T-TYPE AND N-TYPE CALCIUM CHANNELS OF XENOPUS-OOCYTES - EVIDENCE FOR SPECIFIC INTERACTIONS WITH BETA-SUBUNITS

We used amplifying effects of calcium channel beta subunits to identify endogenous calcium channels in Xenopus oocytes. Expression of rat brain beta 4, increased macroscopic endogenous current magnitude with a small effect on kinetics. In contrast, expression of rat brain/cardiac beta(2), produced a much larger increase in current magnitude and dramatically slowed current decay. Low concentrations of omega-conotoxin GVIA irreversibly blocked currents in both uninjected and beta(2)-injected oocytes. Single channel recordings revealed both T- and N-type calcium channels with conductances of 9 and 18 pS, respectively, in uninjected oocytes and in oocytes expressing either beta subunit. Expression of either P subunit slowed average current decay of T-type single channels. Slowing of T-type current decay by expression of beta(2) was due to reopening of the channels. N-type single channel average current decay showed little change with expression of beta(2), whereas expression of beta(2) slowed average current decay.