Outer Medullary Descending Vasa Recta Research Articles

Urea Transporter B and MicroRNA-200c Differ in Kidney Outer Versus Inner Medulla Following Dehydration

BackgroundUrea transporters (UTs) are important in urine concentration and in urea recycling, and UT-B has been implicated in both. In kidney, UT-B was originally localized to outer medullary descending vasa recta, and more recently detected in inner medullary descending vasa recta. Endogenously produced microRNAs (miRs) bind to the 3′UTR of genes and generally inhibit their translation, thus playing a pivotal role gene regulation. MethodsMice were dehydrated for 24 hours then sacrificed. Inner and outer medullas were analyzed by polymerase chain reaction (PCR) and quantitative PCR for miRNA expression and analyzed by western blotting for protein abundance. ResultsMiRNA sequencing analysis of mouse inner medullas showed a 40% increase in miRNA-200c in dehydrated mice compared with controls. An in silico analysis of the targets for miR-200c revealed that miRNA-200c could directly target the gene for UT-B. PCR confirmed that miR-200c is up-regulated in the inner medullas of dehydrated mice while western blot showed that UT-B protein abundance was down-regulated in the same portion of the kidney. However, in the outer medulla, miR-200c was reduced and UT-B protein was increased in dehydrated mice. ConclusionsThis is the first indication that UT-B protein and miR-200c may each be differentially regulated by dehydration within the kidney outer and inner medulla. The inverse correlation between the direction of change in miR-200c and UT-B protein abundance in both the inner and outer medulla suggests that miR-200c may be associated with the change in UT-B protein in these 2 portions of the kidney medulla.

Functional characterization of isolated, perfused outermedullary descending human vasa recta

The renal medulla plays an important role in the control of water and salt balance by the kidney. Outer medullary descending vasa recta (OMDVR) are microscopic vessels providing blood flow to the renal medulla. Data on the physiology of human vasa recta are scarce. Therefore, we established an experimental model of human single isolated, perfused OMDVR and characterized their vasoactivity in response to angiotensin II and to pressure changes. Human non-malignant renal tissue was obtained from patients undergoing nephrectomy due to renal cell carcinoma. OMDVR were dissected under magnification and perfused using concentric microscopic pipettes. The response of OMDVR to angiotensin II and pressure changes was quantified in serial pictures. All patients signed a consent form prior to surgery. Outer medullary descending vasa recta constricted significantly after bolus applications of angiotensin II. OMDVR constriction to angiotensin II was also concentration dependent. Response to luminal pressure changes was different according to the diameter of vessels, with larger OMDVR constricting after pressure increase, while smaller ones did not. Outer medullary descending vasa recta constrict in response to angiotensin II and pressure increases. Our results show that OMDVR may take part in the regulation of medullary blood flow in humans. Our model may be suitable for investigating disturbances of renal medullary circulation in human subjects.

Iodixanol, Constriction of Medullary Descending Vasa Recta, and Risk for Contrast Medium–induced Nephropathy

To determine whether a type of contrast medium (CM), iodixanol, modifies outer medullary descending vasa recta (DVR) vasoreactivity and nitric oxide (NO) production in isolated microperfused DVR. Animal handling conformed to the Animal Care Committee Guidelines of all participating institutions. Single specimens of DVR were isolated from rats and perfused with a buffered solution containing iodixanol. A concentration of 23 mg of iodine per milliliter was chosen to mimic that expected to be used in usual examinations in humans. Luminal diameter was determined by using video microscopy, and NO was measured by using fluorescent techniques. Iodixanol led to 52% reduction of DVR luminal diameter, a narrowing that might interfere with passage of erythrocytes in vivo. Vasoconstriction induced by angiotensin II was enhanced by iodixanol. Moreover, iodixanol decreased NO bioavailability by more than 82%. Use of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (a superoxide dismutase mimetic) prevented both vasoconstriction with iodixanol alone and increased constriction with angiotensin II caused by CM. Iodixanol in doses typically used for coronary interventions constricts DVR, intensifies angiotensin II-induced constriction, and reduces bioavailability of NO. CM-induced nephropathy may be related to these events and scavenging of reactive oxygen species might exert a therapeutic benefit by preventing the adverse effects that a CM has on medullary perfusion.

