Evidence for a concentration gradient favoring outward movement of sodium from the thin loop of Henle

Molecular mechanisms underlying paracellular calcium and magnesium reabsorption in the proximal tubule and thick ascending limb

Calcium and magnesium are the most abundant divalent cations in the body. The plasma level is controlled by coordinated interaction between intestinal absorption, reabsorption in the kidney, and, for calcium at least, bone storage and exchange. The kidney adjusts urinary excretion of these ions in response to alterations in their systemic concentration. Free ionized and anion-complexed calcium and magnesium are filtered at the glomerulus. The majority (i.e., >85%) of filtered divalent cations are reabsorbed via paracellular pathways from the proximal tubule and thick ascending limb (TAL) of the loop of Henle. Interestingly, the largest fraction of filtered calcium is reabsorbed from the proximal tubule (65%), while the largest fraction of filtered magnesium is reclaimed from the TAL (60%). The paracellular pathways mediating these fluxes are composed of tight junctional pores formed by claudins. In the proximal tubule, claudin-2 and claudin-12 confer calcium permeability, while the exact identity of the magnesium pore remains to be determined. Claudin-16 and claudin-19 contribute to the calcium and magnesium permeable pathway in the TAL. In this review, we discuss the data supporting these conclusions and speculate as to why there is greater fractional calcium reabsorption from the proximal tubule and greater fractional magnesium reabsorption from the TAL.

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.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

Transepithelial water and urea permeabilities of isolated perfused Munich-Wistar rat inner medullary thin limbs of Henle's loop

To better understand the role that water and urea fluxes play in the urine concentrating mechanism, we determined transepithelial osmotic water permeability (Pf) and urea permeability (Purea) in isolated perfused Munich-Wistar rat long-loop descending thin limbs (DTLs) and ascending thin limbs (ATLs). Thin limbs were isolated either from 0.5 to 2.5 mm below the outer medulla (upper inner medulla) or from the terminal 2.5 mm of the inner medulla. Segment types were characterized on the basis of structural features and gene expression levels of the water channel aquaporin 1, which was high in the upper DTL (DTLupper), absent in the lower DTL (DTLlower), and absent in ATLs, and the Cl-(1) channel ClCK1, which was absent in DTLs and high in ATLs. DTLupper Pf was high (3,204.5 ± 450.3 μm/s), whereas DTLlower showed very little or no osmotic Pf (207.8 ± 241.3 μm/s). Munich-Wistar rat ATLs have previously been shown to exhibit no Pf. DTLupper Purea was 40.0 ± 7.3 × 10(-5) cm/s and much higher in DTLlower (203.8 ± 30.3 × 10(-5) cm/s), upper ATL (203.8 ± 35.7 × 10(-5) cm/s), and lower ATL (265.1 ± 49.8 × 10(-5) cm/s). Phloretin (0.25 mM) did not reduce DTLupper Purea, suggesting that Purea is not due to urea transporter UT-A2, which is expressed in short-loop DTLs and short portions of some inner medullary DTLs close to the outer medulla. In summary, Purea is similar in all segments having no osmotic Pf but is significantly lower in DTLupper, a segment having high osmotic Pf. These data are inconsistent with the passive mechanism as originally proposed.

Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Axial compartmentation of descending and ascending thin limbs of Henle's loops

In the inner medulla, radial organization of nephrons and blood vessels around collecting duct (CD) clusters leads to two lateral interstitial regions and preferential intersegmental fluid and solute flows. As the descending (DTLs) and ascending thin limbs (ATLs) pass through these regions, their transepithelial fluid and solute flows are influenced by variable transepithelial solute gradients and structure-to-structure interactions. The goal of this study was to quantify structure-to-structure interactions, so as to better understand compartmentation and flows of transepithelial water, NaCl, and urea and generation of the axial osmotic gradient. To accomplish this, we determined lateral distances of AQP1-positive and AQP1-negative DTLs and ATLs from their nearest CDs, so as to gauge interactions with intercluster and intracluster lateral regions and interactions with interstitial nodal spaces (INSs). DTLs express reduced AQP1 and low transepithelial water permeability along their deepest segments. Deep AQP1-null segments, prebend segments, and ATLs lie equally near to CDs. Prebend segments and ATLs abut CDs and INSs throughout much of their descent and ascent, respectively; however, the distal 30% of ATLs of the longest loops lie distant from CDs as they approach the outer medullary boundary and have minimal interaction with INSs. These relationships occur regardless of loop length. Finally, we show that ascending vasa recta separate intercluster AQP1-positive DTLs from descending vasa recta, thereby minimizing dilution of gradients that drive solute secretion. We hypothesize that DTLs and ATLs enter and exit CD clusters in an orchestrated fashion that is important for generation of the corticopapillary solute gradient by minimizing NaCl and urea loss.

