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Improving ewe nutrition even for short periods will increase ovulation rate. The increased nutrients must in some way affect the number of follicles that develop to the pre-ovulatory stage. One possible mechanism is that a nutrient or a metabolic hormone that responds to nutrition might act directly on the ovary to influence follicle development and/or follicle selection. In the study described here, insulin and glucose, alone or together, were infused directly into the ovarian artery of ewes with an autotransplanted ovary, for 13.5 h on day 11 of the oestrous cycle. The pattern of androstenedione and oestradiol secretion in response to a GnRH-stimulated LH pulse was measured 2.5 h before and 12.5 h and 24.5 h after the start of the infusion. Glucose or insulin infused alone had no effect on the secretion of androstenedione and oestradiol. However, when infused together, they decreased significantly the secretion of androstenedione and, to a lesser extent, oestradiol. We suggest that the sudden availability of additional glucose and insulin increases insulin-stimulated glucose uptake by the follicle. This leads to an inhibition of LH-stimulated steroidogenesis by the ovarian follicle which occurs in the absence of any detectable changes in circulating plasma concentrations of FSH. These results show that insulin and glucose act together to influence ovarian function directly and suggest that the effects of short-term nutrition on ovulation rate may be mediated by a direct ovarian action of insulin and glucose.
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Abstract
The positive relationship between nutritional state and ovulation rate in sheep may involve the action of specific nutrients on gonadotrophin release. LH and FSH secretion is controlled in part by hypothalamic GnRH, which is in turn influenced by central adrenergic and serotonergic neuronal systems. In this experiment the branched chain amino acids (BCAAs) leucine, isoleucine and valine were examined for effects on LH and FSH secretion. A mixture of the three amino acids was infused into ewes for 5 days immediately before luteolysis, a time when nutritional effects on ovulation rate occur.
The infusion significantly increased ovulation rate without any associated increase in FSH or LH. However, the infusion did increase plasma insulin concentrations and this effect, together with the high levels of blood urea observed, suggests that these amino acids had increased the supply of energy substrates to the follicles. An increase in insulin-mediated glucose uptake by follicles could be the stimulus responsible for the increase in ovulation rate. The ability of the animal to utilize BCAAs for energy metabolism may be an important component of the ovulation responses to nutrition.
Journal of Endocrinology (1995) 145, 315–323
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Abstract
Aspartic acid is a neurotransmitter in the central nervous system that acts via the glutamate receptor and the analogue, N-methyl-d,l-aspartic acid (NMA) is an agonist that stimulates GnRH secretion. Under normal dietary conditions, the plasma concentration of aspartic acid in ewes is low and if increased by improved nutrition may increase the brain concentration of aspartic acid leading to increased gonadotrophin secretion. In two experiments we investigated the effects of NMA on pituitary hormone concentrations and the effects of aspartic acid on ovulation rate and pituitary hormone concentrations. The intravenous injection of NMA into cycling ewes resulted in an immediate (within 15 min) release of a pulse of LH and of GH and a prolonged (up to 1 h) suppression of prolactin secretion. There were marked differences in responsiveness to NMA between individual ewes. The intravenous infusion of aspartic acid for 5 days in the late luteal phase of the oestrous cycle did not affect ovulation rate but reduced the mean LH (P<0·05) and FSH (P<0·05) concentrations in plasma. The frequency of LH pulses also tended to be lower (P<0·1) in ewes infused with aspartic acid. It is suggested that the decrease in gonadotrophin secretion in ewes infused with aspartic acid is due to effects on the hypothalamus or the anterior pituitary gland which are not related to increased levels of ovarian feedback. These changes are likely to involve decreased GnRH secretion.
Journal of Endocrinology (1996) 149, 65–72
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Abstract
The positive relationship between nutrition and ovulation rate was investigated in sheep infused intravenously with glucose. Ovulation rate increased (2·0±0·0 vs 2·4 ± 0·3) when ewes were given an infusion of glucose (60–65 mm/h) for five days in the late luteal phase of the oestrous cycle. The effect of glucose was obtained without any significant change in LH secretion. The concentration of FSH in glucose-infused ewes was lower during the infusion (luteal phase) but higher during the early follicular phase. These data suggest that the change in ovulation rate occurred without increased gonadotrophin support to the follicle during the late luteal phase, which is the period of the sheep oestrous cycle during which improved nutrition increases ovulation rate. There were no changes in GH or prolactin, but changes in circulating glucose and insulin levels were detected. We conclude that insulin, because of its role in cell growth and metabolism, is involved in mediating ovulation responses to nutritional stimuli, either directly or more likely by the stimulation of insulin-mediated glucose uptake.
Journal of Endocrinology (1995) 146, 403–410
