Whys and wherefores of juxtaglomerular apparatus function

Prognostic value of hypochloremia on mortality in patients with heart failure: a systematic review and meta-analysis

AIMS

Electrolyte imbalances are common in patients with heart failure. Several studies have shown that a low serum chloride level is associated with adverse outcomes in hospitalized patients with acute heart failure and in outpatients with chronic heart failure. We performed a systematic review and meta-analysis to assess the association of hypochloremia with all-cause mortality in patients with heart failure.

METHODS

Data search was conducted from inception through 1 February 2023, using the following MeSH terms: ('chloride' OR 'hypochloremia') AND 'heart failure'. Studies evaluating the association between serum chloride and all-cause mortality in patients with heart failure were included. The predefined primary outcome was all-cause mortality. Pooled hazard ratios and 95% confidence intervals (CIs) were used as effect estimates and calculated with a random-effects model; fixed-effects model and leave-one-out sensitivity analyses were also performed.

RESULTS

A total of 15 studies, involving 25 848 patients, were included. The prevalence of hypochloremia ranged from 8.6 to 31.5%. Follow-up time ranged from 6 to 67 months. Hypochloremia as a categorical variable was associated with an increased risk of all-cause mortality [hazard ratio 1.56; 95% confidence interval (CI) 1.38-1.75; P  < 0.001]. As a continuous variable, serum chloride was associated with all-cause mortality (hazard ratio per mmol/l decrease in serum chloride: 1.06; 95% CI 1.05-1.07; P  < 0.001). Results were confirmed by using several sensitivity analyses.

CONCLUSION

Hypochloremia exhibits a significant prognostic value in patients with heart failure. Serum chloride can be used as an effective tool for risk stratifying in patients with heart failure.

Revisiting the relationship between (Pro)Renin receptor and the intrarenal RAS: focus on the soluble receptor

PURPOSE OF REVIEW

The (pro)renin receptor (PRR), also termed as ATPase H+ transporting accessory protein 2 (ATP6AP2), was originally cloned as a specific receptor for prorenin and renin [together called (pro)renin]. Given the wide tissue distribution of PRR, PRR was further postulated to act as a regulator of tissue renin. However, assigning a physiological role of PRR within the renin-angiotensin system (RAS) has been challenging largely due to its pleotropic functions in regulation of embryogenesis, autophagy, and H+ transport. The current review will summarize recent advances in understanding the roles of sPPR within the intrarenal RAS as well as those outside this local system.

RECENT FINDINGS

Site-1 protease (S1P) is a predominant source of sPPR at least in the kidney. So far most of the known physiological functions of PRR including renal handling of electrolytes and fluid and blood pressure are mediated by sPRR. In particular, sPRR serves as a positive regulator of collecting duct renin to activate the intrarenal RAS during water deprivation or angiotensin-II (AngII) infusion. However, PRR/sPRR can act in renin-independent manner under other circumstances.

SUMMARY

S1P-derived sPRR has emerged as a key regulator of kidney function and blood pressure and its relationship with the intrarenal RAS depends on the physiological context.

Classic and Novel Mechanisms of Diuretic Resistance in Cardiorenal Syndrome

Despite the incompletely understood multiple etiologies and underlying mechanisms, cardiorenal syndrome is characterized by decreased glomerular filtration and sodium avidity. The underlying level of renal sodium avidity is of primary importance in driving a congested heart failure phenotype and ultimately determining the response to diuretic therapy. Historically, mechanisms of kidney sodium avidity and resultant diuretic resistance were primarily extrapolated to cardiorenal syndrome from non-heart failure populations. Yet, the mechanisms appear to differ between these populations. Recent literature in acute decompensated heart failure has refuted several classically accepted diuretic resistance mechanisms and reshaped how we conceptualize diuretic resistance mechanisms in cardiorenal syndrome. Herein, we propose an anatomically based categorization of diuretic resistance mechanisms to establish the relative importance of specific transporters and translate findings toward therapeutic strategies. Within this categorical structure, we discuss classic and novel mechanisms of diuretic resistance.

