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. Author manuscript; available in PMC: 2013 Nov 1.
Published in final edited form as: Hypertension. 2012 Oct 1;60(5):1234–1241. doi: 10.1161/HYPERTENSIONAHA.112.200469

Salt-dependent inhibition of ENaC-mediated sodium reabsorption in the aldosterone-sensitive distal nephron by bradykinin

Mykola Mamenko 1, Oleg Zaika 1, Peter A Doris 2, Oleh Pochynyuk 1
PMCID: PMC3475746  NIHMSID: NIHMS411372  PMID: 23033373

Abstract

We have recently documented that Bradykinin (BK) directly inhibits activity of the Epithelial Na+ Channel (ENaC) via B2R-Gq/11-PLC pathway. In this study, we took advantage of mice genetically engineered to lack bradykinin receptors (B1R,B2R-/-) to probe a physiological role of BK cascade in regulation of ENaC in native tissue, aldosterone-sensitive distal nephron (ASDN). Under normal sodium intake (0.32%Na+), ENaC open probability (Po) was modestly elevated in B1R,B2R-/- mice compared to WT mice. This difference is augmented during elevated Na+ intake (2%Na+) and negated during Na+ restriction (<0.01%Na+). Saturation of systemic mineralocorticoid status with deoxycorticosterone acetate (DOCA) similarly increased ENaC activity in both mouse strains suggesting that the effect of BK on ENaC is independent of aldosterone. It is accepted that angiotensin converting enzyme (ACE) represents the major pathway of BK degradation. Systemic inhibition of ACE with captopril (30 mg/kgBW for 7 days) significantly decreases ENaC activity and Po in WT mice but this effect is diminished in B1R,B2R-/- mice. At the cellular level, acute captopril (100 μM) treatment sensitized BK signaling cascade and greatly potentiated the inhibitory effect of 100 nM BK on ENaC. We concluded that BK cascade has its own specific role in blunting ENaC activity particularly under conditions of elevated sodium intake. Augmentation of BK signaling in the ASDN inhibits ENaC-mediated Na+-reabsorption contributing to the natriuretic and antihypertensive effects of ACE inhibition.

Keywords: kallikrein-kinin system, collecting duct, connecting tubule, ACE inhibition, natriuresis

INTRODUCTION

Maintenance of constant circulating volume and normal blood pressure is of key importance to all higher organisms. Kidneys play a dominant role in regulation of circulating volume by controlling water and Na+ excretion1. Compromised kidney function is tightly linked to disturbances in extracellular fluid volume and, as a consequence, to altered blood pressure regulation1,2. It is becoming recognized that excessive Na+ retention by the kidney is predominantly a failure to appropriately suppress tubular Na+ reabsorption2. The aldosterone-sensitive distal nephron (ASDN) is the final site where tubular Na+ reabsorption is regulated. This segment is formed by the connecting tubule (CNT) and the cortical collecting duct (CCD). The activity of the Epithelial Na+ Channels (ENaC) accounts for electrogenic Na+ reabsorption in the ASDN3,4. ENaC-mediated sodium reabsorption in ASDN is stimulated by the Renin-Angiotensin-Aldosterone system (RAAS) in response to decreased Na+ intake and/or volume contraction5,6. A role of ENaC in regulation of systemic blood pressure is generally recognized. Genetic mutations in humans causing gain-of-function/loss-of-function in ENaC result in monogenic forms of hypertension/hypotension, respectively7.

While the RAAS serves to correct states associated with volume contraction8, its counterpart, the kallikrein-kinin system (KKS), is activated in response to conditions with volume expansion9. KKS lowers blood pressure by promoting vasodilation, natriuresis and diuresis generally opposing the hypertensive effects of RAAS10,11. Moreover, activation of KKS reduces generation of reactive oxygen species and plays a protective role against organ damage in the heart and kidney12.

