Published online before print September 10, 2007, doi: 10.1161/HYPERTENSIONAHA.107.092973
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(Hypertension. 2007;50:958.)
© 2007 American Heart Association, Inc.
Original Articles |
Extracellular Renal Guanosine Cyclic 3'5'-Monophosphate Modulates Nitric Oxide– and Pressure-Induced Natriuresis
From the Division of Endocrinology and Metabolism, Department of Medicine, University of Virginia Health Sciences Center, Charlottesville.
Correspondence to Dr Robert M. Carey, P.O. Box 801414, University of Virginia Health System, Charlottesville, VA 22908-1414. E-mail rmc4c{at}virginia.edu
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Abstract |
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This study addresses the hypothesis that NO- and pressure-induced natriuresis are inhibited when guanosine cyclic 3',5'-monophosphate (cGMP) is prevented from being transported outside its renal synthesizing cells in vivo. Rats received a renal interstitial (RI) infusion of NO donor S-nitroso-N-acetylpenicillamine (SNAP) or SNAP+organic anion transporter inhibitor probenecid (PB) or SNAP+PB+cGMP. SNAP alone increased UNaV (P<0.05 at 1 hour and P<0.005 at 2 hours). In contrast, SNAP failed to increase UNaV when coinfused with PB, but cGMP coinfused with SNAP+probenecid restored the natriuretic response. SNAP alone increased RI cGMP (P<0.05) during the second experimental period. PB abolished the increase in RI cGMP in response to SNAP (P<0.01), but cGMP levels were restored by coinfusion with cGMP. PB also abolished SNAP-induced increases in fractional excretion of Na+ (FENa) and lithium (FELi) (both P<0.01). PB also abolished the rise in RI cGMP and natriuresis induced by raising renal perfusion pressure (RPP) from 100 to 160 mm Hg in rats subjected to a standard pressure-natriuresis protocol and the natriuretic response was rescued by coinfusion with cGMP. RI administration of phosphodiesterase type V (PDE V) reduced both RIcGMP and UNaV in parallel (both P<0.01) without altering RIcAMP. The data demonstrate that export of cGMP from its renal synthesizing cells into the extracellular RI compartment is critical for the natriuretic action of NO donor SNAP or increased RPP and that RI cGMP controls basal Na+ excretion. Extracellular cGMP modulates NO- and pressure-induced natriuresis.
Key Words: sodium • cyclic GMP • nitric oxide • pressure-natriuresis • kidney • interstitial fluid • probenecid
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Introduction |
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Guanosine cyclic 3',5'-monophosphate (cGMP) is a major cell signaling molecule mediating renal sodium (Na+) excretion and blood pressure (BP).1 Studies from our laboratory have demonstrated that renal interstitial (RI) cGMP inhibits renal Na+ reabsorption via a direct action at the renal tubule.2 In vitro studies have suggested that extracellular cGMP controls Na+ transport in renal tubule cells. Chevalier et al3,4 in studies using LLC-PK1 cells in vitro suggested that, during application of an NO donor, cGMP is transported out of renal proximal tubule (RPT) cells by a probenecid (PB)-sensitive organic anion transporter and that extracellular cGMP may act in autocrine or paracrine fashion to inhibit transepithelial Na+ transport. These findings were corroborated by our studies showing that immortalized human RPT cells contain soluble guanylyl cyclase, both synthesize and export cGMP into the extracellular environment and reduce Na+ transport in response to an NO donor.5 Exported cGMP did not reenter the cell, and both the export of cGMP and the inhibition of Na+ transport were abolished by PB. These studies suggested that extracellular extrusion of cGMP may be necessary for inhibition of Na+ transport in vitro. However, the role of extracellular renal cGMP in the control of Na+ excretion in vivo has not yet been established.
