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

Sesti, L.A.; Britt, J.H.

Biology of Reproduction 49(2): 332-339


ISSN/ISBN: 0006-3363
PMID: 8373957
DOI: 10.1095/biolreprod49.2.332
Accession: 002294531

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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 mu-g, 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 cntdot ml-1 cntdot 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. The NMA- and GnRH-induced LH responses diverged in primiparous sows; the NMA-induced LH response increased linearly from Day 1 to 21 whereas the GnRH-induced LH response increased from Day 1 to 7 and then decreased to similar values on Days 14 and 21. In multiparous sows, the NMA- and GnRH-induced responses were greater on Day 21 than on Day 7. Progesterone and estradiol-17-beta were greater (p lt 0.01) on Day 1 than on any other day in primiparous sows; however, both steroids increased (p lt 0.05) from early to late lactation in multiparous sows. These results are consistent with the concept that GnRH accumulates in axonal terminals but is released at a reduced rate during early lactation, thereby permitting increased synthesis and accumulation of LH. As lactation progresses, GnRH release rate increases, raising basal concentrations of LH and decreasing pituitary pools. Secretion of FSH differs from LH secretion in that it increases as lactation progresses without evidence of a strong inhibition by suckling.