Principles of Spatial Transcriptomics Analysis: A Practical Walk-Through in Kidney Tissue

Spatial transcriptomic technologies capture genome-wide readouts across biological tissue space. Moreover, recent advances in this technology, including Slide-seqV2, have achieved spatial transcriptomic data collection at a near-single cell resolution. To-date, a repertoire of computational tools has been developed to discern cell type classes given the transcriptomic profiles of tissue coordinates. Upon applying these tools, we can explore the spatial patterns of distinct cell types and characterize how genes are spatially expressed within different cell type contexts. The kidney is one organ whose function relies upon spatially defined structures consisting of distinct cellular makeup. Thus, the application of Slide-seqV2 to kidney tissue has enabled us to elucidate spatially characteristic cellular and genetic profiles at a scale that remains largely unexplored. Here, we review spatial transcriptomic technologies, as well as computational approaches for cell type mapping and spatial cell type and transcriptomic characterizations. We take kidney tissue as an example to demonstrate how the technologies are applied, while considering the nuances of this architecturally complex tissue.

Hypochloremia and Diuretic Resistance in Heart Failure: Mechanistic Insights

BACKGROUND

Recent epidemiological studies have implicated chloride, rather than sodium, as the driver of poor survival previously attributed to hyponatremia in heart failure. Accumulating basic science evidence has identified chloride as a critical factor in renal salt sensing. Our goal was to probe the physiology bridging this basic and epidemiological literature.

METHODS AND RESULTS

Two heart failure cohorts were included: (1) observational: patients receiving loop diuretics at the Yale Transitional Care Center (N=162) and (2) interventional pilot: stable outpatients receiving ≥80 mg furosemide equivalents were studied before and after 3 days of 115 mmol/d supplemental lysine chloride (N=10). At the Yale Transitional Care Center, 31.5% of patients had hypochloremia (chloride ≤96 mmol/L). Plasma renin concentration correlated with serum chloride (r=-0.46; P<0.001) with no incremental contribution from serum sodium (P=0.49). Hypochloremic versus nonhypochloremic patients exhibited renal wasting of chloride (P=0.04) and of chloride relative to sodium (P=0.01), despite better renal free water excretion (urine osmolality 343±101 mOsm/kg versus 475±136; P<0.001). Hypochloremia was associated with poor diuretic response (odds ratio, 7.3; 95% confidence interval, 3.3-16.1; P<0.001). In the interventional pilot, lysine chloride supplementation was associated with an increase in serum chloride levels of 2.2±2.3 mmol/L, and the majority of participants experienced findings such as hemoconcentration, weight loss, reduction in amino terminal, pro B-type natriuretic peptide, increased plasma renin activity, and increased blood urea nitrogen to creatinine ratio.

CONCLUSIONS

Hypochloremia is associated with neurohormonal activation and diuretic resistance with chloride depletion as a candidate mechanism. Sodium-free chloride supplementation was associated with increases in serum chloride and changes in several cardiorenal parameters.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT02031354.

Role of renal transporters and novel regulatory interactions in the TAL that control blood pressure

Hypertension (HTN), a major public health issue is currently the leading factor in the global burden of disease, where associated complications account for 9.4 million deaths worldwide every year. Excessive dietary salt intake is among the environmental factors that contribute to HTN, known as salt sensitivity. The heterogeneity of salt sensitivity and the multiple mechanisms that link high salt intake to increases in blood pressure are of upmost importance for therapeutic application. A continual increase in the kidney's reabsorption of sodium (Na+) relies on sequential actions at various segments along the nephron. When the distal segments of the nephron fail to regulate Na+, the effects on Na+ homeostasis are unfavorable. We propose that the specific nephron region where increased active uptake occurs as a result of variations in Na+ reabsorption is at the thick ascending limb of the loop of Henle (TAL). The purpose of this review is to urge the consideration of the TAL as contributing to the pathophysiology of salt-sensitive HTN. Further research in this area will enable development of a therapeutic application for targeted treatment.

Hypochloraemia is strongly and independently associated with mortality in patients with chronic heart failure

AIMS

Hyponatraemia is strongly associated with adverse outcomes in heart failure. However, accumulating evidence suggests that chloride may play an important role in renal salt sensing and regulation of neurohormonal and sodium-conserving pathways. Our objective was to determine the prognostic importance of hypochloraemia in patients with heart failure.

