Inhibition of carnitine palmitoyltransferase 1 by phenylalkyloxiranecarboxylic acid and its influence on lipolysis and glucose metabolism in isolated, perfused hearts of streptozotocin-diabetic rats

Abstract

The metabolic action of the new hypoglycemic compound, sodium-2-(5-[4-phenyl]-pentyl)-oxirane carboxylate (POCA), was studied in isolated perfused hearts of control and streptozotocin-diabetic rats. Perfusion with POCA selectively inhibited the activity of carnitine palmitoyltransferase 1, but had no influence on the activities of carnitine palmitoyltransferase 2, pyruvate dehydrogenase, and triglyceride lipase. Perfusing the hearts of streptozotocin-diabetic rats with POCA (10 mumol/L) reduced myocardial lipolysis and accelerated the rate of pyruvate and lactate outflow as well as pyruvate oxidation. The insulin sensitivity of the diabetic hearts with respect to lactate production and glucose oxidation was restored by perfusion with POCA. In contrast, defective glycogen synthesis in the diabetic hearts was not influenced by POCA. These data suggest that: (1) The insulin resistance of the glucose-perfused diabetic heart results from two different post-insulin-receptor defects. Whereas the disturbances of glucose oxidation are mediated by the excessive metabolism of endogenous triglycerides, the reason for the disturbed glycogen synthesis remains unclear. (2) Since in vitro perfusion with POCA partially restored the insulin sensitivity of the diabetic hearts, insulin-receptor defects should be of minor importance for the insulin resistance of diabetic hearts. (3) Since POCA inhibited carnitine palmitoyltransferase 1 and reduced the rate of lipolysis but had no effect on triglyceride lipase activity, we assume that product inhibition plays an important role in the regulation of myocardial lipolysis. In summary, inhibition of carnitine palmitoyltransferase 1 by POCA is suggested to be a useful approach for restoring insulin sensitivity depressed by an excessive metabolism of lipids.