Extravasal albumin concentration modulates contractile responses of renal afferent arterioles

AIM

Afferent arterioles (AA) hold a key position in the regulation of renal blood flow and glomerular filtration rate. Being the effector site of tubuloglomerular feedback, the afferent arteriole contributes to the renal handling of sodium and fluid. Dehydration goes along with increased renal interstitial protein concentration. Here, the hypothesis was tested that extravasal protein concentration directly modulates afferent arteriolar tone, a mechanism which may contribute to body fluid volume control.

METHOD

The effect of increased extravasal albumin concentration on the vascular reactivity was investigated in renal AA and interlobar arteries of mice, in rat renal AA and in pancreatic islet arterioles.

RESULTS

Albumin (2 and 4% in the bath solution) significantly potentiated the contractile response of renal afferent arterioles induced by angiotensin II and adenosine, as well as their combination, compared to the control situation (0.1% albumin). Albumin did not influence the contractility of larger renal vessels or pancreatic islet arterioles. Mimicking the increase in the osmolality induced by 4% albumin by applying mannitol to the bath solution also increased the response of renal arterioles to Ang II. However, the effect was smaller compared to that of albumin. The nitric oxide bioavailability, measured by DAF-FM fluorescence, was reduced in afferent arterioles exposed to 4% albumin.

CONCLUSION

The protein-induced modulation of AA tone is mediated by the increased osmolality as well as by NO scavenging. The results suggest a possible contribution of these mechanisms to the control of extracellular fluid volume via adjustment of renal blood flow and glomerular filtration rate.

Adenosine A1 receptor-dependent and independent pathways in modulating renal vascular responses to angiotensin II

AIM

Renal afferent arterioles are the effector site for autoregulation of glomerular perfusion and filtration. There is synergistic interaction between angiotensin II (ANG II) and adenosine (Ado) in regulating arteriolar contraction; however, the mechanisms are not clear. In this context, this study investigated the contribution of A1 receptor-dependent and independent signalling mechanisms.

METHODS

Isolated perfused afferent arterioles from transgenic mice (A1 (+/+) and A1 (-/-) ) were used for vascular reactivity studies. Cultured vascular smooth muscle cells (VSMC) were used for phosphorylation studies of signalling proteins that induce arteriolar contraction.

RESULTS

Maximal arteriolar contraction to ANG II was attenuated in A1 (-/-) (22%) compared with A1 (+/+) (40%). Simultaneous incubation with low-dose ado (10(-8)  mol L(-1) ) enhanced ANG II-induced contraction in A1 (+/+) (58%), but also in A1 (-/-) (42%). An ado transporter inhibitor (NBTI) abolished this synergistic effect in A1 (-/-) , but not in wild-type mice. Incubation with Ado + ANG II increased p38 phosphorylation in aortic VSMC from both genotypes, but treatment with NBTI only blocked phosphorylation in A1 (-/-) . Combination of ANG II + Ado also increased MLC phosphorylation in A1 (+/+) but not significantly in A1 (-/-) , and NBTI had no effects. In agreement, Ado + ANG II-induced phosphorylation of p38 and MLC in rat pre-glomerular VSMC was not affected by NBTI. However, during pharmacological inhibition of the A1 receptor simultaneous treatment with NBTI reduced phosphorylation of both p38 and MLC to control levels.

CONCLUSION

Interaction between ANG II and Ado in VSMC normally involves A1 receptor signalling, but this can be compensated by receptor independent actions that phosphorylate p38 MAPK and MLC.

Inhibition of sodium-linked glucose reabsorption normalizes diabetes-induced glomerular hyperfiltration in conscious adenosine A₁-receptor deficient mice

AIM

Glomerular hyperfiltration is commonly observed in diabetics early after the onset of the disease and predicts the progression of nephropathy. Sustained hyperglycaemia is also closely associated with kidney hypertrophy and increased electrolyte and glucose reabsorption in the proximal tubule. In this study, we investigated the role of the increased tubular sodium/glucose cotransport for diabetes-induced glomerular hyperfiltration. To eliminate any potential confounding effect of the tubuloglomerular feedback (TGF) mechanism, we used adenosine A₁-receptor deficient (A1AR(-/-)) mice known to lack a functional TGF mechanism and compared the results to corresponding wild-type animals (A1AR(+/+)).

