Abstract
Glucagon and L-epinephrine stimulate gluconeogenesis from 20 mM L-lactate, the effect being about 3 times greater in liver cells from fed rats than in those from fasted rats. The rate of pyruvate kinase flux was estimated to be less than 10% of the rate of gluconeogenesis from lactate in hepatocytes from fasted rats, and neither glucagon nor epinephrine lowered the absolute rate significantly. In hepatocytes from fed rats, however, the rate of pyruvate kinase was nearly one-half that of gluconeogenesis. Glucagon caused a marked depression of pyruvate kinase flux, with 1 muM glucagon lowering the rate to nearly the level found in cells from fasted rats Epinephrine at concentrations from 10(-8) to 10(-6) M actually increased pyruvate kinase flux during gluconeogenesis from lactate in cells from fed rats. These results are in accord with the view that the effects of glucagon and epinephrine on gluconeogenesis are not identical.
Highlights
Glucagon and L-epinephrine stimulate gluconeogenesis from 20 mM L-lactate, the effect being about 3 times greater in liver cells from fed rats than in those from fasted rats
In this paper we examine the extent of pyruvate kinase flux
Isolated rat liver cells were incubated with 20 rn~ L-lactate and either L-[U-W]lactate or NaH’TO, as described under “Materials and Methods.”
Summary
Male Wistar rats were either fasted for 18 to 24 h, or fed ad Zibitum a normal chow diet. (acetate form, 100 to 200 mesh), and the columns were washed with water until a 30.ml fraction was collected An aliquot of this was counted to give the isotopic yield in glucose. In liver cells from fed rats, considerable glucose is formed from glycogen breakdown, and analytical glucose formation cannot be considered a measure of gluconeogenesis In this case, gluconeogenesis is estimated by the isotopic yield in glucose from L-[U-L4Cllactate, correcting for dilution by dividing by the factor 0.66. No attempts were made in this paper to correct the lactate yields for the amount of [14C]pyruvate, formed from 14C-labeled P-enolpyruvate, that is reutilized in the cell instead of being trapped in the large extracellular lactate pool [5] This may cause the pyruvate kinase fluxes to be underestimated by about 25%
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