Localization of aquaporin-7 in rat and mouse kidney using RT-PCR, immunoblotting, and immunocytochemistry

To establish the segmental, cellular, and subcellular localization of AQP7 in rat and mouse kidney, we used RT-PCR, immunocytochemical, and immunoblotting approaches. RT-PCR of rat and mouse kidney zones revealed AQP7 mRNA in cortex and outer stripe of the outer medulla. RT-PCR on microdissected nephron segments revealed AQP7 mRNA in proximal convoluted and straight tubules. Immunoblotting using peptide-derived rabbit antibodies to either rat or mouse AQP7 revealed a 28-kDa band in kidney and testes from rat and mouse, respectively. Immunocytochemistry revealed strong AQP7 labeling of segment 3 proximal tubules and weaker labeling of proximal convoluted tubules in both rat and mouse kidneys. The labeling was almost exclusively confined to the brush border with no basolateral labeling. No labeling was observed of thin descending limbs or collecting duct. Immunolabeling controls were negative. The presence of AQP7 in the proximal tubule brush border indicates a role of AQP7 in proximal tubule water reabsorption.     

Intrarenal localization of degradation of atrial natriuretic peptide in isolated glomeruli and cortical nephron segments

Using isolated glomeruli and nephron segments obtained from collagenase treated rabbit kidneys, we examined the in vitro degradation of alpha-human atrial natriuretic polypeptide (alpha-hANP). The ANP-degrading activity was measured by the amount of immunoreactive ANP remaining after incubation of about 50 fmoles alpha-hANP with each tissue preparation for 7.5 min. The sequence of degrading activity among isolated nephron segments was as follows: proximal straight tubule greater than proximal convoluted tubule greater than cortical collecting tubule greater than distal convoluted tubule greater than cortical thick ascending limb. A single glomerulus exhibited the degrading activity which was comparable to approximately 50% of the activity of 1 mm proximal convoluted tubule. Phosphoramidon, an inhibitor of endopeptidase, prevented the degradation of ANP in proximal convoluted tubule and glomerulus by 68% and 89%, respectively, but not in cortical thick ascending limb and cortical collecting tubule. From these results, we conclude that the degradation of ANP by endopeptidase occurs mainly in the proximal tubule and glomerulus.     

SERUM ALBUMIN UPTAKE IN ISOLATED PERFUSED RENAL TUBULES : Quantitative and Electron Microscope Radioautographic Studies in Three Anatomical Segments of the Rabbit Nephron

Proximal convoluted, proximal straight, and cortical collecting tubular segments isolated from rabbit kidney were perfused with I 125-labeled rabbit serum albumin (RSA-I 125) in ultrafiltrate of serum for up to 3 hr After perfusion, the segments were fixed with glutaraldehyde, embedded in Epon, and either counted with a gamma spectrometer to quantitate protein accumulation or analyzed by electron microscope radioautography to sequentially localize radioactivity Proximal convoluted and proximal straight segments accumulate RSA-I 125 nearly linearly as a function of time whereas cortical collecting segments do not accumulate measurable amounts of protein. The rate of accumulation of RSA-I 125 in the proximal convoluted tubule is 2 6 times as great as that in the proximal straight tubule. Electron microscope radioautography of the isolated proximal tubule demonstrated that RSA-I 125 is taken up via small apical vesicles and tubular invaginations, released into large cytoplasmic vacuoles, and finally concentrated in membrane-bounded structures, some of which are acid phosphatase positive These results show that albumin is absorbed by proximal tubules and may be degraded intracellularly within lysosomes. In addition, less radioactivity was located at all times over the lateral intercellular and basilar labyrinthine spaces, suggesting that labeled albumin and/or its breakdown products may be transported across the peritubular cell membrane.     