METHODS

Diabetes was induced by an intravenous bolus injection of alloxan. Glomerular filtration rate (GFR) was determined in conscious mice by a single bolus injection of inulin. The sodium/glucose cotransporters were inhibited by phlorizin 30 min prior to GFR measurements.

RESULTS

Normoglycaemic animals had a similar GFR independent of genotype (A₁AR(+/+) 233 ± 11 vs. A₁AR(-/-) 241 ± 25 μL min(-1)), and induction of diabetes resulted in glomerular hyperfiltration in both groups (A₁AR(+/+) 380 ± 25 vs. A₁AR(-/-) 336 ± 35 μL min(-1); both P < 0.05). Phlorizin had no effect on GFR in normoglycaemic mice, whereas it reduced GFR in both genotypes during diabetes (A₁AR(+/+) 365 ± 18 to 295 ± 19, A₁AR(-/-) 354 ± 38 to 199 ± 15 μL min(-1); both P < 0.05). Notably, the reduction was more pronounced in the A₁AR(-/-) (P < 0.05).

CONCLUSION

This study demonstrates that increased tubular sodium/glucose reabsorption is important for diabetes-induced hyperfiltration, and that the TGF mechanism is not involved in these alterations, but rather functions to reduce any deviations from a new set-point.

Interactions between adenosine, angiotensin II and nitric oxide on the afferent arteriole influence sensitivity of the tubuloglomerular feedback

Adenosine, via activation of A1 receptors on the afferent arteriole (AA), mediates the tubuloglomerular feedback (TGF) mechanism. Angiotensin II and nitric oxide (NO) can modulate the sensitivity of the TGF mechanism. However, the interaction among these substances in regulating the TGF resetting phenomenon has been debated. Studies in isolated perfused AA have shown a biphasic response to accumulating doses of adenosine alone. In the nanomolar range adenosine has a weak contractile effect (7%), whereas vasodilatation is observed at high concentrations. However, a synergistic interaction between the contractile response by adenosine and that of angiotensin II has been demonstrated. Adenosine in low concentrations strongly enhances the response to angiotensin II. At the same time, angiotensin II in physiological concentrations increases significantly the contractile response to adenosine. Moreover, addition of a NO donor (spermine NONOate) to increase NO bioavailability abolished the contractile response from combined application of angiotensin II and adenosine. These mutual modulating effects of adenosine and angiotensin II, and the effect of NO on the response of AA can contribute to the resetting of the TGF sensitivity.

L-arginine or tempol supplementation improves renal and cardiovascular function in rats with reduced renal mass and chronic high salt intake

AIM

Early life reduction in nephron number and chronic high salt intake cause development of renal and cardiovascular disease, which has been associated with oxidative stress and nitric oxide (NO) deficiency. We investigated the hypothesis that interventions stimulating NO signalling or reducing oxidative stress may restore renal autoregulation, attenuate hypertension and reduce renal and cardiovascular injuries following reduction in renal mass and chronic high salt intake.

METHODS

Male Sprague-Dawley rats were uninephrectomized (UNX) or sham-operated at 3 weeks of age and given either a normal-salt (NS) or high-salt (HS) diet. Effects on renal and cardiovascular functions were assessed in rats supplemented with substrate for NO synthase (L-Arg) or a superoxide dismutase mimetic (Tempol).

RESULTS

Rats with UNX + HS developed hypertension and displayed increased renal NADPH oxidase activity, elevated levels of oxidative stress markers in plasma and urine, and reduced cGMP in plasma. Histological analysis showed signs of cardiac and renal inflammation and fibrosis. These changes were linked with abnormal renal autoregulation, measured as a stronger tubuloglomerular feedback (TGF) response. Simultaneous treatment with L-Arg or Tempol restored cGMP levels in plasma and increased markers of NO signalling in the kidney. This was associated with normalized TGF responses, attenuated hypertension and reduced signs of histopathological changes in the kidney and in the heart.

