AJP - Regulatory, Integrative and Comparative Physiology, Vol 252, Issue 3 587-R593, Copyright © 1987 by American Physiological Society
Glucose turnover in 48-hour-fasted running rats
B. Sonne, K. J. Mikines and H. Galbo
In fed rats, hyperglycemia develops during exercise. This contrasts with
the view based on studies of fasted human and dog that euglycemia is
maintained in exercise and glucose production (Ra) controlled by feedback
mechanisms. Forty-eight-hour-fasted rats (F) were compared to fed rats (C)
and overnight food-restricted (FR) rats. [3-3H]- and [U-14C] glucose were
infused and blood and tissue sampled. During running (21 m/min, 0% grade)
Ra increased most in C and least in F and only in F did Ra not
significantly exceed glucose disappearance. Plasma glucose increased more
in C (3.3 mmol/l) than in FR (1.6 mmol/l) and only modestly (0.6 mmol/l)
and transiently in F. Resting liver glycogen and exercise glycogenolysis
were highest in C and similar in FR and F. Resting muscle glycogen and
exercise glycogenolysis were highest in C and lowest in F. During running,
lactate production and gluconeogenesis were higher in FR than in F. At
least in rats, responses of production and plasma concentration of glucose
to exercise depend on size of liver and muscle glycogen stores; glucose
production matches increase in clearance better in fasted than in fed
states. Probably glucose production is stimulated by "feedforward"
mechanisms and "feedback" mechanisms are added if plasma glucose decreases.
