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

1. In sodium-free solution electrical constants of short Purkinje fibres were similar to those in Tyrode solution. Alterations in the extracellular calcium concentration ([Ca](o) = 0; 1.8; 7.2 mM) had no appreciable effect on these constants, unless the fibres were soaked in calcium-free solution for more than 40 min.2. In sodium-free solution without calcium 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 calcium-containing solution initial outward current was followed by a slow change in current towards zero 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 chloride or magnesium concentrations. The dependence on [Ca](o) suggests that the slow inward current is carried by calcium ions.3. Negative slopes in the steady-state current-voltage relations were obtained in fibres soaked in calcium-containing solutions but were never observed in calcium-free solution.4. The calcium equilibrium potential (E(Ca)) was estimated to be about 150 mV, inside positive.5. In Tyrode solution the slow inward current was smaller than in sodium-free solution and its threshold was shifted to about -20 to -10 mV. It was dependent on [Ca](o) as in sodium-free solution. It was increased by adrenaline and not affected by tetrodotoxin.6. It is concluded that calcium ions carry an appreciable membrane current in the inward direction when the membrane of the Purkinje fibre is depolarized. This calcium current may be involved in excitation-contraction coupling.