Aquaporin-1 water channels in short and long loop descending thin limbs and in descending vasa recta in rat kidney

The localization of aquaporin-1 water channels (AQP-1) in nephron and vascular structures in rat kidney were characterized, because vascular bundles are known to play a key role in urinary concentration. Immunohistochemistry and immunoelectron microscopy were applied on thin cryosections or ultrathin Lowicryl sections, using an optimized freeze-substitution method. Within the vascular bundles, AQP-1 is localized in descending thin limbs (DTL) of short nephrons in apical and basolateral membranes. The expression in DTL of short nephrons is considerably lower compared with the expression in long nephrons, consistent with the known lower osmotic water permeability of this segment. Furthermore, DTL of short nephrons expressing AQP-1 continue abruptly into a thin limb segment without AQP-1. This suggests the existence of a novel thin limb epithelium in the outer medulla. Extensive expression of AQP-1 is observed in apical and basolateral membranes of DTL of long nephrons, which are localized in the periphery of the vascular bundles. The expression decreases along the axis of long nephron DTLs in correlation with the known water permeability characteristics of thin limb segments. DTLs of both short and long nephrons continue abruptly into thin limb segments without AQP-1 expression, revealing an abrupt cell-to-cell transition. In vasa recta, AQP-1 is selectively localized in the nonfenestrated endothelium of descending vasa recta, whereas the fenestrated endothelium of ascending vesa recta and peritubular capillaries do not express AQP-1. AQP-1 is localized in both apical and basolateral plasma membranes, which is logical for transendothelial water transport. Isolated perfused descending vasa recta display high water permeability, and, unlike sodium permeability, diffusional water permeability is partly inhibited by mercurials, thus substantiating the presence of mercurial-sensitive water channels in descending vasa recta. Thus AQP-1 is localized in DTL and descending vasa recta within vascular bundles, and AQP-1 expression in DTL segments is in exact concordance with the known water permeability characteristics, strongly supporting that AQP-1 is the major constitutive water channel of the nephron.

Development of new nephrons in adult kidneys following gentamicin-induced nephrotoxicity

Gentamicin-induced nephrotoxicity results in necrosis of proximal tubular epithelial cells. In mammals, the injured nephron can be repaired by migration and division of surviving cells. We examined this repair process in the fresh-water goldfish, Carassius auratus. Fish were given a single intraperitoneal injection of 50 mg/kg gentamicin and sacrificed at 1, 4, 14, and 21 days. Injured nephrons showed regeneration along the basement membrane several days following gentamicin administration. In addition, 2-3 weeks following the injection, entirely new nephrons formed in the renal interstitium. Development of new nephrons in adult fish kidneys provides an excellent model for studying renal development and toxicity, and could provide insights into new therapies for chronic renal disease.

Effects of prostaglandin E2 on amiloride-blockable Na+ channels in a distal nephron cell line (A6)

We studied the mechanisms by which prostaglandin E2 (PGE2) regulates amiloride-blockable 4-pS Na+ channels in A6 distal nephron cells. With each apical cell-attached patch acting as its own control, acute (3-6 min) basolateral, but not apical, exposure to 1 microM PGE2 inhibited Na+ channel activity by decreasing the open probability (Po). This PGE2-induced inhibition was attenuated by 30 min pretreatment with the protein kinase C (PKC) antagonists 1 microM staurosporine or 100 microM D-sphingosine but was insensitive to pertussis toxin (PTX). Furthermore, the time course for channel inhibition by acute PGE2 correlated with a transient increase in intracellular inositol 1,4,5-trisphosphate (IP3) levels. In contrast, after chronic (10-50 min) exposure of A6 cells to 1 microM basolateral PGE2, channel activity was stimulated compared with controls. This stimulation was due to an increase in the number of apical Na+ channels, similar to the effect of maneuvers that increase intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels in A6 cells (22). Indeed, chronic exposure to basolateral PGE2 correlated with a sustained increase in cAMP levels. In conclusion, 1) the regulation of apical 4-pS highly selective Na+ channel activity by basolateral PGE2 is a complicated biphasic process, which includes inhibition by acute PGE2 and stimulation by chronic PGE2 exposure; 2) acute PGE2 promotes a transient generation of IP3 which activates Ca(2+)-dependent PKC and promotes a decrease in Po; 3) chronic PGE2 promotes a sustained generation of cAMP that leads to an increase in channel density; and 4) both the acute and chronic effects of PGE2 on Na+ channels are PTX-insensitive processes.

Antidiuretic hormone-responding nonselective cation channel in distal nephron epithelium (A6)

Arginine vasotocin (AVT, 70 mU/ml) added from the basolateral side transiently activated a nonselective cation (NSC) channel with a single-channel conductance of 28.5 pS and almost identical selectivity for Na+ and K+ in the apical membrane of distal nephron cells (A6) cultured on permeable supports for 10-12 days in media containing 10% fetal bovine serum without supplemental aldosterone. The open probability (Po) of the NSC channel at the apical resting membrane potential in cell-attached patches was approximately 0.09 and increased when the apical membrane depolarized. The Po of the NSC channel was decreased by a rise in cytosolic Ca2+ concentration within a range of 30 nM-1 microM but not affected by cytosolic pH within a range of 6-8. The channel was activated by the application of negative pressure (10-60 cmH2O) into the patch pipette. Gadolinium (2 microM), an inhibitor of stretch-activated channels, decreased the Po by 40%. This blocking action of gadolinium was more effective after the channel was activated by stretch, i.e., 2 microM gadolinium decreased the Po by 70% when a negative pressure (60 cmH2O) was applied into the patch pipette. Amiloride (10 and 100 microM) showed a blocking action on the channel only when the NSC channel was activated by stretch.

Expression of Na-P(i) cotransport in rat kidney: localization by RT-PCR and immunohistochemistry

We have recently identified a rat kidney cortex Na-dependent transport system for phosphate (P(i)) by expression cloning (NaP(i)-2) (S. Magagnin, A. Werner, D. Markovich, V. Sorribas, G. Stange, J. Biber, and H. Murer. Proc. Natl. Acad. Sci. USA 90: 5979, 1993). In this study we have used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry to establish the sites of expression of the NaP(i)-2-related mRNA and protein. RT-PCR was performed with single microdissected nephron segments. From these experiments we conclude that NaP(i)-2 mRNA is predominantly expressed in the proximal tubules of superficial and deep nephrons. No NaP(i)-2 mRNA was detected in the thick ascending limb of Henle's loop; however, faint NaP(i)-2 related PCR products were also observed in collecting ducts. Expression of the NaP(i)-2 protein was examined with the use of polyclonal antibodies raised against synthetic NaP(i)-2-derived peptides. Strong specific anti-NaP(i)-2 antiserum-mediated immunofluorescence was found in the convoluted part of proximal tubules and gradually decreased along the straight part. Immunofluorescence indicated that the NaP(i)-2 protein is present in the brush border of proximal tubular cells. In addition, NaP(i)-2-specific immunofluorescence was also observed in subapical vesicles. The described distribution of the NaP(i)-2 protein is in agreement with previously described nephron sites of P(i) reabsorption in the rat kidney and therefore suggests that the NaP(i)-2 transport system represents an Na-P(i) cotransporter involved in proximal tubular apical transport of phosphate.

