PARENT SESSION
PLATFORM SESSION 21: REGULATION OF GONADOTROPINS AND THEIR RECEPTORS: II
Chair: LaBarberra, Andrew¹, ¹
Co-chair: Meehan, Thomas¹, ¹
Harborside D
2:00 PM-4:00 PM

(450) STAGE OF ESTROUS CYCLE AND EXOGENOUS STEROID TREATMENT EFFECTS ON SERUM GONADOTROPINS, PITUITARY GONADOTROPIN, AND STEROID RECEPTOR CONTENT IN HEIFERS.

Lane, Elizabeth¹, Roche, James¹, Crowe, Mark¹, ¹ Dept of Animal Husbandry and Production, Dublin, IE

ABSTRACT- To determine the effect of estrous cycle stage and exogenous steroid treatment on the relationship between serum gonadotropins, pituitary gonadotropin and steroid receptor content in heifers. Heifers (n = 40) were slaughtered at 1) 13 h (13 h control; n = 8) after estrous onset, following the primary gonadotropin surge, 2) 30 h (n = 24), during the transient FSH rise and nadir LH, or 3) 66 h (66 h control; n = 8), at nadir FSH and LH. Heifers slaughtered at 30 h were assigned to one of three treatments; no treatment (30 h control; n = 8), progesterone (P) device inserted 12 h after estrous onset until slaughter (30 h P; n = 8) or P with an estradiol benzoate capsule inserted 12 h after estrous onset until slaughter (30 h P+E; n = 8). Serum LH was unaffected by treatment. Mean serum FSH was higher (P < 0.05) in control heifers (0.5 ± 0.06 ng/ml) slaughtered at 30 h than in those slaughtered at 13 (0.2 ± 0.04 ng/ml) or 66 h (0.2 ± 0.04 ng/ml). Exogenous steroids prevented (P < 0.05) this increase. The ratio of pituitary LH immunoreactive to non-immunoreactive (I:NI) cells was unaffected by cycle stage. This ratio was suppressed (P < 0.05) by P (0.05 ± 0.01) compared with untreated controls (0.09 ± 0.02), but was unaffected by treatment with P+E (0.06 ± 0.01). The ratio of pituitary FSH I:NI cells was lower (P < 0.05) in heifers slaughtered at 13 h (0.08 ± 0.01) compared with heifers slaughtered at 30 (0.18 ± 0.02) or 66 h (0.15 ± 0.01). Steroid treatment suppressed (P < 0.05) this ratio (0.12 ± 0.02 and 0.11 ± 0.02 for 30 h P and 30 h P+E, respectively) compared with 30 h controls (0.18 ± 0.02). The mean ratio of I:NI cells containing estrogen receptor (ER) was decreased (P < 0.05) in heifers slaughtered at 30 h (0.04 ± 0.01) compared with those slaughtered at 13 (0.08 ± 0.02) or 66 h (0.09 ± 0.01). Steroid treatment did not affect this ratio. The proportion of cells containing ER was unaffected by treatment. Pituitary progesterone receptors were not detected suggesting that progesterone effects gonadotropin secretion at a level other than the pituitary. Steroidal suppression of serum FSH was associated with decreased pituitary FSH content, but not with changes in steroid receptor content. A change in the expression of ER, but not ER, was associated with differential regulation of gonadotropin secretion during the first transient FSH rise.

KEY WORDS: pituitary, gonadotropins, steroid receptors, heifers