Functional interaction of CaV channel isoforms with ryanodine receptors studied in dysgenic myotubes

Abstract

The L-type Ca2+ channels CaV1.1 (α 1C) and CaV1.2 (α1C) share properties of targeting but differ by their mode of coupling to ryanodine receptors in muscle cells. The brain isoform CaV2.1 (α1A) lacks ryanodine receptor targeting. We studied these three isoforms in myotubes of the α1S-deficient skeletal muscle cell line GLT under voltage-clamp conditions and estimated the flux of Ca2+ (Ca 2+ input flux) resulting from Ca2+ entry and release. Surprisingly, amplitude and kinetics of the input flux were similar for α1C and α1A despite a previously reported strong difference in responsiveness to extracellular stimulation. The kinetic flux characteristics of α1C and α1A resembled those in α1S-expressing cells but the contribution of Ca 2+ entry was much larger. α1C but not α1A-expressing cells revealed a distinct transient flux component sensitive to sarcoplasmic reticulum depletion by 30 μM cyclopiazonic acid and 10 mM caffeine. This component likely results from synchronized Ca2+-induced Ca2+ release that is absent in α1A-expressing myotubes. In cells expressing an α1A-derivative (α1Aas(1592-clip)) containing the putative targeting sequence of α1S, a similar transient component was noticeable. Yet, it was considerably smaller than in α1C, indicating that the local Ca2+ entry produced by the chimera is less effective in triggering Ca2+ release despite similar global Ca2+inward current density. © 2005 by the Biophysical Society.