PS-BPC06-5: EXOGENOUS THYROXINE INCREASES GLUT4 TRANSLOCATION IN MEMBRANE AND IMPROVES CARDIAC METABOLISM INDEPENDENT OF REDUCED SBP IN INSULIN RESISTANT OLETF RATS

Abstract

Objective: During insulin resistance the heart undergoes a metabolic shift where fatty acids (FA) account for about 99% of the ATP production, indicative of impaired glucose metabolism. This results in increased reactive oxygen species and lipotoxicity leading to impaired mitochondrial function. Thyroid disorders such as hyperthyroidism and hypothyroidism both have the potential to lead to hypertension. Thyroid hormones (TH) may improve glucose intolerance by increasing glucose reabsorption and metabolism and therefore improve cardiac function and lower the risk of developing hypertension. However, incongruent data exists showing that TH promote oxidative stress and cardiac hypertrophy. Thus, the beneficial effects of TH in the heart during insulin resistance remains controversial. The glucose transport protein, GLUT4, is a downstream target of the TH response element indicating a possible mechanism by which TH regulate glucose metabolism. Design: Insulin resistant, Otsuka Long Evans Tokushima Fatty (OLETF) rats were used to assess the effects of exogenous thyroxine (T4) on glucose metabolism in cardiac tissue. Rats were assigned to four groups: 1) lean, Long Evans Tokushima Otsuka (LETO; n = 6), 2) LETO + T4 (8 μg/100 g BM/d × 5 wks; n = 7), 3) untreated OLETF (n = 6), and 4) OLETF + T4 (n = 7). T4 was administered via osmotic minipumps implanted subcutaneously. Systolic blood pressure (SBP) was measured weekly in conscious rats by tail-cuff plethysmography. Animals were provided water and food ad libitum. Results: SBP increased by 15% in OLETF compared to lean LETO (138 ± 2 vs. 117 ± 1 mm Hg), but T4 did not alter SBP is OLETF strain. T4 increased GLUT4 gene expression by 85% and GLUT4 protein translocation to the membrane by 328% in OLETF. Additionally, T4 increased phosphofructokinase-1, the rate limiting step in glycolysis, by 98% and decreased CD36 gene expression for FA transport by 20% in OLETF. We expect our recent metabolomics and lipidomics data to help better explain these molecular changes. Conclusions: Five-week dose of exogenous T4 did not reduce SBP is OLETF. The results suggest that increased T4 has the potential to increase glucose reabsorption and metabolism in the heart of insulin resistant rats and restore impaired substrate metabolism, but these improvements in substrate metabolism do not translate to sustained reduction in arterial pressure.