Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats

Actin cytoskeleton and associated myosin motors within the renal epithelium

This review highlights the complexity of renal epithelial cell membrane architectures and organelles through careful review of ultrastructural and physiological studies published over the past several decades. We also showcase the vital roles played by the actin cytoskeleton and actin-associated myosin motor proteins in regulating cell type-specific physiological functions within the cells of the renal epithelium. The purpose of this review is to provide a fresh conceptual framework to explain the structure-function relationships that exist between the actin cytoskeleton, organelle structure, and cargo transport within the mammalian kidney. With recent advances in technologies to visualize the actin cytoskeleton and associated proteins within intact kidneys, it has become increasingly imperative to reimagine the functional roles of these proteins in situ to provide a rationale for their unique, cell type-specific functions that are necessary to establish and maintain complex physiological processes.

Low potassium activation of proximal mTOR/AKT signaling is mediated by Kir4.2

The renal epithelium is sensitive to changes in blood potassium (K+). We identify the basolateral K+ channel, Kir4.2, as a mediator of the proximal tubule response to K+ deficiency. Mice lacking Kir4.2 have a compensated baseline phenotype whereby they increase their distal transport burden to maintain homeostasis. Upon dietary K+ depletion, knockout animals decompensate as evidenced by increased urinary K+ excretion and development of a proximal renal tubular acidosis. Potassium wasting is not proximal in origin but is caused by higher ENaC activity and depends upon increased distal sodium delivery. Three-dimensional imaging reveals Kir4.2 knockouts fail to undergo proximal tubule expansion, while the distal convoluted tubule response is exaggerated. AKT signaling mediates the dietary K+ response, which is blunted in Kir4.2 knockouts. Lastly, we demonstrate in isolated tubules that AKT phosphorylation in response to low K+ depends upon mTORC2 activation by secondary changes in Cl- transport. Data support a proximal role for cell Cl- which, as it does along the distal nephron, responds to K+ changes to activate kinase signaling.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) critically utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) predominantly utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Key points

A positive correlation between salt reabsorption and oxygen consumption in mammalian kidneys hints at a potential interaction between transport activity and mitochondrial respiration in renal tubules.Renal tubules are heterogeneous in transport activity and mitochondrial metabolism, and traditional assays using bulk kidney tissues cannot provide segment-specific information.Here, we applied an extracellular flux analysis to investigate mitochondrial respiration and energy metabolism in isolated renal tubules. This assay is sensitive in detecting oxygen consumption and acid production in centimeter-length renal tubules and reliably recapitulates segment-specific metabolic features.Acute inhibition of transport activity reduces basal and ATP-linked mitochondrial respirations without changing maximal mitochondrial respiratory capacity. Chronic alterations of transport activity further adjust maximal mitochondrial respiratory capacity via regulating mitochondrial biogenesis or non-transcriptional mechanisms.Our findings support the concept that renal tubular cells finely adjust mitochondrial bioenergetics and biogenesis to match the new steady state of transport activity.

Medullary and cortical thick ascending limb: similarities and differences

The thick ascending limb of the loop of Henle (TAL) is the first segment of the distal nephron, extending through the whole outer medulla and cortex, two regions with different composition of the peritubular environment. The TAL plays a critical role in the control of NaCl, water, acid, and divalent cation homeostasis, as illustrated by the consequences of the various monogenic diseases that affect the TAL. It delivers tubular fluid to the distal convoluted tubule and thereby affects the function of the downstream tubular segments. The TAL is commonly considered as a whole. However, many structural and functional differences exist between its medullary and cortical parts. The present review summarizes the available data regarding the similarities and differences between the medullary and cortical parts of the TAL. Both subsegments reabsorb NaCl and have high Na+-K+-ATPase activity and negligible water permeability; however, they express distinct isoforms of the Na+-K+-2Cl−cotransporter at the apical membrane. Ammonia and bicarbonate are mostly reabsorbed in the medullary TAL, whereas Ca2+and Mg2+are mostly reabsorbed in the cortical TAL. The peptidic hormone receptors controlling transport in the TAL are not homogeneously expressed along the cortical and medullary TAL. Besides this axial heterogeneity, structural and functional differences are also apparent between species, which underscores the link between properties and role of the TAL under various environments.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

Protein- and diabetes-induced glomerular hyperfiltration: role of glucagon, vasopressin, and urea

A single protein-rich meal (or an infusion of amino acids) is known to increase the glomerular filtration rate (GFR) for a few hours, a phenomenon known as “hyperfiltration.” It is important to understand the factors that initiate this upregulation because it becomes maladaptive in the long term. Several mediators and paracrine factors have been shown to participate in this upregulation, but they are not directly triggered by protein intake. Here, we explain how a rise in glucagon and in vasopressin secretion, directly induced by protein ingestion, might be the initial factors triggering the hepatic and renal events leading to an increase in the GFR. Their effects include metabolic actions in the liver and stimulation of sodium chloride reabsorption in the thick ascending limb. Glucagon is not only a glucoregulatory hormone. It is also important for the excretion of nitrogen end products by stimulating both urea synthesis in the liver (along with gluconeogenesis from amino acids) and urea excretion by the kidney. Vasopressin allows the concentration of nitrogenous end products (urea, ammonia, etc.) and other protein-associated wastes in a hyperosmotic urine, thus allowing a very significant water economy characteristic of all terrestrial mammals. No hyperfiltration occurs in the absence of one or the other hormone. Experimental results suggest that the combined actions of these two hormones, along with the complex intrarenal handling of urea, lead to alter the composition of the tubular fluid at the macula densa and to reduce the intensity of the signal activating the tubuloglomerular feedback control of GFR, thus allowing GFR to raise. Altogether, glucagon, vasopressin, and urea contribute to set up the best compromise between efficient urea excretion and water economy.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Active urea transport in lower vertebrates and mammals

