Pathophysiology of radiocontrast nephropathy: a role for medullary hypoxia

Recent experimental data underlies the role of hypoxic tubular injury in the pathophysiology of radiocontrast nephropathy. Although systemic transient hypoxemia, increased blood viscosity, and a leftward shift of the oxygen-hemoglobin dissociation curve may all contribute to intrarenal hypoxia, imbalance between oxygen demand and supply plays a major role in radiocontrast-induced outer medullary hypoxic damage. Low oxygen tension normally exists in this renal region, reflecting the precarious regional oxygen supply and a high local metabolic rate and oxygen requirement, resulting from active salt reabsorption by medullary thick ascending limbs of Henle's loop. Radiologic contrast agents markedly aggravate outer medullary physiologic hypoxia. This results from enhanced metabolic activity and oxygen consumption (as a result of osmotic diuresis and increased salt delivery to the distal nephron) because the regional blood flow and the oxygen supply actually increase. The latter effect may result in part from the activation of various regulatory mediators of outer medullary blood flow to ensure maximal regional oxygen supply. Low-osmolar radiocontrast agents may be less nephrotoxic because of the smaller osmotic load and vasomotor alterations. Experimental radiocontrast-induced renal failure requires preconditioning of animals with various insults (for example, congestive heart failure, reduced renal mass, salt depletion, or inhibition of nitric oxide and prostaglandin synthesis). In all these perturbations, which resemble clinical conditions that predispose to contrast nephropathy, outer medullary hypoxic injury results from insufficiency or inactivation of mechanisms designed to preserve regional oxygen balance. This underlines the importance of identifying and ameliorating predisposing factors in the prevention of this iatrogenic disease.

Exaggerated volume expansion natriuresis in rats preloaded with hypertonic saline: a paradoxical enhancement by inhibition of prostaglandin synthesis

In preliminary experiments rats preinfused with hypertonic saline showed exaggerated natriuresis after an additional small volume expansion (SVE). This was systematically studied in anaesthetized Wistar rats prepared for clearance studies of the left kidney and measurements of medullary blood flow (MBF, laser-Doppler technique) and tissue electrical admittance (Y ), an index of interstitial ion concentration. The rats were preinfused i.v. with 3 mL of 5% NaCl during 90 min. A subsequent injection of isotonic saline, 0.5% of body weight, increased sodium excretion (UNaV ) from 2.1 +/- 0.5 to 4.5 +/- 1.1 micromol min-1 and urine flow (V ) from 12.0 +/- 2.3 to 24.3 +/- 5.6 microL min-1 (P < 0.02). The same volume of whole blood increased UNaV from 5.0 +/- 1.4 to 8.7 +/- 1.7 micromol min-1 and V from 22.3 +/- 5.1 to 37.4 +/- 5.9 microL min-1 (P < 0.01). The glomerular filtration rate, MBF and Y did not change. In rats preinfused with 0.9% saline no natriuresis was observed after SVE. To examine if prostaglandins (PG) were involved in SVE natriuresis, indomethacin (Indo), 5 mg kg-1 or sodium meclophenamate (Meclo), 7.5 mg kg-1, were added to the injected 0.9% saline. Paradoxically, both PG synthesis inhibitors enhanced natriuresis to SVE. After Indo UNaV increased from 2.0 +/- 0.6 to 7.6 +/- 1.3 micromol min-1, significantly more than after SVE alone (P < 0.001). At higher baseline UNaV, the increase with Meclo from 4.5 +/- 1.2 to 13.5 +/- 1.8 micromol min-1 was significantly higher than after whole blood infusion (P < 0.001). MBF decreased and Y increased after both inhibitors. Further studies are required to explain the enhancement of natriuresis after blockade of PG synthesis.

Effect of protamine on ion selectivity of superficial and juxtamedullary proximal straight tubules

Protamine is known to inhibit cation selectivity of the paracellular shunt pathway in several leaky nephron segments. Ion permeability of the superficial (SF) proximal straight tubules (PST) is selective to Cl(-), whereas that of the juxtamedullary (JM) PST is selective to Na(+). Protamine was used to estimate the contribution of the paracellular shunt pathway to ion selectivity in these segments. PSTs were isolated from the kidneys of Japanese white rabbit and microperfused in vitro. The ratio of Na(+) to Cl(-) permeability (P(Na)/P(Cl)) was estimated from the diffusion potential (dV(T)) generated by NaCl gradients. When 300 microg/ml protamine was added to the lumen, P(Na)/P(Cl) was decreased from 0.33 to 0.26 in SF-PST and from 1.80 to 1.29 in JM-PST, respectively, as the results of inhibition of cation selectivity. Addition of 30 U/ml heparin to the lumen, which neutralizes protamine, returned the ratios toward the control levels. Protamine exhibited similar effect on P(Na)/P(Cl) in the presence or the absence of ouabain in the bath, indicating that the observed voltage deflections were independent of the transcellular active transport process. Although P(Na)/P(Cl) was different between SF- and JM-PST, protamine inhibits Na(+) permeability in both segments. Preferential Cl(-) permeability in the SF-PST may be intrinsic to the paracellular route of this segment. The mechanism of this anion selectivity remains to be established.

