Regulation of glucose transport by thyroid hormone in rat ovary

Thyroid hormone (TH) plays an important role in regulating ovarian development. However, the mechanism involved remains unclear. Evidence suggests that glucose is essential for ovarian development, and its uptake is mediated by several glucose transporter proteins (Glut). We have investigated the effects of TH on Glut in rat ovary. Immature rats were treated with 6-propyl-2-thiouracil or L-thyroxine to induce hypothyroidism (hypo) or hyperthyroidism (hyper), respectively. Ovarian weights significantly decreased in both treated groups compared with the control group, although the body weights were not markedly altered. Glut1 expression significantly decreased without further changes being detected in the other Glut isforms in the hypo group and was accompanied by minimal change in mRNA content. The expression of Glut1 decreased in the hyper group. In contrast, L-thyroxine significantly increased Glut4 mRNA level and protein content but had little effect on Glut2 and Glut3 expression. Serum glucose concentrations in the hyper group were dramatically reduced compared with those in the control group. However, the serum glucose levels in the hypo group were not significantly changed. In addition, equine chorionic gonadotropin (eCG) increased ovarian weights in both the hypo and hyper groups compared with those in the rats without eCG injection. Glut2-4 protein content was significantly increased by eCG in hyper rats. Only the Glut4 mRNA content was significantly increased by eCG in the hyper group. Although the mRNA levels were not significantly changed by eCG in the hypo group, the protein level of Glut4 was markedly up-regulated. Serum glucose levels were not significantly altered by eCG in the two groups. Thus, dysfunction of the thyroid gland changes Glut expression in rat ovary and ovarian growth, both of which are also regulated by gonadotropin.