Agonist-Induced Release of Gonadotropin-Releasing Hormone, Luteinizing Hormone, and Follicle-Stimulating Hormone and their Associations with Basal Secretion of Luteinizing Hormone and Follicle-Stimulating Hormone throughout Lactation in Sows

Abstract

Our working hypothesis was that the normal increase in basal secretion of LH and FSH during lactation in sows is paralleled by an increase in readily releasable pools of GnRH, LH, and FSH. Sows fitted with indwelling catheters and nursing 9-11 piglets were sampled on Days 1, 7, 14, and 21 (primiparous sows; n = 10; experiment 1) or Days 7 and 21 (multiparous sows; n = 10; experiment 2) of lactation. On each day, blood was collected at 10-min intervals for 2 h before one member of a pair of sows was infused (i.v.) with saline (n = 5) and the other with the neuroexcitatory amino acid N-methyl-DL-aspartic acid (NMA; 10 mg/kg b.w.) to measure readily releasable GnRH as estimated by LH release. Two hours later, each sow that had received saline was given GnRH (100 micrograms, i.v.) for estimation of releasable pools of pituitary LH and FSH. Responses to NMA and GnRH were estimated by area under the curve (ng.ml-1.min) of serum LH profiles for 1 h (NMA group) or 2 h (GnRH group) after treatment. Basal LH secretion in primiparous sows decreased from Day 1 to Day 7 and then returned to Day 1 values by Day 21. Similarly, in multiparous sows, basal LH was lower on Day 7 than on Day 21. Basal FSH secretion increased linearly as lactation progressed in primiparous and multiparous sows.(ABSTRACT TRUNCATED AT 250 WORDS)

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

“…If concentrations of LH in the pituitary increased with the same magnitude in experiment 2 as in experiment 1, one would have expected to see an increase in the readily releasable pools of LH in experiment 2. Indeed, in primiparous sows, we noted [30] that there was a large increase in the readily releasable pool of LH between Days 1 and 7; but in that study the releasable pool of GnRH (assessed by challenges with NMA) also increased modestly from Day 1 to Day 7, in contrast to the situation with multiparous sows in the present study. So primiparous and multiparous sows may differ in the rate of accumulation of LH in the pituitary in the immediate postpartum period.…”

“…Together, these results provide additional evidence that basal FSH secretion is differentially controlled between primiparous and multiparous sows. In primiparous sows, NMA elicited FSH secretion only at Days 14 and 21, not at Days 1 and 7 postpartum [30], whereas multiparous sows showed enhanced FSH release as early as Day 7 of lactation ( [30] and this study). It is not clear why secretion of FSH differs between primiparous and multiparous sows during early lactation, but this might be one of the factors that contribute to a higher rate of postweaning anestrus in primiparous sows [3].…”

“…This is in agreement with the basal LH profiles of experiment 2, in which serum LH decreased from Days 1 to 7 of lactation. Earlier, we [30] reported that basal LH release decreased during the first week of lactation in primiparous sows, and De Rensis et al [31] found that LH was lower in suckled than in zero-weaned primiparous sows by 72-78 h postpartum.…”

“…NMA has been shown to elicit gonadotropin secretion in a number of species, including the pig [14,30,[45][46][47]; and it has been demonstrated that NMA has little direct effect on the pituitary in vivo [14,48]. Thus, it is highly probable that a change in gonadotropin secretion after NMA is due to a change in NMA-induced secretion of GnRH.…”

Peri‐oestrus Hormone Profiles and Follicle Growth in Lactating Sows with Oestrus Induced by Intermittent Suckling

“…1981; Cox et al. 1988; Sesti and Britt 1993b,c). Administration of oestradiol benzoate was found not to induce LH surges in early lactation (Cox et al.…”