Role of dual-site phospholamban phosphorylation in intermittent hypoxia-induced cardioprotection against ischemia-reperfusion injury

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Abstract
Cardioprotection by intermittent high-altitude (IHA) hypoxia against ischemia-reperfusion (I/R) injury is associated with Ca2+overload reduction. Phospholamban (PLB) phosphorylation relieves cardiac sarcoplasmic reticulum (SR) Ca2+-pump ATPase, a critical regulator in intracellular Ca2+cycling, from inhibition. To test the hypothesis that IHA hypoxia increases PLB phosphorylation and that such an effect plays a role in cardioprotection, we compared the time-dependent changes in the PLB phosphorylation at Ser16(PKA site) and Thr17(CaMKII site) in perfused normoxic rat hearts with those in IHA hypoxic rat hearts submitted to 30-min ischemia (I30) followed by 30-min reperfusion (R30). IHA hypoxia improved postischemic contractile recovery, reduced the maximum extent of ischemic contracture, and attenuated I/R-induced depression in Ca2+-pump ATPase activity. Although the PLB protein levels remained constant during I/R in both groups, Ser16phosphorylation increased at I30 and 1 min of reperfusion (R1) but decreased at R30 in normoxic hearts. IHA hypoxia upregulated the increase further at I30 and R1. Thr17phosphorylation decreased at I30, R1, and R30 in normoxic hearts, but IHA hypoxia attenuated the depression at R1 and R30. Moreover, PKA inhibitor H89 abolished IHA hypoxia-induced increase in Ser16phosphorylation, Ca2+-pump ATPase activity, and the recovery of cardiac performance after ischemia. CaMKII inhibitor KN-93 also abolished the beneficial effects of IHA hypoxia on Thr17phosphorylation, Ca2+-pump ATPase activity, and the postischemic contractile recovery. These findings indicate that IHA hypoxia mitigates I/R-induced depression in SR Ca2+-pump ATPase activity by upregulating dual-site PLB phosphorylation, which may consequently contribute to IHA hypoxia-induced cardioprotection against I/R injury.

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