Nutritional State and the Swelling-Induced Inhibition of Proteolysis in Perfused Rat Liver

Abstract

Recent work indicates that cell volume is an important regulator of proteolysis in liver. The antiproteolytic effects of insulin and some amino acids (e.g., glutamine and glycine) are mediated by increases in cell volume. The purpose of the present study was to assess the role of nutritional state in the cell volume-dependent regulation of proteolysis in isolated perfused rat liver. In rats that had been prelabeled by an intraperitoneal injection of L-[4,5-3H]leucine 16–20 h prior to the perfusion experiment, hepatic proteolysis was studied by determination of [3H]label release into effluent perfusate. In separate perfusion experiments [3H]inulin and [14C]urea, acting as markers for extracellular and the sum of extra- plus intracellular space, were employed for determination of effector-induced cell volume changes. Proteolysis in the perfused rat liver was inhibited by insulin-like growth factor-I (IGF-1) and taurocholic acid. Both agonists increased the intracellular water space. The nutritional state of the livers had marked influence on the hormone- and amino acid-dependent regulation of proteolysis. In livers from food-deprived rats for 24 h, the swelling responses to glycine, glutamine and alanine were enhanced, whereas the insulin- and IGF-1-induced increases of cell volume were diminished. A stronger inhibition of proteolysis was observed in livers from food-deprived rats upon addition of the amino acids, whereas the insulin- and IGF-1-mediated inhibition of proteolysis was attenuated. Independent of the nutritional state, a close relationship between the cellular hydration state and the corresponding inhibition of proteolysis was observed, regardless of whether cell volume was modified by amino acids, hormones, hypoosmotic exposure or bile acids. We conclude that the nutritional state markedly modifies the swelling potency of amino acids and hormones in liver and by this means affects proteolysis.