Herbert S. Ormsbee

Action of Serotonin on the Gastrointestinal Tract

Abstract Serotonin is localized in the enterochromaffin cells of the gastrointestinal mucosa and within neurons in the enteric nervous system. It can be released into the blood or into the lumen of the gut. Serotonin inhibits gastric acid secretion and may be an endogenous enterogastrone. It appears to stimulate the production and release of gastric and colonic mucus. When placed on the serosal surface of the rabbit ileum in vitro,serotonin increases short-circuit current and inhibits the mucosal-to-serosal flux of NaCl. Serotonin potentially is involved in the pathogenesis of diarrhea due to amoebae or cholera. As an enteric neurotransmitter, serotonin affects neural modulation of gut smooth muscle function and may act either directly on mesenteric vascular smooth muscle or through enteric nerves to influence gastrointestinal blood flow. Thus, since serotonin may be involved in multiple physiological processes of digestion, this report reviews and summarizes the role of this ubiquitous substance in the major functions of the gastrointestinal system.

Cholecystokinin stimulates neuronal receptors to produce contraction of the canine colon

The motor effects of cholecystokinin 26-33-amide (CCK octapeptide; CCK-OP) and several purported CCK receptor antagonists on canine colonic circular muscle were determined in pentobarbital anesthetized dogs. Intravenous injections of CCK-OP had no effect on colonic motility at doses that contracted the gallbladder, stomach and duodenum. CCK-OP delivered by intraarterial injection to a small segment of the proximal colon produced a dose related increase in colonic motility with one-half maximum response at 12 ng/Kg and maximum response at 50 ng/Kg. The effects of intraarterial injections of several established CCK-receptor antagonists on proximal colonic responses to intraarterial injections of CCK-OP were determined. Proglumide, 10 mg/Kg, did not produce colonic contractions itself, but antagonized CCK-OP-induced responses. Carbobenzyloxy (CBZ)-CCK27-32-amide antagonized CCK-OP-induced colonic responses and also had no effect on basal colonic motility (0.1-1 and 5 micrograms/Kg). Neither compound antagonized acetylcholine- induced colonic responses. Butoxycarbonyl (BOC)-CCK31-33-amide increased basal colonic motility, but did not alter CCK-OP-induced responses at doses of 0.1 and 0.2 mg/Kg. Dibutyryl-cGMP at a dose of 0.1 mg/Kg did not affect basal motility or CCK-OP-induced contractions. At a dose of 1.0 mg/kg it increased basal colonic motility but did not affect CCK-OP-induced contractions. Pentagastrin increased colonic motor activity only at a dose of 5 micrograms/Kg, i.a., a much higher dose than effective doses of CCK-OP. The mechanism of CCK-OP-induced colonic motor effects also was determined. Atropine sulfate, 100 micrograms/Kg, i.v. significantly reduced both intraarterial acetylcholine-and CCK-OP-induced maximum colonic contractions. Tetrodotoxin, at intravenous doses that completely block neuronal activity, did not affect maximum acetylcholine-induced contractions but practically eliminated maximum CCK-OP-induced maximum colonic responses. In conclusion, intraarterial CCK-OP produces circular muscle contraction of the canine proximal colon that is mediated by stimulation of specific CCK receptors which produce the release of acetylcholine from cholinergic enteric neurons. Proglumide and CBZ-CCK27-32-amide are effective CCK receptor antagonists at these colonic neuronal receptors.

Dopamine Agonists/Antagonists in the Treatment of Gastrointestinal Diseases

The smooth muscle relaxant effects of dopamine observed in vascular tissue also extend to the smooth muscle of the gastrointestinal tract. For example, dopamine is known to relax the lower esophageal sphincter and has been suggested as a mediator of receptive relaxation of the stomach (Valenzuela, 1976). In contrast, contraction of GI muscle to dopamine has been reported from several locations in the GI tract in at least two species (Lanfranchi et al., 1978a; Anuras, 1981). Besides these smooth muscle effects, dopamine has a number of other interesting effects on GI function. Dopamine reduces pentagastrin-stimulated acid secretion in man, and it has been implicated in animal experiments to be involved in the pathogenesis of duodenal ulcer disease (Szabo, 1979). In fact, dopamine agonists may be antiulcerogenic drugs. A dopaminergic effect on electrolyte absorption has been demonstrated for the rabbit ileum. An additional effect of dopamine on GI function is its ability to increase blood flow through the arteries supplying most of the stomach and small intestine. Whether some or all of these GI effects of dopamine are physiologically relevant, especially in man, is still very much open to question. The presence of gut dopamine receptors appears to vary not only among species but among organs of the GI tract within one species. A thorough characterization of these receptors has not been performed. The evidence to date will be presented in the text of this chapter.