Pressure-natriuresis is a term describing the physiological relationship by which an increase in arterial pressure induces an increase in Na+ excretion. The mediators of pressure-natriuresis are unknown. However, several studies suggest that renal generation of NO may be an important contributor to pressure-natriuresis.6 Our recent studies in the rat have demonstrated that an increase in renal perfusion pressure (RPP) increases RI cGMP accompanied by a natriuretic response and that both are abolished by selective intrarenal inhibition of NO synthase, soluble guanylyl cyclase, or protein kinase G.7 Because fractional excretion of Na+ and lithium (Li2+) ion were both enhanced by increased RPP and both were abolished by inhibitors of the intrarenal NO-cGMP system, the results suggested the RPT cell as a target for pressure-natriuresis in vivo.7
The present study was conducted to determine the role of extracellular RI cGMP in NO- and pressure-natriuresis in vivo in the rat. We tested the hypotheses that (1) RI administration of an NO donor increases RIcGMP and induces natriuresis and these responses are inhibited by intrarenal administration of PB and rescued by exogenous cGMP; (2) pressure-natriuresis is inhibited when cGMP is prevented from being exported into the extracellular RI space by PB and is rescued by exogenous cGMP; and (3) RI administration of cGMP-specific phosphodiesterase type-5 (PDE V) induces antinatriuresis accompanied by reduced RI cGMP without alteration in RI cAMP. Our results demonstrate that extracellular cGMP maintains basal Na+ excretion and that export of cGMP from its synthesizing cells modulates both NO- and pressure-induced natriuresis.
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Methods |
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Animal Preparation
General Preparation
Experiments were approved by the Animal Care and Use Committee of the University of Virginia School of Medicine and were conducted in Sprague-Dawley rats (n=90) weighing 250 g. On the day of the experiment, the rats were anesthetized with an intraperitoneal injection of pentobarbital (5 mg/100g) [Protocol 1] or ketamine (60 mg/kg) and xylazine (4 mg/kg) [Protocols 2 and 3]. Catheters were placed in the carotid artery for measurement of mean arterial pressure (MAP) and heart rate and in the internal jugular vein for infusion of inulin. Intravenous infusions were performed at 20 µL/min. After unilateral nephrectomy, the remaining ureter was canulated for collection of urine.
Renal Cortical Interstitial Infusion
Through a midline laparotomy, the remaining kidney was exposed. A polyethylene (PE-10) interstitial infusion microtube was inserted under the kidney capsule and stabilized to the surface of the kidney using Vetabond glue (3 mol/L; Animal Products). The infusion microcatheters were connected to a Harvard pump (model 55 to 222) and substances were infused directly into the RI space at 2.5 µL/min.
Renal Interstitial Fluid Microdialysis Technique
For determination of renal interstitial fluid concentrations of cAMP and cGMP, we used a renal interstitial fluid microdialysis technique that has been developed and successfully used in our laboratory.2,7
Measurement of Renal Function
Glomerular filtration rate (GFR) was measured by inulin clearance as published previously.8 Tubular Na+ reabsorption was quantified in selected experiments by fractional excretion of Na+ (FENa) and RPT Na+ reabsorption was estimated by the fractional excretion of lithium (Li2+); FELi).
Pressure-Natriuresis Model
We used the standard pressure-natriuresis model of Roman and Cowley that has been used previously in our laboratory.7,9
Assays
Cyclic Nucleotides
cAMP and cGMP were measured with an enzyme immunoassay (Cayman Chemical) used previously by our laboratory. For cAMP, the assay has a sensitivity of 20 fmol/mL and a specificity of 100%. For cGMP, the assay has a sensitivity of 0.11 pmol/mL and specificity also of 100%. The intra- and interassay cross reactivity is <0.01% for the other cyclic nucleotide. Relative recovery in vitro for cAMP is 72% and for cGMP is 70% in our microdialysis system.
Plasma and Urinary Na+ and Li2+ Concentrations
Plasma and urinary Na+ and Li2+ were measured by IL-943 flame photometer.