Expression and function of arginine-producing and consuming-enzymes in the kidney

The kidney plays a key role in arginine metabolism. Arginine production is controlled by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) which metabolize citrulline and aspartate to arginine and fumarate whereas arginine consumption is dependent on arginine:glycine amidinotransferase (GAT), which mediates creatine and ornithine synthesis. Histological and biochemical techniques have been used to study the distribution and activity of these enzymes in anatomically dissected segments, in isolated fragments of tubules and in whole tissues. ASS and ASL mRNAs and proteins are expressed in the proximal tubule. Within this nephron segment, the proximal convoluted tubule has a higher arginine synthesis capacity than the proximal straight tubules. Furthermore, this arginine-synthesizing portion of the nephron matches perfectly with the site of citrulline reabsorption from the glomerular filtrate. The kidney itself can produce citrulline from methylated arginine, but this capacity is limited. Therefore, intestinal citrulline synthesis is required for renal arginine production. Although the proximal convoluted tubule also expresses a significant amount of GAT, only 10% of renal arginine synthesis is metabolized to guanidinoacetic acid, possibly because GAT has a mitochondrial localization. Kidney arginase (AII) is expressed in the cortical and outer medullary proximal straight tubules and does not degrade significant amounts of newly synthesized arginine. The data presented in this review identify the proximal convoluted tubule as the main site of endogenous arginine biosynthesis.     

Effect of phosphate deprivation on enzymes of the cyclic adenosine monophosphate metabolism in rat proximal convoluted and proximal straight tubules.

Phosphate deprivation causes a resistance to the phosphaturic effect of parathyroid hormone (PTH). The present study determined whether acute phosphate deprivation alters basal or stimulated activities of key enzymes of the cyclic adenosine monophosphate (cAMP) metabolism in microdissected proximal convoluted and proximal straight tubules, since blunted cAMP levels in these proximal subsegments might account for refractoriness to the effect of PTH on phosphate reabsorption in the proximal convoluted and proximal straight tubule segments. In the proximal convoluted tubules of rats fed a normal-phosphate diet (NPD), PTH increased the adenylate cyclase activity by tenfold. In the proximal convoluted tubule of rats fed a low-phosphate diet (LPD), PTH also increased the adenylate cyclase activity by tenfold. In addition, forskolin increased the adenylate cyclase activity to levels similar to PTH in the proximal convoluted tubule of rats fed NPD or LPD. In the proximal straight tubule of rats fed NPD, PTH resulted in an approximately fivefold increase in adenylate cyclase activity. In the proximal straight tubule of rats fed LPD, PTH resulted in a fourfold increase in adenylate cyclase activity. The forskolin-stimulated adenylate cyclase activity was markedly decreased (46%) in the proximal straight tubule of phosphate-deprived rats. The cAMP-phosphodiesterase activity in the proximal convoluted tubule was significantly increased by 26% in phosphate-deprived rats. The cAMP-phosphodiesterase activities in the proximal straight tubules from rats fed NPD or LPD were similar. We conclude that distinct differences in key enzymes of cAMP metabolism exist in the proximal convoluted and proximal straight tubule subsegments. Further, phosphate deprivation affects the cAMP-phosphodiesterase and adenylate cyclase activities differently in these nephron subsegments.(ABSTRACT TRUNCATED AT 250 WORDS)     

金雕肾脏的组织学观察

利用生物显微技术观察了金雕Aquila chrysaetos肾脏的组织结构.结果表明,金雕肾实质由许多肾小叶构成,每个肾小叶可分为皮质和髓质两部分.肾单位由一个肾小体和一条与其相连的肾小管构成.肾小体由肾小囊和肾小球组成.肾小管分为近曲小管、髓袢、远曲小管和连接小管.集合管分为小叶周集合小管和髓质集合管两部分.具有发达的极周细胞.     

Comparison of the distribution of tissue kallikrein and esterase A, a kallikrein-like enzyme, in rat kidney using specific monoclonal antibodies

Tissue kallikrein (E.C. 3.4.21.35) and arginine esterase A, another closely related, kinin-generating serine protease, have been localized by immunocytochemistry in rat kidney, using monoclonal antibodies that do not crossreact with other kallikrein-related enzymes or with tonin. Kallikrein was present primarily in the apical cytoplasm of the connecting tubule and the cortical collecting duct. Esterase A, on the other hand, was present primarily in the basolateral region of both proximal and distal straight tubules in the outer medulla and medullary rays. In addition, esterase A was demonstrable in distal convoluted tubules and, to a lesser extent, in proximal convoluted tubules. The presence of different kinin-generating enzymes at these sites would permit the formation of kinins from appropriate substrates on both the vascular and luminal poles of separate segments of the kidney tubule.     

