Structural adaptation of intercalated cells in rat renal cortex to acute metabolic acidosis and alkalosis

Insights into acidosis-induced regulation of SLC26A4 (pendrin) and SLC4A9 (AE4) transporters using three-dimensional morphometric analysis of β-intercalated cells

The purpose of this study was to examine the three-dimensional (3-D) expression and distribution of anion transporters pendrin (SLC26A4) and anion exchanger (AE)4 (SLC4A9) in β-intercalated cells (β-ICs) of the rabbit cortical collecting duct (CCD) to better characterize the adaptation to acid-base disturbances. Confocal analysis and 3-D reconstruction of β-ICs, using identifiers of the nucleus and zona occludens, permitted the specific orientation of cells from normal, acidotic, and recovering rabbits, so that adaptive changes could be quantified and compared. The pendrin cap likely mediates apical Cl(-)/HCO3 (-) exchange, but it was also found beneath the zona occludens and in early endosomes, some of which may recycle back to the apical membrane via Rab11a(+) vesicles. Acidosis reduced the size of the pendrin cap, observed as a large decrease in cap volume above and below the zona occludens, and the volume of the Rab11a(+) apical recycling compartment. Correction of the acidosis over 12-18 h reversed these changes. Consistent with its proposed function in the basolateral exit of Na(+) via Na(+)-HCO3 (-) cotransport, AE4 was expressed as a barrel-like structure in the lateral membrane of β-ICs. Acidosis reduced AE4 expression in β-ICs, but this was rapidly reversed during the recovery from acidosis. The coordinate regulation of pendrin and AE4 during acidosis and recovery is likely to affect the magnitude of acid-base and possibly Na(+) transport across the CCD. In conclusion, acidosis induces a downregulation of AE expression in β-ICs and a diminished presence of pendrin in apical recycling endosomes.

Regulation of AQP6 mRNA and protein expression in rats in response to altered acid-base or water balance

In the rat, aquaporin-6 (AQP6) is mainly localized in intercalated cells (ICs) in collecting ducts, where it is exclusively associated with intracellular vesicles. In this study, we examined whether AQP6 protein and mRNA expression were regulated in the inner medulla or inner stripe of the outer medulla. Rats treated with dietary alkali or acid load for 7 days with a fixed daily water intake revealed appropriate changes in urine pH but unchanged urine output. AQP6 protein and mRNA abundance were increased in alkali-loaded rats (187 +/- 18 and 151 +/- 17% of control, respectively), whereas no changes were observed in acid-loaded rats. Immunohistochemistry revealed increased IC AQP6 labeling in alkali-loaded rats but not in acid-loaded rats. In contrast, administration of NH(4)Cl in the drinking water for 2 wk (free access to water) revealed a significant increase in AQP6 protein abundance (194 +/- 9% of control), but this was associated with increased water intake. Combined, this suggests that AQP6 expression was not affected by acid loading per se but rather was in response to changes in water intake. Consistent with this, water loading for 48 h was associated with increased AQP6 protein abundance, compared with thirsted rats. Moreover, rats with lithium-induced nephrogenic diabetes insipidus had a threefold increase in both AQP6 protein and mRNA expression. Overall, these results suggest that AQP6 expression in collecting duct ICs is regulated by altered acid/alkali load or water balance. Thus AQP6 may contribute to maintenance of acid-base homeostasis and water balance.

Stereologic methods and their application in kidney research

Stereologic methods are used to obtain quantitative information about three-dimensional structures based on observations from section planes or--to a limited degree--projections. Stereologic methods, which are used in biologic research and especially in the research of normal and pathologic kidneys, will be discussed in this review. Special emphasis will be placed on modern stereologic methods, free of assumptions of the structure, size, and shape, etc., so-called UFAPP (unbiased for all practical purposes) stereologic methods. The basic foundation of all stereology, sampling, will be reviewed in relation to most of the methods discussed. Estimation of error variances and some of the basic problems in stereology will be reviewed briefly. Finally, a few comments will be made about the future directions for stereology in kidney research.

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.

Immunohistochemical localization of atrial natriuretic peptide in the developing and adult mammalian kidney

The discovery, within the last decade, of atrial natriuretic peptide (ANP), a family of peptides with natriuretic/diuretic and vasorelaxant properties, has prompted much research into the mechanisms and sites of action of ANP within the kidney. In the present study, ANP was localized in the kidneys of several mammalian species by immunohistochemical techniques 1) to identify possible sites of synthesis; 2) to compare the localization of ANP to known physiological effects; 3) to determine species differences, if any, in ANP localization; and 4) to study the development of ANP immunoreactivity in the fetal and neonatal rat kidney. Using an antibody against rat ANP, IV, ANP was localized exclusively on the proximal convoluted tubule (PCT) brush border and within intercalated cells of the outer medullary and cortical collecting tubules and ducts of adult mouse, rat, pig, monkey, and human kidneys. The development of ANP immunoreactivity paralleled the differentiation and maturation of collecting duct epithelium in rat fetal kidney. Atrial natriuretic peptide found within intercalated cells of the cortical and outer medullary collecting ducts may be the result of endogenous synthesis and, following secretion, may be available to receptors in the inner medullary collecting ducts.

