Prolactin and Somatotropin Binding by Granulosa Cells of Cows at Their Different Reproductive States

Specific binding of bovine prolactin and somatotropin by granulosa cells from the antral follicles of various diameters was studied in cows at different reproductive states, prepubertal, pubertal, and early gestation. The ability of granulosa cells to bind prolactin did not depend on the reproductive state of an animal. At the same time, the dynamics of somatotropin specific binding by granulosa cells during maturation of the antral follicles differed at dissimilar reproductive states of the cows. When the diameter of follicles increased from 3–5 to 6–10 mm, specific binding of ¹²⁵I-somatotropin decreased in pubertal animals, but remained unchanged in the prepubertal and pregnant animals. The results of Scatchard analysis of the binding data indicate that sexual maturation of cows did not affect the binding of prolactin and somatotropin by granulosa cells from follicles of 1–2 mm in diameter. The data obtained suggest that the decreased sensitivity of granulosa cells to somatotropin at the terminal stages of maturation of the antral follicles is essential for their development and acquisition of the ability for ovulation.