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 from lactating rats, 3–4 days post partum, was via both Na⁺-dependent and Na⁺-independent pathways. The Na⁺-dependent pathway, the predominant route for l-carnitine uptake, was a saturable process: the K_m and V_max were, respectively, 132 μM and 201 pmol/2 h/mg of intracellular water. The Na⁺-independent pathway, which was non-saturable, had a coefficient of 0.26 μl/mg of intracellular water/2 h. The Na⁺-dependent component of l-carnitine uptake by mammary tissue explants was cis-inhibited by d-carnitine and acetyl-l-carnitine, but not by choline or taurine. In contrast, the Na⁺-independent component of l-carnitine uptake was not affected by any of these compounds. The uptake of l-carnitine by mammary tissue isolated from lactating rats, 10–12 days post partum, was qualitatively similar to that by mammary tissue taken from rats during the early stage of lactation. However, l-carnitine uptake was quantitatively lower: this was attributable to a reduction in the Na⁺-dependent component of l-carnitine uptake. l-Carnitine efflux from rat mammary tissue taken from animals 3–4 days post partum, consisted of at least two components; a fast extracellular component and a slow membrane-limited component. Reversing the trans-membrane Na⁺-gradient did not stimulate l-carnitine efflux 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.