Plasticity of functional epithelial polarity

Inhibition of amiloride-sensitive apical Na+ conductance by acetylcholine in rabbit cortical collecting duct perfused in vitro

We examined effects of acetylcholine (ACh) on the electrical parameters and intracellular Ca2+ concentration ([Ca2+]i) in the isolated rabbit cortical collecting duct (CCD) perfused in vitro using the conventional microelectrode technique and microscopic fluorescence spectrophotometry. ACh (10(-8) to 10(-5) M) in the bath caused a positive deflection of the transepithelial voltage (VT) and an increase in [Ca2+]i. Carbachol also showed similar but smaller effects. The effects of ACh were antagonized by muscarinic receptor antagonists. ACh at 10(-6) M hyperpolarized the apical membrane voltage and increased the fractional resistance of the apical membrane of the collecting duct cells accompanied by a positive deflection of VT and an increase in transepithelial resistance, whereas it did not affect these parameters in the beta-intercalated cells. In the presence of 10(-5) M amiloride in the lumen, the effects of ACh were almost completely abolished. The ACh-induced increase in [Ca2+]i is accounted for by the release of Ca2+ from intracellular store and Ca2+ entry from the bath. In the absence of Ca2+ in the bath, the ACh-induced changes in electrophysiological parameters were significantly smaller than those observed in the presence of Ca2+. Both phorbol-12-myristate-13-acetate (PMA) and phorbol-12,13-dibutylate (PDBu), activators of protein kinase C (PKC), also inhibited the apical Na+ conductance. In the presence of PMA or PDBu in the bath, ACh did not show further inhibitory effect. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine, an inhibitor of PKC, partially attenuated the effect of ACh. These observations indicate that ACh inhibits the apical Na+ conductance partly by both increasing [Ca2+]i and activating PKC. Such an action of ACh may partially explain its natriuretic effect.

A 28 kDa sarcolemmal antigen in kidney principal cell basolateral membranes: relationship to orthogonal arrays and MIP26

Two recently cloned water channels, CHIP28 and WCH-CD, are homologous to MIP26, an integral membrane channel-forming protein found in lens fiber plasma membranes. CHIP28 is found in basolateral and apical plasma membranes of kidney proximal tubules and thin descending limbs of Henle, whereas WCH-CD is apically located in collecting duct principal cells. So far, the putative water channel that may be responsible for the high constitutive permeability of principal cell basolateral membranes has not been identified. Interestingly, freeze-fracture electron microscopy has shown that characteristic orthogonal arrays of intramembrane particles (OAPs) are found on the basolateral plasma membranes of collecting duct principal cells, and that morphologically identical OAPs present in lens fiber cell plasma membranes contain the protein MIP26. Similar OAPs have also been detected on plasma membranes of other cell types including gastric parietal cells, astroglial cells and skeletal muscle fibers. By indirect immunofluorescence, western blotting and northern blotting, MIP26 was found only in lens fibers. In addition, functional studies on reconstituted and oocyte-expressed MIP26 excluded the possibility that MIP26 might be a basolateral water channel in the kidney. However, a polyclonal antibody raised against skeletal muscle sarcolemmal vesicles, which are enriched in OAPs, produced an intense staining of principal cell basolateral plasma membranes in kidney collecting duct and immunoprecipitated a 28 kDa protein from kidney papilla. The immunoprecipitated protein from papilla was not recognized by anti-CHIP28 or anti-MIP26 antibodies, indicating that principal cell basolateral membranes contain a novel member of the CHIP/MIP family. Because this antibody also stained brain astrocyte end feet, which are enriched in OAPs, it is possible that the 28 kDa protein is related to these structures. We conclude that OAPs probably contain related but distinct proteins that may have different membrane channel functions in different cell types.

An induced extracellular matrix protein reverses the polarity of band 3 in intercalated epithelial cells

The intercalated epithelial cell exists in two interconvertible forms in vivo, one where band 3 protein is apical and the other where it is basolateral. We seeded an immortalized clone of these cells at low density and found that band 3 was apical at confluence. There was little or no apical endocytosis. But when the cells were plated at high density, band 3 was basolateral, and there was vigorous apical endocytosis. Extracellular matrix produced by high density cells was able to retarget band 3 in low density cells and to induce apical endocytosis, as did a 230 kd protein partially purified from this matrix. Therefore, polarized targeting of some proteins is determined by external cues that might determine their polarity by reorganizing the cytoplasm.

Morphological responses of the rainbow trout (Oncorhynchus mykiss) gill to hyperoxia, base (NaHCO3) and acid (HCl) infusions

Marked morphological responses occur in the gills of freshwater rainbow trout in response to experimental acid-base disturbance and these responses play an important role in acid-base correction. Compensated respiratory acidosis induced by 70h exposure to environmental hyperoxia (elevated water PO2) caused a 33% decrease in branchial chloride cell fractional surface area (CCFA). Metabolic alkalosis induced by normoxic recovery (6h) from hyperoxia (72h) caused a 50% increase in CCFA, whereas metabolic alkalosis induced by infusion (19h) of NaHCO3 caused a 70% rise. However, the largest increase (135%) in CCFA was seen in response to infusion (19h) of HCl. NaCl infusion had no effect. A particular goal was to assess the relative importance of changes in CCFA vs. changes in internal substrate (HCO3 (-)) availability in regulating the activity of the branchial Cl(-)/HCO3 (-) exchange system. For each of the experimental treatments, the accompanying blood acid-base status and branchial transport kinetics (Km, Jmax) for Cl(-) uptake had been determined in earlier studies. In the present study, a positive linear relationship was established between CCFA and J(Cl-) max in individual control fish in the absence of an acid-base disturbance. By reference to this relationship, observed changes in J(Cl-) max during metabolic acid-base disturbances were clearly due to changes in both CCFA and internal substrate levels (plasma [HCO3 (-)]) with the two factors having approximately equal influence.

