Molecular mechanisms of vasoselectivity of the 1,4-dihydropyridine lercanidipine

The effects of (S)- and (R)-lercanidipine on CHO cells stably expressing the cardiac (CA(v)1.2a) or vascular (CA(v)1.2b) splice variant of the L-type calcium channel pore subunit were studied, using whole-cell and single-channel patch-clamp measurements. Lercanidipine block of CA(v)1.2b current was enantioselective. (S)-lercanidipine was 4.1-fold more potent. Experiments using acidic solutions (pH 6.8) revealed a 6.4-fold enhanced inhibitory effect of (S)-lercanidipine compared with physiological conditions (pH 7.4) indicating that the charged form mediates inhibition. At depolarised holding potential (-40 mV), (S)-lercanidipine exhibited a 35-fold greater potency, compared with standard conditions (-80 mV). A comparison of the concentration-dependent inhibition of CA(v)1.2a with CA(v)1.2b subunit currents by (S)-lercanidipine revealed only a 1.8-fold difference in IC(50), but the slope of the dose-response curve was much steeper (n(H)=2.3) with CA(v)1.2a, compared with CA(v)1.2b (n(H)=0.8). This indicates overlap between agonistic and antagonistic effects, predominant with the cardiac CA(v)1.2a subunit. This idea is supported by transient stimulatory effects, and a slight leftward shift of the IV curves. These effects were more prominent for CA(v)1.2a than for CA(v)1.2b. Single-channel experiments confirmed typical features of Calcium Channel agonists such as prolonged channel openings, a component of lengthened openings, and an enhanced open probability in the presence of (S)-lercanidipine. Again, these findings were concentration-dependent and more pronounced for CA(v)1.2a than for CA(v)1.2b. Our data indicate a splice-variant predominant agonism as a new mechanism contributing to the vasoselectivity of lercanidipine, along with marked voltage-dependence of action.