Isolated interstitial nodal spaces may facilitate preferential solute and fluid mixing in the rat renal inner medulla

Recent anatomic findings indicate that in the upper inner medulla of the rodent kidney, tubules, and vessels are organized around clusters of collecting ducts (CDs). Within CD clusters, CDs and some of the ascending vasa recta (AVR) and ascending thin limbs (ATLs), when viewed in transverse sections, form interstitial nodal spaces, which are arrayed at structured intervals throughout the inner medulla. These spaces, or microdomains, are bordered on one side by a single CD, on the opposite side by one or more ATLs, and on the other two sides by AVR. To study the interactions among these CDs, ATLs, and AVR, we have developed a mathematical compartment model, which simulates steady-state solute exchange through the microdomain at a given inner medullary level. Fluid in all compartments contains Na(+), Cl(-), urea and, in the microdomain, negative fixed charges that represent macromolecules (e.g., hyaluronan) balanced by Na(+). Fluid entry into AVR is assumed to be driven by hydraulic and oncotic pressures. Model results suggest that the isolated microdomains facilitate solute and fluid mixing among the CDs, ATLs, and AVR, promote water withdrawal from CDs, and consequently may play an important role in generating the inner medullary osmotic gradient.

A mathematical model of the urine concentrating mechanism in the rat renal medulla. I. Formulation and base-case results

A new, region-based mathematical model of the urine concentrating mechanism of the rat renal medulla was used to investigate the significance of transport and structural properties revealed in anatomic studies. The model simulates preferential interactions among tubules and vessels by representing concentric regions that are centered on a vascular bundle in the outer medulla (OM) and on a collecting duct cluster in the inner medulla (IM). Particularly noteworthy features of this model include highly urea-permeable and water-impermeable segments of the long descending limbs and highly urea-permeable ascending thin limbs. Indeed, this is the first detailed mathematical model of the rat urine concentrating mechanism that represents high long-loop urea permeabilities and that produces a substantial axial osmolality gradient in the IM. That axial osmolality gradient is attributable to the increasing urea concentration gradient. The model equations, which are based on conservation of solutes and water and on standard expressions for transmural transport, were solved to steady state. Model simulations predict that the interstitial NaCl and urea concentrations in adjoining regions differ substantially in the OM but not in the IM. In the OM, active NaCl transport from thick ascending limbs, at rates inferred from the physiological literature, resulted in a concentrating effect such that the intratubular fluid osmolality of the collecting duct increases ~2.5 times along the OM. As a result of the separation of urea from NaCl and the subsequent mixing of that urea and NaCl in the interstitium and vasculature of the IM, collecting duct fluid osmolality further increases by a factor of ~1.55 along the IM.

Effect of sodium delivery on superoxide and nitric oxide in the medullary thick ascending limb

Hypertension is associated with increased levels of oxidative stress and medullary renal injury. Previous studies have shown that elevations in renal perfusion pressure increase Na(+) delivery to the medullary thick ascending limb (mTAL), and enhancement of NaCl transport in the outer medulla has been reported in many experimental forms of hypertension. This study examined the effects of increased Na(+) and fluid delivery in mTAL perfused in vitro on the generation of superoxide. Osmolality was maintained constant between low- and high-Na(+) perfusates by adjusting with choline Cl(-). Real-time fluorescent microscopic techniques were used to determine the generation of superoxide and nitric oxide in individual mTAL cells using dihydroethidium and DAF-FM dyes, respectively. Increasing the Na(+) concentration of the perfusate from 60 to 149 mM or luminal flow rate from 5 to 20 nl/min (with fixed Na(+) concentration of 60 mM) significantly increased superoxide generation and decreased nitric oxide in mTAL. These effects were inhibited when active transport of Na(+) was inhibited by ouabain. We conclude that increases in luminal Na(+) concentration and/or flow rate can increase the generation of superoxide in mTAL and reduce nitric oxide bioavailability. This may lead to reduction in medullary blood flow and promote hypoxia and tubular necrosis within the renal medulla during in hypertension.

Two modes for concentrating urine in rat inner medulla

We used a mathematical model of the urine concentrating mechanism of rat inner medulla (IM) to investigate the implications of experimental studies in which immunohistochemical methods were combined with three-dimensional computerized reconstruction of renal tubules. The mathematical model represents a distribution of loops of Henle with loop bends at all levels of the IM, and the vasculature is represented by means of the central core assumption. Based on immunohistochemical evidence, descending limb portions that reach into the papilla are assumed to be only moderately water permeable or to be water impermeable, and only prebend segments and ascending thin limbs are assumed to be NaCl permeable. Model studies indicate that this configuration favors the targeted delivery of NaCl to loop bends, where a favorable gradient, sustained by urea absorption from collecting ducts, promotes NaCl absorption. We identified two model modes that produce a significant axial osmolality gradient. One mode, suggested by preliminary immunohistochemical findings, assumes that aquaporin-1-null portions of loops of Henle that reach into the papilla have very low urea permeability. The other mode, suggested by perfused tubule experiments from the literature, assumes that these same portions of loops of Henle have very high urea permeabilities. Model studies were conducted to determine the sensitivity of these modes to parameter choices. Model results are compared with extant tissue-slice and micropuncture studies.