Nephron segment and cell-type specific expression of gangliosides in the developing and adult kidney

Despite the increasing knowledge of the role of gangliosides in normal and diseased tissues, little is known of the presence, distribution and functions of these molecules in the kidney. In this study we analyzed the main gangliosides of isolated glomeruli and cortical, medullary and papillary fractions of the human, rat and bovine kidneys biochemically. In addition, we used immunohistochemistry to visualize the distribution of GM1/GM2, GD2, GD3 and O-acetyl GD3 gangliosides along the nephron. Furthermore, we explored the species specific expression of gangliosides by comparing those from the rat, bovine and human kidney, and studied the pattern of ganglioside expression during development. In glomeruli, cortical tubuli, medullae and papillae, a relatively simple pattern of main gangliosides was observed as revealed by thin layer chromatographic (TLC) analysis in each species studied. Furthermore, considerable changes in the glomerular gangliosides during maturation were observed, with a complex type of gangliosides predominating during the fetal age and with a preference to more simple precursors upon maturation. Interestingly, the immunohistochemical detection revealed a distinct pattern of ganglioside compartmentation to various nephron segments or cell types. These findings provide a basis for studying the role of segment- and cell type-specific gangliosides for local functions.

[Functional integration and modulation in the cellular physiology of the nephron]

Continuous modulation of the nephron cell functions is necessary for the water-salt homeostasis. Specifics of the temporal unit responses to various substances from the luminal and basolateral membranes are discussed as well as possible molecular mechanisms of antidiuretic hormone modulators' effect. A new approach to analysis of integration of activity of nephron cell's different components is regarded on example of organic acids secretion.

Histochemical analysis of carbohydrate moieties and sugar-specific acceptors in the kidneys of the laboratory mouse and the golden spiny mouse (Acomys russatus)

The aims of this work were to histochemically compare the pattern of lectin binding and endolectin expression in different portions of nephrons of two rodent species producing either normal hyperosmotic urine (the laboratory mouse) or highly concentrated urine (Acomys russatus, the golden spiny mouse). A panel of biotinylated lectins and neoglycoproteins and the avidin-biotin-peroxidase complex technique were used on Bouin's fixed, paraffin-embedded sections. Various segments of the uriniferous tubule in both species showed differential affinity for labelled lectins and neoglycoproteins. Significant differences were also evident between comparable tubular segments in laboratory and golden spiny mouse kidneys. Whether the histochemical expression of sugar moieties of glycoconjugates as well as endolectins, thus both sides of presumed protein-carbohydrate interactions, may be correlated to the various glycoproteins which would include constituents of the glycocalyx and domains of a variety of transport enzymes deserves further studies.

Heterogeneity of osteopontin expression among nephrons in mouse kidneys and enhanced expression in sclerotic glomeruli

BACKGROUND

Osteopontin (OPN) is a secreted Ca(2+)-binding phosphoprotein able to mediate cell attachment to bone via an RGD sequence and the alpha v beta 3 integrin. OPN mRNA is found at high levels in the kidney, and the protein is found in the urine. Because published reports of where the protein is produced conflict, we undertook a comprehensive study to localize OPN expression.

EXPERIMENTAL DESIGN

In situ hybridization with a mouse cDNA probe and immunohistochemical staining with three different antisera to mouse OPN were used to identify those cells that contained significant levels of mRNA and protein, respectively.

RESULTS

Both methods of analysis revealed that OPN expression in the normal mouse kidney was primarily restricted to the thick ascending limbs of the loop of Henle and to the distal convoluted tubules. Protein was detected predominantly at the apical surface of cells lining the lumen of a subset of tubules. The alpha v beta 3 integrin, which is a receptor for vitronectin and osteopontin, was uniformly localized by immunostaining not on the apical surface but rather to the baso-lateral surface of cells in the distal part of the tubule. OPN expression was not detected in healthy glomeruli, proximal tubules, thin limbs of the loop of Henle, collecting ducts, or interstitial fibroblasts. In contrast to the localization of Tamm-Horsfall protein expression, in all distal nephrons, expression of OPN was detected by both methods of analysis in only some nephrons. OPN expression (relative to male mice) was somewhat increased in female, pregnant and lactating mice and markedly increased in the parietal epithelium of glomeruli undergoing sclerosis in aging mice. OPN was also detected in the macula densa.

CONCLUSIONS

OPN is synthesized and secreted into the tubule fluid by the luminal epithelia of the distal portions of a subset of kidney nephrons. As animals age expression is found in more proximal portions of the tubule. OPN may contribute to, or be a consequence of, glomerular sclerosis, and may be an indicator of subclinical injury or infection.

How to build a kidney

Embryonic kidney is ideal for the study of epithelial morphogenesis--rich in developmental processes, accessible and clinically important. Interactions between mesenchyme and the epithelium invading it cause the epithelium to differentiate and branch to form the collecting ducts, and the mesenchyme to epithelialise into excretory nephrons. Molecular analyses reveal complex programmes of expression of transcription factors and morphoregulatory molecules, correlating with induction and morphogenesis. In developing nephrons especially, the detailed timing of this programme is now being explored and the functions of some molecules involved have been tested by inhibition experiments. New pharmacological methods of induction may also prove useful in solution of the mechanisms by which a kidney is formed.

Polarized membrane movements in A6 kidney cells are regulated by aldosterone and vasopressin/vasotocin

The polarity of cell-surface membrane movements and their regulation by adrenal steroid hormones (10(-6) M aldosterone) and vasopressin or vasotocin were studied in A6 cells. This cell line is derived from the Xenopus laevis distal nephron and displays regulated Na+ reabsorption but is devoid of regulated water transport. Apical and basolateral membrane movements and their hormonal regulation were characterized by measuring the uptake of the fluid phase marker horseradish peroxidase (HRP) and the secretion of proteins on both sides of cell monolayers cultured on filters. The intracellular accumulation of HRP was visualized by electron microscopy and quantified by the measure of cell-associated peroxidase activity. The rate of intracellular HRP accumulation corresponded to 0.01 nl/minute/filter (4.7 cm2) from the apical side and was 20-32 times faster from the basolateral side. In contrast, the level of protein secretion was 3.5 times higher apically than basolaterally. Among the secreted proteins some were found to be secreted essentially apically, and others basolaterally. Vasotocin increased apical endocytosis (1.88-fold) and apical protein secretion (1.49-fold) in cells pretreated with aldosterone. Basolaterally, only the endocytosis was increased, and to a smaller extent (1.36-fold). These effects of vasotocin depended on aldosterone pretreatment and could be mimicked with forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate (BrcAMP). Measurements of intracellular cAMP levels showed that there was a rankorder correlation between the induced level of intracellular cAMP and that of apical endocytosis. This study shows that vasotocin has a polarized stimulatory action on apical endocytosis and protein secretion in A6 cells, and that the mediation of this action by cAMP is aldosterone dependent.

