Nonselective and A2a-Selective Inhibition of Adenosine Receptors Modulates Renal Perfusion and Excretion Depending on the Duration of Streptozotocin-Induced Diabetes in Rats

Long-lasting hyperglycaemia may alter the role of adenosine-dependent receptors (P1R) in the control of kidney function. We investigated how P1R activity affects renal circulation and excretion in diabetic (DM) and normoglycaemic (NG) rats; the receptors' interactions with bioavailable NO and H₂O₂ were also explored. The effects of adenosine deaminase (ADA, nonselective P1R inhibitor) and P1A2a-R-selective antagonist (CSC) were examined in anaesthetised rats, both after short-lasting (2-weeks, DM-14) and established (8-weeks, DM-60) streptozotocin-induced hyperglycaemia, and in normoglycaemic age-matched animals (NG-14, NG-60, respectively). The arterial blood pressure, perfusion of the whole kidney and its regions (cortex, outer-, and inner medulla), and renal excretion were determined, along with the in situ renal tissue NO and H₂O₂ signals (selective electrodes). The ADA treatment helped to assess the P1R-dependent difference in intrarenal baseline vascular tone (vasodilation in DM and vasoconstriction in NG rats), with the difference being more pronounced between DM-60 and NG-60 animals. The CSC treatment showed that in DM-60 rats, A2aR-dependent vasodilator tone was modified differently in individual kidney zones. Renal excretion studies after the ADA and CSC treatments showed that the balance of the opposing effects of A2aRs and other P1Rs on tubular transport, seen in the initial phase, was lost in established hyperglycaemia. Regardless of the duration of the diabetes, we observed a tonic effect of A2aR activity on NO bioavailability. Dissimilarly, the involvement of P1R in tissue production of H₂O₂, observed in normoglycaemia, decreased. Our functional study provides new information on the changing interaction of adenosine in the kidney, as well as its receptors and NO and H₂O₂, in the course of streptozotocin diabetes.

Endothelin type A receptor blockade increases renoprotection in congestive heart failure combined with chronic kidney disease: Studies in 5/6 nephrectomized rats with aorto-caval fistula

BACKGROUND

Association of congestive heart failure (CHF) and chronic kidney disease (CKD) worsens the patient's prognosis and results in poor survival rate. The aim of this study was to examine if addition of endothelin type A (ET_A) receptor antagonist to the angiotensin-converting enzyme inhibitor (ACEi) will bring additional beneficial effects in experimental rats.

METHODS

CKD was induced by 5/6 renal mass reduction (5/6 NX) and CHF was elicited by volume overload achieved by creation of aorto-caval fistula (ACF). The follow-up was 24 weeks after the first intervention (5/6 NX). The treatment regimens were initiated 6 weeks after 5/6 NX and 2 weeks after ACF creation.

RESULTS

The final survival in untreated group was 15%. The treatment with ET_A receptor antagonist alone or ACEi alone and the combined treatment improved the survival rate to 64%, 71% and 75%, respectively, however, the difference between the combination and either single treatment regimen was not significant. The combined treatment exerted best renoprotection, causing additional reduction in albuminuria and reducing renal glomerular and tubulointerstitial injury as compared with ACE inhibition alone.

CONCLUSIONS

Our results show that treatment with ET_A receptor antagonist attenuates the CKD- and CHF-related mortality, and addition of ET_A receptor antagonist to the standard blockade of RAS by ACEi exhibits additional renoprotective actions.

Effects of renal nerves and plasma epoxyeicosatrienoic acids on blood pressure, renal hemodynamics and excretion in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) display deficiency of epoxyeicosatrienoic acids (EETs). Their possible interaction with renal sympathetic nerves remains unexplored; synthesis of EET-A [disodium (S)-2-(13-(3-pentyl)ureido)-tridec-8(Z)-enamido)succinate], a stable 14,15-EET analog, helps clarify the issue. In anesthetized SHR, untreated or pretreated with EET-A, we assessed early responses of blood pressure (MAP), renal hemodynamics and excretion, and indices of nitric oxide (NO) activity, to bilateral noninvasive renal denervation (DNX). DNX significantly decreased MAP, with or without EET-A pretreatment. Renal perfusion decreased in EET-A treated but not in control rats. After EET-A pretreatment DNX decreased renal excretion of sodium and total solutes, compared to increasing tendency in untreated rats. In EET-A treated but not in untreated SHR denervation reduced the excretion of NO metabolites. Antihypertensive action of EET-A in anesthetized SHR was not clearly dependent on renal nerve activity. On the other hand, DNX unmasked the unexpected effect of EET-A to lower renal perfusion. The mechanism of this novel finding is unclear, as is also the simultaneous post-denervation decrease in renal excretion, again, observed only under EET-A treatment. Possibly, the decrease was secondary to falling MAP and renal perfusion. Increased renal excretion of nitric oxide metabolites under EETs elevation strongly suggests facilitation of NO release; the effect that was observed only with intact renal nerve activity.

Epoxyeicosatrienoic Acid Analog and 20-HETE Antagonist Combination Prevent Hypertension Development in Spontaneously Hypertensive Rats

Numerous studies indicate a significant role for cytochrome P-450-dependent arachidonic acid metabolites in blood pressure regulation, vascular tone, and control of renal function. Epoxyeicosatrienoic acids (EETs) exhibit a spectrum of beneficial effects, such as vasodilatory activity and anti-inflammatory, anti-fibrotic, and anti-apoptotic properties. 20-Hydroxyeicosatetraenoic acid (20-HETE) is a potent vasoconstrictor that inhibits sodium reabsorption in the kidney. In the present study, the efficiency of EET-A (a stable analog of 14,15-EET) alone and combined with AAA, a novel receptor antagonist of 20-HETE, was tested in spontaneously hypertensive rats (SHR). Adult SHR (16 weeks old) were treated with two doses of EET-A (10 or 40 mg/kg/day). In the following experiments, we also tested selected substances in the prevention of hypertension development in young SHR (6 weeks old). Young rats were treated with EET-A or the combination of EET-A and AAA (both at 10 mg/kg/day). The substances were administered in drinking water for 4 weeks. Blood pressure was measured by telemetry. Once-a-week observation in metabolic cages was performed; urine, blood, and tissue samples were collected for further analysis. The combined treatment with AAA + EET-A exhibited antihypertensive efficiency in young SHR, which remained normotensive until the end of the observation in comparison to a control group (systolic blood pressure, 134 ± 2 versus 156 ± 5 mmHg, respectively; p < 0.05). Moreover the combined treatment also increased the nitric oxide metabolite excretion. Considering the beneficial impact of the combined treatment with EET-A and AAA in young rats and our previous positive results in adult SHR, we suggest that it is a promising therapeutic strategy not only for the treatment but also for the prevention of hypertension.