Countercurrent exchange in the renal medulla

The microcirculation of the renal medulla traps NaCl and urea deposited to the interstitium by the loops of Henle and collecting ducts. Theories have predicted that countercurrent exchanger efficiency is favored by high permeability to solute. In contrast to the conceptualization of vasa recta as simple "U-tube" diffusive exchangers, many findings have revealed surprising complexity. Tubular-vascular relationships in the outer and inner medulla differ markedly. The wall structure and transport properties of descending vasa recta (DVR) and ascending vasa recta (AVR) are very different. The recent discoveries of aquaporin-1 (AQP1) water channels and the facilitated urea carrier UTB in DVR endothelia show that transcellular as well as paracellular pathways are involved in equilibration of DVR plasma with the interstitium. Efflux of water across AQP1 excludes NaCl and urea, leading to the conclusion that both water abstraction and diffusion contribute to transmural equilibration. Recent theory predicts that loss of water from DVR to the interstitium favors optimization of urinary concentration by shunting water to AVR, secondarily lowering blood flow to the inner medulla. Finally, DVR are vasoactive, arteriolar microvessels that are anatomically positioned to regulate total and regional blood flow to the outer and inner medulla. In this review, we provide historical perspective, describe the current state of knowledge, and suggest areas that are in need of further exploration.

Concentration of solutes in the renal inner medulla: interstitial hyaluronan as a mechano-osmotic transducer

Although the concentrating process in the renal outer medulla is well understood, the concentrating mechanism in the renal inner medulla remains an enigma. The purposes of this review are fourfold. 1) We summarize a theoretical basis for classifying all possible steady-state inner medullary countercurrent concentrating mechanisms based on mass balance principles. 2) We review the major hypotheses that have been proposed to explain the axial osmolality gradient in the interstitium of the renal inner medulla. 3) We summarize and expand on the Schmidt-Nielsen hypothesis that the contractions of the renal pelvocalyceal wall may provide an important energy source for concentration in the inner medulla. 4) We discuss the special properties of hyaluronan, a glycosaminoglycan that is the chief component of a gel-like renal inner medullary interstitial matrix, which may allow it to function as a mechano-osmotic transducer, converting energy from the contractions of the pelvic wall to an axial osmolality gradient in the medulla. These considerations set the stage for renewed experimental investigation of the urinary concentrating process and a new generation of mathematical models of the renal concentrating mechanism, which treat the inner medullary interstitium as a viscoelastic system rather than a purely hydraulic system.

Immunomorphometric study of rat renal inner medulla

We utilized immunofluorescent immunolabeling of renal tissue sections to identify and count tubules at specified depths of the rat renal inner medulla. We used primary antibodies to aquaporin-1 (AQP1; labeling thin descending limbs), aquaporin-2 (AQP2; labeling inner medullary collecting ducts), the rat kidney-specific chloride channel (ClC-K1; labeling thin ascending limbs), and von Willebrand factor (labeling descending vasa recta). Secondary antibodies conjugated to different fluorophores were used, giving up to a three-color display. Labeled structures were then identified and counted. At each level sampled in the inner medulla, many more thin limbs were labeled by ClC-K1 than AQP1. In addition, thin limbs were found to label with antibodies to ClC-K1 on both sides of their hairpin turns. We conclude that the descending thin limbs shift from expressing AQP1 to expressing ClC-K1 some distance before the point where they turn and begin to ascend. Mathematical models can use our quantitative data to explore implications for the urine-concentrating mechanism.

Contribution of the Na+-K+-2Cl- cotransporter NKCC1 to Cl- secretion in rat OMCD

In rat kidney the "secretory" isoform of the Na+-K+-2Cl- cotransporter (NKCC1) localizes to the basolateral membrane of the alpha-intercalated cell. The purpose of this study was to determine whether rat outer medullary collecting duct (OMCD) secretes Cl- and whether transepithelial Cl- transport occurs, in part, through Cl- uptake across the basolateral membrane mediated by NKCC1 in series with Cl- efflux across the apical membrane. OMCD tubules from rats treated with deoxycorticosterone pivalate were perfused in vitro in symmetrical HCO/CO2-buffered solutions. Cl- secretion was observed in this segment, accompanied by a lumen positive transepithelial potential. Bumetanide (100 microM), when added to the bath, reduced Cl- secretion by 78%, although the lumen positive transepithelial potential and fluid flux were unchanged. Bumetanide-sensitive Cl- secretion was dependent on extracellular Na+ and either K+ or NH, consistent with the ion dependency of NKCC1-mediated Cl- transport. In conclusion, OMCD tubules from deoxycorticosterone pivalate-treated rats secrete Cl- into the luminal fluid through NKCC1-mediated Cl- uptake across the basolateral membrane in series with Cl- efflux across the apical membrane. The physiological role of NKCC1-mediated Cl- uptake remains to be determined. However, the role of NKCC1 in the process of fluid secretion could not be demonstrated.