CONCLUSION

Reduction in nephron number during early life followed by chronic HS intake is associated with oxidative stress, impaired renal autoregulation and development of hypertension. Treatment strategies that increase NO bioavailability, or reduce levels of reactive oxygen species, were proven beneficial in this model of renal and cardiovascular disease.

The gel hypothesis applied to the rat renal capillary membranes: a review

In the gel model for the glomerular (and peritubular) capillary membrane, the integrity of the membrane is supposed to result from the fluid reabsorption induced by the osmotic action of the counter-ions attracted to negative fixed charges, increasing the gel pressure such that it becomes the same as in the capillaries. From this point on, the gel will be unaffected by the high capillary pressure. The same fluid reabsorption will also suspend the fibrils in the matrix such that they form a series of grids composed of, for example, horizontal fibrils spaced similarly from one another. The model thereby explains the well-known phenomenon of a uniform 'pore' size, although slits rather than pores constitute the transport routes. The model also explains the fact that the plasma proteins are free to move in the membrane matrix, which is the consequence of a recent finding that a major restriction to albumin is offered by a unique protein, nephrin, located between the podocytes in Bowman's space cells. A large molecule, which may become trapped in a slit between two fibrils, will thus push out the positive counter-ions whereby the charges become free and hence repel one another, widening the slit such that the molecule is free to move in any direction. It is furthermore concluded that the restriction to proteins is also dependent on the width of the slits closest to plasma.

Angiotensin II enhances the afferent arteriolar response to adenosine through increases in cytosolic calcium

AIMS

Angiotensin II (Ang II) is a strong renal vasoconstrictor and modulates the tubuloglomerular feedback (TGF). We hypothesized that Ang II at low concentrations enhances the vasoconstrictor effect of adenosine (Ado), the mediator of TGF.

METHODS

Afferent arterioles of mice were isolated and perfused, and both isotonic contractions and cytosolic calcium transients were measured.

RESULTS

Bolus application of Ang II (10(-12) and 10(-10) M) induced negligible vasoconstrictions, while Ang II at 10(-8) m reduced diameters by 35%. Ang II at 10(-12), 10(-10) and 10(-8) m clearly enhanced the arteriolar response to cumulative applications of Ado (10(-11) to 10(-4) M). Ado application increased the cytosolic calcium concentrations in the vascular smooth muscle, which were higher at 10(-5) M than at 10(-8) M. Ang II (10(-11) to 10(-6) M) also induced concentration-dependent calcium transients, which were attenuated by AT(1) receptor inhibition. Simultaneously applied Ang II (10(-10) M) additively enhanced the calcium transients induced by 10(-8) and 10(-5) M Ado. The transients were partly inhibited by AT(1) or A(1) receptor antagonists, but not significantly by A(2) receptor antagonists.

CONCLUSION

A low dose of Ang II enhances Ado-induced constrictions, partly via AT(1) receptor-mediated calcium increase. Ado increases intracellular calcium by acting on A(1) but not A(2) receptors. The potentiating effect of Ang II on Ado-induced arteriolar vasoconstrictions may involve calcium sensitization of the contractile machinery, as Ang II only additively increased cytosolic calcium concentrations, while its effect on the arteriolar constriction was more than additive. The potentiating effect of Ang II might contribute to the resetting of TGF.

Adenosine increases calcium sensitivity via receptor-independent activation of the p38/MK2 pathway in mesenteric arteries

AIM

Adenosine (Ado) restores desensitized angiotensin II-induced contractions in the renal arterioles via an intracellular, receptor-independent mechanisms including the p38 mitogen-activated protein kinase (MAPK). In the present study we test the hypothesis that MAPK-activated protein kinase 2 (MK2) mediates the Ado effect downstream from p38 MAPK resulting in an increased phosphorylation of the regulatory unit of the myosin light chain (MLC(20)).

