Histamine and abnormal automaticity in barium- and strophanthidin-treated sheep Purkinje fibers

We used intracellular microelectrodes to study the effects of histamine on both normal and abnormal automaticity in sheep Purkinje fibers. Histamine, dimaprit and 4-methylhistamine caused a similar reduction of the action potential duration in driven Purkinje fibers. Histamine (10-6 M) induced spontaneous activity in p previously quiescent preparations more often than did equimolar concentrations of dimaprit and 4-methylhistamine. The effects of histamine on automaticity were enhanced in the presence of barium In fact histamine, at concentrations which were unable to induce automaticity in normal preparations, induced it in the presence of barium. In Purkinje fibers manifesting barium-induced automatic activity, histamine (10-7--10-6M) significantly increased the average number of spontaneous action potentials and shortened the time of their appearance. In the same range of concentrations, histamine dose-dependently increased the iological manifestation of calcium overload. Histamine (10-6--10-4M) increased the OAP amplitude of strophanthidin -treated Purkinje fibers, eventually inducing triggered extrabeats. All these previously described effects were selectively blocked by cimetidine (10-5 M). It is concluded that histamine may induce cardiac arrhythmias under conditions of calcium overload and that this effect may be due to induction or enhancement of oscillatory afterpotentials.