The physiological actions of the KKS result from production of local hormone peptides kinins, such as bradykinin (BK), from a precursor kininogen mainly by the action of the serine protease kallikrein13. BK interacts with G-protein coupled B1 and B2 receptors (B1R and B2R)14,15. The biological effects of BK are mediated mainly through the B2R which are constitutively expressed in smooth muscles, neurons, vascular endothelium and kidney epithelial cells16. Cumulative experimental evidence strongly supports a critical role of KKS in regulation of systemic blood pressure. Disruption of any of the KKS components including: kininogen17, kallikrein18, and B2R19,20 produces hypertension when sodium intake is elevated. Consistently, low urinary kallikrein levels are found in individuals with essential hypertension21. A polymorphism in human B2R gene (+9,+9) is linked to increased cardiovascular risk and higher systolic blood pressure22.

KKS and RAAS are interconnected at the level of the angiotensin converting enzyme (ACE). ACE inhibition is proved to be potent in treatment of hypertension, congestive heart failure and diabetic nephropathy23-25. However, the beneficial actions of ACE inhibition extend beyond blocking Ang II production and inhibition of RAAS. ACE is much more potent in cleaving kinins than in producing Ang II from Ang I9,23. ACE inhibition alters the balance between the KKS and the RAAS in favor of the former which further contributes to lowering of blood pressure. Inhibition of B2R consistently and significantly attenuates the hypotensive effect of ACE inhibition in both normotensive and hypertensive individuals26.

In the kidney, KKS is thought to be involved in regulation of water and electrolyte handling13,27. In perfused rat kidneys, inhibition of B2 receptors decreased urinary Na+ excretion without altering glomerular filtration rate (GFR) or renal blood flow (RBF)28. Renal KKS is localized to the distal portion of the renal nephron. Specifically, kallikrein immunoreactivity is detected almost exclusively in the CNT29. Kininogen and B2R expression are located in CNT and CCD29. This spatial pattern coincides with ENaC localization and argues that the renal KKS is specifically designed to regulate water-salt transport in the ASDN. Indeed, using split-opened ASDN preparation, we recently demonstrated that BK directly inhibits ENaC open probability via activation of B2R-Gq/11-PLC pathway30.

In the current study, we probed physiological aspects of BK regulation of ENaC activity in freshly-isolated murine ASDNs. We determined that genetic deletion of both BK receptors (B1R and B2R) results in increased ENaC activity under conditions of normal and elevated salt intake and this effect is independent of aldosterone status. We propose that inability to properly suppress ENaC activity during dietary sodium excess contributes to the salt-sensitive hypertension observed in mice with deleted BK receptors19, 20. Furthermore, ACE blockade with captopril greatly augments the BK signal to ENaC in murine ASDNs promoting renal sodium excretion. Genetic deletion of BK receptors attenuates the effect of ACE inhibition on sodium handling in the ASDN.

METHODS

Reagents and animals

All chemicals and materials were from Sigma (St. Louis, MO), VWR (Radnor, PA), and Tocris (Ellisville, MO) and were at least of reagent grade. Animal use and welfare adhered to the NIH Guide for the Care and Use of Laboratory Animals following a protocol reviewed and approved by the Animal Care and Use Committee of the University of Texas Health Science Center at Houston (UTHSCH). For experiments, male C57BL/6J mice (Charles River Laboratories, Wilmington, MA) 6-8 weeks old, were used. B1R,B2R-/- mice (inbred into C57BL/6J background) were originally purchased from Jackson Labs (Bar Harbor, ME, USA; strain #012371) and maintained in the animal facility of UTHSCH. To examine effects of salt intake, animals were provided diets containing nominally free (<0.01% Na+, TD.90228), regular (0.32% Na+, TD.7912), and high (2% Na+, TD.92034) sodium for one week. All diets were purchased from Harlan Teklad (Madison, WI, USA). Animals had free access to tap water. In some experiments, captopril (30 mg/kgBW) was added to drinking water for 7 days.

