Choline Uptake by Cerebral Capillary Endothelial Cells in Culture

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Abstract
A passage of choline from blood to brain and vice versa has been demonstrated in vivo. Because of the presence of the blood-brain barrier, such passage takes place necessarily through endothelial cells. To get a better understanding of this phenomenon, the choline transport properties of cerebral capillary endothelial cells have bveen studied in vitro. Bovine endothelial cells in culture were able to incorporate [3H]choline by a carrier-mediated mechanism. Nonlinear regression analysis of the uptake curves sugggested the presence of two transport components in cells preincubated in the absence of choline. One component showed a Km of 7.59 .+-. 0.8 .mu.M and a maximum capacity of 142.7 .+-. 9.4 pmol/2 min/mg of protein, and the other one was not saturable within the concentration range used (1-100 .mu.M). When cells were preincubated in the presence of choline, a single saturable component was observed with a Km of 18.5 .+-. 0.6 .mu.M and a maximum capacity of 452.4 .+-. 42 pmol/2 min/mg of protein. [3H]Choline uptake by endothelial cells was temperature dependent and was inhibited by the choline analogs hemicholinium-3, deanol, and AF64A. The presence of ouabain or 2,4-dinitrophenol did not affect the [3H]choline transport capacity of endothelial cells. Replacement of sodium by lithium and cell depolorization by potassium partially inhibited choline uptake. When cells had been preincubated without choline, recently transported [3H]choline was readily phosphorylated and incorporated into cytidine-5''-diphosphocholine and phospholipids; however, under steady-state conditions most (63%) accumulated [3H]choline was not metabolized within 1 h. Only part of the labelled intracellular free choline was transported back into the incubation medium. Because of their special location, transport properties, and metabolism, cerebral capillary endothelial cells may provide a gating mechanism that can contribute to the physiological control of choline concentration in the brain extracellular fluid.