METHODS AND RESULTS

Contraction experiments were performed in rings of mesenteric arteries under isometric conditions in C57BL6 and MK2 knock out mice (MK2-/-). Ado pretreatment (10(-5) mol L(-1)) strongly increased Ang II sensitivity, calcium sensitivity and the phosphorylation of MLC(20). Treatment with Ado (3 x 10(-6) or 10(-5) mol L(-1) in between successive Ang II applications) enhanced the desensitized Ang II responses (second to fifth application). Ca(2+) transients were not effected by Ado. Further, blockade of type 1 and type 2 Ado receptors during treatment did not influence the effect. Type 3 receptor activation by inosine instead of Ado had no effect. Conversely, inhibition of nitrobenzylthioinosine-sensitive Ado transporters prevented the effects of Ado. Inhibition of p38 MAPK as well as use of MK2-/- mice prevented contractile Ado effects on the mesenteric arteries and the phosphorylation of MLC(20).

CONCLUSION

The study shows that Ado activates the p38 MAPK/MK2 pathway in vascular smooth muscle via an intracellular action, which results in an increased MLC(20) phosphorylation in concert with increased calcium sensitivity of the contractile apparatus. This mechanism can significantly contribute to the regulation of vascular tone, e.g. under post-ischaemic conditions.

Neuronal nitric oxide synthase-deficient mice have impaired renin release but normal blood pressure

BACKGROUND

Nitric oxide deficiency is involved in the development of hypertension, but the mechanisms are currently unclear. This study was conducted to further elucidate the role of neuronal nitric oxide synthase (nNOS) in blood pressure regulation and renin release in relation to different sodium loads.

METHODS

Blood pressure and heart rate were measured telemetrically and assessed during periods of physical activity and inactivity. Urinary solute excretion was measured by metabolism cages and plasma renin concentration (PRC) was determined by radioimmunoassay; all in nNOS knockout (nNOS(-/-)) and wild-type (nNOS(+/+)) mice after 10 days of low (0.01% NaCl) and high (4% NaCl) sodium diets.

RESULTS

The resting heart rate was reduced in nNOS(-/-) mice, but the two genotypes had similar blood pressure during the low (nNOS(+/+) 104 +/- 2 mm Hg; nNOS(-/-) 103 +/- 2 mm Hg) and high (nNOS(+/+) 107 +/- 3 mm Hg; nNOS(-/-) 108 +/- 2 mm Hg) sodium diets. During the high sodium diet, PRC did not differ between the genotypes (nNOS(+/+) 743 +/- 115 10(-5) Goldblatt units; nNOS(-/-) 822 +/- 63 10(-5) Goldblatt units), but during the low sodium diet, nNOS(-/-) mice failed to increase PRC (nNOS(+/+) 2164 +/- 220 10(-5) Goldblatt units; nNOS(-/-) 907 +/- 101 10(-5) Goldblatt units) and renal renin mRNA. On the low sodium diet, nNOS(-/-) mice also showed increased urine flow rate and osmolar excretion, observations not made during a high sodium diet.

CONCLUSIONS

Our results show that nNOS is necessary for stimulation of renin in response to sodium restriction. Furthermore, nNOS(-/-) mice are normotensive, and their blood pressure responds normally to an increased dietary sodium intake, indicating that nNOS deficiency does not cause salt-sensitive hypertension.

Diabetes-induced hyperfiltration in adenosine A(1)-receptor deficient mice lacking the tubuloglomerular feedback mechanism

AIMS

Glomerular hyperfiltration is commonly found in diabetic patients early after the onset of disease. This is one of the first indications of the development of progressive diabetic nephropathy. It has been proposed that glomerular hyperfiltration is caused by decreased delivery of electrolytes to the macula densa due to the increased sodium and glucose reabsorption in the proximal tubule, which would increase the glomerular filtration rate (GFR) via the tubuloglomerular feedback (TGF) mechanism. In this study, we investigated the role of TGF in diabetes-induced glomerular hyperfiltration by inducing diabetes in adenosine A(1)-receptor knockout (A1AR(-/-)) mice known to lack a functional TGF mechanism.

