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The secretory response to 16, 16-dimethyl prostaglandin E2 (DMPG) administered orally in 4 different dosages and to placebo was evaluated in healthy volunteers over a 2-hr period. During stimulation of gastric secretion by histamine, DMPG at the highest dosage (1.5 mug per kg) reduced volume by 47% and acid output by 79%. Pepsin concentration was not affected. At the same dose, DMPG inhibited basal secretion by 54% and 99% for volume and acid output, respectively. There were no side effects secondary to DMPG administration, which indicates that this compound may be useful in treating peptic ulcer disease.

The effects of U-46619, a stable analogue of the prostaglandin endoperoxide PGH2, were studied on canine gastric acid secretion, gastric mucosal blood flow, and secretion of mucus into gastric juice and compared to those of PGE2. U-46619 was approximately four and three times as potent as PGE2 in inhibiting acid secretion and stimulating mucus secretion, respectively. When infused at a low dose, U-46619 inhibited acid secretion directly without causing a decrease in the ratio of mucosal blood flow to volume rate of secretion (R), this effect being similar to that observed for PGE2. However, unlike PGE2, U-46619 when administered in a higher dose caused a decline in R while decreasing acid secretion and mucosal blood flow, suggesting a primary restriction of blood flow. The antisecretory effects of arachidonic acid may be due in part to the endogenous formation of prostaglandin endoperoxides as opposed solely to prostaglandin formation. Considering the antisecretory and mucogenic actions of U-46619, nontoxic analogues of prostaglandin endoperoxides may be of value as antiulcer agents.

Heat-labile enterotoxin preparations obtained from two enteropathogenic strains of Escherichia coli of porcine and human origin were shown to stimulate adenylate cyclase activity of human embryonic intestinal epithelial cells in culture. Comparable results were also obtained when cholera toxin was used. The degree of enzyme stimulation was proportional to the concentration of enterotoxin. Similar preparations from two strains of non-enterotoxigenic E. coli had no effect on adenylate cyclase activity. Cells exposed to enterotoxin could be washed after 1 min of contact time without altering the subsequent course of maximum adenylate cyclase activity, which was maintained for at least 18 h at 37 C. During long periods (18 h) of tissue culture incubation, the determination of adenylate cyclase activity was 200- to 300-fold more sensitive than quantitating fluid accumulation in the adult rabbit ileal loop model. Decreasing the incubation time appreciably reduced the sensitivity of the epithelial cells to enterotoxin. E. coli enterotoxin is an effective activator of nonintestinal adenylate cyclase systems. Treatment of KB and HEp-2 cell lines with enterotoxin also resulted in significant enzyme stimulation. The intestinal epithelial cell tissue culture model provides a sensitive homogenous biological system for studying the response of intestinal adenylate cyclase to enterotoxin while eliminating the numerous cellular and tissue components present in the ligated ileal loop model.

Kinetic behavior most consistent with the presence of two independent , but simultaneously acting, regulatory effector sites for prostaglandins has been presented for adenylate cyclase (EC 4.6.1.1) of rabbit intestinal epithelial cells. One site regulates activation of the catalytic site, while the other site regulates inhibition. A synthetic prostaglandin analogue, 7-oxa-13-prostynoic acid, is recognized at both sites in a concentration-dependent manner. At concentrations of 7-oxa-13-prostynoic acid less than 45 μg/ml, activation is seen, while at higher concentrations, inhibition is seen. Different naturally occurring prostaglandins appear to be site-specific. Prostaglandin E 1 gives only activation of the cyclase, while prostaglandin A 1 gives only inhibition of the activated cyclase. When saturating concentrations of prostaglandin E 1 are used to activate adenylate cyclase, no further activation by 7-oxa-13-prostynoic acid can be elicited, indicating that both molecules activate at the same site. The similarity of inhibition constants for both 7-oxa-13-prostynoic acid and prostaglandin A 1 suggests that the mode of binding is the same for both compounds and that they probably inhibit by acting at the same site. The inhibition by 7-oxa-13-prostynoic acid and by prostaglandin A 1 overrides enzyme activation produced by either Escherichia coli enterotoxin, prostaglandin E 1 , or sodium fluoride, suggesting that in intestinal adenylate cyclase this site is the primary regulatory site (i.e., primary allosteric effector site) for enzyme activity. These data suggest that sites exist on adenylate cyclase which would allow prostaglandins to serve as the intracellular messengers by which the cell controls its adenylate-cyclase-mediated response to extracellular stimulation, as with hormones.