Effects of ibuprofen and indomethacin on the regional circulation in newborn piglets

We investigated the effects of clinically comparable doses of ibuprofen and indomethacin on renal, gastrointestinal and cerebral perfusion in newborn piglets, and hypothesized that ibuprofen would have less effect on regional circulation. Animals were randomly assigned to receive ibuprofen (20 mg/kg, n = 8), indomethacin (0.3 mg/kg, n = 7) or vehicle (n = 6). Fluorescent microspheres were injected prior to and at 20, 40, 60, 90 and 120 min after drug administration. Regional blood flow was measured and vascular resistances were calculated. Cardiovascular and respiratory variables were not significantly affected by either study drug or vehicle. Ibuprofen increased renal cortical and medullary resistance by 44 and 52% (p < 0.05). However, ibuprofen had no significant effects on gastrointestinal or cerebral resistance. Indomethacin raised renal cortical and medullary resistance by 66 and 71% at 60 min postinjection, respectively (p < 0.05). Indomethacin increased duodenojejunal, ileal and colon resistance by 97, 102 and 75% at 60 min, respectively (p < 0.05). Indomethacin increased cerebral cortical and cerebellar resistance by 92 and 86% at 90 min (p < 0. 05). While indomethacin and, to a lesser extent, ibuprofen both increase renal vascular resistance, indomethacin vasoconstricts the gastrointestinal and cerebral circulations which are unaffected by ibuprofen.

Influence of the viscosity of iodixanol on medullary and cortical blood flow in the rat kidney: a potential cause of Nephrotoxicity

The aim of the present study was to investigate the effects of four iodinated contrast media on cortical, inner medullary and outer medullary blood flow in the rat kidney by using laser-Doppler flowmetry. The high-osmolar contrast medium diatrizoate did not significantly modify medullary perfusion but moderately decreased the cortical blood flow when injected at a dose of 1600 mg iodine kg(-1). Similar effects were obtained with the low-osmolar contrast media ioxaglate and iobitridol. In contrast, the new iso-osmolar contrast medium iodixanol induced a dose-dependent reduction of perfusion in all regions tested. This effect was accompanied by concomitant hypotension. The reduction of inner medullary and cortical blood flow induced by iodixanol was partially alleviated by heating the solution prior to injection and subsequently reducing its viscosity. In the outer medulla, however, this procedure did not improve blood flow. These results suggest that lowering the viscosity may palliate the harmful effects of iodixanol on the inner medulla and cortex, but may not protect the outer medulla from hypoxic injury.

Differing effects of enalapril and losartan on renal medullary blood flow and renal interstitial hydrostatic pressure in spontaneously hypertensive rats

OBJECTIVE

To determine the effect of short-term angiotensin converting enzyme inhibition (enalapril) or angiotensin II AT1 receptor blockade (losartan) on medullary hemodynamics in the spontaneously hypertensive rat (SHR).

DESIGN

Laser-Doppler flowmetry allowed for the characterization of medullary blood flow (MBF) over a wide range of renal arterial pressure (RAP), and was used for comparison among treatment groups. Renal interstitial hydrostatic pressure (RIHP) was also determined over a wide range of RAP.

METHOD

Enalapril or losartan was given to male 12-13-week-old SHR for 3 days (25 mg/kg per day in drinking water). Rats were anesthetized with Inactin, renal function was measured at resting levels of RAP and then RAP was varied over a range of 50-150 mmHg in 25 mmHg steps. MBF and RIHP were determined at each pressure.

RESULTS

Resting mean arterial pressure (MAP) (mmHg +/- SE) for enalapril- and for losartan-treated SHR [114 +/- 3 (n = 18) and 124 +/- 3 (n = 20), respectively] were both significantly lower than for untreated SHR [159 +/- 5 (n = 20)]. Renal function at resting levels of MAP was not significantly different among groups. Enalapril and losartan both increased MBF by 30% at levels of RAP of 125 mmHg and over. Enalapril did not alter the relation between RAP and RIHP, but losartan shifted the RAP versus RIHP curve by approximately 40 mmHg to lower levels of RAP. Acute administration of the B2 kinin receptor antagonist HOE 140 [20 microg/kg intravenous (i.v.) bolus, then 10 microg/kg per h i.v.] did not significantly alter MAP in any group. HOE 140 did not significantly alter MBF or RIHP in the untreated or losartan-treated SHR. MBF in enalapril-treated rats receiving HOE 140 was not significantly different from that of the enalapril-only group; however, the relation between RAP and RIHP was shifted to lower levels of RAP by approximately 45 mmHg.

