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 expressio n was assayed following nuclear injection into Xenopus oocytes. Whole cell recordings showed that rbC-II currents (recorded with 40 mM Ba2as the charge carrier) had variable activation rates and minimal inact ivation over an approximately 700 msec depolarizing step pulse. The ph armacological properties of the rbC-II current were consistent with th ose of an L-type calcium channel, being sensitive to dihydropyridines (10 muM nifedipine blocked approximately 85% of the current, 10 muM Ba y K 8644 enhanced the current between 2- and 10-fold) and not affected by the N- and P-type calcium channel antagonists, omega-conotoxin GVI A and omega-agatoxin IVA, respectively. Coexpression of rbC-II with cl oned 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 dependen ce of activation. Taken together with results showing that the neurona l class D alpha1 subunit also encodes an L-type calcium channel [Willi ams M. E., Feldman D. H., McCue A. F., Brenner R., Velicelebi G., Elli s S. B. and Harpold M. M. (1992a) Neuron 8: 71-84], these results indi cate that the mammalian nervous system expresses two distinct genes en coding L-type calcium channels.