Characterization of N-ethylmaleimide-sensitive proton pump in the rat kidney. Localization along the nephron

This study is aimed both at characterizing an ATPase activity in rat kidney equivalent to the proton pump described in bovine kidney medulla and at localizing this enzyme along the nephron. Membrane fractions isolated from kidney homogenates by differential and density gradient centrifugations were enriched 7-fold in ATPase activity sensitive to N-ethylmaleimide (NEM). These fractions also displayed ATP-dependent proton transport. ATPase activity and proton transport in vesicles had similar pharmacological properties as both were insensitive to vanadate and ouabain and had similar sensitivities toward NEM (apparent Ki = 20 microM) and N,N'-dicyclohexylcarbodiimide (apparent Ki = 50 microM). Proton transport was dependent on chloride availability as chloride addition to the extravesicular medium stimulated proton transport in a dose-dependent fashion (apparent K 1/2 = 7 mM). NEM-sensitive ATPase activity displaying similar pharmacological properties as proton transport in vesicles was also found in single segments of nephron. It was insensitive to vanadate and ouabain, was inhibited by similar concentrations of NEM (apparent Ki = 15-20 microM) and N,N'-dicyclohexylcarbodiimide (apparent Ki = 30 microM), and is therefore likely to be a proton pump. NEM-sensitive ATPase was localized in all the segments of the rat nephron; its activity was highest in proximal convoluted tubules; intermediate in proximal straight tubules, thick ascending limbs, and cortical collecting tubules; and lowest in outer medullary collecting tubules.     

Progressive enzyme changes within anatomically defined segments of rabbit nephron: demonstration with a new technique

The kidney is an extremely heterogeneous organ, with morphological, physiological, and metabolic changes occurring from segment to segment along each nephron. To determine the heterogeneity that might exist within discrete anatomical segments of rabbit nephron, we developed a technique for making quantitative enzyme assays in serial samples, about 100 micron long, along identified segments of the nephron. Results for three enzymes in proximal convoluted and straight tubules show that adenylate kinase, an enzyme of high-energy phosphate metabolism, gradually decreases along the S1 and S2 segments of the proximal tubule, with no abrupt changes. Fructose bisphosphatase, a gluconeogenic enzyme, is high along the major portion of the proximal tubule but plummets along the final millimeter of S3. Conversely, phosphofructokinase, a glycolytic enzyme, is very low along the proximal tubule but increases sharply within the final millimeter. These data underscore the biochemical heterogeneity of the nephron, illustrating the enzyme levels may change markedly even within anatomically defined regions. They also suggest the importance of further studies of this type and demonstrate a practical means for such studies.     

甘肃鼢鼠肾结构特征

甘肃鼢鼠(Myospalax cansus)在缺水的黄土高原营严格地下生活,主要从食物中摄取水分.为研究甘肃鼢鼠的调水机制,用组织解剖学方法对其肾的形态结构进行观察.结果显示,甘肃鼢鼠肾为单乳头肾,呈蚕豆形,表面光滑,不分叶;皮质与髓质的厚度比为0.71:1;皮质中髓旁肾单位相对分布密度小于浅表肾单位的相对分布密度;髓旁肾单位中的血管球直径大于浅表肾单位中的血管球直径;近曲小管与远曲小管的截面数量比为2.25:1.结果表明,甘肃鼢鼠肾的重吸收能力较小,其组织形态学结构有显著的生态适应意义.     

