Anatomy and ultrastructure of the excretory system of the lizard, Sceloporus cyanogenys

The anatomy and ultrastructure of the lizard kidney (Sceloporus cyanogenys) have been studied by light and electron microscopy. The number of glomeruli was counted in serial sections and estimated to be 2,000 (in the two kidneys). Beginning with the glomerulus and Bowman's capsule the nephron segments are sequentially: (a) proximal tubule; (b) intermediate ciliated segment consisting of a proximal and distal part; (c) distal tubule, which can be divided into two segments, followed by (d) connecting tubule and (e) initial collecting duct. The initial collecting ducts from several nephrons open into the collecting duct. Tubular epithelium in this lizard has similarities to that of other reptiles. The lateral borders do not overlap like in mammals, but interdigitate by fingerlike projections. The length of the nephron segments was measured in disected tubules and the diameter was measured on light and electron micrographs. From these measurements estimates of inner tubular surface area were made. Together with data from physiological studies (Stolte et al., '76; Schmidt-Nielsen, '76) the estimated surface area was used to calculate transport rates per unit area across the epithelium. Comparisons of structure and transport rates per unit area across the epithelium. Comparisons of structure and transport rates were made between S. cyanogenys and other reptiles and mammals.     

Histogenetic stereological reconstruction of rat basophilic, clear, and oncocytic neoplastic renal cell lesions using carbonic anhydrase type II-PAS double-stained sections

The histogenesis of 3 types of rat renal cell tumors (basophilic cell, clear cell, and oncocytic) was stereologically analyzed, with particular attention paid to transitions from normal tubules. Early nitrosamine-induced preneoplastic lesions, including dysplastic tubules (altered tubules), epithelial hyperplasias, and small adenomas, were reconstructed using serially sectioned specimens processed for carbonic anhydrase type II (CA) and periodic acid-Schiff (PAS) (CA-PAS) double staining to allow easier distinction of the nephron segments: Proximal tubules had a PAS-positive brush border and were weakly positive for CA in the cytoplasm; distal tubules were PAS negative and weakly positive for CA; collecting ducts were PAS negative and strongly positive for CA. Similarly, cytochrome c oxidase (CytOx) and CytOx-PAS double staining was also applied to confirm the character of oncocytic lesions. All basophilic lesions (7 of 7) showed transition to proximal tubules. Clear cell lesions positive for CA, on the other hand, showed transition to distal tubules in 4 of 9 (44.4%) lesions and to collecting ducts in 4 of 9 (44.4%) lesions, but in only 1 of 9 (11%) to a proximal tubule. All oncocytic lesions (16 of 16), characterized by positivity for both CA and CytOx, showed transition to collecting ducts. The results indicate that the origins of renal cell neoplasia are proximal tubules for the basophilic cell lesions, either proximal or distal tubules for their clear cell counterparts, and collecting ducts for oncocytic lesions.     

The differentiation of proximal and distal tubules in the male rat kidney: the appearance of aldolase isozymes, aminopeptidase and alkaline phosphatase during ontogeny