Some unicellular organisms can take up urea from the surrounding fluids by an uphill pumping mechanism. Several active (energy-dependent) urea transporters (AUTs) have been cloned in these organisms. Functional studies show that active urea transport also occurs in elasmobranchs, amphibians, and mammals. In the two former groups, active urea transport may serve to conserve urea in body fluids in order to balance external high ambient osmolarity or prevent desiccation. In mammals, active urea transport may be associated with the need to either store and/or reuse nitrogen in the case of low nitrogen supply, or to excrete nitrogen efficiently in the case of excess nitrogen intake. There are probably two different families of AUTs, one with a high capacity able to establish only a relatively modest transepithelial concentration difference (renal tubule of some frogs, pars recta of the mammalian kidney, early inner medullary collecting duct in some mammals eating protein-poor diets) and others with a low capacity but able to maintain a high transepithelial concentration difference that has been created by another mechanism or in another organ (elasmobranch gills, ventral skin of some toads, and maybe mammalian urinary bladder). Functional characterization of these transporters shows that some are coupled to sodium (symports or antiports) while others are sodium-independent. In humans, only one genetic anomaly, with a mild phenotype (familial azotemia), is suspected to concern one of these transporters. In spite of abundant functional evidence for such transporters in higher organisms, none have been molecularly identified yet.

Chronic activation of vasopressin V2 receptor signalling lowers renal medullary oxygen levels in rats

AIM

In the present study, we aimed to elucidate the effects of chronic vasopressin administration on renal medullary oxygen levels.

METHODS

Adult Sprague Dawley or vasopressin-deficient Brattleboro rats were treated with the vasopressin V2 receptor agonist, desmopressin (5 ng/h; 3d), or its vehicle via osmotic minipumps. Immunostaining for pimonidazole and the transcription factor HIF-1α (hypoxia-inducible factor-1α) were used to identify hypoxic areas. Activation of HIF-target gene expression following desmopressin treatment was studied by microarray analysis.

RESULTS

Pimonidazole staining was detected in the outer and inner medulla of desmopressin-treated rats, whereas staining in control animals was weak or absent. HIF-1α immunostaining demonstrated nuclear accumulation in the papilla of desmopressin-treated animals, whereas no staining was observed in the controls. Gene expression analysis revealed significant enrichment of HIF-target genes in the group of desmopressin-regulated gene products (P = 2.6*10(-21) ). Regulated products included insulin-like growth factor binding proteins 1 and 3, angiopoietin 2, fibronectin, cathepsin D, hexokinase 2 and cyclooxygenase 2.

CONCLUSION

Our results demonstrate that an activation of the renal urine concentrating mechanism by desmopressin causes renal medullary hypoxia and an upregulation of hypoxia-inducible gene expression.

Sustained elevated levels of circulating vasopressin selectively stimulate the proliferation of kidney tubular cells via the activation of V2 receptors

The hypothalamic hormone vasopressin (AVP) has known mitogenic effects on various cell types. This study was designed to determine whether sustained elevated levels of circulating AVP could influence cell proliferation within adult tissues known to express different AVP receptors, including the pituitary, adrenal gland, liver, and kidney. Plasmatic AVP was chronically increased by submitting animals to prolonged hyperosmotic stimulation or implanting them with a AVP-containing osmotic minipump. After several days of either treatment, increased cell proliferation was detected only within the kidney. This kidney cell proliferation was not affected by the administration of selective V1a or V1b receptor antagonists but was either inhibited or mimicked by the administration of a selective V2 receptor antagonist or agonist, respectively. Kidney proliferative cells mostly concerned a subpopulation of differentiated tubular cells known to express the V2 receptors and were associated with the phosphorylation of ERK. These data indicate that in the adult rat, sustained elevated levels of circulating AVP stimulates the proliferation of a subpopulation of kidney tubular cells expressing the V2 receptor, providing the first illustration of a mitogenic effect of AVP via the activation of the V2 receptor subtype.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Vasopressin directly regulates cyst growth in polycystic kidney disease

The polycystic kidney diseases (PKD) are a group of genetic disorders causing renal failure and death from infancy to adulthood. Arginine vasopressin (AVP) V2 receptor antagonists inhibit cystogenesis in animal models of cystic kidney diseases, presumably by downregulating cAMP signaling, cell proliferation, and chloride-driven fluid secretion. For confirmation that the protective effect of these drugs is due to antagonism of AVP, PCK (Pkhd1(-/-)) and Brattleboro (AVP(-/-)) rats were crossed to generate rats with PKD and varying amounts of AVP. At 10 and 20 weeks of age, PCK AVP(-/-) rats had lower renal cAMP and almost complete inhibition of cystogenesis compared with PCK AVP(+/+) and PCK AVP(+/-) rats. The V2 receptor agonist 1-deamino-8-d-arginine vasopressin increased renal cAMP and recovered the full cystic phenotype of PCK AVP(-/-) rats and aggravated the cystic disease of PCK AVP(+/+) rats but did not induce cystic changes in wild-type rats. These observations indicate that AVP is a powerful modulator of cystogenesis and provide further support for clinical trials of V2 receptor antagonists in PKD.

