Ca2+ dependence of gluconeogenesis stimulation by glucagon at different cytosolic NAD (+) -NADH redox potentials
Marques-da-Silva, A.C.; D'Avila, R.B.; Ferrari, A.G.; Kelmer-Bracht, A.M.; Constantin, J.; Yamamoto, N.S.; Bracht, A.
Brazilian Journal of Medical and Biological Research 30(7): 827-836
1997
ISSN/ISBN: 0100-879X PMID: 9361705 Document Number: 477772
The influence of Ca-2+ on hepatic gluconeogenesis was measured in the isolated perfused rat liver at different cytosolic NAD+-NADH potentials. Lactate and pyruvate were the gluconeogenic substrates and the cytosolic NAD+-NADH potentials were changed by varying the lactate to pyruvate ratios from 0.01 to 100. The following results were obtained: a) gluconeogenesis from lactate plus pyruvate was not affected by Ca-2+-free perfusion (no Ca-2+ in the perfusion fluid combined with previous depletion of the intracellular pools); gluconeogenesis was also poorly dependent on the lactate to pyruvate ratios in the range of 0.1 to 100; only for a ratio equal to 0.01 was a significantly smaller gluconeogenic activity observed in comparison to the other ratios. b) In the presence of Ca-2+, the increase in oxygen uptake caused by the infusion of lactate plus pyruvate at a ratio equal to 10 was the most pronounced one; in Ca-2+-free perfusion the increase in oxygen uptake caused by lactate plus pyruvate infusion tended to be higher for all lactate to pyruvate ratios; the most pronounced difference was observed for a lactate/pyruvate ratio equal to 1. c) In the presence of Ca-2+ the effects of glucagon on gluconeogenesis showed a positive correlation with the lactate to pyruvate ratios; for a ratio equal to 0.01 no stimulation occurred, but in the 0.1 to 100 range stimulation increased progressively, producing a clear parabolic dependence between the effects of glucagon and the lactate to pyruvate ratio. d) In the absence of Ca-2+ the relationship between the changes caused by glucagon in gluconeogenesis and the lactate to pyruvate ratio was substantially changed; the dependence curve was no longer parabolic but sigmoidal in shape with a plateau beginning at a lactate/pyruvate ratio equal to 1; there was inhibition at the lactate to pyruvate ratios of 0.01 and 0.1 and a constant stimulation starting with a ratio equal to 1; for the lactate to pyruvate ratios of 10 and 100, stimulation caused by glucagon was much smaller than that found when Ca-2+ was present. e) The effects of glucagon on oxygen uptake in the presence of Ca-2+ showed a parabolic relationship with the lactate to pyruvate ratios which was closely similar to that found in the case of gluconeogenesis; the only difference was that inhibition rather than stimulation of oxygen uptake was observed for a lactate to pyruvate ratio equal to 0.01; prgressive stimulation was observed in the 0.1 to 100 range. f) In the absence of Ca-2+ the effects of glucagon on oxygen uptake were different; the dependence curve was digmoidal at the onset, with a well-defined maximum at a lactate to pyruvate ratio equal to 1; this maximum was followed by a steady decline at higher ratios; at the ratios of 0.01 and 0.1 inhibition took place; oxygen uptake stimulation caused by glucagon was generally lower in the absence of Ca-2+ except when the lactate to pyruvate ratio was equal to 1. The results of the present study demonstrate that stimulation of gluconeogenesis by glucagon depends on Ca-2+. However, Ca-2+ is only effective in helping gluconeogeneisis stimulation by glucagon at highly negative redox potentials of the cystolic NAD+-NADH system. The triple interdependence of glucagon-Ca-2+-NAD+NADH redox potential reveals highly complex interrelations that can only be partially understood at the present stage of knowledge.