CHARACTERISTICS OF L-CARNITINE TRANSPORT BY LACTATING RAT MAMMARY TISSUE

The transport of L-carnitine by lactating rat mammary tissue has been examined. L-Carnitine uptake by rat mammary tissue explants isolated f rom lactating rats, 3-4 days post partum, was via both Na+-dependent a nd Na+-independent pathways. The Na+-dependent pathway, the predominan t route for L-carnitine uptake, was a saturable process: the K-m and V -max were, respectively, 132 mu M and 201 pmol/2 h/mg of intracellular water. The Na+-independent pathway, which was nonsaturable, had a coe fficient of 0.26 mu l/mg of intracellular water/2 h. The Na+-dependent component of L-carnitine uptake by mammary tissue explants was cis-in hibited by D-carnitine and acetyl-L-carnitine, but not by choline or t aurine. In contrast, the Na+-independent component of L-carnitine upta ke was not affected by any of these compounds. The uptake of L-carniti ne by mammary tissue isolated from lactating rats, 10-12 days post par tum, was qualitatively similar to that by mammary tissue taken from ra ts during the early stage of lactation. However, L-carnitine uptake wa s quantitatively lower: this was attributable to a reduction in the Na +-dependent component of L-carnitine uptake. L-Carnitine efflux from r at mammary tissue taken from animals 3-4 days post partum, consisted o f at least two components; a fast extracellular component and a slow m embrane-limited component. Reversing the trans-membrane Na+-gradient d id not stimulate L-carnitine afflux suggesting that the Na+-dependent L-carnitine carrier operates with asymmetrical kinetics. A hyposmotic shock, hence cell-swelling, increased L-carnitine efflux from mammary tissue explants. (C) 1998 Elsevier Science B.V. All rights reserved.