Muscle blood flow during exercise in sedentary and trained
hypothyroid rats.
McAllister, Richard M., Michael D. Delp, Kristina A. Thayer, and M.
Harold Laughlin.
Department of Veterinary Biomedical Sciences, College of Veterinary
Medicine, University of Missouri, Columbia, MO 65211 and Department
of Surgery, Medical College of Pennsylvania & Hahnemann
University, Pittsburgh, PA 15212
APStracts 2:0246H, 1995.
Hypothyroidism is characterized by exercise intolerance. Previous
research, however, has shown that isolated, contracting skeletal
muscle in hypothyroid animals performs normally if provided with
blood flow equal to that for muscle in euthyroid an imals. We
therefore hypothesized that active muscle blood flow during in vivo
exer cise is inadequate in the hypothyroid state. Additionally, we
hypothesized that endur ance exercise training would restore normal
blood flow during acute exercise. To test these hypotheses, rats were
made hypothyroid (HYPO) over 3-4 mo with propylthio uracil. A subset
of HYPO rats was trained (THYPO) on a treadmill at 30 m/min (15%
grade) for 60 min/d, 5 d/wk over 10-15 wk. Hypothyroidism was
evidenced by 80% reductions in plasma T3 levels in both HYPO and
THYPO, and by 40-50% reductions in citrate synthase activities in
high oxidative muscles in HYPO, as compared with euthyroid (EUT)
rats. Training efficacy was indicated by increased (25-100%) citrate
synthase activities in muscles of THYPO vs. HYPO. Regional blood
flows were deter mined by the radiolabelled microsphere method, pre
-exercise and at 1-2 min of tread mill running at 15 m/min (0% grade).
Pre-exercise muscle blood flows were generally similar among groups.
During exercise, however, flows were lower in HYPO than EUT for high
oxidative muscles such as the red section of vastus lateralis (EUT,
277+/-24 ml/min/100 g, mean+/-SE; HYPO, 153+/-13; P<0.01) and
vastus intermedius (EUT, 317+/-32; HYPO, 187+/-20; P<0.01)
muscles. Training (THYPO) did not normalize these flows (168+/-24 and
181+/-24 for red section of vastus lateralis and vastus intermedius
muscles, respectively). Blood flows to low oxidative muscle such as
the white section of vastus lateralis muscle were similar among
groups (EUT, 21+/-5; HYPO, 25+/-4; THYPO, 34+/-7; NS). With
hypothyroidism, some degree of hypoperfusion during exer cise was
present in 26 of 28 hindlimb muscles (HYPO vs. EUT). These findings
indic ate that hypothyroidism is associated with reduced blood flow
to skeletal muscle dur ing exercise, suggesting that impaired
delivery of nutrients to and/or removal of met abolites from skeletal
muscle contribute to the poor exercise tolerance characteristic of
hypothyroidism.
Received 13 March 1995; accepted in final form 6 June 1995.
APS Manuscript Number H242-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.