Threonine metabolism in sheep. I. Threonine catabolism and gluconeogenesis in mature Blackface wethers given poor quality hill herbage

Abstract

In three experiments, mature Blackface wethers were given freeze-stored Agrostis festuca herbage by continuous feeder. In Expt 1, on separate occasions [U-14C]threonine, [U-14C]glucose and NaH14CO3 were infused over 12 h periods to obtain estimates of irreversible loss rate (ILR) of threonine, glucose and carbon dioxide in the plasma and of the exchange of C between these metabolites. In Expts 2 and 3, during periods when glucose and threonine metabolism were examined, glucose loss across the kidneys (23-29 g/d) was induced by infusion of phloridzin. Results from the four sheep used in Expts 1 and 3 are presented as three-pool models. They indicate that threonine ILR (7.8 g/d; 3.1 g C/d) was approximately three times the estimated rate of absorption of exogenous threonine (1 g C/d). Glucose ILR was approximately 76 g/d (mean +/- SE; 30.3 +/- 0.57 g C/d). Only 0.3% of the glucose-C (0.09 g/d) was derived directly from threonine-C (i.e. 3% of the threonine-C ILR). Bicarbonate ILR was 170 +/- 7.3 g C/d, and glucose contributed 11.1 +/- 3.52 g C/d to this, accounting for 51 +/- 4.4% of glucose-C ILR. Threonine contributed 0.20 +/- 0.026 g C/d to the bicarbonate-C ILR, accounting for only 6.4 +/- 0.87% of the threonine-C ILR. When, in Expts 2 and 3, phloridzin was infused, glucose ILR was increased by 28 +/- 1.5% and bicarbonate ILR was increased by 13 +/- 2.4%. Threonine ILR (3.1 g C/d) was not increased, but the metabolic distribution of threonine-C was altered. The transfer of threonine-C into glucose and CO2 was increased by 39 and 69% respectively to 0.125 and 0.45 g C/d, accounting for 4 and 13% of the threonine ILR respectively. Both technical and metabolic considerations which affect interpretation of these results in terms of rates of catabolism of threonine and of quantitative estimates of gluconeogenesis from threonine are discussed.