Flow of glucose carbon into cholesterol and phospholipids in various regions of the adult rat brain: enhanced incorporation into hypothalamic phospholipids.

The contribution of glucose carbon to the biosynthesis of cholesterol and phospholipids in distinct brain regions was studied quantitatively in the adult male rat. Rates of flow of glucose carbon into the lipids in vivo were calculated from two measurements: the curve representing the decrease in plasma 14C-glucose with time and the specific activity of the cerebral lipid 180 minutes after a rapid intravenous injection of a tracer dose of D-U 14C-glucose. The following brain regions were studied: cerebral cortex, hypothalamus, medulla, and corpus callosum and cerebellum. The values for carbon flow into phospholipids were significantly higher in the hypothalamus than in the whole brain, whereas small, but insignificant, regional differences were found for carbon flow into cholesterol. The conversion of U-14C-glucose to individual phospholipids of both hypothalamus and cerebral cortex was further investigated in vitro in order to establish whether the higher rate of carbon flow into hypothalamic phospholipids resulted from enhanced synthesis of a particular phospholipid. In agreement with the results obtained in vivo, the rate of incorporation of 14C into total phospholipids was 60% higher in hypothalamic tissue. The results indicate that the higher rate of carbon flow into hypothalamic phospholipids might be attributed to enhanced incorporation of glucose carbon to phosphatidyl-choline and phosphatidyl-ethanolamine following a faster conversion of glucose to glycerol in this brain region.