Tyrosine kinase regulates epithelial sodium transport in A6 cells

Insulin increases epithelial Na+ reabsorption, and many of its actions involve tyrosine kinase. We used tyrosine kinase inhibitors to examine the role of tyrosine kinase in the action of insulin. Pretreatment of Na+ transporting cells with tyrosine kinase inhibitors attenuates the subsequent action of insulin, suggesting that the action of insulin on epithelial Na+ transport involves tyrosine kinase activity. In addition to their effect on insulin-induced Na+ transport, the tyrosine kinase inhibitors also significantly reduce Na+ transport in Na(+)-transporting epithelial cells, suggesting that there is a significant tonic tyrosine kinase activity that modulates epithelial Na+ transport. Using patch-clamp methods, we found that one inhibitor, genistein, reduces the number of active Na+ channels in cell-attached patches without significantly affecting the open probability of any remaining channels. The effects of the tyrosine kinase inhibitors are not due to inhibition of protein kinase A (PKA), since H89, a PKA inhibitor, does not affect Na+ transport of control cells (as the tyrosine kinase inhibitors do), and the tyrosine kinase inhibitor, genistein or tyrphostin 23, does not alter the stimulation of ion transport by 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate, a membrane-permeable adenosine 3',5'-cyclic monophosphate analogue (as H89 does).

Tubule-tubule and tubule-arteriole contacts in rat kidney distal nephrons. A morphologic study based on computer-assisted three-dimensional reconstructions

BACKGROUND

Functional investigations of the tubulo-glomerular feedback mechanism have indicated the existence of a contact between the distal nephron and the macula densa region. The structural justification of such a contact is investigated.

EXPERIMENTAL DESIGN

Tubule-tubule and tubule-arteriole contacts were investigated in distal nephrons from normal rat kidneys. Computer-assisted three-dimensional reconstructions of distal nephrons were made from serial sections of renal cortical tissue and selected sections were examined by electron microscopy.

RESULTS

In 14 of 15 reconstructed nephrons, the distal convoluted tubule or the connecting tubule approached the macula densa region. A wall-to-wall contact between two tubules corresponding to a three-dimensional distance below 28 microns between the axes of the two tubules was found in only five of the reconstructed tubules. The distal nephron contacts to afferent and efferent arterioles of the same nephron were also examined. The efferent arteriole revealed no consistent contacts but the afferent arteriole contacted the distal convoluted tubule/connecting tubules consistently in all 10 of the superficial nephrons and in 3 of 5 midcortical nephrons. Electron microscopy confirmed a close contact between the distal tubule and the afferent arteriole in superficial nephrons and small nerves were often found at or near the site of contact, but the morphology at the site of contact was not unique. The arteriole contacts were made with late distal convoluted tubules, connecting tubules, or cortical collecting ducts.

CONCLUSIONS

In conclusion, the present study shows that tubule-tubule contacts are inconsistent between the macula densa region and the distal nephron but that the tubule-afferent arteriole contact is consistent and close in superficial nephrons. This morphology is compatible with the existence of a feedback mechanism between the superficial distal nephron and the afferent arteriole, apart from the one located at the juxtaglomerular apparatus.

[Usefulness of microdissection of nephron segments and fluorescent indicator for molecular biological studies of nephron functions]

The kidney consists of numerous functional units called nephrons. Thus, the use of individual nephron segments is essential to characterize their functional properties and to clarify the molecular basis of site-specific functions. Nephron segments can be microdissected from collagenase-treated renal slices under a stereomicroscope. A variety of intracellular ionic concentrations or membrane potential can be determined with various fluorescent probes. Fura-2/AM-loaded nephron segments reveal a transient increase of cytosolic free calcium concentrations by agonists such as angiotensin II, vasopressin, kinins, etc. To localize their receptors or to characterize their subtypes, this technique is especially beneficial, because tiny fragments of the nephron are sufficient by combination with a two-wave length microscope fluorometer. As an example, discovery of a novel vasopressin receptor (Vp) is described.

Na+/H(+)-exchange in A6 cells: polarity and vasopressin regulation

We have analyzed the mechanism of Na(+)-dependent pHi recovery from an acid load in A6 cells (an amphibian distal nephron cell line) by using the intracellular pH indicator 2'7'-bis(2-carboxyethyl)5,6 carboxyfluorescein (BCECF) and single cell microspectrofluorometry. A6 cells were found to express Na+/H(+)-exchange activity only on the basolateral membrane: Na+/H(+)-exchange activity follows simple saturation kinetics with an apparent Km for Na+ of approximately 11 mM; it is inhibited in a competitive manner by ethylisopropylamiloride (EIPA). This Na+/H(+)-exchange activity is inhibited by pharmacological activation of protein kinase A (PKA) as well as of protein kinase C (PKC). Addition of arginine vasopressin (AVP) either at low (subnanomolar) or at high (micromolar) concentrations inhibits Na+/H(+)-exchange activity; AVP stimulates IP3 production at low concentrations, whereas much higher concentrations are required to stimulate cAMP formation. These findings suggest that in A6 cells (i) Na+/H(+)-exchange is located in the basolateral membrane and (ii) PKC activation (heralded by IP3 turnover) is likely to be the mediator of AVP action at low AVP concentrations.

Mechanism of PGE2-induced cell swelling in distal nephron segments

The effects of prostaglandin (PG) E2 on cell swelling were studied in isolated perfused tubules of rabbit kidney. PGE2 (1 microM) added to the bath induced cell swelling by 13.4, 7.2, and 9.6% in the connecting tubule, distal convoluted tubule, and cortical collecting duct, respectively, but it had no effect on the proximal convoluted tubule and cortical thick ascending limb. The response was dose dependent in the range of 1 nM to 1 microM. PGI2 exerted a similar effect, but PGF2 alpha had no effect. The swelling was completely blocked by basolateral Na+ removal and was attenuated by bilateral Cl- removal, suggesting that the swelling was mediated by basolateral Na+ entry in association with Cl- entry. In all segments except proximal tubule, PGE2 caused an initial transient peak followed by a sustained increase in intracellular Ca2+. Intracellular Ca2+ chelation or inhibition of Ca2+ release from intracellular stores abolished the PGE2-induced cell swelling, but extracellular Ca2+ removal did not. An inhibitor of the Na(+)-Ca2+ exchanger (3',4'-dichlorobenzamil, 100 microM) in the bath completely inhibited PGE2-induced cell swelling. Neither furosemide (1 mM) nor amiloride (1 mM) added to bath abolished the response, indicating that neither Na(+)-K(+)-2Cl- cotransport nor Na(+)-H+ exchange is involved in the action of PGE2. The swelling response to PGE2 was observed even in the presence of ouabain, indicating that the effect of PGE2 is independent of Na(+)-K(+)-adenosinetriphosphatase inhibition. Nicardipine added to bath partially inhibited the swelling response.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin activates single amiloride-blockable Na channels in a distal nephron cell line (A6)

