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Abstract
The present study examines the role of histamine in the stimulation of acid secretion induced by vagal nerve stimulation and by the muscarinic Ml agonist McN-A-343 in the totally isolated, vascularly perfused rat stomach. The stimuli were combined with an agent stimulating the cAMP system (isobutyl methylxanthine (IMX) or forskolin), a muscarinic antagonist (atropine or pirenzepine), or a histamine H2 antagonist (ranitidine). IMX and forskolin potentiated McN-A-343-stimulated acid secretion, yielding acid outputs of 280% and 260% of the sum of McN-A-343-and IMX-, or McN-A-343-and forskolin-stimulated outputs, respectively. Ranitidine inhibited acid secretion stimulated by McN-A-343 alone or in combination with IMX, whereas the forskolin-stimulated secretion was not influenced by the H2 antagonist. This strongly indicates that endogenous histamine potentiates muscarinic Mi-stimulated acid secretion by increasing parietal cell cAMP. Vagal nerve stimulation with IMX increased acid output from 12.2 ± 3.0 to 49.2 ± 9.3 μmol/60 min (mean ± SEM). The Ml antagonist pirenzepine and the M1/M2 antagonist atropine both significantly (p < 0.01) inhibited vagally stimulated acid secretion. Histamine output as measured in the venous effluent was unchanged by McN-A-343, whereas nerve stimulation induced a clear increase in venous histamine output, from 101 ± 21 before to 212 ± 28 pmol/min (mean ± SEM) after initiation of nerve stimulation. Histamine release was reduced to base-line levels by atropine but only insignificantly inhibited by pirenzepine, indicating a muscarinic M2 stimulation of histamine release in the rat stomach.
