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Properties of carnitine transport in rat kidney cortex slices

Biochimica et Biophysica Acta (BBA) - Biomembranes, ISSN: 0005-2736, Vol: 602, Issue: 3, Page: 621-634
1980
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The properties of carnitine transport were studied in rat kidney cortex slices. Tissue: medium concentration gradients of 7.9 for L -[ methyl - 14 C]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 = 22nmol/min per ml intracellular fluid; for D -carnitine, Km = 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.

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