Availability and Metabolism of Various Substrates in Ruminants. II. Rate of Acetate Oxidation as Affected by Availability of Substrate

Abstract

The Na and K salts of acetic acid were infused into the external jugular vein of a 455 kg Holstein steer at a rate to supply 20.27, 45.00, and 65.00 g of free acetic acid/hr. for a period of 7 hr. Acetate-1-C14 of high specific activity and of the same salt form was mixed thoroughly with the carrier (unlabeled) acetate before being infused. From the specific activity of the expired CO2 and the total CO2 output, the amount of infused acetate oxidized as well as the rate of oxidation was determined. Of the 142, 315, and 455 g of acetate administered during the low, medium, and high rates of infusion, 103, 188, and 333 g were oxidized to CO2 and expired respectively. The mean rates of acetate oxidation were calculated to be 15.46, 28.12, and 51.38 g/hr- for the 3 rates of infusion respectively. At the highest rate of oxidation (51.38 g/hr.), acetate accounted for 34.2% of the total C in the expired CO2. The contribution of the oxidized acetate toward the maintenance energy requirement of the animal was estimated to be 37.4%. The technique used for the collection, measurement, and sampling of the expired gases is applicable to the study of factors affecting the oxidation of various metabolites to CO2. The simplicity of operation of the mechanical equipment and the normalcy of the animal under the experimental conditions makes the technique extremely advantageous for studies of this type. The large size of the expired gas aliquot and the high counting efficiency of the C14O2 permits the use of low levels of radioactivity thereby reducing hazard from radiation exposure.