a dose- E S T I M A T I ON O F M U C O S A L B L OO1) dependent increase in both acid secretion and F L O W mucosal blood flow but the blood flow component After an overnight fast each volunteer was is relatively resistant to inhibition by cimetidine.4 intubated with a double lumen nasogastric tube

A specific histamine-H,-receptor agonist would be (Sherwood 12-14 French gauge), the position on September 26, 2021 by guest. Protected copyright. of value in the investigation of the role of being determined by a water recovery test.' histamine in the control of gastric acid secretion Overnight secretion was aspirated and discarded. and blood flow in both healthy man and in A 15 minute basal sample was collected as a patients with peptic ulcer disease. Recent studies reference for neutral red standards (see below). have indicated that impromidine is a potent Winged needles were inserted into a vein in each stimulant of gastric secretion in man with forearm; one for drug infusion and the other for relatively fewer side-effects than histamine." 6 blood sampling. A 15 ml venous sample was taken The purpose of the present investigation was to for processing of neutral red standards in whole study the stimulation of acid secretion and blood blood and then a loading dose of neutral red (NR, flow in healthy volunteers using increasing doses 125 gg/kg) was given followed by a constant in- of impromidine and to compare these effects with fusion (250 ,g/kg per h). Blood samples were taken our previous results using histamine.4 at 15 minute intervals for 45 minutes and then at 30 minute intervals thereafter. After a basal Methods period (45 minutes) gastric acid secretion was stimulated by an infusion of impromidine. Dose- In the study group there were nine volunteers, response curves were constructed on two separate mean age 22 years (range 21-26 years). The study days at least one week apart. On day one, *Address for correspondence: Dr R L Mcisaac, Academic Surgical impromidine was given in four sequential doses Unit, St. Mary's Hospital Medical School, London W2 1 PG. (125, 2.5, 5*0, and 10 ig/kg per h) each for one Received for publication 11 December 1980 hour (Fig. 1). Two volunteers received impromi- 529 Gut: first published as 10.1136/gut.22.7.529 on 1 July 1981. Downloaded from 53 Mc!saac, Johnston, and Fielding 600 60

.E a-- mm ------m------. *~ f E- 500 00. 50 T ,o n =2) E 0 400 40 0 1 z E (n-2) 0 300 I- ° 30

U 200 /1 0 OK I- 20 t z T11T2;(=s 100 1 i itn=S) 10 [ -L 'F , I 0 -," . 0 B 1.25 2.5 5.0 10 20 40 80 B 1.25 2.5 5.0 10 20 40 80 Impromidine pjg/kg.h Impromidine pg/kg-h Fig. I Mean (: S E M ) a(id output ini volunteers durinig infiision ol graded doses of inpromidine ( 125-20 Fig. 2 Mean ( t S E M) neutral red clearance in the pg/kgper h). Baval secretion (B) 5444;13 3 pmpioll/nin has same volunteers as in Fig. 1. Basal NRC (B) 20 :t4*3 been subtracted. Tte theoretical maximunm was mIl/min has been subtracted. The calculated maximum calculated to be 530 4 -605 pmol/min and is shown by was 56 1 9-6 ml/min and is shown by the dotted line. the dotted line. (ED,,) and an inhibitory dose,,, (ID,,,) for cime- dine in the dose range 2-5-20 gg/kg per h. On day tidine. Standard errors for these calculations were two, the test was repeated with cimetidine (0 25, also derived from the least squares fit. 0.5, or 1[0 mg/kg) which was randomised among Blood pressure and heart rate were measured three groups of the volunteers (three, three, and electronically by inflating a cuff around the arm two volunteers respectively). The infusion of over the brachial artery (Digital 8000, Ueda cimetidine was begun 30 minutes before Electronics Ltd) and changes were analysed by impromidine (dose-range 5-80 gg/kg per h). means of Student's paired t test. http://gut.bmj.com/ Gastric juice was collected every 10 minutes. The volume of the aspirate was recorded and the Results concentration of H' measured by titration of 3 ml gastric juice to pH 7.0 with 0 1M NaOH using an I M P R O M I I) I N E 1) 0 S E - R E S P O N S E automatic titrator (Radiometer, Copenhagen). S T U I) I E S Acid secretion was expressed as gmol H+/min. Impromidine (1-25-10 Mg/kg per h) caused a

