Repetitive acidosis protects the ischemic heart: Implications for mechanisms in preconditioned hearts

Repetitive brief ischemic episodes (ischemic preconditioning, PC) result in transient intracellular acidosis and protect the heart, from subsequent is chemic injury, potentially through a protein kinase C (PKC)-dependent mecha nism. We hypothesized that repetitive brief acidification of the heart with out concomitant ischemia would also protect the heart from ischemic injury via a PKC-dependent mechanism. Isolated rat hearts underwent 30 min of glob al ischemia following control perfusion (CTL), or after PC or repetitive ac idosis (RA), in the presence of absence of chelerythrine, a specific PKC in hibitor. Intracellular pH, PCr and ATP were measured using P-31 NMR spectro scopy, while intracellular sodium [Na]i was measured using Na-23 spectrosco py. Na,K-ATPase activity was measured prior to ischemia and on reperfusion. Both PC and RA. resulted in transient acidification prior to ischemia. Isc hemic injury, as assessed by creatinine kinase (CK) release on reperfusion, was reduced in both the PC and RA hearts [63 +/- 14 and 16 +/- 4 IU/g dry weight (dw) respectively v 705 +/- 72 IU/gdw for control; P<0.001], and was associated with improved functional recovery on reperfusion. PC and RA eac h significantly reduced Na,K-ATPase activity prior to ischemia (8.18 +/- 0. 47 and 7.76 +/- 0.54 mu mol ADP/h/mg protein) when compared to control (11. 05 +/- 0.54 mu mol ADP/h/mg protein; P<0.05), limited the rate of ATP deple tion during ischemia, and resulted in more rapid normalization of [Na]i on reperfusion. Chelerythrine resulted in intermediate CK release in PC and TW hearts (443 +/- 48 and 375 +/- 72 IU/gdw, P<0.001 v PC, P<0.01 v control), but did not alter the rate of ATP depletion or [Na]i kinetics in either PC or RA hearts. PC and RA each protect the ischemic heart, having in common ATP preservation during ischemia and more rapid normalization of [Na]i on r eperfusion. These effects, not modulated by protein kinase C, are consisten t with the hypothesis that ATP preservation during ischemia provides enhanc ed substrate for sodium efflux via the Na,K-ATPase on reperfusion.