Activation of the Ca 2+ release channel of skeletal muscle sarcoplasmic reticulum by caffeine and related compounds

Abstract

The caffeine-sensitive Ca 2+ release pathway in skeletal muscle was identified and characterized by studying the release of 45Ca 2+ from heavy sarcoplasmic reticulum (SR) vesicles and by incorporating the vesicles or the purified Ca 2+ release channel protein complex into planar lipid bilayers. First-order rate constants for 45Ca 2+ efflux of 1 s −1 were obtained in the presence of 1–10 μ m free Ca 2+ or 2 × 10 −9 m free Ca 2+ plus 20 m m caffeine. Caffeine- and Ca 2+-induced 45Ca 2+ release were potentiated by ATP and Mg·ATP, and were both inhibited by Mg 2+. Dimethylxanthines were similarly (3,9-dimethylxanthine) or more (1,7-, 1,3-, and 3,7-dimethylxanthine) effective than caffeine in increasing the 45Ca 2+ efflux rate. 1,9-Dimethylxanthine and 1,3-dimethyluracil (which lacks the imidazole ring) did not appreciably stimulate 45Ca 2+ efflux. Recordings of calcium ion currents through single channels showed that the Ca 2+- and ATP-gated SR Ca 2+ release channel is activated by addition of caffeine to the cis (cytoplasmic) and not the trans (lumenal) side of the channel in the bilayer. The single channel measurements further revealed that caffeine activated Ca 2+ release by increasing the number and duration of open channel events without a change of unit conductance (107 pS in 50 m m Ca 2+ trans). These results suggest that caffeine exerts its Ca 2+ releasing effects in muscle by activating the high-conductance, ligand-gated Ca 2+ release channel of sarcoplasmic reticulum.