Fractional Water Excretion Research Articles

Impaired free water excretion in child C cirrhosis and ascites: relations to distal tubular function and the vasopressin system

Water retention in advanced cirrhosis and ascites may involve disturbances in renal distal tubular function and in the vasopressin system. Twelve patients with Child B cirrhosis and ascites were compared with 11 patients with Child C cirrhosis and ascites. The subjects were studied during a 400 ml/h oral water load. Child C patients had a lower baseline glomerular filtration rate (32 vs 63 ml/min, P<0.001) and a lower urinary flow rate (V(u)) (0.86 vs 1.95 ml/min, P<0.001) than the Child B patients. However, the free water clearance (C(H2O)) did not differ (-0.60 vs -0.21 ml/min, P=0.20). After the water loading, plasma vasopressin (AVP) decreased significantly in both the groups (P<0.05). The Child B patients had increased V(u) (1.95-3.24 ml/min, P<0.001) and C(H2O) (-0.21-1.21 ml/min, P<0.01) and distal fractional water excretion (10.5 vs 0% in Child C, P=0.01) and aquaporin-2 (AQP2) (P<0.058) after water loading. In contrast, the Child C patients did not have increased V(u) and C(H2O) in response to the water and the decrease in AVP. Furthermore, the markers of distal tubular water regulation, AQP2 excretion and distal fractional water excretion, were unaltered. In Child C cirrhosis, ascites and mild hyponatraemia, there is an impaired ability to excrete solute-free water. The patients are characterised by a low glomerular filtration rate, a low distal tubular flow and an inability to increase free water clearance during water loading. This may be related to a vasopressin-independent production of AQP2.

Antidiuretic Effects of Endothelin A Receptor Antagonism

Selective ET‐A receptor antagonism has diuretic and natriuretic effects which may be potentiated by the unopposed activation of ET‐B receptors by endogenous endothelin. A major side effect of selective ET‐A receptor antagonists is fluid retention and edema. We hypothesized that compounds with high selectivity against ET‐A receptors produce fluid retention at concentrations where non‐specific blockade of ET‐B receptors may occur. Incremental doses of the ET‐A receptor antagonist SPP301 (0.003; 0.03; 3 mg/kg; lower dose to target ET‐A receptor, highest dose to target both ET‐A and ET‐B receptors based on a CHO cell assay) were administered i.v. to anesthetized rats (SD, n=6) undergoing saline diuresis (15 mL/kg/h, 1% BSA). Urine was collected from the bladder while blood pressure (BP) and glomerular filtration rate (GFR) were monitored. SPP301 decreased urine output (49; 46; 32 ul/min, p&lt;0.05) and fractional excretion of water (6; 5; 4 %, p&lt;0.05) in a concentration‐dependent manner. Sodium excretion was initially increased but returned to control values after the highest dose of SPP301. GFR was unchanged while BP was reduced by 10 mmHg only by the highest dose of SPP301. Administration of BQ‐788 (ET‐B receptor antagonist, 3mg/kg) after SPP301 3 mg/kg did not further decrease urine output or water excretion and was without effect on GFR. The highest dose of SPP301 caused a decrease of both 24‐hour diuresis (18.3%) and natriuresis (17.6%) with a consequent hematocrit decrease (8.3%) in a hypertensive transgenic model expressing both human renin and angiotensinogen genes. These data indicate that increasing concentrations of SPP301 may also block ET‐B receptors and cause antidiuresis. This effect could explain why fluid retention and edema occur during treatment with selective ET‐A receptor blockers.