Protocols
Effects of Intrarenal NO Donor Administration With and Without Probenecid or Probenecid+cGMP on RI cGMP and Renal Function
Rats (n=46) were studied during a 1 hour control period during which they received vehicle (V), 2 consecutive 1-hour experimental periods, and a 1-hour postcontrol period during which they again received V infusion. To evaluate the hypothesis that the export of cGMP is important for NO-induced natriuresis, we used a control group (n=8) which received V infusion during the entire 4-hour study; a second group (n=12) that received S-nitroso-N-acetylpenicillamine (SNAP; 0.12 nmol/kg/min) alone during the 2-hour experimental period, a third group (n=9) that received SNAP combined with organic anion transporter inhibitor probenecid (PB; 10 µg/kg/min); a fourth group (n=9) that received PB alone during the experimental period; and a fifth group (n=8) that received cGMP (18 µg/kg per minute) together with SNAP and PB. PB is a prototypical inhibitor of the renal proximal tubule organic anion transport system10,11 and effectively blocks the export of cGMP from cells.4 All RI infusions were at 2.5 µL/min. RIcGMP, UNaV, GFR, FENa, FELi, and MAP were recorded during each control and experimental period.
Effects of Increased Renal Perfusion Pressure on RI cGMP and UNaV in the Presence and Absence of Intrarenal Administration of PB or PB+cGMP
This study was conducted to determine whether inhibition of cGMP export from synthesizing cells into the RI compartment with PB would reduce RIcGMP and abolish pressure natriuresis. Rats (n=27) were uninephrectomized and studied according to our pressure-natriuresis protocol as previously published.7,9 The rats were prepared with a carotid artery catheter for measurement of MAP, a renal infusion micro-catheter for infusion of V or PB or PB+cGMP, and a renal cortical interstitial microdialysis catheter for sampling RIcGMP. RPP was adjusted at approximately 100 mm Hg (normal), or 160 mm Hg (high) by adjusting 2 ultra-micro-Blalock clamps on the aorta, one above the superior mesenteric artery and the other below the left renal artery as previously published,7 and UNaV was quantified in urine collected from an indwelling ureteral catheter. Pressure-natriuresis and RIcGMP were studied with (n=6) and without (n=14) RI infusion of PB or PB+cGMP (n=7). To be sure that volume depletion or RI fluid leakage attributable to renal capsule penetration by microinfusion catheters or microdialysis probes did not influence natriuretic responses to increased RPP, uninephrectomized rats (n=11) were studied under identical conditions except that normal (0.9%) saline was infused intravenously during 2 control collection periods and an experimental period with increased RPP (30 minutes each). During this study, no RI catheters were used; UNaV responses to increased RPP were quantified.
Effects of Intrarenal Administration of PDE V on RIcAMP, RIcGMP, and UNaV
To determine whether a reduction of RIcGMP decreases UNaV, uninephrectomized rats (n=6) were infused directly into the renal cortical interstitial space with PDE V (Calbiochem) at 0.12 U/kg/min or vehicle (control). The infusion rate of PDE V was calculated from a preliminary dose-response study as the dose that induced maximal reduction of RIcGMP without affecting RIcAMP. RIcAMP, RIcGMP, and UNaV were quantified during a 30-minute control period and during 30 minutes of PDE V infusion.
Statistical Analysis
Results are expressed as mean±SE. Data were analyzed by paired Student t test. ANOVA with a repeated measures term was used for multiple comparisons, and P<0.05 was considered significant.
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Results |
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Effects of Intrarenal SNAP With and Without PB, or PB Alone, or SNAP+PB+cGMP on RI cGMP and UNaV
As depicted in Figure 1, SNAP increased RIcGMP from control values of 5.1±0.9 to 8.4±1.7 fmol/min (P<0.05) in the first experimental period and to 10.5±2.3 fmol/min (P<0.05) during the second experimental period, after which RIcGMP returned to baseline levels during the postcontrol period (PC). Overall, SNAP induced an increase in RIcGMP (ANOVA P<0.01). In contrast, PB abolished the SNAP-induced increase in RI cGMP in both experimental periods (P<0.01 from SNAP alone during the first experimental period and P<0.001 during the second experimental period). V time control or PB alone did not influence RI cGMP. RI cGMP coinfusion with SNAP+PB restored RI cGMP levels to those observed with SNAP alone (P<0.0001 from SNAP+PB).