Role of Ang1-7 in renal haemodynamics and excretion in streptozotocin diabetic rats

The contribution of angiotensin (1-7) (Ang1-7) to control of extrarenal and renal function may be modified in diabetes. We investigated the effects of Ang1-7 supplementation on blood pressure, renal circulation and intrarenal reactivity (IVR) to vasoactive agents in normoglycaemic (NG) and streptozotocin diabetic rats (DM). In Sprague Dawley DM and NG rats, 3 weeks after streptozotocin (60 mg/kg i.p.) or solvent injection, Ang1-7 was administered (400 ng/min) over the next 2 weeks using subcutaneously implanted osmotic minipumps. For a period of 5 weeks, blood pressure (BP), 24 h water intake and diuresis were determined weekly. In anaesthetised rats, BP, renal total and cortical (CBF), outer (OMBF) and inner medullary (IMBF) perfusion and urine excretion were determined. To check IVR, a short-time infusion of acetylcholine or norepinephrine was randomly given to the renal artery. Unexpectedly, BP did not differ between NG and DM, and this was not modified by Ang-1-7 supplementation. Baseline IMBF was higher in NG vs. DM, and Ang1-7 treatment did not change it in NG but decreased it in DM. In the latter, Ang1-7 increased cortical IVR to vasoconstrictor and vasodilator stimuli. IMBF decrease after high acetylcholine dose seen in untreated NG was reverted to an increase in Ang1-7 treated rats. Irrespective of the glycaemia level, Ang1-7 did not modify BP. However, it impaired medullary circulation in DM, whereas in NG it rendered the medullary vasculature more sensitive to vasodilators. Possibly, the medullary hypoperfusion in DM was mediated by Ang1-7 activation of angiotensin AT-1 receptors which are upregulated by hyperglycaemia.

Effects of systemic and renal intramedullary endothelin-1 receptor blockade on tissue NO and intrarenal hemodynamics in normotensive and hypertensive rats

Endothelin 1 (ET-1) seems essential in salt-dependent hypertension, and activation of ETA receptors causes renal vasoconstriction. However, the response in the renal medulla and the role of tissue NO availability has never been adequately explored in vivo. We examined effects of ETA and ETB receptor blockade (atrasentan and BQ788) on blood pressure (MAP), medullary blood flow (MBF) and medullary tissue NO. Effects of systemic and intramedullary blocker application were compared in anesthetized normotensive ET-1-pretreated Sprague-Dawley rats (S-D), in salt-dependent hypertension (HS/UNX) and in spontaneously hypertensive rats (SHR). Total renal blood flow (RBF) was measured using a Transonic renal artery probe, MBF as laser-Doppler flux, and tissue NO signal using selective electrodes. In normotensive rats ET-1 significantly increased MAP, decreased RBF (-20%) and renal medullary NO. In HS/UNX rats atrasentan decreased MAP and increased medullary NO, earlier and more profoundly with intravenous infusion. In SHR atrasentan decreased MAP, more effectively with intravenous infusion; the increase in tissue NO (∼10%) was similar with both routes; however, only intramedullary atrasentan increased MBF. No consistent responses to BQ788 were seen. We confirmed dominant role of ETA receptors in regulation of blood pressure and renal hemodynamics in normotensive and hypertensive rats and provided novel evidence for the role of ETA in control of intrarenal NO bioavailability in salt-dependent and spontaneous hypertension. Under conditions of activation of the endothelin system ETB stimulation preserved medullary perfusion.

Kidney Response to Chemotherapy-Induced Heart Failure: mRNA Analysis in Normotensive and Ren-2 Transgenic Hypertensive Rats

The aim of the present study was to perform kidney messenger ribonucleic acid (mRNA) analysis in normotensive, Hannover Sprague-Dawley (HanSD) rats and hypertensive, Ren-2 renin transgenic rats (TGR) after doxorubicin-induced heart failure (HF) with specific focus on genes that are implicated in the pathophysiology of HF-associated cardiorenal syndrome. We found that in both strains renin and angiotensin-converting enzyme mRNA expressions were upregulated indicating that the vasoconstrictor axis of the renin-angiotensin system was activated. We found that pre-proendothelin-1, endothelin-converting enzyme type 1 and endothelin type A receptor mRNA expressions were upregulated in HanSD rats, but not in TGR, suggesting the activation of endothelin system in HanSD rats, but not in TGR. We found that mRNA expression of cytochrome P-450 subfamily 2C23 was downregulated in TGR and not in HanSD rats, suggesting the deficiency in the intrarenal cytochrome P450-dependent pathway of arachidonic acid metabolism in TGR. These results should be the basis for future studies evaluating the pathophysiology of cardiorenal syndrome secondary to chemotherapy-induced HF in order to potentially develop new therapeutic approaches.

Early Renal Vasodilator and Hypotensive Action of Epoxyeicosatrienoic Acid Analog (EET-A) and 20-HETE Receptor Blocker (AAA) in Spontaneously Hypertensive Rats

Cytochrome P450 (CYP-450) metabolites of arachidonic acid: epoxyeicosatrienoic acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE) have established role in regulation of blood pressure (BP) and kidney function. EETs deficiency and increased renal formation of 20-HETE contribute to hypertension in spontaneously hypertensive rats (SHR). We explored the effects of 14,15-EET analog (EET-A) and of 20-HETE receptor blocker (AAA) on BP and kidney function in this model. In anesthetized SHR the responses were determined of mean arterial blood pressure (MABP), total renal (RBF), and cortical (CBF) and inner-medullary blood flows, glomerular filtration rate and renal excretion, to EET-A, 5 mg/kg, infused i.v. for 1 h to rats untreated or after blockade of endogenous EETs degradation with an inhibitor (c-AUCB) of soluble epoxide hydrolase. Also examined were the responses to AAA (10 mg/kg/h), given alone or together with EET-A. EET-A significantly increased RBF and CBF (+30% and 26%, respectively), seen already within first 30 min of infusion. The greatest increases in RBF and CBF (by about 40%) were seen after AAA, similar when given alone or combined with EET-A. MABP decreased after EET-A or AAA but not significantly after the combination thereof. In all groups, RBF, and CBF increases preceded the decrease in MABP. We found that in SHR both EET-A and AAA induced renal vasodilation but, unexpectedly, no additive effect was seen. We suggest that both agents have a definite therapeutic potential and deserve further experimental and clinical testing aimed at introduction of novel antihypertensive therapy.

Kynurenic acid selectively reduces heart rate in spontaneously hypertensive rats

We found previously that intravenous kynurenic acid (KYNA), a native broad spectrum glutamate antagonist, increases renal blood flow and induces natriuresis in anesthetized spontaneously hypertensive rats (SHR). Since such changes may affect systemic circulation and can potentially find therapeutic application, in this study we examined long term influence of orally administered KYNA on systemic and renal hemodynamics and renal excretion in conscious SHR. KYNA was administered in drinking water at a dose of 25 mg/kg/day for 3 weeks. Heart rate (HR), systolic (SBP), and mean arterial pressure (MAP) were measured through telemetry. The records were taken at the beginning of the study (control, day 0), and then on day 7, 14, and 21 of treatment. Diuresis (V), total solute excretion (U_osmV), and sodium excretion (U_NaV) were determined on days 0, 7, and 14. KYNA consistently decreased HR, from 319 ± 8 to 291 ± 5, 299 ± 9 and 284 ± 6 beats/min on day 7, 14, and 21, respectively, (- 9, - 6, and - 11%; p < 0.01-0.0001); HR was stable in the solvent group. SBP, MAP, V, and U_NaV were not affected by KYNA, whereas U_osmV increased modestly. Chronic oral administration of KYNA to conscious SHR decreased HR without affecting MAP. Since tachycardia is an independent risk factor for cardiovascular disorders, and most drugs used to decrease HR have strong inotropic negative or hypotensive effect, such selective action seems of therapeutic potential. Moreover, food supplementation with KYNA can be considered in the prevention of heart diseases.

