Effects of hypoosmolality on whole-body lipolysis in man

Changes in extracellular osmolality, and thus in the cellular hydration sta te, appear to directly influence cell metabolism. The metabolic changes ass ociated with cell swelling are inhibition of glycogenolysis, glycolysis, an d proteolysis. Recent studies in our laboratory demonstrated diminished who re-body protein breakdown in humans during an acute hypoosmolar state. Beca use of the close interrelationship between carbohydrate and fat metabolism we speculated that adipose tissue lipolysis and fatty acid oxidation are re gulated by changes in extracellular osmolality. Therefore, we investigated the effect of artificially induced hypoosmolality on whole-body lipolysis a nd fat oxidation in seven healthy young men. Hypoosmolality was induced by intravenous administration of desmopressin, liberal ingestion of water, and infusion of hypotonic (0.45%) saline solution. Lipolysis was assessed by a stable-isotope method (2-[C-13]-glycerol infusion). The glycerol rate of a ppearance (Ra), reflecting whole-body lipolysis, was higher under hypoosmol ar compared with isoosmolar conditions (2.35 +/- 0.40 v 1.68 +/- 0.21 mu mo l/kg/min, P = .03). This was even more pronounced when lipolysis was suppre ssed during hyperinsulinemia and euglycemic clamping (0.90 +/- 0.08 v 0.61 +/- 0.03 mu mol/kg/min, P = .002). However, plasma free fatty acid (FFA), g lycerol, ketone body, insulin, and glucagon concentrations and carbohydrate and lipid oxidation measured by indirect calorimetry were not significantl y altered by hypoosmolality. Plasma norepinephrine concentrations were lowe r under hypoosmolar conditions (P < .01 v control), In conclusion, hypoosmo larity in vivo results in increased whole-body lipolysis, which is not due to changes in major lipolysis regulating hormones. Copyright (C) 1999 by W. B. Saunders Company.