Myocardial ischemia results in an increase in intracellular
sodium concentration ([Na]i), which may lead to cellular injury via cellular swelling and
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
ischemia, and provide protection from ischemic injury. Isolated rat hearts were subjected to 30 min of global
ischemia with and without the specific PKC inhibitor
chelerythrine. Intracellular pH,
ATP, and [Na]i were assessed using 31P and 23Na NMR spectroscopy, whereas Na-K-
ATPase and PKC activity were determined using biochemical assays.
Na/H exchanger activity was determined using the
ammonium prepulse technique under nonischemic conditions.
Chelerythrine 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 membrane and cytosolic fractions (39% and 28% of control, respectively), and reduced
creatine kinase release on reperfusion (48 +/- 5 IU/g dry wt vs. 689 +/- 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)
chelerythrine: 21.5 +/- 1.2 mM; control: 31.9 +/- 1.2 mM; P < 0.0001), without 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 activity. These data support the hypothesis that pharmacological inhibition of PKC before
ischemia induces cardioprotection by reducing intracellular
sodium overload via an increase in Na-K-
ATPase activity.