Characterization of the Ca 2+ coordination site regulating binding of Ca 2+ channel inhibitors d-cis-diltiazem, (±)bepridil and (−)desmethoxyverapamil to their receptor site in skeletal muscle transverse tubule membranes

Abstract

Ca 2+ inhibits (−)[ 3H]desmethoxyverapamil, d-cis-[ 3H]diltiazem and (±)[ 3H]bepridil binding to skeletal muscle transverse-tubule membranes with a half-maximum inhibition constant, K 0.5 = 5 ± 1 μM. This value is close to that of the high affinity Ca 2+ binding site which controls the ionic selectivity of the Ca 2+ channel found in electrophysiological experiments suggesting that the Ca 2+ coordination site which regulates the ionic selectivity is also the one which alters binding of the Ca 2+ channel inhibitors investigated here. Ca 2+ and (−)D888 bind to distinct sites. Occupation of the Ca 2+ coordination site decreases the affinity of (−)D888 for its receptor by a factor of 5. Other divalent cations have the same type of inhibition behavior with the rank order of potency Ca 2+ (K 0.5 = 5 μM) > Sr 2+ (K 0.5 = 25 μM) > Ba 2+ (K 0.5 = 50 μM) > Mg 2+ (K 0.5 = 170 μM).