Rapid blood pressure increase after renal denervation in anaesthetized rats: interaction of oxidative stress and neuronal nitric oxide synthase

We showed previously that in anaesthetized rats acute noninvasive renal denervation (DNX) induced an increase in arterial blood pressure (MABP), unlike the usual hypotensive effect. Here we aimed to establish the background of such unusual response, especially the role of oxidative stress as suggested by an earlier study. The contribution of oxidative stress was explored by studying the effects on DNX-induced MABP increase of pretreatment with 4-hydroxy-3-methoxyacetophenone (apocynin, APO), a powerful antioxidant and antihypertensive agent, and N(omega)-propyl-L-arginine (L-NPA), a blocker of neuronal nitric oxide synthase (nNOS). In anaesthetized Wistar rats maintained on standard (STD) or high-salt (HS) diet sequential right- and left-side DNX was performed. MABP responses were examined without pretreatment and after APO (20 mg/day on two preceding days) and L-NPA (1 mg/kg/h throughout experiment), given alone or combined. In untreated rats, bilateral DNX increased MABP by 6% on STD and 15% on HS diet (P < 0.01 or less); the difference between MABP responses was highly significant (P = 0.002). In STD rats APO or APO + L-NPA treatment failed to alter post-DNX MABP increases whereas L-NPA alone reversed the response and a significant 7% decrease occurred. In HS rats APO and L-NPA given alone reversed the MABP response and significant decreases of 14% (P = 0.001) and 8% (P = 0.01), were seen. Surprisingly, with L-NPA + APO pretreatment only abolishment (not reversal) of post-DNX pressure increase occurred. The results suggest that both systemic, intrarenal and brain oxidative stress, and excessive nNOS activity, mostly in the brain, determine the unexpected post-DNX pressure increase.

Sex-linked differences in the mortality in Ren-2 transgenic hypertensive rats with aorto-caval fistula: effects of treatment with angiotensin converting enzyme alone and combined with inhibitor of soluble epoxide hydrolase

We found recently that in Ren-2 transgenic hypertensive rats (TGR) addition of soluble epoxide hydrolase inhibitor (sEHi) to treatment with angiotensin-converting enzyme inhibitor (ACEi), surprisingly, increased the mortality due to heart failure (HF) induced by creation of the aorto-caval fistula (ACF). Since TGR exhibit sex-related differences in mortality, we examined here if such differentiation exists also in the response to the treatment with ACEi (trandolapril), alone or combined with sEHi [cis-4-[4-(3-adamantan-1-yl-ureido)cyclohexyloxy]benzoic acid, (c-AUCB)]. ACEi improved survival in males to 74 % (vs. 0 %) and in females to 65 % (vs. 32 %). ACEi and sEHi combined also improved the survival in male ACF TGR, however, it was significantly less (38 %) than after ACEi alone. In contrast, in females the combined treatment significantly improved the final survival rate (84 %). There were no significant sex-linked differences in survival rate in untreated or treated normotensive Hannover Sprague-Dawley rats. In conclusion, in HF patients with co-existing hypertension and RAS hyperactivity, the sex may co-determine the rate of HF progression, and can influence the effectiveness of the therapeutic measures applied. Therefore, in the relevant pre-clinical studies the sex-linked differences should be seriously considered. Our data indicate that TGR might be an optimal model for such studies.

Distinct effects of bosentan on NO-dependent vasodilation and calcium influx in heterozygous Ren-2 transgenic rats on high-salt diet

Our studies in hypertensive Ren-2 transgenic rats (TGR) demonstrated that chronic administration of atrasentan (ETA receptor antagonist) decreased blood pressure by reduced Ca2+ influx through L-type voltage-dependent calcium channels (L-VDCC) and attenuated angiotensin II-dependent vasoconstriction. We were interested whether bosentan (nonselective ET(A)/ET(B) receptor antagonist) would have similar effects. Young 4-week-old (preventive study) and adult 8-week-old (therapeutic study) heterozygous TGR and their normotensive Hannover Sprague-Dawley (HanSD) controls were fed normal-salt (NS, 0.6 % NaCl) or high-salt (HS, 2 % NaCl) diet for 8 weeks. An additional group of TGR fed HS was treated with bosentan (100 mg/kg/day). Bosentan had no effect on BP of TGR fed high-salt diet in both the preventive and therapeutic studies. There was no difference in the contribution of angiotensin II-dependent and sympathetic vasoconstriction in bosentan-treated TGR compared to untreated TGR under the condition of high-salt intake. However, bosentan significantly reduced NO-dependent vasodilation and nifedipine-sensitive BP component in TGR on HS diet. A highly important correlation of nifedipine-induced BP change and the BP after L-NAME administration was demonstrated. Although bosentan did not result in any blood pressure lowering effects, it substantially influenced NO-dependent vasodilation and calcium influx through L-VDCC in the heterozygous TGR fed HS diet. A significant correlation of nifedipine-induced BP change and the BP after L-NAME administration suggests an important role of nitric oxide in the closure of L-type voltage dependent calcium channels.

Arginine and tetrahydrobiopterin supplementation in rats with salt-induced blood pressure increase: minor hypotensive effect but improvement of renal haemodynamics

High salt (HS) intake can lead to hypertension, probably the result of the predominance of vasoconstrictor reactive oxygen species over vasodilator nitric oxide (NO). We aimed to examine if the supposed NO deficiency and the resultant blood pressure increase could be corrected by supplementation of L-arginine, the substrate, and tetrahydrobiopterin (BH4), a co-factor of NO synthases. Wistar rats without known genetic background of salt sensitivity were exposed to HS diet (4%Na) for 10 or 26 days, without or with supplementation with oral L-arginine, 1.4 mg/kg b.w. daily, alone or together with intraperitoneal BH4, 10 mg/kg daily. Systolic blood pressure (SBP, tail-cuff method) was measured repeatedly and found to increase ~40 mmHg after 26 days; L-arginine and BH4 did not significantly attenuate this increase. At the end of chronic studies, in anaesthetized rats the diet- and treatment-induced changes in renal haemodynamics were assessed. HS diet selectively decreased (-30%, P < 0.03) the inner medullary blood flow (IMBF, laser-Doppler flux) without changing total or cortical renal perfusion. Arginine supplementation tended to raise all renal circulatory parameters, and distinctly increased IMBF, to 61% above the HS diet level (P < 0.05). In conclusion, unlike in confirmed genetically determined salt-dependent hypertension, L-arginine and BH4 supplementation failed to attenuate the SBP increase observed after exposure to HS diet. On the other hand, arginine increased total and regional renal perfusion, especially IMBF. This suggests that the delivery of arginine increased intrarenal NO synthesis, an action of renoprotective potential which presumably countered the harmful influence of the local tissue oxidative stress.

Modulating Role of Ang1-7 in Control of Blood Pressure and Renal Function in AngII-infused Hypertensive Rats

BACKGROUND

Indirect evidence suggests that angiotensin 1-7 (Ang1-7) may counterbalance prohypertensive actions of angiotensin II (AngII), via activation of vascular and/or renal tubular receptors to cause vasodilation and natriuresis/diuresis. We examined if Ang1-7 would attenuate the development of hypertension, renal vasoconstriction, and decreased natriuresis in AngII-infused rats and evaluated the mechanisms involved.

METHODS

AngII, alone or with Ang1-7, was infused to conscious Sprague-Dawley rats for 13 days and systolic blood pressure (SBP) and renal excretion were repeatedly determined. In anesthetized rats, acute actions of Ang1-7 and effects of blockade of angiotensin AT1 or Mas receptors (candesartan or A-779) were studied.

RESULTS

Chronic AngII infusion increased SBP from 143 ± 4 to 195 ± 6 mm Hg. With Ang1-7 co-infused, SBP increased from 133 ± 5 to 161 ± 5 mm Hg (increase reduced, P < 0.002); concurrent increases in urine flow (V) and sodium excretion (UNaV) were greater. In anesthetized normotensive or AngII-induced hypertensive rats, Ang1-7 infusion transiently increased mean arterial pressure (MABP), transiently decreased renal blood flow (RBF), and caused increases in UNaV and V. In normotensive rats, candesartan prevented the Ang1-7-induced increases in MABP and UNaV and the decrease in RBF. In anesthetized normotensive, rats intravenous A-779 increased MABP (114 ± 5 to 120 ± 5 mm Hg, P < 0.03) and urine flow. Surprisingly, these changes were not observed with A-779 applied during background Ang1-7 infusion.