METHODS AND RESULTS

Patients in the BEST trial with baseline serum chloride values were evaluated (n = 2699). Hypochloraemia was defined as a serum chloride ≤96 mmol/L and hyponatraemia as serum sodium ≤135 mmol/L. Hypochloraemia was present in 13.0% and hyponatraemia in 13.7% of the population. Chloride and sodium were only modestly correlated (r = 0.53), resulting in only 48.7% of hypochloraemic patients having concurrent hyponatraemia. Both hyponatraemia and hypochloraemia identified a population with greater disease severity; however, renal function tended to be worse and loop diuretic doses higher with hypochloraemia. In univariate analysis, lower serum sodium or serum chloride as continuous parameters were each strongly associated with mortality (P < 0.001). However, when both parameters were included in the same model, serum chloride remained strongly associated with mortality [hazard ratio (HR) 1.3 per standard deviation decrease, 95% confidence interval (CI) 1.18-1.42, P < 0.001], whereas sodium was not (HR 0.97 per standard deviation decrease, 95% CI 0.89-1.06, P = 0.52).

CONCLUSION

Serum chloride is strongly and independently associated with worsened survival in patients with chronic heart failure and accounted for the majority of the risk otherwise attributable to hyponatraemia. Given the critical role of chloride in a number of regulatory pathways central to heart failure pathophysiology, additional research is warranted in this area.

The hidden hand of chloride in hypertension

Among the environmental factors that affect blood pressure, dietary sodium chloride has been studied the most, and there is general consensus that increased sodium chloride intake increases blood pressure. There is accruing evidence that chloride may have a role in blood pressure regulation which may perhaps be even more important than that of Na⁺. Though more than 85 % of Na⁺ is consumed as sodium chloride, there is evidence that Na⁺ and Cl⁻ concentrations do not go necessarily hand in hand since they may originate from different sources. Hence, elucidating the role of Cl⁻ as an independent player in blood pressure regulation will have clinical and public health implications in addition to advancing our understanding of electrolyte-mediated blood pressure regulation. In this review, we describe the evidence that support an independent role for Cl⁻ on hypertension and cardiovascular health.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Role of renal cortical cyclooxygenase-2 expression in hyperfiltration in rats with high-protein intake

Renal cortical cyclooxygenase-2 (COX-2) is restricted to the macula densa and adjacent cortical thick ascending limbs (MD/cTALH). Renal cortical COX-2 increases in response to diabetes and renal ablation, both of which are characterized by hyperfiltration and reduced NaCl delivery to the MD due to increased proximal NaCl reabsorption. High-protein intake also induces hyperfiltration and decreases NaCl delivery to the MD due to increased NaCl reabsorption proximally. We investigated whether high protein induces cortical COX-2 and whether cortical COX-2 contributes to high protein-induced hyperfiltration and increased intrarenal renin biosynthesis. Cortical COX-2 increased after protein loading but decreased after protein restriction. COX-2 inhibition attenuated high protein-induced hyperfiltration but had no effect on high protein-induced intrarenal renin elevation. Therefore, induction of cortical COX-2 contributed to high protein-induced hyperfiltration but not intrarenal renin elevation. In the kidney cortex, neuronal nitric oxide synthase (nNOS) is also localized to the MD, and interactions between intrarenal nNOS and COX-2 systems have been proposed. Cortical COX-2 elevation seen in salt restriction was blocked by nNOS inhibiton. Cortical nNOS expression also increased after protein loading, and inhibition of nNOS activity completely reversed high protein-induced cortical COX-2 elevation and hyperfiltration. These results indicate that NO is a mediator of high protein-induced cortical COX-2 elevation and suggest that both intrarenal nNOS and COX-2 systems appear to regulate afferent arteriolar tone and subsequent hyperfiltration seen in high-protein intake.

