Metabolic Interrelations of Glucose and Lactate in Sheep

Abstract

Entry rates of glucose in sheep have been estimated 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 .propionate available 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 requirements. The existence of the Cori cycle (muscle glycogen-+ lactate-* liver glycogen-+ glucose muscle glycogen) was inferred by Cori from experiments 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 arteriovenous difference measurements of blood glucose, lectate, oxygen and C02 in the resting human forearm, 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

Effect of fasting on the rates of lactate turnover and oxidation in rats

“…They found that lactate accumulates more and glucose decreases less in fasted than in fed rats. They conclude that the flux of lactate from peripheral tissues to the liver is doubled after 24 h of starvation but such a result is not in agreement with the decrease of lactate turnover observed by others [6] and by the present work. These conflicting results are more likely due to the very different experimental conditions and to a deep hormonal stimulation following an important surgical procedure as used by Blackshear et al in their animals [8].…”

“…This observation can be explained by the reduction of glycolysis occurring during fasting due to the well known decrease of glycogen stores and of hepatic glucose production [2,11 ,121 This is in agreement with data reported in sheep [6] but the reduction after a 24 h fasting-period is more pronounced in rats (-38%) than in sheep (-29%). This fact is probably due to the relatively more severe fasting period for rats than for sheep and possibly to species differences.…”

“…It was of interest to know if the same phenomenon occurred for the rates of lactate turnover and oxidation and the extent of such a decrease so that the contribution of lactate to gluconeogenesis in vivo could be estimated in these conditions. It has been reported that lactate turnover as measured with the [ 14C] lactate perfusion technique decreases during fasting in sheep [6] but does not vary in obese human patients [7]. Recently, the study of the effects of functional hepatectomy and of insulin on blood lactate concentration led to the conclusion that the flux of lactate from peripheral tissues to the liver is doubled after a 24 h fasting-period in rats [8] .…”

The Uptake of L-Lactate and D-Glucose by the Mammary Gland of the Cow

“…mgjl00 ml, which is similar to that observed in the unanaesthetized cow (Shaw, Boyd, and Petersen 1938) and sheep (Annison, Lindsay, and White 1963), but about twice that observed in the anaesthetized cow (Powell and Shaw 1942;Shaw 1946). …”

“…It has been estimated that, in sheep, at least 40% of the lactate pool is derived from glucose and a minimum of 15% of the glucose pool is derived from lactate (Annison, Lindsay, and White 1963). If a similar interconversion occurs in the cow, the net uptake ofL-lactate should be considered in assessing the net uptake ofD-glucose.…”