H+/base transport in principal cells characterized by confocal fluorescence imaging

A dual-excitation inverted confocal laser-scanning microscope has been developed for measuring intracellular pH (pHi) using 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) in individual cells in the isolated perfused cortical collecting tubule (CCT). This new microscope has superior depth discrimination, which eliminates the contribution of fluorescence information from cells outside the plane of focus. pHi was monitored in real time from a spot 0.55 microns in diameter within a single cell. Experiments were performed to examine the apical and basolateral membrane H+/base transport properties of single principal cells. The results indicate that principal cells possess a basolateral membrane Na(+)-independent Cl-/base exchanger, a Na(+)-H+ antiporter, and a Na+/base cotransporter. No evidence was found for an apical membrane Na(+)-independent Cl-/base exchanger. The data provide evidence for base efflux pathways in the principal cell and are compatible with the hypothesis that principal cells contribute importantly to H+/base transport in the CCT. The new methodology described in this report can be applied to other epithelia that are optically heterogeneous in the depth dimension.

Lithium-induced structural changes in the cortical distal nephron localized by computer-assisted three-dimensional reconstruction

Lithium treatment is known to cause tubule dilation in distal nephron segments both in rat and in man. However, due to the heterogeneous cell composition of the distal nephron and the cellular changes following lithium treatment, it has been difficult to identify the structurally changed segments. In this study we have therefore applied computer-assisted reconstruction of cortical distal nephron segments. Tubule dilation was demonstrated in connecting and initial collecting tubules and in the first part of cortical collecting ducts (CCD) whereas it was absent from distal straight and distal convoluted tubules. Principal cells (P cells) in the CCD showed swelling of the cytoplasm, accumulation of actin-like microfilaments, and abnormal arrangements of basolateral membranes. Connecting tubule cells (CNT cells) showed similar but less pronounced changes. Intercalated cells (I cells) showed an accumulation of vesicles in the apical cytoplasm and a reduced luminal surface area. Lesions in P and CNT cells may, at least in part, explain the diabetes insipidus and sodium loss found during lithium treatment. Proton secretion in I cells is probably mediated by an ATPase present in the luminal membrane. The reduction in area of this membrane may explain why lithium-treated animals have a lowered ability to excrete an acid load.

Secretion of bicarbonate by rat distal tubules in vivo. Modulation by overnight fasting

We have performed microperfusion studies on distal tubule bicarbonate reabsorption (JtCO2) of fed and fasted rats to extend our previous observations of in vivo bicarbonate secretion and to resolve certain discrepancies between free-flow and microperfusion data. When rats are fasted overnight, as in previous free-flow studies, distal tubule microperfusion with a 28-mM tCO2 solution results in significant JtCO2 (53 +/- 6 pmol.min-1.mm-1) at normal flow and increases briskly (91 +/- 16 pmol.min-1.mm-1) with bicarbonate load. This response is not influenced by the addition of other normal tubular fluid constituents. However, when normally fed rats are used, as in our previous microperfusion studies, distal tubule JtCO2 is not different from zero when a 28-mM tCO2 solution is perfused at normal flow rates but becomes negative (-54 +/- 13 pmol.min-1.mm-1) at high flow rates, which indicates the existence of bicarbonate secretion against a concentration gradient. Alkali loading of fasted rats also elicits bicarbonate secretion at high flow. These results demonstrate for the first time that normal feeding or alkali loading can induce bicarbonate secretion in a mammalian nephron segment in vivo, and resolves previous discrepancies between free-flow and microperfusion data.