Vasodilatation of afferent arterioles and paradoxical increase of renal vascular resistance by furosemide in mice

Loop diuretics like furosemide have been shown to cause renal vasodilatation in dogs and humans, an effect thought to result from both a direct vascular dilator effect and from inhibition of tubuloglomerular feedback. In isolated perfused afferent arterioles preconstricted with angiotensin II or N(G)-nitro-L-arginine methyl ester, furosemide caused a dose-dependent increase of vascular diameter, but it was without effect in vessels from NKCC1-/- mice suggesting that inhibition of NKCC1 mediates dilatation in afferent arterioles. In the intact kidney, however, furosemide (2 mg/kg iv) caused a 50.5 +/- 3% reduction of total renal blood flow (RBF) and a 27% reduction of superficial blood flow (SBF) accompanied by a marked and immediate increase of tubular pressure and volume. At 10 mg/kg, furosemide reduced RBF by 60.4 +/- 2%. Similarly, NKCC1-/- mice responded to furosemide with a 45.4% decrease of RBF and a 29% decrease of SBF. Decreases in RBF and SBF and increases of tubular pressure by furosemide were ameliorated by renal decapsulation. In addition, pretreatment with candesartan (2 mg/kg) or indomethacin (5 mg/kg) attenuated the reduction of RBF and peak urine flows caused by furosemide. Our data indicate that furosemide, despite its direct vasodilator potential in isolated afferent arterioles, causes a marked increase in flow resistance of the vascular bed of the intact mouse kidney. We suggest that generation of angiotensin II and/or a vasoconstrictor prostaglandin combined with compression of peritubular capillaries by the expanding tubular compartment are responsible for the reduction of RBF in vivo.

Intrarenal octreotide treatment prevents sodium retention in liver cirrhotic rats: evidence for direct effects within the thick ascending limb of Henle's loop

We have previously shown that systemic treatment with the somatostatin analog octreotide has marked beneficial effects on renal function in rats with liver cirrhosis induced by common bile duct ligation (CBL; Jonassen TEN, Christensen S, Sørensen AM, Marcussen N, Flyvbjerg A, Andreasen F, and Petersen JS. Hepatology 29: 1387-1395, 1999). In the present study, we tested the hypothesis that octreotide has a direct effect on renal tubular function. Rats (CBL or Sham-CBL) were intrarenally treated with low-dose octreotide in a long-acting release formulation, which had no systemic actions (100 microg/kg body wt as a single dose). Rats receiving low-dose octreotide (sc) were used as controls. The rats were chronically instrumented, and renal function was examined 4 wk after CBL or Sham-CBL. Intrarenal octreotide administration (IROA) prevented sodium retention in CBL rats without changes in renal plasma flow, glomerular filtration rate, or circulating levels of aldosterone and vasopressin. Renal clearance studies revealed that IROA normalized the increased natriuretic efficacy of furosemide found in CBL rats. Furthermore, IROA protected against the development of hypertrophy of the inner stripe of the outer medulla and thereby the increased the volume of thick ascending limb of Henle's loop (TAL) epithelium found in CBL rats. Finally, Western blot analyses of outer medullary homogenates showed increased abundance of the furosemide-sensitive Na-K-2Cl (NKCC2) cotransporter. IROA did not affect the abundance of NCKK2 within the outer medulla. Together with the histological findings, these results indicate that IROA reduces the total number of NKCC2 within the outer medulla. In conclusion, the results indicate a direct intrarenal effect of octreotide on TAL function and morphology in cirrhotic rats.

Mitochondrial reactive oxygen species contribute to high NaCl-induced activation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), resulting in increased expression of genes involved in osmoprotective accumulation of organic osmolytes, including glycine betaine, and in increased expression of osmoprotective heat shock proteins. Our previous studies showed that high NaCl increases reactive oxygen species (ROS), which contribute to activation of TonEBP/OREBP. Mitochondria are a major source of ROS. The purpose of the present study was to examine whether mitochondria produce the ROS that contribute to activation of TonEBP/OREBP. We inhibited mitochondrial ROS production in HEK293 cells with rotenone and myxothiazol, which inhibit mitochondrial complexes I and III, respectively. Rotenone (250 nM) and myxothiazol (12 nM) reduce high NaCl-induced ROS over 40%, whereas apocynin (100 microM), an inhibitor of NADPH oxidase, and allopurinol (100 microM), an inhibitor of xanthine oxidase, have no significant effect. Rotenone and myxothiazol reduce high NaCl-induced increases in TonEBP/OREBP transcriptional activity (ORE/TonE reporter assay) and BGT1 (betaine transporter) mRNA abundance ranging from 53 to 69%. They inhibit high NaCl-induced TonEBP/OREBP transactivating activity, but not its nuclear translocation. Release of ATP into the medium on hypertonic stress has been proposed to be a signal that triggers cellular osmotic responses. However, we do not detect release of ATP into the medium or inhibition of high NaCl-induced ORE/TonE reporter activity by an ATPase, apyrase (20 U/ml), indicating that high NaCl-induced activation of TonEBP/OREBP is not mediated by release of ATP. We conclude that high NaCl increases mitochondrial ROS production, which contributes to the activation of TonEBP/OREBP by increasing its transactivating activity.

Effects of renal denervation on tubular sodium handling in rats with CBL-induced liver cirrhosis

This study was designed to examine the effect of bilateral renal denervation (DNX) on thick ascending limb of Henle's loop (TAL) function in rats with liver cirrhosis induced by common bile duct ligation (CBL). The CBL rats had, as previously shown, sodium retention associated with hypertrophy of the inner stripe of the outer medulla (ISOM) and increased natriuretic effect of furosemide in vivo, and semiquantitative immunoblotting showed increased expression of the furosemide-sensitive Na-K-2Cl cotransporter type 2 (NKCC2) in ISOM from CBL rats. DNX significantly attenuated the sodium retention in the CBL rats, which was associated with normalization of the natriuretic effect of furosemide, as well as a significant reduction in the expression of NKCC2 in the ISOM. However, the marked hypertrophy of the ISOM found in CBL rats was not reversed by DNX. Together, these data indicate that increased renal sympathetic nerve activity known to be present in CBL rats plays a significant role in the formation of sodium retention by stimulating sodium reabsorption in the TAL via increased renal abundance of NKCC2.