Descending outer medullary vasa recta are constricted by perfusion with iodinated contrast media

ObjectiveContrast induced nephropathy (CIN) is a frequent complication of coronary angiography. Outer medullary descending vasa recta (DVR) are contractile microvessels supplying blood flow to the renal medulla ‐ the area of kidney at risk for CIN. Our objective was to test whether perfusion of DVR with a contrast medium (CM) modifies DVR vasoactivity.MethodsDVR from rats were microscopically isolated and perfused with iodixanol (23 mg iodine/ml, calculated after usual doses of CM). DVR luminal diameter was determined by microscopy and nitric oxide (NO) was measured by fluorescent techniques.ResultsCM led to 50% reduction of luminal diameter, and increased vasoconstriction of DVR by angiotensin II (ANG II). Dismutation of superoxide by Tempol prevented these effects. CM also decreased NO bioavailability by >60 percent.DiscussionOur data suggest that CM may lead to renal circulatory derangements described in CIN through a direct effect on DVR. This effect seems to happen via increased oxidative stress. Scavenging of reactive oxygen species might exert a therapeutic effect on CIN by mildering such derangements.Supported by the German Research Foundation, Werner Jackstaedt Foundation, and NIH

Roles of Aquaporins in Kidney Revealed by Transgenic Mice

Transgenic mouse models of aquaporin (AQP) deletion and mutation have been instructive in elucidating the role of AQPs in renal physiology. Mice lacking AQP1 are unable to concentrate their urine because of low water permeability in the proximal tubule, thin descending limb of Henle, and outer medullary descending vasa recta, resulting in defective near-isosmolar fluid absorption in the proximal tubule and defective countercurrent multiplication. Mice lacking functional AQP2, AQP3, or AQP4 manifest various degrees of nephrogenic diabetes insipidus resulting from reduced collecting duct water permeability. Mice lacking AQP7 and AQP8 can concentrate their urine fully, although AQP7 null mice manifest an interesting defect in glycerol reabsorption. Two unexpected renal phenotypes of AQP null mice have been discovered recently, including defective proximal tubule cell migration in AQP1 deficiency, and cystic renal disease in AQP11 deficiency. AQPs thus are important in several aspects of the urinary concentrating mechanism and in functions unrelated to tubular fluid transport. The mouse phenotype data suggest the renal AQPs as targets for the development of aquaretics and potentially for therapy of cystic renal disease and acute renal injury.

Aquaporins in endothelia

Aquaporin-1 (AQP1) water channels are expressed widely in microvascular endothelia outside of the central nervous system, including renal vasa recta and tumor microvessels, as well as in non-vascular endothelia in pleura, peritoneum, cornea, and lymphatics. In kidney, AQP1-facilitated water transport in outer medullary descending vasa recta is likely an important component of the urinary concentrating mechanism. However, in most vascular endothelia outside of kidney, it remains uncertain whether AQP1 expression and high water permeability are physiologically important. AQP1 in non-vascular endothelia at the inner corneal surface is involved in the maintenance of corneal transparency. Recently, a new role of AQP1 in endothelial cell migration was discovered in analyzing the cause of defective tumor angiogenesis in AQP1-deficient mice. AQP1 facilitates endothelial cell migration by a mechanism that may involve facilitated water transport across cell protrusions (lamellipodia). AQP1 inhibitors may thus have aquaretic and antiangiogenic activity.

Mechanisms underlying the differential control of blood flow in the renal medulla and cortex.