1. The specific activities of aminopeptidase, alkaline phosphatase and aldolase isozymes were measured in homogenates of kidneys taken at different stages of ontogeny. The cellular localization of these enzymes was studied in cryostat tissue sections using substrate linked assays for aminopeptidase and alkaline phosphatase and the mixed aggregation immuno-cytochemical technique for aldolase isozymes; local enzyme concentrations were estimated photometrically. 2. The presence of both aldolase-A and aldolase-B was demonstrated in all metanephrogenic cells (and at still higher concentrations in collecting tubule cells) of the rat fetus 16 days after conception and in the undifferentiated cells of the neogenetic zone of kidney up to 8 days after birth; no aminopeptidase or alkaline phosphatase could be found in these cells. 3. Measurements made on stained tissue sections show that the shift towards aldolase-B, seen in homogenate analyses, is due to a change in the relative amounts of proximal tubules. No evidence was seen for repression in the synthesis of aldolase-A or aldolase-B monomers in the different kidney cells during ontogeny. 4. Two transitions in the mode of nephron differentiation were observed: one was shortly after birth, the other followed weaning. Before the first transition the concentrations of the enzymes increased to different degrees, such that the enzymes reached concentrations comparable with those as in the cells of adult rats by 2 to 4 days post partum. After the second transition proximal tubule size and specific activity of brush border membrane enzymes increased 3 fold. In contrast, the distal tubules did not increase significantly in size, but their aldolase-A concentration increased 3 fold. 5. Evidence based on enzyme quantification and morphometry in kidney sections is presented to demonstrate that the proximal tubule cells show functional adaptation by two independent mechanisms: specific amplification of gene expression and hypertrophy. In contrast, the distal tubule shows functional adaptation only by specific amplification of gene expression.     

Morphology of the kidney of the platypus (Ornithorhynchus anatinus: Monotremata)

The platypus kidney shows morphological similarities to those of other mammals. Macroscopically, the cortex is easily distinguishable from the fairly wide medulla. Within the medulla, no clear border is observed between the inner and outer zones. Light and transmission electron microscopically, the glomeruli show quite similar architecture to those of other mammals; however, the glomerular lobulation is very clear. The glomerular tufts are rather simple, but capillary lumen varies widely in size, which is one of the unique features of the platypus kidney. The urinary tubule is generally similar to that of human and other mammals in shape and segmentation; however, the staining specificities of histochemical reactions and the shape of epithelial cells of the Henle's loop differ from those of other mammals. The most conspicuous features are: 1) although no protein casts are found in the tubular lumina, epithelial cells of the proximal convoluted tubule (PCT) have numerous electron-dense vesicles as in human nephrotic kidneys; and 2) the platypus Henle's loop consists of the thick epithelial cells similar to the mammalian type nephron of birds. As compared to those of other mammals such as humans and rats, our observations suggest that the platypus kidney is less developed, in terms of evolution.     

Structural aspects of the decrease in several renal functions in old rats

The proliferation, size and ultrastructure of the renal cells was studied in 32-38-month-old rats. At this age no dividing epithelial cells were found and their size was enlarged (in proximal areas of the nephrons by 30,2%, in the collecting tubes by 22%). The decreased rate of glomerular filtration in aged rats in dependent on the disturbance of hemodynamics in the glomerular capillaries, for which, to certain extent, the hypofunction of the rhenin-secreting complex is responsible. The reduction of microvilli, hypoplasia of the basal folds, changes in the orientation and size of the mitochondria are considered to be the structural basis of the decreased reabsorbtion in the proximal part of the nephron. In the distal part of the nephron the organization of the plasmalemmal-mitochondrial sodium pump is disturbed. In the collecting tube the number of dark cells becomes less, they shrink and disintegrate, which is considered to be responsible for disorders in the function of the hydrogen ions secretion. The decreased level of the physiological regeneration of cells is likely to underlie the disorders in renal functions in aged rats. The lack of proliferation gradually results in a deficiency of epithelial cells, their hypertrophy, functional stress and wear.     

Increased epithelial cell proliferation and abnormal extracellular matrix in rat polycystic kidney disease

