Renal medullary circulation: morphological characteristics of vessels and their organization

The blood vessels of the renal medulla have several particular features. The supplying vessels of the renal medulla are the efferent arterioles of the juxtamedullary glomeruli. Thus, the blood supply is entirely postglomerular. The distribution of the blood within the renal medulla is effected by the descending vasa recta. Descending and ascending vasa recta form the vascular bundles. Descending vasa recta leave the bundles at any level of the medulla to feed the adjacent capillaries; the longest descending vasa recta reach the papillary tip. The capillary plexus are differently developed. A dense capillary plexus is present only in the inner stripe of the outer medulla. In the inner medulla the capillary plexuses are poorly developed. At these sites ascending recta contribute to the capillarisation. The ascending vasa recta originate from capillaries at any level of the medulla and ascend without joining together towards the intrarenal veins at the cortico-medullary border. They are capillary vessels with wide lumina. Within the outer stripe of the outer medulla these vessels are very narrowly associated with the tubules; interstitial spaces are very poorly developed at this site. These relationships are interpreted as a possible weak point of the medullary circulation in pathological situations.     

Genetic and behavioral determinants of waist-hip ratio and waist circumference in women twins

1. The microcirculation of the kidney is arranged in a manner that facilitates separation of blood flow to the cortex, outer medulla and inner medulla. 2. Resistance vessels in the renal vascular circuit include arcuate and interlobular arteries, glomerular afferent and efferent arterioles and descending vasa recta. 3. Vasoactive hormones that regulate smooth muscle cells of the renal circulation can originate outside the kidney (e.g. vasopressin), can be generated from nearby regions within the kidney (e.g. kinins, endothelins, adenosine) or they can be synthesized by adjacent endothelial cells (e.g. nitric oxide, prostacyclin, endothelins). 4. Vasoactive hormones released into the renal inner medullary microcirculation may be trapped by countercurrent exchange to act upon descending vasa recta within outer medullary vascular bundles. 5. Countercurrent blood flow within the renal medulla creates a hypoxic environment. Relative control of inner versus outer medullary blood flow may play a role to abrogate the hypoxia that arises from O2 consumption by the thick ascending limb of Henle. 6. Cortical blood flow is autoregulated. In contrast, the extent of autoregulation of medullary blood flow appears to be influenced by the volume status of the animal. Lack of medullary autoregulation during volume expansion may be part of fundamental processes that regulate salt and water excretion.     

A flavinogenic mutant of the yeast Pichia guilliermondii with impaired iron transport

We recently demonstrated that net fluid uptake occurs in the capillary system of the inner medulla. To define the site of fluid uptake, the concentration of protein was determined in plasma from descending vasa recta at the base and tip of the exposed papilla in Munich-Wister rats. The vasa recta plasma-to-arterial plasma protein concentration ratio (VR/P) was 1.43 +/- 0.09 at the base and 1.66 +/- 0.09 at the tip. These results, which indicate fluid loss from the descending vasa recta, are difficult to explain on the basic of hydraulic and oncotic forces alone. The osmolality of the contents of descending vasa recta increased between base and tip (delta = 72 +/- 30 mosmol/kg H2O). If the increase in osmolality of plasma in descending vasa recta lags behind that of the adjacent medullary interstitium, a transcapillary osmotic driving force exists favoring water loss from descending vessels. It is concluded that fluid uptake by the inner medullary circulation occurs beyond descending vasa recta in interconnecting capillaries or ascending vasa recta. In our view the most likely interpretation of these results is that fluid movement across vasa recta in the inner medulla is influenced by three forces: those owing to transcapillary differences in osmotic, oncotic, and hydraulic pressures.     

Location of an inducible nitric oxide synthase mRNA in the normal kidney

An inducible nitric oxide synthase (iNOS) mRNA was found primarily in the outer medulla of normal rat kidney. Identification of the mRNA was based upon the specificity of the oligonucleotide primers used for PCR amplification, PCR-Southern blot analysis and the nucleic acid sequence of the cloned PCR product. In addition to the outer medulla, glomeruli prepared from normal rat kidney contained significant amounts of an iNOS mRNA. These results suggest that there may be tonic influences in the outer medulla of the normal rat kidney resulting in the "steady-state" presence of an iNOS mRNA. Cortical tubules and the inner medulla were found to contain detectable but lesser amounts of the iNOS mRNA. The outer medulla was microdissected into proximal straight tubule (PST), medullary thick ascending limb (MTAL), medullary collecting duct (MCD) and vasa recta bundle (VRB). The iNOS mRNA was found primarily in the MTAL with minor amounts in the MCD and VRB of normal rat kidney. Animals were injected with lipopolysaccharide (LPS) and sacrificed 24 hours later. Treatment with LPS caused at least a 20-fold increase in the amount of iNOS mRNA in the liver or in macrophages isolated from the peritoneum. Endotoxin treatment led to over a 10-fold increase in iNOS mRNA content in glomeruli and the inner medulla. The iNOS mRNA level of the outer medulla was increased two- to threefold due to LPS treatment.     

