Fatty acids are the preferred substrate of ischemic, reperfused myocardium and may account for the decreased cardiac efficiency during aerobic recovery. Neonatal cardiac myocytes in culture respond to
hypoxia/serum- 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 decline 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 hypoxic
CPT-I for
carnitine, suggesting that the liver
CPT-I
isoform is more dominant following
hypoxia. However, there was no alteration in hypoxic
CPT-I inhibition by
malonyl-CoA. DNP-
etomoxiryl-CoA, a specific inhibitor of the liver
CPT-I
isoform, uncovered identical Michaelis kinetics of the muscle
isoform in control and hypoxic myocytes with activation of the liver
isoform. Northern blotting did not reveal any change 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 associated with a return of
CPT-I activity to control levels. The data demonstrate that
CPT-I is activated upon
ATP depletion. Lower
enzyme activities are present in control and reoxygenated cells where
ATP is abundant or when
phosphatases are inhibited. This is the first suggestion that phosphorylation may modulate the activity of the liver
CPT-I
isoform in heart.