Lipolysis and the antilipolytic effect of insulin in adipocytes from rats adapted to a high-protein diet

Free fatty acid (FFA) mobilization during fasting was investigated in rats fed a high-protein, carbohydrate-free (HP) diet (70% casein, 8% fat, wt/wt) or a balanced diet (66% carbohydrate, 17% casein, 8% fat) for 30 to 40 days. In vivo, rats on the HP diet showed reduced rates of plasma FFA increase during fasting. Their blood sugar remained unchanged and was higher than that of control rats 24 hours after removal of food. In the fed state, serum insulin levels were smaller in HP-fed rats but did not differ significantly in the two experimental groups during fasting. In vitro, the rates of glycerol and FFA release by epididymal fat pads obtained from fasted rats were similar in rats consuming the HP diet. Fat cells isolated from rats on the HP diet also had reduced rates of basal lipolysis. Furthermore, they showed a significant increase in responsiveness to the lipolytic action of noradrenaline and an increase in both sensitivity and responsiveness to the inhibitory effect of insulin on noradrenaline-stimulated lipolysis. Adipocytes from HP-fed and control rats had mean diameters of 51 and 60 mu, respectively, and estimated average volumes of 90 and 142 pL. On the basis of existing data on the correlation between size and lipolytic activity of fat cells, the smaller size of the adipocytes from HP-fed rats might account for the lower rate of basal lipolysis but not for the increased response to the hormones. The increased sensitivity of fat cells to the antilipolytic action of insulin may have been an important factor in the reduced lipomobilization during fasting in rats under the high-protein regimen.