Proliferation of renal tubular epithelial cells is considered a major factor leading to cyst formation in human polycystic kidney disease (PKD). The Han:SPRD rat model for inherited PKD permits a close scrutiny, especially for early stages of the disease, and shows numerous similarities to human autosomal dominant PKD (ADPKD). In this study, the exact in vivo proliferation rate in Han:SPRD rat kidneys was evaluated in a cell type-specific manner, using immunohistochemistry with antibody to proliferating cell nuclear antigen (PCNA). The proliferation index (PI; percentage of PCNA-positive cell nuclei) was determined in normal and cystically altered tissue, and a relationship between proliferative activity and alterations in extracellular matrix expression was established using in situ hybridization for collagen I and IV mRNA. Heterozygously affected rats (cy/+) showed strong increases of PI values in cystically altered nephron portions that were mostly derived from proximal tubule. Cell proliferation obviously preceded cyst formation, because early in the progression of the disease, the normal-appearing tubules from PKD kidneys had markedly increased PI values compared with healthy controls (14.1-fold in 3-mo-old rats and 11.9-fold in 12-mo-old rats; P < 0.05), whereas later stages revealed a more generalized cystic degeneration of the nephron, with increases in PI between 14- and 82-fold, depending on the respective category of cystic epithelia. In cysts with a distal phenotype, changes were less pronounced. No significant differences were encountered between the two age groups. Proliferation was also present in interstitial cells, whereas glomeruli were unchanged. Increases in epithelial and interstitial proliferation coincided with an overexpression of matrix compounds. For comparison, changes in homozygously affected rats (cy/cy) showed up to several hundred-fold elevated PI values. These results indicate that in the Han:SPRD model for ADPKD, cystic malformation of the nephron is preceded by and coincides with enhanced epithelial and interstitial cell proliferation. Altered cell-matrix interactions seem to be directly involved in the disruption of epithelial differentiation.     

Congenital mesoblastic nephroma: an immunohistochemical and lectin study

Eight cases of congenital mesoblastic nephroma (CMN) were examined. Three CMNs were of the classical (typical) variant, two were cellular (atypical), and three showed a mixed pattern. A panel of nephron segment-specific tubular epithelial markers (the lectins Tetragonolobus purpureas, Phaseolus vulgaris erythroagglutinin, and Arachis hypogaea and antibodies to epithelial membrane antigen, cytokeratin, and Tamm-Horsfall protein) were used to differentiate epithelial structures within the tumor. Antibodies against vimentin, desmin, and muscle-specific actin were used as mesenchymal markers. A monoclonal antibody to the long (embryonic) form of polysialic acid (PSA) on the neural cell adhesion molecule was used as a putative renal oncodevelopmental marker. An antibody to proliferating cell nuclear antigen also was applied, which revealed increased proliferative rate in cellular CMNs. In addition to clearly entrapped native renal tubules, CMNs contain tubular structures with immature, dysplastic epithelium and occasional epithelial cell clusters embedded deep within the tumor. These immature tubules and clusters express distal nephron, including collecting duct markers and, occasionally, vimentin and PSA. We propose that these primitive tubules and epithelial structures may originate from the ureteric bud. An epithelial differentiation of the tumor cells also is possible. In one pure cellular CMN and two mixed CMNs the cellular component showed diffuse staining for PSA. The PSA (neural cell adhesion molecule) expression of the cellular component suggests that CMN may originate from the uninduced nephrogenic mesenchyme.     

Intra-renal and subcellular distribution of the human chloride channel, CLC-5, reveals a pathophysiological basis for Dent's disease

Dent's disease, which is a renal tubular disorder characterized by low molecular weight proteinuria, hypercalciuria and nephrolithiasis, is associated with inactivating mutations of the X-linked chloride channel, CLC-5. However, the manner in which a functional loss of CLC-5 leads to such diverse renal abnormalities remains to be defined. In order to elucidate this, we performed studies to determine the segmental expression of CLC-5 in the human kidney and to define its intracellular distribution. We raised and characterized antisera against human CLC-5, and identified by immunoblotting an 83 kDa band corresponding to CLC-5 in human kidney cortex and medulla. Immunohistochemistry revealed CLC-5 expression in the epithelial cells lining the proximal tubules and the thick ascending limbs of Henle's loop, and in intercalated cells of the collecting ducts. Studies of subcellular human kidney fractions established that CLC-5 distribution was associated best with that of Rab4, which is a marker of recycling early endosomes. In addition, confocal microscopy studies using the proximal tubular cell model of opossum kidney cells, which endogenously expressed CLC-5, revealed that CLC-5 co-localized with the albumin-containing endocytic vesicles that form part of the receptor-mediated endocytic pathway. Thus, CLC-5 is expressed at multiple sites in the human nephron and is likely to have a role in the receptor-mediated endocytic pathway. Furthermore, the functional loss of CLC-5 in the proximal tubules and the thick ascending limbs provides an explanation for the occurrences of low molecular weight proteinuria and hypercalciuria, respectively. These results help to elucidate further the patho-physiological basis of the renal tubular defects of Dent's disease.     

