Molecular sieving of albumin by the ascending vasa recta wall.

Molecular sieving of albumin by ascending vasa recta. Evidence exists to support the presence of an extravascular pool of albumin in the renal medullary interstitium. This study used microperfusion in vivo to measure the transport of 125I-labeled albumin from descending (DVR) and ascending vasa recta (AVR) to the papillary interstitium. Perfusions were performed during furosemide diuresis with a buffer containing FITC-labeled dextran (FITC-Dx) 2 x 10(6) mol wt and 125I-albumin. Perfusate albumin and collection pressure were adjusted to induce either zero transcapillary volume flux (Jv) or high volume flux. When Jv was zero, the collectate-to-perfusate ratios of FITC-Dx (RDX) and 125I-albumin (Ralb) in the DVR and AVR were identical implying that diffusive efflux of albumin was immeasurably small. In contrast, when Jv was increased, paired comparison of Ralb and RDX in the same AVR revealed a difference, 1.58 +/- 0.06 vs 1.72 +/- 0.08, respectively (P less than 0.01). AVR perfusions in hydropenic animals showed similar results, Ralb = 1.70 +/- 0.07 and RDX = 2.00 +/- 0.07 (P less than 0.01). These data suggest that albumin transport across vasa recta in vivo is likely to be governed by solvent drag. The reflection coefficient of the AVR wall to 125I-albumin is estimated to be 0.78.

Effect of sodium chloride gradients on water flux in rat descending vasa recta.

In the hydropenic kidney, volume efflux from descending vasa recta (DVR) occurs despite an intracapillary oncotic pressure that exceeds hydraulic pressure. That finding has been attributed to small solute gradients which may provide an additional osmotic driving force favoring water transport from DVR plasma to the papillary interstitium. To test this hypothesis, axial gradients of NaCl and urea in the papilla were eliminated by administration of furosemide and saline. DVR were then blocked with paraffin and microperfused at 10 nl/min with a buffer containing albumin, fluorescein isothiocyanate labeled dextran (FITC-Dx), 22Na, and NaCl in a concentration of 0 (hypotonic to the interstitium), 161 (isotonic) or 322 mM (hypertonic). Collectate was obtained from the perfused DVR by micropuncture and the collectate-to-perfusate ratios of FITC-Dx and 22Na were measured. A mathematical model was employed to determine DVR permeability (Ps) and reflection coefficient to NaCl (sigma NaCl). The rate of transport of water from the DVR lumen to the papillary interstitium was 2.8 +/- 0.3 (Nv = 22), -0.19 +/- 0.4 (Nv = 15), and -2.3 +/- 0.3 nl/min (Nv = 21) (mean +/- SE) when perfusate NaCl was 0, 161, or 322 mM, respectively (Nv = number of DVR perfused). The collectate-to-perfusate 22Na concentration ratios were 0.34 +/- 0.04, 0.36 +/- 0.04 and 0.37 +/- 0.03 for those groups, respectively. Based on these data, Ps is calculated to be 60.4 x 10(-5) +/- 4.0 x 10(-5) cm/s and sigma NaCl less than 0.05. The results of this study confirm that transcapillary NaCl concentrations gradients induce water movement across the wall of the DVR.

Adenosine modulates vasomotor tone in outer medullary descending vasa recta of the rat.

Adenosine is generated within the renal medulla under hypoxic conditions and is known to induce net vasoconstriction within the renal cortex while increasing medullary blood flow and oxygenation. To test the hypothesis that vasoconstriction of outer medullary descending vasa recta (OMDVR) is modulated by adenosine, we examined the effects of adenosine and adenosine Al and A2 receptor subtype agonists on in vitro perfused control and preconstricted rat OMDVR. Constriction with angiotensin II (ANG II, 10(-9) M) was attenuated by adenosine in a concentration-dependent manner (EC50 = 2.0 x 10(-7)M, P < 0.05). Similarly, an adenosine A2 agonist (CGS-21680, 10(-7) M), but not an adenosine Al agonist (cyclohexyladenosine, 10(-6) M), attenuated ANG II-induced vasoconstriction. Under control conditions, ablumenal application of adenosine (10(-12) to 10(-5) M) elicited a biphasic response. Additionally, cyclohexyladenosine (10(-6) M) caused vasoconstriction and CGS-21680 (10(-6) M) had no effect on untreated vessels. Finally, an influence of ANG II receptor stimulation on adenosine Al receptor-mediated vasoconstriction could not be shown. These data suggest that OMDVR possess both Al and A2 adenosine receptors and that they mediate constriction and dilatation, respectively. We conclude that adenosine is a potent modulator of OMDVR vasomotor tone and that its net effect is dependent upon local concentrations.

