Triiodothyronine receptors in porcine granulosa cells

Triiodothyronine has been found to enhance gonadotropin- and insulin-stimulated morphologic luteinization and progesterone production by porcine granulosa cells in culture. The role of triiodothyronine in the above events is not well defined. One possibility is that triiodothyronine effects are direct and are receptor mediated. To test this concept, we undertook this study to investigate the presence of triiodothyronine receptors in granulosa cells aspirated from large (6 to 12 mm), medium (3 to 5 mm), and small (1 to 2 mm) porcine ovarian follicles. Crude nuclei were isolated and tested for iodine 125-triiodothyronine specific binding. Binding was time and temperature dependent and maximal in cells from small follicles at pH 8. Scatchard analysis yielded a mean apparent dissociation constant of 5.5 X 10(-9) mol/L and a mean apparent total number of binding sites of 1.0 pmol/mg of deoxyribonucleic acid. Competition experiments revealed the following relative binding affinities: triiodothyronine greater than L-thyroxine greater than reverse triiodothyronine. Gonadotropins, prostaglandins, epidermal growth factor, and insulin did not compete with 125I-triiodothyronine for binding. We conclude that there are nuclear binding sites in porcine granulosa cells with characteristics expected of a triiodothyronine receptor which might mediate a direct action of triiodothyronine on granulosa cells.