(Hypertension. 2001;38:309.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
Renal Interstitial cGMP Mediates Natriuresis by Direct Tubule Mechanism
Department of Medicine, University of Virginia, School of Medicine, Charlottesville.
Reprint requests to Dr Robert M. Carey, Box 800793, University of Virginia Health System, Charlottesville, VA 22908. E-mail RMC4c{at}virginia.edu
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Abstract |
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Abstract—— The objective of this study was to test the hypothesis that renal interstitial (RI) cGMP is natriuretic in vivo. In conscious rats (n=8), urinary sodium excretion (UNaV) was significantly greater on days 3 and 4 of RI infusion of cGMP (1.17±0.14 and 1.61±0.11 mmol/24 h, respectively) than during vehicle infusion (0.56±0.15 and 0.70±0.17 mmol/24 h, respectively) (P<0.01). Similarly, UNaV was greater on days 3 and 4 of RI infusion of 8-bromo-cGMP (2.15±0.42 and 2.16±0.1 mmol/24 h, respectively). Protein kinase G inhibitor Rp-8-pCPT-cGMPS reduced cGMP-induced and 8-bromo-cGMP-induced UNaV to control levels. Acute RI infusion of L-arginine (L-Arg, 40 mg · kg-1 · min-1), but not D-arginine, caused an increase in UNaV from 1.65±0.11 to 4.07±0.1 µmol/30 min (P<0.01). This increase was blocked by RI infusion of NG-nitro-L-arginine methyl ester (100 ng · kg-1 · min-1) by the phosphodiesterase (PDE II) activator 5,6DMcBIMP (0.01 µmol/µL), by PDE II (0.03 U · kg-1 · min-1) itself, or by the soluble guanylyl cyclase inhibitor 1-H-[1,2,4]oxadiazolo-[4,2-
]quinoxalin-1-one (ODQ, 0.12 mg · kg-1 · min-1). The PDE II activator also blocked L-Arg-stimulated cGMP levels. The NO donor S-nitroso-N-acetylpenicillamine (SNAP, 0.12 µmol · L-1 · kg-1 · min-1) increased UNaV from 1.65±0.11 to 2.93±0.08 µmol/30 min (P<0.01), and this response was blocked completely by ODQ. Renal arterial but not RI administration of the heat-stable enterotoxin of Escherichia coli induced natriuresis. RA infusion of cGMP (3 µg/min) increased UNaV, renal blood flow (RBF), and glomerular filtration rate (GFR). Renal cortical interstitial cGMP infusion increased UNaV with no effect on total RBF, renal cortical blood flow, or GFR. Similarly, the natriuretic actions of renal interstitial L-Arg or SNAP were not accompanied by any change in RBF or GFR. Medullary cGMP infusion had no effect on UNaV, total RBF, or medullary blood flow. Texas red-labeled cGMP infused via the RI space was distributed exclusively to cortical renal tubular cells. The results demonstrate that RI cGMP inhibits renal tubular sodium absorption via protein kinase G independently of hemodynamic changes. These observations indicate that the cortical interstitial compartment provides a potentially important domain for cell-to-cell signaling within the kidney.
Key Words: cyclic GMP • kidney • natriuresis • sodium • protein kinases
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Introduction |
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Two different guanylyl cyclase (GC) enzymes, soluble GC (sGC) and particulate GC (pGC), generate cGMP.1 sGCs are heterodimers containing heme, which serves as an intercellular receptor for NO.2,3 In contrast, pGCs are cell membrane receptors for endogenous peptides, including the atriopeptin family and the guanylin peptides: guanylin, uroguanylin, and the heat-stable enterotoxin of Escherichia coli (STa).4,5 Thus, cGMP may be derived from either cytosolic or cell surface GCs expressed in different cells, depending on the particular hormone or autacoid stimulated.