Redistribution of myosin VI from top to base of proximal tubule microvilli during acute hypertension

During acute hypertension, Na(+)/H(+) exchangers (NHE3) retract from top to base of proximal tubule microvilli (MV) and Na(+) reabsorption decreases in proximal tubule. This study aimed to determine whether the actin-based motor myosin VI coordinately retracts with NHE3 in response to acute hypertension. BP was raised approximately 50 mmHg in rats for 20 to 30 min or sham treated, and kidneys were analyzed by subcellular fractionation or microscopy. During acute hypertension, myosin VI redistributed from low density apical MV-enriched membranes (from 23 +/- 2.4 to 11.4 +/- 2.2%) into higher density membranes (from 23.2 +/- 0.7 to 36.9 +/- 2.6%). By confocal microscopy, myosin VI was detected over the whole length of the MV in controls, then became completely focused at the base of MV during acute hypertension. For electron microscopic analysis using immunogold labeling, MV were divided into five zones from top (z1) to base (z5). In controls, myosin VI was evenly distributed through the five MV zones. In acute hypertension, myosin VI decreased in z1 (from 20.6 +/- 1.9 to 10.5 +/- 2.3%) and z2 (from 21.0 +/- 2.0 to 13.2 +/- 1.4%) and increased in z5 (from 21.1 +/- 3.3 to 38.6 +/- 3.0%). These results provide the first observation that acute hypertension causes myosin VI redistribution and support the idea that myosin VI may serve as the molecular motor for NHE3 retraction from top to base of MV during acute hypertension.

Simultaneous changes of cell volume and cytosolic calcium concentration in macula densa cells caused by alterations of luminal NaCl concentration

Cell volume and cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured in rabbit macula densa (MD) cells loaded with calcein and Fura Red using confocal microscopy. [Ca(2+)](i) was also analysed with Indo-1 and fura-2. We used isolated microperfused thick ascending limbs with attached glomerulus. The results showed that when the luminal NaCl concentration ('NaCl') was decreased from 35 to 10 mM, the cell volume decreased by 10.4%, and [Ca(2+)](i) increased by 9.5%. This increase was inhibited in Ca(2+)-free solution. When luminal [NaCl] was changed from 35 to 135 mM, the cell volume increased by 15.1%, and [Ca(2+)](i) did not change. The cell volume alterations were not different in Ca(2+)-free solutions. Using Indo-1, basal [Ca(2+)](i) in MD cells was 107.8 nM. When luminal [NaCl] was changed from 135 to 10 mm, [Ca(2)](i) increased by 23.5 nM. Using fura-2, the basal [Ca(2+)](i) in MD cells was 115.3 nM, and when luminal [NaCl] was changed from 135 or 35 to 10 mM, [Ca(2+)](i) change was 30.1 or 10.6 nM, respectively. An increase in [NaCl] caused no change in [Ca(2+)](i). In Ca(2+)-free solution, no change in [Ca(2+)](i) occurred. A stepwise decrease in luminal [NaCl] resulted in a sigmoid increase in [Ca(2+)](i) in MD cells. The steepest part of the curve was between 70 and 10 mM. In conclusion, we found that MD cells have cell volume regulation, and that [Ca(2+)](i) elevation caused by decreased luminal [NaCl] is independent of the cell volume.

Should we restrict chloride rather than sodium?

Low-salt diets have potential for prevention and treatment of hypertension, and may also reduce risk for stroke, left ventricular hypertrophy, osteoporosis, renal stones, asthma, cataract, gastric pathology, and possibly even senile dementia. Nonetheless, the fact that salt restriction evokes certain counter-regulatory metabolic responses-- increased production of renin and angiotensin II, as well as increased sympathetic activity--that are potentially inimical to vascular health, has suggested to some observers that salt restriction might not be of unalloyed benefit, and might in fact be contraindicated in some "salt-resistant" subjects. Current epidemiology indicates that lower-salt diets tend to reduce coronary risk quite markedly in obese subjects, whereas the impact of such diets on leaner subjects (who are less likely to be salt sensitive) is equivocal--seemingly consistent with the possibility that salt restriction can exert countervailing effects on vascular health. There is considerable evidence that sodium chloride, rather than sodium per se, is responsible for the known adverse effects of dietary salt. Other non-halide sodium salts, such as sodium citrate or bicarbonate, do not raise plasma volume, increase blood pressure, boost urinary calcium loss, or promote stroke in stroke-prone rats. Nonetheless, these compounds have been shown to blunt the impact of salt restriction on renin, angiotensin II, and sympathetic activity in humans. This may rationalize limited clinical evidence that organic sodium salts can decrease blood pressure in salt-restricted hypertensives. Furthermore, organic sodium salts have an alkalinizing metabolic impact favorable to bone health. These considerations suggest that restricting dietary salt to the extent feasible, while encouraging consumption of organic sodium salts in mineral waters, soft drinks, or other nutraceuticals--preferably in conjunction with organic potassium salts and taurine--may represent a superior strategy for controlling blood pressure, promoting vascular health, and preserving bone density. Further clinical studies should determine whether a moderately salt-restricted diet supplemented with organic sodium salts has a better and more uniform impact on hypertension than salt restriction alone, while rodent studies should examine the comparative impact of these regimens on rodents prone to vascular disease.