NR concentration was determined in 3 ml blood dcse-dependent increase in both acid output and on September 26, 2021 by guest. Protected copyright. and 1 ml gastric juice after alkalinisation and NRC (Figs t and 2). In the two volunteers who extraction into diethyl ether (10 ml Analar grade, received an infusion of impromidine 20 ag/kg BDH Ltd) with further extraction into 0-1M per h, this dose caused a small increase in acid HCI. The aqueous layer was read in a spectro- output above that induced by 10 1Mg/kg per h, photometer (Pye-Unicam SP 1800) at 540 nm but NRC at the high dose began to decrease. against serial standards of NR made up in whole The best fit curves for these data give the follow- blood or gastric juice and processed along with ing calculated values (omitting the data for 20 the samples. Neutral red clearance (NRC) in Mg/kg per h). Maximal acid ouput was 530+60.5 mI/min was calculated from the equation NCR= (SEM) gumol H+/min and the ED>,, 2.3±0.26 (NR). xxV/(NR)h, where (NR). and (NR)b are M£g/kg per h. For NRC the calculated maximal gastric and whole blood concentrations of NR rate was 56+9-6 ml/min and the ED5,,, 1l7+0.34 and V is the volume of gastric aspirate in ml/min. Mg/kg per h. Dose-response curves were constructed using the mean of the last two 10 minute periods at EFFECT OF CIM ETI DINE ON each dose level of impromidine. The data were I M P R 0 M II) I N E- I N I) L' C E I) A C II) used to fit non-linear curves by the method of SECRETION AND NRC: COMk1PARISON least squares"' so as to calculate a best fit maxi- WITH PREVIOUS H ISTAMI NE DATA mum for acid secretion and NCR as well as In a previous study. cimetidine (06 mg/kg per obtaining calculated values for the dose of h) reduced acid secretion induced by histanmine imnromidine lroducing a half-maximal response but had no effect on neutral red clearance.4 Gut: first published as 10.1136/gut.22.7.529 on 1 July 1981. Downloaded from Impromidine and gastric blood flow 531

600 150 ._E E impromidline -. 500 r2= .57 0 E Y -.14X * 23 400 100 [ C * E 30 : ' * ' A A 0. E 300 1, A U C A CA a A C &* ,AA z 0 200 so g, 6A AA * histamine A An, #A A .61 AAA r2yy~ .1ox - 1g AA A 100 A A

0 0 1.25 2.5 5.0 10 20 40 500 1000 Impromidine pg/kg h ACID output pmol/min Fig. 3 The effect or cimetidine given in three doses Fig. 4 Comparison of the acid secretory and NRC 025(E),05( A), and 1-0( *) mg/kgperh on impromidine- responses during infusion ofgraded doses ofhistamine induced gastric acid secretion. The calculated ID50 for (A) and impromidine (*). The calculated regression cimetidine inhibition was 0-10 0014 mg/kg per h. line for histamine was Y=0 IOX - 19, r2=0-61, n=38 (p < 0001) andfor impromidine Y=0-14X -23, In the present study cimetidine caused a parallel, r2-0-57, n =44 (p <0O00J). dose-dependent displacement of the curves for both acid secretion and NRC to the right (Fig. 3, decrease in diastolic pressure of 12+4:1 mm Hg Table). The data are also tabulated for com- (P<0-05 compared with resting levels). Only parison of the effect of cimetidine on both his- small changes in systolic pressure were seen, so tamine- and impromidine-induced acid secretion that the mean arterial blood pressure fall from and NRC (Table). On a molar basis impromidine 92 + 3-3 mm Hg to 80 + 2-3 mm Hg was due is some five times more potent than h:istamine entirely to the above changes in diastolic press- on acid output and NRC. The calcuilated ID,, ure. Concomitant infusion of cimetidine pre- http://gut.bmj.com/ for cimetidine-inhibition of acid secretion was vented these changes but the cardiovascular the same whether histamine or impromidine was responses reappeared at higher doses of impro- used as stimulant but the effects on NRC differed. midine 20, 40, and 80 pg/kg per h.