CONCLUSIONS

The results suggest that in AngII-dependent hypertension, Ang1-7 deficit contributes to sodium and fluid retention and thereby to BP elevation; a correction by Ang1-7 infusion seems mediated by AT1 and not Mas receptors.

Innovative lipid-based carriers containing cationic derivatives of polyisoprenoid alcohols augment the antihypertensive effectiveness of candesartan in spontaneously hypertensive rats

Novel lipid-based carriers, composed of cationic derivatives of polyisoprenoid alcohols (amino-prenols, APrens) and 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), were designed. The carriers, which were previously shown to be nontoxic to living organisms, were now tested if suitable for administration of candesartan, an antihypertensive drug. Spontaneously hypertensive rats (SHR) received injections of candesartan (0.1 mg/kg body weight per day; s.c.) in freshly prepared carriers for two weeks. The rats' arterial pressure was measured by telemetry. Urine and blood collection were performed in metabolic cages. In a separate group of SHR, the pharmacokinetics of the new formulation was evaluated after a single subcutaneous injection. The antihypertensive activity of candesartan administered in DOPE dispersions containing APrens was distinctly greater than that of candesartan dispersions composed of DOPE only or administered in the classic solvent (sodium carbonate). The pharmacokinetic parameters clearly demonstrated that candesartan in APren carriers reached the bloodstream more rapidly and in much greater concentration (almost throughout the whole observation) than the same drug administered in dispersions of DOPE only or in solvent. Serum creatinine (P_Cr) decreased significantly only in the group receiving candesartan in carriers with APrens (from 0.80 ± 0.04 to 0.66 ± 0.09 mg/dl; p < 0.05), whereas in the other groups P_Cr remained at the same level after treatment. Moreover, the new derivatives increased the loading capacity of the carriers, which is a valuable feature for any drug delivery system. Taken together, our findings led us to conclude that APrens are potentially valuable components of lipid-based drug carriers.

Effects of high and low sodium diet on blood pressure and heart rate in mice lacking the functional grainyhead-like 1 gene

Hypertension is a major health problem throughout the world because of its high prevalence and its association with increased risk of cardiovascular disease. Two independent studies discovered a locus conferring susceptibility to essential hypertension on chromosome 2, in the 2p25 region, but the causative gene remains unknown. Grainyhead-like 1 (GRHL1) is one of the genes located in this region. Our experiments determined that the Grhl1 -null mice, when fed standard diet, have the same blood pressure as their wild type littermate controls. However, we discovered that blood pressure of these mice increases following high sodium diet and decreases when they are fed low sodium diet, and similar effects were not observed in the control wild type littermates. This suggests that the Grhl1 -null mice are sensitive to the development of salt-sensitive hypertension. Thus it is possible that the GRHL1 gene is involved in the regulation of blood pressure, and it may be the causative gene for the locus of susceptibility to essential hypertension in the 2p25 region.

An endomorphine analog ([d-Ala(2)]-Endomorphin 2, TAPP) lowers blood pressure and enhances tissue nitric oxide in anesthetized rats

BACKGROUND

Activation of opioid receptors can alter cardiovascular function, an action possibly mediated by nitric oxide (NO). In this study we examined the effects of ([d-Ala(2)]-Endomorphin 2, TAPP), a synthetic opioid μ-receptor agonist, on blood pressure (MABP), tissue NO bioavailability and renal hemodynamics and excretion.

METHODS

In acute experiments with anesthetized normotensive male Sprague-Dawley rats TAPP was given as a short iv infusion at a dose of 1.2 or 12mg/kg and then MABP, renal medullary NO signal (polarographic electrode), total renal blood flow (RBF, renal artery Transonic probe), renal regional perfusion (laser-Doppler fluxes) and renal excretion were simultaneously measured over 2h.

RESULTS

After 1.2mg/kg dose MABP decreased progressively from 121±7 to 114±9mmHg (-6%, p<0.05) while kidney tissue NO signal increased from 29.1±2.7 to 31.7±3.1nA (6%, p<0.04). Both effects were prevented by Naloxone methiodide, a peripheral opioid receptor inhibitor. RBF and renal regional perfusion were not altered by either dose of TAPP; renal sodium excretion changes were highly variable and were not affected by Naloxone pretreatment.

CONCLUSIONS

Briefly, we found that in anesthetized normotensive rats stimulation of peripheral opioid receptors with TAPP caused a prolonged decrease in arterial pressure, a change that was associated and probably causally related to an increase in tissue NO. The data suggest that synthetic opioids that do not penetrate the blood-brain barrier and are potentially non-addictive could be considered for antihypertensive therapy.

Effects of chymostatin, a chymase inhibitor, on blood pressure, plasma and tissue angiotensin II, renal haemodynamics and renal excretion in two models of hypertension in the rat

NEW FINDINGS

What is the central question of this study? We examined, in hypertensive rats, whether the angiotensin-converting enzyme-independent enzymes generating angiotensin II in the tissues modulate blood pressure, peripheral circulation and renal function. What is the main finding and its importance? The results suggest that chymostatin-sensitive enzymes diminish vascular tone in renal and extrarenal vascular beds. Chymase or similar chymostatin-sensitive enzymes have a significant role in the synthesis of angiotensin II in different tissues but do not control blood pressure in the short term, similarly in salt-dependent or Goldblatt-type rat hypertension. In salt-dependent hypertension, chymase blockade protected renal outer medullary perfusion, probably by reducing the angiotensin II content in the kidney. Chymase is presumed to be a crucial enzyme of the non-angiotensin-converting enzyme pathway of angiotensin II (Ang II) generation in tissues, a process involved in vascular remodelling and development of hypertension. We examined the role of chymase in hypertension induced by exposure of uninephrectomized rats to high dietary salt intake (UNX HS) and in the Goldblatt renal artery stenosis (two-kidney, one-clip) model. In acute experiments with anaesthetized rats of either model, chymostatin at 2 mg kg(-1) h(-1) or 0.05% DMSO solvent was infused i.v. Mean arterial blood pressure, heart rate, iliac blood flow (a measure of hindlimb perfusion), total renal blood flow and intrarenal regional perfusion (laser-Doppler technique) were measured continuously, along with the glomerular filtration rate and renal excretion. In both models, chymase blockade distinctly decreased plasma and tissue Ang II without lowering mean blood pressure or consistently altering the other functional parameters measured. Unexpectedly, in Goldblatt hypertensive rats the blockade increased the renal and hindlimb vascular resistances by 51 and 33%, respectively (P < 0.05). In UNX HS hypertensive rats, chymase blockade abolished the solvent-induced decrease in outer medullary blood flow. We conclude that chymase or similar chymostatin-sensitive enzyme(s) has a significant role in the synthesis of Ang II in different tissues but does not participate in short-term control of blood pressure in salt-dependent or Goldblatt-type rat hypertension. In the Goldblatt model, chymase appeared to reduce the renal and hindlimb vascular resistances by an unknown mechanism. In salt-dependent hypertension, chymase blockade protected renal outer medullary perfusion, probably by reducing Ang II content in the kidney.

