Regulation of phospholipid metabolism in differentiating cells from rat brain cerebral hemispheres in culture: ontogenesis of carrier-specific transport of choline and N-methyl-substituted choline analogs.

Abstract

Dimethylaminoethanol was studied both as a substrate and as an inhibitor of choline uptake in long-term cultures of foetal rat cerebral hemispheres. A saturable component with an apparent Km of 28 microM and Vmax of 11 pmol/min/microgram DNA for dimethylaminoethanol, was observed. Like choline, dimethylaminoethanol was also taken up by a second, low-affinity component, the apparent Vmax of which was about 102 pmol/min/microgram DNA. Dimethylaminoethanol inhibited the high-affinity but not the low-affinity choline uptake in a competitive manner with an apparent inhibition constant of 6.0 microM. Monomethylaminoethanol (Ki approximately 60 microM) competitively inhibited high-affinity choline transport. At low concentrations hemicholinium-3, but not ethanolamine, effectively inhibited high-affinity uptake of choline and to a lesser degree the uptake of the dimethylaminoethanol. While the high-affinity uptake of both substrates was inhibited by high concentrations of hemicholinium-3 or ethanolamine, the low-affinity system was not affected by hemicholinium-3. From the kinetics of uptake and inhibition patterns of choline and its related analogs, the methyl group seems to play a major role in determining the affinity rate constants for these substrates. The maximum rate of choline uptake via the high-affinity component increases about sixfold during a period of 2 weeks. In the absence of serum the maximum velocity of the high-affinity component is greatly reduced. These observations suggest that the high-affinity choline uptake component is an integral property, and a useful marker, of the developing cerebral cells.