Phosphorylation of phospholamban in ischemia-reperfusion injury: Functional role of Thr(17) residue.

Phospholamban (PLB) is a sarcoplasmic reticulum (SR) protein that when phosphorylated at Ser(16) by PKA and/or at Thr(17) by CaMKII increases the affinity of the SR Ca(2+) pump for Ca(2+). PLB is therefore, a critical regulator of SR function, myocardial relaxation and myocardial contractility. The present study was undertaken to examine the status of PLB phosphorylation after ischemia and reperfusion and to provide evidence about the possible role of the phosphorylation of Thr(17) PLB residue on the recovery of contractility and relaxation after a period of ischemia. Experiments were performed in Langendorff perfused hearts from Wistar rats. Hearts were submitted to a protocol of global normothermic ischemia and reperfusion. The results showed that (1) the phosphorylation of Ser(16) and Thr(17) residues of PLB increased at the end of the ischemia and the onset of reperfusion, respectively. The increase in Thr(17) phosphorylation was associated with a recovery of relaxation to preischemic values. This recovery occurred in spite of the fact that contractility was depressed. (2) The reperfusion-induced increase in Thr(17) phosphorylation was dependent on Ca(2+) entry to the cardiac cell. This Ca(2+) influx would mainly occur by the coupled activation of the Na(+)/H(+) exchanger and the Na(+)/Ca(2+) exchanger working in the reverse mode, since phosphorylation of Thr(17) was decreased by inhibition of these exchangers and not affected by blockade of the L-type Ca(2+) channels. (3) Specific inhibition of CaMKII by KN93 significantly decreased Thr(17) phosphorylation. This decrease was associated with an impairment of myocardial relaxation. The present study suggests that the phosphorylation of Thr(17) of PLB upon reflow, may favor the full recovery of relaxation after ischemia. (Mol Cell Biochem 263: 131-136, 2004).