CARDIOVASCULAR EFFECTS OF Relationship between NRC and acid secretion I M P R O M I I) I N E during impromidine and histamine infusion

During the infusion of impromidine (5 and 10 There-was a significantly linear relationshi.p be- on September 26, 2021 by guest. Protected copyright. pg/kg per h) there was an in!crease in pulse rate tween neutral red clearance and acid output of 12±3 beats per minute and this was statisti- during impromidine infusion and this is com- cally different from the pretreatment periods or pared with histamine infusion (Fig. 4). The in- the lower doses of impromidine (P<0-05). This tercepts of the two regression lines were not increase in pulse rate was accom.panied by a different but there was a difference in slopes (slope ± 950% confidence limits: impromidine Table Values for dose of agonist producing half 0-14+0-06 and histamine 0-10+0-04, P<0-05). maximal rates (ED50) for impromidine and histamine impromidine producing a higher NRC relative on acid secretion and neutral red clearance (NRC) to secretion than histamine. and inhibitory dose50 (ID50) for cimetidine against these agonists Discussion Impromidine Histamine In 1966, Jacobson, Linford, and Grossman'1 EDM( (j.g/kg per h) ACID 2.310.213 11 1 :2.15 described the use of aminopyrine clearance for NRC 1.7--0339 7.9+ 127 the estimation of gastric mucosal blood flow. IDso (mg/kg per h) ACID 0.101.0 014 0.3 ! 0-15 Since that time experience with th.e clearance NRC 0 10:1-0 027 No effectt markers has provided evidence that mucosal blood flow and acid secretion are in some way The data for histamine are taken from Knight et al.4 linked, this tCimetideine 0 6 mg/kg per h had no effect on the histamine-induced although relationship is not entirely NRC. clear.'2 In addition, some limitations to the use Gut: first published as 10.1136/gut.22.7.529 on 1 July 1981. Downloaded from