CONCLUSIONS

Both enalapril and losartan increase MBF in SHR, suggesting that the medullary circulation of SHR is influenced by endogenous levels of angiotensin II. The failure of enalapril to increase RIHP in parallel with MBF appears to be due to an enhanced effect of kinins.

Hypoperfusion in the renal outer medulla after injection of contrast media in rats

PURPOSE

The effect on regional renal blood was studied after injection of nonionic iso-osmolar iotrolan or ionic high-osmolar iothalamate.

MATERIAL AND METHODS

Laser-Doppler flowmetry was used to measure outer medullary (OMBF) and superficial cortical blood flow (CBF) simultaneously in anesthetized rats. Iotrolan (320 mOsm/kg H2O) was injected i.v. at a dose of 600 mg I/kg b.w. (normal dose) over 2 min or 1,600 (high dose) mg I/kg b.w. over 2 or 8 min. Iothalamate (2,580 mOsm/kg H2O) was injected i.v. at a dose of 1,600 (high dose) or 2,900 (extremely high dose) mg I/kg b.w. over 2 min.

RESULTS

At the normal dose and 2-min injection of iotrolan, OMBF was reduced by 25+/-9% over 20 min. The high dose of iotrolan injected over 8 min resulted in a reduction in OMBF slightly smaller (17+/-9%) than that induced by the normal dose but lasting longer (30 min). Compared to the normal dose, the high dose and fast (2 min) injection of iotrolan resulted in a greater and more prolonged decrease in OMBF (32+/-6% lasting 50 min). After the high dose of iothalamate (1,600 mg I/kg) there was a decrease in OMBF by 21+/-6%, lasting 30 min. An extremely high dose (2,900 mg I/kg b.w.) gave a heterogeneous response with a mean increase in OMBF of 48+/-24% occurring 60 min after the injection.

CONCLUSION

Iso-osmolar and high-osmolar contrast media (CM), at normal and high doses, decrease OMBF, while an extremely high dose of iothalamate may result in an increase. The depression of outer medullary perfusion may have implications for CM-induced acute renal failure in view of the vulnerability of this region to a decrease in oxygen tension.

The effects of angiotensin II on blood perfusion in the rat renal papilla

1. Systemic infusion of angiotensin II (AII) increased papillary blood perfusion (PBP) measured by laser-Doppler flowmetry in rats, aged about 5 weeks. 2. The mechanisms involved in this response were determined by infusion of AII in the presence of systemic doses of losartan (a type 1 AII receptor antagonist), HOE-140 (a bradykinin B2 receptor antagonist), and an inhibitor of NO production - Nomega-nitro-L-arginine (NOLA). 3. Mean arterial blood pressure (MAP) and PBP increased in a dose-dependent manner in response to intravenous infusions of AII. Infusion of losartan abolished these responses to AII but HOE-140 was without effect. Infusion of NOLA abolished the increase in PBP but did not affect the pressor response to AII. Systemic infusion of sodium nitroprusside restored the response to AII in experiments with NOLA infusion. 4. The results indicate that the increase in PBP caused by AII is mediated via angiotensin AT1 receptors and does not involve bradykinin B2 receptors. The AII-induced increase in PBP is dependent upon the presence of NO, thus providing a mechanism for maintenance of papillary perfusion in the face of generalized renal vasoconstriction due to AII.

Effect of i.v. furosemide on pelvic urinary oxygen tension in humans

Urinary oxygen tension may be an index of renal medullary blood flow. The effect of i.v. furosemide on urinary oxygen tension was studied in four patients with indwelling nephrostomy tubes. An intravascular oxygen sensor (Paratrend 7, Biomedical Sensors Ltd, UK) was inserted into the renal pelvis via the nephrostomy and urine oxygen tension measured. In all cases, furosemide 20 mg i.v. produced a decrease in pelvic urinary oxygen. The possible mechanisms and implications are discussed.

