THE LIVER ISOFORM OF CARNITINE PALMITOYLTRANSFERASE-I IS ACTIVATED INNEONATAL RAT CARDIAC MYOCYTES BY HYPOXIA

Fatty acids are the preferred substrate of ischemic, reperfused myocar dium and may account for the decreased cardiac efficiency during aerob ic recovery. Neonatal cardiac myocytes in culture respond to hypoxia/s erum- and glucose-free medium by a slow decline in ATP which reverses upon oxygenation. This model was employed to examine whether carnitine palmitoyltransferase I (CPT-I) modulates high rates of beta-oxidation following oxygen deprivation. After 5 h of hypoxia, ATP levels declin e to 30% control values and CPT-I activity is significantly stimulated in hypoxic myocytes with no alteration in cellular carnitine content or in the release of the mitochondrial matrix marker, citrate synthase . This stimulation was attributed to an increase in the affinity of hy poxic CPT-I for carnitine, suggesting that the liver CPT-I isoform is more dominant following hypoxia. However, there was no alteration in h ypoxic CPT-I inhibition by malonyl-CoA. DNP-etomoxiryl-CoA, a specific inhibitor of the liver CPT-I isoform, uncovered identical Michaelis k inetics of the muscle isoform in control and hypoxic myocytes with act ivation of the liver isoform. Northern blotting did not reveal any cha nge in the relative abundance of mRNA for the liver vs. the muscle CPT -I isoforms. The tyrosine phosphatase inhibitor, pervanadate, reversed the hypoxia-induced activation of CPT-I and returned the affinity of cardiac CPT-I for carnitine to control. Reoxygenation was also associa ted with a return of CPT-I activity to control levels. The data demons trate that CPT-I is activated upon ATP depletion. Lower enzyme activit ies are present in control and reoxygenated cells where ATP is abundan t or when phosphatases are inhibited. This is the first suggestion tha t phosphorylation may modulate the activity of the liver CPT-I isoform in heart.