Molecular Determinants of the Differential Modulation of Ca v 1.2 and Ca v 1.3 by Nifedipine and FPL 64176.

Nifedipine and FPL 64176 (FPL), which block and potentiate L-type voltage-gated Ca2+ channels, respectively, modulate Cav 1.2 more potently than Cav 1.3. 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 Cav 1.2 into Cav 1.3 (Cav 1.3+) reduced the IC50 of nifedipine from 289 to 101 nM, and substitution of S1100 with an A residue, as in Cav 1.2, accounted for this difference. Substituting M1030 in IIIS5 to V in Cav 1.3+ (Cav 1.3+V) further reduced the IC50 of nifedipine to 42 nM. FPL increased current amplitude with an EC50 of 854 nM in Cav 1.3, 103 nM in Cav 1.2, and 99 nM in Cav 1.3+V. In contrast to nifedipine block, substitution of M1030 to V in Cav 1.3 had no effect on potency of FPL potentiation of current amplitude, but slowed deactivation in the presence and absence of 10 μ M FPL. FPL had no effect on deactivation of Cav 1.3/dihydropyridine-insensitive (DHPi), a channel with very low sensitivity to nifedipine block (IC50 ∼93 μ 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 Cav 1.2 and Cav 1.3 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.