Status of Ca2+/calmodulin protein kinase phosphorylation of cardiac SR proteins in ischemia-reperfusion.

Although the sarcoplasmic reticulum (SR) is known to regulate the intracellular concentration of Ca2+ and the SR function has been shown to become abnormal during ischemia-reperfusion in the heart, the mechanisms for this defect are not fully understood. Because phosphorylation of SR proteins plays a crucial role in the regulation of SR function, we investigated the status of endogenous Ca2+/calmodulin-dependent protein kinase (CaMK) and exogenous cAMP-dependent protein kinase (PKA) phosphorylation of the SR proteins in control, ischemic (I), and ischemia-reperfused (I/R) hearts treated or not treated with superoxide dismutase (SOD) plus catalase (CAT). SR and cytosolic fractions were isolated from control, I, and I/R hearts treated or not treated with SOD plus CAT, and the SR protein phosphorylation by CaMK and PKA, the CaMK- and PKA-stimulated Ca2+ uptake, and the CaMK, PKA, and phosphatase activities were studied. The SR CaMK and CaMK-stimulated Ca2+ uptake activities, as well as CaMK phosphorylation of Ca2+ pump ATPase (SERCA2a) and phospholamban (PLB), were significantly decreased in both I and I/R hearts. The PKA phosphorylation of PLB and PKA-stimulated Ca2+ uptake were reduced significantly in the I/R hearts only. Cytosolic CaMK and PKA activities were unaltered, whereas SR phosphatase activity in the I and I/R hearts was depressed. SOD plus CAT treatment prevented the observed alterations in SR CaMK and phosphatase activities, CaMK and PKA phosphorylations, and CaMK- and PKA-stimulated Ca2+ uptake. These results indicate that depressed CaMK phosphorylation and CaMK-stimulated Ca2+ uptake in I/R hearts may be due to a depression in the SR CaMK activity. Furthermore, prevention of the I/R-induced alterations in SR protein phosphorylation by SOD plus CAT treatment is consistent with the role of oxidative stress during ischemia-reperfusion injury in the heart.