METABOLIC INTERRELATIONS OF GLUCOSE AND LACTATE IN SHEEP

Abstract

Entry rates of glucose in sheep have been esti- mated by Annison & White (1961, 1962) by using isotope-dilution techniques.The values obtained were surprisingly large in view of the small absorption -of carbohydrate from the alimentary tract of sheep (Annison & Lewis, 1959) and the limited amount of .propionateavailable for conversion into glucose (Annison, Hill & Lewis,, 1957; Annison & Lindsay, 1962).When interpreting these results we have to consider to what extent a turnover rate, as measured by isotope dilution, may be-equated with glucose requirement.Annison & White (1961) drew attention to the fact that an internal cycle such as that described by Cori & Cori (1928) would be included in such an estimate of turnover rate although it would have no significance in relation to carbohydrate require- ments.The existence of the Cori cycle (muscle glycogen -+ lactate -* liver glycogen -+ glucose muscle glycogen) was inferred by Cori from experi- ments in animals under the action of adrenaline.$uch texts as refer to it (Lovatt Evans, 1945) suggest that it is of importance only under the action of adrenaline or exercise.However, Andres, Cader & Zierler (1956), on the basis of arterio- venous difference measurements of blood glucose, lectate, oxygen and C02 in the resting human fore- arm, suggested that a major fraction of glucose utilized by muscle was dissimilated to lactate.Drury & Wick (1956) used L( + )-[14C]lactate to demonstrate that lactate oxidation at resting concentrations of blood lactate could account for up to 25% of the C0.output of rabbits.These results suggest that the Cori cycle could be more important than is commonly supposed, even under resting conditions.There appears to be no pub.lished evidence relating to ruminants.A constant infusion of uniformly labelled