Properties of carnitine transport in rat kidney cortex slices

Abstract

The properties of carnitine transport were studied in rat kidney cortex slices. Tissue: medium concentration gradients of 7.9 for L-[ methyl- 14C]carnitine were attained after 60-min incubation at 37°C in 40 μM substrate. L- and D-carnitine uptake showed saturability. The concentration curves appeared to consist of (1) a high-affinity component, and (2) a lower affinity site. When corrected for the latter components, the estimated K m for L-carnitine was 90 μM and V = 22 nmol/min per ml intracellular fluid; for D-carnitine, K m = 166 μM and V = 15 nmol/min per ml intracellular fluid. The system was stereospecific for L-carnitine. The uptake of L-carnitine was inhibited by (1) D-carnitine, γ-butyrobetaine, and (2) acetyl- L-carnitine. γ-Butyrobetaine and acetyl- L-carnitine were competitive inhibitors of L-carnitine uptake. Carnitine transport was not significantly reduced by choline, betaine, lysine or γ-aminobutyric acid. Carnitine uptake was inhibited by 2,4-dinitrophenol, carbonyl cyanide m- chlorophenylhydrazone , N 2 atmosphere, KCN, N- ethylmaleimide , low temperature (4°C) and ouabain. Complete replacement of Na + in the medium by Li + reduced L- and D-carnitine uptake by 75 and 60%, respectively. Complete replacement of K + or Ca 2+ in the medium also significantly reduces carnitine uptake. Two roles for the carnitine transport system in kidney are proposed: (1) a renal tubule reabsorption system for the steady-state maintenance of plasma carnitine; and (2) maintenance of normal carnitine levels in kidney cells, which is required for fatty acid oxidation.