Tissue isolation

The procedure for isolation of the ASDNs suitable for electrophysiology and Ca2+-imaging has been described previously31-33. Briefly, mice were sacrificed by CO2 administration followed by cervical dislocation and the kidneys were removed immediately. Kidneys were cut into thin slices (< 1mm) with slices placed into ice-cold physiologic saline solution buffered with HEPES (pH 7.4). The ASDN was identified as merging of CNT into CCD and was mechanically isolated from cortical sections of kidney slices by micro-dissection using watchmaker forceps under a stereomicroscope. Isolated ASDN was attached to a 5×5 mm coverglass coated with poly-L-lysine. A coverglass containing ASDN was placed in a perfusion chamber mounted on an inverted Nikon Eclipse Ti microscope and perfused with room temperature HEPES buffered (pH 7.4) saline solution. ASDNs were split-opened with two sharpened micropipettes, controlled with different micromanipulators, to gain access to the apical membrane. The tubules were used within 1-2 hr of isolation.

Electrophysiology

ENaC activity in principal cells was determined in cell-attached patches on the apical membrane made under voltage-clamp conditions (-Vp = -60 mV) using standard procedures32-34 (see online supplement for more details).

[Ca2+]i measurements

Intracellular calcium levels were measured in cells of the split-opened ASDNs using Fura-2 fluorescence ratiometric imaging as described previously35,36. Split-opened ASDNs were loaded with Fura-2 by incubation with 2 μM Fura-2/AM in a bath solution for 40 min at RT. Subsequently, the ASDNs were washed and incubated for an additional 10-15 minutes prior to experimentation. The ASDNs were then placed in an open-top imaging study chamber (Warner RC-10) with a bottom coverslip viewing window and the chamber attached to the microscope stage of an InCa Imaging Workstation (Intracellular Imaging, Inc.). Cells were imaged with a 20X Nikon Super Fluor objective and regions of interest (ROIs) drawn for individual cells. The Fura-2 fluorescence intensity ratio was determined by excitation (an average for ~300 msec) at 340 nm and 380 nm and calculating the ratio of the emission intensities at 511 nm in the usual manner every 5 seconds. We observed no significant Fura-2 bleaching and minimal Fura-2 leakage at both wavelengths during experiments. The changes in the ratio are converted to intracellular Ca2+ concentrations using the calibration methods as we have done before37,38. Experimental traces from individual cells were inspected visually prior to acceptance.

Data analysis

All summarized data are reported as mean ± SEM. Data from before and after treatment within the same experiment were compared using the paired t-test. Data from different experiments were compared with a Student's (two-tailed) t-test or an One-Way ANOVA as appropriate. P ≤ 0.05 was considered significant.

RESULTS

Genetic deletion of B1 and B2 receptors disrupts BK regulation of ENaC

Our recent report30 demonstrated that BK acutely regulates ENaC Po in the murine ASDN by stimulation of B2R. Genetic deletion of B2R can lead to compensatory upregulation of B1R expression39, 40. To adequately probe the physiological relevance of BK regulation of ENaC, we used mice lacking both BK receptors (B1R,B2R-/-). BK fails to affect ENaC activity in freshly isolated split-opened ASDNs as was assessed with patch clamp electrophysiology (Figure 1A). ENaC Po was 0.47±0.09, 0.47±0.10, and 0.48±0.09 (n=7, N=5 mice) in the control, upon application of 500 nM BK, and followed washout, respectively (Figure 1B). In contrast, the same experimental sequence greatly diminishes ENaC open probability in ASDN cells from wild type (WT) animals as exemplified by the representative patch clamp experiment in Figure 1C. The magnitude of inhibition was 73±8% (n=6, N=4 mice; Figure 1D) which is consistent with our previous report30.

Figure 1. Bradykinin inhibition of ENaC is absent in B1R,B2R-/- mice.