Functional characterization of three intercalated cell subtypes in the rabbit outer cortical collecting duct

The distribution of Na(+)-independent Cl(-)-HCO3- exchange was studied in individual intercalated cells from in vitro perfused rabbit outer CCDs using dual excitation laser scanning confocal microscopy by measuring the pHi response to sequential removal of Cl- from both sides of the tubule. Three patterns of intracellular pH (pHi) response were observed. 39% of intercalated cells had only apical Cl(-)-HCO3- exchange (beta cell), 4% had only basolateral Cl(-)-HCO3- exchange (alpha cell), and 57% had both apical and basolateral Cl(-)-HCO3- exchange (gamma cell). Valinomycin-high K+ voltage clamping had no effect on the pHi response of intercalated cells with bilateral Cl(-)-HCO3- exchange. Although the mean rates of dpHi/dt following apical Cl- removal were similar in beta cells compared to gamma cells, a wide range of apical rates was seen among individual beta and gamma intercalated cells. Neither the apical nor the basolateral Cl(-)-HCO3- exchanger in gamma cells was inhibited by 0.5 mM H2DIDS. Binding of apical peanut lectin was seen both in beta cells and in gamma cells. In 41% of CCDs with four to seven intercalated cells studied, all intercalated cells were of the same subtype. We conclude that the majority of intercalated cells from the rabbit outer CCD have both apical and basolateral Na(+)-independent Cl(-)-HCO3- exchangers (gamma cells), which are stilbene-insensitive. Intercalated cells with only basolateral Cl(-)-HCO3- exchange are very uncommon in the rabbit outer CCD. There is a tendency for all intercalated cells in a given rabbit outer CCD to be of the same subtype (either all beta cells or all gamma cells), suggesting the presence of CCD intertubule heterogeneity at the same cortical level. This finding may account for intertubule differences in transepithelial H(+)-base transport.

Differentiation of intercalated cells in culture

The renal collecting duct is a heterogenous epithelium consisting of intercalated cells (ICCs) and principal cells (PCs). To test the hypothesis that the two cell types might originate from one another and to determine which one of the two is a stem cell, beta-ICCs and PCs were isolated by fluorescence-activated cell sorting and grown on permeable supports. Cultures of sorted PCs maintained their PC phenotype [electrogenic sodium (Na+) reabsorption and potassium (K+) secretion and expression of PC-specific antigens]. In contrast, cultures of sorted beta-ICCs acquired alpha-ICC-specific functions (e.g. proton secretion) and gradually expressed functions specific for PCs (amiloride-blockable Na+ current and K+ secretion). Most cells in cultures of sorted beta-ICCs also acquired a central cilium, a characteristic feature of PCs. Dual-staining of beta-ICC cultures with cell-specific antibodies against surface antigens revealed that approximately 45% of the cells expressed only ICC-specific antigens and approximately 20% expressed only PC antigens. The remainder of the cells were ICC/PC "hybrids" and stained for both markers. Such hybrid cells were also observed in situ, albeit with a lower frequency, on kidney sections dual-stained with cell type-specific markers. The proliferation rate of the two cell types, assessed by pulse labeling cells in S-phase with bromodeoxyuridine or staining with an antibody against a proliferation antigen (KI-67), revealed a significantly higher proliferation rate among beta-ICCs than PCs. In aggregate, these data suggest that beta-ICCs in culture are capable of differentiating into alpha-ICCs and PCs and raise the possibility that beta-ICC is the stem cell of the collecting duct.

Histochemical markers reveal an unexpected heterogeneous composition of the renal embryonic collecting duct epithelium

The ampullary collecting duct epithelium acts as an inductor in the embryonic and neonatal kidney. It induces the formation of all nephron generations and thus determines the whole architecture of the kidney. As the organ matures, the collecting duct epithelium itself transdifferentiates. The ampullary inductor epithelium, which appears homogeneous as revealed by light microscopy, develops into the well-known heterogeneous epithelium of the mature collecting duct consisting of light principal and dark intercalated cells. Up to now the mechanisms initiating and regulating this transdifferentiation step are unknown. Only very few data are available concerning functional characteristics of the ampullary epithelial cells of neonatal rabbit kidney. Therefore, a characterization of the collecting duct ampullary cells was carried out by means of immunohistological techniques using a set of different monoclonal antibodies and the lectin peanut agglutinin. All epithelial cells within the ampullary tip and neck were positive for cytokeratin 19, an intermediate filament protein. On the other hand, the monoclonal antibody CD 7 revealed a clear cut boundary between the ampullary neck and the ampullary tip region. Furthermore, after incubation with the monoclonal antibody BO-7 specifically reacting with intercalated cells of the mature collecting duct, both labeled and unlabeled cells were observed within the whole ampullary epithelium. These results were confirmed by scanning electron microscopical investigations which revealed two distinct epithelial cell populations. Thus, an unexpected heterogeneity of the ampullary epithelium could be demonstrated.

Silver-enhanced colloidal-gold labelling of rabbit kidney collecting-duct cell surfaces imaged by scanning electron microscopy

The luminal cell surfaces of rabbit kidney cortical collecting-duct cells were labelled with peanut lectin (PNA) and investigated by scanning electron microscopy. Labelling was performed either on 20-microns-thick cryostat sections from prefixed and cryoprotected rabbit kidney tissue or on cultured collecting-duct epithelium using biotinylated PNA and a 6-nm colloidal-gold-coupled antibody against biotin. Colloidal-gold labels were detected at low magnification (2000-4000x) using silver enhancement. Coating with chromium allowed simultaneous imaging of both cell-surface morphology and labelling topography in the backscattered electron imaging mode. Our results show that PNA binding is specific for a subtype of intercalated cells equipped with microvilli on the luminal surface. The presented method promises to be useful for the identification of specific cell types in heterogeneous tissues.