There is much evidence that the medullary circulation plays a key role in regulating renal salt and water handling and, accordingly, the long-term level of arterial pressure. It has also recently become clear that various regulatory factors can affect medullary blood flow (MBF) differently from cortical blood flow (CBF). It appears likely that the influence of hormonal and neural factors on the control of arterial pressure is mediated partly through their impact on MBF. In this review, we focus on the mechanisms underlying the differential control of MBF and CBF, particularly the relative insensitivity of MBF to vasoconstrictors such as angiotensin II, endothelin-1 and the sympathetic nerves. The vascular architecture of the kidney appears to be arranged in a way that protects the renal medulla from ischaemic insults, with juxtamedullary arterioles, the source of MBF, having larger calibre than their counterparts in other kidney regions. Indeed, recent studies using vascular casting methodology suggest that juxtamedullary glomerular arterioles are not the chief regulators of MBF, which is consistent with the idea that outer medullary descending vasa recta play a key role in MBF control. Release of vasoactive paracrine factors such as nitric oxide and various eicosanoids from the vascular endothelium, and probably also from the tubular epithelium, appear to differentially modulate responses of MBF and CBF to hormonal and neural factors. The prevailing intrarenal hormonal milieu and existing haemodynamic conditions also appear to strongly modulate these responses, indicating that multiple control systems interact to regulate regional kidney blood flow at an integrative level.

Prostaglandins.

Prostaglandins.

Angiotensin II constriction of rat vasa recta is partially thromboxane dependent.

We tested the hypothesis that thromboxane generation mediates vasoconstriction of isolated outer medullary descending vasa recta (OMDVR) by angiotensin (Ang) II. The lipoxygenase and cyclooxygenase (COX) inhibitor eicosatetraynoic acid (1 micromol/L) and the COX inhibitor indomethacin (1 micromol/L) partially reversed Ang II (1 nmol/L) constriction of in vitro perfused OMDVR. To determine whether thromboxane is a mediator of Ang II-induced vasoconstriction, a thromboxane synthase inhibitor, U63577A (1 micromol/L), and thromboxane receptor antagonists, SQ-29548 or BMS-180,291 (1 micromol/L, each), were introduced into the bath of vessels that had been preconstricted by Ang II (1 nmol/L). These agents significantly inhibited vasoconstriction induced by Ang II. In contrast, SQ-29548 and U63557A did not affect vessels preconstricted by raising extracellular KCl from 5 to 100 mmol/L. The thromboxane receptor agonist U46619 (1 micromol/L) constricted OMDVR, an effect that was blocked by the antagonist BMS-180,291. In separate protocols, microperfused OMDVR were pretreated with U63577A or SQ-29548, after which they were exposed to luminal Ang II to induce vasoconstriction. Both agents inhibited vasoconstriction whether preexposure to them was via the bath or the perfusate. We conclude that Ang II-induced constriction of OMDVR is partly mediated by metabolites of arachidonic acid, including thromboxanes.

Control of descending vasa recta pericyte membrane potential by angiotensin II.

Using nystatin perforated-patch whole cell recording, we investigated the role of Cl(-) conductance in the modulation of outer medullary descending vasa recta (OMDVR) pericyte membrane potential (Psi m) by ANG II. ANG II (10(-11) to 10(-7) M) consistently depolarized OMDVR and induced Psi m oscillations at lower concentrations. The Cl(-) channel blockers anthracene-9-decarboxylate (1 mM) and niflumic acid (10 microM) hyperpolarized resting pericytes and repolarized ANG II-treated pericytes. In voltage-clamp experiments, ANG II-treated pericytes exhibited slowly activating currents that were nearly eliminated by treatment with niflumic acid (10 microM) or removal of extracellular Ca(2+). Those currents reversed at -31 and -10 mV when extracellular Cl(-) concentration was 152 and 34 mM, respectively. In pericytes held at -70 mV, oscillating inward currents were sometimes observed; the reversal potential also shifted with extracellular Cl(-) concentration. We conclude that ANG II activates a Ca(2+)-dependent Cl(-) conductance in OMDVR pericytes to induce membrane depolarization and Psi m oscillations.

