Modification of isolated atrial chronotropic histamine receptor responses by alterations in the calcium concentration of the bathing medium

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.).

Rat gastric secretion "in vitro": interaction between histamine and various antisecretagogues acting by different mechanisms

The interaction between histamine and various antisecretagogues acting by different mechanisms has been investigated in the isolated fundus from the rat stomach. Histamine evoked a concentration-dependent stimulatory effect which was competitively antagonized by the H2-receptor antagonist, ranitidine and non competitively by the H+/K+-ATPase inhibitor, omeprazole. The histamine induced secretion was highly resistant to the action of the calcium entry blocker verapamil, somatostatin and KSCN, but some inhibition was obtained with the calmodulin antagonist, trifluoperazine. Removal of calcium ions from the bathing media (both mucosal and serosal) greatly enhanced histamine-induced gastric secretion. The results suggest that the relationship between receptor stimulation and the intracellular events leading to acid secretion is far from being elucidated.

Altered calcium concentration and rabbit atrial H1-receptors

Rabbit atria were incubated in various concentrations of calcium (0.9, 1.8 and 3.6 mM) and then homogenized and histamine H1-receptors analysed by radioligand binding using (3H)-pyrilamine. Only atria incubated in 3.6 mM calcium showed an alteration in (3H)-pyrilamine binding. In these tissues the Kd of H1-receptors for (3H)-pyrilamine was increased suggesting a decrease in the affinity of the H1-receptor. That this decreased affinity is due to altered calcium concentrations was demonstrated by showing that no change in histamines' chronotropic effects were produced by decreasing the sodium concentration to produce the same calcium/sodium ratio compared to high calcium alone. We conclude that calcium has effects directly on H1-receptor affinity and appears to produce a decrease in the ability of histamine to bind.