[Evaluation of G(i/o) protein signal transduction pathway in cardioprotection of hypoxic preconditioning]

In order to understand the intracellular mechanism of hypoxic preconditioning,we investigated the effects of G(i/o) protein, phospholipase C and adenylyl cyclase/cyclic adenosine monophosphate (cAMP) on the survival rate and lactate dehydrogenase (LDH) release of cultured neonatal rat cardiomyocytes during the reoxygenation following 3 h of hypoxia. Cardioprotection was conferred by a brief episode of hypoxia followed by reoxygenation. The obtained results are as follows: (1) Hypoxic preconditioning (25 min of hypoxia and 30 min of reoxygenation) increased the survival rate and decreased the release of LDH in the myocytes suffering from sustained hypoxia-reoxygenation. (2) Reducing the intracellular cAMP by N-ethylmaleimide, an inhibitor of adenylyl cyclase, could mimic the cardioprotection of the preconditioning. (3) Inhibition of G(i/o) protein by pertussis toxin abolished the cardioprotection of hypoxic preconditioning, with reduction of the survival rate and increase of the release of LDH. (4) U-73122 could not abolish the protective effect of hypoxic preconditioning. (5) When intracellular cAMP was increased by using 8-Br-cAMP, a high membrane permeable cAMP analogue, or forskolin, a specific adenylyl cyclase activator, the survival rate was lower and LDH activity higher than those in preconditioning myocytes. The results indicate that G(i/o) protein is a crucial component, in cardioprotection of hypoxic preconditioning in neonatal rat cardiomyocytes. Activation of phospholipase C does not seem to be involved in the intracellular signal pathway underlying the cardioprotective effect of the preconditioning.