METHODS

Diabetes was induced by alloxan (75 mg kg(-1) bw) injected into the tail vein. The 24-hour urinary electrolyte excretion was measured in metabolic cages, the GFR determined by inulin clearance under isoflurane-anaesthesia, and histological changes evaluated.

RESULTS

All alloxan-treated animals developed hyperglycaemia (> or =20 mm). Normoglycaemic animals had a similar GFR independent of genotype (A1AR(+/+) 9.3 +/- 0.5 vs. A1AR(-/-) 10.1 +/- 0.8 microL min(-1)g(-1) bw) and diabetes resulted in similar glomerular hyperfiltration in both groups (A1AR(+/+) 14.0 +/- 1.7, n = 9 vs. A1AR(-/-) 15.3 +/- 1.9 microL min(-1)g(-1) bw). Diabetic animals had a similar tendency to develop interstitial fibrosis, whereas the glomerular volume was similar in both genotypes, and unaltered by diabetes.

CONCLUSIONS

This study shows that the A1AR(-/-) mice develop diabetes-induced glomerular hyperfiltration, demonstrating that the TGF mechanism is not the major cause of the development of hyperfiltration. Furthermore, the hyperfiltration in the present study was not related to alterations in the glomerular filtration area.

Hydronephrosis causes salt-sensitive hypertension and impaired renal concentrating ability in mice

AIM

Hypertension is a common disease in the industrialized world and approximately 5% of all cases are secondary to kidney malfunction. We have recently shown that hydronephrosis due to partial unilateral ureteral obstruction (PUUO) causes salt-sensitive hypertension in rats. The mechanisms are still unclear, but appear to be intrarenal and primarily located to the diseased kidney. In the present study, we have developed a model for PUUO to study if hydronephrotic mice develop salt-sensitive hypertension.

METHODS

PUUO was created in 3-week-old mice (C57bl/6J). Blood pressure and heart rate were measured telemetrically in adult animals on normal and high salt diets. Metabolism cages were used to study the renal excretion of electrolytes and water. Plasma samples for renin analysis were collected and renal histological changes were evaluated.

RESULTS

All hydronephrotic animals developed salt-sensitive hypertension that correlated to the degree of hydronephrosis. In hydronephrotic animals, blood pressure increased from 114 +/- 1 mmHg on normal salt diet to 120 +/- 2 mmHg on high salt diet, compared with 103 +/- 1 to 104 +/- 1 in controls. Hydronephrotic animals showed increased diuresis and reduced ability to regulate electrolyte concentration. No differences in plasma renin concentration were found between the groups. The parenchymal weight and glomerular area of contralateral kidneys were significantly increased in the hydronephrotic animals. Histopathology of the hydronephrotic kidneys displayed areas with fibrosis, inflammation and glomerular changes.

CONCLUSION

This study provides a model for PUUO in mice and demonstrates the presence of salt-sensitive hypertension and an impaired renal concentrating ability in mice which has not been described before.

Relief of chronic partial ureteral obstruction attenuates salt-sensitive hypertension in rats

AIM

The incidence of hydronephrosis due to ureteropelvic junction obstruction is approx. 0.5%. During the last decade, the management of non-symptomatic hydronephrosis has become much more conservative, but the long-term physiological consequences of this policy are not clear. Previously, we have shown that animals with chronic partial unilateral ureteral obstruction develop salt-sensitive hypertension. In this study, the effects of ipsilateral and contralateral nephrectomy and ureterovesicostomy on blood pressure were studied in hydronephrotic animals.

METHODS

Partial unilateral ureteral obstruction was created in 3-week-old male Sprague-Dawley rats and blood pressure was measured telemetrically 4-6 weeks later during a normal and high salt diet before and after uninephrectomy or ureterovesicostomy. Plasma samples for renin assay were collected during both diets before and after ipsilateral nephrectomy.

