Secretion of gonadotropins and estimated releasable pools of gonadotropin-releasing hormone and gonadotropins during establishment of suckling-induced inhibition of gonadotropin secretion in the sow.

Abstract

Two experiments were conducted to evaluate relationships between changes in hypothalamic GnRH, pituitary and serum gonadotropins, and ovarian function during the first week postpartum in lactating multiparous sows. In experiment 1, our hypothesis was that sows would have active LH secretion immediately after parturition and that suckling would inhibit LH concentrations within the first 3 days postpartum. Sows were killed at Day 1 (n = 5) or Day 7 (n = 5) of lactation, and blood samples were taken every 6 h from 48 h before parturition until 24 h (Day 1 sows) or 156 h (Day 7 sows) after parturition. Preoptic area (POA), medial basal hypothalamus (MBH), stalk median eminence (SME), anterior pituitary (AP), and ovaries were collected at slaughter. On Day 1, sows had fewer (p < 0.05) small follicles (< 4 mm diameter), but more (p < 0.05) medium (4-6 mm) and large (> 6 mm) follicles, than sows on Day 7 (20 +/- 2, 6.5 +/- 1.1, 2.5 +/- 0.7 vs. 26.3 +/- 3, 0, and 0, respectively). Serum LH and FSH decreased (p < 0.04) during the period from 24 to 48 h postpartum and remained low through Day 7. GnRH concentrations in POA, MBH, and SME and the proportion of residual GnRH released in vitro in response to K+ did not differ between Day 1 and 7. Weights of AP were not different, but pituitary concentrations (microgram/mg) of LH and FSH increased (p < 0.05) from Day 1 to 7 (LH: 0.27 +/- 0.03 vs. 0.44 +/- 0.01; FSH: 2.6 +/- 0.07 vs. 7.8 +/- 0.8, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)