THE EFFECT OF THE INFUSION OF INSULIN DURING THE LUTEAL-PHASE OF THE ESTROUS-CYCLE ON THE OVULATION RATE AND ON PLASMA-CONCENTRATIONS OF LH, FSH AND GLUCOSE IN EWES

The role of insulin in mediating pituitary responses to nutrition was investigated in 30 mature Border Leicester X Merino ewes. The ewes wer e infused with saline (n = 15) or bovine insulin at 0.4 IU/kg/d (n = 1 5) for 72 h during the luteal phase of the estrous cycle The ewes were housed in individual pens and were fed, ad libitum, a diet of low qua lity straw. Their estrous cycles were synchronized with prostaglandin (PG), with infusions given over Days 9 to 11 of the estrous cycle. A f urther injection of PG was given at the end of the infusion, and the s ubsequent ovulation rate was determined by endoscopy 12 d later. Blood samples were collected every 4 h from Day 8 until 52 h after the fina l PG injection for the determination of plasma FSH, insulin and glucos e concentrations. On Day 11 blood samples were also taken every 20 min for 24 h for the determination of LH pulse characteristics. During th e infusion of insulin, its concentration rose 4-fold and remained elev ated until the end of infusion, when it fell to pretreatment concentra tions. Glucose concentrations were significantly reduced during the in sulin infusion and rose to pretreatment concentrations after infusion. In control ewes glucose and insulin concentrations did not change. Ov ulation rate of treated ewes was not affected by the insulin (1.9 +/- 0.07) compared with that of control ewes (2.0 +/- 0.10). Neither were FSH concentrations affected by treatment with insulin, although a sign ificant interaction of treatment with time was observed in the 36 h af ter infusion. The pre-ovulatory decline in FSH concentrations was dela yed by about 8 h in the insulin treated ewes. The mean (+/- SEM) LH pu lse frequency (4.3 +/- 0.4 vs 1.8 +/- 0.3 pulses per 24 h) and the mea n (+/- SEM) concentration of LH (0.48 +/- 0.04 vs 0.32 +/- 0.03 ng/ml) were both significantly reduced by insulin. These results indicate th at insulin-induced hypoglycaemia inhibits LH secretion in cyclic ewes and implicates insulin as a mediator of normal hypothalamo-pituitary f unction. (C) 1997 by Elsevier Science Inc.