Effect of the Exogenous Amino Acid Concentration on the Rate of Gluconeogenesis in Liver Slices 1

Abstract

Summary

The steady state rate of gluconeogenesis in rat liver slices incubated in U-¹⁴C-alanine was linearly related to the extracellular alanine concentration over a range of 1-9 mg of alanine/100 ml. The rate of oxidation of alanine to CO₂ was also linearly related to the extracellular alanine concentration and the ratio of alanine oxidized to CO₂ to the alanine converted to glucose was constant with changes in the alanine concentration.

Appendix: CO₂ trapping efficiency correction. Since the quantity of labeled carbon dioxide found in the centerwell of closed incubation flasks following a test injection (step forcing) of Na₂ ¹⁴CO₃ increased exponentially with time, the trapping mechanism was a first order process (15). During incubation the liver slice produced carbon dioxide continuously, therefore, under experimental conditions the first order carbon dioxide trapping process was subjected to a ramp, rather than a step forcing. The equation, predicting the fraction of released ¹⁴CO₂ that would be trapped in the centerwell, is that of a ramp response of a first order process:

F = fraction of ¹⁴CO₂ recovered = 1/t[t-r(l-e^-t/r)], where: r is the time constant of 79 min determined from the test experiment; t is the incubation period of 60 min; e is the base of natural logarithms. The equation predicts that at the end of a 60-min incubation period only 30% of the total carbon dioxide released during that time must have been trapped in the centerwell:

F = 1/60 [60-79 (1-e ^-60/79)] =0.30. Therefore, the ¹⁴CO₂ data obtained must be corrected by multiplying by 3.33.