K+/NH4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla

The mechanism of NH4+ transport in inner medulla is not known. The purpose of these experiments was to study the process that is involved in ammonium (NH4+) transport in cultured inner medullary collecting duct (mIMCD-3) cells. Cells grown on coverslips were exposed to NH4+ and monitored for pHi changes by the use of the pH-sensitive dye BCECF. The rate of cell acidification following the initial cell alkalinization was measured as an index of NH4+ transport. The rate of NH4+ transport was the same in the presence or absence of sodium in the media (0.052 +/- 0.003 vs 0.048 +/- 0.004 pH/min. P > 0.05), indicating that NH4+ entry into the cells was independent of sodium. The presence of ouabain, bumetanide, amiloride, barium, or 4,4'-di-isothiocyanostilbene-2-2'-disulfonic acid (DIDS) did not block the NH4(+)-induced cell acidification, indicating lack of involvement of Na+:K(+)-ATPase, Na+:K+:2Cl- transport, Na+:H+ exchange, K+ channel, or Cl-/base exchange, respectively, in NH4+ transport. The NH4(+)-induced cell acidification was significantly inhibited in the presence of high external [K+] as compared to low external [K+] (0.018 +/- 0.001 vs. 0.049 +/- 0.003 pH/min for 140 mM K+ vs. 1.8 mM K+ in the media, respectively, P < 0.001). Inducing K+ efflux by imposing an outward K+ gradient caused intracellular acidification by approximately 0.3 pH unit in the presence but not the absence of NH4+. This K+ efflux-induced NH4+ entry increased by extracellular NH4+ in a saturable manner with a Km of approximately 5 mM, blocked by increasing extracellular K+ and was not inhibited by barium. The K+ efflux-coupled NH4+ entry was electroneutral as monitored by the use of cell membrane potential probe 3,3'-dipropylthiadicarbocyanine. These results are consistent with the exchange of internal K+ with external NH4+ in a 1:1 ratio. The K(+)-NH4+ antiporter was inhibited by verapamil and Schering 28080 in a dose-dependent manner, was able to work in reverse mode, and did not show any affinity for H+ as a substrate, indicating that it is distinct from other NH4(+)-carrying transporters. We conclude that a unique transporter, a potassium-ammonium (K+/NH4+) antiport, is responsible for NH4+ transport in renal inner medullary collecting duct cells. This antiporter is sensitive to verapamil and Schering 28080, is electroneutral, and is selective for NH4+ and K+ as substrates. The K+/NH4+ antiporter may play a significant role in acid-base regulation by excretion of ammonium and elimination of acid.

Basolateral membrane Na+/H+ exchange enhances HCO3- absorption in rat medullary thick ascending limb: evidence for functional coupling between basolateral and apical membrane Na+/H+ exchangers

The role of basolateral membrane Na+/H+ exchange in transepithelial HCO3- absorption (JHCO3) was examined in the isolated, perfused medullary thick ascending limb (MTAL) of the rat. In Na(+)-free solutions, addition of Na+ to the bath resulted in a rapid, amiloride-sensitive increase in intracellular pH. In MTALs perfused and bathed with solutions containing 146 mM Na+ and 25 mM HCO3-, bath addition of amiloride (1 mM) or 5-(N-ethyl-N-isopropyl) amiloride (EIPA, 50 microM) reversibly inhibited JHCO3 by 50%. Evidence that the inhibition of JHCO3 by bath amiloride was the result of inhibition of Na+/H+ exchange included the following: (i) the IC50 for amiloride was 5-10 microM, (ii) EIPA was a 50-fold more potent inhibitor than amiloride, (iii) the inhibition by bath amiloride was Na+ dependent, and (iv) significant inhibition was observed with EIPA as low as 0.1 microM. Fifty micromolar amiloride or 1 microM EIPA inhibited JHCO3 by 35% when added to the bath but had no effect when added to the tubule lumen, indicating that addition of amiloride to the bath did not directly inhibit apical membrane Na+/H+ exchange. In experiments in which apical Na+/H+ exchange was assessed from the initial rate of cell acidification following luminal EIPA addition, bath EIPA secondarily inhibited apical Na+/H+ exchange activity by 46%. These results demonstrate basolateral membrane Na+/H+ exchange enhances transepithelial HCO3- absorption in the MTAL. This effect appears to be the result of cross-talk in which an increase in basolateral membrane Na+/H+ exchange activity secondarily increases apical membrane Na+/H+ exchange activity.

Countercurrent system

Urinary concentration is achieved by countercurrent multiplication in the inner medulla. The single effect in the outer medulla is active NaCl absorption from the thick ascending limb. While the single effect in the inner medulla is not definitively established, the majority of experimental data favors passive NaCl absorption from the thin ascending limb. Continued experimental studies in inner medullary nephron segments will be needed to elucidate fully the process of urinary concentration.