Regulation of potassium channel Kir 1.1 (ROMK) abundance in the thick ascending limb of Henle's loop

The renal outer medullary potassium channel (ROMK) of the thick ascending limb (TAL) is a critical component of the counter-current multiplication mechanism. In this study, two new antibodies raised to ROMK were used to investigate changes in the renal abundance of ROMK with treatments known to strongly promote TAL function. These antibodies specifically recognized protein of the predicted size of 45 kD in immunoblots of rat kidney or COS cells transfected with ROMK cDNA. Infusion of 1-deamino-(8-D-arginine)-vasopressin (dDAVP), a vasopressin V2 receptor-selective agonist, for 7 d into Brattleboro rats resulted in dramatic increases in apical membrane labeling of ROMK in the TAL of dDAVP-treated rats, as assessed by immunocytochemical analyses. Using immunoblotting, a more than threefold increase in immunoreactive ROMK levels was observed in the outer medulla after dDAVP infusion. Restriction of water intake to increase vasopressin levels also significantly increased TAL ROMK immunolabeling and abundance in immunoblots. In addition, dietary Na(+) levels were varied to determine whether ROMK abundance was also affected under other conditions known to alter TAL transport. Rats fed higher levels of sodium, as either NaCl or NaHCO(3) (8 mEq/250 g body wt per d), exhibited significantly increased density of the 45-kD band, compared with the respective control animals. Moreover, in rats fed a low-NaCl diet (0.25 mEq/250 g body wt per d), a 50% decrease in band density for the 45-kD band was observed (relative to control rats fed 2.75 mEq/250 g body wt per d of NaCl). These results demonstrate that long-term adaptive changes in ROMK abundance occur in the TAL with stimuli that enhance transport by this segment.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

UT-A2: a 55-kDa urea transporter in thin descending limb whose abundance is regulated by vasopressin

The renal urea transporter gene (UT-A) produces different transcripts in the inner medullary collecting ducts (UT-A1) and thin descending limbs of Henle's loop (UT-A2), coding for distinct proteins. Peptide-directed rabbit polyclonal antibodies were used to identify the UT-A2 protein in renal medulla of mouse and rat. In the inner stripe of outer medulla, an antibody directed to the COOH terminus of UT-A recognized a membrane protein of 55 kDa. The abundance of this 55-kDa protein was strongly increased in response to chronic infusion of the vasopressin analog 1-deamino-[8-D-arginine]vasopressin (DDAVP) in Brattleboro rats, consistent with previous evidence that UT-A2 mRNA abundance is markedly increased. Immunofluorescence labeling with the COOH-terminal antibody in Brattleboro rats revealed labeling in the lower portion of descending limbs from short-looped nephrons (in the aquaporin-1-negative portion of this segment). This UT-A labeling was increased in response to DDAVP. Increased labeling was also seen in descending limbs of long-looped nephrons in the base of the inner medulla. These results indicate that UT-A2 is expressed as a 55-kDa protein in portions of the thin descending limbs of Henle's loop and that the abundance of this protein is strongly upregulated by vasopressin.

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.

Effects of chronic octreotide treatment on renal changes during cirrhosis in rats

We examined the effect of a new long-acting release formula (LAR) of the somatostatin analogue, octreotide, on development of sodium retention and functional and structural changes in the thick ascending limb of Henle's loop (TAL) in rats with cirrhosis induced by common bile duct ligation (CBL). CBL and sham-operated control rats were treated with octreotide-LAR (10 mg/kg body weight subcutaneously, as a single dose) or vehicle at the time of CBL or sham-CBL. The rats were instrumented with chronic catheters, and sodium balance and renal function were examined 4 weeks after CBL or sham operation. Octreotide-LAR treatment significantly inhibited sodium retention in CBL rats and prevented renal vasodilatation without changes in glomerular filtration rate (GFR). The natriuretic response to a test dose of furosemide (7.5 mg/kg body weight intravenously) was significantly increased in CBL rats, and when expressed in terms of natriuretic efficiency (mmol Na/mg furosemide in urine), the natriuretic response was increased by 57% relative to sham-operated controls. Stereological examination of kidneys demonstrated a 53% increase in the volume of the inner stripe of the outer medulla and a 108% increase in the volume of TAL epithelium in cirrhotic rats relative to controls. The increased natriuretic efficiency of furosemide as well as the hypertrophy of the inner stripe and the TAL in this renal zone were absent in CBL rats treated with octreotide-LAR. These results suggest that octreotide-LAR treatment inhibits sodium retention in cirrhotic rats, partly by inhibition of increased furosemide-sensitive sodium reabsorption in the TAL.

Vasopressin increases Na-K-2Cl cotransporter expression in thick ascending limb of Henle's loop

To investigate whether the enhancement of thick ascending limb (TAL) NaCl transport in response to long-term increases in circulating vasopressin concentration is associated with increased expression levels of the apical Na-K-2Cl cotransporter in the rat TAL, we have carried out immunoblotting and immunofluorescence studies using affinity-purified, peptide-directed antibodies. Semiquantitative immunoblotting studies demonstrated a marked increase (193% of controls) in Na-K-2Cl cotransporter band density in response to restriction of water intake to 15 ml/day for 7 days. In contrast, the expression levels of two other apical proteins of the TAL (the type 3 Na/H exchanger and Tamm-Horsfall protein) were unchanged in the outer medulla. A 7-day subcutaneous infusion of the V2 receptor-selective vasopressin analog, 1-desamino-[8-D-arginine]vasopressin (DDAVP), to Brattleboro rats also markedly increased Na-K-2Cl cotransporter expression in the outer medulla (183% of controls). Immunofluorescence localization in outer medullary tissue sections confirmed the increase in Na-K-2Cl cotransporter expression in response to DDAVP. We conclude that vasopressin strongly upregulates the expression of the Na-K-2Cl cotransporter of the TAL and that it is likely to play an important role in the long-term regulation of the countercurrent multiplication system.

