Activation by intracellular calcium of a potassium channel in cardiac sarcoplasmic reticulum.

The effects of low (pCa 7.5 to 3) concentrations of intracellular calcium ion on a single potassium channel in the sarcoplasmic reticulum of canine heart ventricular muscle were investigated using a planar lipid bilayer technique. The low concentrations were obtained by mixing EGTA and calcium chloride. By varying the pCa of the cytoplasmic face between 3 to 7.5, two novel effects were observed. First, an increase in the intracellular Ca2+ concentration produced an increase in the unit current amplitude of open states; the voltage-current relationship was ohmic at these concentrations. Second, an increase in the Ca2+ concentration increased the open probability. Both these effects of Ca2+ were dose-dependent, and were consistently observed in all channels tested. Thus, the SR potassium channel observed appears to belong to the class of Ca2(+)-activated potassium channels.