Thiorphan and acetorphan inhibit gastric secretion by a central, non-opioid mechanism in the rat

A comprehensive review of the pharmacodynamics, pharmacokinetics, and clinical effects of the neutral endopeptidase inhibitor racecadotril

Racecadotril, via its active metabolite thiorphan, is an inhibitor of the enzyme neutral endopeptidase (NEP, EC 3.4.24.11), thereby increasing exposure to NEP substrates including enkephalins and atrial natriuretic peptide (ANP). Upon oral administration racecadotril is rapidly and effectively converted into the active metabolite thiorphan, which does not cross the blood–brain-barrier. Racecadotril has mainly been tested in animal models and patients of three therapeutic areas. As an analgesic the effects of racecadotril across animal models were inconsistent. In cardiovascular diseases such as hypertension or congestive heart failure results from animal studies were promising, probably related to increased exposure to ANP, but clinical results have not shown substantial therapeutic benefit over existing treatment options in cardiovascular disease. In contrast, racecadotril was consistently effective in animal models and patients with various forms of acute diarrhea by inhibiting pathologic (but not basal) secretion from the gut without changing gastro-intestinal transit time or motility. This included studies in both adults and children. In direct comparative studies with loperamide in adults and children, racecadotril was at least as effective but exhibited fewer adverse events in most studies, particularly less rebound constipation. Several guidelines recommend the use of racecadotril as addition to oral rehydration treatment in children with acute diarrhea.

Intracisternal neutral endopeptidase-24.11 inhibitors produce inhibition in gastric acid output: independence from opiate, bombesin, or neurotensin-mediated mechanisms

Intracisternal (ic) injection of the neutral endopeptidase-24.11 inhibitor phosphoramidon (1-100 nmol) produced a dose-dependent inhibition of gastric acid secretion in 2-h pylorus-ligated rats. The response resulted from a reduction in acid concentration and volume. Likewise, ic injection of another neutral endopeptidase-24.11 inhibitor Zincov (200 nmol) produced a 63% inhibition in gastric acid output. In contrast, neither intravenous injection of phosphoramidon (100 nmol) nor ic injection of the aminopeptidase inhibitor amastatin (100 nmol) produced any change in gastric acid secretion. The inhibitory effect of ic phosphoramidon (10 nmol) was not reversed by a dose of naloxone sufficient to antagonize the acid inhibitory effects of ic [D-Ala2-D-met5]enkephalinamide (8.5 nmol). Moreover, phosphoramidon-induced inhibition of acid was not reduced by the centrally effective bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 or by reserpine pretreatment at a dose effective to antagonize ic neurotensin-induced inhibition in acid secretion. These results suggest that an endogenous neutral endopeptidase-24.11 sensitive substrate may act in the brain to inhibit gastric acid output by mechanisms independent of CNS opiate, bombesin or neurotensin activity.

Effect of alpha 2-adrenoceptor agonists on gastric pepsin and acid secretion in the rat

1. The purpose of the present study was to analyze the effects of the alpha 2-adrenoceptor agonists clonidine, guanabenz, detomidine and medetomidine on pepsin secretion in conscious rats provided with gastric chronic fistula and to compare this with acid secretion. 2. Basal interdigestive gastric secretion, which is mainly neurally driven in the rat, and the secretion directly stimulated by the two main stimulants of chief cells, cholecystokinin octapeptide (CCK8) and methacholine, were studied. 3. Basal secretion of pepsin and acid was inhibited by all four drugs with comparable EC50S. 4. CCK-stimulated pepsin and acid secretion was less sensitive than basal pepsin and acid secretion to alpha 2-adrenoceptor inhibition. 5. Methacholine-stimulated pepsin and acid secretion was not changed by clonidine and guanabenz; methacholine-stimulated acid was even marginally increased by clonidine. 6. These results do not favour the presence of alpha 2-receptors on chief cells in the rat stomach. They rather suggest that pepsin inhibition by alpha 2-adrenoceptor agonists is indirect and due to central or peripheral inhibition of the discharge of nerve fibres activating pepsin secretion.

Immunoautoradiographic localisation of enkephalinase (EC 3.4.24.11) in rat gastrointestinal tract

Enkephalinase (EC 3.4.24.11, membrane metalloendopeptidase) is a zinc peptidase expressed by neurons and a variety of epithelial cells, and responsible for the inactivation of enkephalins in brain. Its functions in the gastrointestinal (GI) tract are less well understood although enkephalinase inhibitors were reported to induce a constellation of antisecretory and motor responses. Its localisation in various segments of the rat GI tract was established autoradiographically using a 125I-labelled monoclonal antibody. All along the GI tract, the highest immunoreactivity was found in mucosal layers e.g., in intestinal villi, basal epithelial layers of the oesophagus or gastric cardia, muscularis mucosae of the stomach and large intestine. The immunoreactivity was also high in the stomach submucosae and moderate in the muscularis propria of the caecum. A faint patchy immunoreactivity was also observed in several other layers. This distribution suggests that the membrane peptidase is expressed by enterocytes and a variety of other cells. Its high expression in mucosal layers is consistent with its participation in protein digestion and also in the inactivation of endogenous peptides, particularly the enkephalins, acting at this level to control secretory mechanisms and hydroelectrolytic fluxes. Its presence in submucosal layers may account for some naloxone-reversible motor responses elicited by enkephalinase inhibitors.