Impact of cyclo-oxygenase blockade on juxtamedullary microvascular responses to angiotensin II in rat kidney

1. Experiments were designed to evaluate the hypothesis that cyclo-oxygenase products modulate the influence of angiotensin II (AII) on the renal juxtamedullary microvasculature of enalaprilat-treated rats. 2. The in vitro blood-perfused juxtamedullary nephron technique was utilized to provide access to afferent arterioles, efferent arterioles and descending vasa recta located in the outer stripe of the outer medulla. 3. Baseline afferent arteriolar diameter was 20.8 +/- 1.9 microns in kidneys subjected to cyclo-oxygenase blockade (1 mumol/L piroxicam), a value significantly lower than that observed in untreated kidneys (26.1 +/- 1.0 microns). Baseline diameters of efferent arterioles and outer medullary descending vasa recta did not differ between untreated and piroxicam-treated groups. 4. Topical application of 1 nmol/L AII reduced blood flow through outer medullary descending vasa recta by 22 +/- 6% in untreated kidneys and by 24 +/- 7% in piroxicam-treated kidneys. 5. In untreated kidneys, AII (0.01-100 nmol/L) produced concentration-dependent afferent and efferent arteriolar constrictor responses of similar magnitudes. Neither afferent nor efferent arteriolar AII responsiveness was significantly altered in piroxicam-treated kidneys, although afferent responses exceeded efferent responses at AII concentrations > or = 10 nmol/L. 6. We conclude that endogenous cyclo-oxygenase products exert a vasodilator influence on juxtamedullary afferent arterioles under baseline conditions. Although cyclo-oxygenase inhibition had little effect on juxtamedullary microvascular responses to AII, the response to high AII concentrations may be modulated by cyclo-oxygenase products in a manner which delicately alters the relative influence of the peptide on pre- vs postglomerular resistances.     

Localization of ROMK channels in the rat kidney

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

Smaller sized particles are preferentially taken up by alveolar type II pneumocytes

Specific urea transporters are responsible for the rapid urea movements occurring in precise medullary structures of the mammalian kidney. Three of them, ensuring facilitated passive transports, have been cloned yet: UT2-long is responsible for the high vasopressin-dependent urea permeability of the terminal inner medullary collecting ducts; UT2-short is located along a short portion of the thin descending limbs of Henle's loops; UT11 is expressed along the descending vasa recta. These three transporters are involved in the accumulation of urea in the medulla, participating to the corticopapillary osmotic gradient required for urine concentration in the presence of antidiuretic hormone. UT2-long enables diffusion of urea in the inner medulla, and UT2-short and UT11 enable the recycling of this urea by counter-exchange. These transporters could also be involved in nitrogen balance by modulation of their expression according to the need for urea excretion (protein-rich diet), or for nitrogen conservation (protein-poor diet). Several other urea transporters, including active transporters responsible for urea secretion or reabsorption, remain to be cloned and characterized.     

Changes within vascular bundles of rodent kidneys caused by different diets

Morphological effects caused by two different diets (low protein-high water intake, and high protein-restricted water) on the vascular bundles in the outer medullary zone of the kidney were studied in the laboratory white mouse and in the golden spiny mouse (Acomys russatus, Muridae). In both rodents, when on a low protein-high water intake diet, considerable interstitial substance was found between the vasa recta of the bundles. No interstitial substance was found in animals on high protein-low water intake diet; as a result the vasa recta of the vascular bundle adhered closely. The low protein-high water intake diet caused a marked diuresis, low urine osmolality and low urinary urea concentration. It is assumed that the increase in interstitial substance between the vasa recta of the vascular bundle lowers the efficiency of the counter current barrier system for urea in the kidney and, as a consequence, the medullary urea gradient and urine concentrating capacity decreases. In animals on a high protein diet, the closely juxtaposed vasa recta assure an efficient countercurrent exchange, leading to accumulation and maintenance of a large urea gradient in the medulla and maximal urine concentration. It is suggested that the amount of interstitial material between the vasa recta of the vascular bundle might serve as a modulating factor for the urea concentration in the kidney.     

