Histology of the kidney and urinary bladder of Siphonops annulatus (Amphibia-Gymnophiona)

The histology of the kidney and urinary bladder of Siphonops annulatus was studied by light microscopy in semithin sections of tissue embedded in hydrophilic resin. The kidney's nephron comprises the renal corpuscle, neck segment, proximal tubule, intermediate segment, distal tubule and collecting tubule. Nephrostomes are present. This structure, the neck segment, and intermediate tubules present long cilia, and probably play important roles in the propulsion of the peritoneal fluid and glomerular filtrate. The proximal tubule cells possess loosely packed microvilli and contain abundant polymorphic granules and vesicles that assume the aspect of lysosomes in different stages of intracellular digestion. The distal tubules are characterized by large, vertically disposed mitochondria assuming the aspect of ions transporting cells. The urinary bladder is lined with a transitional epithelium, whose aspect varies according to the quantity of urine.     

Mechanism of increased tubular Na-K-ATPase during streptozotocin-induced diabetes

Since the mechanisms responsible for stimulation of kidney Na-K-ATPase during streptozotocin-induced diabetes are unknown, we studied the possible role(s) of kidney hyperfiltration and hypertrophy and of hyperaldosteronism on Na-K-ATPase induction. For this purpose, we studied the relationship between Na-K-ATPase activity in individual nephron segments and alterations of glomerular filtration rate during the early phase of diabetes. Within 2 days after streptozotocin administration, Na-K-ATPase activity markedly increased in the proximal convoluted tubule, medullary thick ascending limb and cortical and outer medullary collecting tubule, but not in the proximal straight tubule, cortical thick ascending limb and distal convoluted tubule. Streptozotocin administration also markedly enhanced the glomerular filtration rate but only after 4 days following initiation of treatment. Changes in Na-K-ATPase were specific since the activity of adenylate cyclase, another marker of basolateral membranes, was not altered. Finally, when animals were adrenalectomized prior to streptozotocin treatment, Na-K-ATPase stimulation was curtailed in the collecting tubule but not in more proximal segments. These results suggest that diabetes alters Na-K-ATPase activity in specific nephron segments independent of alterations of glomerular filtration rate and of kidney hypertrophy, and that the stimulation of collecting tubule Na-K-ATPase is secondary to hyperaldosteronism.     

Sodium transport-related proteins in the mammalian distal nephron – distribution, ontogeny and functional aspects

The mammalian distal nephron plays a pivotal role in adjusting urinary sodium excretion. Successive portions of the renal tubule are formed to adapt to this function, and an axial heterogeneity of the distal segments has been defined. The specific transport properties of these epithelia are accomplished by the expression of proteins (cotransporters, exchangers, channels) governing the movement of ions on either cell side. Molecular cloning of these proteins has had a marked impact on the study of their localization and function in the healthy and diseased kidney. Electroneutral cation-chloride cotransporters [Na(K)CC] have been localized to the thick ascending limb and the distal convoluted tubule using specific probes. Proteins implicated in the function of aldosterone target cells, such as the epithelial Na⁺ channel (ENaC), the mineralocorticoid receptor (MR) and 11β-hydroxysteroid dehydrogenase type 2 (11HSD2), an enzyme that confers mineralocorticoid specificity, have been found in the terminal portion of the nephron and the collecting duct. A mineralocorticoid-sensitive component of thiazide-sensitive NaCl transport has been identified in the distal convoluted tubule. Analysis of the ontogeny of these proteins in the maturing kidney has provided a detailed picture of epithelial differentiation and morphological specialization of the renal tubule. The study of mutations of the proteins related with NaCl transport has led to the identification of the molecular causes of inherited human diseases associated with hypo- or hypertension, and the respective sites of an impaired ion transport could be mapped to the renal tubule. Accepted: 13 April 1999     

The structural organization of the kidney of Typhlonectes compressicaudus (Amphibia, Gymnophiona)

