State-dependent inhibition of L-type Ca2+ channels in A7r5 cells by cilnidipine and its derivatives

Using a whole-cell patch-clamp technique, state-dependent inhibition of dihydropyridines (DHP)s was investigated on L-type channels in A7r5 cells. Cilnidipine, its derivatives (D-342 and D-69) and nimodipine inhibited the Ba2+ current. However, cilnidipine and D-342, but not D-69 or nimodipine, accelerated current decay. The apparent rank order for the effects on the DHP-sensitive decaying component was different from that obtained for inhibition of the peak current. The dissociation constants for cilnidipine in the resting and inactivated states were estimated to be 190 and 12 nM, respectively. Cilnidipine, but not other DHP derivatives, created a faster and voltage-independent component (r= 37 ms). The linear relationship between the tau(-1) of the current decay and the cilnidipine concentration provided a value of 471 nM for the dissociation constant in the open state, suggesting that the current decay is mediated by one-to-one lower affinity binding of cilnidipine molecules to their binding site. The present study demonstrates that structurally related DHPs act in distinct ways to inhibit the L-type channel in the resting, open and inactivated states. Cilnidipine and some related DHPs probably exert their blocking action on the open channel by binding to a receptor distinct from the known DHP-binding site.