No abstract

Specialized techniques, previously used in surgically prepared goats, which simultaneously measure udder metabolism (arteriovenous difference of milk precursors x udder blood flow) and the whole body turnover of the milk precursors, have been successfully transferred to dairy cows. Methods of obtaining representative samples of arterial and mammary venous blood and of measuring udder blood flow are described.The rates of entry into the circulation, as determined by isotope dilution, of glucose, acetate and plasma free fatty acids were 3-3-4-0, 1-7-2-1 and 0-5 kg/day respectively. Acetate and glucose contributed 32-50 and 4-11% respectively of the total CO a output by the animal.Measurement of the uptake of precursors of milk constituents and their transfer into milk showed that there were substantial arteriovenous differences of glucose, acetate, triglyceride and /?-hydroxybutyrate which were not significantly different between breeds or related to milk yield. Isotopic and balance data confirm that glucose is the main precursor of lactose and that the oxidation and transfer of glucose into lactose accounted for 69-98 % of the glucose entry rate. As in the goat, plasma triglycerides and blood acetate accounted for 35-80 % and 25-50 % of the milk triglycerides respectively. Propionate was extracted from plasma but the uptake was only about 8 % of the value for acetate.There was no net arteriovenous difference of phospholipids, cholesterol esters or free fatty acids, but the fall in specific radioactivity of free fatty acids across the mammary gland indicated there was an exchange of free fatty acids between plasma and mammary tissue. In agreement with previous findings, acetate contributed to all the milk fatty acids up to a chain length of C 14 and part of the C 16 fatty acid. Plasma triglycerides contributed to the remainder of the C 16 fatty acid and all the milk fatty acids with a chain length of C 18 or higher.In contrast to the lactating goat, cow plasma contained very few chylomicrons. The majority of the triglycerides taken up by the udder were derived from the lowdensity lipoprotein fraction.The essential amino acids were extracted from blood in amounts sufficient to account for the essential amino acids secreted into milk protein. Although the plasma level of methionine was low, 52-72 % of the material reaching the mammary gland was taken up. The uptake of arginine was far in excess of the requirement for milk protein synthesis.been difficult to interpret such trials accurately because oi our relative ignorance of the complex The economic importance of the dairy cow has processes involved in the body's mechanisms reguled to numerous studies on the effect of diet on the lating the uptake and metabolism of milk preyield and composition of milk. Frequently it has cursors by the udder. However, in fed and fasted • Present address: Biochemistry Department, Lin-goats important quantitative data have been coin College, University of Canterbury, New Zealand. obtained by simultaneously measuring milk out-

No abstract

SUMMARY1. When goats were milked each hour after being given a dose of synthetic oxytocin within the range thought to be released by the pituitary, there was a progressive rise in milk yield becoming statistically significant by 5 hr. The effect was reduced if the milk was not removed from the gland each hour.2. Milking transplanted glands each hour without injecting oxytocin also increased milk yield. The yield of the unmilked glands on the same animals was not affected. Massaging the transplanted glands had no effect on the milk yield. 5. In some goats very small doses of oxytocin caused changes in milk composition and in one such animal these changes were mimicked by the close arterial infusion of bradykinin.6. Reasons are given for believing that the changes in composition are incidental to the main action of oxytocin in expelling milk and could be caused by a small number of leaks between the tight junctions connecting secretary cells.7. The increase in the rate of milk secretion following milk removal is probably of greater physiological significance than the small changes in milk composition and supports Levy's idea of a local negative feed-back via a chemical component of milk.

It is well established that in ruminants dietary carbohydrate is largely fermented to short-chain fatty acids. Acetate, the major product of ruminal fermentation, is absorbed together with smaller amounts of propionate and butyrate to constitute a major source of energy. The paucity of alimentary glucose, when considered with the low concentrations of blood glucose and relative insensitivity to insulin which characterize ruminants, has tended to obscure the importance of glucose in ruminant metabolism. However, recent work based on the dilution of constantly infused isotope (Searle. Strisover & Chaikoff, 1954;Steele, Wall, de Bodo & Altszuler, 1956) has shown that rates of turnover and oxidation of glucose in sheep (Annison & White, 1961Bergman, 1963;Ford, 1963) and cattle (Davis & Brown, 1962) are not markedly lower than in non-ruminants. Similar studies with acetate have confirmed the quantitative importance of this substrate in ruminant metabolism and shown that, relative to glucose, acetate makes a two-to three-fold greater contribution to total oxidative metabolism in the intact animal (Annison, Brown, Leng, Lindsay, West & White, 1963).The dominant role of acetate in the over-all economy of ruminants is not reflected in the metabolism of all individual tissues. McClymont & Setchell (1956) demonstrated the uptake of glucose but not acetate by the sheep brain, and Annison, Scott & Waites (1963) showed that the testis and epididymis in the anaesthetized ram oxidize more glucose than acetate. Extensive studies on the metabolism of acetate and glucose by the isolated perfused mammary gland ofthe lactating goat have also shown that glucose is oxidized about twice as rapidly as acetate and is essential for milk secretion