Age-related changes in thyroid hormone effects on glucose transporter isoforms of rat heart

To determine the age-related changes in thyroid hormone (TH) effects on car diac glucose transporter one (GLUT-1) and four (GLUT-4) isoforms, male Fisc her 344 rats at 4, 12, and 25 months of age were studied at euthyroid, hype rthyroid and hypothyroid conditions. Hyperthyroidism was induced with daily intraperitoneal injections of triiodothyronine (15 mu g/100 gm) for 10 day s. Hypothyroidism was achieved with 0.025% methimazole in the drinking wate r for 4 weeks. Immunoblot analysis indicated that at euthyroid basal condit ions GLUT-1 protein was not significantly altered with age while GLUT-4 pro tein was significantly reduced in 25 month old rats (82.0 +/- 28.8% of a 4 month old rat p <0.01). In 4 months old rats, GLUT-1 was increased in both hypothyroidism (432.5 +/- 208.7% of age-matched euthyroid control) and to a lesser extent in hyperthyroidism (242.0 +/- 93.3% of control) p<0.01. In 2 5 month old rats, hyperthyroidism was also associated with increased GLUT-1 mass (190.8 +/- 117.6% of age-matched euthyroid control) p<0.01. Hypothyro idism in this age group was not associated with significant change in GLUT- 1 protein. The cardiac GLUT-4 protein was increased during both hypothyroid ism and hyperthyroidism. The changes of GLUT-4 in aged rats were similar to those found in young rats. It is concluded that TH effect on GLUT-1 expres sion in the heart is altered with age while TH effects on GLUT-4 are age in dependent.