The GC-activating agents, NO and atrial natriuretic peptide, exert potent effects in the kidney, especially on renal blood flow (RBF), glomerular hemodynamics, and urinary sodium excretion (UNaV).6–10 Atrial natriuretic peptide or inhibition of cGMP phosphodiesterase (PDE) increases cGMP and causes a marked diuresis and natriuresis.11–13 NO also stimulates cGMP generation in the vascular system and the kidney.14,15 NO synthase (NOS) inhibition during regulation of renal perfusion pressure reduces sodium and water excretion, which can be reversed by the cGMP analogue, 8-bromo-cGMP (8-Br-cGMP).8 However, the relationship of NO generation and cGMP production to renal tubular sodium transport is not established. Indeed, controversy exists about whether NO stimulates or inhibits sodium transport in specific nephron segments.16–20 Although several studies indicate that NO inhibits sodium reabsorption,16,17,20 others have suggested that NO and cGMP stimulate proton flux and sodium and bicarbonate transport in proximal tubule cells18,19 and that inhibition of NOS increases fractional sodium excretion.18
The present study was designed to clarify the role of local generation of cGMP in the renal interstitial space in the control of renal sodium excretion. We demonstrate, for the first time to our knowledge, that renal interstitial cGMP induces natriuresis by a direct tubular mechanism mediated by protein kinase G (PKG).
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Methods |
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Animal Preparation
Sprague-Dawley female rats (body weight 200±15 g, Harlan Teklad, Madison, Wis) were used. The rats were provided free access to normal-sodium (0.28% sodium) or low-sodium (0.08% sodium) diets (Bioserve Biotechnologies) and tap water for 12-hour light-dark cycles unless described otherwise. For chronic studies in conscious rats, animals were placed in metabolic cages. Daily UNaV was monitored. For rats undergoing acute studies, the left jugular vein was cannulated. A venous cannula (PE-50) was connected to a Harvard 22 infusion pump (Harvard Apparatus Inc) through which lactated Ringer’s solution (vehicle) or drugs were infused. The University of Virginia School of Medicine Animal Care Committee approved all animal protocols.
Uninephrectomy and Renal Interstitial Osmotic Minipump Implantation
With the animals under general anesthesia with an intraperitoneal injection of ketamine (60 mg/kg) and xylazine (4 mg/kg), by a sterile technique, we conducted a modified renal interstitial catheter implantation method as described for the kidney.21
Renal Interstitial Fluid Microdialysis
We used a microdialysis method previously described for the kidney.22,23
Renal Artery Catheter Placement
For continuous renal arterial infusion of cGMP, after right nephrectomy under a dissecting microscope, a PE-10 polyethylene tube (0.28 mm ID, 0.61 mm OD) was inserted into the right renal artery stump and pushed into the abdominal aorta. The catheter tip was pushed to a point 2 mm above the left renal artery, gently inserted just inside the left renal artery with the aid of microdissection forceps, and then fixed in place with a 4-0 silk suture around the right renal artery stump. The cannula (PE-10) was connected to a Harvard 22 infusion pump (Harvard Apparatus Inc) through which cGMP was continuously infused at a rate 3 of µg/min for 30 minutes.
Total RBF, RCBF, and RMBF Determination and Urine Collection
A midline celiotomy incision was made. The left renal artery was exposed by careful dissection. Monitoring of RBF was accomplished by inserting a renal artery flow probe on the left renal artery by a dual-channel flowmeter (Transonic Systems Inc). Renal cortical blood flow (RCBF) and renal medullary blood flow (RMBF) were monitored by using a laser flowmeter (Advance Laser Flowmeter ALF 21D). To monitor the urinary flow rate from the left (remaining) kidney, a catheter was inserted into the left ureter, and urine was collected from the kidney. After infusion of pharmacological agents, the left renal artery was exposed once again, and RBF was measured.
Glomerular Filtration Rate
The glomerular filtration rate (GFR) was determined as described24 and is reported as milliliters per minute per gram kidney weight.
