Shape of cells and extracellular channels in rabbit cortical collecting ducts

Effect of luminal flow on doming of mpkCCD cells in a 3D perfusable kidney cortical collecting duct model

The cortical collecting duct (CCD) of the mammalian kidney plays a major role in the maintenance of total body electrolyte, acid/base, and fluid homeostasis by tubular reabsorption and excretion. The mammalian CCD is heterogeneous, composed of Na⁺-absorbing principal cells (PCs) and acid-base-transporting intercalated cells (ICs). Perturbations in luminal flow rate alter hydrodynamic forces to which these cells in the cylindrical tubules are exposed. However, most studies of tubular ion transport have been performed in cell monolayers grown on or epithelial sheets affixed to a flat support, since analysis of transepithelial transport in native tubules by in vitro microperfusion requires considerable expertise. Here, we report on the generation and characterization of an in vitro, perfusable three-dimensional kidney CCD model (3D CCD), in which immortalized mouse PC-like mpkCCD cells are seeded within a cylindrical channel embedded within an engineered extracellular matrix and subjected to luminal fluid flow. We find that a tight epithelial barrier composed of differentiated and polarized PCs forms within 1 wk. Immunofluorescence microscopy reveals the apical epithelial Na⁺ channel ENaC and basolateral Na⁺/K⁺-ATPase. On cessation of luminal flow, benzamil-inhibitable cell doming is observed within these 3D CCDs consistent with the presence of ENaC-mediated Na⁺ absorption. Our 3D CCD provides a geometrically and microphysiologically relevant platform for studying the development and physiology of renal tubule segments.

Novel minimal physiologically-based model for the prediction of passive tubular reabsorption and renal excretion clearance

PURPOSE

Develop a minimal mechanistic model based on in vitro-in vivo extrapolation (IVIVE) principles to predict extent of passive tubular reabsorption. Assess the ability of the model developed to predict extent of passive tubular reabsorption (Freab) and renal excretion clearance (CLR) from in vitro permeability data and tubular physiological parameters.

METHODS

Model system parameters were informed by physiological data collated following extensive literature analysis. A database of clinical CLR was collated for 157 drugs. A subset of 45 drugs was selected for model validation; for those, Caco-2 permeability (Papp) data were measured under pH6.5-7.4 gradient conditions and used to predict Freab and subsequently CLR. An empirical calibration approach was proposed to account for the effect of inter-assay/laboratory variation in Papp on the IVIVE of Freab.

RESULTS

The 5-compartmental model accounted for regional differences in tubular surface area and flow rates and successfully predicted the extent of tubular reabsorption of 45 drugs for which filtration and reabsorption were contributing to renal excretion. Subsequently, predicted CLR was within 3-fold of the observed values for 87% of drugs in this dataset, with an overall gmfe of 1.96. Consideration of the empirical calibration method improved overall prediction of CLR (gmfe=1.73 for 34 drugs in the internal validation dataset), in particular for basic drugs and drugs with low extent of tubular reabsorption.

CONCLUSIONS

The novel 5-compartment model represents an important addition to the IVIVE toolbox for physiologically-based prediction of renal tubular reabsorption and CLR. Physiological basis of the model proposed allows its application in future mechanistic kidney models in preclinical species and human.

The renal cortical collecting duct: a secreting epithelium?

KEY POINTS

The cortical collecting duct (CCD) plays an essential role in sodium homeostasis by fine-tuning the amount of sodium that is excreted in the urine. Ex vivo, the microperfused CCD reabsorbs sodium in the absence of lumen-to-bath concentration gradients. In the present study, we show that, in the presence of physiological lumen-to-bath concentration gradients, and in the absence of endocrine, paracrine and neural regulation, the mouse CCD secretes sodium, which represents a paradigm shift. This secretion occurs via the paracellular route, as well as a transcellular pathway that is energized by apical H+ /K+ -ATPase type 2 pumps operating as Na+ /K+ exchangers. The newly identified transcellular secretory pathway represents a physiological target for the regulation of sodium handling and for anti-hypertensive therapeutic agents.