Chronic effects of lovastatin and bezafibrate on cortical and medullary hemodynamics in deoxycorticosterone acetate-salt hypertensive mice

Cholesterol synthesis inhibitors and fibrates both exercise effects that could influence BP and renal function in hypertension. To test this issue, transit-time ultrasound flow probes, implanted optical fibers, and laser-Doppler flowmetry were used for measurements of total and regional renal blood flows in lovastatin (40 mg/kg body wt) and bezafibrate (50 mg/kg body wt) chronically treated deoxycorticosterone acetate (DOCA)-salt hypertensive mice. Total renal blood flow was well autoregulated between 70 and 150 mmHg (approximately 3.5 ml/min per g kidney weight in DOCA-salt mice). Both lovastatin and bezafibrate increased renal blood flow to a range between 4.7 and 5.5 ml/min per g kidney weight. In the renal perfusion pressure ranges investigated, renal vascular resistance increased in lovastin- and bezafibrate-treated DOCA-salt mice, but not as steeply as in vehicle-treated DOCA-salt mice. During a stepwise increase in renal perfusion pressure in lovastatin-treated DOCA-salt mice, medullary blood flow increased up to 130% of baseline values, which was not seen in vehicle- or bezafibrate-treated mice. After extracellular volume expansion with 1% saline, 1 ml over 1 min, total renal blood flow was also higher in lovastatin- or bezafibrate-treated DOCA-salt mice, whereas medullary blood flow increased more steeply in lovastatin-, compared with bezafibrate- or vehicle-treated mice. Systemic BP was significantly decreased in lovastatin-treated DOCA-salt mice compared with vehicle-treated mice. Lovastatin prevented histologic evidence for hemostasis in the medullary circulation of DOCA-salt mice. The results suggest that both lovastatin and bezafibrate diminished DOCA-salt-induced reductions in total renal blood flow. Lovastatin also abolished the perturbed medullary blood flow reactions to increased perfusion pressure or to volume expansion. Finally, lovastatin decreased systemic BP in DOCA-salt mice. These data suggest that cholesterol synthesis inhibition or fibrate treatment improve disturbed renal function in a mouse model of salt-dependent hypertension.

Power Doppler imaging of acute renal transplant rejection

PURPOSE

We evaluated the usefulness of power Doppler imaging (PDI) in diagnosing acute renal-transplant rejection.

METHODS

Twenty-eight patients underwent 33 renal-transplant biopsies for suspected acute rejection. Patterns of renal parenchymal vascularity revealed by PDI in patients with abnormal biopsy results were compared with patterns in a group who had normal biopsy results. PDI examinations were reviewed retrospectively by 2 independent radiologists who had no knowledge of the biopsy results. A PDI diagnosis of acute rejection required marked vascular pruning in both the cortex and medulla. PDI results then were compared with transplant-biopsy results.

RESULTS

The sensitivity and specificity of PDI for diagnosing acute renal-transplant rejection were 40% and 100%, respectively. None of the patients with negative biopsy results had PDI abnormalities. The negative predictive value of PDI was 33%, and the positive predictive value was 100%.

CONCLUSIONS

In our study, an abnormal sonogram was highly predictive of acute transplant rejection. However, a normal sonogram did not exclude the possibility of rejection.

Insertion/deletion angiotensin converting enzyme gene polymorphism affects the microvascular structure of the kidney in patients with nondiabetic renal disease

OBJECTIVE

It has been reported that the deletion allele of the insertion/deletion polymorphism of the angiotensin I converting enzyme gene is associated with increased cardiovascular risk and progressive renal disease, including immunoglobulin A nephropathy. We therefore investigated the relationship between angiotensin converting enzyme polymorphism and intrarenal microvascular structure in 56 patients with nondiabetic renal disease.

METHODS AND RESULTS

We determined various cardiovascular hormones of the renin-angiotensin system and angiotensin converting enzyme gene polymorphism in 56 patients with nondiabetic renal diseases who underwent a renal biopsy. The patients were divided into three groups by angiotensin converting enzyme genotype (insertion/insertion, n = 21; insertion/deletion, n = 23; deletion/deletion, n = 12) using polymerase chain reaction methods. The angiotensin converting enzyme insertion/ deletion and deletion/deletion genotypes were associated with a significantly higher interlobular artery wall : lumen ratio than the insertion/insertion genotype (insertion/insertion 0.27 +/- 0.01, insertion/deletion 0.32 +/- 0.01, deletion/deletion 0.33 +/- 0.02; P < 0.05). Afferent arteriolar and tubulo-interstitial injury scores were similar among the three genotypes. Although serum angiotensin converting enzyme activity was higher in the deletion/deletion than in the other two genotypes (insertion/insertion 9.7 +/- 0.7, insertion/deletion 10.7 +/- 0.9, deletion/deletion 14.0 +/- 2.4 IU/I; P < 0.05), other factors of the renin-angiotensin system, including blood pressure and serum creatinine levels, were not different among the three groups.

CONCLUSIONS

The angiotensin converting enzyme deletion/deletion genotype may be considered a risk factor for the development of microvascular wall thickening in nondiabetic renal diseases.