Effects of terlipressin on the aquaretic system: evidence of antidiuretic effects

The vasopressin analog terlipressin is believed to cause vasoconstriction selectively by V1 receptor stimulation. However, a possible antidiuretic effect by V2 receptor stimulation has never been ruled out. Twenty-two patients with ascites, including seven with refractory ascites, were included. The subjects were studied during a 400 ml/h oral water load before and after infusion of 2 mg of terlipressin (18 patients) or placebo infusion (4 patients). Effects on the V2 receptors were assessed by evaluating aquaporin (AQP)2 excretion, free water clearance (C(H2O)), urine osmolality (Uosm), and fractional distal water excretion (DFeH2O). After terlipressin the excretion of AQP2 increased by 89% [144 ng/mmol creatinine, 95% confidence interval (CI) 73-214 ng/mmol creatinine, P = 0.001]. C(H2O) decreased 1.05 ml/min (from 0.17 to -0.89 ml/min, P = 0.001), and DFeH2O decreased 37% (19 vs. 12; 95% CI 2-11, P = 0.01). Uosm increased by 27% (93 mosmol/kgH2O, 95% CI 23-164 mosmol/kgH2O, P = 0.02). Plasma sodium decreased 1.1 mmol/l (P < 0.01). An increase in AQP2 excretion and a decrease in C(H2O) and distal water excretion after terlipressin despite water loading is a clear indication of activation of the antidiuretic system (V2 receptor effect).

Will changing maintenance intravenous fluid from 0.18% to 0.45% saline do more harm than good?

The recommended change in maintenance intravenous fluid in children from 0.18% to 0.45% saline might cause more children to develop hypernatraemia than it would prevent children from developing hyponatraemia, and...

P1,P4-Diadenosine Tetraphosphate (Ap4A) Inhibits Proximal Tubular Reabsorption of Sodium in Rats

Background/Aims: P¹,P⁴-diadenosine tetraphosphate (Ap₄A) is a vasoactive dinucleotide possessing natriuretic activity. It is unclear, however, which part of the nephron is the target site of action for Ap₄A. Methods: We evaluated the tubular sites of Ap₄A action using the lithium clearance technique. Results: Ap₄A at a priming dose of 2 µmol/kg with subsequent infusion at 20 nmol/kg/min increased fractional water and sodium excretion 2.5- and 5.6-fold, respectively. Moreover, Ap₄A increased lithium clearance 1.9-fold and fractional lithium excretion 2.8-fold. Fractional water and sodium excretion from distal nephron segments was not significantly affected by Ap₄A. Conclusion: These results suggest that Ap₄A induces natriuresis mainly through inhibition of proximal tubular reabsorption of sodium.

Glomerular and tubular function during AT1 receptor blockade in pigs with neonatal induced partial ureteropelvic obstruction

Previously, we showed that neonatal induced chronic partial unilateral ureteral obstruction (PUUO) of the multipapillary pig kidney decreased glomerular filtration rate (GFR) of the obstructed kidney. We hypothesized that ANG II and nitric oxide (NO) are important for the changes in renal function and in the present study we examined the effects of chronic AT1 receptor blockade using CV-11974 (0.12 mg/h candesartan from age 23 to 30 days) on kidney function development after PUUO was induced in 2-day-old piglets. Moreover, the effect of superimposed acute NO inhibition using N(G)-nitro-l-arginine methyl ester (l-NAME; 15 mg/kg) was examined to identify if this has diagnostic potential. PUUO significantly increased GFR in the nonobstructed contralateral kidney independent of candesartan. In candesartan-treated piglets, the l-NAME-induced GFR reduction seen in normal and nonobstructed kidneys was absent in the partial obstructed kidneys. Urine output and fractional excretion of water were increased from the partial obstructed kidneys. Consistent with this immunohistochemical analyses showed a reduced aquaporin-2 labeling in the collecting duct principal cells. Moreover, renal sodium handling was compromised by PUUO evidenced by an increased fractional excretion of sodium which was enhanced by candesartan treatment. In conclusion, our findings suggest that the counterbalance between AT1 receptor-mediated vasoconstriction and NO-mediated vasodilatation which maintain GFR in normal young porcine kidneys is changed by neonatal induced chronic PUUO. This may have diagnostic potential in children with suspected congenital obstruction. Our results also demonstrate compromised tubular functions in response to chronic PUUO despite preservation of glomerular function.

