Acetate utilization in sheep.

Acetate, an important metabolite in most species, has a unique place in ruminant metabolism. As the major product of ruminal and caecal fermentation, acetate provides much of the energy requirements of ruminant tissues. Little information is available, however, on the overall utilization of acetate in sheep. Intravenous-acetate tolerance tests in sheep (Jarrett, Potter & Filsell, 1952; Pugh & Scarisbrick, 1952; Reid, 1958) have indicated that raised concentrations of circulating acetate fall more slowly than in cats (Smyth, 1947) or dogs (Ciaranfi & Fonnesu, 1954). A serious limitation of tolerance tests is that utilization rates can be examined only at high, unphysiological and continuously varying concentrations. In the present studies an isotope-dilution technique has been used to determine overall utilization rates at normal concentration of circulating acetate. The constantinfusion technique employed was similar to that used by Steele, Wall, de Bodo & Altszuler (1956) for the measurement of glucose-utilization rates in dogs. In this technique the rate of entry of acetate into the circulation is measured, and under steadystate conditions this will equal the rate of acetate leaving the plasma, i.e. the utilization rate. In addition, the utilization of acetate has been studied at normal and enhanced levels of blood acetate, high concentrations of circulating acetate being achieved by infusing unlabelled acetate with the 14C-labelled material. Similarly, by infusing either glucose or insulin with labelled acetate, acetate-utilization rates at high and low levels of blood glucose have been measured. Comparison of the specific activity of expired carbon dioxide and plasma acetate during the infusion of [carboxy-14C]acetate and [methyl-14C]acetate has allowed rough estimates to be made of the contribution of acetate to the oxidative metabolism of sheep. A preliminary account of this work has been given to The Biochemical Society (Annison & Lindsay, 1958).