| Abstract: | Although the sarcoplasmic reticulum (SR) is known to regulatethe intracellular concentration ofCa2+ and the SR function has beenshown to become abnormal during ischemia-reperfusion in theheart, the mechanisms for this defect are not fully understood. Becausephosphorylation of SR proteins plays a crucial role in the regulationof SR function, we investigated the status of endogenousCa2+/calmodulin-dependent proteinkinase (CaMK) and exogenous cAMP-dependent protein kinase (PKA)phosphorylation of the SR proteins in control, ischemic (I), andischemia-reperfused (I/R) hearts treated or not treated withsuperoxide dismutase (SOD) plus catalase (CAT). SR and cytosolicfractions were isolated from control, I, and I/R hearts treated or nottreated with SOD plus CAT, and the SR protein phosphorylation by CaMKand PKA, the CaMK- and PKA-stimulated Ca2+ uptake, and the CaMK, PKA,and phosphatase activities were studied. The SR CaMK andCaMK-stimulated Ca2+ uptakeactivities, as well as CaMK phosphorylation ofCa2+ pump ATPase (SERCA2a) andphospholamban (PLB), were significantly decreased in both I and I/Rhearts. The PKA phosphorylation of PLB and PKA-stimulatedCa2+ uptake were reducedsignificantly in the I/R hearts only. Cytosolic CaMK and PKA activitieswere unaltered, whereas SR phosphatase activity in the I and I/R heartswas depressed. SOD plus CAT treatment prevented the observedalterations in SR CaMK and phosphatase activities, CaMK and PKAphosphorylations, and CaMK- and PKA-stimulated Ca2+ uptake. These resultsindicate that depressed CaMK phosphorylation and CaMK-stimulatedCa2+ uptake in I/R hearts may bedue to a depression in the SR CaMK activity. Furthermore, prevention ofthe I/R-induced alterations in SR protein phosphorylation by SOD plusCAT treatment is consistent with the role of oxidative stress duringischemia-reperfusion injury in the heart. |
