Abstract
Chronic hemodynamic overload of the heart induces ventricular hypertrophy that may be either compensatory or progress to decompensation and heart failure. The gradual impairment of ventricular function is, at least in part, the result of a reduction of cardiac thyroid-hormone (TH) action. Here, we examined the proposed roles of increased cardiac expression of the TH-inactivating enzyme deiodinase type 3 (D3) and reduced plasma TH levels in diminishing cardiac TH levels. Using minipumps, mice were infused for one and two weeks with isoproterenol (ISO) alone or in combination with phenylephrine (PE). Remodeling of the heart induced by these adrenergic agonists was assessed by echocardiography. Left ventricular (LV) tissue and plasma TH levels (T4 and T3) were determined using liquid chromatography-tandem mass spectrometry. LV D3 activity was determined by conversion of radiolabeled substrate and quantification following HPLC. The results show that ISO induced compensated LV hypertrophy with maintained cardiac output. Plasma levels of T4 and T3 remained normal, but LV hormone levels were reduced by approximately 30% after two weeks, while LV D3 activity was not significantly increased. ISO + PE induced decompensated LV hypertrophy with diminished cardiac output. Plasma levels of T4 and T3 were substantially reduced after one and two weeks, together with a more than 50% reduction of hormone levels in the LV. D3 activity was increased after one week and returned to control levels after two weeks. These data show for the first time that relative to controls, decompensated LV hypertrophy with diminished cardiac output is associated with a greater reduction of cardiac TH levels than compensated hypertrophy with maintained cardiac output. LV D3 activity is unlikely to account for these reductions after two weeks in either condition. Whereas the mechanism of the mild reduction in compensated hypertrophy is unclear, changes in systemic TH homeostasis appear to determine the marked drop in LV TH levels and associated impairment of ventricular function in decompensated hypertrophy.