Ca2+ dependence of gluconeogenesis stimulation by glucagon at different cytosolic NAD+-NADH redox potentials

The influence of Ca2+ 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 rations from 0.01 to 100. The following results were obtained: a) gluconeogenesis from lactate plus pyruvate was not affected by Ca2+-free perfusion (no Ca2+ in the perfusion fluid combined with previous depletion of the intracellular pools); gluconeogenesis was also poorly dependent on the lactate to pyruvate rations 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 rations. b) In the presence of Ca2+, 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 Ca2+-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 Ca2+ the effects of glucagon on gluconeogenesis showed a positive correlation with the lactate to pyruvate rations; for a ratio equal to 0.01 no stimulation ocurred, 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 Ca2+ 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 Ca2+ was present. e) The effects of glucagon on oxygen uptake in the presence of Ca2+ showed a parabolic relationship with the lactate to pyruvate ratios which was closely similar to that found in the case of gluconeogenesis.

Sensitivity of ketogenesis and citric acid cycle to stevioside inhibition of palmitate transport across the cell membrane

The effect of stevioside, an inhibitor of long-chain fatty acid transport, on ketogenesis and on [14C]CO2 production from [1-14C] palmitate (100-300µM) was investigated in the isolated and hemoglobin-free perfused rat liver. Stevioside (2.5mM), a sweet glycoside found in Stevia rebaudiana leaves, inhibited both parameters, but had a lower effection on [14C]CO2 production. At 300µM palmitate and 150 µM albumin, for example, ketogenesis was inhibited by 66.3%, whereas no significant inhibition of [14C]CO2 was demonstrable. These results were interpreted to reflect 1) different degress of saturation of the citric acid cycle and the ketogenic patheay and 2) changes in the redox state of the mitochondrial NAD+-NADH couple which may also occur upon stevioside infusion

Saturated fatty acids with different chain length as ketogenic substrates in the rat liver

Production of ketone bodies, activation of oxygen uptake and production of [14C] carbon dioxide due to fatty acids of various chain lengths were measured in the isolated perfused rat liver. As ketogenic substrates, the fatty acids investigated can be ordered in the following sequence: strate < palmitate < laureate < decanoate < miristate < octanoate. For activation of oxygen consumptor, the same sequence is observed. A good linear correlation between the increase in oxygen uptake (deltaO2) and production of ketone bodies was found, with a mean deltaO2 of 0.66 mol per mol ketone bodies. This is a relatively low value when confronted with the known stoichiometry of ß-oxidation, and may be an indication of complexities in the metabolism of exogenously added fatty acids in the perfused liver

The influence of albumin on palmitate transport in the isolated perfused rat liver

Unidirectional fluxes (influx and efflux) and the net flux of palmitate across the hepatocyte membrane were measured in the intact rat liver employing the multiple indicator dilution technique. At albumin concentrations in the range between 0.1 and 0.5 mM the influx rate was 2.3 times greater than that of the net flux. The rate of efflux was somewhat higher than the net flux, indicating that palmitate undergoes an exchange process across the liver cell membranes with efflux to the extracellular albumin site being significant. At. lower albumin concentrations, hower, the influx/net flux approached unit, implying that transport becomes a rate-limiting factor for metabolism

Octanoate transport in the isolated, perfused rat liver

The multiple indicator dilution technique was used to investigate octanoate transport in the isolated, perfused rat liver. The form of the octanoate outflow profiles depends on albumin concentration. The steady-state intracellular octanoate concentration and the rate of uptake increase when the albumin concentration is decreased. At physiological albumin concentrations, the intracellular concentration of octanoate is only 7% of the extracellular concentration. Exchange between the intra-and extracellular space, however, is very rapid, irrespective of the albumin concentration and consequently exchange is not rate limiting for metabolism