Characterization of acid-base transport mechanisms in the kidney cell line RCCT-28A

RCCT-28A cells, a continuous cell line of rabbit cortical collecting tubule origin, have been found to exhibit apical peanut-lectin binding, basal band-3 immunostaining and a transepithelial electrical resistance of 246 +/- 37 omega cm2. For the studies reported, confluent monolayers of RCCT-28A cells were grown on permeable wells and incubated in a control solution or in alkaline solutions by lowering PCO2. Equivalent H+ fluxes (JH+) into the apical solution (nmol.min-1.cm2) were measured in the absence of drugs and in the presence of: amiloride (A, 10(-3) M), N-ethylmaleimide (NEM, 5 mM) and omeprazole (OM, 100 microM) in the apical solution. After preincubation in control solutions JH+ was 21 +/- 2 while A had no effect. Addition of NEM diminished JH+ to 12 +/- 2 (P < 0.005), and OM diminished JH+ to 2 +/- 2 (P < 0.001 vs. control). Monolayers incubated at low PCO2 had a basal JH+ of 11 +/- 5. No effect on JH+ could be demonstrated under these conditions by addition of NEM or OM. Removal of K+ from the apical solution diminished apical acidification by 60%. The inhibitor of H+,K(+)-ATPase Schering 28080 (SCH) was tested at different concentrations and an inhibitory effect was demonstrated (JH+ -2 +/- 1 vs. 18 +/- 1, SCH vs. control, respectively). Probenecid and bafilomycin-A also decreased apical acidification and an apical base-equivalent extrusion was apparent under the inhibitors effect. JH+ was abolished by removal of Cl- from the basolateral solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Apical endosomes isolated from kidney collecting duct principal cells lack subunits of the proton pumping ATPase

Endocytic vesicles that are involved in the vasopressin-stimulated recycling of water channels to and from the apical membrane of kidney collecting duct principal cells were isolated from rat renal papilla by differential and Percoll density gradient centrifugation. Fluorescence quenching measurements showed that the isolated vesicles maintained a high, HgCl2-sensitive water permeability, consistent with the presence of vasopressin-sensitive water channels. They did not, however, exhibit ATP-dependent luminal acidification, nor any N-ethylmaleimide-sensitive ATPase activity, properties that are characteristic of most acidic endosomal compartments. Western blotting with specific antibodies showed that the 31- and 70-kD cytoplasmically oriented subunits of the vacuolar proton pump were not detectable in these apical endosomes from the papilla, whereas they were present in endosomes prepared in parallel from the cortex. In contrast, the 56-kD subunit of the proton pump was abundant in papillary endosomes, and was localized at the apical pole of principal cells by immunocytochemistry. Finally, an antibody that recognizes the 16-kD transmembrane subunit of oat tonoplast ATPase cross-reacted with a distinct 16-kD band in cortical endosomes, but no 16-kD band was detectable in endosomes from the papilla. This antibody also recognized a 16-kD band in affinity-purified H+ ATPase preparations from bovine kidney medulla. Therefore, early endosomes derived from the apical plasma membrane of collecting duct principal cells fail to acidify because they lack functionally important subunits of a vacuolar-type proton pumping ATPase, including the 16-kD transmembrane domain that serves as the proton-conducting channel, and the 70-kD cytoplasmic subunit that contains the ATPase catalytic site. This specialized, non-acidic early endosomal compartment appears to be involved primarily in the hormonally induced recycling of water channels to and from the apical plasma membrane of vasopressin-sensitive cells in the kidney collecting duct.

A plasma membrane proton ATPase in specialized cells of rat epididymis

Acidification of the luminal fluid in the epididymis is believed to play an important role in sperm maturation. Previous studies have shown that specialized cells in the epithelium lining the epididymis contain high levels of carbonic anhydrase and that these cells have rod-shaped intramembraneous particles when examined by freeze fracture. Both of these features are characteristic of proton-transporting intercalated cells in the kidney collecting duct. We now show that apical cells in the head of the epididymis and clear cells in the body and tail of the epididymis express high levels of a vacuolar proton-pumping adenosinetriphosphatase on their apical plasma membranes and on intracellular vesicles. By analogy with kidney intercalated cells, these cell types may be specialized for acid secretion in the epididymis.

Maturation of aldose reductase expression in the neonatal rat inner medulla

Newborns are less able to concentrate urine than adults are. With development of the concentrating system and a hypertonic medullary interstitium, there is a need to generate intracellular osmolytes such as sorbitol, which is produced in a reaction catalyzed by the enzyme aldose reductase. We sought to discriminate between two possible mechanisms of aldose reductase induction during development: (a) a response to an osmotic stimulus generated by the concentrating mechanism; or (b) part of the genetic program for development of the kidney. We measured the change in aldose reductase mRNA and activity in terminal inner medullary collecting ducts (IMCDs) microdissected from Sprague-Dawley rats during the first month of life. Aldose reductase mRNA was assayed by Northern analysis of total RNA from inner medulla and by detection of the reverse transcription-polymerase chain reaction (RT-PCR) product obtained from single IMCDs using aldose reductase-specific primers. Aldose reductase activity was measured in IMCDs taken from the same rats using a fluorescent microassay. Newborn rat IMCDs had minimal aldose reductase mRNA or activity, however mRNA was readily detected in IMCDs from rats older than 3 d of age, with peak expression occurring at 1-3 wk of age before decreasing to adult levels. In contrast, the mRNA level for a housekeeping metabolic enzyme, malate dehydrogenase, did not change during maturation. Aldose reductase enzyme activity was readily detectable by 6 d of age, peaked at 20 d, then decreased to adult levels. Urine osmolality remained < 600 mosmol/kg until 16 d, then increased to > 1,100 mosmol/kg after 20 d. Thus, aldose reductase mRNA and activity increased before urinary osmolality reached 870 mosmol/kg. Because urine osmolality may not be indicative of inner medullary osmolality and because mother's milk may provide excessive free water to the pups under 3 wk of age, half of the animals in several litters were separated from their mothers for 1 d and inner medullary osmolality, in addition to urine osmolality, was measured by vapor pressure osmometry, while aldose reductase mRNA was assessed densitometrically in IMCDs after RT-PCR. Although fluid restriction resulted in a near doubling of urine osmolality and a tendency towards increased aldose reductase mRNA, there was no consistently significant increase in aldose reductase mRNA or inner medullary osmolality during the first 13 d of life compared to the suckling animals. On the other hand, 2-3-wk-old rats showed significant increases in aldose reductase mRNA, accompanied by increases in inner medullary osmolality, after fluid restriction. Thus, the dissociation between the increases in aldose reductase expression and inner medullary hyperosmolality indicates that the maturational induction of the aldose reductase gene is not a consequence of osmotic stimulation, but rather, part of the developmental program of the kidney.