Relationship between structure and function in distal tubule and collecting duct

The relationship between structure and function in the distal tubule and collecting duct has been studied with morphologic and physiologic techniques, including morphometric analysis, to identify functionally distinct cell populations. The distal tubule, including the thick ascending limb (TAL) and the distal convoluted tubule (DCT), is involved in active reabsorption of sodium chloride. It is characterized by extensive invaginations of the basolateral plasma membrane, numerous mitochondria, and high Na-K-ATPase activity, features characteristic for an epithelium involved in active transport. Between the distal tubule and the collecting duct is a transition region, the connecting segment or the connecting tubule (CNT), which exhibits species differences with respect to both structure and function. The collecting duct includes the cortical (CCD), the outer medullary (OMCD), and the inner medullary (IMCD) collecting ducts. Principal cells are present throughout the collecting duct, whereas intercalated cells are located mainly in the CCD and OMCD. Morphometric analysis combined with micropuncture and microperfusion studies has provided evidence that the CNT and principal cells are responsible for potassium secretion in the connecting segment and the CCD. The OMCD is a main site of hydrogen ion secretion, and morphometric studies have provided evidence that the intercalated cells in this segment secrete hydrogen ion at least in the rat. Two configurations of intercalated cells exist in the CCD--a type A and a type B. The A cells are similar in ultrastructure to the intercalated cells in the OMCD and are believed to be involved in hydrogen ion secretion. The function of the B cells remains to be established. The inner two-thirds of the IMCD corresponds to the papillary collecting duct, which has a high permeability to urea. The relationship between structure and function in the IMCD has not been studied in detail. This review emphasizes the role of morphometric analysis in establishing the relationship between structure and function in the distal nephron.

Ultrastructure of three-dimensionally localized distal nephron segments in superficial cortex of the rat kidney

The ultrastructure of superficial distal nephron segments was analyzed after precise localization of tubule cross sections using computer-assisted three-dimensional reconstructions. Five systems of tubules, each with three interconnected distal tubules, were reconstructed and the lengths of the post macula densa segment of the distal straight tubule (DST), the distal convoluted tubule (DCT), the connecting tubule (CNT), and the initial collecting tubule (ICT) were determined. Each cortical collecting duct (CCD) was in continuity with only one tubule in contact with the renal capsule. In three of the five reconstructions, the two nonsubcapsular tubules fused and had a common connection to the subcapsular tubule. The length, between the macula densa (MD) and the confluence, of subcapsular tubules (2.68 +/- 0.15 mm) significantly exceeded the length of tubules not in contact with the renal capsule (2.05 +/- 0.10 mm). This difference was mainly due to a longer ICT in subcapsular tubules. Subcapsular tubules always contacted the renal capsule in the early DCT and often again in the ICT. Cells in the early DCT showed more microvilli on the luminal surface and more infoldings of basolateral membranes than cells in the late DCT. The ultrastructure of intercalated cells (I cells) varied within a range of different manifestations and the ultrastructural variation of I cells was similar in all the analyzed tubule segments. Connecting tubule cells and principal cells were similar in ultrastructure in all tubule segments and cortical levels analyzed.

Microheterogeneity of the collecting duct system in rabbit kidney as revealed by monoclonal antibodies

This report describes the immunolocalization of three monoclonal antibodies along the collecting duct system in rabbit kidney. The antibodies were raised against antigens derived from a membrane fraction of homogenized papillary tissue. Western Blot analysis demonstrated that each of the antibodies recognized a single band of about 190,000 (PCD1), 210,000 (PCD2) and 50,000 (PCD3) daltons. In renal tissue, the antibodies bound specifically to the epithelia of the connecting tubule (CNT), the collecting duct (CD) and the papillary surface epithelium. Differences in the binding patterns of the antisera were limited to the cortex. PCD1 labeled only a few scattered cells in the CNT, and exhibited a heterogeneous binding along the cortical collecting duct (CCD). PCD2 and PCD3 binding patterns were similar. In the CNT, these antibodies bound to the intercalated cells (IC-cells) but not to the CNT-cells proper. In the CCD, both IC-cells and principal cells were labeled. The binding to the medullary collecting duct by all three antisera was identical. The ureter was labeled only by PCD2 and PCD3, and none of the antisera bound to the bladder epithelium. The antibody binding patterns provide information concerning tubular axial heterogeneity and embryogenetic aspects of the CNT and the CCD. These antibodies may be used as differentiation markers in studies of the developing kidney and of renal tissue culture systems.

Ultrastructure of distal nephron cells in rat renal cortex

Distal nephron segments in the rat renal cortex contain distal convoluted tubule cells (DCT cells), connecting tubule cells (CNT cells), intercalated cells (I cells), and principal cells (P cells). The present study was carried out to expand present knowledge on the ultrastructure of these cells. The cells were sampled from superficial cortex and analyzed by electron microscopy. Several morphometric parameters were determined and statistical comparison between cell types was performed. Significant structural differences between the cell types were demonstrated. DCT cells showed the highest volume density of mitochondria whereas the amplification of basolateral membranes was higher in CNT cells than in I and P cells. The surface density of the membrane that bounds intermediate vesicles in the apical cytoplasm was twofold higher in I cells than in the other cell types. The morphological differentiation found in the present study adds to available evidence indicating a functional differentiation between the cell types and provides a reference for structure-function correlations in these cells.