Urinary NO3 excretion and renal failure in indinavir-treated patients

Treatment with indinavir (IDV), a protease inhibitor, is frequently associated with renal abnormalities. We determined the incidence of renal failure (creatinine clearance <80 mL min-1 1.73 (m(2))-1) in HIV patients treated with highly active antiretroviral therapy, including IDV, and investigated the possible mechanisms and risk factors of IDV nephrotoxicity. Thirty-six patients receiving IDV were followed for 3 years. All were assessed for age, body weight, duration of infection, duration of IDV treatment, sulfur-derivative use, total cholesterol, triglycerides, magnesium, sodium, potassium, creatinine, and urinalysis. We also determined renal function in terms of creatinine clearance, urine osmolality and fractional excretion of sodium, potassium, and water. Urinary nitrate (NO3) excretion was measured in 18 IDV-treated patients and compared with that of 8 patients treated with efavirenz, a drug without renal side effects. Sterile leukocyturia occurred in 80.5% of the IDV-treated patients. Creatinine clearance <80 mL min-1 1.73 (m(2))-1 was observed in 22 patients (61%) and was associated with low body weight and the use of sulfur-derivatives. These patients also had lower osmolality, lower urine volume and a higher fractional excretion of water compared to the normal renal function group. Urinary NO3 excretion was significantly lower in IDV-treated patients (809 +/- 181 microM NO3-/mg creatinine) than in efavirenz-treated patients (2247 +/- 648 microM NO3-/mg creatinine, P < 0.01). The lower NO3 excretion suggests that IDV decreases nitric oxide production.

Biphasic effects of ANP infusion in conscious, euvolumic rats: roles of AQP2 and ENaC trafficking

Atrial natriuretic peptide (ANP) acutely promotes water and sodium excretion, whereas subchronic effects involve water retention. Renal hemodynamics, water and sodium excretion, and aquaporin-2 (AQP2) and epithelial Na channel (ENaC) subcellular trafficking were determined in response to continuous ANP infusion in conscious rats, where body sodium and fluid balance was constantly maintained. ANP (0.5 microg x kg(-1) x min(-1)) evoked a transient (peak at 10 min) fivefold diuresis followed by reduced urine production to control levels (30- to 90-min period). The fractional distal water excretion was significantly increased initially and then decreased in response to ANP. There was no change in the subcellular localization of AQP2 and AQP2 phosphorylated in PKA consensus site S256 (p-AQP2) 10 min after ANP infusion. In contrast, after 90 min a marked increase in apical labeling of AQP2 and p-AQP2 was observed in the inner and outer medullary collecting ducts but not in cortical collecting ducts. In support of this, ANP induced plasma membrane targeting of AQP2 in transiently AQP2-transfected cells. ANP infusion evoked an instant increase in renal sodium excretion, which persisted for 90 min. Ten minutes of ANP infusion induced no changes in the subcellular localization of ENaC subunits, whereas a marked increase in apical targeting of alpha- and gamma-subunits was observed after 90 min. In conclusion, 1) ANP infusion induced a sustained natriuresis and transient diuresis; 2) there were no changes in the subcellular localization of AQP2 and ENaC subunits after 10 min of ANP infusion; and 3) there was a marked increase in apical targeting of AQP2, p-AQP2, and alpha- and gamma-ENaC after 90 min of ANP infusion. The increased targeting of ENaC and AQP2 likely represents direct or compensatory effects to increase sodium and water reabsorption and to prevent volume depletion in response to prolonged ANP infusion.