Renal alterations induced by Cerastes cerastes cerastes venom

The purpose of the present study was to determine the structural and functional alterations in the kidneys of rats experimentally envenomated with the viper Cerastes cerastes cerastes. The effects of a single i.p. sub-LD50 (0.73 mg Kg-1) for 24 hr and 7 consecutive doses (0.42 mg Kg-1; daily injection) on the kidneys of rats were studied. Histological examination of the kidneys in envenomated rats revealed the presence of atrophied glomeruli, cloudy materials within the proximal and distal convoluted tubules, atrophy and hypertrophy of quite a large number of tubular nuclei and mesangial proliferation. Hemorrhagic areas, dilated thrombosed vessels, fibrosis, and mitotic indices were also among the histopathological alterations in the renal cortex of the venom-injected animal groups. Electron microscopical findings revealed the presence of cell debris and electron dense materials within the lumen of the distal and proximal convoluted tubules. The cell lining of the proximal convoluted tubules underwent variable degrees of degeneration, and most of their intracellular organelles were deteriorated. The glomeruli contained quite large areas of mesangial matrix, and the lumen of most of their blood capillaries enclosed a moderately electron dense matrix. The cells of the endothelial lining of these capillaries were hypertrophied, and most of them lost their integrity and diffused with the surrounding matrix of the capillary lumen. Several leukocytic aggregates were observed in the intertubular spaces and blood capillaries. In addition, the erythrocytes of most of these capillaries displayed different patterns of electron density.     

Renal cell tumours in mice: electron microscopy versus immunohistochemistry

15 renal cell tumours induced in CBA male mice by 1,2-dimethylhydrazine were studied electronmicroscopically (EM). All these tumours earlier were investigated histochemically and immunohistochemically with the use of gamma-glutamyltranspeptidase (GGT) as a marker of the proximal tubules and antigen A6 as a marker of distal tubules and/or collecting ducts. One of the tumours was GGT-positive and antigen A6-negative and ultrastructurally well developed brush border was found. This correlation between immunohistochemistry and EM data allowed to conclude that the site of origin of this tumour were the cells of the proximal tubules. All other tumours were GGT-negative and antigen A6-positive, i.e. the development of these tumours from the distal tubules (and not from the proximal tubules) could be suspected. However in 5 of these neoplasms reduced brush border was found. Microvilli were of a smaller size than in normal proximal tubules and were frequently located not on the apical cell surface but in the narrow spaces between two cells. Invaginations and apical vesicles typical for the normal proximal tubules were also found in some tumour cells. EM and immunohistochemical data combined allow to suggest the origin of these tumours from the common precursor cell capable of differentiation, in the course of tumour progression, into the cells with properties of proximal and/or distal tubules.     