Renal tubular and vascular urea transporters: influence of antidiuretic hormone on messenger RNA expression in Brattleboro rats

In the kidney, facilitated urea transport in precise vascular and tubular structures is mainly involved in water conservation. Three urea transporters have been cloned: UT2-long expressed in terminal inner medullary collecting duct (IMCD), UT2-short expressed in thin descending limb, and UT11 in descending vasa recta. The effect of arginine vasopressin (AVP) administration on mRNA expression of these three transporters was examined in Brattleboro rats with diabetes insipidus. V2 effects were discriminated from combined V1 + V2 effects by comparing treatments with 1-deamino-8-D-AVP (dDAVP) (selective V2 agonism) and AVP (V1 and V2 agonism). Acute and chronic treatments were studied. Abundance of specific mRNA was assessed by quantitative Northern blot analysis of RNA extracted from two regions of inner stripe of outer medulla and from two regions of inner medulla (IM). The results show that mRNA of these urea transporters are differently regulated by AVP. (1) Long-term treatment with either AVP or dDAVP does not alter UT2-long mRNA in tip IM (terminal IMCD) except for a transient initial decrease. (2) Unlike AVP, dDAVP induces the appearance of significant expression of UT2-long mRNA in base IM (initial IMCD), indicating a major V2 effect. (3) UT2-short mRNA in deep inner stripe of outer medulla and base IM (thin descending limb of short and long loops, respectively) is progressively upregulated with duration of AVP or dDAVP treatment. (4) The much higher changes in UT2-long and UT2-short induced by dDAVP compared with AVP suggest that they are dependent mainly on V2 agonism, and likely attenuated by V1 agonism. (5) UT11 mRNA expression in tip IM is equally depressed by AVP and dDAVP, indicating that this vascular transporter is also influenced by AVP and/or urine-concentrating activity, via an indirect mechanism that remains to be determined.     

Autoradiographic localization of vasopressin V1a receptors in the rat kidney using [3H]-SR 49059

Localization and characterization of binding sites of the selective non-peptide vasopressin receptor V1a ligand, [3H]-SR 49059, were investigated in the adult rat kidney by quantitative autoradiography using a fast-detecting radioluminographic phosphor-imaging plate system. [3H]-SR 49059, like the other V1a ligands used, showed a total absence of binding in the papilla, discrete and sparse labeling in the cortex and maximal binding in the outer part of the inner medulla. This labeling seemed to be mainly associated with medullary interstitial cells and vascular elements of the vasa recta. Conversely, [3H]-AVP intensely labeled the V2-enriched medulla-papillary portion of the kidney and, to a lesser extent, the cortical structures. [3H]-SR 49059 binding, quantified in the outer part of the inner medulla in rat kidney sections, was time-dependent, reversible, saturable and a single class of high affinity binding sites (Kd = 1.48 +/- 0.16 nM) was identified. The relative potencies of the reference peptide and non-peptide compounds to inhibit [3H]-SR 49059 binding confirm the V1a nature of the site and the stereospecificity of this binding. Thus, [3H]-SR 49059 allows the mapping and characterization of the V1a receptor population present in the rat kidney. The stability and the highly selective affinity of this non-peptide ligand for rat and human V1a receptors make it a suitable probe for the localization of V1a receptors in organs expressing heterogeneous populations of receptors.     

Localization and regulation of renal Na+/myo-inositol cotransporter in diabetic rats

We have examined the effect of diabetes on sodium/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol content in the kidney to test the hypothesis that diabetes-induced changes in renal myo-inositol levels are due to the regulation of SMIT mRNA levels. In streptozotocin-induced diabetic rats, after 3, 7 and 28 days of diabetes, SMIT mRNA levels in the whole kidney were increased three to fivefold, and remained increased by about twofold after six months of diabetes. Insulin treatment of diabetic rats normalized blood glucose levels and prevented the increase in SMIT mRNA levels. Treating diabetic rats with sorbinil, an aldose reductase inhibitor, corrected the abnormal accumulation of sorbitol but had no effect on the diabetes-induced increase in renal SMIT mRNA levels. The regional distribution of SMIT mRNA from normal rats showed a relative abundance in cortex, outer medulla, and inner medulla of 1.0:3.4:7.0. After seven days of diabetes, the levels of SMIT mRNA and myo-inositol content were significantly increased only in the outer medulla. In situ hybridization studies revealed that SMIT mRNA in the outer medulla was predominately localized to the medullary thick ascending limbs of Henle's loop and was not localized to any specific cell in the inner medulla. This distribution pattern was unchanged in diabetic rats. These studies show that diabetes causes an increase in renal SMIT mRNA, which is primarily localized to the outer medulla. Accumulation of myo-inositol by the thick ascending limb of Henle's loop may account for most of the increase caused by diabetes.     

