Action of divalent cations on membrane potentials and contractility in rat atrium

Abstract

The effects of the chlorides of Ba, Cd, Co, Mg, Mn, Hg, Ni,Sr, and Zn on the transmembrane action potential and isometric tension developed in the rat atrium were observed. The concentrations in the perfusate bathing the atrium varied from 10-3 to 10-6 m. Little alteration was observed in the resting membrane potential or overshoot of the action potential. Ba and Zn significantly prolonged the duration of the action potential, whereas Cu and Cd significantly shortened the duration of the action potential. Ba, Hg, and Sr produced an increase in isometric tension ranging from 60 to 175%, whereas Mn, Mg, Cu, Cd, Zn, Co, and Ni produced a decreasd in contractile tension ranging from 50 to 90%. When the perfu-sion medium was deficient in Ca, the positive inotropic effect of Ba and Hg was not obtained. Sr produced a positive inotropic effect in the absence of Ca but the effect was not sustained. The perfusion of Tyrode solution containing twice the normal concentration of Ca reversed the depressed contractile tension produced by Mn, Mg, Cu, Cd, Zn, Co, and Ni. No consistent correlation was found between the various parameters of the action potential and contractility: this can be explained on the basis that the divalent cations not only alter the transmembrane action potential, but more probably, act on the excitation-contraction coupling. The ability of high concentrations of Ca to reverse the depressed contractile tension produced by certain cations seems to rule out a structural alteration of the contractile proteins, and suggest that the negative inotropic effect is due to an interference with Ca activity which alters the excitation-contraction coupling mechanism resulting in a decrease in contractility.