AJP - Regulatory, Integrative and Comparative Physiology, Vol 237, Issue 3 147-R152, Copyright © 1979 by American Physiological Society
Liver glycogenolysis during exercise without a significant increase in cAMP
W. W. Winder, J. Boullier and R. D. Fell
Liver glycogenolysis may be controlled by glucagon or catecholamine-induced
changes in cAMP or by cAMP-independent mechanisms. The purpose of these
experiments was to determine whether an increase in liver cAMP occurs
during exercise at a time when the rate of liver glycogenolysis is greatly
accelerated. Rats were taught to run on a treadmill 10 min/day for 6 wk.
They were then run continuously for periods of time ranging from 0 to 120
min at 0.8 mph up a 15% grade. Liver glycogen was depleted by the end of 90
min in fed animals and by 20 min in overnight-fasted animals. Liver cAMP
was not significantly increased in fed animals during the first 60 min of
exercise. The major increase in liver cAMP occurred after liver glycogen
was depleted, at which time the rat must rely entirely on gluconeogenesis
for maintenance of blood glucose. This increase in cAMP corresponded to
large increases in plasma glucagon and catecholamines. We conclude that
liver glycogenolysis in the rat can occur during exercise independently
from significant detectable increases in cAMP concentrations.