Vehicle-dependent in situ modification of membrane-controlled drug release

Transmission electron microscopic techniques were used to carry out a quantitative analysis of the density of fenestration in the inner medullary vasa recta of the rat kidney. Measurements were made at 200 microns intervals from the tip to the base of the papilla (1800 microns from the tip). Fenestral diaphragms were estimated to be 65.4 +/- 0.78 nm in diameter (mean +/- S.E.M.), and were arranged in plaques with a mean interfenestral distance of 114.8 +/- 2.6 nm. Near the tip of the papilla there was no correlation between vessel size and degree of fenestration; density of fenestration, however, began to decrease about 1400 microns from the tip. The ratio of fenestrated to non-fenestrated profiles of vasa recta was found to be linear with respect to distance from the tip (r = 0.991), with values ranging from about 40:1 near the tip to 2:1 near the base of the papilla. We have estimated the proportion of the total surface area of a fenestrated vasa recta occupied by fenestral diaphragms to be 0.057 at 1000 microns from the tip. The total potential conductance (K) of a 200 microns segment of fenestrated vessel at 1000 microns from the tip was calculated to be 0.319 microns 3 s-1 cmH2O-1, giving a hydraulic conductivity (Lp) of 0.030 micron s-1 cmH2O-1. We have also examined the reverse question of the conductance of a single fenestra if all the fluid flux across the vessel wall occurred through the fenestrae and none via the intercellular clefts or water channels; single fenestral conductance was estimated to be 1.94 x 10(-3) microns 3 s-1 cmH2O-1.     

Medullary thick limb basolateral Cl- channels

Chloride transport by the medullary thick ascending limb is a major determinant of urinary concentrating and diluting power and also of tubuloglomerular feedback. In this review, we discuss the electrophysiologic, regulatory, and molecular features of a Cl- channel present in basolateral membranes from the medullary thick ascending limb. Functional and immunohistochemical data support the view that rbCIC-Ka, a CIC Cl- channel cloned from rabbit kidney medulla, may be the Cl- channel which mediates basolateral Cl- efflux.     

Concentrating defect in experimental nephrotic syndrone: altered expression of aquaporins and thick ascending limb Na+ transporters

BACKGROUND: Several pathophysiological states associated with deranged water balance are associated with altered expression and/or intracellular distribution of aquaporin water channels. The possible role of dysregulation of thick ascending limb NaCl transporters, which are responsible for countercurrent multiplication in the kidney, has not been evaluated. METHODS: Semiquantitative immunoblotting and immunocytochemistry were carried out in the kidneys of rat with adriamycin-induced nephrotic syndrome and in vehicle-injected control rats. RESULTS: Preliminary studies confirmed the presence of a severe concentrating defect. Semiquantitative immunoblotting of outer medullary homogenates demonstrated a marked decrease in the abundance of three thick ascending limb Na+ transporters in nephrotic rats, namely the bumetanide-sensitive Na-K-2Cl cotransporter (BSC-1), the type 3 Na/H exchanger (NHE-3), and the alpha 1-subunit of the Na-K-ATPase. These results are predictive of a decrease in the NaCl transport capacity of the medullary thick ascending limb and therefore a decrease in countercurrent multiplication. Immunocytochemistry of outer medullary thin sections demonstrated broad (but highly variable) suppression of BSC-1 expression in the outer medullas of adriamycin-nephrotic rats. There was also a large decrease in outer medullary expression of two collecting duct water channels (aquaporin-2 and -3) and the major water channel of the thin descending limb of Henle's loop (aquaporin-1). CONCLUSION: The concentrating defect in adriamycin-induced nephrotic syndrome in rats is a consequence of multiple defects in water and solute transporter expression, which would alter both the generation of medullary interstitial hypertonicity and osmotic equilibration in the collecting duct. Whether a similar widespread defect in transporter expression is present in idiopathic nephrotic syndrome is, at this point, untested.     