Summary The structural organization of the kidney ofTyphlonectes compressicaudus (Amphibia, Gymnophiona) was studied by light microscopic (LM) examination of serial paraffin and semithin Epon sections. The kidney is slender and quite long and has a mesonephric segmental construction; the excretory duct (Wolffian duct), running along the lateral side of the kidney, segmentally receives the terminal trunks of the collecting duct system. The nephron has the following parts: renal corpuscle, neck segment, proximal tubule, intermediate segment, distal tubule and connecting tubule. The distal tubule is located in a ventromedial (central) zone of the kidney; all other tubular segments lie in a dorsolateral (peripheral) zone. The renal corpuscles are found at the border between these two zones.The renal corpuscle is very large; its urinary pole faces the peripheral zone. A small proportion of neck segments receive either a nephrostomal duct or a blind branch. The proximal tubule is a thick, highly convoluted tubule. The intermediate segment is ciliated and makes a few coils. The distal tubule is composed of three portions: a highly convoluted part in the central zone, subsequently an attachment site with the renal corpuscle and a short postattachment-part. The connecting tubule and the collecting duct have a heterogeneous epithelium consisting of light and dark cells. The collecting duct is distinguished by dilated intercellular spaces. The Wolffian duct has a pseudostratified epithelium.The present study correlates the course and segmentation of the renal tubule ofTyphlonectes. The tubule has three major convolutions. The first occurs in the proximal tubule in the peripheral zone; the second is established by the distal tubule and occurs in the central zone; the third is formed by the connecting tubule and is found in the peripheral zone.Research fellow of the Alexander von Humboldt foundation: home address: Department of Anatomy, Faculty of Medicine, University of Tokyo, Tokyo 113, Japan     

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.     

The structure of the kidney from the freshwater teleost Carassius auratus

Summary The structure of the kidney of the crucian carp (Carassius auratus; a freshwater teleost, Cypriniformes) was studied by means of reconstruction from serial paraffin and semithin sections. In C. auratus, the Wolffian duct traverses the entire kidney. At various levels collecting ducts of different length and thickness join the Wolffian duct at right angles. Each collecting duct accepts a large number of connecting tubules, which are established by the joining of many nephrons. A regular pattern concerning the distribution of nephrons and the fusion of renal tubules is not apparent. Four segments have been distinguished in renal tubules; 1) proximal tubule, 2) distal tubule, 3) connecting tubule and 4) collecting duct. A neck and an intermediate segment are absent. The proximal tubule is established by proximal tubule cells which bear a brush border and have a conspicuous apical cytoplasmic rim containing few cell organelles, ciliated cells, mucous cells and dark cells. In the first part of the proximal tubule the brush border and the apical cytoplasmic rim of proximal tubule cells are well developed. Ciliated cells are interposed between proximal tubule cells, decreasing in number toward the end of this part. In the second part ciliated cells are absent and dark cells are numerous. In the third part the brush border and the apical cytoplasmic rim of proximal tubule cells are scarcely developed. Ciliated cells reapear and increase in number toward the distal tubule. The distal and connecting tubule are similar in epithelial structure. Connecting tubules are joined distal tubules and thus they belong to two or more nephrons. The main cells of distal and connecting tubules contain abundant mitochondria, but have no brush border. The connecting tubule becomes a collecting duct before joining the Wolffian duct. The main cells of collecting ducts are characterized by division of their cytoplasm into a dark apical half and a light basal half.     

Localization and regulation of the renal kallikrein kinin system: Possible relations to renal transport functions

Summary The complete renal kallikrein kinin system has recently been localized in defined nephron segments. Kallikrein was found to be formed and secreted by connecting tubule cells in the late distal convoluted tubule, whereas kininogen and a novel kininase were located in collecting tubules. Kinins were shown to act on collecting tubule as well as medullary interstitial cells and the renal vasculature. The literature on interactions of this system with renal sodium transport is conflicting. Renal and urinary kallikrein was found to be increased under sodium restricted conditions, whereas kinin has a diuretic and natriuretic effect in the collecting tubule, when added from the basolateral surface. On the other hand renal kallikrein activity and connecting tubule cell morphology change in parallel with dietary potassium load indicating a coupling to potassium secretion. The possible role of the renal kallikrein kinin system in regulating collecting tubule function by tubular and vascular effects is outlined in spite of many open questions which remain to be answered.     

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin–angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT₁ receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT₁ receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.     

Heterogenous distribution of glycoconjugates in the kidney of dogfish Scyliorhinus caniculus (l.) with reference to changes in the glycosylation pattern during ontogenetic development of the nephron

