Calcium ions inhibit the allosteric interaction between the dihydropyridine and phenylalkylamine binding site on the voltage-gated calcium channel in heart sarcolemma but not in skeletal muscle transverse tubules

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Abstract
Calcium channel blockers bind with high affinity to sites on the voltage-sensitive Ca2+ channel. Radioligand binding studies with various Ca2+ channel blockers have facilitated identification and characterization of binding sites on the channel structure. In the present study we evaluated the relationship between the binding sites for the Ca2+ channel blockers on the voltage-sensitive Ca2+ channel from rabbit heart sarcolemma and rabbit skeletal muscle transverse tubules. [3H]PN200-110 binds with high affinity to a single population of sites on the voltage-sensitive Ca2+ channel in both rabbit heart sarcolemma and skeletal muscle transverse tubules. [3H]PN200-110 binding was not affected by added Ca2+ whereas EGTA and EDTA noncompetitively inhibited binding in both types of membrane preparations. EDTA was a more potent inhibitor of [3H]PN200-110 binding than EGTA. Diltiazem stimulates the binding of [3H]PN200-110 in a temperature-sensitive manner. Verapamil inhibited binding of [3H]PN200-110 to both types of membrane preparations in a negative manner, although this effect was of a complex nature in skeletal muscle transverse tubules. The negative effect of verapamil on [3H]PN200-110 binding in cardiac muscle was completely reversed by Ca2+. On the other hand, Ca2+ was without effect on the negative cooperativity seen between verapamil and [3H]PN200-110 binding in skeletal muscle transverse tubules. Since Ca2+ did not affect [3H]PN200-110 binding to membranes, we would like to suggest that Ca2+ is modulating the negative effect of verapamil on [3H]PN200-110 binding through a distinct Ca2+ binding site. The differential effect of Ca2+ on the negative cooperativity between verapamil and [3H]PN200-110 binding in rabbit heart and skeletal muscle membrane may reflect molecular and functional differences between voltage-sensitive Ca2+ channels in these two types of tissue.Key words: Ca2+ channel, sarcolemma, calcium channel blockers.

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