Diurnal variations in hepatic and plasma lipids and in the activities of hepatic phosphatidate phosphohydrolase and heart lipoprotein lipase

ABSTRACT

Diurnal variations of liver and plasma lipids and of the activities of liver phosphatidate phosphohydrolase and heart lipoprotein lipase were determined in male rats kept for 15 days with alternating 12 h periods of darkness and light. Plasma triacylglycerol concentration increased during the dark period reaching a peak at 0700 h. Hepatic phosphatidate phosphohydrolase activity was also elevated at the early part of the dark with a peak at 2300 h. The circadian rhythms of hepatic triacylglycerol and phospholipids were less pronounced. Heart lipoprotein lipase activity was maximum at 1500 h and its nadir point was at 0700 h. The data were used to calculate definite circadian rhythm for VLDL-triacylglycerol secretion rate which coincided with those of liver phosphatidate phosphohydrolase and plasma triacylglycerol. It is concluded that the change in hepatic phosphatidate phosphohydrolase activity could be responsible for the diurnal variations of VLDL-triacylglycerol secretion rate and that both lipoprotein lipase activity and VLDL secretion rate are the major causes of plasma triacylglycerol diurnal variations