Planar lipid bilayr and patch clamp studies on the single channel currents of cardiac sarcop lasmic reticulum.

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Article 1 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 bilayr 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 Ca^<2+> 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 Ca^<2+> concentration increased the open probability. Both these effects of Ca^<2+> were dose-dependent, and were consistently observed in all channels tested. Thus, the SR potassium channel observed appears to belong to the class of Ca^<2+>-activated potassium channels.Article2 The modulating effects of Ca^<2+> on single K^+ channel currents in canine heart sarcop … More lasmic reticulum were studied using a planar lipid bilayr technique. The open-state probability and the unitary open-state current both decreased gradually as the Ca^<2+> 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 Ca^<2+> within this range was voltage independent. The Ca^<2+> 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, Ca^<2+> 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 Ca^<2+> 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 than cardiac SR K^+ channel currents may be activated or inhibited through various types Ca^<2+> binding sites on and within the channels. Less