Differential sensitivities of CaV1.2 IIS5–S6 mutants to 1,4-dihydropyridine analogs

1,4-Dihydropyridines (DHPs), L-type calcium channel (CaV1) blockers, are known to interact with CaV1.2 subunits through their binding site located at IIIS5–S6 and IVS6 regions. We recently identified two domain II residues (S666 and A752) critical for nifedipine blockade (Kwok et al., 2008). In this study, we examined the blockade effects of two DHP analogues, nemadipine and nicardipine, on wildtype, M1161A (in IIIS6), S666V (in IIS5) and A752T (in IIS6) mutants of the rat α1C subunit transiently expressed with β2a and α2δ in cultured tsA201 cells. We found that the IC50 ratio of the mutants to the wildtype channel was similar in S666V and M1161A mutants for both drugs, but in A752T it was lower for nemadipine than nicardipine (P < 0.05). At saturating drug concentrations, not all the current was completely blocked in the mutants. The residual current recorded in 100 μM nemadipine was ~ 10% of the total current for the A752T channel, which was significantly higher than that in 100 μM nicardipine (~ 2%). In wildtype, S666V and M1161A, there was no significant difference in residual current between nemadipine and nicardipine, although it was greater in S666V (~ 15%) and M1161A (~ 30%) as compared to the wildtype channel (< 5%). Taken together, our findings suggest that the domain II residues alter the DHP effect in a structure-specific manner and may be involved in a pathway downstream of DHP binding.