cGMP/PKG pathway mediates myocardial postconditioning protection in rat hearts by delaying normalization of intracellular acidosis during reperfusion

Ischemic postconditioning has been demonstrated to limit infarct size in patients, but its molecular mechanisms remain incompletely understood. Low intracellular pH (pHi) inhibits mitochondrial permeability transition, calpain activation and hypercontracture. Recently, delayed normalization of pHi during reperfusion has been shown to play an important role in postconditioning protection, but its relation with intracellular protective signaling cascades is unknown. The present study investigates the relation between the rate of pHi normalization and the cGMP/PKG pathway in postconditioned myocardium. In isolated Sprague–Dawley rat hearts submitted to transient ischemia both, postconditioning and acidic reperfusion protocols resulted in a similar delay in pHi recovery measured by 31P-NMR spectroscopy (3.6 ± 0.2 min and 3.5 ± 0.2 min respectively vs. 1.4 ± 0.2 min in control group, P < 0.01) and caused equivalent cardioprotection (48% and 41% of infarct reduction respectively, P < 0.01), but only postconditioning increased myocardial cGMP levels (P = 0.02) and activated PKG. Blockade of cGMP/PKG pathway by the addition of the guanylyl cyclase inhibitor ODQ or the PKG inhibitor KT5823 during reperfusion accelerated pHi recovery and abolished cardioprotection in postconditioned hearts, but had no effect in hearts subjected to acidic reperfusion suggesting that PKG signaling was upstream of delayed pHi normalization in postconditioned hearts. In isolated cardiomyocytes the cGMP analog 8-pCPT-cGMP delayed Na+/H+-exchange mediated pHi normalization after acidification induced by a NH4Cl pulse. These results demonstrate that the cGMP/PKG pathway contributes to postconditioning protection at least in part by delaying normalization of pHi during reperfusion, probably via PKG-dependent inhibition of Na+/H+-exchanger.

Research highlights
► Postconditioning but not acidic perfusion activates cGMP/PKG pathway.
► cGMP/PKG pathway mediates protection by delaying pHi recovery at reperfusion.
► PKG-dependent inhibition of Na+/H+-exchanger modulates pHi normalization.
► Delayed pHi recovery and PKG activation are inter-connected in postconditioning.