532 Mc!saac, Johnston, and Fielding of clearance markers have been uncovered animals the histamine-induced vasodilatation in (although perhaps not widely recognised by those the mesenteric vascular beds has been described who use such agents) and have prompted more as biphasic and time-dependent-the HI-response sceptical interpretations of results.'3-'" An characterised as immediate but short lived and element of uncertainty remains that the clear- the H,-response as slow in onset and of longer ance markers may not accurately reflect mucosal duration.2 More difficult to understand is the blood flow during low secretory rates or when fact that the HI-antagonists can actually increase there are rapid changes in flow. It is important histamine-induced blood flow and acid secretion to keep this in mind when studies are confined in some animals and perhaps in man.12 Two to man and where other methods are not yet explanations for this are possible: first an inhi- available for comparison.15 16 bitory action of the H1-receptor on the gastric The newest clearance marker, neutral red, vasculature to restrain blood flow and hence has been used in both animals and in man.8 In secretion; secoo.d, the systemic hypotensive action animals during brisk acid secretion blood flow of histamine which leads to a reduction in splanch- as estimated by neutral red clearance is of the nic blood flow may limit the gastric mucosal same order as that using other methods.12 We blood flow and secretory responses to histamine."7 have used the clearance of neutral red to esti- In either case, inhibition of the H1-mediated mate gastric mucosal blood flow in man during effects of histamine or use of a specific H2- stimulation of acid secretion by the new agent, agonist would then allow unfettered blood flow impromidine, which acts specifically at the hista- and secretion. In the present study in man a mine-H,-receptor. Impromidine increased neutral comparison of the blood flow responses to hista- red clearance in a dose-dependent manner in mine with those of impromidine showed that addition to its previously described effect to in- for the same level of acid secretion histamine- crease acid secretion.5 As expected, the H2- induced blood flow was less. Also in another receptor antagonist cimetidine produced a paral- study in man18 again using neutral red clearance lel shift to the right of the dose-response curves we found that pentagastrin-induced blood flow for both acid secretion and NRC but did not was greater than histamine-induced flow and that change the maximum. This agrees with our pre- both cimetidine and ranitidine inhibited these http://gut.bmj.com/ vious studies in the dog in which cimetidine responses to pentagastrin. It may be that blood and ranitidine inhibited NRC and acid secretion pressure changes which occur on infusion of to both histamine and imrpromidine.8 However, histamine in animals and in man but which are in man, cimetidine had no effect on the NRC less marked with impromidine or pentagas- response to histamine while reducing the secre- trin5, 19-21 could account for the lesser blood tory response.4 Initially, this suggested to us flow, but clearly histamine-mediated responses that the vasculature was less sensitive than the of the vasculature are complex in man. The parietal cell to cimetidine and that the un- specific H,-receptor agonist, impromidine, has on September 26, 2021 by guest. Protected copyright. changed response of the vasculature during some advantages over histamine as a potent infusion of cimetidine was due to residual stimu- gastric secretogogue in healthy man, producing lation of Hl-receptors by histamine. However, readily definable acid secretory and vascular res- in that study the histamine-H,-receptor antag- ponses which can be competitively inhibited by onist, mepyramine, also failed to alter the cimetidine. Thus, it may have a place in the vascular response to histamine. The study with routine testing of gastric function in patients impromidine and cimetidine reported here in- with peptic ulcer diseases and will also be useful dicated that the above suggestion is untenable for the study of mechanisms of gastric acid and that the sensitivity of the vasculature is secretion and mucosal blood flow in these probably not different from that of the parietal patients. cell, although the relationship between the two is still not completely clear. The effect of hista- We gratefully acknowledge the Research Insti- mine on the parietal cell is undoubtedly via tute, Smith, Kline and French Laboratories stimulation of the H,-receptor but the response Limited, Welwyn Garden City, Herts, for the of the vasculature is more complex. The in- gifts of impromidine and cimetidine and for creased blood flow seen with histamine may financial support for the project. reflect partly the energy requirements of increased acid secretion and partly a direct inter- References action with histamine-H1 and H2-receptors on 'Black JW, Duncan WAM, Durant CJ, Ganellin CR, the vasculature. Using more direct methods in Parsons ME. Definition and antagonism of hista- Gut: first published as 10.1136/gut.22.7.529 on 1 July 1981. Downloaded from

