Changes in Plasma Concentrations of Immunoreactive Inhibin, Estradiol and FSH Associated with Follicular Waves during the Estrous Cycle of the Cow

Abstract

Changes in plasma concentrations of immunoreactive (ir-) inhibin, steroid hormones and gonadotropins were determined in relation to follicular growth throughout the bovine estrous cycle. Three waves of follicular development occurred; the first, second and third waves were observed during the early-luteal, mid-luteal and follicular phases, respectively. A dominant follicle was identified in each wave. During the growth of the dominant follicle, the other follicles ceased their growth, while the emergence of follicular wave was observed after the dominant follicle ceased to grow or ovulated. Plasma concentrations of ir-inhibin increased (p<0.05), concomitantly with the emergence of each wave. A high concentration of ir-inhibin was noted in the growing phase of each dominant follicle. Concentrations of estradiol in the plasma increased (p<0.01) coincidentally with the growth of the first and third dominant follicles. However, during the second wave, plasma estradiol levels did not show any significant rise, suggesting that the second dominant follicle has a very low activity to produce estradiol. Progesterone levels declined (p<0.01) sharply due to the spontaneous luteolysis and increased again during the early luteal phase, reached the high levels during the mid-luteal phase. Concentrations of plasma FSH were high before the emergence of each follicular wave and began to decrease in accordance with the emergence of the wave, then remained low until the dominant follicle regressed or ovulated. These results suggest that the dominant follicle, during its growing phase, lowers the plasma FSH concentration by enhancing the secretion of inhibin alone and or estradiol, and suppresses the recruitment and growth of the other follicles. In addition, progesterone also probably affects FSH secretion during the early-luteal phase in combination with estradiol.