Calcium modulation of single SR potassium channel currents in heart muscle

The modulating effects of Ca2+ on single K+ channel currents in canine heart sarcoplasmic reticulum were studied using a planar lipid bilayer technique. The open-state probability and the unitary open-state current both decreased gradually as the Ca2+ concentration was reduced from pCa 3 to pCa 7.5. Each single-channel I-V curve was ohmic at any pCa: the modulating effect of Ca2+ within this range was voltage independent. The Ca2+ dose-response curves for the conductances and open probabilities were all biphasic in shape for both sides of the channel at the voltages used. However, Ca2+ within the pCa ranges used caused significantly more prominent activation of conductance and gating properties on the cytoplasmic side than it did on the SR luminal side. Furthermore, conductance decreased when cytoplasmic Ca2+ concentrations were greater than pCa 3. The I-V relation in this instance exhibited inward rectification caused by a voltage-dependent fast block. This suggests that cardiac SR K+ channel currents may be activated or inhibited through various types of Ca2+ binding sites on and within the channels.