Characterization of carnitine transport in isolated perfused adult rat hearts

Carnitine transport was characterized in isolated perfused adult rat hearts. Carnitine uptake consisted of both a saturable (carrier-mediated) and nonsaturable (diffusion) component. Perfusion with 0.05 mM mersalyl acid, a sulfhydryl binding agent, inhibited the carrier-mediated transport but did not inhibit diffusion. The saturable transport system exhibited Michaelis-Menten kinetics with a maximum velocity of 154 nmol . g dry wt-1 . h-1 and an apparent Michaelis constant of 24 microM. D-carnitine competitively inhibited L-carnitine transport with an apparent inhibitor dissociation constant of 500 microM. Anoxia and K+ arrest resulted in only a slight inhibition of the saturable transport, suggesting that transport is not adenosine 5'-triphosphate (ATP) dependent. At physiological concentrations of extracellular carnitine (44 microM), total carnitine uptake rate was about 100 nmol . g dry wt-1 . h-1, 80% of which was by carrier-mediated transport. This rate of uptake would require about 60 h to replace the total cellular carnitine. Loss of tissue carnitine also appeared to be a slow process. These results suggest that carnitine is transported across the sarcolemma by both diffusion and carrier-mediated transport.