Using the patch-clamp technique, we studied the effect of insulin on an amiloride-blockable Na channel in the apical membrane of a distal nephron cell line (A6) cultured on permeable collagen films for 10-14 days. NPo (N, number of channels per patch membrane; Po, average value of open probability of individual channels in the patch) under baseline conditions was 0.88 +/- 0.12 (SE)(n = 17). After making cell-attached patches on the apical membrane which contained Na channels, insulin (1 mU/ml) was applied to the serosal bath. While maintaining the cell-attached patch, NPo significantly increased to 1.48 +/- 0.19 (n = 17; P less than 0.001) after 5-10 min of insulin application. The open probability of Na channels was 0.39 +/- 0.01 (n = 38) under baseline condition, and increased to 0.66 +/- 0.03 (n = 38, P less than 0.001) after addition of insulin. The baseline single-channel conductance was 4pS, and neither the single-channel conductance nor the current-voltage relationship was significantly changed by insulin. These results indicate that insulin increases Na absorption in the distal nephron by increasing the open probability of the amiloride-blockable Na channel.

Compartmentation of multiple forms of creatine kinase in the distal nephron of the rat kidney

Creatine kinase enzymes are present in tissues such as muscle and brain to interconvert creatine phosphate and ADP, thus providing a system to interconnect energy production and utilization (Bessman, S. P., and Carpenter, C. L. (1985) Annu. Rev. Biochem. 54, 831-862). Creatine kinase isoenzymes in kidney have received little attention since kidney contains relatively low creatine kinase activity compared with muscle and brain and because there is disagreement regarding the identity of the specific isoforms expressed in kidney. Using a combination of chromatographic and immunological techniques, we have identified two isoforms of creatine kinase in rat kidney supernatants, B creatine kinase, and the non-sarcomeric form of the mitochondrial creatine kinase, which represent 82 and 15%, respectively, of the total creatine kinase activity in this tissue. The identity of the non-muscle form of the mitochondrial creatine kinase was confirmed by N-terminal sequence analysis and compared with recently published cDNA sequences (Haas, R. C., and Strauss, A. W. (1990) J. Biol. Chem. 265, 6921-6927). We prepared multiple antisera specific for each isoform using synthetic peptide immunogens based upon nonhomologous regions from the primary sequence of each creatine kinase isoform. Immunocytochemical results demonstrate that both creatine kinase isoforms are colocalized in the inner stripe of the outer medulla in tubules of the distal nephron. A similar distribution of creatine kinase isoforms was obtained when different layers of the renal cortex and medulla were examined for creatine kinase activity and isozyme content using nondenaturing electrophoresis. In general, the distribution of creatine kinase enzymes in kidney corresponds to the regions of greatest ATP utilization, oxygen consumption, and sodium transport. These results suggest a role for creatine kinase enzymes in the coupling of ion transport and oxidative phosphorylation in the distal nephron of the mammalian kidney.

Streaming kidney

Twenty-five male young adult random bred rats, weighing 250-300 g, were injected with 18.5 kBq/g body weight tritiated thymidine (specific activity 185 GBq/mM). The rats were then killed in groups of five, at the following times: 1 h and 14, 30, 60 and 120 days. The kidneys were processed histologically and dipped into liquid emulsion, exposed for 3 weeks and developed. Kinetic measurements were restricted to juxta-medullary glomeruli, their adjacent convoluted tubules and medullary tubuli. All other nephrons were ignored. The medullary pole of the juxta-medullary glomerulus served as reference point for all measurements and was referred to as the origin. The distance of a labelled cell from the origin was measured with an eye-piece micrometer and expressed in terms of two units: distance (microns) and cell location, defined as the number of cells separating a labelled cell from origin. Since only medullary nephrons were considered, these measurements represent distances directed toward the papilla. One hour after labelling, most cells were in the vicinity of the juxta-medullary glomerulus, not further than 800 microns from the origin in the direction of the papilla. During the following days labelled cells advanced toward the papilla at a daily velocity of 13.8 microns, covering 1.1 locations/day. Kinetically, the juxta-medullary nephron is a two-compartment cell renewal system. Its compartments, the progenitor (P) and the functional (Q), cover locations 0-80 and 81-150, respectively. The first feeds the second with cells. Morphologically, the progenitor compartment includes proximal and distal convoluted epithelia and a part of thick straight tubules. Other nephron portions belong to the Q-compartment. It is assumed that the nephrocyte is a part of a cell stream directed toward the papilla, which probably also includes stroma and vasculature.

Characterization and localization of ouabain-insensitive Na-dependent ATPase activities along the rat nephron

Single segments of rat nephron contain two distinct ouabain-insensitive, K-independent, Na-dependent ATPase activities: a Na-stimulated ATPase and a Na-inhibited ATPase. Na-inhibited ATPase activity is found in the proximal tubule and the thick ascending limb of Henle's loop but is absent in the collecting tubule whereas Na-stimulated ATPase is exclusively located in the proximal convoluted tubule. Na-inhibited ATPase, but not Na-stimulated ATPase, is totally abolished in the presence of 100 microM Ca2+. Conversely, Na-stimulated ATPase, but not Na-inhibited ATPase, is curtailed when nephron segments are preincubated at pH 7.2 whereas it is activated at pH 7.8. Finally, Na-stimulated ATPase displays an apparent Km for Na+ of approximately 10 mM, and is dose-dependently inhibited by the diuretic triflocin (IC50 approximately 6 x 10(-6) M).

Cellular fibronectins are differentially expressed in human fetal and adult kidney

The localization of cellular forms of fibronectin (cFn) was studied in fetal and adult kidneys. We used monoclonal antibodies reacting with the extradomains A and B in cFN (EDAcFn, EDBcFn) as well as with a differentially glycosylated fetal form of the protein (Onc-cFn). In adult human kidney EDAcFn was present in glomerular mesangium and in the walls of larger blood vessels, whereas a polyclonal rabbit fibronectin antiserum widely reacted also with interstitial areas. Immunoreactivity for EDBcFn and Onc-cFn, however, was not found in adult kidney. In the basement membranes and interstitial areas of developing tubules and glomeruli the immunoreactivity for EDAcFn was distinct and detectable in the earlier stages also for EDBcFn. In developing glomeruli, EDAcFn and EDBcFn were detected in teh mesangial areas, but in more mature fetal glomeruli, the mesangial immunoreactivity only persisted for EDAcFn. Both EDAcFn and EDBcFn were found in the basement membranes in the medullary area of all developing kidneys. In fetal kidney, immunoreactivity for EDAcFn and EDBcFn was seen also in small blood vessels, including the capillaries. Immunoreactivity for Onc-cFn was found in mesangial cells of fetal glomeruli as well as in the intima of larger blood vessels but not in the basement membranes. The results show that the three forms of cFn are present in the fetal kidney and have certain differences in their distribution. Conversely, only the EDAcFn was detected in the adult kidney. The different, partially age-related distributions of the three types of cFns suggest that they may also differ in their functions.

