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. 1968 Jul;197(2):445–459. doi: 10.1113/jphysiol.1968.sp008569

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

E F Annison, J L Linzell, C E West
PMCID: PMC1351809  PMID: 5716853

Abstract

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-14C]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 O2 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 β-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 CO2. The udder took up 10·7-16·1 mg/min/kg of tissue and 8-10% of mammary CO2 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 CO2 was derived from it. The udder used 0·8-2·4 mg/min/kg of tissue and 9-13% of mammary CO2 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 C14 which in fed animals are synthesized from acetate and β-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 CO2 was derived from each. The udder took up 3·0-5·7 mg/min/kg of tissue and 4-8% of mammary CO2 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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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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