FUNCTIONAL EXPRESSION AND CHARACTERIZATION OF SKELETAL-MUSCLE DIHYDROPYRIDINE RECEPTORS IN XENOPUS OOCYTES

Dihydropyridine receptors in vertebrate skeletal muscle serve a dual r ole: as voltage sensors for excitation-contraction coupling and as vol tage-activated calcium channels. Although they were the first of six c lasses of calcium channels to be cloned, skeletal muscle dihydropyridi ne receptors remain the only ones not functionally expressed as calciu m channels in Xenopus oocytes, leading to the hypothesis that an inter acting component is missing. Using beta(1b), an isoform previously fou nd in brain, we have for the first time reconstituted skeletal muscle calcium channel function in Xenopus oocytes. We show that this beta su bunit is necessary for functional expression and that the alpha(2) del ta subunit significantly enhances the expressed current. The majority of the alpha(1) subunit in skeletal muscle is a truncated form. Here w e show that both the full-length and truncated forms produce functiona l calcium channels in Xenopus oocytes, but the truncated form gives si gnificantly larger currents. In addition, we show that the beta(1b) tr anscript is expressed in rat skeletal muscle, although at a much lower level than the abundant beta(1a) isoform.