Ischemic preconditioning reduces Na+ accumulation and cell killing in isolated rat hepatocytes exposed to hypoxia

Abstract

Short periods of ischemia followed up by reperfusion are known to protect the heart against injury caused by a subsequent sustained ischemia. This phenomenon, known as ischemic preconditioning, has also been recently shown to reduce ischemic liver damage, but the mechanisms involved are still unknown. By using isolated hepatocytes as an in vitro model of liver preconditioning, we have investigated the possible effect of preconditioning on intracellular pH and Na ؉ homeostasis. Freshly isolated rat hepatocytes were preconditioned by 10 minutes of incubation under hypoxic conditions followed up by 10 minutes of reoxygenation and subsequently exposed to 90 minutes of hypoxia. Although preconditioning did not ameliorate adenosine triphosphate (ATP) depletion, preconditioned hepatocytes exhibited an increased resistance to cell killing during hypoxic incubation. Intracellular acidosis and Na ؉ accumulation developing during hypoxia were appreciably reduced in preconditioned cells. The effects of preconditioning on intracellular pH, Na ؉ homeostasis, and citotoxicity were mimicked by stimulating protein kinase C (PKC) with 4␤-phorbol-12-myristate-13-acetate (PMA) or 1,2 dioctanoyl-glycerol (1,2 DOG). Conversely, inhibiting PKC with chelerythrine or blocking vacuolar proton ATPase (V-ATPase) with bafilomycin A 1 abolished the protection given by preconditioning or by PMA treatment on hypoxic acidosis, Na ؉ overload, and hepatocyte killing. Similarly, the addition of Na ؉ ionophore monensin also reverted the cytoprotection exerted by preconditioning. This indicated that ischemic preconditioning of isolated hepatocytes decreased cell killing during hypoxia by preventing intracellular Na ؉ accumulation. We propose that, after preconditioning, the stimulation of PKC might activate proton extrusion through V-ATPase, thus, limiting intracellular acidosis and Na ؉ overload promoted by Na ؉ -dependent acid buffering systems. (HEPATOLOGY 2000;31:166-172.)In 1986, Murry et al. 1 reported that a short period of ischemia led to an unexpected resistance of the myocardium to a subsequent prolonged ischemia. Since then, the resistance to ischemic injury acquired after 1 or more brief periods of ischemia followed up by reperfusion has been termed ischemic preconditioning. 1,2 In the myocardium, ischemic preconditioning occurs in 2 phases: an early phase (early preconditioning) that immediately follows the transient hypoxia and lasts 2 to 3 hours, and a late phase (late preconditioning) that begins 12 to 24 hours from the transient ischemia and lasts for 3 to 4 days. 3 Besides the heart, ischemic preconditioning has been shown in several organs including the brain, the skeletal muscles, and the small intestine. 3 Recently, the development of preconditioning has also been observed in livers exposed to brief interruptions of blood perfusion. [4][5][6] Hepatic preconditioning prevents hepatocellular damage caused by both warm and cold ischemia and improves liver transplantation in rats. [4][5][6] Extensive studies in the myocardium have s...

Signal Pathway Involved in the Development of Hypoxic Preconditioning in Rat Hepatocytes

“…8 Indeed, the V-ATPase inhibitor, bafilomycin A 1 , abolished the cytoprotection and reverted the effects of preconditioning of hepatocyte pH and Na ϩ levels. 8 Nonetheless, other mechanisms might also contribute to the development of cellular resistance to hypoxia and reoxygenation damage in preconditioned livers. For instance, Yadav and coworkers have shown that preconditioning prevented hepatocyte and endothelial-cell apoptosis by inhibiting caspase 3 activation.…”

“…In accordance to the in vivo experiments, hepatocyte preconditioning develops following a transient hypoxia lasting more than 5 minutes and not exceeding 10 minutes. 8 Moreover, we have observed that the prevention of intracellular acidosis and of cytosolic Na ϩ increase during hypoxia is responsible for reducing hypoxic injury in preconditioned hepatocytes. 8 These effects have been ascribed to the activation of H ϩ extrusion by vacuolar proton-ATPase (V-ATPase) that, by reducing hypoxic acidosis, limited Na ϩ influx promoted by the operation of Na ϩ /H ϩ exchanger and Na ϩ -HCO 3 -cotransporter.…”

“…8 Moreover, we have observed that the prevention of intracellular acidosis and of cytosolic Na ϩ increase during hypoxia is responsible for reducing hypoxic injury in preconditioned hepatocytes. 8 These effects have been ascribed to the activation of H ϩ extrusion by vacuolar proton-ATPase (V-ATPase) that, by reducing hypoxic acidosis, limited Na ϩ influx promoted by the operation of Na ϩ /H ϩ exchanger and Na ϩ -HCO 3 -cotransporter. 8 Indeed, the V-ATPase inhibitor, bafilomycin A 1 , abolished the cytoprotection and reverted the effects of preconditioning of hepatocyte pH and Na ϩ levels.…”

“…3 Recent studies have shown that the same phenomenon could also be observed in the liver. [4][5][6][7][8][9][10] In particular, 10-minute interruption of liver blood supply in anesthetized rats followed by 10 minutes of reperfusion reduces transaminases released during a subsequent 90-minute period of ischemia and 90-minute reoxygenation. 5 A similar effect has also been observed in steatosic livers following heat shock preconditioning.…”

“…8 These effects have been ascribed to the activation of H ϩ extrusion by vacuolar proton-ATPase (V-ATPase) that, by reducing hypoxic acidosis, limited Na ϩ influx promoted by the operation of Na ϩ /H ϩ exchanger and Na ϩ -HCO 3 -cotransporter. 8 Indeed, the V-ATPase inhibitor, bafilomycin A 1 , abolished the cytoprotection and reverted the effects of preconditioning of hepatocyte pH and Na ϩ levels. 8 Nonetheless, other mechanisms might also contribute to the development of cellular resistance to hypoxia and reoxygenation damage in preconditioned livers.…”