The metabolism of acetate and glucose by the isolated perfused udder. 2. The contribution of acetate and glucose to carbon dioxide and milk constituents

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Abstract
A substrate mixture of glucose, acetate and amino acids containing either uniformly labelled [Cl4]glucose ([U-Cl4]glucose) or [2-C14]-acetate was infused for 6-9 hours into the perfusion fluid of 4 isolated perfused mammary glands of lactating goats. In an experiment with [U-Cl4]glucose, 77% of the milk lactose, 23% of milk triglyceride glycerol, 41% of milk citrate, 0.5% of milk triglyceride fatty acids and 39% of the expired CO2 came from added glucose. In another experiment with [2-C14]acetate, only 0.3% of the milk lactose, 2% of milk triglyceride glycerol and 14% of expired CO2 came from added acetate. However, 48% of the milk triglyceride fatty acids, 90% of the volatile fatty acids and about 33% of the milk citrate came from this precursor. The labelling of milk fatty acids by [2-C14]acetate but not by [U-C14]-glucose is contrasted with the equal labelling of milk citrate by both. The possibilities of two acetyl-CoA pools and of the carboxylation of pyruvate are discussed. The slow initial rise in the specific radioactivities of both milk glycerol and fatty acids, as compared with that of lactose, suggests the existence of larger stores of triglyceride and precursors than of lactose and precursors.