Prostaglandin E2 abrogates endothelin-induced vasoconstriction in renal outer medullary descending vasa recta of the rat.

Endothelins (ET) and prostaglandin E2 are synthesized in the inner medulla by collecting duct epithelium and interstitial cells, respectively. All ascending vasa recta (AVR) blood returns from the inner medulla to the cortex in outer medullary vascular bundles. We reasoned that hormones might influence medullary blood flow by diffusing across AVR fenestrations to modulate vasoconstriction of outer medullary descending vasa recta (OMDVR). To investigate this possibility, OMDVR dissected from vascular bundles were exposed to ET-1, 2, or 3. Each endothelin isoform induced stable vasoconstriction with potency, ET-1 > ET-2 > ET-3 (EC50, 1.8 x 10(-15), 5.9 x 10(-12), and 8.8 x 10(-10) M, respectively). The ETA receptor antagonist BQ-123 and BQ-610 (10(-6) M), as well as an ETA and ETB receptor antagonist combination, attenuated vasoconstriction due to ET-1 (10(-12) M). BQ-123 had no effect on the response to ET-3 (10(-8) M). The ETB receptor antagonist BQ-788 (10(-6) M) attenuated the response to ET-3 (10(-10) M), but not that to ET-1 (10(-12) M). Finally, PGE2 (10(-6) M) reversibly dilated OMDVR preconstricted with ET-1 (10(-12) M) or ET-3 (10(-8) M) but not ET-1 (10(-10) M). We conclude that ET-1,2, and 3 are potent constrictors of OMDVR and the response to ET-1 is mainly ETA receptor subtype mediated, while ET-3 acts via the ETB. PGE2 modulates ET induced constriction. These findings are consistent with interactive feedback and control of medullary perfusion by locally synthesized hormones.

Transport of sodium and urea in outer medullary descending vasa recta.

We dissected and perfused outer medullary vasa recta (OMVR) from vascular bundles in the rat. Permeabilities of sodium (PNa) and urea (Pu) were simultaneously determined from the lumen-to-bath efflux of 22Na and [14C]urea. PNa and Pu were also measured by in vivo microperfusion of descending (DVR) and ascending vasa recta (AVR) at the papillary tip of Munich-Wistar rats. In some OMVR PNa was indistinguishable from zero. The mean +/- SE of PNa (x 10(-5), cm/s) in OMVR was 76 +/- 9. Pu in OMVR was always very high (x 10(-5), cm/s), 360 +/- 14. There was no correlation between OMVR PNa and Pu. Inner medullary AVR and DVR had PNa of 115 +/- 10 and 75 +/- 10, respectively, and Pu of 121 +/- 10 and 76 +/- 11, respectively. PNa and Pu in papillary vasa recta were always nearly identical and highly correlated. Transport of [14C] urea in OMVR was reversibly inhibited by addition of unlabeled urea or phloretin to the bath and lumen, providing evidence for carrier-mediated transport. These data suggest that sodium and urea might traverse the wall of inner medullary vasa recta by a paracellular pathway while urea also crosses by a transcellular route in OMVR. Electron microscopic examination of seven in vitro perfused OMVR revealed no fenestrations and exposure of these vessels to 10 microM calcium ionophore A23187 or 1 nM angiotensin II resulted in reversible contraction, suggesting that in vitro perfused OMVR are DVR only.

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.

The renal medullary microcirculation.

Blood flow to the renal medulla is supplied through descending vasa recta (DVR), which are derived from the efferent arterioles of juxtamedullary glomeruli. In addition to their role as conduits for blood flow, it is accepted that the vasa recta are countercurrent exchangers. That process, however, involves events which are more complicated than paracellular diffusive exchange of NaCl and urea. Urea transport in DVR is accommodated through the combined expression of endothelial and erythrocyte facilitated carriers while transport of water involves solute driven efflux across water channels. Unlike DVR, which have a continuous endothelium, ascending vasa recta (AVR) are fenestrated with a very high hydraulic conductivity. Transport of water in AVR is probably governed by transmural hydraulic and oncotic pressure gradients. The parallel arrangement of DVR in outer medullary vascular bundles coupled with their capacity for vasomotion implies a role for regulation of the regional distribution of blood flow within the medulla The importance of the latter process in the urinary concentrating mechanism and the exchange of nutrients and O2 is poorly defined. The large number of hormones and autacoids that influence DVR vasomotion, however, suggests that DVR have evolved to optimize the functions of the renal medulla.

