Potentiation by Carbachol and Aminophylline of Histamine‐ and db‐cAMP‐Induced Parietal Cell Activity in Isolated Gastric Glands

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Abstract
The response to combinations of gastric acid secretagogues was studied in isolated glands from the rabbit gastric mucosa in terms of changes in oxygen consumption and accumulation of the weak base aminopyrine (AP). The latter reflects the acid secreting status of the glands. The following secretagogues were investigated: histamine, carbachol, aminophylline and db‐cAMP. The histamine respiratory dose‐response curve was shifted to the left in the presence of the phosphodiesterase inhibitor aminophylline. Both ED‐50 and maximum response were significantly increased. Histamine‐induced AP accumulation was also strongly enhanced by aminophylline (5 × 10‐4 M). These results are consistent with the hypothesis that histamine stimulation of acid secretion is mediated by cyclic AMP. Carbachol‐stimulated oxygen consumption could not be potentiated by aminophylline and the combined effect was only additive. The response to a combination of histamine and carbachol was a significant increase in oxygen consumption above what could be expected from an additive effect alone. Carbachol addition to glands prestimulated with histamine gave a rapid increase in the respiratory rate resulting in a new steady state level within 10–15 min, as compared with a time constant of about 40 min when both drugs were added simultaneously. Likewise AP accumulation increased more rapidly and reached a higher value after addition of histamine + carbachol as compared with histamine alone. The db‐cAMP‐stimulated oxygen consumption was in all respects equally affected by carbachol as was histamine stimulation. This indicates that the well known cholinergic potentiation of histamine stimulation is not due to an increased sensitivity of the histamine receptor but is of a more general nature. A mechanism involving intracellular availability of Ca2+ is proposed as one possible explanation of this potentiation.