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As shown in Figure 2, RI SNAP infusion increased UNaV from 0.046±0.009 to 0.087±0.022 µmol/min (P<0.05) after 1 hour and to 0.105±0.018 µmol/min (P<0.01) at 2 hours of SNAP infusion followed by a reduction to near-baseline levels (P=NS from control) during PC. SNAP induced an overall increase in UNaV (ANOVA P<0.001). In contrast, SNAP failed to increase UNaV (P=NS) when coinfused with PB during either experimental period. However, the natriuretic response to SNAP alone was rescued when cGMP was coinfused with SNAP+PB (P<0.001 from SNAP+PB). Neither V time control nor PB alone influenced UNaV.
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MAP was not significantly affected by any of the RI infusions (data not shown).
As shown in Figure 3A, GFR was not influenced by intrarenal infusion of SNAP alone, SNAP+PB, V, or PB alone. As shown in Figure 3B, SNAP increased FENa from 1.2±0.3 to 2.0±0.6% (P=NS) in Experimental Period 1 and to 2.9±0.9% (P<0.05) in Experimental Period 2. PB, while not influencing FENa when given alone, abolished the SNAP-induced increase in FENa in Period 2 (P=NS from control and P<0.01 from SNAP alone). As shown in Figure 3C, SNAP increased FELi from 27±3 to 42±7% (P<0.05) in Experimental Period 1 and to 47±6% (P<0.01) in Experimental Period 2 followed by return to baseline levels in PC. PB abolished this response (P=NS from control; P<0.05 from SNAP alone in Period 1 and P=NS from control; P<0.01 from SNAP alone in Period 2). V or PB alone did not affect FELi.
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Effects of Increased Renal Perfusion Pressure on RI cGMP and UNaV in the Presence and Absence of Exogenous Intrarenal PB With or Without cGMP
As shown in Figure 4A and 4D, an increase in renal perfusion pressure (RPP) from 102±2.2 to 157±4.4 mm Hg (P< 0.00001) increased RI cGMP from 4.6±1.2 to 11.0±1.9 fmol/min (P<0.01). However, increasing RPP from 100±2 to 159±3 mm Hg (P<0.00001) during RI infusion of PB abolished the increase in RI cGMP attributable to increased RPP (P=NS from control; P<0.01 from increased RPP alone). However, coinfusion of cGMP with PB restored the cGMP response to increased RPP (P<0.0001 from control; P<0.01 from increased RPP+PB).
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As indicated in Figure 4B, the increase in RPP from 102±2 to 157±4 mm Hg increased UNaV from 0.047±0.01 to 0.49±0.16 µmol/min (P<0.01). The pressure- natriuresis was completely abolished by intrarenal administration of PB (P=NS from RPP 100±2 mm Hg; P<0.01 from RPP 159±3 mm Hg in the absence of PB). However, coinfusion of cGMP+PB rescued the natriuretic response to increased RPP (P<0.01 from RPP 100; P<0.05 from PB alone). As shown in Figure 4C, urine flow rates changed in parallel with UNaV in response to increased RPP in the presence or absence of PB and PB+cGMP.
To demonstrate that volume depletion and the presence of indwelling RI microcatheters did not alter natriuretic responses to increased RPP, we infused normal saline intravenously and repeated the pressure-natriuresis studies in the absence of renal catheters. UNaV was steady at 0.17±0.08 and 0.16±0.04 µmol/min (P=NS) during 2 consecutive 30-minute control periods and increased to 0.56±0.20 µmol/min (P<0.05) in response to increasing RPP from 105±4 to 140±4 mm Hg (P<0.0001).
