Regulation of protein synthesis and enzyme accumulation in the rat pancreas by amount and timing of dietary protein.

A 24-hour study in rats evidenced a clear rhythmicity of both synthesis and storage of pancreatic hydrolases. Synthesis measured by incorporation of 3H leucine into proteins was maximal during the night, reaching 21.3 mg/g tissue at 2400 hours against 4.1 mg/g at 0900 hours. Amylase and chymotrypsinogen contents, on the contrary, were 2-fold higher during the day (resting period) than at night (feeding period), while trypsinogen did not vary significantly. The diametrical opposition between the variations in synthesis and enzyme contents shows that, during periods of active feeding, stimulated synthesis merely balances excretion, while during periods of spontaneous fasting, basal synthesis is greater than basal secretion resulting in a preprandial accumulation of hydrolases. The effect of dietary proteins was investigated by feeding them as a separate meal at different times of the day while providing a protein-free diet ad libitum. In this case the general pattern of synthesis was biphasic. Rates of protein synthesis increased rapidly 2- to 3-fold after the protein meal, while tissue amino acids concentrations dropped. This first peak was tentatively attributed to the action of digestive hormones released after protein ingestion. The second peak occurred 15-18 hours, later together with a rise in tissue amino-acids due to limited endogenous proteolysis. This suggests that digestive hormones and amino-acid supply act independently to stimulate the synthesis of hydrolases in the pancreas. The amount of enzyme stored depends on the timing of the protein meal with respect to the period of most intense feeding, i.e. on the timing of maximal synthesis with respect to maximal secretion.