Differential traffic of proximal tubule Na+transporters during hypertension or PTH: NHE3 to base of microvilli vs. NaPi2 to endosomes

We previously reported that Na+/H+exchanger type 3 (NHE3) and NaPi2 are acutely retracted from the proximal tubule (PT) microvilli (MV) during acute hypertension [high blood pressure (BP)] or parathyroid hormone (PTH) treatment. By subcellular membrane fractionation, NHE3 and NaPi2 show indistinguishable redistribution patterns out of light-density into heavy-density membranes in response to either treatment consistent with a retraction from the apical MV to the intermicrovillar cleft region. This study aimed to examine the redistribution of PT NHE3 vs. NaPi2 by confocal and electron microscopy during high BP and during PTH treatment to determine whether their respective destinations overlap or are distinct. High-BP protocol: systolic BP was increased 50–60 mmHg by increasing peripheral resistance for 20 min; PTH protocol: rats were infused with 6.6 μg/kg iv of PTH followed by 0.1 μg·kg−1·min−1infusion for 1 h. For light microscopy, rats were infused with 25 mg of horseradish peroxidase (HRP) 10 min before kidney fixation. Kidney slices were dual labeled with either NHE3 or NaPi2 and either clathrin-coated vesicle adaptor protein AP2 or endosome marker HRP. The results demonstrate retraction of NHE3 from the MV to the base of MV during either high-BP or PTH treatment: NHE3 staining did not retract below the AP2-stained domain or to HRP-labeled endosomes in either model. In comparison, NaPi2 was retracted from MV to below the AP2-stained region in both models, a little colocalizing with HRP staining. At the electron microscopic level with immunogold labeling, during high BP NHE3 was concentrated in a distinct domain in the base of the MV while NaPi2 moved to endosomes. The results demonstrate that there are divergent routes of retraction of PT NHE3 and NaPi2 from the MV during acute hypertension or PTH treatment: NHE3 is not internalized but remains at the base of the MV while NaPi2 is internalized.

Sodium glomerulopathy: tubuloglomerular feedback and renal injury in African Americans

African Americans are prone to develop not only essential hypertension but also progressive renal injury. We present a simple model to explain salt-induced renal injury (sodium glomerulopathy) in African Americans, the central features of which are the tubuloglomerular feedback and the balance/imbalance between the vascular tones of the afferent and efferent glomerular arterioles. We propose that in African Americans, habitual consumption of high salt causes chronic intermittent tubular hyperperfusion of the macula densa, resulting in a rightward and upward resetting of the operating point for the tubuloglomerular feedback. The resetting of the operating point causes an imbalance between the vascular tones of the afferent/efferent arterioles, a rise in the glomerular capillary hydraulic pressure, and consequent hyperfiltration. Increased susceptibility to glomerular hyperfiltration of African Americans on a high salt intake may explain their proclivity to progressive renal injury associated with essential hypertension.

Does membrane trafficking play a role in regulating the sodium/hydrogen exchanger isoform 3 in the proximal tubule?

PURPOSE OF REVIEW

The proximal tubule sodium/hydrogen exchanger continuously reabsorbs the bulk of the filtered sodium, controlling salt delivery to the distal nephron which is critical for tubuloglomerular feedback autoregulation and for fine control of salt excretion in the distal nephron. This review focuses on recent studies of the mechanisms of regulation of sodium transport in the proximal tubule, and addresses whether results from studies in proximal tubule cell lines are applicable to the proximal tubule in situ.

