Quantitative evaluation of two pathways for phosphatidylcholine biosynthesis in rat brain in vivo

[Me-(3)H] choline and [(32)P] orthophosphate were injected intraventricularly into adult female rats. After variable intervals from injection (1-10 min) the animals were sacrificed by means of a microwave apparatus, and phosphorylcholine and choline phosphoglycerides extracted from brain and counted for radioactivity content after separation. The kinetic constants (K) for phosphorylcholine incorporation into lipids were determined both for [(32)P] and [(3)H] labeling. From the data obtained by these procedures it is concluded that base-exchange reactions for choline incorporation into lipids are operating in rat brain in vivo and that they represent a rapidly equilibrating system.