Molecular Determinants of the Differential Modulation of Ca1.2 and Ca1.3 by Nifedipine and FPL 64176.

Nifedipine and FPL 64176 (FPL), which block and potentiate L-type voltage-gated Ca channels, respectively, modulate Ca1.2 more potently than Ca1.3. To identify potential strategies for developing subtype-selective inhibitors, we investigated the role of divergent amino acid residues in transmembrane domains IIIS5 and the extracellular IIIS5-3P loop region in modulation of these channels by nifedipine and FPL. Insertion of the extracellular IIIS5-3P loop from Ca1.2 into Ca1.3 (Ca1.3+) reduced the IC of nifedipine from 289 to 101 nM, and substitution of S1100 with an A residue, as in Ca1.2, accounted for this difference. Substituting M1030 in IIIS5 to V in Ca1.3+ (Ca1.3+V) further reduced the IC of nifedipine to 42 nM. FPL increased current amplitude with an EC of 854 nM in Ca1.3, 103 nM in Ca1.2, and 99 nM in Ca1.3+V. In contrast to nifedipine block, substitution of M1030 to V in Ca1.3 had no effect on potency of FPL potentiation of current amplitude, but slowed deactivation in the presence and absence of 10 M FPL. FPL had no effect on deactivation of Ca1.3/dihydropyridine-insensitive (DHPi), a channel with very low sensitivity to nifedipine block (IC ∼93 M), but did shift the voltage-dependence of activation by ∼-10 mV. We conclude that the M/V variation in IIIS5 and the S/A variation in the IIIS5-3P loop of Ca1.2 and Ca1.3 largely determine the difference in nifedipine potency between these two channels, but the difference in FPL potency is determined by divergent amino acids in the IIIS5-3P loop.