The dependence of slow inward current in Purkinje fibres on the extracellular calcium‐concentration

Abstract

In Na-free solution electrical constants of short Purkinje fibers were similar to those in Tyrode solution. Alterations in the extracellular Ca concentration ([Ca]0 = 0; 1.8; 7.2 mM) had no appreciable effect on these constants, unless the fibers were soaked in Ca-free solution for more than 40 min. In Na-free solution without Ca there was constant or increasing outward current in response to sudden depolarizations (voltage-clamp technique) over the whole voltage range -85 to +40 mV. In Ca-containing solution initial outward current was followed by a slow change in current towards 0 which was sometimes large enough to produce a net inward current. This current had a threshold in the voltage range -60 to -40 mV. It was not affected by alterations in the extracellular Cl or Mg concentrations. The dependence on [Ca]0 suggests that the slow inward current is carried by Ca ions. Negative slopes in the steady-state current-voltage relations were obtained in fibers soaked in Ca-containing solutions but were never observed in Ca-free solution. The Ca equilibrium potential (ECa) was estimated as about 150 mV, inside positive. In Tyrode solution the slow inward current was smaller than in Na-free solution and its threshold was shifted to about -20 to -10 mV. It was dependent on [Ca]0 as in Na-free solution. It was increased by epinephrine and not affected by tetrodotoxin. Ca ions carry an appreciable membrane current in the inward direction when the membrane of the Purkinje fiber is depolarized. This Ca current may be involved in excitation-contraction coupling.