Functional Interaction of Ca sub(V) Channel Isoforms with Ryanodine Receptors Studied in Dysgenic Myotubes

• Published in: Biophysical journal 2005-03, Vol.88 (3), p.1765-1777
• Main Authors: Schuhmeier, R P, Gouadon, E, Ursu, D, Kasielke, N, Flucher, B E, Grabner, M, Melzer, W
• Format: Article
• Language: English
• ISSN: 0006-3495
• DOI: 10.1529/biophysj.104.051318

The L-type Ca super(2+) channels Ca sub(V)1.1 ( alpha sub(1S)) and Ca sub(V)1.2 ( alpha sub(1C)) share properties of targeting but differ by their mode of coupling to ryanodine receptors in muscle cells. The brain isoform Ca sub(V)2.1 ( alpha sub(1A)) lacks ryanodine receptor targeting. We studied these three isoforms in myotubes of the alpha sub(1S)-deficient skeletal muscle cell line GLT under voltage-clamp conditions and estimated the flux of Ca super(2+) (Ca super(2+) input flux) resulting from Ca super(2+) entry and release. Surprisingly, amplitude and kinetics of the input flux were similar for alpha sub(1C) and alpha sub(1A) despite a previously reported strong difference in responsiveness to extracellular stimulation. The kinetic flux characteristics of alpha sub(1C) and alpha sub(1A) resembled those in alpha sub(1S)-expressing cells but the contribution of Ca super(2+) entry was much larger. alpha sub(1C) but not alpha sub(1A)-expressing cells revealed a distinct transient flux component sensitive to sarcoplasmic reticulum depletion by 30 mu M cyclopiazonic acid and 10 mM caffeine. This component likely results from synchronized Ca super(2+)-induced Ca super(2+) release that is absent in alpha sub(1A)-expressing myotubes. In cells expressing an alpha sub(1A)-derivative ( alpha sub(1)Aas(1592-clip)) containing the putative targeting sequence of alpha sub(1S), a similar transient component was noticeable. Yet, it was considerably smaller than in alpha sub(1C), indicating that the local Ca super(2+) entry produced by the chimera is less effective in triggering Ca super(2+) release despite similar global Ca super(2+) inward current density.