Consequences of the loss of the Grainyhead-like 1 gene for renal gene expression, regulation of blood pressure and heart rate in a mouse model

AIM

The Grainyhead-like 1 (GRHL1) transcription factor is tissue-specific and is very highly expressed in the kidney. In humans the GRHL1 gene is located at the chromosomal position 2p25. A locus conferring increased susceptibility to essential hypertension has been mapped to 2p25 in two independent studies, but the causative gene has never been identified. Furthermore, a statistically significant association has been found between a polymorphism in the GRHL1 gene and heart rate regulation. The aim of our study was to investigate the physiological consequences of Grhl1 loss in a mouse model and ascertain whether Grhl1 may be involved in the regulation of blood pressure and heart rate.

EXPERIMENTAL APPROACH

In our research we employed the Grhl1 "knock-out" mouse strain. We analyzed renal gene expression, blood pressure and heart rate in the Grhl1-null mice in comparison with their "wild-type" littermate controls. Most important results: The expression of many genes is altered in the Grhl1(-/-) kidneys. Some of these genes have previously been linked to blood pressure regulation. Despite this, the Grhl1-null mice have normal blood pressure and interestingly, increased heart rate.

CONCLUSIONS

Our work did not discover any new evidence to suggest any involvement of Grhl1 in blood pressure regulation. However, we determined that the loss of Grhl1 influences the regulation of heart rate in a mouse model.

High salt intake increases blood pressure in normal rats: putative role of 20-HETE and no evidence on changes in renal vascular reactivity

Background/Aims

High salt (HS) intake may elevate blood pressure (BP), also in animals without genetic salt sensitivity. The development of salt-dependent hypertension could be mediated by endogenous vasoactive agents; here we examined the role of vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE).

Methods

In conscious Wistar rats on HS diet systolic BP (SBP) was examined after chronic elevation of EETs using 4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB), a blocker of soluble epoxide hydrolase, or after inhibition of 20-HETE with 1-aminobenzotriazole (ABT). Thereafter, in acute experiments the responses of renal artery blood flow (Transonic probe) and renal regional perfusion (laser-Doppler) to intrarenal acetylcholine (ACh) or norepinephrine were determined.

Results

HS diet increased urinary 20-HETE excretion. The SBP increase was not reduced by c-AUCB but prevented by ABT until day 5 of HS exposure. Renal vasomotor responses to ACh or norepinephrine were similar on standard and HS diet. ABT but not c-AUCB abolished the responses to ACh.

Conclusions

20-HETE seems to mediate the early-phase HS diet-induced BP increase while EETs are not engaged in the process. Since HS exposure did not alter renal vasodilator responses to Ach, endothelial dysfunction is not a critical factor in the mechanism of salt-induced blood pressure elevation.

Vascular effects of a tripeptide fragment of novokinine in hypertensive rats: Mechanism of the hypotensive action

BACKGROUND

Activation of angiotensin AT2 receptors (AT2R) counteracts vasoconstrictor effects of AT1R stimulation and contributes to blood pressure control. We examined effects on mean arterial pressure (MAP) and renal hemodynamics of LKP, a tripeptide fragment of novokinine, an established AT2R agonist.

METHODS

Effects of intravenous LKP infusion and then superimposed losartan (AT1R antagonist) on MAP, total renal (RBF, Transonic probe) and renal medullary blood flows (laser-Doppler), and on renal excretion, were examined in anesthetized (1) Wistar rats with acute norepinephrine-induced hypertension, untreated or pretreated with AT2R antagonist (PD 123319) and (2) spontaneously hypertensive rats (SHR) maintained on standard or high-sodium (HS) diet.

RESULTS

In Wistar rats LKP decreased MAP (-4%, p<0.01) and increased renal medullary perfusion, these effects were abolished in rats pretreated with PD 123319 in which a post-LKP increase in MAP (+6%, p<0.02) occurred. LKP did not alter MAP in SHR; in those on HS diet RBF decreased (-14%, p<0.02), the response that was reverted by losartan. Addition of losartan always decreased or tended to decrease MAP.

CONCLUSIONS

LKP lowered MAP in norepinephrine-induced hypertension, probably via activation of AT2R. At reduced availability of AT2R, as in SHR, LKP appeared to bind to different receptors, possibly AT1, and induced systemic or renal vasoconstriction. Compared to the parent novokinine, a smaller LKP molecule might be easier absorbed after oral application and more useful in therapy.

Oxidative stress and neuronal NOS activity: putative determinants of rapid blood pressure increase after renal denervation in anesthetized rats

Long-term effects of renal denervation (DNX) commonly include a decrease in blood pressure (BP), observed in both normotensive animals and various models of hypertension. On the other hand, short term BP responses vary. We examined how post-DNX increase in BP observed in this study depends on baseline metabolic and functional status of animals, with a special interest for the role of oxidative stress. Anesthetized Wistar rats on standard (STD), low-sodium (LS) or high-sodium (HS) diet were used, untreated or pre-treated with tempol, a superoxide scavenger, or N(omega)-propyl-L-arginine (L-NPA), an inhibitor of neuronal NOS (nNOS). Early BP and renal hemodynamic responses were examined to right- and then left-side DNX performed using an own relatively non-invasive technique. Left kidney cortical, outer- and inner-medullary blood flows (CBF, OMBF, IMBF) were continuously recorded as laser-Doppler fluxes. Sequential denervations significantly increased BP to final 19 %, 12 %, and 6 % above control level in HS, LS, and STD groups, respectively. CBF, a measure of total renal perfusion, increased in LS and STD but not in HS rats. Tempol pretreatment prevented the post-denervation BP increase on each diet. Selective inhibition of nNOS prevented BP increase in STD and HS groups, a modest increase persisted in LS rats. We propose that enhanced afferent impulsation from intrarenal chemoreceptors related to oxidative stress in the kidney was the background for acute BP increase after DNX. The response was triggered by a release of brain sympatho-excitatory centers from inhibition by renal afferents, this was followed by widespread sympathetic cardiovascular stimulation.

Interaction of nitric oxide and the cytochrome P-450 system on blood pressure and renal function in the rat: dependence on sodium intake

AIM

Interaction was examined of nitric oxide (NO) and cytochrome P-450 (CYP-450)-dependent arachidonic acid derivatives, 20-HETE and EETs, in control of arterial pressure (MABP) and renal function. Modification of this interaction by changing sodium intake was also studied.

METHODS

On low, standard or high Na diet (LS, STD and HS rats respectively) effects of sequential blockade of NO synthases (NOS) and CYP-450 enzyme activity on MABP, renal blood flow (RBF, Transonic probe), renal medullary perfusion (MBF, laser-Doppler technique), medullary tissue NO (selective electrode) and renal excretion were examined in anaesthetized rats. All NOS were blocked with N(ϖ) -nitro-l-arginine methyl ester (l-NAME), the neuronal NOS with S-methyl-l-thiocitrulline (SMTC), and CYP-450 with 1-aminobenzotriazole (ABT).

RESULTS

In each diet group the baseline MABP was highest in rats pre-treated with l-NAME. CYP-450 inhibition significantly decreased MABP only in LS (-9%) and HS rats (-22%) pre-treated with l-NAME. This MABP decrease correlated directly with the dietary sodium content (r = 0.644, P < 0.001). CYP-450 inhibition decreased RBF in LS and HS rats (not in HS pre-treated with l-NAME). Acute exclusion of CYP-450 significantly increased MBF only in STD, SMTC pre-treated rats; in HS group it significantly increased medullary tissue NO by about 1.0 nA. The post-ABT changes in renal excretion occurred in LS and HS rats, irrespective of the status of NO synthesis.

CONCLUSIONS

Both NO- and CYP-450-dependent agents contribute to blood pressure and kidney function control, however, the role of 20-HETE and EETs becomes crucial only under conditions of high sodium intake or after NOS inhibition.

