VOLTAGE-DEPENDENT CESIUM BLOCKADE OF A CATION CHANNEL FROM FRAGMENTED SARCOPLASMIC-RETICULUM

Cs+ ions block the K+ conductance of a voltage-gated ionic channel derived from fragmented sarcoplasmic reticulum (SR) and incorporated into an artificial phospholipid bilayer. The block follows a single-site titration curve with a voltage-dependent dissociation constant of 18 mM at +50 mV. Blockade observed with the macroscopic conductance can be fully accounted for by the reduction of the apparent single-channel conductance in the presence of Cs+. The voltage dependence is explained by assuming that Cs+ binds to the open state of a single channel on a site located about 40% of the way through the membrane from the cis side (the side to which the SR vesicles are added). The blocking site is accessible only from the cis side. © 1979 Nature Publishing Group.