Functional expression and characterization of skeletal muscle dihydropyridine receptors in Xenopus oocytes.

Dihydropyridine receptors in vertebrate skeletal muscle serve a dual role: as voltage sensors for excitation-contraction coupling and as voltage-activated calcium channels. Although they were the first of six classes of calcium channels to be cloned, skeletal muscle dihydropyridine receptors remain the only ones not functionally expressed as calcium channels in Xenopus oocytes, leading to the hypothesis that an interacting component is missing. Using beta1b, an isoform previously found in brain, we have for the first time reconstituted skeletal muscle calcium channel function in Xenopus oocytes. We show that this beta subunit is necessary for functional expression and that the alpha2delta subunit significantly enhances the expressed current. The majority of the alpha1 subunit in skeletal muscle is a truncated form. Here we show that both the full-length and truncated forms produce functional calcium channels in Xenopus oocytes, but the truncated form gives significantly larger currents. In addition, we show that the beta1b transcript is expressed in rat skeletal muscle, although at a much lower level than the abundant beta1a isoform.