Figure 1

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(A) A representative continuous current trace from a cell-attached patch containing single ENaC in the control condition, under application of 500 nM BK, and following washout with regular bath solution. The patch was formed on the apical plasma membrane of a principal cell within a split-opened area of ASDN isolated from B1R,B2R-/- mice. The patch was held at a test potential of Vh=-Vp=-60 mV. Areas control (1) and upon BK treatment (2) are shown below at an expanded time scale. Inward Li+ currents are downward. Dashed lines indicate the respective current state with C denoting the closed state. (B) Summary graph of ENaC Po changes in response to BK and following washing out from paired patch clamp experiments similar to that shown in Figure 1A. (C) A representative continuous current trace from a cell-attached patch monitoring ENaC activity in ASDN isolated from WT mice in the control condition, under addition of 500 nM BK, and following washout of BK with regular bath solution. (D) Summary graph of relative ENaC activity after treatment with 500 nM BK for WT and B1R,B2R-/- mice, respectively. ENaC activity was normalized to the corresponding values before treatment. Here and below, numbers of experiments for each dataset are shown. #-significant change versus WT.

Ablation of BK signaling results in salt-sensitive augmentation of ENaC activity

We next carefully determined the consequences of the disruption of BK cascade on ENaC activity in mice kept on regular Na+ intake (0.32% Na+). Figure 2A shows representative current traces of ENaC activity in WT and B1R,B2R-/- mice. As is clear, ENaC activity is modestly but significantly increased in mutant mice (Figure 2B). This elevation was attributed to a greater ENaC Po but not to changes in functional ENaC levels (fN). We concluded that dysfunctional BK signaling results in ENaC hyperactivity due to augmentation of ENaC gating.

Figure 2. Genetic deletion of BK receptors augments basal ENaC activity.

Figure 2

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(A) Representative current traces of ENaC activity in ASDN from WT (top) and B1R,B2R-/- mice (bottom). All other conditions are identical to those described in Figure 1A. (B) Summary graphs comparing total ENaC activity (fNPo, left), ENaC open probability (Po, middle), and functional ENaC expression (fN, right) for split-opened ASDNs from WT and B1R,B2R-/- mice. #-significant increase versus Wild Type.

We further probed whether disruption of BK signaling affects adaptation of ENaC activity to changes in salt intake. Indeed, increased Na+ intake (2% Na+) exacerbates the difference in ENaC Po between B1R,B2R-/- and WT mice (Figure 3A). In contrast, we found no significant difference in ENaC activity when animals were placed on a sodium deficient diet (<0.01% Na+). Of note, we did not detect significant differences in functional ENaC levels in WT and B1R,B2R-/- mice under these conditions. We concluded that BK signaling is critical to suppress ENaC activity when sodium intake is elevated.

Figure 3. Effect of BK on ENaC is independent of aldosterone status.

Figure 3

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(A) Summary graph of ENaC Po for WT and B1R,B2R-/- mice maintained on sodium deficient (<0.01% Na+) and high sodium (2% Na+) diet for 1 week prior to the experimentations. #-significant increase versus Wild Type. (B) Summary graph comparing total ENaC activity for WT and B1R,B2R-/- mice kept on high sodium intake and subcutaneously injected with DOCA for 3 consecutive days (2.4 mg/injection/animal). #-significant increase versus Wild Type 2%Na+; ##-significant increase versus B1R,B2R-/- 2%Na+.

The blunted modulation of ENaC activity by salt intake in B1R,B2R-/- mice may arise from altered sensitivity to aldosterone. To test this, we placed animals on high salt intake to suppress endogenous aldosterone secretion and exogenously saturated mineralocorticoid signaling with deoxycorticosterone acetate (DOCA) injections. As summarized in Figure 3B, DOCA induces comparable increases in ENaC activity in both strains. This suggests that BK cascade does not overlap with aldosterone regulation of ENaC.

BK regulation of ENaC is under control of ACE

Newly generated kinins are known to be rapidly degraded via ACE-dependent cleavage. Since ACE activity is present in the ASDN41,42, we investigated whether ACE interferes with BK regulation of ENaC. Systemic inhibition of ACE with captopril (30 mg/kgBW for 7 days) significantly decreases total ENaC activity and Po in WT mice kept on regular salt regimen (Figure 4). Importantly, genetic deletion of BK receptors diminishes ACE-dependent ENaC inhibition. Snapshot measurements of urinary sodium and creatinine concentrations in the bladder (Supplementary Figure S1) support the conception that natriuretic effect of captopril is blunted in B1R,B2R-/-mice. More careful analysis is required to definitively prove this. Overall, our results suggest that tonic ACE activity limits BK inhibition of ENaC-mediated sodium reabsorption.