Eight fluorochrome-coupled lectins with different sugar specificities were applied to cryosections of dogfish kidney. Despite profound differences in renal architecture between clasmobranch fish and other vertebrates, the sequence of nephron segments as revealed by the lectin-binding pattern was rather similar to that of tetrapodes. Wheat germ agglutinin (WGA) bound to cell membranes of epithelial cells of glomeruli, proximal and distal tubules, their basement membranes, the collecting tubule, and epithelial cells. Among other broadly binding lectins were Ricinus communis agglutinin I (RCA-I), soybean agglutinin (SBA), peanut agglutinin (PNA), Lycopersicon esculentum agglutinin (LEA), and Jacalin, all of which marked proximal as well as distal portions of the renal tubule. Dolichos biflorus agglutinin (DBA) did not react with any renal structure. Ulex europaeus agglutinin I (UEA-I), which indicates the presence of α-L-fucose, very strongly and specifically marked single epithelial cells of the early distal nephron, all epithelial cells of the late distal tubule, the beginning of the collecting tubule in the mesial tissue zone, and single cells in the end portion of the collecting tubule in the lateral bundles. Binding of UEA-I to receptors of distal nephron cells could be useful for the identification of these cells in functional studies employing teased tubule and/or isolated cell preparations. Binding of UEA-I to dogfish kidney structures resembles staining with UEA-I conjugates of late distal tubules and collecting tubules in the kidneys of frog and other, higher vertebrates. Epithelial cells of early developmental stages showed, very rarely, binding sites for most lectinfluorochrome conjugates. A large number of lectin binding sites was observed in the extracellular matrix of fibroblast layers surrounding the early anlage and the S-shaped body. Lectin binding sites of the nephron epithelia appeared in a sequential manner in the next stages of development of the nephron. Ontogenetic and phylogenetic aspects of the merging region between nephron proper (late distal tubule) and collecting system (collecting tubule) are discussed. © 1993 Wiley-Liss, Inc.     

Hormonal control of distal nephron function

Summary The distal tubule and collecting tubules are important control sites of fluid and electrolyte excretion. In our presentation we consider the cell mechanisms of transport of sodium and potassium ions and the effects of several hormones. Aldosterone and antidiuretics stimulate potassium secretion directly, and the available evidence strongly suggests that this effect involves the principal cell population. Epinephrine inhibits potassium secretion at sites beyond the distal tubule. In addition to such direct effects, secondary factors such as hormone-induced changes in flow rate along the distal tubule and changes in the plasma potassium level play an important modifying role. Several examples are presented to demonstrate that interaction of several control components uncouples potassium secretion from distal flow rate and tends to stabilize urinary potassium excretion during changes in sodium and water balance.Work in the authors laboratory was supported by NIH Grant No AM-17433     

A multinephron model of renal blood flow autoregulation by tubuloglomerular feedback and myogenic response

Tubuloglomerular feedback implies that a primary increase in arterial pressure, renal blood flow, glomerular filtration and increased flow rate in the distal tubule increase preglomerular resistance and thereby counteract the primary rise in glomerular filtration rate and renal blood flow. Tubuloglomerular feedback has therefore been assumed to play a role in renal autoregulation, i.e., the constancy of renal blood flow and glomerular filtration at varying arterial pressure. In evaluating this hypothesis, the numerous tubular and vascular mechanisms involved have called for mathematical models. Based on a single nephron model we have previously concluded that tubuloglomerular feedback can account for only a small part of blood flow autoregulation. We now present a more realistic multinephron model, consisting of one interlobular artery with an arbitrary number of evenly spaced afferent arterioles. Feedback from the distal tubule was simulated by letting glomerular blood flow exert a positive feedback on preglomerular resistance, in each case requiring compatibility with experimental open-loop responses in the most superficial nephron. The coupling together of 10 nephrons per se impairs autoregulation of renal blood flow compared to that of a single nephron model, but this effect is more than outweighed by greater control resistance in deep arterioles. Some further improvement was obtained by letting the contractile response spread from each afferent arteriole to the nearest interlobular artery segment. Even better autoregulation was provided by spreading of full strength contraction also to the nearest upstream or downstream afferent arteriole, and spread to both caused a renal blood flow autoregulation approaching experimental observations. However, when the spread effect was reduced to 25% of that in each stimulated afferent arteriole, more compatible with recent experimental observations, the autoregulation was greatly impaired. Some additional mechanism seems necessary, and we found that combined myogenic response in interlobular artery and tubuloglomerular feedback regulation of afferent arterioles can mimic experimental pressure-flow curves.     

Ciliar functions in the nephron

The primary cilium is a microtubule-based nonmotile organelle that is found on most cells in the mammalian body. Once regarded as a vestigial organelle, it has been recently shown to play unforeseen roles in mammalian physiology and tissue homeostasis. In kidney epithelial cells, the primary cilium plays a fundamental role in tubule organization and function and it is now considered to serve as a versatile mechanosensor and chemosensor. Diseases related to kidney primary cilia include autosomal polycystic kidney disease, recessive polycystic kidney disease, Bardet–Biedl syndrome, and nephronophthisis. Multiple proteins whose functions are disrupted in cystic kidney diseases have been localized in the primary cilium. This review provides a general introduction to the cell biology and function of renal primary cilia and an overview of cilia-related kidney diseases.     

