Biphasic modulation of choline uptake and phosphatidylcholine biosynthesis by vasopressin in rat cardiac myocytes.

The effect of vasopressin on choline uptake and phosphatidylcholine biosynthesis in isolated rat heart myocytes was investigated. Myocytes were incubated with labelled choline in the presence of 0.05-1.0 microM vasopressin. Uptake of choline was enhanced (25%) by a low concentration (0.2 microM) of vasopressin, but was attenuated (19%) by a higher vasopressin concentration (1.0 microM). The biosynthesis of phosphatidylcholine was also affected by vasopressin in a biphasic manner. At low concentrations of vasopressin, a general increase in cytosine triphosphate:phosphocholine cytidylyltransferase activity was observed that caused an enhanced conversion of phosphocholine to phosphatidylcholine via the cytidine diphosphocholine pathway. At high vasopressin concentrations, a decrease in the activity of cytidylyltransferase was detected, which was caused by the translocation of the enzyme from the microsomal fraction to the cytosolic fraction. The decrease in enzyme activity coincides with a reduction in the conversion of labelled phosphocholine to phosphatidylcholine. In view of the fact that phospholipid biosynthesis in rat hepatocytes is inhibited by vasopressin at all concentrations, the biphasic modulation of phosphatidylcholine biosynthesis in rat heart myocytes illustrates the diverse effects of this hormone in different mammalian tissues.