Expression of aquaporin-1 (AQP1) in the adult and developing sheep kidney

The distributions of aquaporin-1 mRNA and protein were studied by hybridization histochemistry with a homologous riboprobe and immunohistochemistry, in the adult sheep kidney. Heaviest labelling occurred in the thin descending limb (DTL) of the loop of Henle in the inner stripe of the outer medulla, with apparent decreasing expression in the inner medulla, outer stripe of the outer medulla and cortex, but no quantitation was performed. Only proximal tubules (PT) (convoluted and straight) and DTL labelled. The glomerulus showed no labelling, consistent with the pattern in the rat but different to that in the human. During ontogeny, no labelling occurred in the mesonephros at 27 or 41 days of gestation (term = 145-150 days) but other structures did label at 27 days (heart, lung bud, blood vessels surrounding developing spinal cord). Labelling first occurred faintly in the metanephros at 41 days of gestation and increased throughout gestation consistent with morphological development of nephrons.     

Effect of oxygen and solute on PGE and PGF production by rat kidney slices

Increasing oxygen from 5% to 95% resulted in an increases production of both PGE's and PGF's. The release of prostaglandins from slices of rat kidney cortex and outer and inner medulla was measured. Prostaglandin production was observed predominantly in the inner medulla, was close to the lower limit of detection in the outer medulla, and was undetectable in the cortex. Increasing oxygen concentration resulted in a threefold increase in inner medullary prostaglandin production. Synthesis at 95% O2 was less at 2100 mOsm than at 300 mOsm, while synthesis at 5% O2 was not affected by high solute concentration. The implications of these results with respect to kidney function are discussed.     

Two-chain factor VIIa generated in the pericardium during surgery with cardiopulmonary bypass : relationship to increased thrombin generation and heparin concentration

The effect of changes in medullary extracellular tonicity on mRNA expression for aldose reductase (AR), sorbitol dehydrogenase (SDH), Na+/Cl-/betaine (BGT) and Na+/myo-inositol (SMIT) cotransporter in different kidney zones was studied using Northern blot analysis and non-radioactive in situ hybridization in four groups of rats: controls, acute diuresis (the loop diuretic furosemide was administered), chronic diuresis (5 days of diuresis), and antidiuresis [5 days of diuresis followed by 24 h deamino-Cys1, d-Arg8 vasopressin (dDAVP)]. Acute administration of the loop diuretic furosemide significantly reduced AR, SMIT and BGT gene expression in the inner and outer medulla compared with controls. Administration of dDAVP to chronically diuretic rats raised the expression of these three mRNAs in the inner but not the outer medulla compared with the chronically diuretic rats. None of these alterations in medullary tonicity significantly changed SDH expression. The in situ hybridization studies showed AR, BGT and SMIT mRNAs to be expressed in both epithelial and non-epithelial cells of the outer and inner medulla. The various cell types (epithelial, endothelial and interstitial cells) differed in their expression pattern and intensity of AR, SDH, BGT and SMIT mRNA, but the inner medullary cells responded uniformly to a decrease in extracellular tonicity with a reduction, and to an increase with enhancement of their AR, BGT and SMIT expression.     

Changes in the countercurrent system in the renal papilla: diuresis increases pH and HCO3- gradients between collecting duct and vasa recta

This study was designed to elucidate the acid-base balance local to the collecting duct urine (CD) and vasa recta blood (VR) in the rat renal papilla in diuresis. The pH changes were measured in both a furosemide-induced and a volume-load-induced diuresis, whereas the PCO2 (i.e., CO2 tension) and HCO3- concentration were measured only in a furosemide-induced diuresis. In an antidiuresis, the pH of the VR was more acidic than that of the systemic arterial blood (DeltapH = 0.44-0.73). Additionally, the pH of the ascending VR was significantly lower than that of the descending VR (DeltapH = 0.14-0. 16). In diuresis, the pH of the CD decreased (DeltapH = 0.81-0.97), while the pH of the descending and the ascending VR increased; however, the increase was only significant in the ascending VR (DeltapH = 0.23-0.30). Consequently, the significant difference in the pH gradient between the descending and the ascending VR was eliminated. The PCO2 values in the CD and the ascending VR were not different from those in antidiuresis, while the HCO3- concentration in the CD and the ascending VR, respectively, decreased and increased significantly. Thus, in diuresis, the decrease in the pH of the CD and the increase in the pH of the ascending VR result, respectively, from the decrease and the increase in the HCO3- concentration, with no changes in the PCO2 values.