Ovulation rate and the concentrations of LH, FSH, GH, prolactin and insulin in ewes infused with tryptophan, tyrosine or tyrosine plus phenylalanine during the luteal phase of the oestrous cycle

Dietary amino acid precursors for cathecholamineric and serotonergic neurotransmitters may be important in the mechanism of nutritional effects on ovulation rate. This paper reports the results of three experiments that examined the effect of such amino acids on ovulation rate and the concentrations of FSH and LH in sheep. In three separate experiments, groups of ewes were infused, over Days 9 to 13 of the oestrous cycle, with either tryptophan (n = 11), tyrosine (n = 11) or a mixture of tyrosine and phenylalanine (n = 11). Control ewes (n = 12 in each experiment) were infused with a vehicle over the same period. None of the amino acids infused effected ovulation rate or plasma concentrations of LH, FSH, GH or prolactin. The infusion of a mixture of tyrosine and phenylalanine increased insulin concentrations. The infusion of these amino acids was not associated with changes in gonadotrophin concentrations and therefore the effect of nutrition on ovulation rate in ewes does not seem to involve an increase in the availability of tryptophan, tyrosine or phenylalanine. Increasing the uptake of other amino acids that compete with tryptophan, tyrosine or phenylalanine for the large neutral amino acid transporter may cause a decrease in the availability of tryptophan, tyrosine or phenylalanine thereby eliciting the effects of nutrition on ovulation rate. However, this hypothesis remains to be tested.