Resistance of ascending vasa recta to transport of water.

A study was undertaken to determine the effect of increasing capillary pressure on volume flux in ascending vasa recta (AVR). In one experiment (group I), AVR were blocked by a single injection of paraffin wax and subjected to free-flow microperfusion at 10 nl/min. Collected fluid was obtained from the perfused vessels by micropuncture. In a second experiment (group II), AVR segments were isolated between two paraffin blocks and perfused at 10 nl/min. In group II, the collection pipette was pressurized to 0, 10, or 20 mmHg. Transmembrane volume flux was determined by measuring the change in concentration of fluorescein isothiocyanate-labeled dextran (2 x 10(6) mol wt) from perfusate to collected fluid. In group I, measurements revealed a capillary pressure of 10.3 +/- 0.5 (SE) mmHg and volume flux of 4.3 +/- 1.0 nl.mm-1.min-1. In group II, volume flux was 1.8 +/- 1.3, 5.9 +/- 1.0, and 11.2 +/- 1.1 nl.mm-1.min-1 at collection pressures of 0, 10, or 20 mmHg, respectively. Based on these data and an AVR diameter of 20 microns, AVR hydraulic conductivity is between 12.5 x 10(-6) and 18.7 x 10(-6) cm.s-1.mmHg-1. The papillary AVR have a high hydraulic conductivity. This is consistent with their role as the sole conduit for removal of water from the papillary interstitium.

Characterization of the urea transporter in outer medullary descending vasa recta.

Partially because of facilitated transport of urea, urea permeability (Pu) of the outer medullary descending vasa recta (OMDVR) frequently exceeds sodium permeability by more than an order of magnitude. This study characterizes the OMDVR urea transporter. Application of the urea analogue thiourea (200 mM) to the abluminal surface of microperfused OMDVR inhibited Pu by 33%. When osmolarity due to thiourea was balanced by addition of mannitol or thiourea, similar results were obtained. Thiourea produced graded inhibition of Pu from 343 +/- 54 (SE) to 191 +/- 43 x 10(-5) cm/s as concentration was increased from 0 to 100 mM. The thiourea concentration needed for half-maximal inhibition was 19 mM. The abilities of urea analogues to reduce Pu were compared by addition of 50 mM concentrations to the bath and perfusate. Thiourea and methylurea produced 32 and 34% inhibition of Pu, respectively, whereas urea and acetamide produced only 3 and 11% inhibition, respectively. The transporter showed negligible saturation as the transmural urea gradient was increased from 0 to 200 mM. Phloretin and p-chloromercuribenzenesulfonate inhibited Pu in a concentration-dependent fashion. It is concluded that a transporter confers high Pu to OMDVR. Pu is equally high when measured by urea influx or efflux. Properties of the transporter are similar to those expressed by the inner medullary collecting duct.

Vasoconstriction of outer medullary vasa recta by angiotensin II is modulated by prostaglandin E2.

Vasa recta were dissected from outer medullary vascular bundles in the rat and perfused in vitro. Examination by transmission electron microscopy reveals them to be only outer medullary descending vasa recta (OM-DVR). To establish a method for systematic examination of vasoconstriction, OMDVR were perfused at 5 nl/min with collection pressure increased to 5 mmHg. Under these conditions, transmembrane volume flux was found to be near zero, and the transmural hydraulic pressure gradient was found to be < 15 mmHg. Over a concentration range of 10(-12) to 10(-8) M, abluminal application of angiotensin II (ANG II) caused graded focal vasoconstriction of OMDVR that is blocked by saralasin. Luminal application of ANG II over the same concentration range was much less effective. Abluminal application of prostaglandin E2 (PGE2) shifted the vasoconstrictor response of OMDVR to higher ANG II concentrations. PGE2 reversibly dilated OMDVR that had been preconstricted by ANG II. These results demonstrate that OMDVR are vasoactive segments. Their anatomical arrangement suggests that they play a key role in the regulation of total and regional blood flow to the renal medulla.