ABSTRACT

In vitro microperfusion experiments have demonstrated that cortical collecting ducts (CCDs) reabsorb sodium via principal and type B intercalated cells under sodium-depleted conditions and thereby contribute to sodium and blood pressure homeostasis. However, these experiments were performed in the absence of the transepithelial ion concentration gradients that prevail in vivo and determine paracellular transport. The present study aimed to characterize Na+ , K+ and Cl- fluxes in the mouse CCD in the presence of physiological transepithelial concentration gradients. For this purpose, we combined in vitro measurements of ion fluxes across microperfused CCDs of sodium-depleted mice with the predictions of a mathematical model. When NaCl transport was inhibited in all cells, CCDs secreted Na+ and reabsorbed K+ ; Cl- transport was negligible. Removing inhibitors of type A and B intercalated cells increased Na+ secretion in wild-type (WT) mice but not in H+ /K+ -ATPase type 2 (HKA2) knockout mice. Further inhibition of basolateral NaCl entry via the Na+ -K+ -2Cl- cotransporter in type A intercalated cells reduced Na+ secretion in WT mice to the levels observed in HKA2-/- mice. With no inhibitors, WT mouse CCDs still secreted Na+ and reabsorbed K+ . In vivo, HKA2-/- mice excreted less Na+ than WT mice after switching to a high-salt diet. Taken together, our results indicate that type A intercalated cells secrete Na+ via basolateral Na+ -K+ -2Cl- cotransporters in tandem with apical HKA2 pumps. They also suggest that the CCD can mediate overall Na+ secretion, and that its ability to reabsorb NaCl in vivo depends on the presence of acute regulatory factors.

Lack of an effect of collecting duct-specific deletion of adenylyl cyclase 3 on renal Na+ and water excretion or arterial pressure

cAMP is a key mediator of connecting tubule and collecting duct (CD) Na(+) and water reabsorption. Studies performed in vitro have suggested that CD adenylyl cyclase (AC)3 partly mediates the actions of vasopressin; however, the physiological role of CD AC3 has not been determined. To assess this, mice were developed with CD-specific disruption of AC3 [CD AC3 knockout (KO)]. Inner medullary CDs from these mice exhibited 100% target gene recombination and had reduced ANG II- but not vasopressin-induced cAMP accumulation. However, there were no differences in urine volume, urinary urea excretion, or urine osmolality between KO and control mice during normal water intake or varying degrees of water restriction in the presence or absence of chronic vasopressin administration. There were no differences between CD AC3 KO and control mice in arterial pressure or urinary Na(+) or K(+) excretion during a normal or high-salt diet, whereas plasma renin and vasopressin concentrations were similar between the two genotypes. Patch-clamp analysis of split-open cortical CDs revealed no difference in epithelial Na(+) channel activity in the presence or absence of vasopressin. Compensatory changes in AC6 were not responsible for the lack of a renal phenotype in CD AC3 KO mice since combined CD AC3/AC6 KO mice had similar arterial pressure and renal Na(+) and water handling compared with CD AC6 KO mice. In summary, these data do not support a significant role for CD AC3 in the regulation of renal Na(+) and water excretion in general or vasopressin regulation of CD function in particular.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Effect of flow and stretch on the [Ca2+]i response of principal and intercalated cells in cortical collecting duct

An acute increase in tubular fluid flow rate in the microperfused cortical collecting duct (CCD), associated with a approximately 20% increase in tubular diameter, leads to an increase in intracellular Ca2+ concentration ([Ca2+]i)in both principal and intercalated cells (Woda CB, Leite M Jr, Rohatgi R, and Satlin LM. Am J Physiol Renal Physiol 283: F437-F446, 2002). The apical cilium present in principal but not intercalated cells has been proposed to be a flow sensor. To determine whether flow across the cilium and/or epithelial stretch mediates the [Ca2+]i response, CCDs from New Zealand White rabbits were microperfused in vitro, split-open (to isolate the effect of flow across cilia), or occluded (to examine the effect of stretch and duration/magnitude of the flow impulse), and [Ca2+]i was measured using fura 2. In perfused and occluded CCDs, a rapid (<1 s) but not slow (>3 min) increase in luminal flow rate and/or circumferential stretch led to an approximately threefold increase in [Ca2+]i in both principal and intercalated cells within approximately 10 s. This response was mediated by external Ca2+ entry and inositol 1,4,5-trisphosphate-mediated release of cell Ca2+ stores. In split-open CCDs, an increase in superfusate flow led to an approximately twofold increase in [Ca2+]i in both cell types within approximately 30 s. These experimental findings are interpreted using mathematical models to predict the fluid stress on the apical membranes of the CCD and the forces and torques on and deformation of the cilia. We conclude that rapid increases in luminal flow rate and circumferential stretch, leading to shear or hydrodynamic impulses at the cilium or apical membrane, lead to increases in [Ca2+]i in both principal and intercalated cells.

