Decamethonium and hexamethonium block K+ channels of sarcoplasmic reticulum

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Abstract
The sarcoplasmic reticulum membrane (SR) of skeletal muscle contains cation-selective channels which were detected by isotope fluxes in fragmented SR vesicles, fluorimetric dyes and direct incorporation of SR vesicles to planar phospholipid bilayers. SR channels incorporated in bilayers have a single open-state conductance of 140 pS [picosiemens] in 0.1 M K+. Blockade of the SR channel by Cs+, a low-affinity blocker with a zero-voltage dissociation constant of 40 mM, was previously reported. Increasing Cs+ concentrations reduced the open-channel conductance, increased the mean open time and conferred voltage dependence on the open-state conductance. The blockade induced by the cholinergic drugs decamethonium and hexamethonium on the SR channel is reported here. Although blockade by hexamethonium is similar to that of Cs+, decamethonium blocks with a much higher affinity and induces flickering events which are probably due to the interaction of single drug molecules with the open state.