Figure 4. Systemic inhibition of ACE decreases ENaC activity in WT but not in B1R,B2R-/- mice.

Figure 4

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Summary graph of total ENaC activity (left) and open probability (right) for WT and B1R,B2R-/- mice in the control condition and after treatment with captopril for 7 days. Captopril (30 mg/kgBW) was given with drinking water. #-significant decrease versus Wild Type; ##-significant increase versus Wild Type+captopril.

We next directly probed if ACE blockade potentiates BK signal to ENaC in the ASDN. Since 500 nM BK maximally decreases ENaC activity (by approximately 75%, Figure 1D, and30), we compared the inhibitory effect of a lower concentration of BK (100 nM) on ENaC in the absence and presence of acute ACE inhibition with 100 μM captopril (Figure 5). 100 nM of BK significantly decreased ENaC Po in a reversible manner in control conditions (Figure 5A). The magnitude of this effect (inhibition by 43±10%) is considerably smaller than observed with 500 nM BK (Figure 1D). Acute captopril application had no measurable effect on ENaC (Figure 5B). However, as is clear from the representative current trace (Figure 5B) and the summary graph (Figure 5C), BK had a significantly stronger action on ENaC when ACE is blocked (83±5%). The comparison of BK inhibition of ENaC Po is shown in Figure 5D.

Figure 5. Acute ACE inhibition potentiates BK inhibition of ENaC.

Figure 5

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(A) A representative continuous current trace from a cell-attached patch containing single ENaC in the control condition, under application of 100 nM BK, and following washout with regular bath solution. All other conditions are identical to those described in Figure 1A. The patch was formed on the apical plasma membrane of a principal cell within a split-opened area of ASDN isolated from Wild Type mice. (B) A representative continuous current trace from a cell-attached patch monitoring ENaC activity in the control condition, under application of 100 μM captopril, following 100 nM BK in the presence of captopril, and washout with regular bath solution. Drugs application time are shown with respective bars on the top. (C) Summary graph of ENaC Po changes in response to captopril and following BK+captopril from paired patch clamp experiments similar to that shown in Figure 5B. (D) Summary graph of relative ENaC activity after treatment with 100 nM in the absence and presence of pretreatment with captopril. ENaC activity was normalized to the corresponding values before treatment. #-significant change versus 100 nM BK.

Activation of BK receptors is known to increase [Ca2+]i via PLC-IP3 dependent mechanism15. To assess the functional status of the receptors, we used Ca2+-sensitive dye Fura 2 to directly monitor changes in [Ca2+]i in individual cells within a split-opened ASDN in response to activation of BK signaling. As demonstrated on the average timecourse of changes in [Ca2+]i (Figure 6), 100 nM BK elicits a remarkably greater Ca2+ response after pretreatment with captopril. In contrast, we did not observe any significant changes in the magnitude of the [Ca2+]i transient elicited by subsequent BK application (data not shown).

Figure 6. Augmentation of BK-mediated [Ca2+]i- responses by captopril.

Figure 6

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The average time course of [Ca2+]i changes in individual cells of ASDN in response to repetitive 2 min applications of 100 nM BK (shown with gray bars) and 100 μM captopril (shown with a black bar).

Overall, our results strongly suggest that ACE blockade augments BK signaling in the ASDN leading to a greater ENaC inhibition likely contributing to the increased renal sodium excretion by captopril.