RECENT FINDINGS

Recent in-vivo studies provided evidence that sodium/hydrogen exchanger isoform 3 can move into and out of the apical microvilli accompanied by parallel changes in renal sodium transport: the exchanger is retracted from the microvilli in response to hypertension, parathyroid hormone or dopamine treatment and moved into the microvilli in response to sympathetic nervous system stimulation, puromycin aminonucleoside induced nephritic syndrome, and insulin treatment. Studies in cultured opossum kidney proximal tubule cells provided evidence for clathrin coated vesicle mediated, dynamin dependent, cytoskeleton dependent internalization of sodium/hydrogen exchanger isoform 3 from the surface to an endosomal pool in response to dopamine or parathyroid hormone. In the intact proximal tubule there is evidence for a two-step internalization process: (1) from villi to the intermicrovillar cleft region and (2) to a higher density membrane pool that may be either below the microvilli or deep in intermicrovillar clefts. Recent studies have described a significant inactive pool of the exchanger in the intermicrovillar region in vivo that may serve as a storage and recruitable pool.

SUMMARY

The molecular mechanisms responsible for increasing or decreasing sodium transport in the proximal tubule appear to include redistribution of sodium/hydrogen exchanger isoform 3 to or from the microvillar region. Detailed studies in cultured proximal tubule cell lines provide evidence for endocytosis and exocytosis of the exchanger dependent on cytoskeleton and clathrin coated vesicles. In vivo, the apical membrane is differentiated into discrete villar and intermicrovillar membrane domains and the intermicrovillar domain, not observed in cultured cells, may serve as a recruitable storage pool for sodium/hydrogen exchanger isoform 3.

Chronic renal injury-induced hypertension alters renal NHE3 distribution and abundance

Renal cortical phenol injection provokes acute sympathetic nervous system-dependent hypertension and a shift of proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3) and Na(+)-P(i) cotransporter type 2 (NaPi2) to apical microvilli. This study aimed to determine whether proximal tubule (PT) Na(+) transporter redistribution persists chronically and whether the pool sizes of renal Na(+) transporters are altered. At 5 wk after a 50-microl 10% phenol injection, blood pressure is elevated: 154 +/- 8 vs. 113 +/- 11 mmHg after saline injection. Cortical membranes were fractionated into three "windows" enriched in apical brush border (WI), mixed apical and intermicrovillar cleft (WII), and intracellular membranes (WIII). NHE3 relative distribution in these windows, assessed by immunoblots and expressed as %total, remained shifted to apical from intracellular membranes (WI: 25.3 +/- 3 in phenol vs.12.7 +/- 3% in saline and WIII: 9.1 +/- 1.3 in phenol vs. 18.9 +/- 3% in saline). NaPi2 and dipeptidyl-peptidase IV also remained shifted to WI, and alkaline phosphatase activity increased 100.9 +/- 29.7 (WI) and 51.4 +/- 17.5% (WII) in phenol-injected membranes. Na(+) transporter total abundance [NHE3, NaPi2, thiazide-sensitive Na-Cl cotransporter, bumetanide-sensitive Na-K-2Cl cotransporter, Na-K-ATPase alpha(1)- and beta(1)-subunits, and epithelial Na(+) channel (ENaC) alpha- and beta-subunits] was profiled by immunoblotting. Only cortical NHE3 abundance was altered, decreasing to 0.56 +/- 0.06. The results demonstrate that phenol injury provokes a persistant shift of PT NHE3 and NaPi2 to the apical microvilli, along with a 44% decrease in total NHE3, evidence for an escape mechanism that would counteract the redistribution of a larger fraction of NHE3 to the apical surface by normalizing the total amount of NHE3 in apical membranes.

Responses of proximal tubule sodium transporters to acute injury-induced hypertension

