Cardiac glucose utilization in mice with mutated α- and β-thyroid hormone receptors
American Journal of Physiology. Endocrinology and Metabolism
Abnormal thyroid function is usually associated with altered cardiac function. Mutations in the thyroid hormone (TH)-binding region of the TH ␤-receptor (TR␤) that eliminate its TH-binding ability lead to the thyroid hormone resistance syndrome (RTH) in humans, which is characterized by high blood TH levels, goiter, hyperactivity, and tachycardia. Mice with "knock-in" mutations in the TH ␣-receptor (TR␣) or TR␤ that remove their TH-binding ability have been developed, and those with the mutated
... TR␤ (TR␤ PV/PV ) appear to provide a model for RTH. These two types of mutants show different effects on cerebral energy metabolism, e.g., negligible change in glucose utilization (CMRGlc) in TR␤ PV/PV mice and markedly reduced CMRGlc, like that found in cretinous rats, in the mice (TR␣ PV/ϩ ) with the knock-in mutation of the TR␣ gene. Studies in knockout mice have indicated that the TR␣ may also influence heart rate. Because mutations in both receptor genes appear to affect some parameters of cardiac function and because cardiac functional activity and energy metabolism are linked, we measured heart glucose utilization (HMRGlc) in both the TR␤ PV/PV and TR␣ PV/ϩ mutants. Compared with values in normal wild-type mice, HMRGlc was reduced (Ϫ77 to Ϫ95%) in TR␣ PV/ϩ mutants and increased (87 to 340%) in TR␤ PV/PV mutants, the degree depending on the region of the heart. Thus the TR␣ PV/ϩ and TR␤ PV/PV mutations lead, respectively, to opposite effects on energy metabolism in the heart that are consistent with the bradycardia seen in hypothyroidism and the tachycardia associated with hyperthyroidism and RTH.