A combined SEM and TEM study on the basal labyrinth of the collecting duct in the rat kidney

The three-dimensional ultrastructure of principal cells in the rat renal collecting duct was studied by scanning electron microscopy (SEM) using the NaOH digestion technique and the aldehyde prefixosmium-DMSO-osmium method, as well as by transmission electron microscopy (TEM). Special reference was given to the basal labyrinth of the cells and its numerous ruffles and infoldings of the basal plasma membrane. Observations showed that, as the collecting duct descends from the cortical collecting duct (CCD) to the terminal portion of the inner medullary collecting duct (t-IMCD), the pattern of the labyrinth gradually simplified and the ruffles grew thinner. In the CCD, the labyrinth was conspicuously complicated in structure, being formed of tightly arranged ruffles of uniform shape and thickness (70 nm). From the outer medullary collecting duct (OMCD) to the initial portion of inner medullary collecting duct (i-IMCD), the labyrinth became less complicated due to the mingling of wide flattened ruffles. Also, the basal infoldings were reduced in depth (from 700 nm in CCD to 500-600 nm in i-IMCD). In the t-IMCD, the labyrinth was rudimental, and instead presented small grooves (300 nm in depth) which corresponded to indentions of the basal plasma membrane. The regional simplification of the labyrinth was accompanied by morphological changes in mitochondria suggesting their functional decline: the electron density and number of cristae were reduced, these being changed in shape from plate-like to vesicular. These morphological data readily account for the potential for active transport by the collecting duct, which is highest in the CCD and is decreased towards the t-IMCD, and which may function merely as an excretory duct of urine from the papilla. The present study three-dimensionally demonstrates fine-structural heterogeneity in different segments of the collecting duct of the rat kidney.

A mathematical model of rat cortical collecting duct: determinants of the transtubular potassium gradient

In assessing disorders of potassium excretion, urine composition is used to calculate the transtubular gradient (TTKG), as an estimate of tubule fluid concentration, at a point when the fluid was last isotonic to plasma, namely, within the cortical collecting duct (CCD). A mathematical model of the CCD has been developed, consisting of principal cells and α- and β-intercalated cells, and which includes Na+, K+, Cl−, HCO[Formula: see text], CO2, H2CO3, phosphate, ammonia, and urea. Parameters have been selected to achieve fluxes and permeabilities compatible with data obtained from perfusion studies of rat CCD under the influence of both antidiuretic hormone and mineralocorticoid. Both epithelial (flat sheet) and tubule models have been configured, and model calculations have focused on the determinants of the TTKG. Using the epithelial model, luminal K+ concentrations can be computed at which K+secretion ceases (0-flux equilibrium), and this luminal concentration derives from the magnitude of principal cell peritubular uptake of K+ via the Na-K-ATPase, relative to principal cell peritubular membrane K+ permeability. When the model is configured as a tubule and examined in the context of conditions in vivo, osmotic equilibration of luminal fluid produces a doubling of the initial K+ concentration, which, depending on delivered load, may be substantially greater than the zero-flux equilibrium value. Under such circumstances, the CCD will be a site for K+ reabsorption, although the relatively low permeability ensures that this reabsorptive flux is likely to be small. Osmotic equilibration may also raise luminal NH3 concentrations well above those in cortical blood. In this situation, diffusive reabsorption of NH3 provides a mechanism for base reclamation without the metabolic cost of active proton secretion.

A mathematical model of the outer medullary collecting duct of the rat

A mathematical model of the outer medullary collecting duct (OMCD) has been developed, consisting of alpha-intercalated cells and a paracellular pathway, and which includes Na(+), K(+), Cl(-), HCO(3)(-), CO(2), H(2)CO(3), phosphate, ammonia, and urea. Proton secretion across the luminal cell membrane is mediated by both H(+)-ATPase and H-K-ATPase, with fluxes through the H-K-ATPase given by a previously developed kinetic model (Weinstein AM. Am J Physiol Renal Physiol 274: F856-F867, 1998). The flux across each ATPase is substantial, and variation in abundance of either pump can be used to control OMCD proton secretion. In comparison with the H(+)-ATPase, flux through the H-K-ATPase is relatively insensitive to changes in lumen pH, so as luminal acidification proceeds, proton secretion shifts toward this pathway. Peritubular HCO(3)(-) exit is via a conductive pathway and via the Cl(-)/HCO(3)(-) exchanger, AE1. To represent AE1, a kinetic model has been developed based on transport studies obtained at 38 degrees C in red blood cells. (Gasbjerg PK, Knauf PA, and Brahm J. J Gen Physiol 108: 565-575, 1996; Knauf PA, Gasbjerg PK, and Brahm J. J Gen Physiol 108: 577-589, 1996). Model calculations indicate that if all of the chloride entry via AE1 recycles across a peritubular chloride channel and if this channel is anything other than highly selective for chloride, then it should conduct a substantial fraction of the bicarbonate exit. Since both luminal membrane proton pumps are sensitive to small changes in cytosolic pH, variation in density of either AE1 or peritubular anion conductance can modulate OMCD proton secretory rate. With respect to the OMCD in situ, available buffer is predicted to be abundant, including delivered HCO(3)(-) and HPO(4)(2-), as well as peritubular NH(3). Thus, buffer availability is unlikely to exert a regulatory role in total proton secretion by this tubule segment.