Effect of protein intake and urea on sodium excretion during inappropriate antidiuresis in rats

Administration of urea to patients with the syndrome of inappropriate antidiuresis (SIAD) is thought to ameliorate hyponatremia by both producing an osmotic diuresis and diminishing ongoing natriuresis. The present study evaluated these effects in a rat model of SIAD utilizing dilutional hyponatremia induced by continuous infusion of 1-deamino-[8-D-arginine] vasopressin. Following 48 hours of sustained hyponatremia, separate groups of rats were then refed with either: (1) 5% dextrose alone, (2) a 20% protein chow, (3) an isocaloric protein deficient (0%) chow, or (4) the isocaloric protein-deficient chow supplemented with oral urea. Our results demonstrate that rats refed a 20% protein diet significantly improved their plasma [Na+] as compared to rats refed protein deficient diets, and this improvement was accompanied by decreases in natriuresis despite an increased glomerular filtration rate and an unchanged negative free water clearance. Identical effects were observed in rats refed a protein deficient diet but supplemented with oral urea, suggesting that urea generation from catabolism of dietary protein is responsible for the effect of protein refeeding to decrease urinary sodium excretion. Both the protein and urea refed rats had significantly higher inner medullary urea contents and concentrations compared to rats refed protein-deficient diets and also to rats studied immediately before protein refeeding, supporting the hypothesis that urea and dietary protein decrease natriuresis in patients with SIAD in association with increased inner medullary urea concentrations.

Ammonium transport in medullary thick ascending limb of rabbit kidney: involvement of the Na+,K+,Cl(-)-cotransporter

In order to investigate the question whether ammonium reabsorption in the thick ascending limb of Henle's loop (TALH) proceeds via the Na+,K+,Cl(-)-cotransporter, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and the effect of NH+4 on their transport properties was investigated. It was found that, in the presence of a 78-mmol/liter NaCl gradient, 5 mmol/liter NH+4 inhibited bumetanide-sensitive rubidium flux by 86%; a similar decrease was observed for 5 mmol/liter, K+. Inhibition of bumetanide-sensitive rubidium uptake by NH+4 was competitive and an apparent Ki of 1.9 mmol/liter was found. Bumetanide-sensitive sodium uptake measured in the presence of a 83 mmol/liter KCl gradient was not inhibited by 5 mmol/liter NH+4. A 100-mmol/liter NH4Cl gradient was, however, capable of stimulating bumetanide-sensitive sodium uptake to the same extent as a KCl gradient. These data suggest that NH+4 is accepted by the K+ site of the Na+,K+,Cl-cotransport system and that the transporter can function in a Na+,NH+4,2Cl mode. Since the affinity of the transporter for NH+4 lies in the concentration range found in the TALH lumen in vivo, it is concluded that Na+,NH+4,2Cl-cotransport can contribute to the NH+4 reabsorption in this tubular segment.

Osmoregulation of renal papillary cells

Element concentrations were determined in various extra- and intracellular compartments of the rat renal papilla in antidiuresis and after furosemide-induced diuresis using electron microprobe analysis to elucidate further how the cells adapt osmotically to different osmolalities. In antidiuresis and diuresis the sum of intracellular cations (sodium and potassium), accompanying anions and urea was insufficient in both cases to provide cell osmolalities similar to those in extracellular compartments. This finding provides further evidence that the papillary cells achieve osmoadaptation to widely differing extracellular electrolyte concentrations mainly by varying the cellular concentrations of osmotically-active substances other than urea and electrolytes.

Internephron heterogeneity for carbonic anhydrase-independent bicarbonate reabsorption in the rat

