Effects of Ca2+ ions on gastric acid secretion by the rat isolated stomach

The role of Ca2+ in the stimulatory action of histamine has been evaluated in the isolated gastric fundus from immature rats, by changing the concentration of calcium ions in the bathing solutions. Lowering Ca2+ to 1.2 mM greatly enhanced the secretory response to histamine, while leaving unaffected that to the H2-receptor agonist, dimaprit. The effect of histamine was competitively antagonized by ranitidine (pA2 = 6.78) in normal solutions; conversely in 1.2 mM Ca2+, the antagonism by ranitidine became unsurmountable. Basal rates of acid secretion did not change in low Ca2+ solutions, whereas they were reduced approximately by 50% in Ca2+-free media. Finally, the secretory response to theophylline was significantly lower in low Ca2+ solutions in comparison with that in control conditions. From the above results it may be concluded that changes in the concentration of Ca2+ ions caused different changes in the secretory response of the rat stomach in the various experimental conditions. The marked enhancement of the response to histamine observed in low Ca2+ is unlikely to be connected with H2-receptors, as suggested by the lack of interference in the response to dimaprit, but it could be related to intracellular mechanisms (H+/K+-ATPase, carbonic anhydrase activation etc.).