Cardiac Purkinje fibres

The pacemaker tail current of sheep Purkinje fibres became larger when CaCl₂ was injected intracellularly by pressure, whereas it was reduced or even abolished by injection of K-EGTA. The analysis of the [Ca²⁺]_i-induced changes in the steady state activations (S _∞) curve suggested that the effect could not be explained on the basis of internal screening of surface charges. The position of theS _∞-curve on the potential axis remained almost unchanged after the injection of CaCl₂ or K-EGTA. On the other hand, the vertical amplitude of the s_∞-curve on the current axis, proportional to \(\bar g_{{\text{K2}}}\) , was enlarged during CaCl₂ injections and strongly diminished by K-EGTA injections. Ca injection displaced the fully deactivated part of thes _∞-curve to large outward current which corresponds to an increase ing _K1. EGTA injection reducedg _K1. The results correspond to a [Ca²⁺]_i-sensitive potassium permeability. A possible interpretation is that [Ca²⁺]_i controls Ca-binding near the inner mouth of a potassium channel and that the binding is followed by a change in the channel's configuration from a closed to an open state.