Experimental Protocols
Effect of Chronic Renal Interstitial Administration of cGMP, 8-Br-cGMP, and/or Rp-8-pCPT-cGMPS on UNaV in Conscious Rats
To evaluate whether micro-osmotic pump delivery of cGMP or its membrane-permeable analogue 8-Br-cGMP into renal cortical interstitial fluid increases UNaV, we infused rats (n=6 in each group) on normal or low sodium intake for 6 days to increase renal interstitial cGMP continuously beginning on the day of interstitial catheter placement (day 0) for 4 consecutive days. After recovery from the catheter placement procedure, UNaV was quantified on days 3 and 4 after catheter placement. The following drug groups were used: vehicle, cGMP (1 µg/h), 8-Br-cGMP (1 µg/h), cGMP or 8-Br-cGMP plus the PKG inhibitor Rp-8-parachlorophenylthiolguanosine cyclic 3',5'-monophosphothioate (Rp-8-pCPT-cGMPS) (1 µg/h), and Rp-8-pCPT-cGMPS alone.25 For chronic studies in conscious animals, the renal interstitial infusion rate was reduced from that used in the acute studies to approximate the endogenous physiological level of cGMP.15,22,23 All agents were infused at a volume flow rate of 1 µL/h. In rats on low sodium intake, in addition to cGMP and 8-Br-cGMP alone or combined with Rp-8-pCPT-cGMPS, the protein kinase G inhibitor was infused alone to determine whether blockade of cGMP action decreases sodium excretion when endogenous renal cGMP is increased.22 Body weight and sodium and fluid intake during vehicle infusion (control group) and in response to cGMP, 8-Br-cGMP, Rp-8-pCPT-cGMPS, or the combination on days 3 or 4 were quantified. Renal interstitial cGMP levels were measured on day 4 via the microdialysis sampling method in the above treatment groups. Arterial blood pressure was monitored daily for cGMP-infused and 8-Br-cGMP-infused rats by the tail-cuff method with use of a model 179 blood pressure analyzer (IITC Inc)
Effect of Acute Renal Interstitial Infusion of L-Arg, D-Arg, L-NAME, SNAP, cGMP, ODQ, Rp-8-pCPT-cGMPS, and PDE II or PDE Activator on UNaV in Anesthetized Rats
This study was conducted to examine whether renal NO production induces natriuresis and, if so, by which mechanism. Rats (n=6 in each group; total, 84 rats for 14 infusion protocols) were infused directly into the renal cortical interstitial space with 1 of the following pharmacological agents: NOS substrate L-arginine (L-Arg, 10 mg · kg-1 · min-1), inactive substrate D-arginine (D-Arg, 10 mg · kg-1 · min-1), NO donor S-nitroso-N-acetylpenicillamine (SNAP, 0.12 µmol · L-1 · kg-1 · min-1), or cGMP (12 µg · kg-1 · min-1) for 30 minutes. The infusion rate of cGMP was selected as a pharmacological dose that was 4-fold greater than that used in our previous studies in the rat jejunum.26 In separate experiments, the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 100 ng · kg-1 · min-1), cGMP-specific PDE (PDE II, 0.03 U · kg-1 · min-1; a dose selected from the literature that did not alter basal cGMP), soluble guanylyl cyclase inhibitor 1-H-[1,2,4]oxadiazolo-[4,2-]quinoxalin-1-one (ODQ, 0.12 mg · kg-1 · min-1),27 PKG inhibitor Rp-8-pCPT-cGMPS, or PDE II activator 5,6-dimethylbenzimidazole riboside-3',5'-monophosphate (5,6DMcBIMP) (0.12 µmol/L · kg-1 · min-1) was infused alone or combined with L-Arg or SNAP. For all infusion protocols, each rat received a renal interstitial infusion of vehicle for 30 minutes, followed by infusion of the experimental agent for 30 minutes. Thus, each rat served as its own control. Measurements of renal interstitial fluid cGMP levels in response to renal interstitial administration of L-Arg and/or 5,6DMcBIMP also were conducted to determine whether L-Arg mediates sodium transport via cGMP production and whether the interstitial cGMP level was reduced by a specific PDE. Arterial blood pressure was monitored by direct measurement with a catheter in the carotid artery with use of a blood pressure analyzer (Micro-Med Inc) during experiments in which cGMP and L-Arg were infused into the renal cortical interstitium.
