Caffeine-induced release of intracellular Ca²⁺ from chinese hamster ovary cells expressing skeletal muscle ryanodine receptor: Effects on full- length and carboxyl-terminal portion of Ca²⁺ release channels

The ryanodine receptor (RyR)/Ca²⁺ release channel is an essential component of excitation-contraction coupling in striated muscle cells. To study the function and regulation of the Ca²⁺ release channel, we tested the effect of caffeine on the full-length and carboxyl-terminal portion of skeletal muscle RyE expressed in a Chinese hamster ovary (CHO) cell line. Caffeine induced openings of the full length RyR channels in a concentration- dependent manner, but it had no effect on the carboxyl-terminal RyE channels. CHO cells expressing the carboxyl-terminal RyE proteins displayed spontaneous changes of intracellular [Ca²⁺]. Unlike the native RyE channels in muscle cells, which display localized Ca²⁺ release events (i.e., 'Ca²⁺ sparks' in cardiac muscle and 'local release events' in skeletal muscle), CHO cells expressing the full length RyE proteins did not exhibit detectable spontaneous or caffeine-induced local Ca²⁺ release events. Our data suggest that the binding site for caffeine is likely to reside within the amino- terminal portion of RyR, and the localized Ca²⁺ release events observed in muscle cells may involve gating of a group of Ca²⁺ release channels and/or interaction of RyE with muscle-specific proteins.