Effect of Adaptation to Meal-Feeding on Insulin, Glucagon and the Cyclic Nucleotide-Protein Kinase System in Rats

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Abstract
Diurnal changes in insulin, glucagon, and the cyclic nucleotide-protein kinase system were examined in rats trained to eat a 2-hour daily meal and in control rats fed ad libitum. Sharp increases in both insulin and glucagon were observed in response to feeding in trained rats. However, throughout most of the rest of the day, the plasma concentrations of both hormones were lower in meal-fed than in control rats. In adipose tissue, diurnal changes in cyclic AMP concentration were inversely correlated with changes in plasma insulin concentration. In general, cyclic AMP concentrations were depressed and cyclic GMP elevated in adipose tissue of meal-fed rats compared with those fed ad libitum. Diurnal changes in cyclic GMP concentration tended to parallel those of cyclic AMP. Cyclic AMP-activated protein kinase was elevated in adipose tissue of meal-fed rats. However, with the exception of fasting rats, the percentage of the enzyme in the active form was decreased. In liver, there was no clear relation between the feeding pattern and changes in either cyclic nucleotide, and no significant differences were observed with the protein kinase. It can be concluded that the magnitude of the adaptive response of the cyclic nucleotide-protein kinase system to meal-feeding in rats is greater in adipose tissue than in liver.