Effect of Acute Renal Interstitial or Renal Arterial Infusion of STa
This study was conducted to determine whether pGC can be activated via the renal interstitial space or via a renal arterial route or both. Anesthetized rats (n=6 in each group) were infused with STa (3 U/min for 30 minutes) either directly into the renal cortical interstitial space or into the renal artery, and both UNaV and interstitial cGMP levels were monitored.
Effect of Acute Renal Interstitial or Renal Arterial Administration of cGMP on UNaV, RBF, GFR, RCBF, and RMBF
To determine whether renal sodium transport is mediated by a tubular mechanism or via hemodynamic alterations or both, we infused cGMP (12 µg · kg-1 · min-1) via either the renal cortical or the renal medullary interstitium or via the renal artery for 30 minutes. UNaV, RBF, GFR, RCBF, and RMBF were recorded.
Effect of Acute Renal Interstitial Administration of PDE II on UNaV, cGMP, and cAMP in Anesthetized Rats
To determine whether reduction in renal interstitial cGMP decreases UNaV, rats (n=6) were infused directly into the cortical interstitial space with PDE II at 0.12 U · kg-1 · min-1, a 4-fold increase in infusion rate over that used to block UNaV responses to L-Arg above. This infusion rate of PDE II was obtained from a dose-response study as the dose that induced maximal reduction in cGMP without influencing cAMP. UNaV and renal interstitial cGMP and cAMP were measured during a control period and during 30 minutes of PDE II infusion.
Distribution of cGMP After Renal Interstitial Administration
Rats (n=3) in each group were anesthetized and subjected to right nephrectomy. A renal interstitial microdialysis catheter was implanted in the renal cortex of the left kidney as described above. Ringer’s solution (30 µL) containing either Texas red-conjugated cGMP (1 µg · kg-1 · min-1) or Texas red alone was infused during a 10-minute period into the renal cortical interstitium through the implanted catheter. The kidney was collected, and 5-µm frozen sections were examined immediately with a Nikon PCM 2000 laser scanning confocal microscope by using an HeNe laser with an excitation wavelength of 543 nm and an emission wavelength of 570 nm.
Analytical Methods
A Nova Analyzer (Nova Biomedical) was used to measured UNaV. Renal interstitial fluid cGMP levels were measured with an enzyme immunoassay kit (Cayman Chemical).
Statistical Analysis
Results are presented as mean±1 SE. Data were analyzed by Student’s paired or unpaired t test as appropriate. ANOVA was used for multiple comparisons, and a value of P<0.05 was considered statistically significant.
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Intrarenal Distribution of cGMP via Renal Cortical Interstitial Infusion
After renal cortical interstitial infusion, Texas red-conjugated cGMP was localized on cortical renal tubular cells (Figure 1A). No fluorescence signal was detected in blood vessels, glomeruli, or the medullary regions of the kidney. High-power examination demonstrated that the Texas red-conjugated cGMP was localized largely to the basolateral membrane regions of the tubular cells (Figure 1B) and that little, if any, signal was present on the apical (luminal) surfaces or intracellularly. As a control, when unconjugated Texas red was infused, no signal was detected (Figure 1C).
