GONADOTROPIN-RELEASING-HORMONE RECEPTORS IN ANTERIOR PITUITARIES OF CATTLE DURING THE ESTROUS-CYCLE

Gonadotropin-releasing hormone (GnRH) receptors in anterior pituitaries of cows during the estrous cycle and those with ovarian cysts were examined. Cows were classified according to expected day of the estrous cycle (Day 1 = day of ovulation, Day 20 = day of estrus): Stage 1 (Days 1-4), Stage 2 (Days 5-10), Stage 3 (Days 11-17), and Stage 4 (Days 18-20), and ovarian cysts, based upon the morphological features of the ovaries. The ovarian cysts were further divided into follicular cysts (plasma concentration of progesterone < 1 ng ml-1) and luteinized cysts (plasma concentrations of progesterone greater-than-or-equal to 1 ng ml-1). Numbers and affinities of GnRH receptors were determined by Scatchard analysis. Concentrations of progesterone in plasma were measured by radioimmunoassay. The numbers of GnRH receptors were relatively lower in Stages 1 and 2, compared with a significant increase (P < 0.05) in Stage 3. Subsequently, the number of GnRH receptors did not differ significantly between Stages 3 and 4, and decreased significantly from Stage 3 to Stage 1. The concentrations of progesterone in plasma of cows with follicular cysts were comparable to those in Stage 4, while those with luteinized cysts did not differ significantly from those in Stages 1-3. The number of GnRH receptors in cows with follicular cysts did not differ significantly from those in Stage 4 of the cycle. The number of GnRH receptors in cows with luteinized cysts did not differ significantly from those in Stages 1-3 of the cycle. The Ka value of GnRH receptors did not differ significantly among all the groups. These results suggest that numbers of GnRH receptors in anterior pituitaries decrease from the late-mid luteal to the early luteal phase in cows. They also indicate that cows with ovarian cysts do not have remarkable abnormality in GnRH receptors of the anterior pituitary.