The differential effect of progesterone on concentrations of luteinizing hormone and follicle-stimulating hormone in heifers.

The effects of progesterone on LH and FSH concentrations in cattle are not clearly understood. The objective of this experiment was to determine the effect of various concentrations of progesterone on serum concentrations of LH and FSH in heifers. After estrous synchrony, animals were assigned to four different treatment groups (n = 10/group). Heifers received vaginal coils containing either 2% or 6.75% progesterone between days 8-10 (luteal phase) or days 17-18 (follicular phase) after estrus. Coils remained in place for 7 days. One day before removal of coils, all animals received an injection of prostaglandin-F2 alpha to cause regression of existing corpora lutea. Both the stage of the cycle and the percentage of progesterone in the coil affected serum concentrations of progesterone while the coils were in the vagina. Concentrations of FSH during progesterone treatment were not different for animals on different treatments. While coils were in the vagina, there was a significant increase in serum LH concentrations in heifers in the follicular phase given 2% progesterone coils, but no increase in LH was observed in the other three treatment groups. After removal of the progesterone coils, but before the preovulatory gonadotropin surges, LH increased but not FSH. The LH and FSH surges occurred synchronously. After the surges, FSH concentrations increased at a faster rate than LH. These data suggest that differential feedback mechanisms exist for LH and FSH in heifers. (Endocrinology 108: 568, 1981)