HYDROGEN-PEROXIDE ACTIVATES MITOGEN-ACTIVATED PROTEIN-KINASES AND NA-H+ EXCHANGE IN NEONATAL RAT CARDIAC MYOCYTES()

Reperfusion of cardiac tissue after an ischemic episode is associated with metabolic and contractile dysfunction, including reduced tension development and activation of the Na+-He exchanger (NHE). Oxygen-deriv ed free radicals are key mediators of reperfusion abnormalities, altho ugh the cellular mechanisms involved have not been fully defined. In t he present study, the effects of free radicals on mitogen-activated pr otein (MAP) kinase function were investigated using cultured neonatal rat ventricular myocytes. Acute exposure of spontaneously beating myoc ytes to 50 mu mol/L hydrogen peroxide (H2O2) caused a sustained decrea se in contraction amplitude (80% of control). MAP kinase activity was measured by in-gel kinase assays and Western blot analysis. Acute expo sure to H2O2 (100 mu mol/L, 5 minutes) resulted in sustained MAP kinas e activation that persisted for 60 minutes. Catalase, but not superoxi de dismutase, completely inhibited MAP kinase activation by H2O2. Pret reatment with chelerythrine (10 mu mol/L, 45 minutes), a protein kinas e C inhibitor, or genistein (75 mu mol/L, 45 minutes) or herbimycin A (3 mu mol/L, 45 minutes), tyrosine kinase inhibitors, caused significa nt inhibition of H2O2-stimulated MAP kinase activity (51%, 78%, and 45 %, respectively, at 20 minutes). Brief exposure to H2O2 also stimulate d NHE activity. This effect was completely abolished by pretreatment w ith the MAP kinase kinase inhibitor PD 98059 (30 mu mol/L, 60 minutes) . These results suggest that low doses of H2O2 induce MAP kinase-depen dent pathways that regulate NHE activity during reperfusion injury.