Mitochondrial biogenesis and development of respiratory chain enzymes in kidney cells: role of glucocorticoids

We have previously shown that the activity of several mitochondrial oxidative enzymes increased greatly in the medullary thick ascending limb of Henle's loop (MTAL) of developing rat kidney between 16 days after birth and the adult stage. These changes were triggered by the postnatal rise in circulating glucocorticoids. To determine whether these increases are related to a mitochondrial biogenesis we have studied the changes in mitochondrial density of MTAL cells and mitochondrial DNA content in the inner stripe of the outer medulla during the postnatal period. The activities of respiratory chain (RC) complexes II and IV, located in the inner mitochondrial membrane (IMM), were also assayed, and developmental changes in the ultrastructure of the IMM were quantified. Quantitative electron-microscopic data showed a doubling in mitochondrial density from day 16 to the adult, accompanied by twofold increase in mitochondrial DNA, as determined by slot-blot experiments. The surface density of IMM increased by 77%, and there was a concomitant large rise in the activity of both RC enzymes. A possible role of glucocorticoids on the regulation of mitochondrial biogenesis was examined in similar experiments performed in adrenalectomized rat pups. The data demonstrated that glucocorticoids are essential for the rise in RC enzymes and the development of IMM but do not regulate postnatal changes in mitochondrial density and mitochondrial DNA content. Finally, ontogenic changes in oxidative capacities of MTAL cells could be a critical factor in the development of kidney urine concentrating mechanisms in weanling rats.

Diuretic drugs and the treatment of edema: from clinic to bench and back again

Despite wide variations in dietary NaCl intake, homeostatic mechanisms ensure that renal NaCl excretion matches intake at steady state. This does not imply, however, that extracellular fluid volume is maintained within narrow limits. In contrast with blood pressure, which appears to be tightly controlled, extracellular fluid volume varies significantly, even in normal individuals, when dietary NaCl intake changes. Cardiac, liver, or renal disease can perturb the relationship between NaCl intake and extracellular fluid volume and lead to symptomatic edema. All major classes of diuretic drugs in use today were developed between 1950 and 1970. These drugs were developed empirically, without knowledge of specific ion transport pathways, but experimental work during the past 15 years has shown that each major class of diuretic inhibits a specific ion transport protein in the kidney. These transport proteins have been characterized physiologically and the mechanisms by which each diuretic drug inhibits ion transport have been defined. Antibodies directed against these transport proteins have localized ion transport pathways to specific cell types along the nephron. Most recently, isoforms of each class of diuretic-sensitive Na transport pathway have been cloned. Ongoing experimental work is aimed at exploring relationships between families of transporters, determining the structural prerequisites for ion transport, and studying molecular mechanisms of transport regulation. Treatment of edema with diuretics is often straightforward, but can lead to adaptive changes in nephron structure and function. These adaptations can limit the effectiveness of diuretic drugs; maneuvers aimed at blocking these processes can be effective approaches to patients who are resistant to diuretic drugs.

Dietary protein-induced renal growth: correlation between renal IGF-I synthesis and hyperplasia

Insulin-like growth factor I (IGF-I) and IGF binding protein 1 (IGFBP-1) mRNAs are colocalized in the medullary thick ascending limb (MTAL) of the rat nephron, a segment that undergoes selective growth in response to elevated dietary protein. In the present study, rats were fed isocaloric diets containing variable protein content (6-40%) for 1-7 days, and changes in fractional renal weight, MTAL length, and regional DNA synthesis were assayed and compared with local changes in IGF-I/IGFBP-1 mRNAs, as determined by quantitative in situ hybridization. Rats switched to high-protein diets demonstrated increased IGF-I and decreased IGFBP-1 mRNA levels in MTALs, whereas those switched to low protein showed inverse changes. The increase in renal IGF-I mRNA was maximal at 2 days and was closely paralleled by significant increases in fractional renal weight, DNA synthesis, and MTAL length. Similar changes were seen in vasopressin-deficient Brattleboro and growth hormone (GH)-deficient dwarf rats in response to high-protein diets, suggesting that the effects of dietary protein in this model are not mediated by vasopressin or GH. The close spatial and temporal correlation between changes in renal IGF-I expression and changes in regional growth parameters strongly supports a role for locally produced IGF-I in the induction of protein-induced renal growth.

Influence of oxytocin on renal hemodynamics and sodium excretion

Acute administration of physiological doses of synthetic OT to conscious Long-Evans and Brattleboro homozygous diabetes insipidus rats produced a modest increase in GFR and effective filtration fraction. Chronic administration of OT to DI rats for 9 days in dosages that were antidiuretic (plasma OT ca. 100 pg/ml) increased both GFR and ERPF by 40%. Table 1 summarizes these renal hemodynamic changes and compares them to the renal effects of VP. Further investigation is needed to define the mechanisms responsible for the changes in GFR and/or ERPF produced by acute and chronic administration of OT to conscious rats. Acute administration of physiological doses of synthetic OT to conscious LE and DI rats also produced a brisk natriuresis with a marked increase in the fractional excretion of sodium. A natriuresis was also observed in conscious Sprague-Dawley rats administered physiological amounts of OT by subcutaneous osmotic minipump. The natriuretic effect of the hormone was short lived, however, being observed only during the first 24-hr period of treatment. The nephron site where OT exerts its natriuretic action, either directly or indirectly, is unknown. Renal prostaglandins may contribute to OT-induced natriuresis, but other mechanisms such as increased renal production of nitric oxide and cGMP have not been tested. Although the natriuretic response to OT has also been described for conscious dogs, it probably does not occur in humans and nonhuman primates. Precise localization of specific renal OT receptors has recently been reported for the rat. OT receptors were identified in the macula densa cells of the adult, rat kidney. This location suggests a possible role for OT in the regulation of tubuloglomerular feedback and solute transport. The signal transduction of the renal OT receptor has been recently evaluated in various kidney epithelial cells in culture. OT stimulates phosphoinositide hydrolysis and increases cytosolic calcium concentrations. In fact, VP produces similar cellular responses in renal epithelia, possibly through the OT receptor. Also, OT stimulates soluble guanylate cyclase and increases intracellular cGMP. Whether OT activates soluble guanylate cyclase secondarily through the production of nitric oxide has not been tested. An important role for OT in renal sodium homeostasis under basal conditions is likely, at least for the rat. Moreover, OT possibly mediates dehydration natriuresis in lower animal species. The contribution of OT to renal physiology in humans and in nonhuman primates, if any, remains uncertain.