The effects of Escherichia coli enterotoxin on the activity of adenyl cyclase in intestinal mucosa were determined in the small intestine of the rabbit. In vivo incubation of enterotoxin in isolated intestinal loops for 2-4 hr led to an increase in the activity of adenyl cyclase in mucosal epithelial cells. The degree of stimulation of the enzyme was proportional to the concentration of enterotoxin. The levels of activity were additive when enterotoxin-treated, homogenized mucosal cells were exposed to prostaglandin E1. The activity of adenyl cyclase in both enterotoxin-treated and control preparations was stimulated markedly by NaF. Phosphodiesterase activity was not altered in enterotoxin-treated loops. Enterotoxin was partially inactivated when held at 60 C for 15 min; it was completely inactivated at 100 C. Differences in the degree of stimulation of adenyl cyclase were noted among strains of E. coli. Enterotoxin from a strain isolated in an adult in Calcutta with choleralike symptoms was a more potent stimulator of adenyl cyclase than enterotoxin from a strain of E. coli associated with a mild case of diarrhea in an infant in Chicago.

The response of the cyclic nucleotide system (cAMP, cGMP, adenylate cyclase, guanylate cyclase, and specific phosphodiesterases) to two gastric acid secretagogues, histamine and acetylcholine, and two secretory inhibitors, prostaglandin E2 and secretin, was studied in vivo and in vitro in canine gastric fundic mucosa. Histamine and acetylcholine in vivo failed to stimulate cAMP but significantly increased cGMP; in vitro they affected neither adenylate cyclase nor guanylate cyclase. Prostaglandin E2 and secretin, however, increased cAMP in vivo and significantly stimulated adenylate cyclase in vitro. Specific phosphodiesterases were unaffected by these compounds. The changes, while not specifically localized to the acid-producing cells, are consistent with the suggestion that the control of canine gastric acid secretion may be mediated by changes in mucosal cAMP and cGMP.

The effects of indomethacin on intestine mucosal cAMP, intestinal fluid secretion, and mucosal and fluid PGE were studied in rabbits in vivo following challenge with cholera toxin. Indomethacin had no effect on cholera toxin-induced fluid secretion or cAMP accumulation. Inhibition of PGE synthesis was achieved by the administration of two but not one injection of indomethacin. These studies provide evidence against a role for PGE in mediating cholera toxin-induced secretion and point out the need to measure prostaglandin levels when using prostaglandin synthetase inhibitors in vivo.

The effects of orally administered prostaglandin E2, 16,16-dimethyl prostaglandin E2 and U-46619, an analogue of the prostaglandin endoperoxide PGH2, on gastric secretory volume, acid and mucus were studied in the rat. All of the compounds significantly increased the volume of gastric secretion, mucus secretion, measured as N-acetylneuraminic acid and mucus synthesis measured as the incorporation of [3H]-glucosamine into mucosal glycoprotein; however, only PGE2 and 16,16-dimethyl PGE2 inhibited acid secretion. U-46619, 1.5 mg/kg provided significant protection against ethanol-induced gastric ulcers, an effect that has been previously shown for the other two compounds. These studies provide additional evidence that prostaglandin induced mucosal protection may by related to an effect on mucus and on stimulation of nonparietal cell gastric secretion. Further study of these parameters may be important in the development of antiulcer drugs for long term clinical use.