(Wilson, Winnar, Quertermus & Tao, 1975), Rhesus monkey (Dajani & Collison,

Transcription

1 J. Physiol. (1978), 278, pp With 7 text -figure8 Printed in Great Britain EXOGENOUS PROSTAGLANDNS AND GASTR SERETON N THE AT BY R. BAKER, B. M. JAFFE,* J. D. REED, B. SHAW AND. W. VENABLES From the Departments of Surgery and Physiology, University of Newcastle upon Tyne, England, and *Washington University, St Louis, Mo., U.S.A. (Received 14 November 1977) SUMMARY 1. n the conscious gastric fistula cat PGE2 was shown to produce dose-related inhibition of gastric acid and pepsin secreted in response to pentagastrin, histamine and insulin. 2. PGF2. had little effect on pentagastrin-stimulated gastric acid and pepsin secretion. 3. PGE2 delayed the tachyphylaxis of gastric acid stimulated by pentagastrin and histamine, but not that stimulated by insulin. 4. Although not delaying tachyphylaxis of insulin stimulated acid, PGE2 delayed tachyphylaxis of insulin stimulated pepsin. NTRODUTON Prostaglandins of the E group are known to inhibit gastric acid secretion in man (Wilson, Winnar, Quertermus & Tao, 1975), Rhesus monkey (Dajani & ollison, 1976), dog (Robert, Schultz, Nezamis & Lancaster, 1976), rat (Dajani, Driskill, Bianchi & ollins, 1975) and cat (Konturek, Radecki & Pucher, 1976), and to inhibit pepsin secretion in the dog (Nezamis, Robert & Stowe, 1971) and cat (Konturek et al. 1976). Since PGE compounds occur naturally in the gastric mucosa and secretions of these mammalian species (Bennett, Friedmann & Vane, 1967; Bennett, Murray & Wyllie, 1968), it has been postulated that PGE compounds may play a role in the control of gastric secretion (Ramwell & Shaw, 1968) by means of a negative feed-back mechanism (Horton, 1969). The prostaglandins of the F series have been reported as not inhibiting gastric secretion (Robert, Nezamis & Phillips, 1967), or having only a transitory effect (Newman, De Moraes-Filho, Philippakos & Misiewicz, 1975). These studies were undertaken to evaluate the role of PGE in gastric secretion of acid and pepsin. nitially we studied the effect of exogenous PGE2 in the cat and characterized its effect on gastric secretion stimulated by pentagastrin, histamine acid phosphate and insulin, in both dose-response and plateaux experiments. This series of experiments was necessary to characterize the effects of exogenous PGE2 before the study of endogenous levels of prostaglandins during gastric secretion in the same animals. Both studies were performed to test the hypothesis that prostaglandins are involved in the physiological control of gastric acid and pepsin secretion.