Renal carbonic anhydrase in the quail Coturnix coturnix japonica: I. Activity and distribution in male and female metanephros

Carbonic anhydrase activity was studied in the quail metanephros by means of histochemical, histophotometrical and biochemical methods. Male and female samples were examined separately in order to show sex-related differences in enzyme activity and localization. The staining patterns revealed differential distribution of reaction product in the different tubular segments. The initial portion of proximal tubules showed positivity on the brush border in female kidneys only. Extra situ investigations provided further evidence of sexual dimorphism resulting in higher values of enzyme activity for female than for male kidneys. In both sexes, marked staining was detected at the distal tubule level where histophotometric analysis confirmed the highest amount of reaction product. Moreover, the intracellular staining distribution at this site proved to be similar to that observed for mammalian proximal convoluted tubules. In the collecting ducts, a mosaic-like pattern was found with respect to both carbonic anhydrase staining and metachromatic properties. The functional significance of the presence of enzyme in the different renal tubules is discussed by comparison with the mammalian kidney. A model is proposed whereby the distal tubules represent the main sites of urinary acidification and bicarbonate reabsorption.

Cellular basis of an avian countercurrent multiplier system

A kidney from the budgerigar (budgie, parakeet; Melopsittacus undulatus) is composed of cortical reptilian-type nephrons (without loops of Henle) and mammalian-type nephrons (with loops) grouped together in medullary cones. The loop of the mammalian-type nephrons has a descending segment composed of thin and highly interdigitated cells. These thin limb cells have few mitochondria (15% of cell volume), undetectable Na+,K(+)-ATPase activity, and virtually no basolateral surface amplification. Prior to the hairpin turn, the descending limb thickens, but the cells continue to lack basolateral amplification. Cells just prior to and within the hairpin turn resemble cells of the entire ascending limb. These cells are thick (there is no thin ascending segment in the avian loop), with extensive infoldings of the basolateral membrane surrounding numerous mitochondria (45% of cell volume). The area of basolateral membrane is 25 times that of the apical membrane. The basolateral membrane (but not the apical membrane) is enriched in Na+,K(+)-ATPase activity. The structure of the avian mammalian-type nephron (as epitomized by the budgie nephron) and the fact that NaCl accounts for over 90% of the osmotic activity of avian urine leads to the conclusion that the countercurrent multiplier of the avian kidney functions by active NaCl transport from the entire ascending limb. No explanation is offered for the transport specializations found in the thick descending segment of the loop, just prior to the hairpin turn.

Cell volume regulation in the nephron

Nearly every cell in the kidney can volume regulate in response to a hypertonic challenge. Some are able to respond immediately to hyperosmotic media by a RVI. Other cells require stimulation prior to exposure to hyperosmolarity to demonstrate RVI. An increase of intracellular osmolytes during RVI usually occurs by an increase of NaCl influx either via the activation of parallel Na(+)-H+ and Cl(-)-HCO3- exchangers, or Na(+)-K(+)-2Cl- cotransporters. Medullary and papillary cells use organic solutes as well to increase the intracellular concentration of osmolytes. In response to a hypotonic challenge, a RVD response has been demonstrated in the majority of the kidney cells. The efflux of solute during RVD is usually via K+ loss by activation of conductance pathways. Stretch-activated K+ channels and Ca2(+)-activated-K+ channels have been shown to be stimulated in cells exposed in hyposmotic solutions and could thus be involved in RVD. The accompanying anion loss is less well-defined but could be either Cl- or HCO3- in different segments of the nephron. In some cells, the reduction of intracellular solute content is via an efflux of organic osmolytes. Thus it appears that cells in all segments of the nephron volume regulate in response to osmotic stresses. This regulation may be an essential part of transepithelial transport since the cells have to survive transcellular fluxes of osmolytes. It may be particularly important in the kidney to control cell volume both in response to changes in osmolarity and transcellular fluxes of solute in order to maintain proper flow of fluid through the nephron.

Measurement of Na-K-ATPase-mediated rubidium influx in single segments of rat nephron

To determine the functioning rate of Na-K-ATPase in the rat nephron, a micromethod was developed to measure the rate of rubidium uptake in single nephron segments microdissected from collagenase-treated kidneys. Because the hydrolytic activity of Na-K-ATPase displayed the same apparent affinity for K and Rb ions, whereas the Vmax elicited by K was higher than that in the presence of Rb, experiments were performed in the presence of cold Rb plus 86Rb. Before the assay, tubules were preincubated for 10 min at 37 degrees C to restore the normal transmembrane cation gradients. 86Rb uptake was measured after washing out extracellular cations by rinsing the tubules in ice-cold choline chloride solution containing Ba2+. Rb uptake increased quasi-linearly as a function of incubation time up to 30 s in the thick ascending limb, 1 min in the proximal convoluted tubule, and 5 min in the collecting tubule, and reached an equilibrium after 5-30 min. The initial rates of Rb uptake increased in a saturable fashion as Rb concentration in the medium rose from 0.25 to 5 mM. In medullary thick ascending limb, the initial rate of Rb uptake was inhibited by greater than 90% by 2.5 mM ouabain and by 10(-5) M of the metabolic inhibitor carbonyl cyanide trifluoromethoxyphenylhydrazone. Correlation of Na-K-ATPase hydrolytic activity at Vmax and initial rates of ouabain-sensitive Rb uptake in the successive segments of nephron indicates that in intact cells the pump works at approximately 20-30% of its Vmax. Increasing intracellular Na concentration by tubule preincubation in a Rb- and K-free medium increased the initial rates of Rb intake up to the Vmax of the hydrolytic activity of the pump.

Polypeptide growth factors in metanephric growth and segmental nephron differentiation

Although the developing nephron expresses receptors for various polypeptide growth factors, the specific roles of such factors in renal organogenesis are unknown. Therefore, the effects of epidermal growth factor (EGF) (8.2 x 10(-11) M-1.6 x 10(-8) M), multiplication stimulating activity (MSA) (6.6 x 10(-10) M-1.3 x 10(-8) M) and transforming growth factor beta (TGF-beta) (1 x 10(-12) M-1 x 10(-9) M) on organotypic renal growth and segmental nephron differentiation were studied in a serum-free hormone-supplemented, murine metanephric organ culture system. Following culture in control or growth-factor-supplemented medium, explant growth was assessed, and explant growth and differentiation were determined morphometrically in four defined neprhon segments which were identified morphologically or immunohistologically with segment-specific antibodies and/or lectins: glomeruli, proximal tubules, thick ascending limb-early distal tubules, and collecting tubules. Results showed that EGF increased overall renal growth and specific differentiation of distal elements, but retarded differentiation of glomeruli and proximal tubules. EGF also induced hyperplastic cystic malformation in proximal tubules. MSA stimulated explant growth and promoted segmental differentiation of all tubular segments. TGF-beta globally retarded in vitro nephrogenesis. Such data demonstrate that polypeptide growth factors have multiple and often disparate effects on overall renal growth in relation to differentiation of discrete nephron segments and provide insight into the factors which may regulate normal and abnormal renal embryogenesis.