DISCUSSION

The initial evidence that BK regulates sodium handling in the distal nephron was provided by Tomita et al43. Although, it was suggested that BK inhibits electroneutral Na+ and Cl- transport44, this does not exclude an effect of BK on ENaC since changes in ENaC activity can occur in the absence of changes in trans-epithelial membrane potential in perfused CCDs45. Indeed, we reported direct inhibitory action of nanomolar concentrations of BK on ENaC in native distal nephrons and cultured principal cells with patch clamp electrophysiology30. In the Tomita's study, nanomolar BK concentrations inhibited NaCl transport only from the basolateral side in the perfused rat CCD. In our experiments, we applied BK apically, although “back-leak” to the basolateral side is also generally recognized for split-opened ASDN preparations. In addition, differences in species and preconditioning might potentially cause the discrepancies about the apical versus basolateral BK actions on ENaC and sodium transport, respectively.

Kidney is capable of producing a significant amount of BK. Interstitial fluid BK levels in the rat kidney are in the 10-100 nM range and these values are higher in the cortex than those in the medulla46. Furthermore, urinary BK and kallikrein levels are elevated during high salt diet47. This suggests that physiologically relevant concentrations of BK in the kidney are sufficient to suppress ENaC activity. Indeed, in the current study we directly demonstrate that 100 nM BK inhibits ENaC activity in the distal nephron (Figure 5) and that disruption of BK signaling augments ENaC activity. This becomes particularly apparent during volume expanded conditions, such as elevated Na+ intake. Thus, genetic deletion of BK receptors recapitulates the state of gain-of-function mutations in ENaC causing hypertension in humans48,49. This strongly suggests that BK signaling plays an important physiological role by decreasing ENaC activity in the ASDN during euvolemic and volume expanded states to avoid excessive sodium retention. Our results resonate with previously reported observation that inhibition of B2 receptors HOE-140 (icatibant) in perfused kidney increases tubular Na+ reabsorption in rats fed normal, but not sodium deficient diet28. Therefore, it is reasonable to propose that enhanced ENaC activity contributes to the salt-sensitive hypertension observed when BK receptors are deleted19,20. Since BK receptors are also expressed in vasculature16, future studies are necessary to carefully determine the extent of this contribution.

We also found that BK regulation of ENaC does not overlap with salt-dependent regulation of ENaC by mineralocorticoids. Despite the long-standing dogma that aldosterone is the major determinant of ENaC activity, we increasingly appreciate that regulation of sodium handling in the distal nephron is much more complex and requires integrated inputs from several sources. Recent evidence identifies endothelin-150,51 and purinergic33,52 signaling cascades as important inhibitors of ENaC during elevated salt intake. In contrary, we38 and others53 pointed to a direct effect of Ang II in stimulation of ENaC that may occur during volume restriction. It is clear that the ENaC-mediated sodium handling in the ASDN is a critical component of sodium homeostasis. Gain-of-function mutations of the channel cause hypertension and volume expansion (Liddle syndrome)54. Loss-of-function mutations, in contrast, lead to salt wasting and low BP (pseudohypoaldosteronism type I)54. Water and electrolyte transport in the tubule cannot be further compensated downstream of the ASDN and sodium reabsorption in this segment is not under negative feedback control as occurs in proximal tubular segments. Thus, it is tempting to speculate that multiple signaling inputs are designed to fine-tune functional properties of epithelial cells in the ASDN protecting the kidney from unopposed modulation of transport rates. At the same time, these regulatory inputs are not redundant since disruption of either of them results in mild volume imbalance as reported in mice lacking purinergic P2Y2 receptor52, or with disrupted ATP release in the distal nephron (Cx30-/- mice,55) and mice lacking endothelin ETB receptors50. In the current study we provide strong evidence about functional role of BK signaling in the ASDN with its disruption leading to the salt-sensitive ENaC activation.