RESULTS

All hydronephrotic animals developed salt-sensitive hypertension, of different degrees. Before nephrectomy the plasma renin concentration was significantly higher in the hydronephrotic animals than in controls (160 +/- 15 microGU mL(-1) vs. 96 +/- 12 microGU mL(-1), respectively), but after the ipsilateral nephrectomy no differences were found between the groups. In the hydronephrotic animals both ipsilateral nephrectomy and ureterovesicostomy reduced the blood pressure and salt-sensitivity but the former still differed significantly from the controls. In contralaterally, nephrectomized hydronephrotic animals the salt-sensitive hypertension became more pronounced.

CONCLUSION

Hydronephrosis in rats causes salt-sensitive hypertension that can be markedly reduced by removing the hydronephrotic kidney or relieving the obstruction by ureterovesicostomy. The mechanisms appear to be intrarenal and primarily located in the diseased kidney, but a secondary mechanism is also present.

Contribution of adenosine receptors in the control of arteriolar tone and adenosine–angiotensin II interaction

Adenosine (Ado) mediates vasoconstriction via A(1)-Ado receptors and vasodilation via A(2)-Ado receptors in the kidney. It interacts with angiotensin II (Ang II), which is important for renal hemodynamics and tubuloglomerular feedback (TGF). The aim was to investigate the function of Ado receptors in the Ado-Ang II interaction in mouse microperfused, afferent arterioles. Ado (10(-11)-10(-4) mol/l) caused a biphasic response: arteriolar diameters were reduced (-7%) at Ado 10(-11)-10(-9) mol/l and returned to control values at higher concentrations. Treatment with Ang II (10(-10) mol/l) transformed the response into a concentration-dependent constriction. N(6)-cyclopentyladenosine (A(1)-Ado receptor agonist) reduced diameters (12% at 10(-6) mol/l). Application of CGS21680 (10(-12)-10(-4) mol/l, A(2A) receptor agonist) increased the diameter by 13%. Pretreatment with ZM241385 (A(2A)-Ado receptor antagonist) alone or in combination with MRS1706 (A(2B)-Ado receptor antagonist) resulted in a pure constriction upon Ado, whereas 8-cyclopentyltheophylline (CPT) (A(1)-Ado receptor antagonist) inhibited the constrictor response. Afferent arterioles of mice lacking A(1)-Ado receptor did not show constriction upon Ado. Treatment with Ado (10(-8) mol/l) increased the response upon Ang II, which was blocked by CPT. Ado (10(-5) mol/l) did not influence the Ang II response, but an additional blockade of A(2)-Ado receptors enhanced it. The action of Ado on constrictor A(1)-Ado receptors and dilatory A(2)-Ado receptors modulates the interaction with Ang II. Both directions of Ado-Ang II interaction, which predominantly leads to an amplification of the contractile response, are important for the operation of the TGF.

Mechanisms for macula densa cell release of renin

The juxtaglomerular apparatus in the kidney is important in controlling extracellular fluid volume and renin release. The fluid load to the distal tubule is first sensed at the macula densa site via the entry of NaCl, through a Na, K, 2Cl co-transport mechanism. The next step is unclear, but there is recent evidence of an increased macula densa cell calcium concentration with a reduction in fluid load to the macula densa. An increase in macula densa cell calcium could activate phospholipase A2 to release arachidonic acid, the rate-limiting step in the formation of prostaglandins. Recent evidence suggests that the prostaglandin formed is PGE2, a potent stimulator for renin release. Recent evidence has also shown that adenosine has an important function in the juxtaglomerular apparatus. It stimulates calcium release in afferent arteriolar smooth muscle cells, leading to contraction of the afferent arteriole as part of the tubuloglomerular feedback mechanism, and inhibits renin release. Thus, renin release from the afferent arteriole is mediated partly through formation of PGE2, and partly through the reduction of adenosine formation that inhibits renin production.