Kinin influences on renal regional blood flow responses to angiotensin-converting enzyme inhibition in dogs

The relative roles of ANG II and bradykinin (BK) in the regulation of renal medullary circulation have remained unclear. We compared the contributions of ANG II and BK to the renal medullary blood flow (MBF) responses to angiotensin-converting enzyme (ACE) inhibition (enalaprilat, 33 micrograms . kg-1. min-1) in dogs maintained on a normal-salt diet (0.63%, 3 days, n = 14; group 1) with those fed a low-salt diet (0.01%, 5 days, n = 14; group 2), which upregulates both the kallikrein-kinin and the renin-angiotensin systems. MBF responses to ACE inhibition were evaluated either before (n = 7) or after (n = 7) treatment with the BK B2 receptor blocker icatibant (100-300 micergrams) in both groups. Laser-Doppler needle flow probes were used to determine relative changes in MBF and cortical blood flow (CBF). ACE inhibition increased MBF (group 1, 33 +/- 9%, P </= 0.01; group 2, 24 +/- 9%, P </= 0.005) as well as CBF (group 1, 23 +/- 2%, P </= 0.006; group 2, 28 +/- 10%, P </= 0.05). These responses were prevented by prior blockade of B2 receptors in group 2, but not in group 1. These data indicate that under normal sodium intake, increases in MBF and CBF caused by ACE inhibition are primarily due to reduced intrarenal ANG II levels. In contrast, the renal vasodilatory responses to ACE inhibition in dogs on low salt intake were markedly dependent on the activation of BK B2 receptors.

Neuronal nitric oxide synthase-dependent afferent arteriolar function in angiotensin II-induced hypertension

This study was designed to determine the influence of neuronal nitric oxide synthase (nNOS) in tubular flow-dependent regulation of afferent arteriolar diameter in hypertensive Sprague-Dawley rats that received 60 ng/min angiotensin II (Ang II) subcutaneously for 13 days. Systolic blood pressure of control and Ang II-infused rats averaged 122+/-2 (n=23) and 194+/-2 mm Hg (n=24). Afferent arteriolar responses to the nNOS inhibitor S-methyl-L-thiocitrulline (L-SMTC; 0.1 to 10 micromol/L) and the nonselective NOS inhibitor Nomega-nitro-L-arginine (L-NNA; 1 to 100 micromol/L) were assessed in vitro using the blood-perfused juxtamedullary nephron preparation. At a perfusion pressure of 160 mm Hg, afferent arteriolar diameters from control and Ang II-infused rats averaged 18.7+/-1.1 microm (n=8) and 18.1+/-1.1 microm (n=9), respectively, and decreased by 19. 9+/-1.5% and 11.8+/-1.1%, respectively, in response to 10 micromol/L L-SMTC. The L-SMTC-induced afferent arteriolar constriction was significantly greater in control than in Ang II-infused rats. In contrast, 100 micromol/L L-NNA constricted afferent arterioles similarly in both control (n=8) and Ang II-infused (n=7) rats. After transection of the loops of Henle to interrupt flow to the macula densa, the vasoconstrictor responses to L-SMTC but not to L-NNA were reversed. Increasing distal volume delivery by addition of 10 mmol/L acetazolamide to the blood perfusate significantly enhanced the afferent arteriolar constrictor responses to 10 micromol/L L-SMTC (34.5+/-4.8%, n=7) in normotensive rats. In contrast, in Ang II-infused rats, acetazolamide treatment did not enhance the responses to L-SMTC (n=8). These results indicate that chronic Ang II infusion reduces the ability of nNOS-derived nitric oxide to counteract the afferent arteriolar response to increased distal tubular flow.

Local renal medullary L-NAME infusion enhances the effect of long-term angiotensin II treatment

We hypothesized that the relatively high doses of angiotensin (Ang) II required to produce hypertension in rats were related to stimulation of renal medullary nitric oxide production, which in turn blunted reductions in medullary blood flow and the development of hypertension. Ang II was infused (5 days at 3 ng. kg-1. min-1 IV) to uninephrectomized Sprague-Dawley rats in the presence and absence of a continuous medullary interstitial NG-nitro-L-arginine methyl ester (L-NAME) infusion. Renal cortical and medullary blood flows were determined with the use of implanted optical fibers and laser-Doppler flowmetry. Ang II in the absence of medullary nitric oxide synthase inhibition did not change cortical or medullary blood flow or mean arterial pressure. A threshold dose of L-NAME was determined (75 microg. kg-1. h-1) that did not produce significant short- or long-term changes in medullary blood flow and mean arterial pressure. In rats with blunted medullary nitric oxide synthase activity, Ang II infused intravenously resulted in a 30% reduction in medullary blood flow (from 1.3 to 0.9+/-0.2V) and approximately 20 mm Hg increase in mean arterial pressure with Ang II infusion over 5 days. During 70 minutes after the start of intravenous Ang II, there was an immediate reduction in medullary blood flow, with no changes in cortical blood flow or mean arterial pressure. We conclude that the relative insensitivity of rats to long-term elevations of circulating Ang II is due to the potent counterregulatory actions of the nitric oxide system, specifically within the renal medulla. The results provide novel insights of how the organism attempts to protect itself from the hypertensive effects of Ang II.