Renal injury-induced by phenol injection activates renal sympathetic afferent pathways, increases norepinephrine release from the posterior hypothalamus, activates renal efferent pathways, and provokes a rapid and persistent hypertension. This study aimed to determine whether phenol injury provoked a redistribution of proximal Na(+) transporters from internal stores to the apical cell surface mediated by sympathetic activation, a response that could contribute to generation or maintenance of hypertension. Anesthetized rats were cannulated for arterial blood pressure tracing and saline infusion and then 50 microl 10% phenol or saline was injected into one renal cortex (n = 7 each). Fifty minutes after injection, kidneys were removed and renal cortex membranes from injected kidneys were fractionated on sorbitol gradients and pooled into three windows (WI-WIII) that contained enriched apical brush border (WI); mixed apical, intermicrovillar cleft and dense apical tubules (WII); and intracellular membranes (WIII). Na(+) transporter distributions were determined by immunoblot and expressed as percentage of total in gradient. Acute phenol injury increased blood pressure 20-30 mmHg and led to redistribution of Na(+)/H(+) exchanger type 3 (NHE3) out of WIII (from 22.79 +/- 4.75 to 10.79 +/- 2.01% of total) to WI (13.07 +/- 1.97 to 27.15 +/- 4.08%), Na(+)-P(i) cotransporter 2 out of WII (68.72 +/- 1.95 to 59.76 +/- 2.21%) into WI (9.5 +/- 1.62 to 18.7 +/- 1.45%), and a similar realignment of dipeptidyl-peptidase IV immunoreactivity and alkaline phosphatase activity to WI. Renal denervation before phenol injection prevented the NHE3 redistribution. By confocal microscopy, NHE3 localized to the brush border after phenol injection. The results indicate that phenol injury provokes redistribution of Na(+) transporters from intermicrovillar cleft/intracellular membrane pools to apical membranes associated with sympathetic nervous system activation, which may contribute to phenol injury-induced hypertension.

Changes of cell volume and nitric oxide concentration in macula densa cells caused by changes in luminal NaCl concentration

The luminal NaCl concentration ([NaCl]) at the macula densa (MD) controls both tubuloglomerular feedback (TGF) and renin release. Nitric oxide (NO) inhibits TGF sensitivity to a great extent. The NO concentration in the MD cells is not known. This study measured this concentration in MD cells with confocal microscopy in the isolated perfused thick ascending limb using a NO-sensitive fluorophore 4,5-diaminofluorescein (DAF-2). Calcein was used to measure cell volume changes. The loop perfusion fluid was a modified Ringer solution containing 10, 35, or 135 mM NaCl with a constant total osmolarity (290 mOsm), and the bath was perfused with the 135 mM NaCl solution. The results show that MD cell volume and NO concentration measured with DAF-2 DA increased considerably with increasing luminal [NaCl] and with calcium-free solutions in the lumen and bath. L-arginine (5 mM) increased NO concentration in the MD cells by 30%. 7-nitroindazole could totally inhibit the NO production caused by L-arginine and by increased luminal [NaCl]. In conclusion, the MD cell volume changes caused by the changes of luminal [NaCl] were quantitatively measured, and it was found that increasing the luminal [NaCl] resulted in an increase in cell volume. It was also found that NO formation in MD cells could be measured with DAF-2 and that NO production was increased through neuronal NO synthase activation with an increased luminal [NaCl]. An increased NO production will inhibit the vasoconstriction induced by the TGF and at the same time will reduce TGF sensitivity.

Angiotensin II clamp prevents the second step in renal apical NHE3 internalization during acute hypertension

Acute hypertension inhibits proximal tubule (PT) sodium reabsorption. The resultant increase in NaCl delivery to the macula densa suppresses renin release. We tested whether the sustained pressure-induced inhibition of PT sodium reabsorption requires a renin-mediated decrease in ANG II levels. Plasma ANG II concentration of anesthesized Sprague-Dawley rats was clamped by simultaneous infusion of the ANG I-converting enzyme inhibitor captopril (12 microg/min) and ANG II (20 ng. kg(-1). min(-1)). Blood pressure was increased 50 mmHg for 20 min by arterial constriction +/- ANG II clamp or by sham operation. This acute hypertension increased urine output and endogenous Li(+) clearance, and these responses were blunted 40-50% in ANG II clamped rats. Acute hypertension provoked a rapid redistribution of Na(+)/H(+) exchanger isoform 3 (NHE3) out of apical brush-border membranes (21 +/- 4% decrease of total NHE3 abundance) to endosomal/lysosomal membranes (16 +/- 6% increase of total). In ANG II-clamped rats, acute hypertension also provoked disappearance of NHE3 from the apical membranes (27 +/- 2% decrease of total), but NHE3 was shifted to membranes enriched in intermicrovillar cleft and dense apical tubules (step 1) rather than endosomal/lysosomal membranes (step 2). This difference was independently confirmed by confocal analysis. In contrast, the pressure-induced redistribution of Na(+)-P(i) cotransporter type 2 was not altered by ANG II clamp. We conclude that the responses to acute hypertension, including diuresis and redistribution of PT NHE3 into intracellular membranes, require a responsive renin-angiotensin system and that the responses may be induced by the sustained increase in NaCl delivery to the macula densa during acute hypertension.