Selective zonal change of the renomedullary interstitial cells in hypokalemic rats

Electron microscopic morphometry was undertaken to quantitate the morphological change of the renomedullary interstitial cells (RIC) of hypokalemic rats by using large montages. Two weeks of potassium depletion resulted in an increase of the RIC, which were restricted to the interbundle region of the inner stripe of the outer medulla. The increase of the RIC is characterized by a preferential increase in volume density (+340.0%; P < 0.01) and numerical density (+61.4%; P < 0.01) in the interbundle region but not in the vascular bundle of the inner stripe or in the inner medulla. The increased RIC in the interbundle region of the inner stripe demonstrated an increase of lipid droplets, which are known to contain prostaglandin precursors. The selective zonal change of RIC with increased lipid droplets is a characteristic lesion of hypokalemic rats and suggests an enhanced vasoactive function of RIC associated with hypokalemic nephropathy.

Cyclosporine nephropathy: morphometric analysis of the medullary thick ascending limb

The effects of cyclosporine A (CsA) (12.5 mg/kg/d) on the medullary thick ascending limb (mTAL) were studied in five experimental groups: vehicle-treated control (C), salt depletion (SD), cyclosporine (CsA), and the combination of both salt depletion and cyclosporine for 3 (CsA-SD:S) and 8 (CsA-SD:L) weeks. Evaluation was performed on 1-micron plastic horizontal sections. mTALs were classified as either atrophic or nonatrophic by assessing mitochondrial density. The mean cross-sectional area of atrophic mTALs was found to be significantly smaller than the mean of nonatrophic mTALs in all treatment groups. The percentage of atrophic tubules was found to be significantly increased in both CsA-SD groups as compared with the other three treatment groups (P less than 0.01). Regression analysis indicated a rectangular hyperbolic relationship between the percentage of atrophic tubules and mean nonatrophic tubule cross-sectional area (P less than 0.0001). Thus, low levels of injury are associated with a rapid increase in cross-sectional tubular area (hypertrophy), and this response plateaued with increasing degrees of injury. Terminal plasma creatinine correlated with nonatrophic tubular cross-section area (r = 0.52, P less than 0.003). These studies indicate that CsA induces mTAL atrophy, which is more extensive with salt depletion. With limited injury, hypertrophy develops. However, the hypertrophic response cannot be sustained with increasing degrees of injury. The phenomenon of mTAL atrophy and hypertrophy is particularly important, since hypertrophy itself is a known risk factor for mTAL injury.

Surface areas of basolateral membranes in renal distal tubules estimated by vertical sections

The surface area of the Na,K-ATPase-rich basolateral membranes in the medullary thick ascending limb of the distal tubule in the rabbit kidney was determined stereologically using the method of 'vertical sections', whereby unbiased surface estimates are obtained by imposing a cycloid test-lattice on micrographs of ultrathin sections cut roughly longitudinal to the tubular axis. The unbiased estimate of the surface area of basolateral membranes per tubule length in the thick ascending limb was 1.45 x 10(6) microns 2/mm. The results are compared with previous surface area measurements in this segment of the tubule and discussed with respect to the contributions from all sampling levels to the real biological variation. An optimized sampling scheme with a roughly fourfold reduction in workload is suggested.

Vasopressin-dependent kidney hypertrophy: role of urinary concentration in protein-induced hypertrophy and in the progression of chronic renal failure

Recent experiments have shown that the kidney adapts to chronic variations in urine concentration. Glomerular filtration rate (GFR), kidney weight relative to body weight, thickness of inner stripe of the outer medulla, volume of epithelium in early thick ascending limb, and internephron heterogeneity are all decreased by chronic water diuresis and increased by chronic stimulation of urine concentration. It was further shown that the intrarenal pattern of hypertrophy observed after high protein (HP) intake, but not that observed after compensatory hypertrophy or normal growth with age, is exactly similar to that observed after chronic stimulation of urine concentration. Since solute-free water reabsorption (TcH2O) is markedly enhanced by HP diet, this suggests that the increases in GFR and renal mass observed after HP intake are, at least in part, an adaptive response of the kidney to increased urinary concentrating activity. The beneficial effects are induced by protein restriction in chronic renal failure (CRF) could thus be due, in part, to the reduction of this concentrating activity. This hypothesis was confirmed by an experiment performed in rats with experimental chronic renal failure (CRF) in which a chronic increase in water intake, reducing urine osmolality and TcH2O, without any change in food composition or consumption, reduced proteinuria, systemic hypertension, kidney hypertrophy, incidence of glomerulosclerosis, and mortality.

Localization of the protein product of the immediate early growth response gene, Egr-1, in the kidney after ischemia and reperfusion

Egr-1 is an "immediate early" gene that is induced by growth factors and agents that induce differentiation and encodes a protein with a "zinc-finger" motif. This protein is believed to be involved in transcriptional regulation. Because the fate of the kidney, and hence the organism, after an ischemic insult is dependent upon cellular repair, differentiation, and proliferation, we examined whether there was expression of the Egr-1 protein after an ischemic insult to the rat kidney. We have previously reported that Egr-1 mRNA accumulates to high levels in mouse kidneys after 30 min of ischemia and 1 h of reperfusion. In the present study, performed in rats, we show that Egr-1 mRNA transiently accumulates to very high levels after 40 min of ischemia and 1 h of reperfusion, is decreased by 3 h, and is nondetectable by 24 h of reperfusion. Reperfusion is required for Egr-1 protein accumulation to occur. The Egr-1 protein was localized by immunohistochemical techniques primarily to the nuclei of the thick ascending limbs and principal cells of the collecting ducts in the cortex and medulla. The subcellular localization was exclusively nuclear. There was some staining of the glomerular tuft and staining was particularly prominent in the parietal epithelial cells. In parallel to the accumulation of Egr-1 mRNA, the expression of the protein was transient and was no longer apparent after 5 h of reperfusion. The Egr-1 protein may play an important role in regulation of the response to ischemia of those segments of the nephron that are highly susceptible to oxygen deprivation and have a high level of intrinsic plasticity. It is possible that this protein may modulate cellular processes important for the ultimate ability of these critical nephron segments to recover from an ischemic insult.

