Ontogenic and nutritionally induced changes in fetal metabolism in the horse.

1. Using the Fick principle and tracer methodology, the metabolic rates of chronically catheterized fetal foals (n = 24) were measured at different gestational ages during the second half of gestation and in response to maternal fasting for 36 h in late gestation (n = 4, term approximately 335 days). 2. Absolute rates of umbilical blood flow, fetal glucose utilization and umbilical uptake of oxygen (O2) and glucose increased from mid-gestation to 300 days and then plateauxed until term. The absolute rate of umbilical lactate uptake was significant after, but not before, 280 days of gestation. Weight specific rates of umbilical uptake and fetal utilization of glucose decreased progressively throughout the second half of gestation. 3. Weight specific rates of CO2 production from glucose carbon were also inversely correlated with gestational age. Umbilical lactate uptake per kilogram of fetus was lower after 320 days than between 281 and 300 days. In contrast, no gestational trends were observed in the weight specific rates of fetal O2 uptake and urea production. Glucose production by the fetal horse was negligible, even very close to term. 4. Maternal fasting for 36 h reduced the rate of umbilical uptake and utilization of glucose production but had no effect on the rates of endogenous glucose production and umbilical uptake of oxygen and lactate by the horse fetus. 5. The observations show that fetal metabolism is highly dependent on glucose in the horse compared to the sheep in mid- and late gestation. Different species therefore adopt different strategies in meeting the nutritional demands of the growing fetus during the second half of gestation.