Studies on the carbohydrate metabolism of sheep. XIII. The interpretation of changes in the levels of metabolic intermediates after intravenous injection of propionate, succinate and malate

Abstract

Blood levels of pyruvic, citric, and ß-oxoglutaric acids in sheep are similar to those in man. Citric acid declines markedly on fasting; levels in ewes with pregnancy toxaemia are not significantly different from those in fasted, pregnant ewes. The presence of low citric acid levels in ewes with pregnancy toxaemia in which blood glucose levels are normal or above may further support the suggestion that there is an interference with glucose utilization in pregnancy toxaemia. Intravenous injection of propionate, succinate, and malate into fasted, pregnant ewes always lowered blood ketones, but it is not known to what extent this response is merely a consequence of increased blood glucose following injection. Injected propionate disappeared more rapidly than injected acetate. Propionate injection into fed, non-pregnant ewes is followed by an immediate increase in blood glucose to high levels; the disappearance of excess glucose appears to follow the pattern of a normal glucose tolerance curve. Blood pyruvic acid increases markedly with blood glucose. Succinate and malate injections are followed by small increases in blood glucose and pyruvic acid. The difficulty of interpreting changes in blood glucose in inadequately trained experimental animals is emphasized. Consistent, significant increases in blood oxalaoetic acid did not occur in response to either propionate, succinate, or malate injection. Blood citric and a-oxoglutaric acids increased markedly after succinate and malate, but not after propionate injection. The data are discussed in relation to metabolic pathways in sheep tissues; it is concluded that this type of in vivo experiment is of limited value.