Uterine prostaglandin F metabolite production in relation to glucose availability in late pregnancy and a possible influence of diet on time of delivery in the mare.

The production of uterine PGFM during different dietary states has been investigated in pregnant mares in late gestation. Arterial and uterine venous plasma concentrations of PGFM rose when food was withdrawn for 12-30 h and the V-A difference widened significantly. There was an inverse correlation between the rise in PGFM and the fall in plasma glucose during a fast, and a significant decrease in the A-V plasma glucose differences across the uterus. Plasma PGFM and free fatty acid concentrations before and during food withdrawal were also correlated but no uterine A-V difference in free fatty acids could be detected. An immediate fall in uterine PGFM levels occurred upon refeeding or infusion of glucose, which was more closely correlated with the rise in plasma glucose than the fall in free fatty acids, both of which occurred after restoration of nutrient. No detectable alterations in uterine blood flow occurred during a fast/feed cycle and hence A-V or V-A differences across the uterus gave an indication of uptake or output. PGFM production increased 5 times during a fast and glucose uptake decreased by about 70%. Basal levels were restored by 1-3 h after feeding or glucose infusion. Daily fluctuations in plasma PGFM concentrations were closely associated with the time of the morning feed of concentrates. The overnight rise in PGFM was greatest in late gestation. Five of the 8 mares delivered prematurely within 1 week of the last period of food withdrawal. Attention is drawn to the consequences of inadequate or intermittent feeding on premature delivery.