Fate of intraluminal glutamine in the perfused renal tubule

Extracellular glutamate flux regulates intracellular glutaminase activity in LLC-PK1-F+ cells

The role of extracellular glutamate flux in regulating intracellular glutaminase activity was assessed in confluent monolayers of proximal tubule-like LLC-PK1-F+ cells grown on porous supports. Glutamate is a well-known inhibitor of phosphate-dependent glutaminase (PDG). We hypothesized that, by restricting the flux of glutamate from the extracellular media, cellular level would fall, effecting deinhibition of the cellular glutaminase activity. To test this, cellular glutamate uptake and extracellular production were inhibited for 18 h by the addition of D-aspartate (10 mM) or acivicin (0.7 mM) to both apical and basal media. Inhibiting glutamate flux depressed cellular glutamate content 43 and 41%, respectively. Intracellular relative glutaminase activity, monitored as the breakdown of 14C-radiolabeled glutamine to glutamate, measured over 60 s in the presence of D-aspartate or acivicin showed a 2- to 2.5-fold increase with the fall in cellular glutamate. Interestingly, enhanced glutamine uptake after PDG deinhibition was predominantly expressed on the basal surface. Indeed, measuring glutamine utilization after gamma-glutamyltranspeptidase inhibition over the entire 18-h time course revealed inhibition at the apical surface but relative enhancement of uptake at the basal surface. The increased intracellular glutaminase pathway was also reflected in increased alanine production measured over the 18-h time course, despite the reduction in overall glutamine utilization. These results point to a major role for extracellular glutamate fluxes in regulating cellular glutamine metabolism and suggest that the intracellular pathway may be suppressed under these conditions.

Gamma glutamyltransferase contribution to renal ammoniagenesis in vivo

The role of gamma-glutamyltransferase (gamma-GT) in renal ammoniagenesis and glutamine utilization was evaluated in the intact functioning rat kidney. Total NH4+ released, as the sum of renal venous and urinary NH4+, was measured under conditions of chronic metabolic acidosis and paraminohippurate infusion. Ammonia derived from extracellular gamma-GT hydrolysis of glutamine was differentiated from that produced by intracellular phosphate dependent glutaminase (PDG) by employing acivicin, a gamma-GT inhibitor. In non-acidotic animals acivicin administration inhibited gamma-GT 95% and renal venous NH4+ release 48%; NH4+ release into the urine was not inhibited. Chronic metabolic acidosis elevated total NH4+ release 2.5fold, associated with adaptive increase in both gamma-GT and PDG; acivicin reduced total NH4+ released 36% with both renal venous and urinary release effected. The contribution of gamma-GT to total NH4+ production doubles in metabolic acidosis in agreement with the adaptive rise in the in vitro assayed gamma-GT activity. Luminal ammoniagenesis increases in chronic acidosis associated with a fall in urinary glutamine concentration and a rise in the blood to urine glutamine concentration gradient; gamma-GT inhibition eliminates this gradient suggesting luminal ammoniagenesis is largely dependent upon the paracellular glutamine flux. In support of this, paraminohippurate (PAH) infusion increased total renal NH4+ release due entirely to enhanced NH4+ excretion. PAH stimulated luminal ammoniagenesis was associated with an acceleration of renal glutamine extraction and a steeper blood to urine glutamine diffusion gradient; acivicin blocked this response consistent with PAH secretion coupled to activation of intraluminal gamma-GT and glutamine hydrolysis.

Formation and excretion of NH3----NH4+. New aspects of an old problem

The proximal tubule cell is the major site of renal ammoniagenesis. Glutamine is the major substrate. Deamidation by mitochondrial glutaminase yields glutamate- and NH4+ (not NH3, as traditionally taught). A second NH4+ ion is obtained by deamination of glutamate- to 2-oxo-glutarate2-. NH4+ preferentially enters the tubule lumen primarily, but probably not exclusively, by non-ionic diffusion of NH3. For each NH3 formed in the cell one H+ ion is left behind. H+ and NH3 are secreted on separate routes, but recombine in the lumen to NH4+ and reach the final urine in this form. This process per se does not net-remove H+ from the organism. For this purpose, the anionic products of ammoniagenesis (2-oxo-glutarate2- and others) have to be converted into neutral compounds (CO2, glucose). This metabolism again takes place usually in the tubule cell. For each negative charge one HCO3- is formed which enters the peritubular blood. Luminal gamma-glutamyl transferase-mediated ammoniagenesis contributes to NH4+ accumulation in the proximal tubule to a small extent. The endproximal NH4+ delivery exceeds the filtered load by a factor of 9. Only 1/3 of it reaches the distal convoluted tubule mainly because NH+4 as such is reabsorbed from the thick ascending limb of Henle's loop by secondary active transport or electrodiffusion. Both processes are energized by the active Na+ transport in this segment. Thereby NH3----NH4+ is accumulated in the medullary interstitium, which establishes the chemical gradient for non-ionic diffusion of NH3 into the lumen of the collecting ducts. This is favoured by the acidic disequilibrium pH in the lumen of this segment.(ABSTRACT TRUNCATED AT 250 WORDS)