Shennan D B, Grant A, Ramsay R R, Burns C, Zammit V A
Hannah Research Institute, Ayr KA6 5HL, UK.
Biochim Biophys Acta. 1998 Jul 31;1393(1):49-56. doi: 10.1016/s0005-2760(98)00056-3.
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 Km and Vmax were, respectively, 132 microM and 201 pmol/2 h/mg of intracellular water. The Na+-independent pathway, which was non-saturable, had a coefficient of 0.26 microl/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.
对哺乳期大鼠乳腺组织中左旋肉碱的转运进行了研究。从产后3 - 4天的哺乳期大鼠分离出的大鼠乳腺组织外植体对左旋肉碱的摄取通过钠依赖性和非钠依赖性途径进行。钠依赖性途径是左旋肉碱摄取的主要途径，是一个可饱和过程：米氏常数（Km）和最大反应速度（Vmax）分别为132微摩尔和201皮摩尔/2小时/毫克细胞内水。非钠依赖性途径不可饱和，系数为0.26微升/毫克细胞内水/2小时。乳腺组织外植体对左旋肉碱摄取的钠依赖性成分受到D - 肉碱和乙酰 - 左旋肉碱的顺式抑制，但不受胆碱或牛磺酸的抑制。相比之下，左旋肉碱摄取的非钠依赖性成分不受这些化合物中任何一种的影响。从产后10 - 12天的哺乳期大鼠分离出的乳腺组织对左旋肉碱的摄取在性质上与哺乳期早期大鼠的乳腺组织相似。然而，左旋肉碱摄取量在数量上较低：这归因于左旋肉碱摄取的钠依赖性成分减少。产后3 - 4天的动物的大鼠乳腺组织中左旋肉碱的流出至少由两个成分组成；一个快速的细胞外成分和一个缓慢的膜限制成分。逆转跨膜钠梯度并未刺激左旋肉碱流出，这表明钠依赖性左旋肉碱载体以不对称动力学运行。低渗休克，即细胞肿胀，增加了乳腺组织外植体中左旋肉碱的流出。
