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pubmed:abstractText |
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.
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