The role of the kidney in the maintenance of water balance

This chapter shows how the mammalian kidney is able to regulate the excretion of water independently from that of solutes. For this function, which derives from several evolutionary steps among vertebrates, it takes advantage of the diluting ability of the thick ascending limb to produce osmotic energy which is then used to concentrate solutes in the urine. This concentration is permitted by a highly sophisticated architecture of nephrons and vessels in the renal medulla, combined with special permeability characteristics of the different nephron segments and specific hormonal regulation. Two different types of loops of Henle and several well-insulated vascular compartments contribute to this process. The major nitrogenous waste product, urea, is concentrated by an indirect process involving a transfer of osmotic energy from the outer to the inner medulla. As known for several decades, concentrating function is primarily regulated by the effect of antidiuretic hormone (ADH) on water permeability of the collecting duct. However, as discovered more recently, it is also largely dependent upon the effect of the same hormone on urea permeability in the terminal collecting duct. In addition, recent investigations have revealed a much more complex hormonal regulation of the concentrating process than previously thought. ADH itself acts on many other structures in the kidney, and many other hormones and mediators, the secretion of which is not thought to be influenced by the water status, do affect urine concentration either directly or by their interaction with ADH. Rodents display a wide spectrum of morphological and functional renal adaptations improving water conservation. Their study has brought a better understanding of the significant steps and anatomical structures that contribute to the concentrating process. Finally, it is also apparent that the capacity to concentrate urine is influenced in individual animals of a given species by the availability of water, by specific feeding patterns, and by the protein content of the diet.

Antidiuretic hormone reduces the high PGE2 synthesis in papillary collecting duct of DI rats

PGE2 synthesis was measured along the nephron of Brattleboro (DI) rats, lacking ADH, and control LE rats, using an enzyme immunoassay. Experiments were performed in vitro, in the absence of exogenous arachidonic acid, using microdissected tubular segments. The effect of a chronic treatment of dDAVP was tested on three ADH sensitive tubular segments, medullary thick ascending limb (MTAL), medullary collecting tubule (OMCD) and papillary collecting duct (IMCD). No difference in PGE2 synthesis was present between LE and DI in glomerulus and tubular segments up to OMCD. In both strains, values were low in the proximal tubule and the loop of Henle, and gradually increased along the collecting tubule. In IMCD, PGE2 synthesis was much higher in DI (12.8 +/- 2.0 pg per 30 min per mm tubular length) than in LE (3.8 +/- 0.5, LE vs. DI p less than 0.001). In MTAL and OMCD, dDAVP treatment did not affect PGE2 synthesis. In IMCD, dDAVP reduced PGE2 synthesis to values (5.3 +/- 0.8 pg per 30 min per mm tubular length), which were not significantly different from those of LE. Neither oxytocin, which has been shown to be elevated in DI rats, nor furosemide, that reduced papillary osmolarity to values comparable to those of DI rats, were able to increase PGE2 synthesis in IMCD of LE rats. The mechanism of the increase in PGE2 synthesis in IMCD of DI rats, and of the inhibitory effect of dDAVP is yet unknown; it may participate to compensate for the lack of ADH in the Brattleboro rat.

Micropuncture comparison of nephron function during acute and chronic ADH excess in the rat

1. The aim of this study was to compare the effect of acute versus chronic ADH administration on the handling of sodium, potassium and water by the nephron. Simultaneous clearance and free-flow micropuncture experiments were performed on rats, infused with hypotonic Ringer solution, following either 1-3 h ('acute') or 10-12 days ('chronic') of continuous treatment with arginine vasopressin, 50 mU/h. A third group of animals receiving no exogenous ADH acted as controls. 2. Chronic ADH treatment led to more profound concentration of the urine and antidiuresis than acute treatment, due to enhanced extraction of water in the renal medulla. 3. Fractional sodium excretion during this protocol was increased after 'acute' but not 'chronic' ADH treatment. The acute natriuretic response was brought about principally along the length of the proximal tubule, probably due to volume expansion and increased arterial blood pressure. 4. Fractional excretion of potassium was also increased by 'acute' ADH, but this response was due to stimulation of potassium secretion in the late distal tubule. 5. It is concluded that acute and chronic exposure to ADH have different effects on nephron function, as a result of both direct and indirect actions.

Adaptation of the distal convoluted tubule of the rat. Structural and functional effects of dietary salt intake and chronic diuretic infusion

We studied the effects of dietary NaCl intake on the renal distal tubule by feeding rats high or low NaCl chow or by chronically infusing furosemide. Furosemide-treated animals were offered saline as drinking fluid to replace urinary losses. Effects of naCl intake were evaluated using free-flow micropuncture, in vivo microperfusion, and morphometric techniques. Dietary NaCl restriction did not affect NaCl delivery to the early distal tubule but markedly increased the capacity of the distal convoluted tubule to transport Na and Cl. Chronic furosemide infusion increased NaCl delivery to the early distal tubule and also increased the rates of Na and Cl transport above the rates observed in low NaCl diet rats. When compared with high NaCl intake alone, chronic furosemide infusion with saline ingestion increased the fractional volume of distal convoluted tubule cells by nearly 100%, whereas dietary NaCl restriction had no effect. The results are consistent with the hypotheses that (a) chronic NaCl restriction increases the transport ability of the distal convoluted tubule independent of changes in tubule structure, (b) high rates of ion delivery to the distal nephron cause tubule hypertrophy, and (c) tubule hypertrophy is associated with increases in ion transport capacity. They indicate that the distal tubule adapts functionally and structurally to perturbations in dietary Na and Cl intake.

