ISOFORM-SPECIFIC INHIBITION OF L-TYPE CALCIUM CHANNELS BY DIHYDROPYRIDINES IS INDEPENDENT OF ISOFORM-SPECIFIC GATING PROPERTIES

Dihydropyridines (DHPs) block L-type Ca2+ channels more potently at de polarized membrane potentials, consistent with high affinity binding t o the inactivated state. Nisoldipine (a DHP antagonist) blocks the smo oth muscle channel more potently than the cardiac one, a phenomenon ob served not only in native channels but also in expressed channels. We examined whether this tissue specificity was attributable to differenc es of inactivation in the two channel types. We expressed cardiac or s mooth muscle alpha 1C subunits in combination with beta 2a and alpha 2 /delta subunits in human embryonic kidney cells, and used 2 mM Ca2+ as the permeant ion. This system thus reproduces the in vivo topology an d charge carrier of the channels while facilitating comparison of the two alpha 1C splice variants. Both voltage-dependent and isoform-speci fic sensitivity of 10 nM nisoldipine inhibition of the channel were de monstrated, with the use of -100 mV as the holding potential for fully reprimed channels and -65 mV to populate the inactivated state. Under drug-free conditions, we characterized fast inactivation (I-sec prepu lses) and slow inactivation (3 min prepulses) in the two isoforms. Ina ctivation parameters were not statistically different in the two chann el isoforms; if anything, cardiac channels tended to inactivate more t han the smooth muscle channels at relevant voltages. Likewise, the vol tage-dependent activation was identical in the two isoforms. We thus c onclude that the more potent nisoldipine inhibition of smooth muscle v ersus cardiac L-type Ca2+ channels is not attributable to differences in channel inactivation or activation. Intrinsic, gating-independent D HP receptor binding affinity differences must be invoked to explain th e isoform-specific sensitivity of the DHP block.