Alterations of the renal function and oxidative stress in renal tissue from rats chronically treated with aluminium during the initial phase of hepatic regeneration

Various indices of renal functions during the early stage of hepatic injury were studied in rats chronically treated with aluminum (Al) lactate. Tubular and hemodynamic parameters were analyzed four days after producing a 65% partial hepatectomy (PH). Water and sodium balances were also studied. Oxidative stress and the activity of Na–K–ATPase were determined in renal tissue. The rats were distributed in four groups: control, Al, PH, Al + PH. Al did not modify the hemodynamic renal functions and the PH-group reduced the glomerular filtrate rate (GFR). The Al + PH group presented a decrease in the renal blood flow and accentuated the GFR fall as compared with PH. The fractional excretion (FE) of water and sodium increased in the PH group. The rats chronically treated with Al and then submitted to the PH protocol developed a further increase in FE of water but a reduction in FE of sodium. Both PH and Al promoted an increase in the aldosterone. PH and Al induced a similar increase of the lipoperoxidation status with reduction of glutathione (GSH) and the activity of glutathione peroxidase (GSH-Px). The data indicated that Al is an inhibitor of catalase. The GSH and GSH-Px activity in the Al + PH group demonstrated a synergic effect of Al and PH. This work demonstrates that rats treated chronically with Al and submitted to another injury (such as hepatic damage) can aggravate renal functions, probably by increasing the oxidative state, at least in kidneys.

Altered renal response to acute volume expansion in transgenic rats harboring the human tissue kallikrein gene

The renal response to acute volume expansion was investigated in transgenic (TGR) rats harboring the human tissue kallikrein gene. After a primer injection of 0.9% NaCl (3 ml/100 g, i.v), Sprague–Dawley (SD) or TGR rats received a continuous infusion of 0.9% NaCl (15 μl/100 g/min, i.a.) through a catheter placed into the carotid artery. Acute volume expansion was produced by a second injection of 0.9% NaCl (2 ml/100 g, i.v.) 65 min after the first injection. Plasma vasopressin (AVP) and atrial natriuretic peptide (ANP) concentration was measured before and within 10 min of volume expansion. TGR animals presented a blunted response to acute volume expansion evidenced by an attenuated increase in total and fractional water and sodium excretion. Before or after volume expansion, plasma AVP and ANP did not differ between SD and TGR. Pre-treatment with the BK-B 2 antagonist HOE-140 (7.5 μg/100 g. i.a) partially improved the renal response of TGRs and severely blunted the response in SD rats. These data show that TGR (hKLK1) rats have an impaired renal response to acute volume expansion that can not be accounted for by changes in the release of AVP or ANP.

Effects of maternal hypercholesterolemia on pregnancy and development of offspring.

The effect of maternal hypercholesterolemia on the course of pregnancy and the development of offspring was investigated. Rats were fed either an enriched-cholesterol diet (HC) or a standard diet (control) from 1 week before mating until weaning of offspring. Compared with the control group, HC dams showed a fourfold increase in abortions, a twofold increase in neonatal mortality, smaller litter size, and lower birth weight of pups. At weaning, Na(+),K(+)-ATPase activity in the outer renal medulla was reduced in HC pups compared with control pups, suggesting retarded or impaired development of medullary nephron segments. At this point, to better examine the adverse effects of maternal hypercholesterolemia, the HC pups were divided into two groups: one fed a cholesterol-enriched diet (HC/hc) and the other a standard diet (HC/nc), while control pups were maintained on the standard diet. In adulthood, the HC/hc group showed growth impairment and reduced renal function, demonstrated by low creatinine clearance (0.24+/-0.04 ml/min per 100 g body weight) and high fractional excretion of sodium, potassium, and water ( P<0.05 vs. control). These effects were partially reversed in the HC/nc group. In this study, neither dams nor offspring developed hypertension. Thus, maternal hypercholesterolemia adversely affected pregnancy outcomes and the development of offspring by inducing abnormalities and thereby reducing renal function.