2 442 R. BAKER AND OTHERS METHODS Experiments were carried out in six conscious cats (mean wt kg) with cannulated gastric fistulae prepared at least 1 yr earlier. The animals were fasted for 36 hr before each experiment but were allowed free access to water. Gastric secretion was collected continuously, measured at 15 min intervals, and the acid output determined by electrometric titration to ph 7 0 with 0.1 m-naoh (Radiometer, Denmark). The acid output was expressed as,uequiv. H+ kg body wt.-" 15 min-. Pepsin activity was measured by a haemoglobin digestion method (hiang, Sanchez-hiang, Wolf & Tang, 1966) and calculated as Beg bovine pepsin kg body wt.-l. 15 or 30 min-'. A percutaneous needle (Butterfly 21G, Abbott Ltd.) was inserted into a cephalic vein and 0-9 % saline infused at 12 ml. hr-1. Gastric secretion was stimulated by pentagastrin (Peptavlon,..., England), histamine acid phosphate (B.D.H., England) or insulin added to this infusion. Prostaglandin E2 (PGE2) and PGF. were added to the respective test. 1. Dose-responwe experiments A. Pentagadtrin stimulated gastri secretion Pentagastrin was infused at rates of 1, 2, 4, 8 and 16 jag kg-1 hr-l. The first dose of pentagastrin given in each experiment was infused for 45 min; subsequent doses were infused for 30 min. The control acid response to each dose was expressed as the mean response occurring in the final 15 min of the infusion of that dose of pentagastrin; the pepsin response was expressed as the output during the final 30 min. The effect of PGE2 on pentagastrin-stimulated secretion was determined by repeating the experiments adding a constant background infusion of PGE2 at rates of 10, 20 and 40 jug kg-1 hr-1. The effect of PGF2. was similarly determined in an experiment using a constant background infusion of PGF ag kg-l hr-1. B. Histamine stimulaed gastric secretion Histamine acid phosphate was infused at rates of 50, 125, 250 and 375 jug kg-l hr-l. Each dose was infused for 45 min and the acid response was expressed as the mean response occurring in the final 15 min of the infusion of each dose. n these experiments pepsin output was not measured as histamine is a poor stimulant of pepsin secretion in the cat (Albinus, Blair, Hirst, Reed, Schally & Shaw, 1977). The effect of PGE2 was determined by repeating these experiments while adding a constant background infusion of PGE2 at rates of 10, 20 and 40 jug kg-1 hr-1. Results were calculated as mean + 1 s.e. for each dose of agonist used and the mean results analysed for correlation by the method of least squares to give a log dose-response line (r = correlation coefficient). The significance of differences between slopes of regression lines was calculated by analysis of variance of the regressions and the significance of difference between doses of PGE2 calculated at the mean dose of agonist. Significance of difference between means was set at P < Plateaux experimente n the same six cats gastric secretion was stimulated on separate occasions by 5 hr infusions of pentagastrin 1 ug kg-1 hr-l, histamine acid phosphate 125jg kg-1 hr-1, and insulin 0-2 u. kg-1 hr-1. PGE2 was infused for the first 2 hr of these experiments at doses of 0 (control), 20 and 40 jug kg-1 hr-1 during histamine and pentagastrin infusion, and 0, 6 and 20 jug kg-l hr-1 during insulin stimulation. Gastric secretion was collected at 15 min intervals and acid output determined as above. Pepsin output was measured only during insulin infusion. All the results were expressed as meant 1 s.e. Significance of differences between means was calculated at each 15 min interval using Student's t test for paired data. RESULTS 1. Dose-response experiments A. Pentagastrin stimulated gastric secretion Pentagastrin infused at rates of 1-16 ug kg-1 hr-1 produced a significant log dose-response line for both acid (r = ) and pepsin (r = ) output (Fig. 1). ncreasing doses of PGE2 produced progressive inhibition of gastric acid secretion as

3 EXOGENOUS PROSTAGLANDNS 443 shown by the significant progressive parallel displacement to the right of the log dose-response lines (Fig. 1A). All three doses of PGE2 produce near complete inhibition of pepsin output (Fig. 1 B). nfusion of PGF2. at 40 jug kg-' hr-1 produced a significant displacement of the pentagastrin dose-acid response line (Fig. 2A) to the right. However, this inhibition was less than that produced by PGE2 10 jug kg-' hr-1. Pepsin output was slightly 40 jig kg-' hr-1 (Fig. 2B). but significantly increased by PGF2.-._ r- 400 k H A pg kg-' hr-' B P5 alone, r= _ 2200K / 1900/ / 1700r / 1600 X 1500 l / 800 / 700h 600 / 500 _ ~~ T P5+PGE2 = P5+PGE r= P5+PGE240 P r= g kg-' hr-' Fig. 1. The effect of PGE2 (10, 20 and 40 jug kg-" hr-1 i.v.) on gastric acid (A) and pepsin (B) secreted in response to incremental doses of i.v. pentagastrin in the same six conscious cats. Results in this Figure and all subsequent figures are expressed as mean + 1 s.e. of mean of the output per kilogram body wt. of the animal. The solid lines represent significant linear correlations (r correlation coefficient) between log = dose of agonist and response. B. Histamine stimulated gastric secretion As seen with pentagastrin, histamine acid phosphate infused at rates of ug kg-' hr-1 produced a significant log dose-response line for acid output (r = ) and increasing doses of PGE2 produced significant dose-related inhibition of gastric acid secretion (Fig. 3).

