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.