We also demonstrate that ACE determines functional status of BK signaling in the ASDN. ACE activity constitutes one of the major pathways responsible for cleavage of kinins. Emerging evidence suggests that sufficient ACE activity is present at the apical plasma membrane of principal cells41,42. We found that under regular salt diet prolonged ACE blockade markedly decreases ENaC activity and this effect was greatly attenuated by disruption of BK receptors. Importantly, we were also able to correlate changes in ENaC activity with changes in urinary Na+ excretion under these conditions indicating physiological relevance of this regulation. It should be noted, though, that a part of captopril actions on ENaC could be mediated via decreased Ang II and aldosterone levels. However, animals are not volume depleted under normal sodium intake and the concentrations of these hormones are low. In addition, Ang II can be also formed by enzymes other than ACE, such as chymase56. In our preliminary observations, we found a mild effect of systemic inhibition of MR receptors with spironolactone on ENaC activity in mice kept on regular salt diet. However, the effect was associated with decreased number of functional channels but not Po as observed during captopril treatment in this study (Figure 4). We also found that acute ACE blockade with captopril potentiates the inhibitory effect of BK on ENaC. Furthermore, this treatment augments activation of B2 receptors in response to external application of the same concentration of BK (Figure 6). It is possible that close association of ACE with B2R, as was reported for different cell types57, could decrease the actual concentration of the agonist in the vicinity of the receptors due to tonic kininase activity of ACE. Alternatively, ACE inhibitors were suggested to act as allosteric enhancers of BK receptor function58. While it is not feasible to precisely determine which scenario actually takes place, our results favor the concept that BK-mediated inhibition of ENaC activity contributes to the natriuretic and antihypertensive effects of ACE inhibition.

Interestingly, the effects of renal KKS on Na+ transport could be more complex and occur independently of BK production. Tissue kallikrein is a serine protease which can proteolytically cleave ENaC at the prostasin site to augment its activity59,60. It has been proposed that this may activate of ENaC during low Na+ and high K+ intakes59. However, the physiological relevance of this regulation requires further verification because the renal phenotype in animal models with low kallikrein levels18 is similar to that resulting from deletion of B2R19,20. Furthermore, overexpression of human kallikrein in rats and mice causes hypotension61,62. On the other hand, similar dual actions on Na+ handling in the kidney were reported for prostaglandins. Despite the fact that renal prostaglandins cause natriuresis and diuresis63, they also play a critical role in promoting renin secretion64. It is possible that such opposite effects serve to partially balance each other, thus, protecting from extreme disturbances in kidney function.

PERSPECTIVES

Detailed understanding of discrete systems controlling sodium homeostasis is fundamental to understanding physiology and treating diseases, such as hypertension. ENaC-mediated Na+-reabsorption in the distal nephron finalizes adjustments of renal sodium excretion to match dietary sodium intake and maintain sodium balance. This is known to be critical for normal blood pressure control. The current study defines a previously underappreciated role of the renal kallikrein-kinin system in regulation of sodium handling and specifically of ENaC activity in the distal nephron by dietary salt intake. Disruption of this regulation leads to overactive ENaC which is detrimental under sodium loaded conditions. Functional bradykinin signaling in the distal nephron also appears to be a critical component of the natriuresis promoted by ACE inhibition. It is possible that genetic polymorphism in genes encoding B2R and other functional components of KKS may contribute for different susceptibility of patients to the anti-hypertensive actions of ACE blockade.

Supplementary Material

01

NOVELTY AND SIGNIFICANCE.

What is new:

  • Mice lacking BK receptors have elevated ENaC activity in the ASDN and blunted ENaC regulation by dietary salt intake.

  • Inhibition of ACE with captopril decreases ENaC activity by enhancing BK signaling in the ASDN.

What is relevant:

ENaC activity in the distal part of the renal nephron determines urinary sodium excretion. Therefore, investigation of the cellular and molecular mechanisms, such as BK signaling, affecting ENaC activity is directly relevant to understanding normal blood pressure control and pathophysiology of hypertension

Summary:

  • Functional BK signaling is essential for adaptation of ENaC-mediated sodium reabsorption in the ASDN to variations in salt intake.

  • Natriuretic effects of ACE blockade in the distal nephron are caused by potentiation of functional status of BK cascade.

Acknowledgments

SOURCES OF FUNDING

This research was partially supported by the American Heart Association SDG2230391 (to O.Pochynyuk), the Carl W. Gottschalk research scholar grant from the American Society of Nephrology (to O.Pochynyuk) and NIH-NIDDK DK09502 (to O.Pochynyuk).

Footnotes

DISCLOSURES

None.

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