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

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Abstract
Dimethylaminoethanol was studied both as a substrate and as an inhibitor of choline uptake in long-term cultures of fetal rat cerebral hemispheres. A saturable component with an apparent Km of 28 .mu.M and Vmax of 11 pmol/min per .mu.g 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 per .mu.g 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 .mu.M. Monomethylaminoethanol (Ki .apprx. 60 .mu.M) 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 6-fold during a period of 2 wk. In the absence of serum the maximum velocity of the high-affinity component is greatly reduced. The high-affinity choline uptake component is perhaps an integral property and a useful marker of the developing cerebral cells.