Effects of hyper- and hypoosmolality on whole body protein and glucose kinetics in humans

To investigate the effect of acute changes of extracellular osmolality on w hole body protein and glucose metabolism, we studied 10 male subjects durin g three conditions: hyperosmolality was induced by fluid restriction and in travenous infusion of hypertonic NaCl[2-5%; (wt/vol)] during 17 h; hypoosmo lality was produced by intravenous administration of desmopressin, liberal water drinking, and infusion of hypotonic saline (0.4%); and the isoosmolal ity study consisted of ad libitum oral water intake by the subjects. Leucin e flux ([1-C-13]leucine infusion technique), a parameter of whole body prot ein breakdown, decreased during the hypoosmolality study (P < 0.02 vs. isoo smolality). The leucine oxidation rate decreased during the hypoosmolality study (P < 0.005 vs. isoosmolality). Metabolic clearance rate of glucose du ring hyperinsulinemic-euglycemic clamping increased less during the hypoosm olality study than during the isoosmolality study (P < 0.04). Plasma insuli n decreased, and plasma nonesterified fatty acids, glycerol, and ketone bod y concentrations and lipid oxidation increased during the hypoosmolality st udy. It is concluded that acute alterations of plasma osmolality influence whole body protein, glucose, and lipid metabolism; hypoosmolality results i n protein sparing associated with increased lipolysis and lipid oxidation a nd impaired insulin sensitivity.