Heparin and heparan sulfate delimit nephron formation in fetal metanephric kidneys

Formation of nephrons from primitive mesenchyme in fetal kidneys is induced by ureteric buds. Nephron induction is closely coordinated with branching morphogenesis of the ureteric bud. Having previously shown that branching of the primitive ureter is associated with de novo synthesis of chondroitin sulfate proteoglycan and release of free heparan sulfate glycosaminoglycan chains, we asked whether glycosaminoglycans influence nephron development. Fetal mouse kidneys were incubated in organ cultures containing heparan sulfate, heparin, chondroitin sulfate, or hyaluronate. After 48 hr the number of nephrons at each developmental stage was enumerated by light microscopic analysis of serial tissue sections. Kidneys incubated in heparin or in heparan sulfate contained up to 10-fold fewer nephrons than did kidneys incubated in control conditions or in chondroitin sulfate or hyaluronic acid. Maturation of nephrons, however, was unaffected. Inhibition of nephron development was associated with binding of labeled heparin to primitive mesenchyme and altered tissue distribution of fibronectin. Branching morphogenesis was impaired in kidneys exposed to heparin but not to heparan sulfate or to de-N-sulfated, N-acetylated heparin. The capacity of glycosaminoglycans to inhibit nephron formation depended on sugar composition and O-sulfation but not GAG chain size or charge density. Thus, heparan sulfate may have the capacity to specifically control formation of nephrons in fetal metanephric kidneys in vitro.

Presence of vesicular bodies and thick basal laminae in the nephron of the desert gerbil Meriones crassus

Kidney samples of the adult gerbil Meriones crassus were aldehyde fixed and Epon embedded for studies of the general features of various parts of the nephrons, with particular attention to their basal laminae in all regions. Results obtained showed the presence of thick basal laminae (2-6 microns) in the parietal layer of Bowman's capsule, proximal convoluted tubule, thin loop of Henle and distal convoluted tubule. With the aid of the electron microscope, extracellular vesicular bodies were observed within the thick basal laminae in the previous regions. The bodies (50-500 nm in diameter) were found at various levels of the basal laminae. Some of them appeared to have been pinched off directly from the epithelial layer and to have moved to the underlying basal lamina, which may suggest that these vesicular bodies originated from the epithelial layer. The bodies, with a variable electron opacity, may be found either in small groups or as a single structure surrounded by a clear halo of basal lamina.

Membrane pinch-off and reinsertion observed in living cells of Drosophila

The garland cell of Drosophila is a nephrocyte which takes up waste products from the haemolymph. Endocytosis is thought to occur by the pinch-off of coated vesicles from deep invaginations of the plasma membrane called labyrinthine channels. Electron microscopic studies show that the length of these channels is variable, depending on the relative rates of membrane pinch-off and reinsertion (recycling). Thus, in wild-type garland cells, if the temperature is raised from 19 degrees C to 30 degrees C, the channels shorten, because at high temperature the pinch-off rate exceeds the reinsertion rate. On the other hand, in garland cells of the temperature-sensitive, single-gene mutant shibirets1 (shi), in which endocytosis is reversibly blocked at the pinch-off stage at 30 degrees C, the labyrinthine channels elongate considerably, as membrane insertion proceeds while pinch-off is blocked. The rates of membrane pinch-off and insertion were quantitated in living garland cells by observing the changes in the capacitance of the whole cell membrane which occur as a result of changes in the total area of the plasma membrane. In wild-type cells, the capacitance gradually decreased as the temperature was raised to 30 degrees C, reflecting the shortening of the channels. In shi cells, the capacitance decreased between 19 degrees C and 26 degrees C but then began to increase at higher temperatures as the blockage of endocytosis caused by the shi gene took effect, causing the channels to elongate. The observations suggest that in shi cells the surface area of the cell more than doubles in 12 min by channel elongation. Estimates of the amount of membrane which is pinched off and reinserted were made.

A structural study of the rat proximal and distal nephron: effect of peptide and thyroid hormones

The effects of the absence of various hormones (antidiuretic hormone, thyroid hormone, parathyroid hormone, and calcitonin) on proximal and distal structures were studied in diabetes insipidus (DI) Brattleboro rats. The cross-sectional area of the first segment of proximal convoluted tubules (S1) was significantly reduced in thyroparathyroidectomized (TPTX) DI rats compared with Long-Evans rats (the strain of origin of DI rats) and untreated DI rats. Administration of triiodothyronine (T3, 10 micrograms/day for 7 days) to TPTX-DI rats restored the proximal tubule structure. In the distal convoluted tubule (DCT) the cross-sectional area of the epithelium and the number of nuclei per cross-sectional area were significantly greater in untreated ADH-deficient DI rats than in the control Long-Evans rats. Daily administration of 1-desamino-8-D-arginine vasopressin (dDAVP, 500 ng/day for 3 wk) significantly reduced the size and the number of DCT cells in DI rats. Cortical micropuncture data indicated that the Na+ concentration in the fluid delivered to the DCT and the absolute amount of Na+ reabsorbed along the DCT were higher in DI than in dDAVP-treated DI rats. It is concluded that functional changes in the PCT, subsequent to chronic TPTX, are accompanied by marked alteration of the cell anatomy of this nephron segment, and that the processes that modify the Na load delivered to the DCT and the Na transport in the DCT are accompanied by structural modifications of this segment.

Renal blood vascular system in the elasmobranch, Raja erinacea Mitchill, in relation to kidney zones

The arrangement and structure of renal blood vessels were studied in a marine skate with injection of silicone rubber and methacrylate resin after intravenous administration of epinephrine and perfusion fixation. The methacrylate casts were investigated with the scanning electron microscope. Histology was performed by light and transmission electron microscopy of serial sections. The course of the blood vessels is described in relation to the renal zones of lateral bundles and mesial tissue. Each nephron performs two loops in the lateral bundles and two coilings in the mesial tissue before it joins the collecting duct system. The lateral bundles contain an elaborate countercurrent arrangement of neck segment, proximal tubule segment PIa, early distal tubule segment, and collecting tubule. Within the bundles, the nephron portions are associated with a blind-ended central vessel, which is connected with the venous sinuses of the mesial tissue. The microcirculatory bed around the bundles is supplied with arterial blood via small bundle arteries that originate from the intrarenal arteries in parallel to the afferent arterioles of the glomeruli. The efferent arterioles of the glomeruli convey their blood to the peritubular sinuses of the mesial tissue, which is largely irrigated with venous blood of the renal portal system. The mesial tissue, containing the proximal tubule segments PIb and PII, intermediate segment, and late distal tubule segment LDTb, receives venous blood from the caudal vein and the lateral musculature via afferent renal and intrarenal veins and from the efferent arterioles of the glomeruli and venules of the microcirculation of the bundles. The sinuses are drained by efferent renal veins via efferent intrarenal veins. By comparing the renal structures of the skate with those of dogfish, a unique type of circulation--as related to nephron segments, renal zones, and fine structure of the wall of the vessels--is revealed in marine elasmobranchs of different evolutionary levels.