Expression of SV40 receptors on apical surfaces of polarized epithelial cells

We have investigated the interaction of SV40 virions with polarized monkey kidney epithelial cells. Virions were tagged with biotin to facilitate their detection and were found to retain full infectivity. When polarized Vero C1008 cells were incubated with biotinylated virions followed by a strepavidin-rhodamine conjugate, distinct cell populations were identified which expressed very different levels of SV40 receptors. The parental Vero C1008 cells yielded three types of cell clones which exhibited low, moderate, or predominantly high levels of SV40 binding. Virus-binding assays to each of these clones as well as to parental Vero C1008 cells indicated that the level of SV40 receptor expression is cell-cycle-dependent. The cellular receptors for influenza A virus (WSN strain) were also found to be distributed heterogeneously on polarized epithelial cells. In contrast, in several types of nonpolarized cells, SV40 receptors were found to be uniformly distributed over the monolayer. SV40 binding was not found to correlate with HLA expression on Vero C1008 cells or other cell types. Also in contrast to SV40 receptor expression, which is restricted to the apical domain, HLA was found to be distributed on both apical and basolateral domains of Vero C1008 cells.

Mechanisms of ion and acid-base regulation at the gills of freshwater fish

This review examines the branchial mechanisms utilized by freshwater fish to regulate internal acid-base status and presents a model to explain the underlying basis of the compensatory processes. Rainbow trout, Oncorhynchus mykiss, and brown bullhead, Ictalurus nebulosus, were examined under a variety of experimental treatments which induced respiratory and metabolic acid-base disturbances. Acid-base regulation was achieved by appropriate adjustments of Na+ and Cl- net fluxes across the gills which, in turn, were accomplished by variable contributions of three different branchial mechanisms: 1) differential changes in Na+ and Cl- diffusive effluxes, 2) changes in internal substrate (H+, HCO3-) availability, and 3) morphological adjustments to the gill epithelium. Differential diffusive efflux of Na+ over Cl- was involved only during periods of metabolic alkalosis. The importance of internal substrate availability was demonstrated using a two-substrate model. According to the model, ionic flux rates (J(in)Cl-, J(in)Na+) are determined not only by the concentration of the external ion (Na+, Cl-) but also by the concentration of the internal counterion (H+, HCO3-). This system provides for an "automatic negative feedback" to aid in the compensation of metabolic acid-base disturbances. Morphological alteration of the gill epithelia and the associated regulation of chloride cell (CC) fractional area is an essential third mechanism which is especially important during respiratory acid-base disturbances. Specifically, fish vary the availability of the CC associated Cl-/HCO3- exchange mechanism by physical covering/uncovering of CCs by adjacent pavement cells.

The structure and biochemistry of the vacuolar H+ ATPase in proximal and distal urinary acidification

Vacuolar H+ ATPases participate in renal hydrogen ion secretion in both the proximal and distal nephron. These plasma membrane forms of the vacuolar H+ ATPase are regulated physiologically to maintain the acid-base balance of the organism. Proton transporting renal cells have requirements for constitutive acidification of intracellular compartments for normal endocytic and secretory functions. Recent experiments have begun to reveal how the kidney regulates these proton pumps independently. Vacuolar H+ ATPases are a family of structurally similar enzyme which differ in the composition of specific subunits. Cytosolic regulatory enzymes are present in renal cells which may affect vacuolar H+ ATPases in certain membrane compartments selectively. The vacuolar H+ ATPase in the plasma membrane of intercalated cells resides in a specialized proton-transporting apparatus that translocates the enzyme between an intracellular membrane pool and the plasma membrane in response to physiologic stimuli.

Conditional immortalization of bicarbonate-secreting intercalated cells from rabbit

We have derived an immortalized cell line from primary cultures of bicarbonate-secreting intercalated cells from rabbit. Cells were transfected with a plasmid encoding a temperature-sensitive large T antigen of SV40 plus the neomycin resistance gene under the control of an SV40 promoter. Transfectants were selected for resistance to G418. One stably transfected clone, designated IC250, was subcloned to ensure clonality, and a subclone (clone C) was characterized in detail. The cells divide continuously at permissive temperature. At restrictive temperature, they cease dividing and assume morphological and transport properties of true bicarbonate-secreting intercalated cells. They express appropriate ultrastructural features, bind peanut lectin in an apical pattern, are rich in carbonic anhydrase, stain for proton-adenosinetriphosphatase in a basolateral pattern, and do not stain with antibodies to erythrocyte band 3. Most monolayers of transformed type B intercalated cells do not achieve a significant transepithelial resistance; those monolayers that are sufficiently electrically tight for electrophysiological studies are capable of chloride-dependent bicarbonate transport.