Lipopolysaccharide-Induced Acute Renal Failure in Conscious Rats: Effects of Specific Phosphodiesterase Type 3 and 4 Inhibition

In conscious, chronically instrumented rats we examined 1) renal tubular functional changes involved in lipopolysaccharide (LPS)-induced acute renal failure; 2) the effects of LPS on the expression of selected renal tubular water and sodium transporters; and 3) effects of milrinone, a phosphodiesterase type 3 (PDE3) inhibitor, and Ro-20-1724, a PDE4 inhibitor, on LPS-induced changes in renal function. Intravenous infusion of LPS (4 mg/kg b.wt. over 1 h) caused an immediate decrease in glomerular filtration rate (GFR) and proximal tubular outflow without changes in mean arterial pressure (MAP). LPS-induced fall in GFR and proximal tubular outflow were sustained on day 2. Furthermore, LPS-treated rats showed a marked increase in fractional distal water excretion, despite significantly elevated levels of plasma vasopressin (AVP). Semiquantitative immunoblotting showed that LPS increased the expression of the Na(+),K(+),2Cl(-)-cotransporter (BSC1) in the thick ascending limb, whereas the expression of the AVP-regulated water channel aquaporin-2 in the collecting duct (CD) was unchanged. Pretreatment with milrinone or Ro-20-1724 enhanced LPS-induced increases in plasma tumor necrosis factor-alpha and lactate, inhibited the LPS-induced tachycardia, and exacerbated the acute LPS-induced fall in GFR. Furthermore, Ro-20-1724-treated rats were unable to maintain MAP. We conclude 1) PDE3 or PDE4 inhibition exacerbates LPS-induced renal failure in conscious rats; and 2) LPS treated rats develop an escape from AVP in the CDs, which could be aimed to protect against water intoxication in septic conditions associated with decreased GFR and high levels of AVP.

RAT RENAL FUNCTION FOUR DAYS AFTER BILE-DUCT LIGATION: EFFECTS OF INDOMETHACIN AND VASOACTIVE AGENTS

The purpose of this study was to assess the effects of the cyclooxygenase inhibitor, indomethacin, and some vasoactive agents on the renal functional parameters during the early liver injury induced by four days bile duct ligation (BDL). Wistar rats with four days-BDL and control-sham operated were used. Renal function was measured in anesthetized rat treated with a single dose of indomethacin (control, 0.3, 1.0, 3.0 mg/kg b.w.; i.v.) one hour before clearance studies. Sulindac effects were also evaluated (5 mg/kg b.w., i.p). Isolated rat kidney preparations from control and BDL donor rats were used to study renal vascular response to noradrenaline, carbachol or sodium nitroprusside. The bile duct ligation promoted a diminished renal cortical plasma flow (RCPF) on the fourth day post surgery accompanied with a diminution in the glomerular filtration rate (GFR), increased filtration fraction and increased fractional excretion of water and sodium. Indomethacin 0.3 mg/kg induced an increase in GFR and RCPF, maintaining the high filtration fraction in BDL rats. The other doses did not alter these parameters as compared with bile duct ligated rats without treatment, but indomethacin 3 mg/kg caused a significant increase in filtration fraction. Indomethacin induced dose-dependent diminution in natriuresis in sham and BDL groups. Sulindac did not modify hemodynamic parameters, but induced antinatriuresis and antidiuresis in both experimental groups. Maximal vascular responses to noradrenaline measured in isolated rat kidneys were statistically diminished in BDL-rats as compared with controls (C, n = 7; 35.0 ± 2.3 mmHg ml−1 min; BDL-rats, n = 5; 23.8 ± 0.7 mmHg ml−1 min; p<0.02), without changes in EC15. Maximal relaxation induced by sodium nitroprusside in the phenylephrine (PHE)-pre-constricted renal vasculature in control preparations did not differ from that observed in BDL group (C: n = 6; 49.5 ± 2.3%). Values of EC50 were 1.26 ± 0.07 µM (n = 6) in control preparations and 0.34 ± 0.03 µM (n = 4) in kidneys from BDL-rats (p<0.001). Carbachol induced a biphasic relaxation of PHE-pre-constricted renal vasculature. No differences in maximal responses were found. EC50 value of the second phase in BDL group was significantly decreased compared to control preparations (C: n = 6, 0.47 ± 0.05 µM; BDL: n = 6, 0.22 ± 0.03 µM; p<0.001). The present results show that the altered renal function after a short time post bile duct ligation is determined, at least in part, by increased release of arachidonic derivatives in vascular bed and tubular cells. At this stage of liver injury, the alteration in the renal vascular response to different vasoactive agents is remarkable.