The present experiments were designed to localize the sites of carbonic anhydrase-independent bicarbonate reabsorption in the rat kidney and to examine some of its mechanisms. Young Munich-Wistar rats were studied using standard cortical and papillary free-flow micropuncture techniques. Total CO2 (tCO2) was determined using microcalorimetry. In control rats both superficial and juxtamedullary proximal nephrons reabsorbed approximately 95% of the filtered load of bicarbonate. The administration of acetazolamide (20 mg/kg body weight [bw]/h) decreased proximal reabsorption to 65.6% of the filtered load in superficial nephrons (32% was reabsorbed by the proximal convoluted tubule while 31.7% was reabsorbed by the loop segment), and to 38.4% in juxtamedullary nephrons. Absolute reabsorption of bicarbonate was also significantly higher in superficial than in juxtamedullary nephrons after administration of acetazolamide (727 +/- 82 vs. 346 +/- 126 pmol/min; P less than 0.05). The infusion of amiloride (2.5 mg/kg bw/h) to acetazolamide-treated rats increased the fractional excretion of bicarbonate as compared with animals treated with acetazolamide alone (34.9 +/- 1.9 vs. 42.9 +/- 2.1%; P less than 0.01), and induced net addition of bicarbonate between the superficial early distal tubule and the final urine (34.8 +/- 3.0 vs. 42.9 +/- 2.1%; P less than 0.05). Amiloride at this dose did not affect proximal water or bicarbonate transport; our studies localize its site of action to the terminal nephron. Vasa recta (VR) plasma and loop of Henle (LH) tubular fluid tCO2 were determined in control and acetazolamide-treated rats in order to identify possible driving forces for carbonic anhydrase-independent bicarbonate reabsorption in the rat papilla. Control animals showed a tCO2 gradient favoring secretion (LH tCO2, 7.4 +/- 1.7 mM vs. VR tCO2, 19.1 +/- 2.3 mM; P less than 0.005). Acetazolamide administration reversed this chemical concentration gradient, inducing a driving force favoring reabsorption of bicarbonate (LH tCO2, 27.0 +/- 1.4 mM vs. VR tCO2, 20.4 +/- 1.0 mM; P less than 0.005). Our study shows that in addition to the superficial proximal convoluted tubule, the loop segment and the collecting duct show acetazolamide-insensitive bicarbonate reabsorption. No internephron heterogeneity for bicarbonate transport was found in controls. The infusion of acetazolamide, however, induced significant internephron heterogeneity for bicarbonate reabsorption, with superficial nephrons reabsorbing a higher fractional and absolute load of bicarbonate than juxtamedullary nephrons. We think that the net addition of bicarbonate induced by amiloride is secondary to inhibition of voltage-dependent, carbonic anhydrase-independent bicarbonate reabsorption at the level of the collecting duct, which uncovers a greater delivery of carbonate from deeper nephrons to the collecting duct. Finally, our results suggest that carbonic anhydrase-independent bicarbonate reabsorption is partly passive, driven by favorable chemical gradients in the papillary tubular structures, and partly voltage-dependent, in the collecting duct.

Intra- and extracellular element concentrations of rat renal papilla in antidiuresis

The element concentrations in various intra- and extracellular compartments of the tip of the rat renal papilla were determined during antidiuresis using electron microprobe analysis. Urinary concentrations (means +/- SEM) were: urea, 1509 +/- 116; potassium, 268 +/- 32; sodium, 62 +/- 19 mmoles X 1(-1); and osmolality, 2548 +/- 141 mOsm X kg-1. Electrolyte concentrations in the interstitial space were: sodium, 437 +/- 19; chloride, 438 +/- 20; and potassium, 35 +/- 2 mmoles X kg-1 wet wt. The vasa recta plasma exhibited almost identical element concentrations. The values in the papillary collecting duct cells were: sodium, 28 +/- 1; chloride, 76 +/- 3; potassium, 135 +/- 3; and phosphorus, 316 +/- 7 mmoles X kg-1 wet wt. Similar concentrations were observed in the papillary epithelial cells. In interstitial cells potassium and phosphorus concentrations were virtually identical to those of the collecting duct cells, whereas sodium and chloride concentrations were higher by about 30 mmoles X kg-1 wet wt. The element composition of the various papillary cells is, thus, not substantially different from that of proximal tubular cells. This finding demonstrates that cellular accumulation of electrolytes is not the regulatory mechanism by which papillary cells adapt osmotically to their high environmental osmolality and sodium chloride concentration.

Renal countercurrent system: role of collecting duct convergence and pelvic urea predicted from a mathematical model

A differential equation model of the renal countercurrent system has been developed and physiological data from nephron segments were incorporated together with recently suggested urea recycling from renal pelvis to inner medulla and, particularly, an exponential reduction in the number of collecting tubules towards the renal papilla. The role of these features for the countercurrent concentrating mechanism has been studied by simulation runs. The computations, using the multiple shooting method, provide predictions about concentration profiles for salt and urea in tubes (nephron segments) and in the central core along the entire medullary countercurrent system. The results indicate that this model, without active salt or urea transport in the inner medulla, yields concentration gradients along the medullary axis compatible with those measured in the tissue.

Consequences of potassium recycling in the renal medulla. Effects of ion transport by the medullary thick ascending limb of Henle's loop

The consequences of K recycling and accumulation in the renal medulla were examined by measuring the effect of elevated K concentration on ion transport by the medullary thick ascending limb of Henle's loop. Perfused and bathed in vitro, thick limbs from both mouse and rabbit displayed a graded, reversible reduction of transepithelial voltage after increasing K concentration from 5 to 10, 15, or 25 mM. The effect was reproducible whether osmolality was 328 or 445 mosmol/kg H2O, and whether K replaced Na or choline. Net chloride absorption and transepithelial voltage were reduced by almost 90% when ambient K concentration was 25 mM. When either lumen or bath K was increased to 25 mM, net Na absorption was reduced. There was spontaneous net K absorption when perfusate and bath K concentration was 5 mM. Analysis of transepithelial K transfer after imposition of chemical gradients demonstrated rectification in the absorptive direction. Absorption of K by this segment provides a means to maintain high medullary interstitial concentration. Accumulation of K in the outer medulla, by reducing NaCl absorption, would increase volume flow through the loop of Henle and increase Na and water delivery to the distal nephron. K recycling thus might provide optimum conditions for K secretion by the distal nephron.