Differentiation of renal beta-intercalated cells to alpha-intercalated and principal cells in culture

The renal collecting duct is a heterogenous epithelium consisting of intercalated cells (ICC) and principal cells (PC). The origin of this cellular heterogeneity is not clear. To test the hypothesis that the two cell types might originate from one another, pure populations of ICC (beta subtype) and PC were isolated by fluorescence-activated cell sorting and grown on permeable supports. After the monolayers reached confluence, the expression of ICC- and PC-specific functions and antigens was monitored. Cultures of sorted beta-ICC, in addition to expressing ICC-specific functions (such as an electrogenic H+ secretion) and antigens, progressively acquired PC functions (amiloride-sensitive Na+ transport and K+ secretion). On day 6, cultures of sorted beta-ICC exhibited a lumen-negative transepithelial potential difference of 83 +/- 4 mV and a short circuit current of 107 +/- 15 microA/cm2 and created a lumen-to-bath K+ concentration ratio of approximately 10. The percentage of cells staining with two PC-specific antibodies was 53% and 65%. On the other hand, cultures of sorted PC failed to acquire ICC-specific functions while maintaining PC characteristics. To rule out preferential proliferation of a few contaminating PC as an explanation of these results, we have generated a continuous collecting duct cell line (M-1) originating from mice transgenic for the early region of simian virus 40. Cell lines cloned from M-1 cells exhibit both PC and ICC functions and show mutually exclusive heterogenous expression of PC and ICC antigens, demonstrating a common origin of the two cell types. These data indicate that while beta-ICC in culture can give rise to both alpha-ICC and PC, PC cannot convert to ICC, which raises the possibility that beta-ICC is the stem cell in the renal collecting duct. Differentiation of ICC to PC may explain the cellular heterogeneity in the cortical collecting duct.

Complete nucleotide sequence of band 3 related anion transport protein AE2 from human kidney

The cDNA coding for the complete human band 3 related anion exchange protein AE2 has been cloned from human kidney mRNA. The protein is encoded by a mRNA of approx. 3885 nucleotides containing an open reading frame of 3720 nucleotides. The AE2 protein consists of 1240 amino acid residues and has a molecular mass of 136,805 Da.

Regulation of principal cell pH by Na/H exchange in rabbit cortical collecting tubule

Changes in intracellular pH (pHi) were measured using the pH indicator, BCECF, in principal cells from split opened cortical collecting tubules (CCTs) derived from rabbits maintained on a normal diet. This monolayer preparation has the advantage of allowing us to visualize the morphological differences in the two major cell types in this nephron segment under transmitted light. The visual identification of the cell types was verified using emission measurements taken from single principal and intercalated cells in the opened tubule which had been exposed to fluorescein isothiocyanate (FITC)-labeled peanut lectin. We confirmed the existence of an amiloride-sensitive Na/H exchange process activated during intracellular acidosis in principal cells. In addition, the exchanger was active under basal conditions and over a wide range of pHi. Because the exchanger was active under basal conditions we tested the hypothesis that changes in intracellular Na (Nai) would alter pHi in a predictable way. Maneuvers designed to alter Nai were without significant effects within a 10-min time frame. Specifically, addition of 100 microM ouabain to increase Nai or exposure of the tubules to 10(-5) M amiloride to decrease luminal Na entry and reduce Nai did not have an effect on pHi. In some experiments we did observe however, after a 30-min exposure to ouabain, a small decrease in pHi. These results suggest that Na/H exchange is a major regulator of pHi in principal cells. However, regulation of Na transport by changes in pHi in principal cells of rabbit CCT via the activity of a Na/H exchanger do not seem to contribute to the feedback control of Na transport.

Expression and distribution of renal vacuolar proton-translocating adenosine triphosphatase in response to chronic acid and alkali loads in the rat

Renal hydrogen ion excretion increases with chronic acid loads and decreases with alkali loads. We examined the mechanism of adaptation by analyzing vacuolar proton-translocating adenosine triphosphatase (H+ ATPase) 31-kD subunit protein and mRNA levels, and immunocytochemical distribution in kidneys from rats subjected to acid or alkali loads for 1, 3, 5, 7, and 14 d. Acid- and alkali-loaded rats exhibited adaptive responses in acid excretion, but showed no significant changes in H+ ATPase protein or mRNA levels in either cortex or medulla. In contrast, there were profound adaptive changes in the immunocytochemical distribution of H+ ATPase in collecting duct intercalated cells. In the medulla, H+ ATPase staining in acid-loaded rats shifted from cytoplasmic vesicles to plasma membrane, whereas in alkali-loaded rats, cytoplasmic vesicle staining was enhanced, and staining of plasma membrane disappeared. In the cortical collecting tubule, acid loading increased the number of intercalated cells showing enhanced apical H+ ATPase staining and decreased the number of cells with basolateral or poorly polarized apical staining. The results indicate that both medulla and cortex participate in the adaptive response to acid and alkali loading by changing the steady-state distribution of H+ ATPase, employing mechanisms that do not necessitate postulating interconversion of intercalated cells with opposing polarities.

Immunolocalization of 15-kDa membrane proteins in the kidneys of normal and acidotic rats

Proteins with apparent molecular masses between 15 kDa and 17 kDa were enriched from rat renal brush-border membranes by preparative gel electrophoresis and used for immunization of rabbits. The serum of one of the rabbits reacted in Western blots of separated renal brush-border proteins with a single 15-kDa band. A comparably strong reaction is seen with a 15-kDa band of renal endosomal proteins. Basolateral membranes show a much weaker reaction. In light- and electron-microscopic studies the serum stains brush-border membranes and endosomes in rat proximal tubule cells, but not mitochondria and basolateral membranes. In cortical collecting ducts, principal cells are not stained with the antiserum. alpha-type (H(+)-secreting) intercalated cells bind the antibodies at apical tubulovesicles. The luminal membrane is scarcely labelled. Conversely, beta-type (HCO3(-)-secreting) intercalated cells exhibit antibody binding to their basolateral membrane. Thus, the antiserum detects 15-kDa proteins differently sorted in alpha- and beta-intercalated cells. After induction of an acute (6 h) metabolic acidosis, the antibody-binding pattern changes only in intercalated cells, type alpha, and occurs at the markedly enlarged luminal plasma membrane. The amount of alpha-type intercalated cells with enlarged luminal membrane ("secreting cell") increases at the expense of alpha cells with apical tubulovesicles ("resting cell"). Taken together, the antiserum detects 15-kDa proteins, the localization and adaptive changes to metabolic acidosis of which are similar to H(+)-ATPases. The functional role of the 15-kDa proteins needs to be established in further studies.

