FUNCTIONAL-PROPERTIES OF A NEURONAL CLASS-C L-TYPE CALCIUM-CHANNEL

The rat brain class C calcium channel alpha1 subunit cDNA, rbC-II, was subcloned into a vertebrate expression vector and transient expression was assayed following nuclear injection into Xenopus oocytes. Whole cell recordings showed that rbC-II currents (recorded with 40 mM Ba2+ as the charge carrier) had variable activation rates and minimal inactivation over an approximately 700 msec depolarizing step pulse. The pharmacological properties of the rbC-II current were consistent with those of an L-type calcium channel, being sensitive to dihydropyridines (10 muM nifedipine blocked approximately 85% of the current, 10 muM Bay K 8644 enhanced the current between 2- and 10-fold) and not affected by the N- and P-type calcium channel antagonists, omega-conotoxin GVIA and omega-agatoxin IVA, respectively. Coexpression of rbC-II with cloned rat neuronal calcium channel alpha2 and beta subunits resulted in several changes to the electrophysiological properties of the rbC-II current including, an increased whole cell peak current, an increased rate of activation and a hyperpolarizing shift in the voltage dependence of activation. Taken together with results showing that the neuronal class D alpha1 subunit also encodes an L-type calcium channel [Williams M. E., Feldman D. H., McCue A. F., Brenner R., Velicelebi G., Ellis S. B. and Harpold M. M. (1992a) Neuron 8: 71-84], these results indicate that the mammalian nervous system expresses two distinct genes encoding L-type calcium channels.