Impromidine and gastric bloodflow 533 mine H2-receptors. Nature 1972; 236:385-90. studied by a clearance technic. J Clin Invest 1966; 2Owen DAA, Harvey CA, Johnston BM, Shaw KD. 45:1-13. Histamine-induced increase in gastric blood flow in '2McIsaac RL. Mucosal blood flow in gastric cats: Comparison of findings with different experi- physiology. In: Fielding LP, ed. Gastro-intestinal mental techniques. In: Fielding LP, ed. Gastro- mucosal blood flow. Edinburgh: Churchill Living- intestinal mucosal blood flow. Edinburgh: Churchill stone, 1980: 147-56. Livingstone, 1980: 125-46. :"Davenport HW, Munro D. Aminopyrine clearance 3Pawlik W, Tague LL, Tepperman BL, Miller TA, in the damaged gastric mucosa: reconciliation of Jacobson ED. Histamine Hi-and H2-receptor vaso- conflicting data. Gastroenterology 1973; 65:512-4. dilatation of canine intestinal circulation. Am J 14Moody FG. The basis of the methods to estimate Physiol 1977; 233:E219-24. mucosal blood flow. In: Fielding LP, ed. Gastro- 4Knight SE, Mclsaac RL, Rennie CD. The effect of intestinal mucosal blood flow. Edinburgh: Churchill histamine and histamine antagonists on gastric acid Livingstone, 1980: 83-6. secretion and mucosal blood flow in man. Br J Surg 15Sonnenberg A, Blum AL. Limitations to measure- 1980; 67:266-8. ment of gastric mucosal blood flow by 14-C-amino- 5Hunt RH, Mills JG, Beresford J, Billings JA, pyrine clearance. In: Fielding LP, ed. Gastro- Burland WL, Milton-Thompson GJ. Gastric secre- intestinal blood flow. Edinburgh: Churchill Living- tory studies in humans with impromidine (SK&F stone, 1980: 43-58. 92676)-a specific histamine H2-receptor agonist. ";Ivarsson LE, Darle N, Hulten L, Lindhagen J, Gastroenterology 1980; 78:505-11. Lundgren 0. Gastric blood flow in man: 85-Kr ,Mills JG, Hunt RH, Bangerter U, Halter F, elimination technique. In: Fielding LP, ed. Gastro- Burland WL, Milton-Thompson GJ. The effects of intestinal mucosal blood flow. Edinburgh: Churchill Impromidine, a specific H2-receptor agonist, on Livingstone, 1980: 89-93. gastric secretion in man. In: Torsoli A et al., eds. 17Jacobson ED. Physiologic aspects of the intestinal European symposium on further experience with circulation. In: Boley SJ, Schwartz S, Williams LF, H2-receptor antagonists in peptic ulcer disease and eds. Vascular disorders of the intestine. New York: progress in histamine research. Capri, October 1979. Appleton-Century-Crofts, 1971: 19-24. Amsterdam: Excerpta Medica 1980:264-70. 18McIsaac RL, Johnston BJ, Fielding LP. Gastric 7Declaration of Helsinki, Human Experimentation. mucosal blood flow in gastric secretory disorders: Code of ethics of the World Medical Association. Effect of therapeutic agents. In: Konturek SJ, ed. Br Med J 1964; 2:177. Workshop on the pharmacology of the gastric http://gut.bmj.com/ mucosa. XI International Congress of Gastro- 8Knight SE, Mclsaac RL, Rennie CD, Flannery enterology, Hamburg, July 1980. Stuttgart: Georg MC, Fielding LP. Studies of gastric mucosal blood Thieme. (In press.) flow: neutral red clearance and dose-response curve 1'Durant GJ, Duncan WAM, Ganellin CR, Parsons analysis. In: Fielding LP, ed. Gastro-intestinal ME, Blakemore RC, Rasmussen AC. Impromidine mucosal blood flow. Edinburgh: Churchill Living- (SK&F 92676) is a very potent and specific agonist stone, 1980: 105-24. for histamine-H,-receptors. Nature 1978; 276:403-5. 9Hassan MA, Hobsley M. Positioning of subject and 20Boyce MJ, Owen DAA, Wareham K. Modification on September 26, 2021 by guest. Protected copyright. of nasogastric tube during a gastric secretion study. of cardiovascular effects of histamine infusions in Br Med J 1970; 1:456-60. man by chlorpheniramine and cimetidine. Br J 10Waud DR. Analysis of dose-response curves. In: Pharmacol 1980; 70:110P. Daniel EE, Paton DM, eds. Methods in pharma- 21Boyce MJ, Balasubramanian V, Wareham K. cology. New York: Plenum Press, 1975: Chap. 27. Cardiovascular effects in man of impromidine, a 11Jacobson ED, Linford RH, Grossman MI. Gastric novel and specific histamine H2-receptor agonist. secretion in relationship to mucosal blood flow Br J Pharmacol 1980; 70:157P.