Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism

Wade H. Martin, Robert J. Spina, Ellen Korte, Kevin E. Yarasheski, Theodore J. Angelopoulos, Patti M. Nemeth, Jeffrey E. Saffitz

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Abstract

To investigate the mechanism of reduced exercise tolerance in hyperthyroidism, we characterized cardiovascular function and determinants of skeletal muscle metabolism in 18 healthy subjects aged 26±1 yr (mean±SE) before and after 2 wk of daily ingestion of 100 μg of triiodothyronine (T3). Resting oxygen uptake, heart rate, and cardiac output increased and heart rate and cardiac output at the same submaximal exercise intensity were higher in the hyperthyroid state (P < 0.05). However, maximal oxygen uptake decreased after T3 administration (3.087±0.17 vs. 2.94±0.19 1/min; P < 0.001) despite increased heart rate and cardiac output at maximal exercise (P < 0.05). Plasma lactic acid concentration at an equivalent submaximal exercise intensity was elevated 25% (P < 0.01) and the arteriovenous oxygen difference at maximal effort was reduced (P < 0.05) in the hyperthyroid state. These effects were associated with a 21-37% decline in activities of oxidative (P < 0.001) and glycolytic (P < 0.05) enzymes in skeletal muscle and a 15% decrease in type IIA muscle fiber cross-sectional area (P < 0.05). Lean body mass was reduced (P < 0.001) and the rates of whole body leucine oxidation and protein breakdown were enhanced (P < 0.05). Thus, exercise tolerance is impaired in short duration hyperthyroidism because of decreased skeletal muscle mass and oxidative capacity related to accelerated protein catabolism but cardiac pump function is not reduced.

Original languageEnglish
Pages (from-to)2047-2053
Number of pages7
JournalJournal of Clinical Investigation
Volume88
Issue number6
DOIs
StatePublished - 1991

Keywords

  • Cardiac output
  • Oxidative capacity
  • Protein kinetics
  • Skeletal muscle
  • Thyrotoxicosis

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    Martin, W. H., Spina, R. J., Korte, E., Yarasheski, K. E., Angelopoulos, T. J., Nemeth, P. M., & Saffitz, J. E. (1991). Mechanisms of impaired exercise capacity in short duration experimental hyperthyroidism. Journal of Clinical Investigation, 88(6), 2047-2053. https://doi.org/10.1172/JCI115533