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4. HISTAMINE AND THE CARDIOVASCULAR SYSTEM Effects of H 1- and H2-reeeptor antagonists on calcium-dependent action potentials produced by histamine in rabbit left aurieles 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/1) 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  10 -7 mol/1) nor H2-receptor antagonist, cimetidine (2  10 -6 mol/l) but was antagonized by Ha-receptor blocker, mepyramine (10 -5 mol/1) and by the selective Ca+:-channel blocker, D-600 (2 x 10 -6 mol/1). The results suggest that in this species histamine may stimulate the slow inward Ca 2+ current mediated by Hi-receptors.