Photon-counting microcolorimeter with 40-nl cuvette.

A colorimeter with a 40-nl cuvette has been constructed. The wall of standard capillary glass was perforated to produce a sample injection port. A section of the capillary glass was drawn to a length of 1/2 cm and 80 microns ID by heating on a microforge. This produced a cuvette volume of approximately 40 nl. Two fiber-optic filaments, 80 microns in diameter, were fixed into the cuvette to transmit and receive light from the sample. The output of the colorimeter was measured with a microscope photon-counting detection assembly. It has been shown that the colorimeter enables a reduction of the plasma volume requirements of the Lowry microprotein assay from several nanoliters to 60 pl. The linearity and reproducibility of the microcolorimeter when used with the Lowry assay has been verified. The colorimeter output is several orders of magnitude above the lower limit of detection of the photon counter.

Transport of sodium chloride and water in rat ascending vasa recta.

Experiments were undertaken to test the hypothesis that transcapillary small solute (NaCl and urea) gradients drive water across ascending vasa recta (AVR). Axial gradients of NaCl and urea were eliminated with furosemide. AVR were perfused with buffer containing fluorescein isothiocyanate-labeled dextran and 22Na. Perfusion of AVR with isotonic buffer at 10 and 20 nl/min yielded collectate-to-perfusate 22Na ratios of 0.17 +/- 0.05 and 0.34 +/- 0.03, respectively, in AVR of 601 +/- 56 and 583 +/- 46 microns mean length, respectively. A 22Na permeability of 113.2 +/- 12.8 x 10(-5) cm/s was determined. AVR were perfused at 20 nl/min with buffer NaCl of 0 (hypotonic to papilla), 161 (isotonic), or 500 mM (hypertonic). Transcapillary volume flux was not significantly different in these groups (3.8 +/- 1.5, 4.6 +/- 1.5, and 2.1 +/- 1.4 nl.min-1.mm-1, respectively). AVR were perfused in the hydropenic kidney at 5 nl/min antegrade from tip to base and retrograde from base to tip, which was a maneuver designed to impose physiological transcapillary NaCl and urea gradients of opposite direction. Volume fluxes were -1.4 +/- 0.05 and -1.3 +/- 0.04 nl.min-1.mm-1 in these groups, respectively. These data demonstrate that the AVR are highly permeable to NaCl and that physiological small solute gradients do not influence water movement across the AVR wall.

A simplified device for injection of paraffin wax blockades.

Microperfusion of renal tubules and microvessels in vivo requires occlusion of the lumen with a reliable blockade. Paraffin wax serves this purpose but requires specialized hydraulic or mechanical devices to stably hold a micropipette while this solid material is extruded through micron-sized tips. This communication describes the construction of a simple and very inexpensive wax injector from commercially available parts. The rear of an aluminum holder is threaded so that a thumbscrew can be used to push a stiff wire into the lumen of a micropipette. A solder cast of the micropipette lumen is employed as a tight-fitting piston to generate the high pressures required for wax extrusion. Several wax injectors can be made at negligible cost. This device has been used in this laboratory to introduce over 1,000 wax blocks into papillary vasa recta.

Extravascular protein in the renal medulla: analysis by two methods.

Two methods have been used to test for the presence of extravascular protein in the interstitium of the renal inner medulla. First, ascending vasa recta (AVR) segments were perfused with buffer containing 5 g/dl of albumin. The hydraulic pressure in the perfused vessel was varied to control transmembrane volume flux (Jv) to the interstitium. Interpolation to the point of zero Jv was employed to estimate interstitial Starling forces in the hydropenic rat papilla. Analysis of those experiments predicts that interstitial protein concentration (Ci) is high. When AVR segments are filled with oil, the oil column spontaneously breaks up as fluid is secreted into the lumen from the papillary interstitium. To obtain a lower limit on Ci, isolated AVR segments (IAS) filled with oil were sampled to measure protein concentration in the secreted fluid. In hydropenic rats, protein concentration was 3.4 +/- 0.5 and 5.2 +/- 0.2 g/dl in IAS and adjacent free-flowing AVR, respectively (P < 0.01). In rats subjected to furosemide and saline diuresis, the values were nearly identical, 4.7 +/- 0.2 and 5.2 +/- 0.2 g/dl, respectively. These separate experimental approaches corroborate a high concentration of protein in the renal inner medullary interstitium.

