Effects of H1- and H2-receptor antagonists on calcium-dependent action potentials produced by histamine in rabbit left auricles depolarized by potassium.

After inactivation of the fast sodium channels by partial depolarization (25 mmol/l K+ Tyrode solution) histamine (10(-6)-10(-5) mol/l) restored the electrical and mechanical activity in isolated electrically paced left auricle of rabbit. Transmembrane action potentials restored by histamine in this condition have a nearly normal height of overshoot with a low resting potential and with a very low rate of rise. The AP-restoring effect of histamine was affected neither by beta-adrenergic blocking agent, Pindolol (4 X 10(-7) mol/l) nor H2-receptor antagonist, cimetidine by (2 X 10(-6) mol/l) but was antagonized by H1-receptor blocker, mepyramine (10(-5) mol/l) and by the selective Ca/2-channel blocker D-600 (2 X 10(-6) mol/l). The results suggest that in this species histamine may stimulate the slow inward Ca2+ current mediated by H1-receptors.