Cardiac Purkinje fibres: [Ca2+]i controls steady state potassium conductance.

The influence of intracellular calcium concentration [Ca2+]i on the steady state membrane currents i infinity was studied in a range of clamp potentials between -20 and -100 mV. Injection of CaCl2 or Ca-EGTA (pCa less than or equal to 6) increased i infinity whereas injection of K-EGTA diminished it. The changes delta i infinity were attributed to a change in steady state potassium conductance, gK infinity, by four arguments: delta i infinity was restricted to potentials negative to -20 mV and depended on clamp potential in an inward rectifying manner. delta i infinity displayed a reversal potential, Erev' which followed log [K+]o with 60 mV for a tenfold change. Since Erev obtained during Ca injection agreed with Erev observed during EGTA injection the potassium driving force had to be constant. When gK was blocked by superfusion with 20 mM Cesium neither CaCl2 nor K-EGTA injection modified i infinity.