Na-K]ATPase activity in proximal and distal tubules of the rat kidney: modification and application of a quantitative cytochemical technique

A modified cytochemical assay for [Na-K]ATPase in cryostat sections of kidney was further characterized and used to quantify activity in seven functionally distinct sites along the rat nephron. The activity of [Na-K]ATPase was defined as the difference in ATPase activity in specifically identified tubules contained in serial sections incubated with and without ouabain. Preincubation of sections with ouabain was required for maximal inhibition of [Na-K]ATPase activity in several distal sites. The concentration of ouabain necessary for maximal inhibition of activity was 3.0 mM and half-maximal inhibition was obtained in all regions with 30-100 microM ouabain. In distal sites, [Na-K]ATPase formed a higher proportion of total ATPase activity (60-80 per cent) than in proximal sites (20-40 per cent). Enzyme activity was quantified using two different methods. The first measured activity over the basal region of tubules and gave an index of the concentration of [Na-K]ATPase over the basal lateral infoldings of cells composing the tubule. The second read activity over the entire cross section of tubules and provided an estimate of [Na-K]ATPase per length of tubule. The highest activities over the basal basal region were obtained from tubules of the distal nephron including the inner (MALin) and outer (MALout) medullary ascending limb, distal convoluted tubule (DCT) and connecting segment (CS). Lower activities were obtained in proximal convoluted (PCT) tubules, proximal straight (PS) tubules and the papillary collecting duct (PD). Distal convoluted tubules contained the highest activity per length of tubule. Other sites contained lower levels of activity in the following order: MALin greater than MALout greater than PCT greater than PD greater than PS. The modifications introduced increase the sensitivity and precision of this assay and permit the application of this technique to studies of [Na-K]ATPase activity in the major functional regions of the rat nephron.     

Neuropeptide Y inhibits cAMP accumulation in renal proximal convoluted tubules

The effect of neuropeptide Y (NPY) on cAMP accumulation in various segments of the rabbit nephron was examined. NPY inhibited parathyroid hormone-stimulated cAMP accumulation in the proximal convoluted tubule in a concentration-dependent manner. NPY also inhibited forskolin-stimulated cAMP production in this segment of the nephron. In contrast, NPY had no effect on parathyroid hormone or forskolin-stimulated cAMP accumulation in the proximal straight tubule. Similarly, NPY had no effect on forskolin-stimulated cAMP levels along the rest of the nephron. These results are consistent with previous studies which have localized NPY receptors to the proximal convoluted tubule, and suggest that NPY via its effects on cAMP metabolism may play a role in proximal tubule transport.     

Angiotensin converting enzyme predominates in the inner cortex and medulla of the rat kidney

Regional distribution of angiotensin converting enzyme(ACE) in the rat kidney was studied. The ACE activities in the inner cortex and outer medulla were about 10 and 5 times those in the outer cortex, respectively. The activity in the inner medulla or papilla was much the same as that in the outer cortex. Immunofluorescence was greatest in the proximal tubules in the inner cortex, while the outer medulla and the inner medulla or papilla showed a weak fluorescence. The brush border membranes isolated from the inner cortex also possessed about 10 times the ACE activity seen in the outer cortex. The results indicate that the major source of renal ACE is not the proximal convoluted tubules in the outer cortex, but rather the brush border membranes of proximal tubules in the inner cortex. The contribution of ACE in the inner cortex would therefore be predominant.     

Presence of an extramitochondrial anion-stimulated ATPase in the rabbit kidney: Localization along the nephron and effect of corticosteroids

Summary To determine whether kidney membrane fractions contain an extramitochondrial anion-stimulated ATPase, we compared the pharmacological and kinetic properties of HCO₃-ATPase activities in mitochondrial and microsomal fractions prepared from rabbit kidney cortex and outer medulla. The results indicated that this activity differed markedly in each type of fraction. Microsomal HCO₃-ATPase was less sensitive than mitochondrial ATPase to azide, oligomycin, DCCD and thiocyanate, but was more sensitive to filipin and displayed different dependency towards ATP, magnesium and pH. Microsomal ATPase activity was stimulated by sulfite much more strongly than by bicarbonate, whereas mitochondrial activity was stimulated by both these anions to a similar extent. These results demonstrate the presence of an extramitochondrial HCO₃-ATPase in kidney membrane fractions. HCO₃-ATPase was also measured in single microdissected segments of the rabbit nephron using a radiochemical microassay previously developed for tubular Na, K-ATPase activity. An enzyme with the pharmacological and kinetic properties of the microsomal enzyme was detected in both proximal tubule, distal convoluted tubule and collecting duct, but the thick ascending limb was devoid of any detectable activity. Long-term DOCA administration markedly increased HCO₃-ATPase activity in the distal convoluted and collecting tubule. The insensitivity of microsomal HCO₃-ATPase to vanadate indicates that it belongs to the F₀–F₁ class of ATPases, and might therefore be involved in proton transport. This hypothesis is also supported by the localization of tubular HCO₃-ATPase activity at the sites of urinary acidification.     