Effect of medullary tonicity on urinary sodium excretion in the rat

In previous reports from this laboratory we have suggested that a reduction in medullary tonicity decreases the thin ascending loop of Henle sodium reabsorption and is in part responsible for the magnitude for the natriuresis accompanying 10% body weight Ringer loading. According to this postulate, one would expect that the medullary washout associated with water diuresis would also result in a natriuresis, but this does not occur. It is possible, however, that increased delivery from the proximal tubule is necessary to demonstrate an effect of medullary tonicity on urinary sodium excretion. Micropuncture studies were designed to test that possibility by increasing distal delivery by 2% Ringer loading in animals with and without reduced medullary tonicity. In an initial series of experiments the alpha-adrenergic agonist clonidine was used to induce a water diuresis. When given alone, this agent caused a marked decrease in urine osmolality and an increase in urine flow rate but had no effect on proximal reabsorption in either superficial or juxtamedullary nephrons, and did not alter urinary sodium excretion. Volume expansion with 2% body weight Ringer solution resulted in a significant fall in proximal reabsorption and a trivial increment in sodium excretion. When this same degree of volume expansion was conferred on animals undergoing a water diuresis, a marked increase in absolute and fractional sodium excretion occurred. In a second group of studies medullary tonicity was reduced in the left kidney only by removal of the left ureter 1 h before micropuncture. When these animals were infused with 2% body weight Ringer solution, proximal reabsorption was decreased in juxtamedullary nephrons, and a marked increase in sodium excretion was observed only from the left kidney. Finally, the effect of water diuresis on fractional sodium delivery to the early and late distal tubule of superficial nephrons during 2% Ringer loading was evaluated. Delivery to both of these sites was comparable after 2% Ringer loading alone and during 2% Ringer loading plus water diuresis. From these data, we conclude that medullary tonicity does influence renal sodium handling but that this effect is manifest in the final urine only under conditions in which proximal reabsorption is decreased. The data also suggest that this effect is limited to juxtamedullary nephrons and is probably localized to the thin ascending limb of the loop of Henle.

Suppression of potassium-recycling in the renal medulla by short-term potassium deprivation

Recently we proposed that potassium, like urea, normally undergoes medullary recycling from collecting tubule to the pars recta or descending limb of the juxtamedullary nephron and suggested that the extent of recycling is a function of the concentration of potassium in collecting tubule fluid. To test this hypothesis further, we fed young rats a potassium-free diet for 3 days and then prepared them for micropuncture of the left renal papilla. Compared to findings in normally fed animals, potassium deprivation caused a significant fall in plasma potassium and urinary excretion of potassium. There was a striking decrease in the fraction of filtered potassium remaining at the end of the justamedullary descending limb for 94 +/- 11% to 38 +/- 3% (P less than 0.001). The latter value is not significantly different from the fraction of filtered sodium remaining (36 +/- 4%) and suggests that net addition of potassium to the pars recta or descending limb was completely abolished. A correlation was observed between the fraction of filtered potassium remaining at the end of the descending limb and either urinary potassium excretion (P less than 0.001) or urinary potassium concentration(P less than 0.001) in the contralateral unexposed kidney. These results lend further support to the hypothesis of medullary recycling of potassium.

The clinical physiology of water metabolism. Part II: Renal mechanisms for urinary concentration; diabetes insipidus

The renal reabsorption of water independent of solute is the result of the coordinated function of the collecting duct and the ascending limb of the loop of Henle. The unique juxtaposition of the ascending and descending portions of the loop of Henle and of the vasa recta permits the function of a counter-current multiplier system in which water is removed from the tubular lumen and reabsorbed into the circulation. The driving force for reabsorption is the osmotic gradient in the renal medulla which is dependent, in part, on chloride (followed by sodium) pumping from the thick ascending loop of Henle. Urea trapping is also thought to play an important role in the generation of a hypertonic medullary interstitium. Arginine vasopressin (AVP) acts by binding to receptors on the cell membrane and activating adenylate cyclase. This, inturn, results in the intracellular accumulation of cyclic adenosine monophosphate (AMP) which in some fashion abruptly increases the water permeability of the luminal membrane of cells in the collecting duct. As a consequence, water flows along an osmotic gradient out of the tubular lumen into the medullary interstitium. Diabetes insipidus is the clinical condition associated with either a deficiency of or a resistance to AVP. Central diabetes insipidus is due to diminished release of AVP following damage to either the neurosecretory nuclei or the pituitary stalk. Possible causes include idiopathic, familial, trauma, tumor, infection or vascular lesions. Patients present with polyuria, usually beginning over a period of a few days. The diagnosis is made by showing that urinary concentration is impaired after water restriction but that there is a good response to exogenous vasopressin therapy. Nephrogenic diabetes insipidus can be identified by a patient's lack of response to AVP. Nephrogenic diabetes insipidus is caused by a familial defect, although milder forms can be acquired as a result of various forms of renal disease. Central diabetes insipidus is eminently responsive to replacement therapy, particularly with dDAVP, a long lasting analogue of AVP. Nephrogenic diabetes insipidus is best treated with a combination of thiazide diuretics as well as a diet low in sodium and protein.