Visualization of nitric oxide production and intracellular calcium in juxtamedullary afferent arteriolar endothelial cells

AIM

Nitric oxide (NO) is an important signal transmitter with multiple haemodynamic functions in the kidney. Study of these is complicated by the difficulty in measuring NO directly or visualizing its production. Recently the synthesis of a group of new NO-sensitive fluorescent dyes, diaminofluoresceins (DAF), suitable for imaging applications has been reported. We attempted to use one DAF (DAF-2 DA) to investigate the relationship between endothelial calcium, NO production and afferent arteriolar reactivity.

METHODS

We used the isolated, perfused juxtamedullary nephron preparation (JMN) and loaded the afferent arteriolar endothelium with Fura-2 AM and DAF-2 DA (4,5-diaminofluorescein-2-diacetyl). After in vitro calibration of the imaging system, we measured Fura-2 and DAF-2 fluorescence in single endothelial cells of afferent arterioles (AA) perfused at a pressure of 100 mmHg.

RESULTS

Carboxy-2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO) (10-3 m), a specific NO scavenger, decreased DAF-2 fluorescence in the endothelium by 16.1% and the mid-afferent arteriolar diameter by 10.2%, and increased endothelial calcium by 17.8%. Nomega-nitro-l-arginine methyl ester (l-NAME) (10-4 m) decreased fluorescence intensity of DAF-2 by 18.6%, increased cellular calcium level by 19.7% and constricted the vessels by 11.6%. Addition of carbachol (10-4 m) increased average DAF-2 fluorescence by 22.8% and endothelial calcium concentration by 28.9%, whereas the arteriolar diameter remained essentially unchanged. Carbachol failed to increase DAF-2 fluorescence when administered after l-NAME pre-treatment.

CONCLUSION

We conclude that endothelial NO homeostasis is an important determinant of AA reactivity and suggest that DAF are suitable for real-time imaging of afferent arteriolar NO production in the isolated, perfused JMN and may be used in combination with calcium-sensitive fluorophores. We have found that NO reduction by carboxy-PTIO or l-NAME increases endothelial calcium, suggesting involvement of calcium signalling in an autocrine NO production feedback in the endothelium. This method should help to further clarify the role of endothelial NO in renal haemodynamics.

Perfusate composition influences nitric oxide homeostasis in rat juxtamedullary afferent arterioles

AIM

Vascular diameters in isolated juxtamedullary nephron preparations perfused with cell-free solutions differ from those perfused with blood. In the present study, the effects of the albumin content of the perfusate on the afferent arteriolar diameter and endothelial nitric oxide were investigated in the isolated juxtamedullary nephron preparation perfused with Krebs-Ringer-bicarbonate buffer containing albumin in different concentrations.

METHODS

The endothelium was loaded with DAF-FM DA, a nitric oxide-sensitive fluoroprobe. Perfusion was maintained either with 4% (control group), 10 or 20% albumin in the perfusate or with L-NAME (10-4 m) added to the perfusate. Fluorescent images were obtained and stored for evaluation of DAF-FM fluorescence and vascular diameters (in mid-afferent arterioles) immediately before perfusate change and every 15 min thereafter, for a 2-h period.

RESULTS

Increasing the albumin concentration resulted in a decrease in fluorescence. The most rapid decline of fluorescence was obtained following L-NAME administration (relative fluorescence after 2 h: 4% albumin 92.4 +/- 5.3%; 10% albumin 79.5 +/- 4.2%; 20% albumin 66.2 +/- 2.6%; L-NAME 55.4 +/- 3.0%; mean +/- SD, n = 5). A dose-dependent constriction of the afferent arterioles was observed (normalized diameter: 4% albumin 99.8 +/- 3.0%; 10% albumin 80.3 +/- 3.3%; 20% albumin 74.3 +/- 3.2%; L-NAME 70.6 +/- 3.5%).

