Arteriovenous 14CO2 differences and the pentose cycle in the cow's udder

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Abstract
The metabolism of the lactating cows udder was investigated by determinations of arteriovenous differences in the isotope concentrations of C14O2 or Cl3o2 of the udder during a continuous injection of [1-Cl4]acetate or [1-C14] glucose and [1-Cl3]acetate. The isotope concentrations of the blood glucose and acetate were also determined in the venous blood of the udder. The relative rates of oxidation of the C-l of glucose and of the C-l of acetate to CO2 were compared on the basis of the ratios of the isotope concentrations of the substrates and of the CO2- By this method C-1 of acetate was estimated to be oxidized to CO2 in the udder at about the same rate as is C-1 of glucose. This estimate may be subject to considerable error because of assumptions made in the calculation. [1-C14]- and [6-Cl4]-Glucose were alternately injected into the pudic artery of a cow and the specific activities of the CO2 of the venous blood of the udder were determined. After injection of [1-Cl4]giucose the specific activity of the CO2 was much greater than it was after [6-C14]glucose. These results indicate that the pentose cycle occurs in the udder of the lactating cow. The equilibration between the glucose acetate and bicarbonate of the blood and that of the pools of the udder was investigated. This equilibration alters the specific activity of the compounds and makes it difficult to apply tracer-dilution methods in the intact animals.