Effects of histamine on electrophysiological properties and triggered activity in guinea pig papillary muscles.

The effects of histamine on action potentials (APs) and on early afterdepolarizations (EADs) were studied with standard microelectrode and computer in guinea pig papillary muscles. Experiments offer evidence that histamine (6.0 mumol/l) results in a significant diminution in action potential duration at 50% repolarization (APD50) and 90% repolarization (APD90) (data taken from no abnormal automaticity and oscillatory potential cells, 10 out of 19 preparations). Particularly, histamine can induce EADs (about 58%), oscillatory potentials and triggered activity. EADs frequently occur at low stimulation rates of 0.2-1.0 Hz and with prolongation of the action potential duration. EAD-induced triggered activity may exhibit fast spontaneous APs. Cimetidine (10 mumol/l, n = 6), an H2-receptor antagonist, antagonizes the effects of histamine. In the presence of cimetidine no EADs are observed by perfusing histamine. An H1-receptor antagonist, chlorpheniramine (10 mumol/l, n = 9), has no effect on the alterations of APs induced by histamine in ventricular myocardium. The results suggest that the mechanisms of histamine-induced shortness of AP duration, EADs and triggered activity may be related to the H2 receptor-mediated enhancement of the slow inward calcium current.