CONCLUSION

We propose that albumin interferes with arteriolar nitric oxide homeostasis, probably by scavenging nitric oxide intra-luminally. In this respect, albumin acts similarly to red blood cells in the circulation. The magnitude of the scavenging determines the effectiveness of autoregulation in the perfused preglomerular vessels. The scavenging properties of the perfusing fluid are important in setting operating levels of endothelial nitric oxide.

Blood flow distribution during elevated intraperitoneal pressure in the rat

AIM

Oliguria is seen during elevated intraperitoneal pressure, but the physiological mechanisms are not yet clarified. The purpose of the present study was to investigate the changes in renal function, cardiac output and distribution of systemic blood flow (BF) that occur in connection with an elevation of intra-abdominal pressure (IAP) in a rat model by isotope-labelled microsphere technique.

METHODS

A 5 or 10 mmHg IAP was created by CO2 insufflation and maintained for 90 min in anaesthetized and mechanically ventilated rats. Rats with normal IAP served as controls. Blood flow and cardiac output measurements by injection of isotope-labelled microspheres were conducted at three time points. Acid-base balance, urine output, glomerular filtration rate (GFR) and urinary excretion products were also followed.

RESULTS

Glomerular filtration rate decreased [0.7-0.1 mL min(-1) g(-1) kidney weight (KW)] with elevated IAP, as did urine output (8.5-0.6 microL min(-1) g(-1) KW). Dramatic decreases were seen in renal excretion of sodium (by 97%), potassium (by 94%) and osmotic active substances (by 93%). Cardiac output was diminished by 54% at 5 mmHg and by 65% at 10 mmHg intraperitoneal pressure and systemic vascular resistance (SVR) was elevated threefold.

CONCLUSION

Cardiac output, measured by microsphere technique, decreased during elevated intraperitoneal pressure by CO2 in anaesthetized rats, while SVR was elevated and renal excretory functions were decreased to a large extent.

Reduced rat renal vascular response to angiotensin II after chronic inhibition of nNOS

The neuronal isoform of nitric oxide synthase (nNOS) in the kidney is predominantly located in the macula densa (MD) cells. These cells are known to be the sensor in the tubuloglomerular feedback, which influences the tonus of the afferent arteriole. This study investigated the effect of angiotensin II (Ang II) after chronic inhibition of nNOS on renal blood flow (RBF) and cytosolic calcium concentration [Ca(2+)]i in smooth muscle cells from afferent arterioles. Measurements of RBF were made in two control groups and two groups treated with a nNOS inhibitor, 7-nitro indazole (7-NI), for 1 and 4 weeks. At the time of the experiment Ang II bolus was given in the renal artery before and during i.v. l-NNA. [Ca(2+)]i was measured in arterioles from control rats and from rats treated for 1 week with 7-NI. RBF decreased after bolus Ang II by 60 +/- 11% in the control vs. 23 +/- 8% in the 1 week 7-NI treated group. The decreased sensitivity to Ang II after 1 week of 7-NI treatment compared with control rats persisted after l-NNA infusion. There were no differences from control in the group treated for 4 weeks. Ang II gave a transient [Ca(2+)]i increase in vessels from control rats whereas this response was absent in 1 week 7-NI-treated rats. A possible explanation for these findings could be a down regulation of Ang II receptors. The renal vasculature of rats exhibits a diminished RBF and [Ca(2+)]i response to Ang II after 1 week blockade of nNOS.

Nitric oxide modulates and adenosine mediates the tubuloglomerular feedback mechanism

The tubuloglomerular feedback (TGF) mechanism is an important regulator of the glomerular filtration rate (GFR) and urine excretion rate. It operates by sensing the distal delivery of fluid at the macula densa site and adjusting the tone of the glomerular arterioles to control GFR. We found evidence that nitric oxide is an important modulator of the setting of the sensitivity of the TGF mechanism. Studies on adenosine A1 receptor deficient mice have shown that these animals lack the TGF response and have an increased renin release. These findings show the important role of adenosine as a mediator of the signal for the TGF mechanism and as an inhibitor of renin release.