Cytochrome P-450 metabolites in renal circulation and excretion--interaction with the nitric oxide (NO) system

The role of CYP-450 dependent arachidonic acid (AA) metabolites (vasoconstrictor 20-HETE and vasodilator EETs) and NO in control of blood pressure (MABP) and kidney function remains unclear. NO affects the activity of heme-containing enzymes, like CYP-450 related monooxygenases, moreover, their activity depends on Na(+) intake. The focus of this review and underlying studies is on the role of high sodium intake (pro-hypertensive factor) in interrelation between CYP-450 and NOS. The acute vs. chronic non-selective inhibition of CYP-450 AA metabolites (ABT), and selective inhibition of 20-HETE (HET 0016) has also been tested. The renal artery flow (RBF, Transonic probe), medullary blood flow (MBF, laser-Doppler flux), renal excretion, and medullary tissue NO (selective electrode) were measured in male anaesthetized Wistar rats. We conclude that on standard Na(+) intake, opposed effects of 20-HETE and EETs are almost in equilibrium; however, in the renal circulation the vasodilator EETs influence slightly prevails. High sodium intake stimulates NOS, which limits CYP-450 impact on MABP and kidney function. However, this protection disappears after prolonged sodium intake. Long-lasting high sodium intake lowers NO bioavailability and promotes systemic and intrarenal vasoconstrictor activity of 20-HETE. Opposed effects of NO and AA metabolites of CYP-450 on water and solute excretion are also described.

Renal nerves and nNOS: roles in natriuresis of acute isovolumetric sodium loading in conscious rats

It was hypothesized that renal sympathetic nerve activity (RSNA) and neuronal nitric oxide synthase (nNOS) are involved in the acute inhibition of renin secretion and the natriuresis following slow NaCl loading (NaLoad) and that RSNA participates in the regulation of arterial blood pressure (MABP). This was tested by NaLoad after chronic renal denervation with and without inhibition of nNOS by S-methyl-thiocitrulline (SMTC). In addition, the acute effects of renal denervation on MABP and sodium balance were assessed. Rats were investigated in the conscious, catheterized state, in metabolic cages, and acutely during anesthesia. NaLoad was performed over 2 h by intravenous infusion of hypertonic solution (50 μmol·min−1·kg body mass−1) at constant body volume conditions. SMTC was coinfused in amounts (20 μg·min−1·kg−1) reported to selectively inhibit nNOS. Directly measured MABPs of acutely and chronically denervated rats were less than control (15% and 9%, respectively, P < 0.005). Plasma renin concentration (PRC) was reduced by renal denervation (14.5 ± 0.2 vs. 19.3 ± 1.3 mIU/l, P < 0.005) and by nNOS inhibition (12.4 ± 2.3 vs. 19.6 ± 1.6 mlU/l, P < 0.005). NaLoad reduced PRC ( P < 0.05) and elevated MABP modestly ( P < 0.05) and increased sodium excretion six-fold, irrespective of renal denervation and SMTC. The metabolic data demonstrated that renal denervation lowered sodium balance during the first days after denervation ( P < 0.001). These data show that renal denervation decreases MABP and renin secretion. However, neither renal denervation nor nNOS inhibition affects either the renin down-regulation or the natriuretic response to acute sodium loading. Acute sodium-driven renin regulation seems independent of RSNA and nNOS under the present conditions.

Effects of ATP on rat renal haemodynamics and excretion: role of sodium intake, nitric oxide and cytochrome P450

AIM

Adenosine-5'-triphosphate (ATP) affects intrarenal vascular tone and tubular transport via P2 receptors; however, the actual role of the system in regulation of renal perfusion and excretion remains unclear and is the subject of this whole-kidney study.

METHODS

Effects of suprarenal aortic ATP infusion, 0.6-1.2 mg kg(-1) h(-1), were examined in anaesthetised rats maintained on low- (LS) or high-sodium (HS) diet. Renal artery blood flow (RBF, transonic flow probe) and the perfusion (laser-Doppler flux) of the superficial cortex (CBF) and outer and inner medulla (OM-BF, IM-BF) were measured, together with sodium and water excretion and urine osmolality.

RESULTS

Adenosine-5'-triphosphate did not change arterial pressure, RBF or CBF while the effects on medullary perfusion depended on sodium intake. In LS rats ATP increased IM-BF 19 +/- 6%, the effect was prevented by inhibition of nitric oxide (NO) with N-nitro-l-arginine methyl ester. In HS rats ATP decreased OM-BF 16 +/- 3% and IM-BF (7 +/- 4%, not significant); previous inhibition of cytochrome P450 with 1-aminobenzotriazol blunted the OM-BF decrease and reversed the previous decrease of IM-BF to a 13 +/- 8% increase. Inhibition of P2 receptors with pyridoxal derivative (PPADS) abolished medullary vascular responses to ATP. In HS rats pre-treated with PPADS, ATP increased tubular reabsorption, probably via adenosine formation and stimulation of P1 receptors.

CONCLUSION

The data indicate a potential role of ATP in the selective control of renal medullary perfusion, different in sodium depleted and sodium replete rats. The action of ATP appears to be mediated by the NO system and the cytochrome P450 dependent vasoactive metabolites.

Cytochrome P-450 monooxygenases in control of renal haemodynamics and arterial pressure in anaesthetized rats

The renal regulatory role of cytochrome P450 dependent metabolites of arachidonic acid (AA), vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), was examined in anaesthetised rats. We measured renal artery flow (RBF), cortical (CBF) and medullary (MBF) perfusion (laser-Doppler) and medullary tissue nitric oxide (NO, selective electrode), after non-selective inhibition of CYP-450 pathway with 1-aminobenzotriazole (ABT, 10 mg/kg i.v.) or after selective inhibition of 20-HETE synthesis with HET0016 (Taisho Co, Yoshino-cho, Japan), infused into renal artery at 0.3 mg/kg/h or into renal medulla at rates increasing from 0.15 to 1.5 mg/kg/h. ABT caused significant (by 13.7%) decrease in RBF without changing MBF. Renal arterial HET0016 increased MBF (not RBF or CBF) from 152+/-12 to 174+/-12 perfusion units (+16%, P<0.001), while medullary tissue nitric oxide was significantly increased (P<0.001). After renal medullary HET0016, renal perfusion indices were significantly higher than after HET0016 solvent (beta-cyclodextrin). Total renal blood flow seems to be under vasodilator control of EETs whereas renal medullary perfusion under tonic suppression by 20-HETE. The data document, for the first in the whole kidney studies, the functional antagonism of 20-HETE and NO.

Effects of sodium intake on plasma potassium and renin angiotensin aldosterone system in conscious dogs

AIMS

The operating range of the renin-angiotensin-aldosterone system is ill-defined. This study quantifies renin-angiotensin-aldosterone system activity as a function of sodium intake.

METHODS

Renin-angiotensin-aldosterone system variables were measured daily after a sudden reduction in sodium intake (3.0-0.5 mmol kg(-1) day(-1)) or at steady states generated by eight levels of sodium intake (0.5-8.0 mmol kg(-1) day(-1)). Potassium intake was 2.79 +/- 0.03 mmol kg(-1) day(-1). Arterial blood pressure was measured invasively. Hormone concentrations were determined by radioimmunoassays. Glomerular filtration rate and plasma volume were determined by standard methods.