4 444 R. BAKER AND OTHERS 2. Plateaux experiments A. Pentagastrin stimulated gastric secretion nfusion of pentagastrin 1 jug kg-' hr-' for 5 hr produced a rapid rise to peak acid output at 45 min followed by a progressive fall to approximately 50% of peak output ce l,. A H 200 H le.0 dp ip O P5 alone, r= P5+PGF2a40 r= cr 100l 0 c , B P5 (pg kg-' hr-1) Ps+PGF2a 40 r= P5 alone r= R 1000 c 0. n 0. 0 _ Ps (pg kg-' hr-1) Fig. 2. The effect of PGF. (40 jug kg-' hr-1.v.) on gastric acid (A) and pepsin (B) secreted in response to incremental doses of i.v. pentagastrin r ntact HAP alone r= HAP+PGE2 10 r= HAP+PGE2 20 r= HAP+ PGE2 40 r= c _ HA HAP pg kg-1 hr--l Fig. 3. The effect of PGE2 (10, 20 and 40 jug kg-1 hr'.v.) on gastric acid secreted in response to incremental doses of i.v. histamine acid phosphate.

5 EXOGENOUS PROSTAGLANDNS 445 at 5 hr (Fig. 4A). The addition of PGE2 at doses of 20 and 40 jug kg-' hr-1 for the first 2 hr of similar experiments reduced the peak acid output during the first 2 hr of the experiments by approximately 50 and 75 % respectively. Following cessation of the simultaneous PGE2 infusion the acid output rapidly rose to levels significantly higher than in the control experiment at the corresponding time point, and equal to the peak level occurring in the control experiment (Fig. 4B and ). _ A m 300 _ 0 ~~~P5 1 Opg kg-' hr-1 _ ~B i B c /9k P5 1-0 kg- hr-1 6, 300-0_ *** **.; *k;: jl: ij* PGE24Oug kg-' hr-1 PGE240 8 kg- hr p5 1-0 pg kg-' hr Time (hr) Fig. 4. The effect of PGE2 (20 (B) and 40 () jug kg-' hr-1 i.v.) on pentagastrin (1 jug kg-' hr-1) stimulated gastric acid secretion compared with control infusions of pentagastrin (A). * Significant differences between test and control responses. B. Histamine stimulated gastric secretion Histamine acid phosphate infused at the rate of 125,sg kg-' hr-' for 5 hr produced a more gradual rise to peak acid output, which occurred at approximately 90 min, and which was followed by a progressive fall to approximately 50 % of peak acid output at 5 hr (Fig. 5A). Addition of PGE2 in doses of 20 and 40Rg kg-' hr-1 during the first 2 hr of similar experiments produced inhibition of acid output by approximately 50 and 75 % respectively. As in the pentagastrin experiments, cessation of PGE2 infusion produced a rise in acid output to levels significantly higher than in the control experiment at the corresponding time point, and equal to the peak level occurring in the control experiment (Fig. 5B, ).