A mathematical model of the inner medullary collecting duct of the rat: pathways for Na and K transport

A mathematical model of the inner medullary collecting duct (IMCD) of the rat has been developed representing Na+, K+, Cl-, HCO3-, CO2, H2CO3, phosphate, ammonia, and urea. Novel model features include: finite rates of hydration of CO2, a kinetic representation of the H-K-ATPase within the luminal cell membrane, cellular osmolytes that are regulated in defense of cell volume, and the repeated coalescing of IMCD tubule segments to yield the ducts of Bellini. Model transport is such that when entering Na+ is 4% of filtered Na+, approximately 75% of this load is reabsorbed. This requirement renders the area-specific transport rate for Na+ comparable to that for proximal tubule. With respect to the luminal membrane, there is experimental evidence for both NaCl cotransport and an Na+ channel in parallel. The experimental constraints that transepithelial potential difference is small and that the fractional apical resistance is greater than 85% mandate that more than 75% of luminal Na+ entry be electrically silent. When Na+ delivery is limited, an NaCl cotransporter can be effective at reducing luminal Na+ concentration to the observed low urinary values. Given the rate of transcellular Na+ reabsorption, there is necessarily a high rate of peritubular K+ recycling; also, given the lower bound on luminal membrane Cl- reabsorption, substantial peritubular Cl- flux must be present. Thus, if realistic limits on cell membrane electrical resistance are observed, then this model predicts a requirement for peritubular electroneutral KCl exit.

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.

Renal tubular differentiation in mouse and mouse metanephric culture. I. Ultrastructural studies

We studied the morphologic features of renal tubular differentiation in transfilter metanephric culture. Differentiation of portions of the S-shaped loop into proximal convoluted tubule was detected shortly after 72 h of culture by the appearance of microvilli, coated membrane invaginations, and an apical vacuolar-microtubular network. These features developed synchronously, and the microvilli became progressively more numerous and more compact to form a brush border. Computer-assisted morphometric analysis showed only minor differences between proximal tubular cells from 18-day embryos and tubular cells from 7-day cultures of blastema taken from 11-day embryos. Segments of tubule corresponding to distal convoluted tubules were lined with relatively simple cells that contained few differentiating characteristics. Morphometric modeling of the tubular cells indicated a simple shape consistent with an inherent transport function.

Relationship between structure and function in renal proximal tubule

Epithelia which support large transepithelial fluid movements are generally found to have histologic specializations which increase the surface areas of the cell membranes across which flows occur. A relationship between structure and function seems obvious in those cases. On the other hand, the area increasing specializations may also result in complicated shapes for the cells and their adjacent intercellular channels. In this paper we review the means for examining cell shape by quantitative stereologic techniques and the results obtained for the epithelium of the proximal renal tubule. We conclude that cell shape not only is a critical ingredient in any structure-function correlation for that tissue but also a "fingerprint" and a powerful tool with which one can predict and study epithelial absorptive flows and their driving forces.

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.

Isolation by monoclonal antibody of intercalated cells of rabbit kidney

We produced a monoclonal antibody, gamma G6, that reacts only with one cell type in the connecting (CNT) and collecting tubules (CT) of rabbit kidney. The gamma G6 antibody-reactive cells revealed carbonic anhydrase activity, showing one of the characteristics of intercalated (IC) cells. Using immunoelectron microscopy, we demonstrated that IC cells in cortical CT consist of the gamma G6 antibody-reactive and non-reactive cells, whereas all IC cells in medullary CT were reactive with the gamma G6 antibody. We used a cell sorter to enrich this cell type from the isolated kidney cell suspension. When we measured hormone-sensitive adenylate cyclase (ACase) activities of the sorted cells, the presence of parathyroid hormone (PTH) and isoproterenol (ISO) almost doubled ACase activities when compared with the basal values; however, no additive effect of PTH and ISO was observed. They showed no calcitonin-sensitive ACase and negligible arginine vasopressin (AVP)-sensitive ACase. We suggest that the IC cells recognized by the gamma G6 monoclonal antibody possess a receptor(s) for PTH and/or ISO but not for AVP in the CNT and CT, although it remains to be clarified whether the reactivities to PTH and ISO in these cells originate from single or dual cells.