Dimethylphenylpiperazinium(DMPP)-induced relaxation and elevation of cyclic GMP content in canine lower esophageal sphincter (LES)

Cyclic GMP has been proposed as an intracellular mediator of neuronally-induced relaxation in lower esophageal sphincter (LES) smooth muscle. If cyclic GMP is indeed an intracellular messenger, then agents that activate enteric neurons of the sphincter [e.g. the ganglionic nicotinic receptor agonist dimethylphenylpiperazinium (DMPP)] also should cause a relaxation that is associated with an increase in cyclic GMP content. In isolated smooth muscle strips of canine LES, DMPP produced a rapid relaxation that was accompanied by a significant (P less than 0.05) increase in cyclic GMP content with no change in cyclic AMP content. Pretreatment of tissues with either tetrodotoxin or hexamethonium antagonized both the DMPP-induced relaxation and the associated increase in cyclic GMP. The combination of phentolamine and meclofenamic acid also antagonized both the relaxation and the elevation of cyclic GMP produced by DMPP. Electrical field stimulation (EFS)-induced relaxation and elevation in cyclic GMP was unaltered by meclofenamic acid and phentolamine. In conclusion, DMPP (like neuronal electrical activation) relaxed isolated canine LES through an enteric neuronal inhibitory pathway. The presence of phentolamine and meclofenamic acid did not affect EFS-induced effects, but blocked both the relaxation and the increase in cyclic GMP produced by DMPP, suggesting a more complicated pathway for DMPP in the release of inhibitory transmitter.

Effects of KC 2450 on the lower esophageal sphincter in vivo and in vitro

The effect of KC 2450 (racemic 3,5-cis-3-methylamino-2,3,4,5-tetrahydro-1-benzoxepine-5-ol hydrochloride) on lower esophageal sphincter pressure in pentobarbital-anesthetized dogs was determined and compared to the effect of metoclopramide. The ED20 value (i.e. the dose that increased lower esophageal sphincter pressure 20 mm Hg) was 0.72 (0.45-1.04) mg/kg i.v. for KC 2450, significantly different from 2.18 (1.30-3.42) mg/kg i.v. for metoclopramide (P less than 0.01). The superior potency of KC 2450 over metoclopramide also was demonstrated at a dose of 2 mg/kg i.v.; KC 2450 produced an increase in sphincter pressure of 43.2 +/- 4.4 mm Hg and metoclopramide produced an increase in sphincter pressure of only 28.5 +/- 5.4 mm Hg (P less than 0.05). Intraduodenally administered KC 2450 increased lower esophageal sphincter pressure at a threshold dose of 2 mg/kg with 10 mg/kg producing an increase in pressure of 53.2 +/- 9.9 mm Hg. KC 2450-induced increases in sphincter pressure were not affected by bilateral cervical vagotomy or ketanserin, but were eliminated by atropine and reduced by neuronal blockade using tetrodotoxin (TTX). KC 2450 effects also were determined in isolated circular strips of lower esophageal sphincter muscle. KC 2450 produced a concentration-related increase in canine (EC50 = 27 microM) and opossum (EC50 = 199 microM) sphincter muscle strip tension. The KC 2450 concentration-response curve was antagonized by atropine in canine and opossum sphincter muscle strips. Neuronal blockade of canine sphincter muscle with TTX antagonized the KC 2450 concentration-response curve in a non-competitive manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of cyclic amp dependent protein kinase (cA-PK) is associated with relaxation of canine lower esophageal sphincter (LES) by isoproterenol but not by electrical field stimulation (EFS)

ACTIVATION OF CYCLIC AMP DEPENDENT PROTEIN KINASB (CA-PK) IS ASSOCIATED WITH RELAXATION OF CANINE LOWER ESOPHAGEAL SPHINCTER (LES) BY ISOPROTERENOL BUT NOT BY ELECTRICAL FIELD STIMULATION (EFS). M.S. Bsmette, M. Graus, T.J. Torphy and H.S. Ormsbee, III. Departmentofmaxgy, Smith Kline 6 French Laboratories, Philadelphia, PA 19101. Relaxation of LES smooth muscle is associated with elevations of either CAMP or cGMP content. Since the biological effects of the cyclic nucleotides are produced through the activation of their respective kinases, we have begun to examine the ability of several drugs and EFS to increase the activity of CA-PK. The gastroesophageal junction was removed from anesthetized dogs. The tissue was opened along the lesser curvature and the mucosa removed. Strips of circular smooth muscle (1.0-1.5 cm hy 0.3 cm) were cut and placed in a” oxygenated Krebs solution at 37“C with a” initial tension of 2 g. Muscle strips were electrically stimulated (4.0 Hz, 1.0 msec, 65V) and only those that gained tqne spontaneously and relaxed to EFS were used. Frequency-response ~“1‘“~s to EFS and cumulative cancentration-response curves to sodium nitroprusside (SNP), and to selective phosphodiesterase inhibitors (M&B 22,948 and SK&F 94120) were constructed in the presence of atropine (luH), phentolamine (IouM), propranolol (luM), and meclofenamic acid (1uM). In experiments examining the action of isoproterenol, propranolol was not added to the buffer. At the conclusion of the concentration-response studies, tissues were flashfrozen for determination of :yclic nucleotide content and CA-PK activity. Cyclic nucleotida’content was measured by RIA. Protein kinase activity was measured by the incorporation of 32Pi into histone (Type IIA) in the presence and absence of CAMP and the activity ratios (-CANP/+CAMP) were determined. I” tissues treated with isoproterenol and SK&F 94120, the activity ratio of CA-Pd was significantly increased. On the other hand EFS, SNP, or M&B 22,94R did not produce a significant change in the activity ratio of CA-PK. This is the first demonstration qf a” activation of PK in smooth muscle of canine LES and supports OUT hypothesis that there exist both CAMP and cGMP mediated pathways of relaxation in LES.