Nitric oxide generation by isolated descending vasa recta.

Nitric oxide (NO) generation by the outer medullary descending vasa recta (OMDVR) was measured with the fluorescent dye 4,5-diaminofluoroscein (DAF-2) during 30-min incubations. Addition of 0.1 or 1.0 mM L-arginine to the incubation buffer increased the DAF-2 signal by 8.7 and 13.6% (P = 0.08 and P < 0.05), respectively. Compared with L-arginine alone (0.1 mM), bradykinin (BK; 1 x 10(-7) M) enhanced the DAF-2 signal by 11.1% (P < 0.05). The NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (0.1 mM) reversed the BK-stimulated NO generation as measured with either DAF-2 or by the oxidation of Fe(2+) hemoglobin. Using 1 mM 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol), a cell-permeant superoxide dismutase mimetic, we tested whether reduction of superoxide anion increases intracellular NO. Tempol increased DAF-2 fluorescence by 12 and 23.3%, respectively, over BK-stimulated or control vessels. Tempol also vasodilated ANG II (1 x 10(-8) M)-preconstricted OMDVR (P < 0.05). We conclude that NO generation by isolated OMDVR can be increased by L-arginine, that the endothelium-dependent vasodilator BK enhances NO production, and that NO consumption by superoxide plays a role in the determination of cellular NO concentrations.

Role of chloride in constriction of descending vasa recta by angiotensin II.

We investigated the dependence of ANG II (10(-8) M)-induced constriction of outer medullary descending vasa recta (OMDVR) on membrane potential (Psim) and chloride ion. ANG II depolarized OMDVR, as measured by fully loading them with the voltage-sensitive dye bis[1,3-dibutylbarbituric acid-(5)] trimethineoxonol [DiBAC(4)(3)] or selectively loading their pericytes. ANG II was also observed to depolarize pericytes from a resting value of -55.6 +/- 2.6 to -26.2 +/- 5.4 mV when measured with gramicidin D-perforated patches. When measured with DiBAC(4)(3) in unstimulated vessels, neither changing extracellular Cl(-) concentration ([Cl(-)]) nor exposure to the chloride channel blocker indanyloxyacetic acid 94 (IAA-94; 30 microM) affected Psim. In contrast, IAA-94 repolarized OMDVR pretreated with ANG II. Neither IAA-94 (30 microM) nor niflumic acid (30 microM, 1 mM) affected the vasoactivity of unstimulated OMDVR, whereas both dilated ANG II-preconstricted vessels. Reduction of extracellular [Cl(-)] from 150 to 30 meq/l enhanced ANG II-induced constriction. Finally, we identified a Cl(-) channel in OMDVR pericytes that is activated by ANG II or by excision into extracellular buffer. We conclude that constriction of OMDVR by ANG II involves pericyte depolarization due, in part, to increased activity of chloride channels.

Adenosine signaling in outer medullary descending vasa recta.

We tested whether dilation of outer medullary descending vasa recta (OMDVR) is mediated by cAMP, nitric oxide (NO), and cyclooxygenase (COX). Adenosine (A; 10(-6) M)-induced vasodilation of ANG II (10(-9) M)-preconstricted OMDVR was mimicked by the cAMP analog 8-bromoadenosine 3',5'-cyclic monophosphate (10(-10) to 10(-4) M) and reversed by the adenylate cyclase inhibitor SQ-22536. Adenosine (10(-4) M) stimulated OMDVR cAMP production greater than threefold. NO synthase blockade with N(G)-nitro-L-arginine methyl ester and N(G)-monomethyl-L-arginine (10(-4) M) did not affect adenosine vasodilation. Adenosine induced endothelial cytoplasmic calcium transients that were small. Indomethacin (10(-6) M) reversed adenonsine-induced dilation of OMDVR preconstricted with ANG II, endothelin, 4-bromo-calcium ionophore A23187, or carbocyclic thromboxane A(2). In contrast, selective A(2)-receptor activation dilated endothelin-preconstricted OMDVR even in the presence of indomethacin. We conclude that OMDVR vasodilation by adenosine involves cAMP and COX but not NO. COX blockade does not fully inhibit selective A(2) receptor-mediated OMDVR dilation.