Postnatal maturation of the rabbit cortical collecting duct

The mature, fully differentiated cortical collecting duct plays a major role in the final renal regulation of Na+, K+ and H+ transport. To characterize the growth of this segment, we measured the outer diameter and the dry weight of cortical collecting ducts isolated from newborn, 1-month-old, and adult rabbits. During the 1st month of life no significant changes were observed; however, there was a 60% increase in both parameters after the 4th week of life. Growth-related accretion of K+ was demonstrated by showing tubular K+ content to increase by 60% with maturation. Concomitant with the increase in tubular size, total cell number per millimeter of tubular length rose by 30%. Approximately 50% of the observed increment in tubular size could be accounted for by cell hyperplasia, with the remaining increase resulting from cell hypertrophy. Hypertrophy of principal cells was confirmed by scanning electron microscopy, which demonstrated a doubling of the circumferential width without any change in longitudinal length. Hyperplasia was confirmed, using a fluorescent chromatin stain, by our finding of a mitotic frequency of 3/1000 cells in the neonatal mid-cortical collecting duct; the observed number of mitoses was 10-fold higher at the most cortical end (ampulla). The number of intercalated cells per millimeter of tubule length, identified by bright green fluorescence after cortical collecting ducts were stained with 6-carboxyfluorescein diacetate, was found to double during maturation, the increase being significant only after the 4th postnatal week. We conclude that maturation of the mid-cortical collecting duct results from both cellular hyperplasia and hypertrophy. It is unlikely that this segment plays a major role in regulating Na+, K+, and H+ transport in the neonatal kidney.

Cell specificity of vasopressin binding in renal collecting duct: computer-enhanced imaging of a fluorescent hormone analog

A noninvasive microscopic method was used to assess the cell specificity of vasopressin binding within the heterogeneous collecting duct. The binding of a fluorescent vasopressin analog (1-desamino-8-rhodamine-L-lysine vasopressin) to cells of the microperfused rabbit cortical collecting tubule was visualized and quantitated with image-intensified video microscopy and digital image processing. Binding to the basolateral membranes of a subpopulation of cells could be detected within 1-2 min of addition of the fluorescent analog (10 nM) to the peritubular bath. Binding could be prevented or reversed by the addition of a 10-fold excess of the native hormone, which indicates that the fluorescent analog binds specifically to vasopressin receptors. The time course of binding paralleled and slightly preceded hyperpolarization of the lumen-negative transepithelial voltage, an electrical response that is also elicited by the native hormone. Double-label experiments in which the intercalated cell population was stained with fluorescein-labeled peanut lectin revealed that binding of the vasopressin analog was localized to the remaining cell type, the principal cell. Our results support the following conclusions. First, the principal cell constitutes the primary target cell for vasopressin in the rabbit cortical collecting tubule, although the intercalated cell may possess a limited number of receptors at a density below the detection limit of this optical approach. Second, computer-enhanced video microscopy is a powerful, noninvasive method for assessing the kinetics and spatial pattern of hormone binding.

Structural and functional comparison of mesonephric and metanephric proximal tubules

Mesonephric kidneys of the mid-gestation fetus and metanephric kidneys of the adult are known to differ in many gross features but to be qualitatively similar in their cellular morphology. In the present studies, quantitative morphometric and in vitro microperfusion procedures were used to examine further the physical and transport properties of mesonephric and metanephric (S2) proximal tubule segments from rabbit. In perfused mesonephric proximal tubules the volume transport rate was 0.91 +/- 0.08 nl/min per mm tubule length, 2.7 +/- 0.2 nl/min per mm2 basement membrane (outer tubule) surface area, and 0.19 +/- 0.03 nl/min per mm2 basolateral cell membrane surface areas. For comparison, the S2 portion of metanephric proximal tubules is reported to transport 0.33 +/- 0.02 to 0.53 +/- 0.01 nl/min per mm length or, based on the present findings, 2.8 +/- 0.2 to 4.4 +/- 0.8 nl/min per mm2 basement membrane area and 0.16 +/- 0.02 to 0.25 +/- 0.07 nl/min per mm2 basolateral membrane area. Thus, proximal mesonephric and S2 metanephric segments exhibit approximately the same transport per unit area of basement membrane and per unit area of the basolateral cell membranes across which active transport presumably occurs.

