OUABAIN-SENSITIVE CHOLINE TRANSPORT-SYSTEM IN CAPILLARIES ISOLATED FROM BOVINE BRAIN

In physiological conditions, there is a net transport of choline from brain to blood, despite the fact that the choline concentration is higher in plasma than in CSF. Because of the blood-brain barrier characteristics, such passage against the concentration gradient takes place necessarily through endothelial cells. To get a better understanding of this phenomenon, [H-3]choline uptake properties have been analyzed in capillaries isolated from bovine brain. [H-3]Choline uptake was linear with time for up to 1 h. Nonlinear regression analysis of the uptake rates at different substrate concentrations gave the best fit to a system of two components, one of which was saturable (K(m) = 17.8 +/- 4.8-mu-M; V(max) = 11.3 +/- 3.4 pmol/min/mg of protein) and the other of which was nonsaturable at concentrations up to 200-mu-M. The [H-3]choline transport was significantly reduced in the absence of sodium and after incubation with 10(-4) M ouabain for 30 min. Ouabain also inhibited choline uptake in purified cerebral endothelial cells, but not in the endothelium isolated from bovine aorta. Accordingly, cerebral endothelial cells were able to concentrate [H-3]choline, with this effect being abolished by ouabain, whereas in aortic endothelial cells the [H-3]choline intracellular concentration was never higher than that of the incubation medium. These results suggest that the blood-brain barrier endothelium is specifically provided with an energy-dependent choline transport system, which may explain the choline efflux from the brain and the maintenance of a low choline concentration in the cerebral extracellular space.