Regulation of Cl-/HCO3- exchange in the rabbit cortical collecting tubule

Cl-/HCO3- exchange is present in all three cell types of the rabbit cortical collecting tubule, yet may mediate a different function in each cell type. The purpose of this study was to characterize further the location, function, and regulation of Cl-/HCO3- exchange in two cell types using measurements of intracellular pH (pHi). In the principal cell there was no evidence for apical Cl-/HCO3- exchange, including no change in pHi with increases in luminal HCO3-. The principal cell possesses a basolateral Cl-/HCO3- exchanger that is inactive normally but stimulated by intracellular alkalosis. Decreased PCO2 results in increased pHi associated with activation of Cl-/HCO3- exchange and partial recovery of pHi. In contrast, the beta-intercalated cell possesses an apical Cl-/HCO3- exchanger and alkalinizes with increases in luminal HCO3-. Also in contrast to the principal cell, the beta-intercalated cell apical Cl-/HCO3- exchanger does not appear to be involved in pHi regulation and may be specifically modified for transcellular HCO3- transport. In conclusion, the separate Cl-/HCO3- exchangers in the principal cell and the beta-intercalated cell not only have opposite polarity but are regulated differently.

Hypotonic stress-induced release of KHCO3 in fused renal epitheloid (MDCK) cells

Mechanisms of cell volume regulation induced by the reduction of the osmolality of the Ringer solution by one-third were studied in fused Madin-Darby canine kidney (MDCK) cells. Intracellular HCO3-, K+ and Cl- concentrations [ion]i in parallel with cell membrane potential (PD), cell membrane conductance (Gm) and conductances of individual ions (Gmion) were evaluated with microelectrode techniques. Fused cells regulate their cell volume by about 50%. Gm increased from 0.43 +/- 0.03 mS/cm2 in isotonic Ringer solution to 4.3 +/-0.3 mS/cm2 in the steady state phase of cell swelling. GmCl was 0.31 +/- 0.03 mS/cm2 in isotonic Ringer solution and thus was the dominant individual ion conductance. In the initial phase of cell swelling GmK increased transiently 64-fold to 0.32 +/- 0.03 mS/cm2, and consequently PD hyperpolarized. At peak hyperpolarization GmCl transiently decreased by 15%. Cell swelling increased GmCl 11-fold and GmHCO3 28-fold to 0.95 +/- 0.1 mS/cm2 in the steady state phase of cell swelling. In this phase GmCl and GmHCO3 were dominating, whereas GmK was only slightly increased compared to isotonic conditions. The hyperpolarization of PD was paralleled by cytoplasmic acidification. At peak acidification [HCO3-]i decreased by 6.4 mmol/kg H2O. Cl- extrusion was not detectable in the initial phase of cell swelling. In isotonic Ringer solution [K+]i was 125 +/- 5 mmol/kg H2O. During the initial phase of cell swelling 23 +/- 5 mmol/kg H2O K+ was extruded, indicating that yet unknown anions participated in cell volume regulation in this phase of cell swelling. In the steady state phase of cell swelling [pH]i was normalized by replenishing [HCO3-]i, whereas Cl- was extruded. We conclude that fused renal epitheloid cells acutely release KHCO3 in response to hypotonicity, but then regain pH homeostasis in the steady state phase of cell swelling.

Differentiation of renal intercalated cells in fetal and postnatal rats

An ultrastructural study was conducted on the kidneys from rat fetuses and pups from ages ranging from birth to 8 weeks to identify the time of appearance of each of the two intercalated cell types. With transmission electron microscopy. A-intercalated cells were recognized by their large apical microvilli and microplicae as well as by the numerous subapical vesicles. Their identification was confirmed by the presence of typical studs at the cytoplasmic face of the apical plasma membrane. By scanning electron microscopy the cells were recognized by their typical microplicae at the apical surface. In 19-day-old fetuses and newborns. A-intercalated cells were numerous in the epithelium lining the renal pelvis and inner medullary intercalated ducts. Two weeks after birth they disappeared from these regions but became numerous at the outer medullary collecting ducts and also at the cortical collecting ducts although to a lesser degree. B-intercalated cells were recognized by the scarcity of microvilli, the absence of microplicae, and the large number of basal infoldings. Their identification was confirmed by the presence of studs at the cytoplasmic face of the basolateral membrane. B-cells started to appear 3 weeks after birth and increased thereafter. We speculate that the particular stages at which the two cell types differentiate might be related to changes in acid-base status.