6 446 R. BAKER AND OTHERS. nsulin stimulated gastric secretion nfusion of insulin at the rate of 0-2 u. kg-' hr-' over 5 hr produced significant hypoglycaemia and a significant stimulation of gastric secretion. Peak acid output occurred at 60 min and was followed by a rapid fall to approximately 30 % of the peak; this level was sustained over the remainder of the 5 hr experiment (Fig. 6A). Pepsin output followed a similar pattern, with peak output occurring at 60 min but v t_ m 300 w. E 200 _ A HAP 125ujg kg-' hr-' _ 400 B > e: 300 ^' u O _ -PGE2 PE2 20 2Ojg //9 kg -hrk9hap 125 jig kg-' hrq _ 0 f* * ** * * * ** * * *** ** * PGE24Ouigkg- hr-1 * HAP 125 jgkg-' hr,' Time (hr) Fig. 5. The effect of PGE2 (20 (B) and 40 () #ug kg-' hr- i.v.) on histamine acid phosphate (250 ug kg-1 hr-1 i.v.) stimulated gastric acid secretion compared with control infusions of histamine acid phosphate (A). * Significant differences between test and control. reduced to approximately 15% of peak at 5 hr (Fig. 7A). PGE2 infused at rates of 6 and 20 jug kg-1 hr-1 for the first 2 hr of the experiment reduced both acid and pepsin output. However after cessation of PGE2 infusion the acid output did not rise above control levels, whereas pepsin secretion was significantly greater than at the corresponding time point in the control experiments (Figs. 6B,, 7B, 0).

7 EXOGENOUS PROSTAGLANDNS 447 0) _ >7 : c A E 200 Lo T + >r _u + - LO E 400 B 0, nsulin 0 2 u. kg - hr- 200 r A PgE2 6 ji 0l- 0 _ PgE2 20 /, kg l hr l nsulin 0 2 u. kg-1 hr-'1 i L L J Time (h) Fig c 'ce W- 0_ cm T r _ c._ en a) 0-0) W- E LOl r 3000 F 2000k 10o0o OL B 4000 r 3000 F kg-' hr- L PgE2 20 jg kg-' hr- nsulin 0-2 u. kg-' hr Time (hr) Fig. 7 Fig. 6. The effect of PGE2 (6 (B) and 20 (),ug kg-' hr-1 i.v.) on insulin (0-2 u. kg-, hr-'.v.) stimulated gastric acid secretion compared with control infusions of insulin (A). * Significant differences between test and control. Fig. 7. The effect of PGE2 (6 (B) and 20 () jug kg-' hr-1.v.) on insulin (0.2 u. kg-1 hr-') stimulated gastric pepsin secretion compared with control infusions of insulin (A). * Significant differences between test and control. DSUSSON These results demonstrate that in the cat intravenous PGE2 inhibits gastric acid output stimulated by pentagastrin, histamine acid phosphate and insulin, as well as pepsin output stimulated by pentagastrin and insulin. The inhibition of acid output is dose dependent and the progressive parallel displacement of the log dose-response lines is compatible with a competitive type of inhibition. The design of these experiments, however, is such that the full criteria of competitive inhibition cannot be demonstrated. The degree of inhibition of pentagastrin-stimulated pepsin secretion by PGE2