Role of the urinary concentrating process in the renal effects of high protein intake

High protein diet is known to increase glomerular filtration rate (GFR) and induce kidney hypertrophy. The mechanisms underlying these changes are not understood. Since the mammalian kidney comprises different nephron segments located in well-delineated zones, it is conceivable that the hypertrophy does not affect all kidney zones and all nephron segments uniformly. The present experiments were designed to study the chronic effects of high or low isocaloric protein diets (HP = 32% or LP = 10% casein, respectively) on kidney function and morphology in Sprague-Dawley rats. HP diet induced significant increases in kidney mass, GFR, free water clearance, and maximum urine concentrating ability. Kidney hypertrophy was characterized by: 1. a preferential increase in thickness of the inner stripe of the outer medulla (IS) (+54%, P less than 0.001, while total kidney height, from cortex to papillary tip, increased only by 18%); 2. a marked hypertrophy of the thick ascending limbs (TAL) in the inner stripe (+40% epithelium volume/unit tubular length, P less than 0.05) but not in the outer stripe nor in the cortex; 3. an increase in heterogeneity of glomeruli between superficial (S) and deep (D) nephrons (D/S = 1.47 in HP vs. 1.17 in LP, P less than 0.05). In contrast, normal kidney growth with age and kidney hypertrophy induced by uninephrectomy were not accompanied by preferential enlargement of IS structures. The morphologic changes induced by high protein intake parallel those we previously reported in rats fed a normal diet (25% protein) but in which the operation of the urine concentrating mechanism was chronically stimulated by ADH infusion or by reduction in water intake.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of the rat kidney to altered water intake and urine concentration

Previous experiments in Brattleboro rats with hereditary diabetes insipidus revealed that absence of ADH led to several alterations in kidney anatomy, which could be reversed by chronic ADH treatment. Present experiments were undertaken to determine if similar alterations were observable in normal Wistar rats when endogenous ADH level was varied by manipulating water intake or when exogenous ADH was infused. Water intake was increased by giving food with a high water content ad libitum and offering 5% glucose solution to drink (HWI rats), or decreased by reducing water intake to 1/3 of spontaneous intake (RWI rats). An additional group received chronic ADH infusion with Alzet osmotic minipumps (ADH rats). Results were compared to those obtained in control rats (CON) drinking ad libitum. RWI, CON, and ADH rats ate dry pellets ad libitum. After 6 weeks on these regimens kidneys were perfusion fixed and serial sections were cut for morphometric measurements by light microscopy. Results in the four groups showed that kidney weight relative to body weight was influenced by the operation of urinary concentrating mechanism, with HWI less than CON less than RWI less than ADH. The increase in kidney weight in rats with high urine concentration was not homogeneously distributed throughout the different kidney zones and the different nephron segments. The inner stripe of the outer medulla (IS) increased more in relative height and volume than other kidney zones and, within this zone, the volume of epithelium of thick ascending limb of Henle's loops (TAL) increased more than expected from the whole kidney weight increase. In outer stripe of outer medulla (OS) and in cortex (C), TAL hypertrophy was equal to or lower than expected from whole kidney weight increase. Collecting duct epithelium in C, OS, and IS increased in proportion to whole kidney weight. The MTAL hypertrophy in IS was due to an increase in size of preexisting cells, except in the ADH group where an increase in cell number was also observed. Internephron heterogeneity with regard to glomerular size was greater in RWI and ADH than in CON and HWI rats. The marked hypertrophy of the deep TAL in the IS of rats in which urine concentration was stimulated could be related to an increase in salt transport in this nephron segment, triggered both by a direct stimulation by ADH, and by an increased salt recycling. The elongation of the inner stripe provides a greater length for the operation of the countercurrent multiplier system responsible for building up of the osmotic pressure gradient in the medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship between structure and function in distal tubule and collecting duct

The relationship between structure and function in the distal tubule and collecting duct has been studied with morphologic and physiologic techniques, including morphometric analysis, to identify functionally distinct cell populations. The distal tubule, including the thick ascending limb (TAL) and the distal convoluted tubule (DCT), is involved in active reabsorption of sodium chloride. It is characterized by extensive invaginations of the basolateral plasma membrane, numerous mitochondria, and high Na-K-ATPase activity, features characteristic for an epithelium involved in active transport. Between the distal tubule and the collecting duct is a transition region, the connecting segment or the connecting tubule (CNT), which exhibits species differences with respect to both structure and function. The collecting duct includes the cortical (CCD), the outer medullary (OMCD), and the inner medullary (IMCD) collecting ducts. Principal cells are present throughout the collecting duct, whereas intercalated cells are located mainly in the CCD and OMCD. Morphometric analysis combined with micropuncture and microperfusion studies has provided evidence that the CNT and principal cells are responsible for potassium secretion in the connecting segment and the CCD. The OMCD is a main site of hydrogen ion secretion, and morphometric studies have provided evidence that the intercalated cells in this segment secrete hydrogen ion at least in the rat. Two configurations of intercalated cells exist in the CCD--a type A and a type B. The A cells are similar in ultrastructure to the intercalated cells in the OMCD and are believed to be involved in hydrogen ion secretion. The function of the B cells remains to be established. The inner two-thirds of the IMCD corresponds to the papillary collecting duct, which has a high permeability to urea. The relationship between structure and function in the IMCD has not been studied in detail. This review emphasizes the role of morphometric analysis in establishing the relationship between structure and function in the distal nephron.