Effects of activation of vasopressin-V1-receptors on regional kidney blood flow and glomerular arteriole diameters.

We tested whether vasoconstriction of juxtamedullary glomerular arterioles contributes to vasopressin V1 -receptor-mediated reductions in medullary perfusion (MBF). The left kidney of pentobarbitone anaesthetized rabbits was denervated, a perivascular flow probe placed around the renal artery and laser-Doppler flow probes positioned in the inner medulla and on the cortical surface. Rabbits then received a 30 min intravenous infusion of [Phe2,Ile3,Orn8]vasopressin (V1 -AG; 30 ng/kg per min; n = 7) or its vehicle (n = 7). Kidneys were perfusion fixed at the final recorded mean arterial pressure (MAP) and filled with methacrylate casting material. Diameters of afferent and efferent arterioles were determined by scanning electron microscopy. V1 -AG increased MAP (19 +/- 3%) and reduced MBF (30 +/- 8%) but not cortical perfusion or total renal blood flow. Vehicle-treatment did not significantly affect these variables. After vehicle- and V1-AG-treatment, juxtamedullary afferent arteriole luminal diameter averaged 15.35 +/- 1.31 and 15.88 +/- 1.86 microm, respectively (P= 0.92), while juxtamedullary efferent arteriole luminal diameter averaged 17.75 +/- 1.86 and 18.36 +/- 2.24 microm, respectively (P= 0.93). V1-AG reduced MBF but did not significantly affect juxtamedullary arteriolar diameter. Our results therefore do not support a role for juxtamedullary arterioles in producing V1-receptor-mediated reductions in MBF, suggesting that downstream vascular elements (e.g. outer medullary descending vasa recta) might be involved.

Inhibition of calcium signaling in descending vasa recta endothelia by ANG II.

The intracellular calcium ([Ca(2+)](i)) response of outer medullary descending vasa recta (OMDVR) endothelia to ANG II was examined in fura 2-loaded vessels. Abluminal ANG II (10(-8) M) caused [Ca(2+)](i) to fall in proportion to the resting [Ca(2+)](i) (r = 0. 82) of the endothelium. ANG II (10(-8) M) also inhibited both phases of the [Ca(2+)](i) response generated by bradykinin (BK, 10(-7) M), 835 +/- 201 versus 159 +/- 30 nM (peak phase) and 169 +/- 26 versus 103 +/- 14 nM (plateau phase) (means +/- SE). Luminal ANG II reduced BK (10(-7) M)-stimulated plateau [Ca(2+)](i) from 180 +/- 40 to 134 +/- 22 nM without causing vasoconstriction. Abluminal ANG II added to the bath after luminal application further reduced [Ca(2+)](i) to 113 +/- 9 nM and constricted the vessels. After thapsigargin (TG) pretreatment, ANG II (10(-8) M) caused [Ca(2+)](i) to fall from 352 +/- 149 to 105 +/- 37 nM. This effect occurred at a threshold ANG II concentration of 10(-10) M and was maximal at 10(-8) M. ANG II inhibited both the rate of Ca(2+) entry into [Ca(2+)](i)-depleted endothelia and the rate of Mn(2+) entry into [Ca(2+)](i)-replete endothelia. In contrast, ANG II raised [Ca(2+)](i) in the medullary thick ascending limb and outer medullary collecting duct, increasing [Ca(2+)](i) from baselines of 99 +/- 33 and 53 +/- 11 to peaks of 200 +/- 47 and 65 +/- 11 nM, respectively. We conclude that OMDVR endothelia are unlikely to be the source of ANG II-stimulated NO production in the medulla but that interbundle nephrons might release Ca(2+)-dependent vasodilators to modulate vasomotor tone in vascular bundles.

