Differential Ca2+ sensitivity of skeletal and cardiac muscle ryanodine receptors in the presence of calmodulin

Calmodulin (CaM) activates the skeletal muscle ryanodine receptor Ca2+ release channel (RyR1) in the presence of nanomolar Ca2+ concentrations. However, the role of CaM activation in the mechanisms that control Ca2+ release from the sarcoplasmic reticulum (SR) in skeletal muscle and in the heart remains unclear. In media that contained 100 nM Ca2+, the rate of45Ca2+ release from porcine skeletal muscle SR vesicles was increased approximately threefold in the presence of CaM (1 μM). In contrast, cardiac SR vesicle45Ca2+ release was unaffected by CaM, suggesting that CaM activated the skeletal RyR1 but not the cardiac RyR2 channel isoform. The activation of RyR1 by CaM was associated with an approximately sixfold increase in the Ca2+ sensitivity of [3H]ryanodine binding to skeletal muscle SR, whereas the Ca2+ sensitivity of cardiac SR [3H]ryanodine binding was similar in the absence and presence of CaM. Cross-linking experiments identified both RyR1 and RyR2 as predominant CaM binding proteins in skeletal and cardiac SR, respectively, and [35S]CaM binding determinations further indicated comparable CaM binding to the two isoforms in the presence of micromolar Ca2+. In nanomolar Ca2+, however, the affinity and stoichiometry of RyR2 [35S]CaM binding was reduced compared with that of RyR1. Together, our results indicate that CaM activates RyR1 by increasing the Ca2+ sensitivity of the channel, and further suggest differences in CaM's functional interactions with the RyR1 and RyR2 isoforms that may potentially contribute to differences in the Ca2+ dependence of channel activation in skeletal and cardiac muscle.