Cell membrane water permeability of rabbit cortical collecting duct

The water permeability (Posm) of the cell membranes of isolated perfused rabbit cortical collecting ducts was measured by quantitative light microscopy. Water permeability of the basolateral membrane, corrected for surface area, was 66 microns X sec-1 for principal cells and 62.3 microns X sec-1 for intercalated cells. Apical membrane Posm values corrected for surface area, were 19.2 and 25 microns X sec-1 for principal and intercalated cells, respectively, in the absence of antidiuretic hormone (ADH). Principal and intercalated cells both responded to ADH by increasing Posm of their apical membranes to 92.2 and 86.2 microns X sec-1, respectively. The ratio of the total basolateral cell membrane osmotic water permeability to that of the apical cell membrane was approximately 27:1 in the absence of ADH and approximately 7:1 in the presence of the hormone for both cell types. This asymmetry in water permeability is most likely due to the fact that basolateral membrane surface area is at least 7 to 8 times greater than that of the apical membrane. Both cell types exhibited volume regulatory decrease when exposed to dilute serosal bathing solutions. Upon exposure to a hyperosmotic serosal bath (390 mosM), principal cells did not volume regulate while two physiologically distinct groups of intercalated cells were observed. One group of intercalated cells failed to volume regulate; the second group showed almost complete volume regulatory increase behavior.

Apical and basolateral endocytosis in Madin-Darby canine kidney (MDCK) cells grown on nitrocellulose filters

Madin-Darby canine kidney (MDCK) cells (strain I) grown on 0.45 micron pore size nitrocellulose filters formed monolayers which were highly polarized and had high transepithelial electrical resistance (greater than 3000 ohm X cm2). Morphometric analysis showed that the area of the basolateral surface domain was 7.6 times larger than that of the apical. The uptake of fluid-phase markers [3H]inulin and horseradish peroxidase (HRP) was studied from the apical and the basal side of the monolayer. Uptake of [3H]inulin was biphasic and the rate during the first 40 min corresponded to a fluid phase uptake of 20.5 X 10(-8) nl/min per cell from the basolateral side, and 1.0 X 10(-8) nl/min per cell from the apical side. Electron micrographs of the monolayers after HRP uptake showed that the marker was rapidly delivered into endosome-like vesicles and into multivesicular bodies. No labelling of the Golgi complex could be observed during 2 h of uptake. Evidence was obtained for the transport of fluid phase markers across the cell. HRP and fluorescein isothiocyanate-dextran crossed the monolayers in either direction at a rate corresponding to approximately 3 X 10(-8) nl of fluid/min/cell. Adding the transcytosis rate to the rate of fluid accumulation into the cell yielded a total basolateral endocytic rate which was 6-fold greater than the apical rate. When the uptake rates were normalized for membrane area the apical and basolateral endocytic rates were about equal per unit cell surface area.

Scanning electron microscopy of basolateral surfaces of rat renal tubules isolated by sequential digestion

Renal tubular cells and segments isolated by a trypsin, pepsin, pronase E digestion procedure were studied with scanning electron microscopy. The basal and lateral surfaces of S1, S2, S3 proximal tubular (PT) segments, descending and ascending thin limbs of Henle (TL), distal ascending thick limb of Henle, or distal straight tubule (DST) and distal convoluted tubule (DCT) segments, connecting tubules (CNT), and collecting ducts (CD) were identified and characterized. The basal processes of the S1 and S2 PT cells were fan shaped, were oriented in a circumferential direction, and terminated in microvilli at the basement membrane. S3 PT cells had microvillous basal processes mainly on the lateral edges of the cells. The basal processes of DST and DCT were similar to PT in orientation but terminated on the basement membrane with flattened, thin attachments. The long-loop descending TL and the ascending TL exhibited distinctive interdigitating cell processes. TL segments with simple contours were present in smaller numbers and were characteristic of short-loop descending limbs. CNT showed some cells with basal surfaces resembling DCT cells and others resembling CD cells. Both cortical and medullary CD segments exhibited intercalated cells with round basal contours and a sparse pattern of basal infolding clefts. The cortical CD principal cells revealed a much more elaborate mosaic of plicae, clefts, and microvilli than those of the medullary CD. These observations extend the previous knowledge gained from transmission electron microscopy and assist in the interpretation of that knowledge.

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.