Localization of ROMK channels in the rat kidney

Renal potassium secretion occurs in the distal segments of the nephron through apically located secretory potassium (SK) channels. SK may correspond to the ROMK channels cloned from rat kidney. In this study, the localization of ROMK at the cellular level in the rat kidney was examined using an affinity-purified polyclonal antibody raised against a C-terminal peptide of ROMK. The specificity of the antibody was demonstrated by immunoblots of membranes of Xenopus oocytes expressing ROMK2. Immunoblots of homogenates from rat renal outer medulla and cortex revealed predominant bands of 70 to 75 kD, which were ablated by preadsorption with an excess of peptide. These bands were specific for the rat kidney. Immunolocalization studies revealed that ROMK is expressed in specific nephron segments in both the cortex and medulla. In the cortex, ROMK was found in the apical domain of the thick ascending limb of Henle's loop, the connecting tubule, and in some, but not all, cells of cortical collecting tubules. In the medulla, expression in the apical membrane of the thick ascending limbs of Henle's loop was strong, whereas outer medullary collecting ducts were weakly stained. Expression in the thick ascending limb was also heterogeneous; some cells that expressed the Na-K-Cl cotransporter were weakly stained with the anti-ROMK antibody. No staining of glomeruli, proximal tubules, or inner medullary collecting ducts was found. The localization of ROMK agrees well with the findings of SK in patch-clamp studies and supports the view that ROMK is the SK channel of the distal segments of the nephron.     

Evidence for distal tubular inhibition of calcium efflux by nisoldipine in the SHR rat

Nisoldipine, a calcium channel blocking agent, is known to have antihypertensive, renal tubular and hemodynamic effects. The present studies were designed to examine the effects of this drug on the renal tubular transport of calcium in 12 saline-loaded SHR rats. Calcium-45 was infused into three different nephron segments: early proximal, late proximal and early distal sites with or without nisoldipine. Calcium efflux averaged 93.6 +/- 4.9 and 90.5 +/- 8.7% after early and late proximal administration, respectively, indicating that the proximal tubule and the loop of Henle are highly efficient in transporting calcium out of the tubule. In distal nephron segments, calcium transport was limited to 41.1 +/- 4.8% of the amount delivered to these tubules. Nisoldipine inhibited the efflux of simultaneously infused calcium. This apparent inhibitory effect occurred predominantly in distal nephron segments where the drug reduced calcium efflux from 41.1 +/- 4.8 to 22.5 +/- 2.7%, indicating a 45.3% reduction in net calcium reabsorption. The results are consistent with the interpretation that nisoldipine-induced reduction in the tubular efflux of calcium was secondary to a direct inhibition of voltage-sensitive, L-type calcium channels or to a blunting of the rate of phosphorylation of channel proteins by protein kinase C in the distal tubular epithelial cells.     

Evolutionary transformations of the esophageal lining in vertebrates and man in light of the theory of phylembryogenesis

The comparative histological study of processes of embryonic histogenesis of the oesophagus epithelium in certain representatives of vertebrate animals (fishes, amphibia, reptiles, birds, mammals) and man was carried out in the light of the academician A. N. Severtsov's theory of phylembryogenesis. It has been shown that phylogenetic changes of the oesophagus lining of vertebrates related with its evolutionary transformations are regularly reflected in the stages or steps of the oesophagus epithelium histogenesis in ontogenesis of each systematic unit of the subtype. The first step is the phylembryogenesis of the oesophagus lining in all vertebrates is the monolayer epithelium of the endodermal origin; the secone step is the striated muco-ciliated (in low vertebrates and most reptiles) and pseudostratified (embryos of higher vertebrates) epithelium; the third step is the pseudostratified mucous epithelium (bony fishes) and stratified lining (prefetuses of higher vertebrates). The forth step is the stratified mucous (steppe turtle) and stratified muco-ciliated epithelium (fetuses of birds, many mammals and man). The fifth step--stratified squamous epithelium (birds, mammals, man). The academician A. N. Severtsov's theory of phylembryogenesis clearly points to the origins (the entoderm), routs and mechanisms of the transformation of the oesophagus lining in its evolutionary development.     