Renal water handling in rats with decompensated liver cirrhosis.

The present study was performed to investigate the renal handling of water in rats with decompensated liver cirrhosis. Liver cirrhosis was induced by intraperitoneal administration of carbon tetrachloride twice weekly for 16 wk. Control rats were treated with vehicle. The cirrhotic rats developed severe disturbances in water homeostasis: urine production was decreased and hyperosmotic, the rats had significantly decreased plasma sodium concentration and ascites, and the ability to excrete an intravenous water load was significantly impaired. Plasma concentrations of vasopressin and aldosterone were increased. Mean arterial pressure, glomerular filtration rate (GFR), and fractional lithium excretion were decreased. Acute vasopressin type 2-receptor blockade with the selective nonpeptide antagonist OPC-31260 (800 microg. kg(-1). h(-1)) was performed during conditions whereby volume depletion was prevented by computer-driven, servo-controlled intravenous volume replacement with 150 mM glucose. The aquaretic response to OPC-31260 was similar in cirrhotic and control rats. However, the OPC 31260-induced rises in fractional water excretion (delta V/GFR; +24%) and fractional distal water excretion (delta V/C(Li); +46%) were significantly increased in the cirrhotic rats, where V is flow rate and delta is change. This suggests that vasopressin-mediated renal water reabsorption capacity was increased in the cirrhotic rats. Semiquantitative immunoblotting revealed that the expression of the vasopressin-regulated water channel aquaporin-2 was unchanged in membrane fractions of both whole kidney and inner medulla from cirrhotic rats. Together, these results suggest a relative escape from vasopressin on collecting duct water reabsorption in rats with decompensated liver cirrhosis.

Contribution of endogenous oxytocin to sodium excretion in anaesthetized, surgically operated rats.

In order to determine the possible role of endogenous oxytocin in controlling electrolyte and water excretion in animals whose renal function is being assessed by invasive techniques, rats were anaesthetized and subjected to micropuncture surgery. Clearance measurements were made in the presence and absence of the potent oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2), Thr(4), Orn(8), Tyr(NH(2))(9)]-vasotocin. In rats infused with vehicle alone, glomerular filtration rate (GFR), sodium excretion and urine flow rate remained stable. In contrast, in antagonist-treated rats GFR was modestly reduced (P<0.05), and there were large falls in both absolute and fractional sodium excretion (P<0.01 in each case) and absolute and fractional water excretion (P<0.05 in each case), indicating effects on both filtered load and fractional tubular reabsorption. The antinatriuresis was not accompanied by a change in the fractional excretion of lithium, suggesting that proximal tubular function is unaffected by oxytocin receptor antagonism; nor was it accompanied by a change in the fractional excretion of potassium, suggesting that the tubular effect is located beyond the potassium secretory site, i.e. downstream of the cortical collecting tubule. We conclude that circulating plasma concentrations of oxytocin during anaesthesia and moderate surgery are sufficient to enhance GFR and reduce fractional tubular sodium and water reabsorption. This has important implications for the interpretation of invasive studies such as micropuncture.