Lactate oxidation by three segments of the rabbit proximal tubule

Oxidation of [U14C]lactate to 14CO2 was measured in vitro, in nonperfused anatomically defined segments of rabbit proximal tubule (S1, proximal convoluted, and S2 and S3, proximal straight tubules). The rate of lactate oxidation was similar in S2 and S3 segments, and within the range of lactate oxidation rates measured in vivo. In contrast, the oxidation rate of S1 segments was significantly lower than that of S2 or S3. In proximal straight tubules, lactate oxidation was inhibited by incubation at 0 degrees C, or by application of 1 mM ouabain. To determine if the rate of transepithelial transport affected the rate of lactate oxidation, lactate oxidation was measured in proximal straight tubules after the lumen had been opened by perfusion with Ringer's containing 10 mM polyethylene glycol. No difference in lactate oxidation rate was observed between tubules with patent lumina and nonperfused tubules. These results suggest that the various segments of the renal proximal tubule have different metabolic characteristics, and that the rate of substrate oxidation is related to the activity of the Na+, K+-ATPase.     

Kallikrein (kininogenase) in the mouse nephron: effect of dietary potassium

Kininogenase activity of kallikrein was measured in microdissected mouse nephron segments using kininogen from dog plasma and a radioimmunoassay for bradykinin. When single nephron segments were examined, results showed a large scatter. This was found to be due to heterogeneity of distal convoluted tubules (DCT) from different nephrons, since replicate measurements in pools of DCT structures did not show this degree of variation. Nearly 20% of activity was accessible to extracellular substrate when freshly dissected segments were incubated in isoosmotic media. Freezing and thawing which markedly releases activity of intracellular enzymes, did not significantly elevate kininogenase activity. On the other hand deoxycholate and trypsin treatment increased tubular kininogenase activity in an additive fashion. A detailed analysis of microdissected tubule fragments revealed that kallikrein is concentrated in late distal convoluted tubule before entering a branching point (connecting tubule). In contrast initial portions of distal convoluted tubules and cortical collecting tubules contained only little kallikrein activity. Potassium rich diet increased basal and total activity 5-fold, when compared to a potassium poor diet.     

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.     

Immunohistochemical identification of tubular segments in percutaneous renal biopsies

Summary To identify the renal cortical tubular segments involved in tubulo-interstitial disease in formalin-fixed, paraffin-embedded percutaneous kidney biopsies, we developed multiple immunolabeling protocols using segment-specific tubular markers. The present study of biopsies from patients with minimal change or thin basement membrane nephropathy provides a baseline for interpretation of histopathology. Proximal tubules were stained either by the PAS reaction or by the biotinylated Phaseolus vulgaris erythroagglutinin (PHA-E)-streptavidin-gold-silver system (brush borders black). The anti-Tamm-Horsfall (THP) antibody-immunoperoxidase (aminoethylcarbazole, AEC-IPO), and anti-epidermal cytokeratins (ECK) antibodies-immunoalkaline-Fast Blue BB methods marked the distal straight tubules and the cortical collecting system red-brown and blue, respectively. When these immunolabelings were combined, the coapplication of AEC-PO-labeled peanut agglutinin (PNA) or anti-epithelial membrane antigen antibody-AEC-IPO technique (both are markers for distal nephron) visualized the apical membranes of distal convoluted tubules. In the protocol PHA-E + PNA + THP + ECK, the tubular basement membranes were outlined by the anti-laminin antibody-AEC-IPO staining, carried out simultaneously. The protocol PNA + THP + ECK + PAS was found to be a quite appropriate multiple immunolabeling method for the tubules, and is recommended for use as a tool in the study of tubulo-interstitial diseases.Abbreviations PAS periodic acid-Schiff reaction - PHAE Phaseolus vulgaris erythroagglutinin - PNA Peanut agglutinin - EMA epithelial membrane antigen - THP Tamm-Horsfall glycoprotein - ECK epidermal cytokeratins - PO peroxidase - Biot-PHA-E biotinylated PHA-E - APAAP complexes of alkaline phosphatase and mouse monoclonal anti-alkaline phosphatase - SWARI swine anti-rabbit immunoglobulins - FCS fetal calf serum - TBS Tris-buffered saline - AEC aminoethylcarbazole - DAB diaminobenzidine - FBBB Fast Blue BB - IA immunoalkaline - GL glomerulus - PT proximal tubule - DST distal straight tubule - DCT distal convoluted tubule - CCS cortical collecting system - CT connecting tubule - CD collecting duct     

