1. Male Wistar rats were injected with tri-iodothyronine intraperitoneally for either 3 or 5 days.

2. Using 31P nuclear magnetic resonance spectroscopy, skeletal muscle intracellular bioenergetics of the leg were studied at rest and during sciatic nerve stimulation.

3. Tri-iodothyronine caused intracellular alkalinization of the cell, implying that administration of tri-iodothyronine induces an alteration in the set point for the sodium/proton antiport, the chief controller of intracellular pH.

4. During stimulation, there was less acidification of the skeletal muscle cell in the tri-iodothyronine-injected rats, implying either increased intrinsic buffering capacity of the muscle or increased proton efflux possibly due to increased activity of the sodium/proton antiport.

5. Effective proton efflux from the skeletal muscle cell may be sensitive to circulating levels of tri-iodothyronine.

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