Effect of suckling on postpartum changes in 13,14-dihydro-15-keto-PGF2 alpha and progesterone and induced release of gonadotropins in autumn-lambing ewes.

Postpartum changes in concentrations of 13, 14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) and progesterone and gonadotropin-releasing hormone (GnRH) induced release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were studied in two experiments on suckled and nonsuckled autumn-lambing ewes. In both experiments, one group of ewes had lambs weaned on d 3 +/- .5 postpartum and was compared with a second group of ewes that suckled one lamb each. In Exp. 1, jugular blood samples were collected daily from the day after lambing until d 50 postpartum for assay of PGFM and progesterone. On d 21 +/- .5 postpartum, ewes (eight suckled and eight nonsuckled) received GnRH (100 micrograms), and LH and FSH were measured in blood samples collected over a 340 min period. In Exp. 2, jugular blood samples were collected from the day of lambing until d 22 +/- .6 postpartum for assay of PGFM and progesterone. Ewes (six suckled and seven nonsuckled) received GnRH (100 micrograms) on d 22 +/- .6 postpartum, and LH and FSH were quantified in blood samples taken over a 185 min period. The pituitary was removed from each ewe 190 min after GnRH for LH and FSH determinations. Postpartum changes in concentrations of PGFM and progesterone did not differ with suckling in either experiment. In both experiments PGFM concentrations were high on d 1 postpartum, but declined to basal values by d 11. The release of LH after GnRH in Exp. 1 was greater (P less than .001) in suckled than in nonsuckled ewes. In Exp. 2, LH release after GnRH was not affected by suckling, but in both experiments suckled ewes had a greater (P less than .01) release of FSH than did nonsuckled ewes. Pituitaries from suckled ewes contained more FSH (P less than .01) than pituitaries from nonsuckled ewes. The resumption of ovarian cyclicity in well-fed autumn-lambing ewes appeared to be neither altered by suckling nor limited by the ability of the pituitary to respond to GnRH.