RESULTS

Sudden sodium intake reduction doubled plasma renin activity and angiotensin II, and tripled aldosterone on day 1 with only small non-significant additional changes on the following days. Different levels of sodium intake did not affect arterial blood pressure, heart rate, and plasma concentrations of sodium, angiotensinogen, atrial natriuretic peptide, vasopressin, glomerular filtration rate and diuresis. With increasing sodium intake, plasma volume increased by 0.47 +/- 0.04 mL (kg body mass)(-1) (unit increase in Na intake)(-1) (P < 0.01), and plasma potassium decreased with the slope -0.038 mm [(mmol Na+ intake) (kg body mass)(-1) day(-1)](-1) (P = 0.001) while plasma renin-activity, angiotensin II, and aldosterone decreased systematically as expected.

CONCLUSIONS

A step reduction in sodium intake alters renin-angiotensin-aldosterone system activity on day 1 with little further change the subsequent 4 days. Week-long increases in sodium intake decreases renin-angiotensin-aldosterone system activity, increases plasma volume, and decreases plasma potassium. Isolated decreases in sodium intake increase aldosterone secretion via volume-mediated action on the renin-angiotensin system and via increases in plasma potassium.

Effects of renal nerve stimulation on intrarenal blood flow in rats with intact or inactivated NO synthases

AIM

We studied a possible action of nitric oxide (NO), an intrarenal vasodilator, to buffer a decrease in renal perfusion induced by electrical stimulation of renal nerves (RNS).

METHODS

In anaesthetized rats RNS was performed (15 V, 2 ms pulse duration) for 10 s at the frequencies of 2, 3.5, 5 and 7.5 Hz. The total renal blood flow (RBF), an index of cortical perfusion, was measured using a Transonic probe on the renal artery. The outer and inner medullary blood flow (OMBF, IMBF) was measured by laser-Doppler flowmetry. The effect of RNS on RBF, OMBF and IMBF was determined in rats which were either untreated or pre-treated with L-NAME (0.6 or 1.8 mg kg(-1) i.v.), or S-methyl thiocitrulline (SMTC, 20 microg kg(-1) min(-1) i.v.), a selective inhibitor of neuronal NO synthase (nNOS).

RESULTS

In untreated rats, RNS decreased IMBF significantly less than RBF and OMBF. High-dose L-NAME treatment significantly enhanced the RNS induced decrease of RBF but not of OMBF or IMBF. SMTC treatment significantly enhanced the decrease of IMBF, without affecting the response of RBF or OMBF.

CONCLUSION

At intact NO synthesis the inner medullary circulation is not controlled by renal nerves to the extent observed for the outer medulla or cortex. NO generated by all NOS isoforms present in the kidney buffers partly the intrarenal vasoconstriction triggered by electrical RNS. The NO derived from nNOS seems of particular importance in the control of inner medullary perfusion, interacting with NO generated by endothelial NOS and renal nerves.

Role of prostaglandin cyclooxygenase and cytochrome P450 pathways in the mechanism of natriuresis which follows hypertonic saline infusion in the rat

AIM

The prostaglandin cyclooxygenase (COX) and P450 cytochrome (CYP450) pathways of arachidonic acid metabolism are functionally interrelated and both engaged in control of sodium excretion; the study focused on their contribution to the natriuresis which follows hypertonic saline infusion in the rat.

METHODS

In anaesthetized rats, clearance studies were conducted, supplemented with laser-Doppler measurements of the cortical and medullary blood flow (CBF, MBF), and measurement of medullary tissue admittance (Y), an index of interstitial ion concentration.

RESULTS

Indomethacin (Indo), 5 mg kg(-1) i.v. paradoxically enhanced the natriuresis secondary to intra-aortic suprarenal 5% saline load, further increasing sodium excretion by 385 +/- 73% (P < 0.01). After acute clotrimazole, 10 mg kg(-1) i.v. an inhibitor of CYP450 epoxygenase, the increase in natriuresis was smaller and did not differ from that observed after the drug's ethanol solvent. In rats pre-treated with clotrimazole for 3 days, hypertonic saline loading increased sodium excretion (U(Na)V) to 0.94 +/- 0.22 micromol min(-1) , compared with a significantly greater (P < 0.05) increase to 2.76 +/- 0.48 micromol min(-1) measured in untreated controls. Indo increased U(Na)V twofold, similarly in the clotrimazole and in the control group; in the absence or presence of clotrimazole treatment, COX blockade significantly decreased MBF and increased Y.

CONCLUSION

The data indicate that blockade of the CYP450 epoxygenase significantly impairs excretion of sodium in rats acutely loaded with hypertonic NaCl solution. The paradoxical post-Indo natriuresis is preserved in clotrimazole treated rats, which speaks against the role of CYP450 pathway in the response.

Early effects of renal denervation in the anaesthetised rat: natriuresis and increased cortical blood flow

A novel method of renal denervation was developed based on electro-coagulation of tissue containing most of the sympathetic fibres travelling towards the kidney. Kidney tissue noradrenaline was decreased to 4.7 % of the content measured in the contralateral innervated kidney when studied 3 days postdenervation. The method was utilised in anaesthetised rats to examine the effects of denervation within the heretofore unexplored first 75 min period postdenervation. Sodium excretion (UNaV) increased significantly (+82 %, P < 0.03) over the 25-50 min after denervation. In a parallel group, with a lower baseline UNaV, there was also a significant increase in UNaV (+54 %, P < 0.03) within the first 25 min. The renal perfusion pressure was maintained at a constant value and the glomerular filtration rate did not change after denervation. Renal cortical and medullary blood flows (CBF, MBF) were estimated as laser Doppler flux and medullary tissue ion concentration was estimated as electrical admittance (Y). Following denervation, in both groups CBF increased significantly within the first 25 min (+12 %, P < 0.01 and +8 %, P < 0.05, respectively) while MBF did not change or decreased slightly; Y did not change. The data document the development of natriuresis within the first 25-50 min after denervation. The increase in CBF indicated that, prior to denervation, the cortical, but not medullary, circulation was under a tonic vasoconstrictor influence of the renal nerves. Such a dissociation of neural effects on the renal cortical vs. medullary vasculature has not been previously described.

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.

Mechanism of vasopressin natriuresis in the dog: role of vasopressin receptors and prostaglandins

Renal effects of physiological amounts of vasopressin were studied in conscious dogs during servocontrolled overhydration (2% body wt). During infusion of vasopressin (50 pg . min-1 . kg body wt-1), plasma vasopressin concentration increased to 2.30 +/- 0.20 pg/ml compared with 0.12 +/- 0.03 pg/ml during control (water diuresis). With vasopressin infusion, urine flow was significantly lower (0.30 +/- 0.10 ml/min) and sodium excretion (UNaV) was significantly higher (58.0 +/- 15.8 micromol/min) than without vasopressin (4.6 +/- 0.4 ml/min and 14.4 +/- 4.1 micromol/min, respectively). Deamino-[Cys1,D-Arg8]vasopressin, a V2 receptor agonist (4 pg . min-1 . kg-1), mimicked the antidiuretic response (0.20 +/- 0.03 ml/min) without changing UNaV (9.7 +/- 4.4 micromol/min). Indomethacin given during arginine vasopressin (AVP) infusion suppressed prostaglandin E2 excretion, intensified the antidiuresis (0.10 +/- 0.02 ml/min), and abolished the natriuresis (13.4 +/- 3.7 micromol/min). During AVP infusion, UNaV was highly correlated (r = 0.85) with prostaglandin E2 excretion. Blood pressure, glomerular filtration rate, plasma atrial natriuretic peptide concentration, and the rate of proximal tubule reabsorption (derived from lithium clearance) were similar in all series. The data indicate that, in the dog, physiological amounts of vasopressin can induce natriuresis, probably through activation of non-V2 receptors and the intrarenal synthesis of prostaglandins.