The ultrastructure of the protonephridial tubules of the freshwater planarian Bdellocephala brunnea

A systematic electron microscopic investigation was undertaken to elucidate the regional differentiation of the protonephridial tubules of the freshwater planarian Bdellocephala brunnea. The excretory system consists of many independent protonephridia, each made up of the repeatedly branching "proximal tubules," and highly tortuous "distal tubule." The proximal tubules are composed of a simple ciliated epithelium except in the trunk which lacks ciliation. The cytoplasmic features of the ciliated cell closely resemble those of the flame cell, indicating highly active protein reabsorption by pinocytosis. Pinocytosis seems also active in the trunk of the proximal tubules and in the distal tubule, resulting in cytoplasmic vesicles of typical appearance. Salient features of the distal tubule cell are the increased density of the cytoplasmic matrix and the presence of many basal compartments of cytoplasm with mitochondria, a pattern common to many epithelia with osmoregulatory function. The morphology and possible functions of the planarian protonephridium are discussed in comparison with the vertebrate nephron.     

A non-aversive approach to reducing hospital absconding in a head-injured adolescent boy

Human envenomation caused by bee or wasp stings has been reported to cause acute renal failure (ARF), usually due to acute tubular necrosis (ATN), as a frequent complication. The pathogenetic mechanisms of ATN occurring in these accidents are still unclear. In the present study, female Wistar rats weighing 150-200 g were injected intravenously with Africanized bee venom at a dose of 0.4 microl/100 g body weight and used in functional and light microscopy studies. The animals were divided into two groups: the early group was studied 3-8 h after inoculation, and the late group was studied 24-30 h thereafter. The animals showed ARF characterized by reduction of glomerular filtration rate with increasing levels of plasma creatinine. They also showed increased fractional sodium and potassium excretions, suggesting changes in the proximal portion of the nephron. The water transport through collecting tubules was reduced, with consequent diuresis, indicating functional changes in the distal portion of the nephron. These functional changes were more marked in the early group, with recovery tending to occur after 24 h. Albuminuria was also observed in this group. Light microscopy showed ATN mainly in cortex and outer medulla, with isolated necrosis in cells or small groups of cells and cast formation in the distal and collecting tubules. After 24 h frequent mitotic figures were found in the tubular epithelium. The observed ARF was due to ATN which in turn was probably caused by multiple effects, mainly hemodynamic changes secondary to cardiotoxicity and systemic vasodilation caused by the venom, myohemoglobinuria, and the direct action of the venom on tubular cells.     

Freeze-fracture analysis of junctional complexes in the nephron of the garter snake, Thamnophis sirtalis

Zonulae occludentes are shown by freeze-fracture to be pleomorphic along the garter snake nephron. In the neck and proximal segments the occluding junctions are moderately complex with frequent discontinuities in their junctional fibrils. Junctional depth and complexity are maximal in the distal and collecting segments and discontinuities in fibrils are absent. Comparison of these results with similar observations on other tissues indicates that the zonulae occludentes in the neck and proximal segments are "intermediate" to "leaky" and that they may be "very light" in the distal and collecting segments. The findings suggest that in the garter snake nephron transepithelial flow of fluid may occur primarily by passive diffusion through the zonulae occludentes in the neck and proximal segments and by cell-mediated osmotic flow in the distal and collecting segments. Gap junctions occur only in the proximal tubule and areprobably involved in low resistance, intercellular movement of ions.     