Requirement of aquaporin-1 for NaCl-driven water transport across descending vasa recta.

Deletion of AQP1 in mice results in diminished urinary concentrating ability, possibly related to reduced NaCl- and urea gradient-driven water transport across the outer medullary descending vasa recta (OMDVR). To quantify the role of AQP1 in OMDVR water transport, we measured osmotically driven water permeability in vitro in microperfused OMDVR from wild-type, AQP1 heterozygous, and AQP1 knockout mice. OMDVR diameters in AQP1(-/-) mice were 1.9-fold greater than in AQP1(+/+) mice. Osmotic water permeability (P(f)) in response to a 200 mM NaCl gradient (bath > lumen) was reduced about 2-fold in AQP1(+/-) mice and by more than 50-fold in AQP1(-/-) mice. P(f) increased from 1015 to 2527 microm/s in AQP1(+/+) mice and from 22 to 1104 microm/s in AQP1(-/-) mice when a raffinose rather than an NaCl gradient was used. This information, together with p-chloromercuribenzenesulfonate inhibition measurements, suggests that nearly all NaCl-driven water transport occurs by a transcellular route through AQP1, whereas raffinose-driven water transport also involves a parallel, AQP1-independent, mercurial-insensitive pathway. Interestingly, urea was also able to drive water movement across the AQP1-independent pathway. Diffusional permeabilities to small hydrophilic solutes were comparable in AQP1(+/+) and AQP1(-/-) mice but higher than those previously measured in rats. In a mathematical model of the medullary microcirculation, deletion of AQP1 resulted in diminished concentrating ability due to enhancement of medullary blood flow, partially accounting for the observed urine-concentrating defect.

Distribution of angiotensin AT1 and AT2 receptor subtypes in the rat kidney.

ANG II contributes importantly to the regulation of renal vascular resistance, glomerular filtration, and tubular epithelial transport, yet there remains a paucity of information regarding the localization of the ANG II type 1 and 2 (AT1 and AT2) receptors within the rat kidney particularly within the vasculature. The present study was designed to localize the transcriptional and translational site(s) of AT1 and AT2 receptor (AT1R and AT2R, respectively) expression within the rat kidney. Using immunohistochemistry, we detected the AT(1)R translational sites throughout the kidney, with the strongest labeling found in the vasculature of the renal cortex and the proximal tubules of the outer medulla. The AT2R protein expression was found throughout the rat kidney, although there was little to no expression found in the glomerulus and medullary thick ascending limbs of Henle (TAL). Gene-specific primers were then designed to distinguish between the receptor subtypes within microdissected renal tubular and vascular segments using RT-PCR. AT1AR, AT1BR, and AT2R mRNA were found within the renal vasculature (afferent arterioles, arcuate artery, and outer medullary descending vasa recta). The mRNA for both the AT1R isoforms was also detected in the glomeruli and the renal tubules (proximal tubules, TAL, and collecting ducts); however, no AT2R mRNA was detected within the glomerulus and was inconsistently found within the medullary TAL (MTAL). Taken together, these data show that mRNA for the AT1R subtypes was located in all of the renal tubular and vascular segments. Evidence for AT2R mRNA was also found in all but two of the vascular and tubular segments, the MTAL, and the glomeruli. These results are consistent with the whole tissue immunohistochemically localized receptors.

Response of isolated rat descending vasa recta to bradykinin.