Inhibition of pressure natriuresis in mice lacking the AT2 receptor

Inhibition of pressure natriuresis in mice lacking the AT2 receptor. Angiotensin II type 2 (AT2) receptor knockout mice have higher blood pressures than wild-type mice; however, the hypertension is imperfectly defined. We tested the hypothesis that renal mechanisms could be contributory. We conducted pressure-natriuresis-diuresis experiments, measured renal cortical and medullary blood flow by laser Doppler methods, and explored cytochrome P450-dependent arachidonic acid metabolism by means of reverse transcription-polymerase chain reaction. Blood pressure was 15 mm Hg higher in AT2 receptor knockout mice than in controls, and pressure diuresis and natriuresis curves were shifted rightward. At similar renal perfusion pressures (113 to 118 mm Hg), wild-type mice excreted threefold more sodium and water than AT2 receptor knockout mice. Fractional sodium and water excretion curves were shifted rightward in parallel. Renal blood flow ranged between 6.72 and 7.88 mL/min/g kidney wet weight (kwt) in wild-type and between 5.84 and 6.15 mL/min/g kwt in AT2 receptor knockout mice. Renal vascular resistance was increased in AT2A receptor knockout mice. Cortical blood flow readings leveled at 2.5 V in wild-type and 1.5 V in AT2 receptor knockout mice. Medullary blood flow readings ranged between 0.8 and 1.0 V and increased 116% in wild-type mice as renal perfusion pressure was increased. This increase did not occur in AT2 receptor knockout mice. The glomerular filtration rate (GFR) was similar in both groups at approximately 1 mL/min/g kwt. Renal microsomes from AT2 receptor knockout mice had less activity in hydroxylating arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-meter) than controls, whereas renal AT1 receptor gene expression was increased in AT2 receptor knockout mice. Hemodynamic and tubular factors modify renal sodium handling in AT2 receptor knockout mice and may cause hypertension. AT2 receptor disruption induces alterations of other regulatory systems, including altered arachidonic acid metabolism, that may contribute to the intrarenal differences observed between AT2 receptor knockout and wild-type mice.

Effect of crude extract of Stevia rebaudiana on renal water and electrolytes excretion.

To evaluate the effect of crude extract of Stevia rebaudiana on renal water, Na+ and K+ excretion, male Wistar rats (250-350 g each) under antidiuresis or water diuresis conditions, were evaluated. During intravenous infusion of the extract (0.05 mg/min/100 g) no significant differences were detected in mean arterial pressure or renal hemodynamics parameters. In contrast, fractional water and sodium excretion and solute clearance increased significantly, in both groups of animals. In antidiuresis rats the extract significantly increased reabsorption of water by the collecting duct and in water diuresis animals the extract significantly increased free water clearance. The data suggest preferential action of the extract in the proximal tubular cells involved with salt transport mechanism.

Angiotensin-converting enzyme inhibition and AT1 receptor blockade modify the pressure-natriuresis relationship by additive mechanisms in rats with human renin and angiotensinogen genes.

The intrarenal factors responsible for hypertension in double-transgenic rats (dTGR) harboring human renin and human angiotensinogen genes are unclear. The pressure-natriuresis and -diuresis relationships in response to chronic angiotensin-converting enzyme (ACE) inhibition and AT1 receptor blockade were evaluated. Renal renin-angiotensin and nitric oxide (NO) system gene expression was also investigated. Six-week-old dTGR were treated for 3 wk with submaximal doses of cilazapril (10 mg/kg, orally) or losartan (10 mg/kg, orally) or with the drug combination. In untreated dTGR, pressure-natriuresis relationships were maximally shifted rightward by approximately 70 to 80 mmHg, and both renal blood flow (RBF) and GFR were markedly decreased. Submaximal cilazapril and losartan dosages both decreased systolic BP by 30 mmHg and shifted the pressure-natriuresis curves leftward by 25 to 30 mmHg. Cilazapril increased RBF and GFR to values observed in normotensive control animals but did not significantly affect fractional sodium excretion (FENa) or fractional water excretion (FEH2O) curves. In contrast, losartan had no significant effect on RBF or GFR but shifted the FENa and FEH2O curves leftward. The cilazapril and losartan combination completely normalized BP and shifted the pressure-natriuresis curves leftward more than did either drug alone. When cilazapril and losartan were administered at higher doses (30 mg/kg, orally), the two drugs equally shifted the pressure-natriuresis curves leftward, by 50 mmHg. Both drugs increased RBF and GFR; however, only losartan shifted FENa and FEH2O curves leftward. Human and rat renin and angiotensinogen genes were downregulated in dTGR and were increased by losartan and cilazapril treatments, whereas no changes in the expression of rat ACE and AT1A receptor genes were observed. Endothelial NO synthase expression was increased by cilazapril but not by losartan. Neither inducible NO synthase nor neural NO synthase gene expression was affected by drug treatments. Therefore, submaximal ACE inhibition enhanced sodium excretion mainly by increasing RBF and GFR, whereas submaximal AT1 receptor blockade decreased tubular sodium and water reabsorption. The combination of the two drugs produced an additive effect. The ACE inhibitor effects may involve increased endothelial NO synthase expression, perhaps related to the inhibition of bradykinin degradation.