IGF-I immunoreactivity is expressed by regenerating renal tubular cells after ischaemic injury in the rat

The immunohistochemical expression of the trophic peptide insulin-like growth factor I (IGF-I; somatomedin C) was investigated in the post-ischaemic regenerating rat kidney. The part of the nephron most severely injured by the ischaemic insult was the pars rectae or S3 region of the proximal tubules. In this part of the nephron no specific IGF-I immunoreactivity could be demonstrated under control conditions. Three days after injury, however, low regenerating tubular cells expressing IGF-I immunoreactivity could be demonstrated invading the damaged region. Five and 7 days post-ischaemia some of the regenerating cells showed IGF-I immunoreactivity while adjacent and similar-looking cells did not. In parallel with an increasing differentiation of the regenerating cells the IGF-I immunoreactivity vanished. Fourteen days after the injury the S3 cells had regained their normal morphology, and by this time the IGF-I immunoreactivity had returned to a normal undetectable level. It is concluded that there is a transient increase in the expression of IGF-I mainly confined to the early phase of post-ischaemic rat kidney regeneration. It is proposed that IGF-I exerts its trophic role by paracrine mechanisms.

Localisation of immunoreactive kininogen and tissue kallikrein in the human nephron

The cellular localisation of kininogen and its relationships with tissue kallikrein containing cells was studied in the human kidney by the peroxidase-antiperoxidase method using antisera to human LMW kininogen and to human tissue kallikrein. Immunoreactive kininogen was localised in the principal cells of collecting ducts. Immunoreactive tissue kallikrein was detected in the connecting tubule cells, segment of the nephron preceding the cortical collecting ducts. The co-existence of tissue kallikrein and kininogen in the same transitional tubule, but in different cells, was established by the use of serial sections and double immunostaining. This anatomical relationship is in accordance with known studies that describe intermingling of principal cells and connecting tubule cells where connecting tubules merge into cortical collecting ducts in the human nephron. The close relationship between cells that contain tissue kallikrein and its substrate, kininogen, suggests that kinins could be generated in the lumen of distal cortical segments of the human nephron.

Neonatal rabbit kidney cortex in culture as tool for the study of collecting duct formation and nephron differentiation

By stripping off the capsula fibrosa of neonatal rabbit kidneys a consistently thin tissue layer consisting of collecting duct anlagen, S-shaped bodies and nephrogenic blastema is obtained. This thin layer seems to be an excellent object for investigation of epithelium formation and nephron differentiation. Three different tissue culture protocols are described: 1. A polarly differentiated collecting duct epithelium with 'tight' characteristics consisting only of principal cells, grown on specific renal support 2. A morphologically dedifferentiated collecting duct principal cell monolayer grown on the unspecific bottom of a plastic culture dish 3. An embryonic tissue layer with numerous S-shaped bodies which might be a suitable model for investigation of the development of maturing nephron structures in serum-free culture medium.

A distal nephron glycoprotein that has different cell surface distributions on MDCK cell sublines

Monoclonal antibodies that recognize three distinct epitopes of a 23-kDa glycoprotein (gp23) on the plasma membrane of MDCK cells were used to study cell-surface polarity. Immunofluorescence microscopy of MDCK cells obtained from the American Type Culture Collection demonstrated that gp23 was nonpolarized in approximately 50% of the cells (on both apical and basolateral membranes), whereas, in the remaining cells, gp23 had a polarized distribution (basolateral only). This heterogeneous gp23 cell-surface staining was not observed in a variety of other MDCK sublines. Instead, gp23 was found to have a nonpolarized distribution on MDCK cells that produced monolayers with low transepithelial electrical resistances (less than 220 omega X cm2) and was localized only to the basolateral membrane of MDCK cell lines capable of generating considerably higher transepithelial electrical resistances (770-2,220 omega X cm2). Immunofluorescence and immunoelectron microscopy of dog, rat, and rabbit kidney demonstrated that gp23 is a nephron segment-specific glycoprotein localized only to the distal and collecting tubules. These observations provide further evidence for the heterogeneity of the MDCK cell line. They also support a proposal that the origin of MDCK cells is the renal distal nephron.

Myosin content and vasoconstrictive ability of the proximal and distal (renin-positive) segments of the preglomerular arteriole

The PAP-technique and antibodies to myosin were used to demonstrate the prerequisites for vasoconstriction in the juxtaglomerular part of the preglomerular arteriole as compared with its proximal segment in rats and mice. In contrast with the myosin-positive/renin-negative proximal part of the afferent arteriole no myosin-like activity could be demonstrated in its distal, renin-positive part. In accordance, no thick myofilaments were found in fully differentiated juxtaglomerular epithelioid cells replete with mature secretory granules. Stimulation of the renin-angiotensin system was followed by an increase of the renin-positive/myosin-negative portions of the preglomerular arteriole. Marked interspecies and internephron variations in the length of this vessel segment under control and stimulated conditions were observed. The juxtaglomerular part of the preglomerular arteriole close to the macula densa seems therefore to have only limited capabilities for vasoconstriction. This finding may be of importance regarding the tubulo-glomerular feedback, a mechanism allegedly triggered by the so-called 'macula densa-signal'. It is suggested that this non-contractile segment of the afferent arteriole may represent the renal vascular receptor responsible for the increase of renin secretion during pressure reduction. Unlike the afferent arterioles, most of the efferent arterioles showed the highest level of their weak but distinct myosin-like immunoreactivity in the juxtaglomerular region, indicating some efferent juxtaglomerular vasoconstrictive ability.

Regulatory volume decrease in perfused proximal nephron: evidence for a dumping of cell K+

We utilized the microscopic and morphometric procedures described in the preceding paper to examine the role of a swelling-activated dumping of K-salt in the reversal of hyposmotic cell swelling in the perfused proximal nephron. The rate of the regulatory volume decrease that follows cell swelling in dilute solutions was reduced by two maneuvers that attenuate the K+ chemical potential difference across the basolateral membrane; inhibiting the Na+-K+ pump (e.g., with ouabain) and raising the peritubular K+ concentration. The rate of the regulatory volume decrease was also inhibited by peritubular quinine, which blocks K channels and volume regulation for a number of mammalian cells. Additionally, exposure to hyposmotic solutions resulted in a sustained and quinine-sensitive increase in the apparent permeability of the basolateral membrane to K+ salt, which was monitored qualitatively as the rate of cell volume change that was induced by a perturbation in the peritubular K+ concentration. The simplest interpretation of these results is that the reversal of hyposmotic cell swelling in the proximal nephron is referable at least in part to a swelling-activated loss of K-salt and water from the cells.