The pig mesonephros. III. Distal tubule, collecting tubule, and Wolffian duct: SEM- and TEM-studies

The ultrastructure of the distal and collecting tubules of mature pig mesonephroi (41st gestational day) was studied in perfusion-fixed embryos. In the distal tubule, the three subsegments postulated on the basis of enzyme histochemistry show only minor differences of their luminal surfaces, mostly of cell size. TEM photographs reveal a single cell type with interdigitating basolateral processes, frequently flattened to 30-120 nm lamellae devoid of organelles. Larger interdigitating processes harbor vertically oriented mitochondria in the form of indented plates. The macula densa cells are small, do not interdigitate, and have distended intercellular spaces. The collecting tubule starts with a dorsal convolution, in which intercalated cells (with apical microfolds and numerous mitochondria) occur in addition to interdigitating cells. Further down this segment, the interdigitating cells are gradually replaced by principal cells characterized by interlocking lateral microvilli, basal infoldings, and relatively few organelles. Intercalated cells extend into the Wolffian duct. Although the pig mesonephros has the most differentiated nephron of the mammals studied so far, with metanephros-like cells, its intrinsic urinary concentrating capacity appears to be low in view of its vascularization pattern and nephron architecture.

Electrophysiological properties of cellular and paracellular conductive pathways of the rabbit cortical collecting duct

Microelectrode techniques were applied to the rabbit isolated perfused cortical collecting duct to provide an initial quantitation and characterization of the cell membrane and tight junction conductances. Initial studies demonstrated that the fractional resistance (ratio of the resistance of the apical cell membrane to the sum of the resistances of the apical and basolateral membranes) was usually independent of the point along the tubule of microelectrode impalement--implicating little cell-to-cell coupling--supporting the application of quantitative techniques to the cortical collecting duct. It was demonstrated that in the presence of amiloride, either reduction in the luminal pH or the addition of barium to the perfusate selectively reduced the apical membrane potassium conductance. From the changes in Gte and fractional resistance upon reducing the luminal pH or addition of barium to the perfusate, the transepithelial, apical membrane, basolateral membrane and tight junction conductances were estimated to be 9.3, 6.7, 8.1 and 6.0 mS cm-2, respectively. Ninety to ninety-five percent of the apical membrane conductance reflected the barium-sensitive potassium conductance in the presence of amiloride with an estimated potassium permeability of 1.1 X 10(-4) cm sec-1. Reduction in the perfusate pH to 4.0 caused a 70% decrease in the apical membrane potassium conductance, implying a blocking site with an acidic group having a pKa near 4.4. It is concluded that both the transcellular and paracellular pathways of the cortical collecting tubule have high ionic conductances, and that the apical membrane conductance primarily reflects a high potassium conductance. Furthermore, both reduction in the perfusate pH and addition of barium to the perfusate selectively block the apical potassium channels, although the site of inhibition likely differs since the two ions display markedly different voltage-dependent blocks of the channel.

Quantitative analysis of the structural events associated with antidiuretic hormone-induced volume reabsorption in the rabbit cortical collecting tubule

We quantitatively examined the influence of antidiuretic hormone (ADH)-dependent volume reabsorption on the morphology of the rabbit cortical collecting tubule. Estimates of cell volume and the geometry of the lateral intercellular spaces were extracted from differential interference contrast images of perfused nephron segments using the morphometric procedures described in the preceding paper (K.L. Kirk , D.R. DiBona and J.A. Schafer, J. Membrane Biol. 79:53-64, 1984). The results indicate that ADH addition in the presence, but not absence, of a lumen-to-bath osmotic gradient (130 to 290 mOsm) stimulated transepithelial volume flow and simultaneously increased the volumes of both the cells (+28%) and the lateral intercellular spaces (+78%). In addition, the formation of cytoplasmic vacuoles could be observed during the latter stages of the swelling response, and vacuole formation continued well after new steady-state values for transepithelial water flow and cell volume had been reached. Two main conclusions can be drawn from these results. First, the cytoplasmic vacuoles comprise a slowly filling compartment that lies in parallel to the transepithelial pathway for ADH-stimulated volume reabsorption. Second, from the magnitude of the cell volume increase, we estimate that the hydraulic conductivities of the opposing cell membranes are nearly equal during maximal ADH stimulation.