Simultaneous recording of tissue ion content and blood flow in rat renal medulla: evidence on interdependence

The relationship of renal medullary tissue ion concentration and medullary blood flow (MBF) has never been closely evaluated because of limitations of available measuring methods. In an attempt to overcome this difficulty, an integrated probe was developed for simultaneous recording in rat renal medulla of tissue electrical admittance (Y), an index of interstitial ion concentration, and tissue perfusion with blood (laser-Doppler method). During spontaneous-selective MBF variations tissue Y showed inverse changes (r = -0.77, P < 0.001). The inverse correlation of the two variables was also seen after MBF has been reduced (-43%) by indomethacin, 5 mg/kg body wt iv (r = -0.77, P < 0.01). A modest selective MBF reduction (15%) induced by glibenclamide, an inhibitor of ATP-dependent K channels, did not alter medullary tissue admittance. The data support experimentally the concept that the rate of medullary tissue perfusion with blood is one determinant of interstitial solute concentration; however, changes in the latter were demonstrable only with major alterations of the MBF.

Reinvestigation of water diuresis in the rat: role of anesthesia and acute surgery

Renal effects of water loading on urine flow rate (V) and concentration (Uosm) were studied in Wistar rats subjected to minor or extensive surgery, under different kinds and level of anesthesia. In the explanted kidney of chronically prepared rats, under light chloralose anesthesia, 1 h.i.v. hypotonic fluid load increased V of the experimental (left) kidney 11 fold while Uosm decreased from 1181 +/- 142 to 210 +/- 80 mosm kg/H2O. In the acutely exposed kidney, under Inactin anesthesia, V increased 5 fold and Uosm decreased from 785 +/- 170 to 204 +/- 57 mosm/kg H2O. In similar experiments under Nembutal anesthesia V increased 4 fold but the urine did not become hypotonic. After acute surgery under Inactin anesthesia water diuresis can be induced but urine concentration before water loading is impaired. Urine hypotonicity after water loading is not achieved in acute experiments under Nembutal--an unsuitable approach to studies of water diuresis. Thus, a transition from antidiuresis to urine dilution can best be accomplished in the chronic explanted kidney model under light anesthesia and without invasive surgery.

Access to rat kidney with minimal anesthesia and surgery: a new experimental model

Chronic surgical explantation of the left rat kidney out of the abdominal cavity under the flank skin enabled easy access to the organ and ipsilateral urine collection under light chloralose anesthesia without virtually any surgical intervention. The glomerular filtration rate, urine flow and osmolality, sodium excretion, and medullary tissue hypertonicity were similar in the explanted and in the contralateral kidney, whereas p-aminohippurate clearance was 14% lower. The function of the explanted kidney was also compared with that of the kidney acutely exposed in rats under thiobutabarbital anesthesia and rendered euvolemic by isoncotic albumin infusion. Again, in both preparations renal function was comparable except that over time urine osmolality remained stable in the former and fell from 1,385 +/- 195 to 835 +/- 167 mosmol/kgH2O (P < 0.02) in the latter, indicating deterioration of urine concentration. Laser-Doppler probes could be easily applied in the explanted kidney to measure cortical and medullary blood flow. The new experimental model offers some advantages, both over studies using conscious rats and over experiments involving deep anesthesia and acute surgery.

Glucagon increases medullary interstitial electrolyte concentration in rat kidney

In anesthetized rats, tissue electrical admittance of the inner medulla (a measure of total ion concentration in the interstitium), medullary blood flow (laser Doppler technique), and renal clearances were measured simultaneously before and during i.v. infusion of glucagon at 110 and 330 ng.min-1.kg-1 body weight. Admittance increased modestly, 5.4% after a large glucagon dose (p < 0.01), whereas medullary blood flow was stable. Glomerular filtration rate increased transiently and then fell during high-dose glucagon infusion. The increase in tissue electrolyte (mostly NaCl) concentration in the medulla observed with stable medullary blood flow and decreasing glomerular filtration rate indicates that stimulation of NaCl reabsorption in the medullary ascending limb of Henle's loop by glucagon was the mechanism underlying augmentation of medullary ionic hypertonicity. This suggests that glucagon can contribute to the urine concentration process.

Modulation of renal medullary ionic hypertonicity by prostaglandins: data from tissue admittance studies in the rat

1. Modulation of the cortico-papillary electrolyte gradient by prostaglandins (PG) was studied in the kidney of anaesthetized rats. The intrarenal PG activity was varied by synthesis blockade with indomethacin (Ind) or meclophenamate (Me) and by intrarenal infusion of prostaglandin E2 (PGE2). 2. The interstitial electrolyte concentration in the medulla was continuously recorded in the kidney in situ as tissue electrical admittance (reciprocal impedance); the total renal blood flow (RBF), inulin clearance (Cin) and renal excretion were measured simultaneously. 3. Indomethacin and Me (15 mg kg-1 h-1) increased tissue admittance 15-20% in the inner and 12-15% in the outer medulla (P < 0.001) whereas PGE2 (300 ng kg-1 min-1) decreased admittance 14 and 8%, respectively (P < 0.01). 4. Renal blood flow and Cin were not affected by intrarenal PG activity changes. There was an increase in urine concentration after PG blockade and a delayed decrease after PGE2 infusion. 5. A joint analysis of the dynamics of medullary tissue admittance, renal haemodynamics and renal excretion provides evidence that PGs modify the medullary ionic hypertonicity by affecting NaCl transport in the ascending limb of the loop of Henle.

Role of vasopressin V2 receptors in modulation of the renal cortico-papillary NaCl gradient

In order to examine if modulation by vasopressin of NaCl transport in Henle's loops (via V2 receptors) can significantly modify medullary ionic hypertonicity, the effects of stimulation or inhibition of these receptors were studied in anaesthetized Wistar rats. Total electrolyte concentration in the medullary interstitium was continuously measured as tissue admittance (reciprocal impedance), using needle electrodes recording from the inner and outer medulla of the in situ kidney. Deamino-[Cys1, D-Arg8]vasopressin (dDAVP], a V2 agonist, infused i.v. at 7.5 ng.min-1.kg-1, significantly increased admittance by 9% and 8% in the inner and outer medulla, respectively. A slightly pressor i.v. infusion of natural arginine vasopressin (AVP) induced pressure natriuresis and did not affect medullary electrolyte concentration. Inhibition of V2 receptors with [d(CH2)5, D-Phe2, Ile4]-AVP, infused i.v. at 133 micrograms.h-1 kg-1 in indomethacin-treated rats, decreased admittance (significant in the inner medulla). Neither of the three agents used caused significant changes in the renal blood flow (RBF) or clearance of inulin (Cin). The demonstration that changing activity of V2 receptors affects the corticopapillary NaCl gradient indicates that, at least in rodents, stimulation of loop salt transport by AVP may represent an additional mechanism enhancing urine concentration.

Conductance studies of rat renal medulla for rapid estimation of extracellular electrolyte concentration

A method is described for estimation of tissue extracellular electrolytes in fresh excised rat renal medulla on the basis of its electrical conductance. The inner medulla was placed between two standard flat electrodes and the conductance was measured at a frequency of 3.5 kHz. The data were corrected for the surface area and thickness of the tissue fragment. The standard conductance so determined clearly correlated with total electrolyte concentration, and Na+ and 2(Na+ + K+) concentrations in the whole tissue (r = 0.84, 0.81 and 0.77, respectively; P < 0.001). The method enables a rapid and simple estimation of medullary extracellular electrolytes, a variable that is physiologically more meaningful than whole tissue electrolyte concentration.