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

Calmodulin (CaM) activates the skeletal muscle ryanodine receptor Ca2+ rele ase channel (RyR1) in the presence of nanomolar Ca2+ concentrations. Howeve r, the role of CaM activation in the mechanisms that control Ca2+ release f rom the sarcoplasmic reticulum (SR) in skeletal muscle and in the heart rem ains unclear. In media that contained 100 nM Ca2+, the rate of Ca-45(2+) re lease from porcine skeletal muscle SR vesicles was increased approximately threefold in the presence of CaM (1 mu M). In contrast, cardiac SR vesicle Ca-45(2+) release was unaffected by CaM, suggesting that CaM activated the skeletal RyR1 but not the cardiac RyR2 channel isoform. The activation of R yR1 by CaM was associated with an approximately sixfold increase in the Ca2 + sensitivity of [H-3]ryanodine binding to skeletal muscle SR, whereas the Ca2+ sensitivity of cardiac SR [H-3]ryanodine binding was similar in the ab sence and presence of CaM. Cross-linking experiments identified both RyR1 a nd RyR2 as predominant CaM binding proteins in skeletal and cardiac SR, res pectively, and [S-35]CaM binding determinations further indicated comparabl e CaM binding to the two isoforms in the presence of micromolar Ca2+. In na nomolar Ca2+, however, the affinity and stoichiometry of RyR2 [S-35]CaM bin ding 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 th e Ca2+ dependence of channel activation in skeletal and cardiac muscle.