Effects of Intrarenal Administration of PDE V on RIcAMP, RIcGMP, and UNaV
Figure 5A depicts the change in RIcGMP in response to intrarenal administration of PDE V for 30 minutes. PDE V reduced RIcGMP by 60% (P<0.01). In contrast (Figure 5B), PDE V did not alter RIcAMP levels (P=NS). As shown in Figure 5C, PDE V administration reduced UNaV by 60% (P<0.01). Vehicle time control (data not shown) did not alter RIcGMP, RIcAMP, or UNaV.
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Discussion |
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Previous in vitro studies have shown that RPT cells are able to produce and export cGMP in response to NO donor SNAP and that, when cellular export of cGMP is inhibited with PB, the SNAP-induced reduction in cellular Na+ uptake is inhibited.5 In addition, previous in vivo studies have demonstrated that RI cGMP engenders natriuresis and modulates pressure-natriuresis by a direct tubule mechanism.2,7 The present study was conducted to determine whether extracellular cGMP influences natriuresis and pressure-natriuresis in vivo. Our results demonstrate that (1) RIcGMP levels are increased in response to NO donor SNAP; (2) a quantitative relationship exists between RIcGMP and natriuresis; (3) intrarenal inhibition of the cellular export of cGMP from its synthesizing cells with probenecid inhibits both NO-induced natriuresis and pressure-natriuresis; (4) natriuretic responses to intrarenal SNAP and increased RPP that are abolished by PB can be rescued by exogenous cGMP administration that restores RI cGMP levels to those in the absence of PB; and (5) selective intrarenal inhibition of RIcGMP with cGMP-specific PDE V induces antinatriuresis without change in RIcAMP. These results collectively suggest that extracellular RIcGMP is an important modulator of Na+ excretion by NO and RPP.
The soluble guanylyl cyclase activating molecule NO has been shown to exert potent effects on the kidney, especially increasing renal blood flow, GFR, and UNaV.12–16 Inhibition of cGMP-selective PDE increases cGMP and causes a marked diuresis and natriuresis.17 NO synthase (NOS) inhibition during regulation of RPP reduces Na+ and water excretion, which can be reversed by cGMP analogue, 8-Br-cGMP.14 However, the relationship of NO generation and cGMP production in renal Na+ transport in vivo is not established.18–20 Indeed, controversy exists as to whether NO stimulates or inhibits Na+ transport in specific nephron segments.21–26 Whereas several studies indicate that NO inhibits Na+ transport,21–23 others have suggested that NO and cGMP stimulate proton flux and Na+ and bicarbonate transport in RPT cells,24 that inducible NOS (iNOS) upregulates Na+ transport,25 and that inhibition of NOS increases FENa.26,27 Furthermore, there is little or no published information on the compartmentalization of cGMP within the kidney or the possible role of extracellular RI cGMP in the control of Na+ excretion.28 Our current results demonstrate that extracellular RIcGMP plays an important modulatory role in the physiological control of Na+ excretion in response to changes in renal NO production or RPP. Our studies extend the in vitro results of Chevalier et al3,4 and our own laboratory5 indicating that cGMP is transported out of RPT cells by a PB-sensitive organic anion transporter and that extracellular cGMP acts in autocrine or paracrine fashion to inhibit epithelial Na+ transport.
The present study is the first to demonstrate a PB-sensitive organic anion transporter for cGMP in vivo in the kidney, wherein the export of cGMP appears to modulate natriuretic responses. The importance of RI cGMP was evident both for NO- and pressure-induced natriuresis. Regarding pressure-natriuresis, we demonstrated no difference in UNaV responses whether normal saline or lactated Ringer’s solution was infused or whether microinfusion catheters and microdialysis probes were present or absent. Therefore, it is highly unlikely that RI fluid leakage or hydropenia could have influenced our results. We also demonstrated a reduction in UNaV when RI cGMP was selectively reduced with RI PDE V administration. Because the infused PDE V cannot enter cells because of its molecular size (92 kDa), our results suggest that extracellular RI cGMP plays a role in the control of basal Na+ excretion under normal physiological conditions.
