Porcine skeletal and cardiac muscle sarcoplasmic reticulum (SR) vesicle fractions enriched in the ryanodine receptor were phosphorylated in the presence of [γ-32P]MgATP and either exogenous cAMP-dependent protein kinase (cAMP-PK), or Ca2+ plus calmodulin. Phosphorylation of the cardiac muscle ryanodine receptor in the presence of either cAMP-PK or calmodulin (6.4 and 10.6 pmol Pi/mg SR respectively) was approximately equal to or twice the [3H]ryanodine binding activity of this preparation (5.2 pmol/mg). Furthermore, cardiac muscle ryanodine receptor Pi incorporation catalyzed by cAMP-PK and calmodulin was approximately additive. In skeletal muscle SR, however, the level of cAMP-PK or calmodulin catalyzed phosphorylation of the intact ryanodine receptor (0.2 or 2.9 pmol Pi/mg SR, respectively) was much less than the [3H]ryanodine binding activity of this fraction (11.6 pmol/mg). Furthermore, Pi incorporation into the intact skeletal muscle ryanodine receptor was 3-8-fold less than that incorporated into a component of slightly lower Mr. Although this latter component comigrated with an immunoreactive fragment of the ryanodine receptor on polyacrylamide gels, it did not appear to be derived from the ryanodine receptor. We conclude that the significant phosphorylation of the cardiac muscle SR ryanodine receptor indicates a likely physiological role for protein kinase-mediated regulation of this Ca2+-channel. In contrast, the minimal phosphorylation of the skeletal muscle SR ryanodine receptor indicates that such a role of protein kinases is unlikely in this tissue.
Bibliographical noteFunding Information:
We would like to thank Dr. William E. Rempel, of the Department of Animal Science, for the supply of pigs, Ms. Lynn Litterer for performing one of the reported experiments, and Dr. Tony Lai for his generous gift of ryanodine receptor antibody. This work was supported by a grant from the American Heart Association, Minnesota Affiliate (to J.R.M) and NIH grant GM 31382 (to C.F.L).
- Calcium ion channel
- Ryanodine receptor
- Sarcoplasmic reticulum