[Determination of cell surface antigens on cryostat sections with monoclonal antibodies]

Most monoclonal antibodies recognize antigens which do not survive conventional tissue processing: the use of frozen tissue sections and the immunofluorescence method overcome this obstacle but introduce other problems. Three improvements are reported here: the use of serum-free (substitute) "Ultroser Hy" as a culture medium for hybridomas, in order to diminish background staining and the diffusion artifact; the use of freon for freezing tissue sections, so as to slightly increase cellular morphology and staining; the use of a new immunofluorescent slide-mounting medium to enhance histologic preservation and immunohistologic contrast and to diminish fading of immunofluorescence.

Retention of differentiated characteristics by cultures of defined rabbit kidney epithelia

Rabbit nephron segments of proximal convoluted tubules (PCT); proximal straight tubules (PST); cortical and medullary thick ascending limbs of Henle's loop (CAL, MAL); and cortical, outer medullary, and inner medullary collecting tubules (CCT, OMCT, IMCT) were individually microdissected and grown in monolayer culture in hormone supplemented, defined media. Factors favoring a rapid onset of proliferation included young donor age, distal tubule origin, and the addition of 3% fetal calf serum to the medium. All primary cultures had polarized morphology with apical microvilli facing the medium and basement membrane-like material adjacent to the dish. Differentiated properties characteristic of the tubular epithelium of origin retained in cultures included ultrastructural characteristics and cytochemically demonstrable marker enzyme proportions. PCT and PST were rich in alkaline phosphatase; CAL stained strongly for NaK-ATPase; CCT contained two cell populations with regard to cytochrome oxidase reaction. A CCT-specific anti-keratin antibody (aLEA) was immunolocalized in CCT cultures, and a PST cytokeratin antibody stained PST cultures. The biochemical response of adenylate cyclase to putative stimulating agents was the same in primary cultures as in freshly isolated tubules. In PCT and PST adenylate cyclase activity was stimulated by parathyroid hormone (PTH) but not by arginine vasopressin (AVP); CAL and MAL adenylate cyclase was stimulated by neither PTH nor AVP; CCT, OMCT, and IMCT adenylate cyclase was stimulated by AVP but not by PTH. NaF stimulated adenylate cyclase activity in every cultured segment. It is concluded that primary cultures of individually microdissected rabbit PCT, PST, CAL, MAL, CCT, OMCT, and IMCT retain differentiated characteristics with regard to ultrastructure, marker enzymes, cytoskeletal proteins, and hormone response of adenylate cyclase and provide a new system for studying normal and abnormal functions of the heterogeneous tubular epithelia in the kidney.

Sequential cell and tissue interactions governing organogenesis of the kidney

The complex development of the metanephric kidney illustrates the decisive role of sequential morphogenetic interactions of varying type in organogenesis. In this review the following steps were singled out from this continuous, strictly controlled organogenesis: determination of the mesoderm during the blastula stage, induction of pronephric nephrons during gastrulation, guided migration of the pronephric duct, mesenchyme-controlled branching of the ureter, induced aggregation of the mesenchymal cells of the metanephric blastema, homotypically controlled polarization of the cells in the renal vesicle and remodelling of the vesicle into the S-shaped body, guided migration of endothelial cells into the nephric blastema and the glomerular crevice, and the matrix interaction(s) completing the formation of the glomerular basement membrane with dual origin.

Wandering cells in the nephron of the mudskipper Periophthalmus koelreuteri (Pallas)

Wandering cells that infiltrate between the principal cells of the nephron of the mudskipper Periophthalmus koelreuteri were examined by transmission electron microscope. These cells were found at various levels between the tubule cells and were more abundant in the proximal and collecting segments. These cells have cytoplasmic processes and large spherical nuclei. Their cytoplasm appears electron dense and contains many lysosomes, rough endoplasmic reticulum, free ribosomes, some mitochondria and Golgi profiles. Their ultrastructural features indicate that they may be phagocytic.

ATPase activity in macula densa cells of the rabbit kidney

Na-K- and Mg-activated ATPase activities were determined in maculae densae and glomeruli dissected from both superficial and juxtamedullary nephrons of normal rabbits, using an ultramicro method including a cycling reaction. Activities were expressed as Pi generated per macula densa or per glomerulus and normalized for tissue volume. Results indicate that the mean volume of superficial and juxtamedullary macula densa samples was not statistically different, while glomeruli from deep nephrons had sample volumes that were 29% larger than those from superficial nephrons (P less than 0.001). Correcting for volume both superficial and juxtamedullary macula densa samples had an Na-K-ATPase activity of 0.37 +/- 0.21 fmol X h-1 X (micron3)-1 X Mg-ATPase activity in both pools was also similar [0.41 +/- 0.07 and 0.52 +/- 0.1 fmol X h-1 X (micron3)-1]. Na-K-ATPase activity in macula densa cells is estimated to be about 1/40th the activity of surrounding cortical thick ascending limb cells. Total glomerular ATPase per unit volume was significantly higher in glomeruli from superficial than from deep nephrons [0.41 +/- 0.04 vs. 0.28 +/- 0.04 fmol X h-1 X (micron3)-1, P less than 0.05]. There was no statistically significant activity of Na-K-ATPase in either superficial or deep glomeruli. These results suggest that in contrast to previous reports, the macula densa contains Na-K-ATPase, but at a low level relative to surrounding tubular cells. Further, in normal rabbits, this activity is invariant in superficial and juxtamedullary samples.

Chemical and morphological differences in the kidney zones of the elasmobranch, Raja erinacea mitch

The histological investigation of the kidney of the skate Raja erinacea revealed a thin cap of dorsal bundles, which contain segments of single nephrons that are arranged separately in a countercurrent manner, and a large ventral zone, where the second proximal segments (PII) and parts of the lower nephron are located. This zonation is apparent in fresh, unfixed material and makes it possible to separate small tissue samples under a dissecting microscope. The osmolality in both zones does not differ. The dorsal bundle zone had a lower urea concentration and a higher sodium concentration than the ventral zone. The differences in the mean concentrations of the tissue samples indicate a gradient for the two substances along the bundles. Determinations of amounts of water and solutes per mg solute-free, dry tissue of the two zones, showed that the amounts of water, total osmolytes, Na and K were greater in the bundle zone than in the ventral zone, while the amount of urea was identical in the two zones. This indicates that the lower urea concentration in the bundle zone is established through an accumulation of Na and water in the interstitium. The countercurrent arrangement of very early and late segments of single renal tubules supports the concept of passive reabsorption of urea in the kidney of the marine elasmobranch.