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Effect of Chronic Renal Cortical Interstitial Administration of cGMP, 8-Br-cGMP, and/or Rp-8-pCPT-cGMPS on UNaV in Conscious Rats
As shown in Figure 2, in rats on normal sodium intake, UNaV was significantly greater during interstitial cGMP and 8-Br-cGMP infusion than during vehicle infusion. During vehicle infusion, UNaV was 0.56±0.15 and 0.7±0.17 mmol/24 h on days 3 and 4, respectively. During cGMP infusion, UNaV was 1.17±0.14 and 1.61±0.11 mmol/24 h (both P<0.01 from respective vehicle) on days 3 and 4, respectively. During 8-Br-cGMP infusion, UNaV was 2.15±0.42 mmol/24 h (P<0.01 from vehicle) and 2.16±0.1 mmol/24 h (P<0.001 from vehicle) on days 3 and 4, respectively. The PKG inhibitor Rp-8-pCPT-cGMPS completely blocked the natriuretic action of both cGMP and 8-Br-cGMP during normal sodium intake on days 3 and 4 (Figure 2). On day 4, renal interstitial fluid cGMP levels were increased in response to both cGMP (P<0.01) and 8-Br-cGMP (P<0.01) infusion and were not increased further by Rp-8-pCPT-cGMPS (Figure 2, inset). There were no significant changes in systemic arterial blood pressure from control values during vehicle infusion (106±6 mm Hg) in response to 8-Br-cGMP on either day 3 (105±7 mm Hg) or day 4 (108±8 mm Hg). In rats on low sodium intake, UNaV decreased from 0.56±0.15 and 0.70±0.17 to 0.25±0.06 and 0.13±0.08 mmol/30 min (P<0.01) on days 3 and 4, respectively. Neither cGMP nor Rp-8-pCPT-cGMPS by interstitial infusion influenced UNaV in sodium-deficient rats (data not shown).
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Effect of Acute Renal Cortical Interstitial Infusion of L-Arg, L-NAME, SNAP, ODQ, PDE, cGMP, Rp-8-pCPT-cGMPS, and STa on UNaV in Anesthetized Rats During Normal Sodium Intake
As shown in Figure 3, L-Arg increased UNaV from a control value of 2.47±0.3 to 4.07±0.11 µmol/30 min (P<0.01). D-Arg, the inactive enantiomer of L-Arg, had no effect on UNaV. The increase in UNaV due to L-Arg was blocked completely by L-NAME, by cGMP phosphodiesterase activator 5,6DMcBIMP, by PDE II itself, and by ODQ. The NO donor SNAP and cGMP increased UNaV from a control value of 2.03±0.28 to 3.11±0.1 and 2.57±0.12 µmol/30 min (P<0.01), respectively. The natriuretic response to SNAP was blocked completely by ODQ, and the natriuretic response to cGMP was blocked by Rp-8-pCPT-cGMPS. L-NAME, PDE II, 5,6DMcBIMP, and ODQ at the same infusion rates alone did not influence UNaV. Systemic arterial blood pressures were unchanged in response to unilateral nephrectomy, cGMP, or L-Arg administration: control 119±5 mm Hg, 10 days after unilateral nephrectomy 117±6 mm Hg, cGMP 114±7 mm Hg, and L-Arg 117±6 mm Hg (P=NS). As depicted in Figure 4, STa infused into the renal interstitial space had no effect on UNaV. STa infused into the renal artery increased UNaV from a control value of 1.3±0.3 to 2.3±0.2 µmol/30 min (P<0.01). However, renal artery STa infusion did not change renal interstitial fluid cGMP levels (Figure 4).
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Effect of Acute Renal Cortical, Medullary Interstitial, or Renal Arterial Administration cGMP on UNaV, RBF, and GFR in Anesthetized Rats
Renal arterial or renal cortical interstitial cGMP infusion increased UNaV from 3.3±0.03 to 6.0±0.6 µmol/30 min (P<0.05) or to 9.3±0.3 µmol/min (P<0.01), respectively (Figure 5A). However, renal medullary infusion of cGMP did not alter UNaV. Renal arterial infusion of cGMP increased RBF from 8±0.2 to 10.2±0.5 mL/min (P<0.01, Figure 5B). Neither renal cortical nor medullary cGMP infusion altered RBF. Renal arterial cGMP infusion increased GFR from 1.8±0.7 to 3.4±0.9 mL · min-1 · g kidney wt-1 (P<0.01), whereas renal cortical infusion of cGMP had no effect on GFR (Figure 5C).