8 448 R. BAKER AND OTHERS appears greater than the degree of inhibition of acid. This difference could be explained as a greater sensitivity of the peptic cell to PGE2, or as a lesser sensitivity of the peptic cell to the secretagogue used, as compared with the parietal cell. The massive inhibition of pentagastrin-stimulated pepsin secretion seen in the doseresponse experiments can be explained partly on the lesser sensitivity of the peptic cell to pentagastrin. At the greatest dose of pentagastrin used (16,ug kg-' hr-1), acid output was approximately 90 % of the maximal response seen with histamine (Figs. 1, 2) whereas pepsin output was approximately 60 % of the maximal response seen with insulin (Figs. 1, 6). n the insulin plateaux experiments pepsin secretion was high and the percentage inhibition of acid and pepsin output was similar. PGF2. appeared to have only approximately 25 % of the inhibitory effect of PGE2 on pentagastrin-stimulated acid secretion and actually had a slight stimulatory effect on pepsin secretion (Figs. 1, 2). Recently Albinus et al. (1977) have suggested that the tachyphylaxis of gastric acid stimulated by pentagastrin is a consequence of occupation of the gastrin receptor and that somatostatin, but not metiamide, delays tachyphylaxis. n these experiments PGE2 produced a delay in the tachyphylaxis of pentagastrin- and histaminestimulated gastric acid secretion. This suggests that PGE2 either occupies both the gastrin and histamine receptors or alters the affinities of the receptors for gastrin and histamine, so preventing tachyphylaxis. f this hypothesis is applied to insulin stimulated gastric acid tachyphylaxis then it would appear that PGE2 does not influence the cholinergic receptor since no delay in tachyphylaxis was seen. However, the manner in which insulin stimulates gastric secretion is complex and this hypothesis would be best tested with acid secreted in response to direct vagal stimulation or to infusion of stable cholinergic esters. Since PGE2 delayed the tachyphylaxis of pepsin but not that of acid stimulated by insulin, differences in the secretary mechanisms of acid and pepsin in response to insulin may exist. t is possible that the declining pepsin output, unlike the declining output of acid, is the outcome of depletion of stored secretary precursors. f this were so, then any inhibitor which prevented secretion of pepsinogen would be expected to delay this depletion in response to either pentagastrin or insulin. However, the pepsin output in response to pentagastrin declined as did that in response to insulin. Moreover, the amount of pepsin secreted in response to pentagastrin was very much less than that stimulated by insulin, which argues against depletion as the cause of tachyphylaxis. The concentration of pepsin in gastric juice secreted in response to both insulin and pentagastrin was significantly reduced by PGE2. Therefore, the inhibition of pepsin output was not only secondary to a reduction of secretary volume, as reported with the H2 receptor antagonists in cat (Albinus et al. 1977) and man (Konturek, Biernat & Oleksy, 1974). As shown by clearance studies in which the blood flow to acid output ratio increased, the reduced gastric mucosal blood flow seen during prostaglandin administration (Wilson & Levine, 1969) has been established as the result, and not the cause, of inhibition of gastric secretion (Jacobson, 1970; Main & Whittle, 1972). Reed & Smy (1976) suggested that gastric acid tachyphylaxis to gastrin may be partly explained by a primary tachyphylaxis of gastric inucosal blood flow to gastrin.

9 EXOGEN'O US PROSTAGLANDNS 449 f this were the case, the delay in tachyphylaxis of pentagastrin and histamine stimulated acid produced by PGE2 may partly be explained by persistence of the increased mucosal blood flow after the acid inhibitory effects have ceased. Robert (1975) has suggested that the inhibition of acid secretion by PGE is the result of a direct action on. the parietal cell. n the isolated parietal cell preparation PGE has been shown to inhibit histamine stimulated production Qf cyclic AMP (Major & Scholes, 1977). The inhibition by PGE of gastric secretion in reponse to all known stimuli, with the exception of cyclic AMP where results have been conflicting (Ranmwell & Shaw, 1968; Hohnke, 1974), is analogous to the situation produced by histamine H2 site antagonists and atropine. However, since the organization of the receptor mechanisms controlling gastric acid secretion are as yet unknown, it is impossible to state exactly where prostaglandins act. t is possible that they act to modify binding of the various agonists with their receptors or that they act at a more final and common step in the excitation-secretory coupling mechanism. PGE2 and PGF2:, were gifts from Dr Pike, Upjohn Ltd., Kalamazoo, U.S.A., and Upjohn Ltd., rawley, Sussex. We thank MNr K. Elliott, Mrs P. Dunn and Mrs W. Waller for excellent technical assistance. R. B. was supported by a grant from the Scientific and Research ommittee, Newcastle Area Health Authority (Teaching). REFERENES ALBNuS, MN.. BLAR, E. L., HRST, B. H., REED, J. D., SHALLY, A. V. & SHAW, B. (1977). The effects of somatostatin and metiamide on tachyphylaxis of pentagastrin stimulated gastric acid and pepsin secretion in the conscious cat. J. Physiol. 266, BENNETT, A., FREDMANN,. A. & VANE, J. R. (1967). Release of prostaglandin E from the rat stomach. Nature, Lond. 216, BENNETT, A., MURRAY, J. G. & WYLLE, J. M. (1968). Occurrence of prostaglandin E2 in the human stomach, and a study of its effects on human isolated gastric muscle. Br. J. Pharmac. 32, HANG. L., SANHEZ-HANG, L., WOLF. S. & TANG, J. (1966). The separate determination of human pepsin and gastricsin. Proc. Soc. exp. Biol. Med. 122, DAJ.TAN, E. Z. & OLLNSON, D. A. (1976). Gastric antisecretory actions of (15S)-15-methyl prostaglandin E2 methyl ester and natural prostaglandin E2 in Rhesus monkeys. Prostaglatmdins8 11, DAJAN, E. Z., DRSKLL, D. R., BANH, R. G. & OLLNS, P. W. (1975). omparative gastric antisecretory and antiulcer effects of prostaglandin El and its methyl ester in animals. Prostaglandins 10, HOHNKE, L. A. (1974). nteraction of dibutyryl cyclic AMP and PGE2 on stimulated gastric acid secretion in rats. Fedn Proc. 33, 329. HORTON, E. WV. (1969). Hypotheses on physiological roles of prostaglandins. Physiol. Rev. 49, JAOBSON, E. D. (1970). omparison of prostaglandin E, and norepinephrine on the gastric mucosal circulation. Proc. Soc. exp. Biol. med. 133, KONTLREK, S. J., BERNAT, J. & OLEKSY, J. (1974). Effect of metiamide, a histamine H2- receptor antagonist, on gastric response to histamine, pentagastrin, insulin, and peptone meal in man. Dig. Dis KONTUREK, S. J., RADEK, T. & PUHER, A. (1976). omparison of natural and synthetic prostaglandins on gastric and pancreatic secretions and peptic ulcer formation in conscious cats. Digestion 14, MAJOR, J. S. & SHOLES, P. (1978). The localization of a histamine H2 receptor adenylate cyclase system in canine parietal cells and its inhibition by prostaglandins. Agents & Actions. (n the Press). MAN,. H. Ml. & WHTTLE, B. J. R. (1972). Effects of prostaglandins E2 on rat gastric mucosal blood flow, as determined by 14-anllililie clearance. Br. J. Pharmnac. 44, P. 5 PHY' 278

