Diurnal changes in lipolytic activity of isolated fat cells and their increased responsiveness to epinephrine and theophylline with meal feeding in rats.

Diurnal changes in glycogen stores of adipose tissues and in vitro lipolytic activity of isolated epididymal fat cells, and their lipolytic responsiveness to epinephrine and theophylline were examined in rats adapted to a 2-h daily meal feeding (20.00-22.00 h; darkness between 20.00-08.00 h) for 3 weeks and in control rats fed ad libitum. The fat cells from both groups of animals showed the peak of lipolytic activity at the mid-dark period (03.00 h), but the peak values and the average values at 6 or 7 time points examined within the 24-h feeding cycle were significantly higher (p less than 0.025) in meal-fed rats. Basal, epinephrine-stimulated, and epinephrine-induced lipolysis of fat cells from control rats showed diurnal changes, and the rhythms and their amplitude were affected by meal feeding. Changes in lipolytic activity of fat cells did not seem to relate directly to those of glycogen stores in adipose tissue. The over-all 24-h means of lipolytic activity of fat cells were significantly increased (p less than 0.001) with meal feeding. Mean cell size of epididymal fat pads was significantly smaller (p less than 0.001) in meal-fed rats, but lipolytic responsiveness to the graded concentrations of epinephrine and theophylline in the incubation medium was significantly greater (p less than 0.001) in meal-fed rats than in rats fed ad libitum. Thus, these findings suggest that lipolytic activity of the cAMP-hormone sensitive lipase system in fat cells might be increased with meal feeding in rats. Furthermore, the present results may give a new idea to consider the discrepancy that many workers have not been able to observe the increase in body fat deposition with meal feeding, which has been frequently reported to enhance lipogenesis in rats.