Outer medullary descending vasa recta (OMDVR) were dissected from the outer medullary vascular bundles of young rats, perfused in vitro, and loaded with fura 2 for measurement of intracellular calcium concentration ([Ca2+]i) by fluorescent ratio imaging. Fluorescent video images revealed that fura 2 selectively loads into endothelial cells but not pericytes. Bradykinin (BK), at concentrations > 10(-11) M, elicited an increase in [Ca2+]i from baseline values in the range from 50 to 100 nM to peak values of 600-800 nM followed by a sustained plateau of 150-250 nM. The vasopressin V1-receptor agonist [Phe2,Ile3,Orn8]vasopressin constricted OMDVR but yielded no observable [Ca2+]i response, a finding that is consistent with an endothelial cell origin for the fura 2 fluorescent signal. The BK [Ca2+]i response was blocked by the selective BK B2-receptor antagonists D-Arg-[Hyp3,Thi5.8,D-Phe7]BK and D-Arg-[Hyp3,D-Phe7,Leu8]BK but not the B1 antagonist des-Arg9-[Leu8]BK. BK vasodilated microperfused OMDVR that had been preconstricted with 10(-8) M angiotensin II. We conclude that the [Ca2+]i response of OMDVR endothelia can be selectively studied with fura 2, that BK increases endothelial [Ca2+]i via the B2 receptor, and that BK can vasodilate descending vasa recta.

Evidence for the presence of smooth muscle alpha-actin within pericytes of the renal medulla.

This study was designed to determine whether smooth muscle alpha-actin mRNA and smooth muscle alpha-actin contractile protein elements were present within the renal medullary pericytes. Extraction of total RNA from microdissected outer medullary descending vasa recta allowed for the detection of smooth muscle alpha-actin mRNA expression using reverse transcription-polymerase chain reaction (RT-PCR). Expression of smooth muscle alpha-actin was specific to the descending vasa recta and not a result of tubular contamination because RT-PCR amplification of the vasopressin V2 receptor, which is a specific tubular marker, did not occur. To determine the exact cell type(s) that translate the mRNA into protein, we performed immunohistochemistry on the renal outer and inner medulla using a monoclonal smooth muscle alpha-actin antibody, whose specificity was determined by immunoblot analysis. Smooth muscle alpha-actin protein was found selectively within the pericytes surrounding the descending vasa recta from the outer and inner medullary tissue sections. This study demonstrates that the pericytes alone that surround the descending vasa recta within the outer and inner medulla contain smooth muscle alpha-actin mRNA and protein and are therefore the site of the contractile elements that could play a vasomodulatory role in the control of renal medullary blood flow and its distribution within the renal medulla.

Molecular sieving of small solutes by outer medullary descending vasa recta.

Molecular sieving of small solutes by outer medullary descending vasa recta (OMDVR). Descending vasa recta (DVR) plasma equilibrates with the medullary interstitium by volume efflux (Jv), as well as by influx of solutes. Jv is driven by transmural osmotic pressure gradients due to small hydrophilic solutes (delta pi s), NaCl and urea. DVR endothelium probably contains a "water-only" pathway most likely mediated by the aquaporin-1 (AQP1) water channel. We measured the ability of microperfused OMDVR to concentrate lumenal 22Na and [3H]raffinose when Jv was driven by transmural NaCl gradients. Collectate-to-perfusate ratios of 2 x 10(6) M(r) fluorescein isothiocyanate-labeled dextran volume marker (RDx), 22Na (RNa), and [3H]raffinose (Rraf) were measured in the absence and presence of Jv. During volume efflux (Jv > 0), RDx was 1.37 +/- 0.31. RNa increased from 0.64 +/- 0.03 when Jv = 0 to 0.82 +/- 0.05 when Jv > 0 and Rraf increased from 0.83 +/- 0.03 to 1.13 +/- 0.05: Mathematical simulations predict RNa and Rraf most accurately when the OMDVR reflection coefficient to the tracers is assigned a value near unity. This indicates that the OMDVR wall contains a pathway for osmotic volume flux that excludes small hydrophilic solutes, a behavior consistent with that of aquaporins.