Cardiac sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA2a) promotes Ca(2+) 
uptake in the SR. Dephosphorylated phospholamban (PLB) inhibits SERCA2a 
activity. We found a distinct dephosphorylation of PLB at Thr(17) and Ser(16) 
after 20-30min of ischemia produced by coronary artery occlusion in rats. The 
aim of the study was to investigate how PLB is dephosphorylated in ischemia and 
to determine whether PLB dephosphorylation causes myocardial hypercontraction 
and calpain activation through Ca(2+) overload in reperfusion. Protein 
inhibitor-1 (I-1) specifically inhibits protein phosphatase 1 (PP1), the 
predominant PLB phosphatase in heart. A Ca(2+)-dependent phosphatase calcineurin 
may also induce PLB dephosphorylation. Ischemia for 30min induced PKC-α 
translocation, resulting in inactivation of I-1 through PKC-α-dependent 
phosphorylation at Ser(67). The PP1 activation following I-1 inactivation was 
thought to induce PLB dephosphorylation in ischemia. Ischemia for 30min 
activated calcineurin, and pre-treatment with a calcineurin inhibitor, 
cyclosporine A (CsA), inhibited PKC-α translocation, I-1 phosphorylation at 
Ser(67), and PLB dephosphorylation in ischemia. Reperfusion for 5min following 
30min of ischemia induced spreading of contraction bands (CBs) and proteolysis 
of fodrin by calpain. Both CsA and an anti-PLB antibody that inhibits binding of 
PLB to SERCA2a reduced the CB area and fodrin breakdown after reperfusion. These 
results reveal a novel pathway via which ischemia induces calcineurin-dependent 
activation of PKC-α, inactivation of I-1 through PKC-α-dependent phosphorylation 
at Ser(67), and PP1-dependent PLB dephosphorylation. The pathway contributes to 
the spreading of CBs and calpain activation through Ca(2+) overload in early 
reperfusion.