Axial heterogeneity and filtered-load dependence of proximal bicarbonate reabsorption

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

Immunoelectron microscopic localization of the electrogenic Na/HCO(3) cotransporter in rat and ambystoma kidney

Immunofluorescence analysis has revealed that electrogenic Na(+)/HCO(3)(-) (NBC1) is expressed in the proximal tubule of rat kidney and in the proximal and distal tubules of the salamander AMBYSTOMA: tigrinum kidney. The present study was undertaken to define the detailed subcellular localization of the NBC1 in rat and AMBYSTOMA: kidney using high-resolution immunoelectron microscopy. For this purpose, two rabbit polyclonal antibodies raised against amino acids 928 to 1035 and amino acids 1021 to 1035 of the C-terminus of rat kidney (rkNBC1) were developed. The affinity-purified antibodies revealed a strong band of approximately 140 kD in immunoblots of membranes from rat kidney cortex but no signal in membranes isolated from outer and inner medulla. Deglycosylation reduced the apparent molecular weight to approximately 120 kD, corresponding to the predicted molecular weight. A similar but weaker band was also present in membranes isolated from the lateral part of Ambystoma: kidney. In rat kidney, immunohistochemistry confirmed the presence of rkNBC1 in convoluted segments of the proximal tubules. In ultrathin cryosections or Lowicryl HM20 sections from rat kidney cortex, distinct immunogold labeling was associated with the basolateral plasma membrane of segments S1 and S2 of proximal tubules, whereas in S3 no labeling was observed. The labeling density was similar at the basal and lateral plasma membrane and was specifically associated with the inner surface of the membrane consistent with the internal position of the C-terminus of the transporter. In contrast, rkNBC1 was absent from the apical plasma membrane and not observed in intracellular vesicles, including those closely associated with basolateral plasma membrane. In Ambystoma: kidney, a weak labeling was present in the basolateral membrane of the proximal tubule and stronger labeling was observed in the late distal segment. The results demonstrate that rkNBC1 is expressed only in segment S1 and segment S2 of rat proximal tubule as well as Ambystoma: proximal and late distal tubule and that rkNBC1 is present in both basal and lateral plasma membranes and absent in intracellular vesicles of the apical plasma membrane.

A kinetic model of rat proximal tubule transport--load-dependent bicarbonate reabsorption along the tubule

A model is presented of solute and water reabsorption along the proximal tubule of the rat kidney based on kinetic descriptions of the main membrane transport systems, in order to assess the extent to which these kinetics suffice to explain certain aspects of the global transport behaviour in this segment, especially with respect to bicarbonate reabsorption. The model includes in the apical membrane, an active proton pump, Na+/H+ antiport, Na-coupled transport of organic solutes, Cl-/formate exchange with formic acid recycling, and membrane conductances to protons and K+. In the baso-lateral membrane, besides the Na+/K+ pump, the model includes Na(+)-3HCO3- and electroneutral K(+)-Cl- cotransporters, and membrane conductances for K+, H+, and, optionally, for Cl-. Appropriate passive diffusional pathways were included in both cell membranes and in the paracellular pathway. Using mass balance and electroneutrality constraints, these transport systems were built into an epithelial model which was then integrated (by finite difference approximation) into a model of a longitudinal tubule. Simulated cellular solute concentrations and luminal concentration profiles were in good agreement with reported experimental observations. We show that, given the reported transport kinetics for the Na+/H+ antiporter, a hitherto unexplained observation concerning load-dependent bicarbonate reabsorption can be shown mainly to result from the nonlinear longitudinal concentration profile for bicarbonate and pH. We also discuss problems of transcellular Cl- transport in the light of recent reports of basolateral Cl- conductance and observations relevant to apical Cl-/formate (or other base) exchange.