Carbohydrate metabolism during exercise: influence of circulating
fat availability.
Bracy, Deanna P., Bradley A. Zinker, Julia C. Jacobs, D. Brooks Lacy,
and David H. Wasserman.
Department of Molecular Physiology and Biophysics, Vanderbilt
University School of Medicine, Nashville, TN 37232
APStracts 2:0121A, 1995.
To examine the role of circulating fat in the regulation of
carbohydrate metabolism, dogs were studied during rest and 90 min of
moderate treadmill exercise with nicotinic acid infused to suppress
lipolysis in the presence (+Fat, n=5) or absence (-Fat, n=5) of
Intralipid. Isotopic (3-3H-glucose, U-14C- glucose) and hindlimb
arteriovenous methods were used to assess carbohydrate metabolism.
Limb blood flow was assessed using a Doppler technique. Plasma
glucose was similar in both protocols during rest and exercise.
Exercise values reported below represent those observed after 30 or
40 min of muscular work since peak responses were seen at this time.
Arterial FFA levels were 1129+/-253 and 272+/-17 [mu]Eq/l at rest in
+Fat and -Fat and 756+/-145 and 269+/-51 [mu]Eq/l with exercise.
Differences in insulin, catecholamines, and cortisol between the two
groups were insignificant. Arterial plasma glucagon was 50% greater
during both rest and exercise in -Fat. Hepatic glucose production was
4.2 +/-0.3 and 5.0+/-0.4 mg/kg x min at rest and 8.5+/-1.3 and
11.4+/-0.6 mg/kg x min with exercise in +Fat and -Fat, respectively.
Glucose utilization was 4.3 +/-0.3 and 5.3+/-0.2 mg/kg x min at rest
and 9.2+/-1.2 and 12.7+/-0.8 mg/kg x min with exercise in +Fat and
-Fat. Significant differences in glucose fluxes were present during
both rest and exercise. Limb glucose uptake rose similarly, from
29+/-7 to 82+/-22 [mu]mol/min and 28+/-7 to 88+/-16 [mu]mol/min with
exercise, in +Fat and -Fat. Arterial blood lactate was 50 to 100%
greater in -Fat compared to +Fat. Net limb lactate uptake (LLU) was
5+/-8 [mu]mol/min at rest in +Fat and was unaffected by exercise. In
contrast, LLU rose from 18+/-8 to 54+/-18 [mu]mol/min with exercise
in - Fat. Limb glucose and lactate oxidation rose more in -Fat (11+/
-5 to 69+/-7 [mu]mol/min) than in +Fat (8+/-3 to 45+/-8 [mu]mol/min).
Non-oxidative metabolism was similar at rest and was unaffected by
exercise in either protocol. In summary, compensation for a reduction
in circulating fat availability is accomplished by (a) increasing
glucagon levels; (b) accelerating glucose fluxes; (c) a greater LLU
by the working limb; and (d) a greater rate of glucose and lactate
oxidation by the working limb. The greater effectiveness of
hyperinsulinemia, hyperglycemia, and beta-adrenergic blockade in
stimulating carbohydrate metabolism during exercise may be, at least
in part, mediated by their antilipolytic actions.
Received 29 December 1994; accepted in final form 17 March 1995.
APS Manuscript Number A1377-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 28 March 1995.