Renal Hypersensitivity to Vasopressin in Congestive Heart Failure

Plasma vasopressin (AVP) levels are often elevated in congestive heart failure (CHF). To determine the significance of AVP in CHF, we performed clearance studies on the UM-X7.1 strain of cardiomyopathic (CM) hamsters with moderate heart failure and age-matched healthy controls. Exogenous AVP or a selective V₂ agonist (0.3 ng·kg^–1·min^–1) reduced the fractional excretion of sodium (FE_Na) and water (FE_H2O) by 40–46% in the control group. Although the CM hamsters exhibited a blunted physiological response to the V₂ agonist, their urinary cAMP levels were fivefold that of normal and reflect an altered regulation of V₂ receptor signalling during CHF. Additional studies also showed that infusion of a V₂ antagonist (0.3 ng·kg^–1·min^–1) produced natriuresis and diuresis in CM hamsters (FE_Na: 7.9 ± 1.1 vs. 4.8 ± 0.6%, p < 0.05; FE_H2O: 2.2 ± 3 vs. 1.5 ± 0.2%, p < 0.05) but did not decrease fluid reabsorption in the normal hamsters. In conclusion, the attenuated renal response to exogenous AVP in CM hamsters may be attributed to an enhanced endogenous AVP response during CHF.

Intrarenal dopamine participation in frusemide diuretic and natriuretic responses to frusemide.

1. Dopamine (DA) involvement in the renal response to frusemide was analysed using the isolated perfused rat kidney preparation. Endogenous DA levels were increased through the infusion of its precursor levodopa (LD) (0.5 or 1 microM) whereas benserazide (50 or 100 microM), an inhibitor of the enzyme L-dopa-decarboxylase, was used to decrease DA synthesis. 2. Frusemide administration (0.3-20 nmols) induced a dose-dependent increase in fractional excretion of water (FE H2O), sodium (FE Na+) and glucose (FEG). FE Na+ elicited by the diuretic was enhanced 30-40% by 0.5 microM of LD (n = 5, P < 0.05), and 60-80% by LD 1 microM (n = 5, P < 0.05). FE H2O produced by the diuretic was enlarged 80-100% by 0.5 microM (n = 5, P = 0.05), and 130-170% by 1 microM of LD (n = 5, P < 0.01). The increase produced by both concentrations of LD on FEG was 200% for the lowest dose of the diuretic (n = 5, P < 0.01), and 90% for the highest (n = 5, P < 0.05). 3. Benserazide (BZ) (100 microM) decreased the F.E. Na+ induced by frusemide (n = 5, P < 0.05) by 32-42%, and completely abolished frusemide effects on FE H2O and FEG (n = 5). 4. In conclusion, our results suggest that endogenous dopamine participates in the frusemide-induced diuresis and sodium excretion within the kidney, and that the participation of extrarenal factors is not essential for this effect. Dopamine may be involved in frusemide-induced inhibition of proximal sodium reabsorption.