AJP - Endocrinology and Metabolism, Vol 258, Issue 6 E907-E917, Copyright © 1990 by American Physiological Society
Role of insulin and branched-chain amino acids in regulating protein metabolism during fasting
M. Frexes-Steed, M. L. Warner, N. Bulus, P. Flakoll and N. N. Abumrad
Department of Surgery, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
This study examines the independent effects of insulin and amino acids on
protein metabolism after a 12-h and 4-day fast in healthy volunteers.
Leucine (Leu) kinetics were examined during sequential insulin infusions of
0 (group I) or 0.0125 (groups II and III), 1.2, and 10 mU.kg-1.min-1.
Plasma Leu was maintained at 12-h fasted levels in groups I and II and at
84-h fasted levels in group III. Four-day fast (vs. 1 day, P less than
0.01) was associated with a 79% drop in plasma insulin and elevations in
plasma Leu (122%), Leu rates of appearance (Ra) (21%), and Leu oxidation
(56%), and no change in nonoxidative rates of disappearance (Rd). Insulin
resulted in a dose-dependent suppression of endogenous Leu Ra with group
III = I greater than II. Leu oxidation rose 1.7-fold in group III at the
highest insulin dose but remained stable in the two other groups. In
conclusion, 4-day fasting is associated with enhanced proteolysis and Leu
oxidation with no change in nonoxidative Rd (protein synthesis). Elevated
branched-chain (and other) amino acids were required to restore tissue
sensitivity and specificity to the effects of insulin on protein metabolism
after 4 days of fasting.
