The IVS6 segment of the L-type calcium channel is critical for the action of dihydropyridines and phenylalkylamines.

The current through the L-type calcium channel is inhibited and stimulated by distinct dihydropyridines at very low concentrations. The molecular determinants for the high affinity block and stimulation were investigated using chimeras between the class C and E calcium channels. Mutation of three amino acids in the last putative transmembrane segment (IVS6) of the alpha1C subunit decreased the affinity for (+)isradipine 100-fold without significantly affecting the basic properties of the expressed channel. Mutation of two of these three amino acids completely abolished the stimulatory effect of the calcium channel agonist Bay K 8644. These mutations only slightly affected the blocking efficacy of mibefradil and the phenylalkylamine devapamil. Three distinct but adjacently located amino acids mediated the high affinity block by devapamil. These results suggest that the IVS6 segment of the alpha1C subunit is critical for the high affinity interaction between the L-type calcium channel and the calcium channel agonist Bay K 8644 and the two antagonists isradipine and devapamil.