10 450 R. BAKER AND OTHERS NEW]NAN, A., DE AORAES-FLHO, J. P. P., PHLPPAKOS, D. & MSEWZ, J. J. (1975). The effect of intravenous infusions of prostaglandins E2 and F2a on human gastric function. Gut 16, NNEZATMS, J. E., ROBERT, A. & STOWE, D. F. (1971). nhibition by prostaglandin E, of gastric secretion in the dog. J. Physiol. 218, RAMWELL, P. NV. & SHAM, J. E. (1968). Prostaglandin inhibition of gastric secretion. J. Physiol. 195, 34-36P. REED, J. D. & Siiy, J. R. (1976). Effect of isopropylnoradrenaline on tachyphylaxis of gastrin stimulated gastric acid secretion and miucosal blood flow in the anaesthetised cat. J. Physiol. 254, ROBERT, A. (1975). The role of prostaglandins in the etiology and treatment of gastrointestinal diseases. Proc. Sixth nt. ongr. Pharmacol. 5, ROBERT, A., NEZAMS, J. E. & PHLLPS, J. P. (1967). nhibition of gastric secretion by prostaglandins. Am. J. dig. Dis. 12, ROBERT, A., SHULTZ, J. R., NEZAMS, J. E. & LANASTER,. (1976). Gastric antisecretory and antiulcer properties of PGE2, 15-methyl PGE2 and 16,16-dimethyl PGE2. Gastroenterology 70, WLSON, D. E. & LEVNE, R. A. (1969). Decreased canine gastric mucosal blood flow induced by prostaglandin E,: a mechanism for its inhibitory effect on gastric secretion. Gastroenterology 56, WLSON, D. E., XVNNAN, G., QUERTERMUS, J. & TAO, P. (1975). Effects of an orally administered prostaglandin analogue (16,16-dimethyl prostaglandin E2) on human gastric secretion. Gastroenterology 69,