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Effect of Acute Renal Cortical or Medullary Interstitial Infusion of cGMP on UNaV, RCBF, or RMBF
Figure 6 shows that renal cortical interstitial infusion of cGMP increased UNaV from 2.07±0.11 to 3.19±0.09 µmol/30 min (P<0.01) but did not alter RCBF. Renal medullary infusion of cGMP did not affect either UNaV or RMBF (Figure 6).
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Measurement of Renal Interstitial Fluid cGMP in Response to Renal Interstitial Administration of L-Arg and/or 5,6DMcBIMP
L-Arg increased cGMP from a control of 0.23±0.03 to 0.38±0.02 pmol/30 min (P<0.01). This response was blocked by 5,6DMcBIMP to 0.18±0.04 pmol/30 min, but 5,6DMcBIMP at the same infusion rate alone did not alter renal cGMP (0.17±0.03 pmol/30 min).
Effect of Acute Renal Cortical Interstitial Infusion of L-Arg on UNaV, RBF, RCBF, RMBF, and GFR
L-Arg increased UNaV from 2.0±0.4 to 3.5±0.8 µmol/30 min (P<0.01) (Figure 7). The increase in UNaV was unaccompanied by any change in RBF, RCBF, RMBF, or GFR.
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Effect of Acute Renal Cortical Interstitial Infusion of PDE II on UNaV, cGMP, and cAMP
PDE II at 4-fold the infusion rate that blocked UNaV responses to L-Arg reduced basal cGMP levels (Figure 8A). This infusion rate of PDE II caused a significant decrease in UNaV (Figure 8C) but did not alter cAMP (Figure 8B).
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Discussion |
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cGMP is a major signaling molecule mediating the biological actions of NO. NO stimulates sGC, the mRNA of which has been detected in the interlobular arteries of the kidney, the afferent arterioles, proximal tubule, and cortical collecting duct cells.28,29 Functional evidence for the enzyme in proximal tubular cells also has been provided by Roczniak and Burns,17 who determined that inhibition of sGC abrogated the stimulation of cGMP production by NO.
Recently, Eitle et al30 demonstrated in isolated proximal tubules and in split-droplet micropuncture studies that NO stimulated cGMP production, which not only inhibited angiotensin II-stimulated proximal sodium and water absorption but also reduced fluid uptake in the absence of peritubular angiotensin II. The present study suggested that cGMP may have a direct action to inhibit renal tubular sodium absorption. However, the role of cGMP and its mechanism of action in the control of tubular cell sodium transport have been controversial,16–20 and the role of cGMP in the control of sodium excretion in vivo is not established.
We administered cGMP and analogues directly into the renal interstitial space because past studies from our laboratory have indicated that the cyclic nucleotide is pumped out of renal cells into the interstitial fluid in large quantities in response to angiotensin II via stimulation of the type 2 angiotensin receptor.22,23 To determine the renal cellular distribution of cGMP that was extruded into the interstitial space, we infused Texas red-conjugated cGMP into the renal cortical interstitium. The cyclic nucleotide was distributed on proximal renal tubular cells throughout the cortex, and no cGMP was localized on blood vessels or glomerular cells or in the medullary regions of the kidney. Because Texas red has a large molecular size (molecular mass 625 kDa), we infer that the conjugated cGMP was localized to cell membranes of renal tubular cells and did not cross into the cytosol. Indeed, we were able to demonstrate that the majority of the conjugated cGMP was localized to the basolateral regions of the tubular cells. This distribution of infused cGMP is consistent with a direct renal tubular action in the control of sodium transport.