Calcium transport in the nephron.

Ionized and complexed calcium are filtered at the glomerulus and more than 95% of the filtered load is reabsorbed along the length of the nephron. In the proximal convoluted tubule calcium is absorbed in proportion to sodium and water, suggesting a passive mechanism. The high permeability of this segment is compatible with passive transport, but evidence for active transport has been advanced. A role for Ca2+-ATPase and/or for a Ca2+/Na+ antiport has also been proposed. The straight portion of the proximal tubule appears to transport calcium actively but little is known about the mechanism and regulation of calcium absorption in this segment. Both passive and active transport of calcium in the thick ascending limb have been demonstrated, and heterogeneity in the function of medullary and cortical segments has been proposed. Definite evidence has been advanced for avid active calcium absorption in the distal convoluted tubule. Both chlorothiazide and parathyroid hormone enhance the transport of calcium in this segment. The granular portion of the collecting tubule resembles in its properties and function the distal convoluted tubule. The light portion, however, is incapable of transporting calcium. The distal tubule and collecting tubule may be the final regulators of urinary excretion of calcium but much more data are required before this view can be adopted.     

Aquaporins in the kidney: from molecules to medicine.

The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration. Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption. Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this. Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane. The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.     

The pattern of distribution of selected ATP-sensitive P2 receptor subtypes in normal rat kidney: an immunohistological study

Using immunohistological techniques and available polyclonal antibodies, we have identified several ATP-sensitive P2 receptor subtypes in specific structures of the normal rat kidney. Of the P2 receptor subtypes examined, P2X1, P2X2 and P2Y1 receptors were found in the smooth muscle layer of intrarenal vessels. The P2Y1 receptor was also found on glomerular mesangial cells, the brush border membrane of the proximal straight tubule and on peritubular fibroblasts. In the cortex, P2Y4 receptors were found on the tubule epithelium of the proximal convoluted tubule, and P2Y2 receptors on glomerular epithelial cells (podocytes). P2X4 and P2X6 receptors were present throughout the renal tubule epithelium from the proximal tubule to the collecting duct. P2X5 receptors were expressed on medullary collecting duct cells and the apical membrane of the S3 segment of the proximal tubule. Possible functions of these receptor subtypes in normal rat kidney are discussed.     

硒酸锌金属自显影技术检测游离锌离子在小鼠肾脏的分布

目的研究游离锌离子在小鼠肾脏的定位分布。方法应用硒酸锌金属自显影技术(ZnSeAMG)检测小鼠肾脏内的游离锌离子分布。结果游离锌离子在肾脏内分布广泛,皮质中有大量AMG反应阳性颗粒,髓质中的AMG阳性颗粒较少。其中,近曲小管、远曲小管、近直小管和远直小管上皮细胞近腔侧均分布有大量的棕黑色AMG阳性颗粒,肾小体、细段和集合管上皮细胞中AMG阳性颗粒较少。结论小鼠肾脏内含有丰富的游离锌离子,锌离子可能参与肾脏的功能。     

Morphological and morphometrical study of the kidney of the male tropical lizard Tropidurus torquatus

A morphological and morphometrical study of the adult male Tropidurus torquatus kidney was undertaken. The nephron is composed of the following segments: renal corpuscle, neck segment, proximal convoluted tubule, intermediate segment, and distal tubule. The nephron is continued into the collecting duct and sexual segment. A large number of ciliated cells in the intermediate segment, the presence of 2 kinds of cells in the collecting ducts and a well developed permanently retained sexual segment were recorded as special features of this organ. The components of the renal parenchyma had the following relative volumes: proximal convoluted tubule = 56.4%, intermediate segment = 5.1%, distal tubule = 13.0%, collecting duct = 5.2%, and sexual segment = 11.6%.     

扬子鳄（Alligator sinensis）肾的超微结构

用透射电镜观察扬子鳄肾的超微结构。它的近曲小管和收集管上皮细胞无质膜内褶，远曲小管上皮细胞有较少的质膜内褶。扬子鳄肾小管的这些结构特征与它们在淡水生活有关。本文还就扬子鳄的栖息环境及其肾的超微结构与关咸水生活的美洲鳄进行了比较。