Vol. 53, Issue 5, 902-907, May 1998

Isoform-Specific Inhibition of L-Type Calcium Channels by Dihydropyridines Is Independent of Isoform-Specific Gating Properties

Hai Hu and Eduardo Marban

Section of Molecular and Cellular Cardiology, Department of Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

Dihydropyridines (DHPs) block L-type Ca²⁺ channels more potently at depolarized membrane potentials, consistent with high affinity binding to the inactivated state. Nisoldipine (a DHP antagonist) blocks the smooth muscle channel more potently than the cardiac one, a phenomenon observed not only in native channels but also in expressed channels. We examined whether this tissue specificity was attributable to differences of inactivation in the two channel types. We expressed cardiac or smooth muscle 1C subunits in combination with 2a and 2/ subunits in human embryonic kidney cells, and used 2 mM Ca²⁺ as the permeant ion. This system thus reproduces the in vivo topology and charge carrier of the channels while facilitating comparison of the two 1C splice variants. Both voltage-dependent and isoform-specific sensitivity of 10 nM nisoldipine inhibition of the channel were demonstrated, 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 (1-sec prepulses) and slow inactivation (3 min prepulses) in the two isoforms. Inactivation parameters were not statistically different in the two channel isoforms; if anything, cardiac channels tended to inactivate more than the smooth muscle channels at relevant voltages. Likewise, the voltage-dependent activation was identical in the two isoforms. We thus conclude that the more potent nisoldipine inhibition of smooth muscle versus cardiac L-type Ca²⁺ channels is not attributable to differences in channel inactivation or activation. Intrinsic, gating-independent DHP receptor binding affinity differences must be invoked to explain the isoform-specific sensitivity of the DHP block.