To our knowledge, the present study demonstrates for the first time that cGMP and its membrane-permeable analogue, 8-Br-cGMP, exert a direct tubular action to increase renal sodium excretion in vivo. The natriuretic action of cGMP was specific, inasmuch as it was blocked completely by the PKG inhibitor Rp-8-pCPT-cGMPS, whereas the PKG inhibitor alone had no significant effect on sodium excretion.
Because administration of cGMP via the renal interstitial space induced natriuresis, we wished to demonstrate whether this was due to the well-known vasodilator action of cGMP or to a direct action of cGMP on renal tubular sodium transport. We found that cGMP administered into the interstitial space induced natriuresis without altering RBF, GFR, RCBF, or RMBF, strongly suggesting that cGMP has a direct action on renal tubular sodium and fluid transport. In addition to demonstrating the natriuretic action of cGMP in conscious animals, we were able to show that both L-Arg, the substrate for NOS, and SNAP, an NO donor, administered interstitially increased sodium excretion without changing RBF or GFR. Taken together, the results suggest that interstitial cGMP induces natriuresis by a direct action at the renal tubule. On the other hand, cGMP administered into the renal artery caused both renal vasodilatation and increased glomerular filtration associated with a similar degree of natriuresis. Furthermore, we demonstrated that only renal cortical and not renal medullary infusion of cGMP resulted in a natriuretic response. These observations argue strongly for compartmentalization of cGMP responses within the kidney with the cortical interstitial compartment playing an important regulatory role. The results also suggest that sodium transport may be regulated by autacoids presented to the basolateral surface of transporting epithelial cells. These surfaces are exposed directly to renal interstitial fluid,31 which, our studies would indicate, serves as an intrarenal vehicle for the cell-to-cell transport of messenger molecules.
The present study demonstrated that renal NO induces natriuresis via cGMP. We determined that the substrate for NOS, L-Arg, and the NO donor SNAP induced natriuresis when infused into the interstitial space. L-Arg-induced natriuresis was accompanied by a significant increase in renal interstitial fluid cGMP, and both the natriuresis and the increase in cGMP were blocked by NOS inhibition by L-NAME, demonstrating the involvement of NO, or by activation of cGMP PDE with 5,6DMcBIMP or inhibition of sGC with ODQ, indicating the involvement of sGC and cGMP. SNAP-induced natriuresis also was blocked with ODQ, confirming the sGC and cGMP pathway. These results confirm the renal actions of interstitial NO to induce natriuresis.
The recent identification of the guanylin family of peptides (guanylin, uroguanylin, and Sta) has provided insight into cellular mechanisms for the cGMP-mediated control of renal function.5,32,33 These peptides act to stimulate pGC, which serves as a membrane receptor on the apical surface of proximal tubular cells.5 Because cGMP does not cross the tubular cell membrane, we conducted a study to determine whether interstitial cGMP modulates sodium reabsorption by stimulation via the tubular basolateral membrane. We demonstrated the compartmental nature of the renal response to STa, wherein STa engendered natriuresis when infused into the renal artery but not via the renal cortical interstitial space. Thus, it is highly likely that pGC, the STa receptor, is exposed only via the apical surface of these transporting epithelial cells and that the natriuretic responses to sGC and pGC are physically segregated. Although we did not measure renal hemodynamic function in our study with STa, on the basis of the literature, it is highly likely that the natriuresis after renal arterial administration of the peptide was mediated by decreased proximal tubular reabsorption of sodium.34,35
The present study has also demonstrated that renal interstitial cGMP maintains renal sodium excretion in the basal state, inasmuch as a significant reduction in cGMP with PDE II engendered a reduction in UNaV. This effect was not due to an action of PDE II on cAMP, which was unaltered during this experiment. The results suggest that renal interstitial cGMP may be important in the physiological regulation of sodium excretion.
Received February 10, 2001; first decision March 2, 2001; accepted March 9, 2001.
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