Long-chain acyl-CoA and acylcarnitine hydrolase activities in normal and ischemic rabbit heart

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Myocardial fatty acid oxidation during ischemia and reperfusion

Inhibition of fatty acid oxidation is an early event in myocardial ischemia that most likely contributes to tissue injury by the accumulation of potentially toxic intermediates such as acylCoA and acylcarnitine. After reperfusion both myocardial oxygen consumption and fatty acid oxidation may rapidly recover to preischemic levels, even when contractile function remains depressed. The mechanisms underlying the apparent dissociation between contractile function and oxidative metabolism early during reperfusion are still controversial. In isolated rat hearts subjected to 60 min of no-flow ischemia myocardial oxygen consumption and oxidation of palmitate were lowered during reperfusion by 3 mM of NiCl2 and by 6 microM of ruthenium red. The results provide indirect evidence for the hypothesis that intracellular calcium transport may be involved in the mechanisms responsible for the high oxidative metabolic rate early after reperfusion.

Hepatic hydrolysis of octanoyl-CoA and valproyl-CoA in control and valproate-fed animals

1. Medium-chain acyl-CoA hydrolase activities were determined in liver from control and valproate-fed rats and rabbits. 2. Octanoyl-CoA was readily hydrolyzed by all control liver preparations. 3. Animals that had been fed diets containing 0.5 or 1.0% (w/w) valproic acid had significantly elevated octanoyl-CoA hydrolase activities. 4. The CoA ester of valproic acid, a branched-chain isomer of octanoic acid, was poorly hydrolyzed by liver preparations from both control and valproate-fed animals. 5. Livers from the valproate-fed animals contained high levels of medium-chain acyl-CoA; total CoA content was also increased. 6. The inefficiency of hepatic valproyl-CoA hydrolysis may play a role in the toxicity of valproic acid.

Accumulation of lipids and lipid-intermediates in the heart during ischaemia

The content of non-esterified fatty acids (NEFA) and their CoA and carnitine esters is low in normoxic cardiac tissue. The majority of fatty acids is esterified in the triacylglycerol and phosphoglyceride pool. During myocardial ischaemia beta-oxidation of fatty acids is inhibited. In addition, turnover of the esterified fatty acid pools is most likely disturbed. Accumulation of hydroxy fatty acids, acylCoA and acylcarnitine rapidly occurs after the onset of ischaemia. The accumulation of NEFA is a slower process. In addition to extracellular sources, NEFA originate also from intracellular lipid pools, most likely from phosphoglycerides. Although it has been suggested that activation of phospholipase A2 occurs in ischaemic tissue, the mechanism underlying the enhanced degradation of phosphoglycerides ist still incompletely understood.

Cardiac and hepatic acyl-CoA and acylcarnitine hydrolase activities in clofibrate-fed rabbits

Catalase activity in the heart of male rabbits was 21% of that found in the liver; clofibrate feeding (0.3% w/w for 10 days) resulted in an 80% increase in both cardiac and hepatic catalase activities. Fatty acyl-CoA oxidase activity in control heart was 11% of that found in control liver; this peroxisomal activity did not increase subsequent to clofibrate feeding. Only acyl-CoA hydrolase activity in the cardiac supernatant was elevated by clofibrate feeding. Acylcarnitine hydrolase activity was increased significantly in the homogenate, extract and supernatant of both heart and liver from the clofibrate-fed rabbit. Clofibrate feeding increased CoASH and carnitine tissue levels in heart and liver.