Journal of Applied Physiology, Vol 63, Issue 2 589-596, Copyright © 1987 by American Physiological Society
Metabolic and hormonal responses in theophylline-increased cold resistance in males
L. C. Wang, S. F. Man and A. N. Belcastro
Department of Zoology, University of Alberta, Edmonton, Canada.
The effects of theophylline (a phosphodiesterase inhibitor-adenosine
receptor antagonist) and substrate feeding (Ensure, 250 kcal/235 ml) on
cold resistance were studied in seminude males undertaking submaximal (50%
maximum O2 consumption), intermittent (34% of total time) exercise in the
cold (-5 to 15 degrees C, individually adjusted) for 3 h. Each subject (n =
7) served as his own control and was tested on a weekly schedule. Under
control treatment, rectal temperature (Tre) decreased by 0.9 degrees C to
approximately 36.2 degrees C after cold exposure, whereas under
theophylline and Ensure, the decrease of Tre was only 0.4 degrees C,
indicating a significant increase (P less than 0.05) in cold resistance
(50% better than control). The plasma concentration of theophylline was
4.8-5.9 micrograms/ml and was positively correlated with plasma
concentration of free fatty acids. Plasma norepinephrine (NE) increased
significantly during cold exposure; the absolute concentration was
significantly higher after theophylline pretreatment. The plasma
concentrations of glucose, epinephrine, cortisol, and adenosine
3',5'-cyclic monophosphate did not change and the changes of free thyroxine
and triiodothyronine were minor. Together, the effectiveness of
theophylline + Ensure in acutely increasing cold resistance may be due to
increased substrate availability for thermogenesis, part of which, through
theophylline's potentiation of both sympathetic release of NE and
NE-stimulated lipolysis and part of which, through supplementary feeding of
Ensure.
