Cardiac Purkinje fibres: [Ca2+]i controls the potassium permeability via the conductance components gK1 and gK2.

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+]k-induced changes in the steady state activation (s infinity) curve suggested that the effect could not be explained on the basis of internal screening of surface charges. The position of the s infinity-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 infinity-curve on the current axis, proportional to gK2, was enlarged during CaCl2 injections and strongly diminished by K-EGTA injections. Ca injection displaced the fully deactivated part of the s infinity-curve to larger outward current which corresponds to an increase in gK1. EGTA injection reduced gI1. 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.