Renal adrenergic receptors: localization of [125I]prazosin binding sites along the microdissected rat nephron.

Norepinephrine stimulates renal tubular sodium reabsorption, probably through an alpha 1-adrenoceptor-mediated mechanism. Although the distribution of alpha 1-adrenoceptors in the kidney has been studied with autoradiography, the precise location of these receptors in isolated nephron segments is unclear. Using a microassay we determined the specific binding of [125I]iodoarylazidoprazosin ([125I]prazosin), a high specific radioactivity analog of the selective alpha 1-antagonist prazosin, to microdissected glomeruli and tubule segments. Specific binding of [125I]prazosin (3 nM) in the proximal convoluted tubule was time- and concentration-dependent, saturable, and reversible. In this segment the apparent KD by association and dissociation rate constants of [125I]prazosin binding was 0.47 nM, and the maximum receptor density was approximately 0.19 fmol/mm, or 720 fmol/mg protein. Binding specificity was verified in competition studies with excess (3 microM) unlabeled prazosin and probes for alpha 2- (yohimbine), beta- (propranolol), dopamine1- (SCH23390), and dopamine2- (S-sulpiride) receptors. [125I]Prazosin binding was inhibited significantly only by unlabeled prazosin. Mapping of prazosin binding along the nephron revealed that the highest density was in the proximal convoluted tubule, followed by the proximal straight tubule. Lesser binding was found in the thick ascending limb and in the distal convoluted tubule, whereas in the cortical and outer medullary collecting duct and in glomeruli, binding was not significantly different from zero.(ABSTRACT TRUNCATED AT 250 WORDS)     

A cytophotometric analysis of the activity of oxidative enzymes and Na, K-ATPASE in vertebrate nephrons at different levels of sodium transport in the kidney.]

Using quantitative cytochemistry, activities of Na, K-ATPase, succinate dehydrogenase (SDH) and alpha-keto-glutarate dehydrogenase (alpha-KDH) was investigated in cells of renal tubules at different levels of sodium reabsorption in the kidney. The activity of these enzymes in mammals and birds renal tubule cells was found to be higher than in the cells of corresponding renal tubules of cold-blooded vertebrates. This corresponds to the increased total amount of reabsorbed sodium in the kidney of warm-blooded animals. The summer frogs, as compared to the winter ones, exhibit higher activities of SDH and Na,K-ATPase in the proximal tubule cells where changes in sodium reabsorption are also noted. In the kidney of marine teleosts, a negative correlation between U/PNa and the activity of SDH and Na,K-ATPase in the cells of proximal and distal tubule was observed. Aldosterone was found to stimulate sodium reabsorption and to activate Na,K-ATPase.SDH and alpha-KDH mainly in the distal convoluted tubule. Furosemide was observed to inhibit sodium reabsorption and to reduce SDH and Na,K-ATPase activities in cells of the proximal tubule and Henle's loop. In the kidney of adrenalectomized rats, both sodium reabsorption and activities of Na,K-ATPase, SDH, alpha-KDH decreased in all the segments of the nephron. The data obtained suggest that changes in sodium reabsorption may be coupled with those in the activities of the investigated enzymes.