Chelerythrine increases Na-K-ATPase activity and limits ischemic injury inisolated rat hearts

Myocardial ischemia results in an increase in intracellular sodium concentr ation ([Na](i)), which may lead to cellular injury via cellular swelling an d calcium overload. Because protein kinase C (PKC) has been shown to reduce Na-K-ATPase activity, we postulated that pharmacological inhibition of PKC would directly increase Na-K-ATPase activity, reduce [Na](i) during ischem ia, and provide protection from ischemic injury. Isolated rat hearts were s ubjected to 30 min of global ischemia with and without the specific PKC inh ibitor chelerythrine. Intracellular pH, ATP, and [Na](i) were assessed usin g P-31 and Na-23 NMR spectroscopy, whereas Na-K-ATPase and PKC activity wer e determined using biochemical assays. Na/H exchanger activity was determin ed using the ammonium prepulse technique under nonischemic conditions. Chel erythrine increased Na-K-ATPase activity (13.76 +/- 0.89 vs. 10.89 +/- 0.80 mg ADP.h(-1).mg protein(-1); P = 0.01), reduced PKC activity in both the m embrane and cytosolic fractions (39% and 28% of control, respectively), and reduced creatine kinase release on reperfusion (48 +/- 5 IU/g dry wt vs. 6 89 +/- 63 IU/g dry wt; P = 0.008). The rise in [Na](i) during ischemia was significantly reduced in hearts treated with chelerythrine (peak [Na](i) ch elerythrine: 21.5 +/- 1.2 mM; control: 31.9 +/- 1.2 mM; P = 0.0001), withou t an effect on either acidosis (nadir pH 6.16 +/- 0.05 for chelerythrine vs . 6.08 +/- 0.04 for control), the rate of ATP depletion or Na/H exchanger a ctivity. These data support the hypothesis that pharmacological inhibition of PKC before ischemia induces cardioprotection by reducing intracellular s odium overload via an increase in Na-K-ATPase activity.