The Ca2+-dependent slow K+ conductance in cultured rat muscle cells: characterization with apamin

The interaction of apamin, a bee venom neurotoxin, with rat skeletal muscle cell membranes has been followed using both an electrophysiological and a biochemical approach. Voltage-clamp analyses have shown that apamin, at low concentrations, specifically blocks the Ca2+-dependent slow K+ conductance in rat myotubes and myosacs . A specific binding site for apamin in rat muscle cell membranes has been characterized with the use of a highly radiolabelled apamin derivative [( 125I]apamin). The dissociation constant for the apamin-receptor complex is 36-60 pM and the maximal binding capacity is 3.5 fmol/mg of protein. [125I]Apamin binding to rat muscle membranes is displaced by quinine and quinidine with K0.5 values of 110 microM and 200 microM, respectively.