Editorial review. Recent formulations of the urinary concentrating mechanism: a status report

The status of the purely passive mode of solute concentration as of 1979 appears to be similar to that of the original countercurrent hypothesis 10 years ago. The passive mode concept has advanced our understanding of the concentrating process by qualitatively incorporating the permeability characteristics of tubule segments and the lack of an active transport process in the thin loop of Henle into a mechanism which has attractive economy and explanatory value. But in the final analysis some assumptions are not borne out by experimental findings (for example, the high urea concentration of fluid in the rat and hamster end-descending limb; the likelihood of net transepithelial addition of sodium chloride to the Psammomys descending limb; the removal of sodium chloride from the hamster ascending limb against an apparent electrochemical gradient under certain circumstances; and the osmotic lag between vasa recta blood and interstitium in the rat). Furthermore, when the known permeability and transport characteristics of the renal tubule are incorporated into a mathematic model of the passive operating mode, numerical simulations fail to establish a progressively hyperosmotic inner medulla. This does not rule out the applicability of the more general model (Eq. 1), particularly if evidence for some form of active transport in the inner medulla, heretofore lacking, is forthcoming.

Effect of chronic potassium loading on potassium secretion by the pars recta or descending limb of the juxtamedullary nephron in the rat

Recently we demonstrated potassium secretion by the pars recta or by the descending limb of the juxtamedullary nephron. The purpose of this present investigation is to study the effect of a chronic high-potassium intake on this phenomenon. Fractional reabsorption of water and sodium by the juxtamedullary proximal nephron was decreased when compared to that in normal hydropenic rats. There was a striking increase in the fraction of filtered potassium at the end of the juxtamedullary descending limb from 94+/11% to 180+/18%, which was principally a result of enhanced potassium secretion. When the concentration of potassium in the collecting tubule fluid of potassium-loaded rats was reduced after the administration of amiloride, a sharp fall was observed in the amount of potassium which reached the end of the descending limb (64+/8%). A direct correlation was observed between the fraction of filtered potassium at the descending limb and the potassium concentration in the final urine (P less than 0.001). The findings suggest that potassium, like urea, normally undergoes medullary recycling, which is enhanced by chronic potassium loading.

Further studies on segmental sodium transport in the rat kidney during expansion of the extracellular fluid volume

The present studies were designed to further investigate the possibility of heterogeneity of nephron function during Ringer loading in the rat, and to determine the specific nephron segment responsible for this finding. As in previous studies from this laboratory with smaller rats (50-125 g), net addition of sodium between late distal tubule and papillary base (6.9 vs. 10.4% of the filtered load, respectively, P <0.005) was found in more mature rats (170-230 g). In contrast, there was net reabsorption of sodium between these two segments in nonvolume-expanded animals, 1.70 vs. 0.45% of the filtered sodium load, P <0.005. Because nephron heterogeneity of sodium transport during extracellular volume expansion is the most likely explanation for these findings, further studies were performed to determine the specific juxtamedullary nephron segment responsible for the net addition pattern between late distal tubule and papillary base in Ringer-loaded animals. First, a comparison was made of sodium delivery to the late proximal tubule of superficial nephrons vs. the delivery rate to the bend of Henle's loop of juxtamedullary nephrons in both hydropenia and Ringer loading. Fractional sodium delivery was quite comparable between the superficial and juxtamedullary nephrons in both hydropenia and Ringer loading although the absolute level was much greater in both groups of nephrons in the Ringer studies. Chlorothiazide (15 mg/kg loading and 15 mg/kg per h) given during Ringer loading markedly increased late distal sodium delivery, 19% of the filtered load, but did not prevent net addition of sodium at the papillary base. In contrast, furosemide (5 mg/kg loading and 5/mg/kg per h) given during Ringer loading completely reversed the segmental pattern, 35.5 and 28.8% at late distal tubule and papillary base, respectively, P <0.005. These studies demonstrate that the net addition of sodium between late distal tubule and papillary base during Ringer loading is not limited to immature rats and that the segmental pattern does not occur in non-volume-expanded animals. Further, the reversal of the net addition pattern with furosemide, but not chlorothiazide, and the comparable proximal nephron delivery rates in Ringer loading suggest that the loop of Henle of juxtamedullary nephrons reabsorbs less sodium than the same portion of superficial nephrons in this setting. A model is proposed to explain this finding.