CA2+ RELEASE CHANNELS IN RAT DENERVATED SKELETAL-MUSCLES

Sarcoplasmic reticulum (SR) Ca2+ release channel-ryanodine receptors (RYR1) from rat fast-twitch skeletal muscle were studied by incorporating heavy sarcoplasmic reticulum membranes into a lipid bilayer. Channels from normal and denervated muscles had the same conductance as that reported for rabbits (about 500 pS) in 250:250 mM cis:trans caesium methanesulphonate. Caffeine (0.1 mM) induced a larger increase in the open probability (P-o) in denervated than in normal channels. The caffeine effect was caused by changes in mean open and burst time distributions. Longer opening and burst events were detected in the presence of caffeine. High caffeine concentrations (4 mM) gave similar results in channels from normal and denervated muscles. In denervated muscle, unlike intact muscle, the Ca2+ release channel was not activated at millimolar Ca2+ concentrations; this is similar to the cardiac isoform of the channel. Maximal channel activation was shifted to higher Ca2+ concentrations (pCa 4) and the channel remained activated at millimolar Ca2+ concentrations. The main effect of millimolar Ca2+ concentrations upon Ca2+ release channels from denervated muscles was an increase in the mean open time, with a concomitant increment of the mean burst duration. Alterations in channel gating properties in calcium and caffeine account for changes in the mechanical response after skeletal muscle denervation.