The effect of hormones on proton compartmentation in hepatocytes

Abstract

Liver mitochondria isolated from rats treated acutely with glucagon exhibit higher respiration-dependent H+ ion gradients across the mitochondrial inner membrane than mitochondria from control rats. It has been suggested that similar increases in mitochondrial delta pH in situ could stimulate gluconeogenesis, chiefly because the transport of pyruvate into mitochondria would increase in response to the increase in mitochondrial matrix pH. In order to determine whether the increased delta pH observed in vitro in isolated mitochondria also occurs in situ, the effect of glucagon on the pH in the cytosol and mitochondria matrix spaces of isolated hepatocytes was determined. For qualitative results, the spectral responses of intracellularly trapped 6-carboxyfluorescein was used to monitor cytosol pH, while fluorescein-loaded hepatocytes were used to monitor the mitochondrial pH. Hepatocytes were incubated with the diacetate ester derivatives of these dyes. The esters are permeable to the cell membranes, but are rapidly hydrolyzed in the cells. The free unesterified dyes are relatively impermeable to the cell membranes. After being trapped in the cell, 6-carboxyfluorescein remains localized in the cell cytosol, whereas fluorescein is taken up by the mitochondria as a function of the mitochondrial delta pH. In order to quantitate the actual pH in these compartments, the spectral responses (490-465 nm) of 6-carboxyfluorescein-loaded hepatocytes were used to determine the cytosolic pH. Calibration of these responses was obtained within the cell by determination of the dye's differential absorption coefficient (epsilon 490-465 nm) in various high K+ buffers after equilibration of the internal and external pH with valinomycin and the uncoupler 1799. All absorbance values were corrected for dye leakage. Equal hematocrits of unloaded cells were used to correct for absorbance contributions from cellular constituents. The mitochondrial pH was determined by a combination of the indicator dye and [14C]5,5'-demethyloxazolidine-2, 4-dione (DMO) distribution ratio methods. The weak acid DMO freely distributes across the plasma membrane and mitochondrial membrane in whole cells according to the pH gradient across each membrane. Knowledge of the cytoplasmic pH from the 6-carboxyfluorescein data allows the expected distribution of DMO across the plasma membrane to be calculated. The excess accumulation of DMO in intact hepatocytes over that predicted from the plasma membrane pH gradient alone was then used to calculate the pH gradient across the mitochondrial inner membrane. The effects of valinomycin, uncouplers, and hormones on the pH in cytosolic and mitochondrial compartm