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TRANSLATIONAL PHYSIOLOGY

Hyperthyroidism and cation pumps in human skeletal muscle

¹Medical Department M (Endocrinology and Diabetes) and ²Medical Research Laboratories, Aarhus University Hospital, and ³Institute of Physiology, University of Aarhus, Aarhus, Denmark

Submitted 2 November 2004 ; accepted in final form 6 December 2004

Skeletal muscle constitutes the major target organ for the thermogenic action of thyroid hormone. We examined the possible relation between energy expenditure (EE), thyroid status, and the contents of Ca²⁺-ATPase and Na⁺-K⁺-ATPasein human skeletal muscle. Eleven hyperthyroid patients with Graves' disease were studied before and after medical treatment with methimazole and compared with eight healthy subjects. Muscle biopsies were taken from the vastus lateralis muscle, and EE was determined by indirect calorimetry. Before treatment, the patients had two- to fivefold elevated total plasma T₃ and 41% elevated EE compared with when euthyroidism had been achieved. In hyperthyroidism, the content of Ca²⁺-ATPase was increased: (mean ± SD) 6,555 ± 604 vs. 5,212 ± 1,580 pmol/g in euthyroidism (P = 0.04) and 4,523 ± 1,311 pmol/g in healthy controls (P = 0.0005). The content of Na⁺-K⁺-ATPase showed 89% increase in hyperthyroidism: 558 ± 101 vs. 296 ± 34 pmol/g (P = 0.0001) in euthyroidism and 278 ± 52 pmol/g in healthy controls (P < 0.0001). In euthyroidism, the contents of both cation pumps did not differ from those of healthy controls. The Ca²⁺-ATPase content was significantly correlated to plasma T₃ and resting EE. This provides the first evidence that, in human skeletal muscle, the capacity for Ca²⁺ recycling and active Na⁺-K⁺ transport are correlated to EE and thyroid status.

Ca²⁺-activated adenosine triphosphatase; Na⁺-K⁺-activated adenosine triphosphatase; energy expenditure