The role of endogenous adenosine 3',5'-cyclic monophosphate (cAMP) in the mechanisms of action of gastrin and carbachol on aminopyrine accumulation in isolated pig and rat parietal cells was investigated. In pig cells, pentagastrin (100 nM) alone stimulated aminopyrine accumulation, an action significantly reduced by the protein kinase A inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMP[S]; 100 mu M), In rat cells, gastrin-17 (100 nM) was incapable of stimulating aminopyrine accumulation, but it potentiated the action of histamine (100 mu M). Carbachol (10 mu M) stimulated aminopyrine accumulation and potentiated the action of histamine, and its action was potentiated in a dose-dependent manner by Sp-adenosine 3',5'-cyclic monophosphothioate (Sp-cAMP[S]; a cAMP analogue) in both species. The effect of carbachol was dose dependently reduced by Rp-cAMP[S]. The basal cAMP in pig parietal cells was 3.5-fold higher than that in rat parietal cells. Histamine (100 mu M) and 3-isobutyl-1-methylxanthine (IBMX; 100 mu M) only slightly elevated the cAMP content (1.2- to 2.9-fold the basal level) in both pig and rat parietal cells. Their combination, however, increased the cAMP level by 8- to 38-fold, but it did not increase aminopyrine accumulation above that elicited by histamine alone. Gastrin did not alter the cAMP levels in parietal cells of either of the two species. Both gastrin and carbachol increased cytosolic free Ca2+ in enriched pig and rat parietal cells. These results indicated that in isolated pig and rat parietal cells 1) secretagogues that elevate intracellular free Ca2+, such as gastrin and carbachol, require a certain cAMP level to be effective in stimulating aminopyrine accumulation; 2) a further increase over a certain level of cAMP does not result in an increased aminopyrine accumulation; and 3) parallel activation of Ca2+- and cAMP-dependent pathways appears to be necessary to effectively elicit aminopyrine accumulation.
