Mammary and whole animal metabolism of glucose and fatty acids in fasting lactating goats.

1. Measurements were made of milk yield, mammary blood flow and mammary arteriovenous differences during the measurement of substrate entry rate by the isotope dilution method using [U-(14)C]glucose, acetate, palmitate, stearate or oleate in conscious lactating goats after 24 hr starvation.2. As previously reported, in fasting, milk yield fell to 40 +/- 3.4 (S.E.)%, lactose secretion to 31 +/- 3.4%, milk fat secretion to 81 +/- 6.7% and mammary blood flow fell to 53 +/- 7.5% of the values before fasting. Mammary O(2) uptake was only 45 +/- 5% of the mean value in fed animals and there were marked falls in the uptakes of glucose, acetate and triglycerides, a smaller fall in beta-hydroxybutyrate uptake, and a large increase in free fatty acid uptake.3. Glucose was found to enter the circulation of the fasting animal at 1-1.6 mg/min/kg body wt. (entry rate) and it gave rise to 3-5% of the total CO(2). The udder took up 10.7-16.1 mg/min/kg of tissue and 8-10% of mammary CO(2) was derived from glucose, although only 5-10% was oxidized. Mammary uptake accounted for 35-43% of the total glucose entering the circulation.4. In the whole animal acetate entry rate was 1-1.4 mg/min/kg and 9-10% of total CO(2) was derived from it. The udder used 0.8-2.4 mg/min/kg of tissue and 9-13% of mammary CO(2) was derived from acetate, 46-79% of that taken up being oxidized. Mammary uptake accounted for only 2-6% of the total acetate entry rate. Negligible quantities of isotope were found in milk fatty acids and there was a fall in the proportion of milk fatty acids of chain length up to C(14) which in fed animals are synthesized from acetate and beta-hydroxybutyrate.5. Palmitate, stearate and oleate entered the circulation as free fatty acids at 0.94-6.8 mg/min/kg and 6-9% of total CO(2) was derived from each. The udder took up 3.0-5.7 mg/min/kg of tissue and 4-8% of mammary CO(2) was derived from each acid. In the udder 8 and 5.5% of stearate and oleate were oxidized and 25% of palmitate. Mammary uptake of stearate was 31.5% of the total entry rate, palmitate 1%, and oleate 7.5%. Only long chain milk fatty acids were labelled.6. During fasting the mammary R.Q. was 0.85 +/- 0.045 compared with a value in fed animals of 1.24 +/- 0.02, when the udder is synthesizing fatty acids from acetate. The total mammary uptake of lipid precursors was only 74% of the rate of milk fat secretion and there was an 18% shrinkage in empty udder volume, suggesting the use of endogenous mammary tissue substrates.