Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle

Abstract

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

Figure 1

Principle of ATP synthesis rate measurement by ³¹P NMR spectroscopy. (a) Control spectrum in one volunteer: the saturation pulse is applied symmetrically to the γATP Larmor frequency. (b) Spectrum with γATP saturation acquired in the same volunteer (NS = 128). Subtraction of spectrum (b) from (a) gives the fraction of P_i involved in the synthesis of ATP, as shown on the P_i peak. ppm, parts per million.

Figure 2

¹³C NMR data. (a) ¹³C spectra acquired in a volunteer before and at the end of the [2-¹³C]acetate infusion. (b) Time course of glutamate C4 and glutamate C2 peak areas during the same experiment. Glutamate C4 appears soon after the beginning of the [2-¹³C]acetate infusion. The increase in glutamate C2 is delayed in comparison to that of glutamate C4, reflecting the extra turn of the TCA cycle required to randomize the ¹³C-labeled carbon into this position.

Figure 3

Typical model fit of the ¹³C NMR data of glutamate C4 in skeletal muscle (R² = 0.93).

Figure 4

Muscle TCA cycle and ATP synthesis fluxes measured in seven volunteers before (basal) and after T₃ treatment (post-T₃). Thyroid hormone induces a significant increase in muscle TCA cycle flux, whereas the rate of ATP synthesis does not show any significant change. The resulting ATP/TCA ratio (normalized to the basal state) reflects a decrease in mitochondrial energy coupling following T₃ treatment.