Continuous arteriovenous hemofiltration: an in vitro simulation and mathematical model.

In vitro and mathematical models of continuous arteriovenous hemofiltration (CAVH) have been developed. Human erythrocytes resuspended in normal saline containing 5% bovine albumin were used to perfuse the circuit from a gravity driven pressure source. Membrane hydraulic permeability was observed to decline from 31.2 x 10(-5) +/- 11.9 x 10(-5) cm/(min.mm Hg) before use to 12.3 x 10(-5) +/- 3.3 x 10(-5) (mean +/- SD) after use. This fall occurred during the first one to two hours whether perfused with blood or 5% albumin alone. Pressure-flow relationships of each circuit component, measured with 40% sucrose as a calibration medium, conformed to Poiseuille's equation. Use of high resistance blood access on the venous end of the circuit resulted in a low blood flow rate and high filtration fraction. The same access, when placed on the arterial end, produced both low blood flow rate and low filtration fraction. These results were a consequence of pressure distribution within the circuit as demonstrated by measurements of perfusion, prefilter, and postfilter pressures. The importance of negative pressure applied to the filter chamber in order to maintain favorable Starling forces, when the system was operated with a small bore arterial access, was demonstrated by similar methods. Enhancement of urea clearance by predilution was verified. Model simulations suggest that predilution will be of less benefit or even detrimental for other solutes which fail to distribute across the erythrocyte membrane. Comparison of results with predictions of a mathematical model demonstrated good agreement, but with some tendency to overestimate filtrate production. The latter was attributed to neglect of concentration polarization of plasma proteins in model development.

Effect of ureteral excision on inner medullary solute concentration in rats.

To elucidate the role of the ureter in urinary concentration we studied the effect of partial and complete ureteral excision on urinary osmolality and papillary interstitial osmolality and on sodium, potassium, and urea concentrations in the antidiuretic rat. Urine and descending vasa recta (DVR) plasma samples were obtained by micropuncture of the left renal papilla before (period 1) and 45 min after (period 2) complete (group 1, n = 10 rats) or partial (group 2, n = 10 rats) ureteral excision. Urine osmolality fell from 2,063 +/- 156 (mean +/- SE) to 736 +/- 116 mosmol/kgH2O after complete ureteral excision (P less than 0.01). After partial ureteral excision, the fall was less than half as great, from 2,038 +/- 167 to 1,551 +/- 162 mosmol/kgH2O (P less than 0.01). Vasa recta plasma osmolality decreased from 1,742 +/- 133 to 860 +/- 119 mosmol/kgH2O after complete excision (P less than 0.01) but only from 1,830 +/- 146 to 1,504 +/- 154 mosmol/kgH2O after partial excision (P less than 0.05). Mean DVR plasma sodium concentration declined from 339 +/- 25 to 211 +/- 25 meq/l (P less than 0.01) in group 1 but did not change in group 2 (348 +/- 21 to 347 +/- 28 meq/l). The fraction of DVR plasma osmolality accounted for by urea decreased significantly from 0.59 +/- 0.01 to 0.46 +/- 0.02 mM/(mosmol/kgH2O) in group 1 and from 0.59 +/- 0.02 to 0.49 +/- 0.03 mM/(mosmol/kgH2O) for group 2 (P less than 0.01, both groups). We interpret these findings to show that the remnant ureter moderates the fall in interstitial osmolality at least in part through preservation of the corticomedullary sodium chloride gradient.

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.

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.

Pericyte regulation of renal medullary blood flow.

Pericytes are contractile smooth muscle-like cells that surround descending vasa recta (DVR) and provide their capability for vasomotion. The importance of the medullary pericyte derives from the role of DVR to distribute most or all of the blood flow from juxtamedullary cortex to the renal inner and outer medulla. Physiological processes that are likely to be influenced by pericyte constriction of DVR include the urinary concentrating mechanism and pressure natriuresis. Oxygen tensions in the medulla are low, so that subtle variation of pericyte vasomotion might play a role to abrogate hypoxia and prevent insult to the medullary thick ascending limb of Henle. Known vasoconstrictors of DVR include angiotensin II, endothelins, norepinephrine, acetylcholine, and adenosine. Vasodilators include prostaglandin E2, adenosine, acetylcholine, bradykinin, and nitric oxide.