Morphologic response of the rabbit cortical collecting tubule to peritubular hypotonicity: quantitative examination with differential interference contrast microscopy

The isolated and perfused cortical collecting tubule of the rabbit was examined by differential interference contrast microscopy in order to characterize the morphologic response of this nephron segment to peritubular hypotonicity. Computer-assisted, morphometric procedures were developed to obtain measurements of cell volume and lateral intercellular space geometry from interference contrast images of perfused nephron segments. Following dilution of the bath from 290 to 190 mOsm in the absence of antidiuretic hormone (T = 25 degrees C), the cells swelled rapidly to a new steady-state volume which was maintained for at least 20 to 30 min and which was about 90% of that predicted for ideal osmometric behavior. The increase in cell volume was accomplished entirely by bulging of the cells into the lumen; lateral space width and outside tubule diameter were unaffected by peritubular hypotonicity. In addition, the swelling of the cells was associated with an apparent swelling of intracellular organelles, e.g., nuclei and mitochondria. Our results indicate that cells of the mammalian collecting tubule swell without the capacity for significant volume regulation at 25 degrees C and without the cytoplasmic vacuolation and dilation of the lateral intercellular spaces observed following the onset of antidiuretic hormone-dependent volume reabsorption (E. Ganote , J. Grantham , H. Moses, M. Burg and J. Orloff , J. Cell Biol. 36:355, 1968).

Morphometric comparison of rabbit cortical connecting tubules and collecting ducts

Connecting tubule (CNT) segments of the rabbit distal nephron were examined by scanning electron microscopy and computer-assisted morphometric analysis of transmission electron micrographs. CNT were very similar to the cortical collecting ducts (CCD) described previously. The epithelium of both segments contains two cell types, both of which can be modeled as simple cuboidal cells, and two distinct systems of extracellular channels. The lateral intercellular channels are comparable to the spaces between simple cuboidal cells but are modified by short projecting microvilli which produce a modest increase in lateral cell surface area. The basal infolded channels are best developed in the connecting tubule cells of CNT and contribute 63% of all channel-associated membranes in CNT. Total membrane areas are similar in CNT and CCD. The two segments differ only in the degree of extracellular channel dilation and the distribution of infolded membrane relative to cell height in the connecting tubule and principal cells. The relatively minor morphometric differences between CNT and CCD do not correlate well with the marked difference in transtubular volume flow induced in the two segments by ADH and an osmotic gradient.

Histotopography and ultrastructure of the thin limbs of the loop of Henle in the hamster

In the kidney of the Syrian hamster the descending thin limbs of both the short and long loops of Henle are not spatially separated from each other and descend between the vascular bundles. Ultrastructurally, five different epithelial types are distinguished in the thin limbs of the short and long loops of Henle. Short loops possess only a descending thin limb with a simply organized epithelium (type 1). Long loops comprise an upper and a lower part of the descending thin limb and the ascending thin limb. The upper part of the long descending thin limb is equipped with a complex and highly interdigitating epithelium with shallow junctions (type 2), which gradually transforms into the simple noninterdigitating type-3 epithelium of the lower part. In a minor portion of long descending thin limbs, however, the upper part begins with an even more complexly organized epithelium (type 2a) than type 2. Type-2a epithelium is conspicuously thicker and possesses a more elaborate mode of cellular interdigitation. Along the descent of this tubular part through the inner stripe of the outer medulla, type-2a epithelium transforms into type-2 epithelium. It is suggested that the long descending thin limbs, which start with type-2a epithelium, belong to the longest loops. The type-4 epithelium of the ascending thin limbs is characterized by flat and extensively interdigitating cells with shallow junctions. The unique pattern of the type-2a epithelium favors the assumption that solute secretion essentially contributes to the increase in concentration of tubular fluid in long descending thin limbs.

Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake. I. Structural changes

The baso-lateral cell-membrane area in kidney tubules appears to be associated with the capacity for electrolyte transport; in the rabbit, it decreases from the distal convoluted tubule (DCT-cells) over the connecting tubule (CNT-cells) to the cortical collecting duct (principal cells). Adaptation to low Na-, high K-intake changes this pattern: CNT-cells at the beginning of the connecting tubule have the highest membrane area, which decreases along the segment, but remains two-fold higher than in controls. Principal cells have a four-fold higher membrane area than in controls. Simultaneous treatment with the antimineralocorticoid canrenoate-K inhibits the structural changes in CNT-cells only in end-portions of the connecting tubule and in principal cells. After prolonged high Na-, low K-intake DCT-cells display a two-fold higher membrane area than controls, while CNT-cells and principal cells are not affected. Simultaneous treatment with DOCA does not affect the DCT-cells but provokes a moderate increase in membrane area in CNT-cells, and a 5.5-fold increase in principal cells. The data provide evidence that DCT-, CNT- and principal cells are functionally different cell types. The baso-lateral cell-membrane area, associated with electrolyte-transport capacity, appears to be influenced in DCT-cells mainly by Na-intake, in CNT-cells mainly by K-intake and in part also by mineralocorticoids, and in principal cells mainly by mineralocorticoids.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.