Maturation of renal potassium transport

The studies outlined in this review suggest that the immaturity of distal nephron segments may hinder urinary excretion of potassium early in life. Among the factors that may limit potassium secretion by principal cells in the neonatal cortical collecting duct are an unfavorable electrochemical gradient (reduced Ki, Na(+)-K(+)-ATPase activity and/or Vte), limited membrane permeability to potassium and sodium, low tubular fluid flow rate, reduced luminal sodium concentration, or increased paracellular backleak. Alternatively, enhanced potassium absorption by other relatively well-differentiated distal nephron segments may contribute in part to a reduced net potassium excretory rate in the newborn. It should be kept in mind, however, that the limited potassium secretory capacity of the immature kidney becomes clinically relevant only under conditions of potassium excess. Under normal circumstances, the tendency of the newborn to retain potassium is an appropriate and necessary condition for growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunolocalization of gluco- and mineralocorticoid receptors in rabbit kidney

The localization of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) was determined in the rabbit kidney by immunohistochemistry with the use of a monoclonal, anti-GR antibody and a monoclonal, anti-idiotypic, anti-MR antibody. Immunostaining was performed on serial histological sections from normal and adrenalectomized rabbits. The specificity of immunostaining was assessed for MR by in situ competition studies with steroids and for GR by presaturation of the antibody with GR preparation. Immunostaining by both the anti-MR and the anti-GR antibodies was present in all parts of the distal nephron (beyond proximal tubule) and absent in the glomerulus and proximal tubule. The absence of staining by the anti-GR antibody in the proximal tubule suggests that the effects of glucocorticoids in this structure involve either a GR different from that of distal structures or a non-receptor mediated mechanism of action. MR immunostaining predominates in the distal and all along the collecting tubule in its cortical, medullary, and papillary portions. GR immunostaining was most abundant in the medullary ascending limb and distal tubule. Immunostaining by both antibodies was present in papillary interstitial cells and cells of the epithelium lining the papilla. Fifteen to twenty percent of the cells of the cortical collecting tubule, presumably intercalated cells, were devoid of MR and GR immunostaining. Immunostaining was present in both nuclear and cytoplasmic cell compartments. No clear difference was observed between normal and adrenalectomized rabbits. This study is the first report on renal immunolocalization of GR compared with MR. In addition, we show evidence for new targets for corticosteroid hormones such as papillary interstitial cells and papillary epithelium.

Acute changes in channel density of amphibian diluting segment

Intracellular pH is a well-established modulator of the apical membrane potassium conductance of the amphibian diluting segment (early distal tubule). We investigated the modulation of this apical potassium conductance at the single-channel level in everted early distal tubules of the frog. Alkalinization of the bath fluid increased mean channel open probability (NPo) both in the presence and absence of the potassium-hydrogen ionophore nigericin. Reciprocal changes were seen with acidification. Because these effects were observed in cell-attached patches, where the composition of the fluid in the pipette is assumed to remain constant, the observed changes in channel activity were attributed to changes in intracellular pH. Further analysis of the data revealed that the changes in channel activity were produced exclusively by changes in the functional number of channels within the patch (N). We were unable to detect any significant changes in the single-channel open probability (Po). This suggests that the density of channels within a membrane may be far more dynamic than previously assumed.

Acid transport by intracellular vesicles

Many intracellular organelles contain a unique primary, electrogenic proton pump termed the vacuolar H(+)-ATPase. This pump, found in many endocytic, secretory, and storage vesicles in fungal, plant and animal cells, functions, in conjunction with a chloride conductance, to acidify the vesicle interior. Although remotely related to the mitochondrial ATP synthase, the vacuolar H(+)-ATPase is a distinct pump which differs in inhibitor sensitivity, subunit composition and function. The vacuolar H(+)-ATPase transports only protons, and permeable anions (chloride) are required for optimal vesicle acidification. Allosteric and regulatory effects are not yet fully understood. Vesicle acidification appears to be essential for receptor-mediated endocytosis, protein synthesis, and secretion and storage of small solutes such as neurotransmitters. A similar plasma membrane-located H(+)-ATPase may contribute to urinary acidification and cell pH regulation.

Organization of collecting duct intercalated cells

Our understanding of the mechanisms by which the collecting duct transports HCO3 continues to evolve rapidly. The models put forth in Figure 1, though esthetically pleasing by virtue of their simplicity, will undoubtedly require modification as the above areas and others continue to be explored. It should be noted that a large percentage of the citations in this review emanate from colleagues of Dr. Donald Seldin who have been or currently are nephrologists at Southwestern Medical School in Dallas. The length of this list is testimony to the large number of investigators in the field of renal acid-base research who have been intellectually stimulated by their contact with Dr. Seldin.

Endocytic vesicles from renal papilla which retrieve the vasopressin-sensitive water channel do not contain a functional H+ ATPase

The water permeability of the kidney collecting duct epithelium is regulated by vasopressin (VP)-induced recycling of water channels between an intracellular vesicular compartment and the plasma membrane of principal cells. To test whether the water channels pass through an acidic endosomal compartment during the endocytic portion of this pathway, we measured ATP-dependent acidification of FITC-dextran-labeled endosomes in isolated microsomal fractions from different regions of Brattleboro rat kidneys. Both VP-deficient controls and rat treated with exogenous VP were examined. ATP-dependent acidification was not detectable in endosomes containing water channels from distal papilla (osmotic water permeability Pf = 0.038 +/- 0.004 cm/s). In contrast, the addition of ATP resulted in a strong acidification of renal cortical endosomes (pHmin = 5.8, initial rate = 0.18-0.25 pH U/s). Acidification of cortical endosomes was reversed with nigericin and strongly inhibited by N-ethyl-maleimide. Passive proton permeability was similar and low in both cortical and papillary endosomes from rats treated or not treated with VP. The fraction of labeled endosomes present in microsomal preparations was determined by fluorescence imaging microscopy of microsomes nonspecifically bound to poly-l-lysine-coated coverslips and was 25% in cortical preparations compared to 14% (+VP) and 9% (-VP) in papillary preparations. The fraction of cortical endosomes was enriched 1.5-fold by immunoabsorption to coverslips coated with mAbs against the bovine vacuolar proton pump. In contrast, the fraction of papillary endosomes was depleted more than twofold by immunoabsorption to identical coverslips. Finally, sections of distal papilla stained with antibodies against the lysosomal glycoprotein LGP120 showed that most of the entrapped FITC-dextran did not colocalize with this lysosomal protein. These results demonstrate that vesicles which internalize water channels in kidney collecting duct principal cells lack functional proton pumps, and do not deliver the bulk of their FITC-dextran content to lysosomes. The data suggest that the principal cell contains a specialized nonacidic apical endocytic compartment which functions primarily to recycle membrane components, including water channels, to the plasma membrane.