Nuclear distribution in Alternaria tenuis

Renal functional abnormalities constituting the syndrome of postobstructive diuresis imply both altered tubular and glomerular membrane properties. To determine the morphologic and ultrastructural correlates of this disorder a rat model was developed and 32 postobstructed kidneys were studied by light and electron microscopy at the midpoint of diuresis and compared to 22 controls. The abnormal morphology was: dilated distal tubules and collecting ducts, isolated proximal and distal tubule cells that allowed free access of luminal contents to the basement membrane, widened terminal bars and intercellular spaces, thickening of the glomerular basement membrane and, depending upon the portion of nephron, normal or reduced adenosine triphosphatase and acid phosphatase content. In order to confirm the functional nature of the nephrons studied as well as to assess glomerular and tubular permeability, horseradish peroxidase and cytochrome c were infused. These tracers, normally permeable to the glomerular basement membrane, were found in the intercellular spaces and to a lesser extent within cell organelles in the postobstructed diuretic animals whereas controls demonstrated a retarded filtration of horseradish peroxidase, no tracer in the intercellular spaces and large amounts of tracer contained within cell organelles. Absence of enzyme activity in the medulla and reduced dark to light cell ratios in the cortical collecting ducts correlated with prior observations made by others of diminished concentration and acidification processes, respectively. An increase in adenosine triphosphatase activity and renin granules within the juxtaglomerular cells indicated increased renin activity. These observations suggest that the renal functional abnormalities of postobstructive diuresis are attributable to altered glomerular and tubular permeabilities as well as with changes in metabolic activity.     

Familial renal tubular dysgenesis: a disorder not isolated to proximal convoluted tubules

The autopsy findings of a newborn with renal tubular dysgenesis, born to first cousins of Moslim Arab descent, are described. Hypocalvaria and hyperflexible joints were noted in addition to the renal lesion. A microdissection study demonstrated marked shortening of all the nephron segments from the glomeruli to the collecting tubules, rather than an isolated abnormality of the proximal convoluted tubules.     

Epithelial galectin-3 during human nephrogenesis and childhood cystic diseases

Galectin-3 is a beta-galactoside-binding protein with putative roles in development, oncogenesis, and inflammation. Its expression in human nephrogenesis has not been previously reported. This study examines galectin-3 expression in early human embryos by Western blot and immunohistochemistry. This 33-kD protein was detected in the apical domain of distal tubules of the mesonephros and also in the mesonephric duct. In the metanephros, the adult kidney precursor, galectin-3 was detected in the apical domains of ureteric bud branches, and there was intense expression in fetal medullary and papillary collecting ducts in both the cytoplasm and plasma membranes. Low levels of galectin-3 were detected in the cytoplasm of a subset of cells in adult collecting ducts; these were alpha-intercalated cells because they expressed basal band 3 protein. In human multicystic dysplastic kidneys, all diseased epithelia had an embryonic apical expression pattern of galectin-3 and, in addition, all cystic epithelia in autosomal recessive polycystic kidneys expressed this molecule. It is concluded that galectin-3 is expressed by cells of the mesonephric duct/ureteric bud lineage, and it is speculated that the different subcellular locations may be implicated in both the regulation of normal growth and differentiation of this lineage, as well as in the pathogenesis of cystic epithelia.     

Lithium secretion in kidneys of amphibians and reptiles under hydrated conditions

Renal lithium transport was studied at different hydration levels in five species of anuran amphibians (Bufo bufo, B. danatensis, B. viridis, Rana ridibunda, and R. temporaria), two species of urodeles (Triturus vulgaris and T. cristatus) and four species of reptiles (lizards Eremias multiocellata, Lacerta vivipara, Trapelus sanguinolentus, and Teratoscincus scincus). Under dehydration conditions, Li+ was reabsorbed in the kidneys of amphibians ans reptiles, but to a lesser degree than in mammalian kidneys: the ratio of lithium clearance (CLi) to glomerular filtration rate (GFR)--fractional lithium excretion--in dehydrated animals was in the range 0.5-0.8. The transition to the hydrated state resulted in a cessation of net renal lithium reabsorption. Under condition of high hydration, all the animals studied, except for urodeles, showed net renal secretion of Li+, i.e., CLi exceeded GFR. The ratio CLi/GFR was 1.2-1.3 in hydrated anurans and 1.7-2.3 in hydrated lizards. In urodeles, this ratio was approximately unity. It is suggested that renal lithium secretion in hydrated amphibians and reptiles reflects fluid secretion in the proximal tubule, which is additional to the glomerular filtration mechanism of fluid delivery to nephron under water loading.