We were interested to determine whether the nephron site of RI cGMP action is the RPT, because our past studies suggested that the RPT cell is likely to be the target cell for RI cGMP in pressure-natriuresis.7 In addition, most studies indicate a major role for the RPT in pressure-natriuresis.6,29,30 Our present data show that NO-induced natriuresis is accompanied by no change in GFR and that the natriuresis is sustained by an increase in both FENa and FELi. Similar to the results with pressure-natriuresis,7 our data suggest a major role for the RPT in NO-induced natriuresis. Although Li2+ clearance studies reflect RPT Na+ transport with a maximum error of 4%,31 micropuncture studies would have to be used to determine the precise nephron segment(s) involved in NO- and pressure-induced natriuresis.
Although the present studies suggest that extracellular RIcGMP is a potentially important modulator of NO- and pressure-induced natriuresis, the physiological, cellular, and molecular mechanisms by which this occurs are unclear. Pressure-natriuresis is accompanied by an increase in renal interstitial pressure,6,32 and whether the pressure-induced rise in RIcGMP is a cause or an effect of this physiological change is unknown. In addition, the biochemical identity of the probenecid-sensitive organic anion transporter responsible for pumping cGMP out of its renal cell of origin is unknown. Several organic anion transporters have been identified,33 and recently multidrug resistance protein-5 (MRP-5) has been found to be highly expressed in the kidney on tubule cell basolateral membranes. MRP-5 mediates ATP-dependent active transport of cGMP across these membranes and would be an appropriate exporter of cAMP and cGMP into the RI compartment.34
Probenecid is a highly lipid soluble benoic acid derivative the actions of which are confined to inhibition of the transport of organic anions across epithelial barriers.35 Probenecid is a uricosuric agent, increasing the renal excretion of uric acid by inhibiting its tubular reabsorption.35 In addition, probenecid has been demonstrated to inhibit the renal tubular export of cGMP.4 There is no available information in the literature on the effects of oral probenecid administration on salt-sensitivity or arterial blood pressure in humans. However, a recent clinical trial has demonstrated that PDE V inhibition with sildenafil reduced blood pressure both acutely and chronically in patients with primary hypertension.36 The magnitude of the blood pressure reduction was similar to that of other commonly used antihypertensive agents.36,37 Because endothelial function as measured by flow-mediated vasodilation was unchanged in that study, the results are consistent with an effect of increased cGMP, possibly in the kidney, to reduce blood pressure.36
The mechanisms whereby extracellular cGMP mediates an inhibition of Na+ transport also are unknown. Natriuretic responses to RIcGMP in vivo have been inhibited by pharmacological inhibitors of protein kinase G,2,7 but activation of PKG residing in the intracellular compartment would not readily explain an extracellular action of cGMP. Observatons from this and previous studies suggest the possibility of a cGMP binding site on basolateral membranes of the renal tubule cells mediating Na+ transport. The intracellular mechanisms whereby such a putative binding site would induce cell signaling to inhibit Na+ reabsorption at present are speculative and await future investigation.
Perspectives
We have shown that both NO- and pressure-induced natriuresis are accompanied by a rise in cGMP levels within the RI compartment. We have demonstrated that intrarenal administration of PB, an organic anion transporter blocker, prevents RIcGMP from rising and abolishes the natriuretic response, which can be rescued by exogenous cGMP. We also showed that reduction of RI cGMP levels with extracellular administration of PDE V, which does not cross cell membranes, engenders antinatriuresis. These results suggest that extracellular RI cGMP plays a role in the control of basal Na+ excretion and is an important modulator of natriuretic responses to renal NO and blood pressure.
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Acknowledgments |
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Source of Funding
This work was supported by NIH grant 1-RO1-HL-081891 (to R.M.C.).
Disclosures
None.
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Footnotes |
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F.A. and B.A.K. contributed equally to the work.
Received May 1, 2007; first decision May 21, 2007; accepted August 20, 2007.
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- Jin X-H, Siragy HM, Carey RM. Renal interstitial guanosine cyclic 3',5'-monophosphate induces natriuresis by a direct tubule mechanism. Hypertension. 2001; 38: 309–316.
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