Madin-Darby canine kidney cells. II. Aldosterone stimulates Na+/H+ and Cl-/HCO3- exchange

Experiments in dome epithelium of Madin-Darby canine kidney (MDCK) cells were performed to elucidate aldosterone action on acid-base transport. By means of pH-sensitive microelectrodes the pH of the dome fluid was measured while the apical plasma membrane was superfused. In the absence of HCO3- the dome fluid (facing the basolateral cell membrane) alkalinized in response to 10(-7) mol/l aldosterone. Amiloride (10(-3) mol/l) inhibited dome formation and pH recovery of the dome fluid from an extracellular acid load. In the presence of HCO3- dome fluid acidified in response to aldosterone. The stilbene derivative diisothiocyanate-stilbene-2,2'-disulphonic acid (DIDS) or removal of Cl- from the apical perfusate inhibited this dome acidification. In aldosterone-depleted MDCK monolayers HCO3- was actively accumulated in the dome fluid in contrast to aldosterone-supplemented cells. The results indicate that aldosterone stimulates both amiloride-sensitive Na+/H+ exchange and DIDS-sensitive Cl-/HCO3- exchange in the apical cell membrane of MDCK cells. In the absence of aldosterone the HCO3- extrusion process is localized in the basolateral membrane in series with apical Na+/H+ exchange, while in the presence of aldosterone Cl-/HCO3- is mainly localized in the apical membrane in parallel with Na+/H+ exchange. Cl- exits the cell through apical Cl- channels and is absorbed via the paracellular route.

Differentiation of proton-pumping activity in cultured renal inner medullary collecting duct cells

Cultured inner medullary collecting duct (IMCD) cells have been shown to secrete protons (H+) by two mechanisms: an N-ethylmaleimide- and dicyclohexyl-carbodiimide-sensitive electrogenic H(+)-ATPase or H+ pump, and an amiloride-sensitive, secondary active Na+H+ exchanger. These cells also express Cl-/HCO3- exchange and carbonic anhydrase activity in common with other renal epithelial cells involved in acid-base transport. Video fluorescence microscopy of individual cells using 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein has demonstrated that adjacent-cultured IMCD cells show substantial functional intercellular heterogeneity. The development of H(+)-pumping activity is associated with high-baseline intracellular pH and peanut agglutinin (PNA) affinity, and loss of mitotic activity and of Na+/H+ exchange. The H(+)-pumping activity may be further enhanced by removal of fetal calf serum for 6-54 h or by selecting cells with high PNA affinity. IMCD cells in their most differentiated state form domes, which consistently showed the highest rates of H(+)-pumping activity, as well as high affinity for peanut lectin. When IMCD were plated at low density, domes developed relatively late (2-4 weeks), at which time cells located in the center of nests of contiguously growing cells were quiescent and showed H(+)-pumping activity but no Na+/H+ exchange. On the other hand, dense plating was associated with early development of domes (end of 1st week), at which time adjacent cells showed a high mitotic activity and Na+/H+ exchange, but no H(+)-pumping activity. We speculate that differentiation of IMCD cells results in the development of cell polarity.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarized insertion of an intracellular glycoprotein pool into the apical membrane of MDCK cells

A monoclonal antibody that recognizes a 135-kDa glycoprotein (GP135) on the apical membrane of Madin-Darby canine kidney (MDCK) cells was used to identify and characterize an intracellular pool of GP135. Mild trypsin treatment at 4 degrees C removed approximately 95% of the GP135, and after warming to 37 degrees C, the reappearance of GP135 on the apical membrane was monitored by radioimmunoassay. Incorporation of GP135 into the apical cell surface after trypsin treatment consisted of two components, a rapidly inserted, cycloheximide-insensitive portion (defined as the GP135 pool), which leveled off within 1 h, followed by a slower insertion of newly synthesized GP135. Immunogold electron microscopy demonstrated that the GP135 pool was targeted in a polarized manner and was only detected on the apical membrane. Temperature shift and retrypsinization experiments provided evidence that the GP135 pool consisted of intracellular vesicles that could fuse with the plasma membrane. This was confirmed by immunofluorescence microscopy demonstrating that GP135 was localized within large cytoplasmic vesicles residing at varying distances from the apical cell surface. These data provide evidence for the presence of a regulated pathway in MDCK cells and support the possibility that the GP135 pool functions as an intracellular reserve which can exhibit polarized insertion into the plasma membranes similar to that described for other epithelial cells.

FITC-dextran as a probe for endosome function and localization in kidney

Fluorescein isothiocyanate (FITC)-labeled endosomes were localized in kidney epithelial cells after tissue fixation and sectioning, and specific membrane transport properties of isolated endocytic vesicles were measured using the same probe. Rats were infused intravenously with 10 kDa FITC-dextran, and kidneys were fixed with paraformaldehyde lysine periodate. FITC-labeled vesicles were visualized in semithin (1 micron) frozen sections of excised tissue by epifluorescent microscopy and by electron microscopy after a photoconversion reaction. Most FITC-labeled endosomes were apically located in epithelial cells lining the urinary tubules. By immunocytochemistry the anti-lysosomal glycoprotein LGP 120 was absent from most of the FITC-labeled vesicles, although some colocalization was noted. The limiting membrane of FITC-labeled endosomes contained a vacuolar proton pump (pHmin = 6.23 +/- 0.033) and a water channel (osmotic water permeability coefficient, Pf = 0.052 +/- 0.005 cm/s) and was highly permeable to ethylene glycol and urea but relatively impermeable to glucose. Methods allowing the attribution of specific membrane functions to vesicles that can be visualized in the apical endocytic pathway of epithelial cells should be of general use for the study of endocytic pathways in a variety of systems.