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

Recent work has emphasized that both glucose and acetate are important metabolites in the ruminant. Annison & Lindsay (1961) estimate that 33 % of expired C02 is derived directly from acetate in the fed sheep, and Annison & White (1961) find that, under these conditions, 23 % comes from glucose; both these values are sensitive to the nutritional state of the animal. Considerable quantities of both glucose and acetate are taken up by the goat udder in vivo [Linzell (1960); some of the values for glucose uptake in this paper were wrong and an erratum has been published in J. Physiol. (1962), 163, 1], but it is not easy to determine in the whole animal the relative amounts of C02 coming from each substance in the metabolism of the udder itself (Wood, Gillespie, Hansen, Wood & Hardenbrooke, 1959). It is technically easier in the isolated organ because (a) the addition of substrates can be controlled, (b) radioactive substances can readily be infused for long periods, so that steady-state levels are more likely to be reached, (c) the blood flow is known, and (d) the metabolism of only one organ is involved. Although glucose is not a significant precursor of milk fatty acids in the ruminant (Balmain, Folley & Glascock, 1954; Kleiber et al. 1955), considerable amounts of milk citrate come from glucose in the cow (Kleiber et al. 1955; Tombropoulos & Kleiber, 1961), and half the C02 from glucose may be produced by routes other than the pentose phosphate pathway (Black, Kleiber, Butterworth, Brubacher & Kaneko, 1957a). These discordant results suggest that the relationship between glucose, acetyl-CoA and the tricarboxylic acid cycle in the ruminant udder is not clear. Further work (D. C. Hardwick, J. L. Linzell & S. M. Price, unpublished work) to elucidate the metabolic routes of these substances by using inhibitors and substituents for glucose gave results that were difficult to interpret. In the presence of acetate and amino acids, the following substances were tested as glucose substitutes: lactose, galactose, mannose, fructose, ribose, glycerol, pyruvate, ,B-hydroxybutyrate, propionate, fumarate and malate. None supported milk secretion, and only * Part 1: Hardwick, Linzell & Price (1961). mannose and propionate caused an oxygen uptake similar to that with glucose. Lactose, galactose, fructose, pyruvate and fl-hydroxybutyrate were taken up by the gland, but at a rate lower than that for glucose. Transport into the cell or, in some cases, phosphorylation may have been limiting for many of these substances. With inhibitors, although milk secretion was decreased by potassium cyanide, 2,4-dinitrophenol and sodium fluoride, other inhibitors (e.g. fluoroacetate and fluorocitrate) could not be assessed because they had non-specific effects on other tissues of the gland (e.g. they caused vasoconstriction and oedema). Hardwick, Linzell & Price (1961) have shown that in the isolated udder maximal milk production of normal composition depends on both glucose and acetate; it was not clearly shown, however, that lactose and fat were synthesized from the added glucose and acetate. We have therefore now studied the transfer of 14C from [14C]glucose and [14C]_ acetate, not only to C02, but also to lactose, triglyceride fatty acids, triglyceride glycerol and citrate in the milk formed in vitro by the perfused udder.