Divergent effects of ruthenium red and ryanodine on Ca2+/calmodulin-dependent phosphorylation of the Ca2+ release channel (ryanodine receptor) in cardiac sarcoplasmic reticulum

Abstract

In cardiac muscle, a Ca2+/calmodulin-dependent protein kinase (CaM kinase) associated with the sarcoplasmic reticulum (SR) is known to phosphorylate the membrane proteins phospholamban, Ca(2+)-ATPase, and Ca(2+)-release channel (ryanodine receptor). Phosphorylation of phospholamban and Ca(2+)-ATPase is recognized to stimulate Ca2+ sequestration by the SR but the functional consequence of Ca2+ channel phosphorylation has not been clearly established. In this study, we investigated the effects of the SR Ca(2+)-release inhibitor, ruthenium red (RR), and the SR Ca(2+)-release activator, ryanodine (at submicromolar concentrations), on CaM kinase-mediated phosphorylation of the Ca(2+)-cycling proteins in rabbit cardiac SR. Incubation of SR with RR (5-30 microM) for 3 min at 37 degrees C resulted in marked (up to 85%) inhibition of Ca2+ channel phosphorylation (50% inhibition with 15 +/- 2 microM RR) by the endogenous membrane-associated CaM kinase. Phosphorylation of the Ca2+ channel by exogenously added multifunctional alpha CaM kinase II was also inhibited similarly by RR. Phosphorylation of the Ca(2+)-ATPase by endogenous and exogenous CaM kinase was inhibited only modestly (25-30%) by RR, and phospholamban phosphorylation was unaffected by RR. The magnitude of RR-induced inhibition of Ca2+ channel phosphorylation did not differ appreciably at saturating or subsaturating concentrations of Ca2+ or calmodulin, and in the absence or presence of protein phosphatase inhibitors. In contrast to the effects of RR, low concentrations of ryanodine (0.25-1 microM) caused significant stimulation (up to approximately 50%) of Ca2+ channel phosphorylation but had no effect on Ca(2+)-ATPase and phospholamban phosphorylation. These findings suggest that interaction of RR with the ryanodine receptor induces a "nonphosphorylatable state" of the Ca(2+)-release channel, likely through a conformational change involving occlusion of the CaM kinase phosphorylation site. On the other hand, ryanodine binding to the receptor may serve to maintain an open, "phosphorylatable state" of the channel.