Renal intramedullary infusion of L-arginine prevents reduction of medullary blood flow and hypertension in Dahl salt-sensitive rats

A role for reduced renal nitric oxide production has been proposed as a mechanism responsible for hypertension in Dahl "salt-sensitive" rats. The present study had 2 goals: first, to determine the relationship between changes in mean arterial pressure and renal cortical and medullary blood flows in unanesthetized Dahl/Rapp salt-sensitive (S) and Dahl/Rapp salt-resistant (R) rats as daily salt intake was increased from 0.4% to 4.0%; second, to determine if delivery of L- or D-arginine into the renal medulla of Dahl S rats would change the responses to high salt. Optical fibers were implanted into the renal cortex and inner medulla for daily recording of cortical and medullary blood flows using laser-Doppler flowmetry. Indwelling aortic catheters were used to record arterial pressure. Increasing salt intake to 4.0% in Dahl S rats increased mean arterial pressure from 128+/-2.0 to 155+/-5.0 mm Hg by day 5 of high salt diet; medullary blood flow was reduced 13% by day 2, 24% by day 3 (P<0.05), and 31% by day 5 (P<0.05), whereas cortical blood flow was unchanged. In Dahl R rats, mean arterial pressure averaged 117+/-5 mm Hg during the 0.4% salt control period and remained unchanged (as did cortical and medullary blood flows) during 5 days of 4.0% salt intake. Dahl S rats that received medullary L-arginine (300 microg. kg-1. min-1) exhibited no changes of mean arterial pressure or regional renal blood flow during the 5 days of 4.0% salt intake. Medullary infusion of D-arginine (300 microg. kg-1. min-1) did not prevent the development of hypertension in Dahl S rats that received 4.0% salt. The results are consistent with the view that Dahl S rats have a reduced capacity to generate nitric oxide within the renal medulla under conditions of high salt, which the administration of L-arginine can normalize. Furthermore, early reductions of medullary blood flow in Dahl S rats with high salt intake probably contribute to the development of hypertension.

Short- and long-term enalapril affect renal medullary hemodynamics in the spontaneously hypertensive rat

Long-term angiotensin-converting enzyme (ACE) inhibition in the spontaneously hypertensive rat (SHR) resets pressure natriuresis and shifts the relationship between renal arterial pressure (RAP) and renal interstitial hydrostatic pressure (RIHP) to lower levels of arterial pressure. These effects persist after withdrawal of treatment. The purpose of this study was to determine the effect of short- and long-term ACE inhibition on medullary blood flow (MBF). Enalapril (25 mg. kg-1. day-1 in drinking water) was given to male SHR from 4 to 14 wk of age. Four weeks after stopping treatment, we measured MBF over a wide range of RAP using laser-Doppler flowmetry in anesthetized rats. Additional rats, either untreated or previously treated for 10 wk, received 3-day enalapril treatment just before the experiment. MAP (mmHg +/- SE) was 178 +/- 6 (n = 8), 134 +/- 6 (n = 8), 138 +/- 5 (n = 9), and 111 +/- 6 mmHg (n = 9) for the untreated, 3 day, 10 wk, and 10 wk + 3 day groups, respectively. Total renal blood flow for the groups receiving 3-day treatment was significantly higher when compared with that in rats with an intact renin-angiotensin system. Three-day treatment had no effect on the relationship between RAP and RIHP, whereas that in rats receiving 10-wk treatment was shifted to lower levels of RAP by approximately 30 mmHg. Both 10-wk and 3-day treatment independently increased the slope of the RAP versus MBF relationship at values of RAP > 100 mmHg. The slopes in perfusion units/mmHg were 0.12 +/- 0.01 (n = 8), 0.26 +/- 0.01 (n = 8), 0.27 +/- 0.01 (n = 9), and 0.30 +/- 0.02 (n = 9) for the untreated, 3 day, 10 wk, and 10 wk + 3 day groups, respectively. These results indicate that the effect of short-term and the persistent effect of long-term enalapril alter renal medullary hemodynamics in a way that may contribute to the resetting of the pressure-natriuresis relationship in treated rats.

