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Follicular Phase Gonadotropin Secretion in Cyclic Postpartum Beef Cows with Phlorizin-Induced Hypoglycemia^1,2,3,

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

Abstract

Twenty mature, lactating Hereford-cross cows were used to determine the effect of phlorizin-induced hypoglycemia on gonadotropin secretion following prostaglandin-induced luteolysis. Cows were 43 to 108 d postpartum and had a functional corpus luteum (CL) at the start of infusion treatment (d 1). Infusions consisted of either saline (control) or 3 g/d of phlorizin infused continuously from the time of prostaglandin injection at 1000 on d 1 until 0800 on d 5. Blood samples were collected for determination of plasma concentrations of insulin, glucose and free fatty acids (FFA) and for serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone. Plasma concentrations of insulin (P < .05) and glucose (P < .05) were lower, whereas FFA concentrations increased (day x treatment, P < .05) over the infusion period in phlorizin-treated cows compared with saline-infused controls. Mean serum concentrations of LH (1.17 ± .10 vs 1.53 ± .20 ng/ml; P < .05) and LH pulse amplitude (1.69 ± .14 vs 2.47 ± .37 ng/ml; P < .10) were lower in phlorizin-infused compared with saline-infused cows during the 0 to 24-h period immediately preceding the ovulatory gonadotropin surge. The FSH pulse frequency in creased (.33 ± .11 to .55 ± .12 pulses/h) in saline-infused cows, but decreased (.61 ± .10 to .41 ± .11 pulses/h) in phlorizin-infused cows before the gonadotropin surge. Other characteristics of gonadotropin secretion were similar among phlorizin-infused and saline-infused cows. All but one phlorizin-infused cow ovulated and formed functional CL similar to controls. This study has shown that phlorizin infusion and the resulting change in metabolic environment specifically suppresses the normal increase in LH pulse amplitude during the preovulatory period in cyclic postpartum beef cows.

¹ This work was funded by Alberta Dept. of Agric, Farming for the Future.

² The authors gratefully acknowledge the technical assistance of Anita Unruh, Colleen Zelky and W. Olson.

³ Dept. of Vet. Physiol. Sci.

⁴ Address reprint request to this author. Current address: Beef Cattle and Sheep Branch, Alberta Agriculture, #204, 7000-113 Street, Edmonton, Alberta, Canada, T6H 5T6.

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