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Changes in Metabolic and Reproductive Characteristics Associated with Lactation and Glucose Infusion in the Postpartum Ewe^1,2,3,4,

University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0W0

Abstract

This study examined mechanisms whereby the metabolic environment interacts with basic reproductive function. Ewes lambing during the breeding season were fed to maintain (MAINT, n = 10) or gain (GAIN, n = 11) body weight during the last 4 mo of gestation. From d 7 to 22 postpartum, ewes were infused iv with saline (n = 10) or glucose at a rate calculated to increase normal glucose entry rate by 75% (n = 11). Blood samples were collected daily to determine plasma concentrations of nutritive metabolites and insulin and at frequent intervals on d 14 and 21 to determine serum gonadotropin concentrations. Hypothalami and pituitaries were collected on d 22 to determine hormone content and receptor concentrations. Plasma concentrations of nutritive metabolites and insulin indicated that MAINT ewes mobilized more (P<.01) body fat and protein reserves during gestation and early lactation than ewes in the GAIN group. Glucose infusion elevated plasma concentrations of glucose (P<.05) and insulin (P<.07) and reduced (P<.05) fat and protein mobilization, even though it depressed feed intake (P<.001), compared with saline infusion. Hypothalamic gonadotropin-releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) content and pituitary GnRH receptor concentration were similar between treatments. Ewes fed to gain weight had greater mean serum concentrations of FSH than MAINT (27 ± 4.3 vs 19 ± 1.2 ng/ml, P<.10), but LH was similar between treatments. The number of ewes ovulating by d 22 was similar between GAIN and MAINT, but more saline- than glucose-infused ewes ovulated (60 vs 18%, P<.05). Glucose infusion in adequately fed ewes did not enhance return to ovarian cyclicity, but may have caused a metabolic disturbance which suppressed reproductive function. Alteration of metabolic characteristics did not cause changes in hypothalamic and pituitary characteristics.

¹ This work was funded by the Alberta Agr. Res. Council.

² The authors gratefully acknowledge the outstanding technical assistance provided by L. M. Clark, P. J. Lewing and B. Trowel. Special thanks are due Bob Ford and staff of The Provincial Dairy Lab., Regina, Saskatchewan, for analyses of the ewe milk composition.

³ Part of these data has been reported in preliminary form in J. Anim. Sci. 59 (Suppl. 1):365.

⁴ Dept. of Vet. Physiol. Sci.

⁵ Address reprint requests to this author.