Afferent arteriolar vasodilation to the sulfonimide analog of 11, 12-epoxyeicosatrienoic acid involves protein kinase A

The current study determined the contribution of protein kinase-A (PKA) and protein kinase-G (PKG) to the vasodilation elicited by the N-methylsulfonimide analog of 11,12-epoxyeicosatrienoic acid (11, 12-EET). Experiments were performed, in vitro, using the juxtamedullary nephron preparation combined with videomicroscopy. The response of afferent arterioles to the sulfonimide analog of 11, 12-EET, was determined before and after inhibition of PKA, PKG, or guanylyl cyclase. Afferent arterioles, preconstricted with 0.5 micromol/L norepinephrine, averaged 18+/-1 microm (n=25) at a renal perfusion pressure of 100 mm Hg. Superfusion with 0.01 to 100 nmol/L of the 11,12-EET analog caused a graded increase in diameter of the afferent arteriole. Vessel diameter increased by 11+/-1% and 15+/-1%, respectively, in response to 10 and 100 nmol/L of the 11,12-EET analog. The afferent arteriolar response to 10 and 100 nmol/L of the 11,12-EET analog was significantly attenuated during inhibition of PKA with 10 micromol/L H-89 (n=7) or 5 micromol/L myristolated PKI (n=6), such that afferent arteriolar diameter increased by only 5+/-2% and 2+/-1%, respectively, in response to 100 nmol/L of the 11, 12-EET analog. In contrast, the afferent arteriolar vasodilatory response to the 11,12-EET analog was unaffected by PKG or guanylyl cyclase inhibition. In the presence of 200 micromol/L histone H2B (n=5) or 10 micromol/L ODQ (n=7), the afferent arteriolar diameter increased by 16+/-3% and 12+/-2%, respectively, in response to 100 nmol/L of the 11,12-EET analog. These results demonstrate that activation of PKA is an important mechanism responsible for the afferent arteriolar vasodilation elicited by the sulfonimide analog of 11,12-EET.

Renal haemodynamic effects of endothelin-1 and the ETA/ETB antagonist TAK-044 in anaesthetized rabbits

OBJECTIVE

The aim of this study was to test the effects of exogenous endothelin-1 (ET-1) on regional kidney blood flow and renal function, and the renal haemodynamic effects of endogenous ET, in anaesthetized rabbits.

METHODS

ET-1 was infused into the left renal artery at 2 ng/kg/min for 30 min, then at 1 ng/kg/min. Cumulative doses of TAK-044 (0.1-3 mg/kg, i.v.) or its vehicle were given at 30-min intervals. In other rabbits, an extracorporeal circuit was established to adjust renal arterial pressure (RAP) independently of systemic arterial pressure (MAP). RAP was set at 65 mmHg, and either TAK-044 (3 mg/kg, i.v.) or its vehicle was administered.

RESULTS

In the infused kidney ET-1 (2 ng/kg/min) reduced renal blood flow (RBFprobe; 52+/-8%), cortical perfusion (37+/-7%), glomerular filtration rate (GFR; 49+/-8%), urine flow (47+/-14%) and sodium excretion (49+/-13%), but not medullary perfusion (5+/-6%). No effects of ET-1 on MAP or on the contralateral kidney were observed. TAK-044 dose-dependently reversed the effects of ET-1 on RBFprobe and cortical perfusion. TAK-044 also reduced MAP (by up to 11+/-3%) and increased effective renal blood flow in the contralateral kidney (by up to 46+/-27%). In the extracorporeal circuit model, TAK-044 decreased MAP by 12+/-2% and RAP by 10+/-3%, and increased RBF by 9+/-3%.

CONCLUSION

Exogenous ET-1 reduces cortical more than medullary perfusion, and reduces GFR without affecting net tubular sodium and fluid reabsorption. TAK-044 antagonizes local renal vascular responses to ET-1. Endogenous ETs appear to contribute markedly to resting renal vasomotor tone and MAP.

Effects of long-term vasopressin receptor stimulation on medullary blood flow and arterial pressure

Studies were carried out using instrumented unanesthetized rats to determine the long-term effects of arginine vasopressin (AVP) and a specific vasopressin V1 receptor agonist (V1AG; [Phe2, Ile3, Orn8]- vasopressin) on the renal medullary blood flow and arterial blood pressure. It was hypothesized that the hypertension observed with chronic medullary infusion of a V1 receptor agonist may be associated with a sustained reduction of blood flow, whereas infusion of AVP may fail to produce a sustained reduction of blood flow and thereby be unable to produce hypertension. Uninephrectomized Sprague-Dawley rats were prepared with implanted renal cortical and medullary optical fibers for daily measurements of cortical and medullary blood flow using laser-Doppler flowmetry techniques. An implanted renal medullary interstitial infusion catheter delivered either AVP or a specific V1AG at a dose of 2 ng . kg-1 . min-1 over a period of 5 days. The V1AG produced no change of cortical blood flow but a chronic 35% reduction of medullary blood flow (P < 0.05) and mild hypertension (11 +/- 4 mmHg, P < 0.05). AVP produced only an initial, nonsignificant 1- to 2-day reduction of medullary blood flow (-13%) and failed to raise arterial pressure significantly. We conclude that a sustained V1AG response is necessary to achieve a chronic reduction of medullary blood flow and hypertension. The present data are consistent with the idea that chronic stimulation of V2 receptors by AVP offsets the vasoconstrictor and hypertension actions of AVP-induced stimulation of medullary V1 receptors.

