Propionyl-L-carnitine-mediated improvement in contractile function of rat hearts oxidizing acetoacetate

Prior evidence has suggested that propionyl-L-carnitine improves function in ischemic hearts by providing carnitine for dissipation of acyl-CoA derivatives and propionate for enrichment of the citric acid cycle. Because contractile failure in hearts oxidizing ketone bodies is due to sequestration of free coenzyme A, which can be reversed by the addition of anaplerotic substrates that enrich the citric acid cycle, experiments were performed to determine whether the addition of propionyl-L-carnitine (2 mM) can improve performance in working rat hearts utilizing acetoacetate (7.5 mM). Whereas the addition of propionyl-L-carnitine to acetoacetate resulted in a sustained improvement in the work output of the heart, the addition of propionate (2 mM) or L-carnitine (2 mM) alone to acetoacetate had negligible effects on contractile function. Propionyl-L-carnitine increased the uptake of acetoacetate by 130%, whereas beta-hydroxybutyrate release was minimal and unchanged compared with other groups. These observations show that rates of acetoacetate oxidation are increased commensurate with increased contractile function. Tissue metabolite data indicate that the utilization of propionyl-L-carnitine did not lead to accumulation of citric acid cycle intermediates in the span from citrate to 2-oxoglutarate but to an increase in the tissue content of malate. The results show that addition of propionyl-L-carnitine in hearts oxidizing acetoacetate results in improved mechanical performance that is comparable to the mechanical performance of hearts perfused with glucose as the only substrate. This improvement is most likely conferred by anaplerosis, as suggested by enhanced rates of acetoacetate utilization and citric acid flux.