Effects of the vasopressin V1 agonist [Phe2,Ile3,Orn8]] vasopressin on regional kidney perfusion and renal excretory function in anesthetized rabbits

To test whether renal V1-receptors selectively influence blood flow in the renal medulla, we compared the effects of infusion of [Phe2,Ile3,Orn8]vasopressin (3-30 ng/kg/min) by the intravenous, renal arterial, and renal medullary interstitial routes in anesthetized rabbits. Intravenous [Phe2,Ile3,Orn8]vasopressin (30 ng/kg/min) reduced renal medullary perfusion (MBF) by 36 +/- 5% but did not significantly affect cortical perfusion (CBF). MBF was also reduced with the renal arterial (35 +/- 5%) and renal medullary interstitial (40 +/- 7%) routes but, in contrast to the intravenous infusion, CBF was also reduced, by 21 +/- 3% and 15 +/- 3%, respectively. Urine flow and sodium excretion were increased by [Phe2,Ile3,Orn8]vasopressin, and with direct intrarenal administration, this effect was similar for both the infused (left) and noninfused (right) kidneys. After a 20-min renal medullary interstitial infusion of [3H]norepinephrine, radiolabel concentration was approximately fivefold greater in the left medulla than in the left cortex. We conclude that [Phe2,Ile3,Orn8]vasopressin acts on V1-receptors to alter regional kidney blood flow and tubular salt and water handling. The V1-receptors involved are almost certainly within the kidney itself, but given the contrasting effects of the different infusion routes on MBF and CBF, we cannot exclude the possibility that some of the observed effects of [Phe2,Ile3,Orn8]vasopressin are mediated by activation of extra-renal V1-receptors.

Massive gross hematuria in a sickle cell trait patient with renal papillary necrosis. Conservative approach using a balloon ureteral catheter to tamponade the papilla bleeding

We report the case of a patient with heterozygous sickle cell trait presenting with massive gross hematuria. Ureteroscopy revealed bleeding from the tip of papilla of the inferior calica and allowed us to tamponade the bleeding with a balloon ureteral dilator set (Cook(R) Urology). We show the value of the endourological approach in the conservative management of these patients at high risk of recurrent hematuria.

[Influence of sevoflurane on renal medullary blood flow in humans]

Proper anesthetic management is necessary to preserve renal function during anesthesia and surgery. Using ultra-sound color Doppler, we examined the influence of sevoflurane on renal medullary blood flow in 20 adult patients without renal dysfunction. After identifying an interlobar artery in the outer medulla, we measured the velocity of the arterial blood flow before induction of anesthesia, and during sevoflurane anesthesia (1 MAC, 1.5 MAC). The minimum velocity of the interlobar arterial blood flow (Vmin) during wakefulness correlated significantly with creatinine clearance measured preoperatively. We did not find any significant change in Vmin after induction of sevoflurane anesthesia, despite significant decreases in mean arterial blood pressure.

Intrarenal blood flow: microvascular anatomy and the regulation of medullary perfusion

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

Osmotic hypertonicity of the renal medulla during changes in renal perfusion pressure in the rat

1. The relationship between renal perfusion pressure (RPP) and ion concentration in renal medulla was studied in anaesthetized rats. RPP was changed in steps within the pressure range 130-80 mmHg, while tissue electrical admittance (Y, index of interstitial ion concentration) and medullary and cortical blood flow (MBF and CBF; laser Doppler flowmetry) were measured, along with glomerular filtration rate (C in) and renal excretion. 2. With a RPP reduction from 130 to 120 mmHg, tissue Y remained stable; at 100 and 80 mmHg, Y was 5 and 17 % lower, respectively, than at 120 mmHg. 3. CBF fell less than RPP (partial autoregulation) in the range 130-100 mmHg only. MBF was autoregulated within 120-100 mmHg, but not above or below this range. 4. Each step of RPP reduction was followed by a decrease in sodium and water excretion (UNaV and V). The osmolality of excised inner medulla fragments was similar at 120 and 105 mmHg (586 +/- 45 and 618 +/- 35 mosmol (kg H2O)-1, respectively) but lower at 80 mmHg (434 +/- 31 mosmol (kg H2O)-1, P < 0.01); the ion concentration changed in parallel. 5. The data show that medullary hypertonicity was well preserved during RPP fluctuations within 130-100 mmHg, but not below this range. RPP-dependent changes of UNaV and V were not clearly associated with changes in solute concentration in medullary tissue.