Diuretic response to acute hypertension is blunted during angiotensin II clamp

Acute hypertension inhibits proximal tubule (PT) fluid reabsorption. The resultant increase in end proximal flow rate provides the error signal to mediate tubuloglomerular feedback autoregulation of renal blood flow and glomerular filtration rate and suppresses renal renin secretion. To test whether the suppression of the renin-angiotensin system during acute hypertension affects the magnitude of the inhibition of PT fluid and sodium reabsorption, plasma ANG II levels were clamped by infusion of the angiotensin-converting enzyme (ACE) inhibitor captopril (12 microg/min) and ANG II after pretreatment with the bradykinin B(2) receptor blocker HOE-140 (100 microg/kg bolus). Because ACE also degrades bradykinin, HOE-140 was included to block effect of accumulating vasodilatory bradykinins during captopril infusion. HOE-140 increased the sensitivity of arterial blood pressure to ANG II: after captopril infusion without HOE-140, 20 ng x kg(-1) x min(-1) ANG II had no pressor effect, whereas with HOE-140, 20 ng x kg(-1) x min(-1) ANG II increased blood pressure from 104 +/- 4 to 140 +/- 6 mmHg. ANG II infused at 2 ng x kg(-1) x min(-1) had no pressor effect after captopril and HOE-140 infusion ("ANG II clamp"). When blood pressure was acutely increased 50-60 mmHg by arterial constriction without ANG II clamp, urine output and endogenous lithium clearance increased 4.0- and 6.7-fold, respectively. With ANG II clamp, the effects of acute hypertension were reduced 50%: urine output and endogenous lithium clearance increased two- and threefold, respectively. We conclude that HOE-140, an inhibitor of the B(2) receptor, potentiates the sensitivity of arterial pressure to ANG II and that clamping systemic ANG II levels during acute hypertension blunts the magnitude of the pressure diuretic response.

Acute hypertension provokes internalization of proximal tubule NHE3 without inhibition of transport activity

Acute hypertension rapidly decreases proximal tubule (PT) Na(+) reabsorption, facilitated by a redistribution of PT Na(+)/H(+) exchangers (NHE3) out of the apical brush border, increasing NaCl at the macula densa, the signal for autoregulation of renal blood flow and GFR. This study aimed to determine whether NHE3 activity per transporter decreases during acute hypertension and the time dependence of the response. Blood pressure was elevated by 50-60 mmHg in male Sprague-Dawley rats for 5 or 30 min by constricting arteries. Renal cortical membranes were fractionated by density gradient centrifugation. NHE3 transport activity was assayed as the rate of appearance of acridine orange (AO) from AO-loaded vesicles in response to an inwardly directed Na(+) gradient. After 5-min hypertension, 20% of total NHE3 protein, assayed by immunoblot, redistributed from low-density apical membranes to middensity membranes enriched in intermicrovillar cleft markers; by 30 min, a similar percentage shifted to heavier density membranes containing markers of endosomes. NHE3 activity shifted to higher density membranes along with NHE3 protein, that is, no change in activity/transporter during acute hypertension. Confocal analysis of NHE3 distribution also verified removal from apical microvilli and appearance in subapical vesicles. We conclude that the decrease in renal PT Na(+) transport during acute hypertension is mediated by removal of transport-competent NHE3 from the apical brush border to subapical and internal reserves.

Impaired renal autoregulation: implications for the genesis of hypertension and hypertension-induced renal injury

In summary, autoregulation of the renal vasculature provides a mechanism by which renal function is maintained relatively constant despite variations in systemic blood pressure. This system also provides a means for changes in blood pressure to occur without causing inappropriate alterations in urinary NaCl excretion. Alterations in the autoregulatory response can have clinical consequences. Increased activity of the TGF mechanism may be causally related to the development of some forms of hypertension. Decreased activity of TGF or an impaired myogenic response may help explain the increased susceptibility that certain patient groups exhibit toward hypertension-induced renal injury. The aggressive treatment of hypertension in patients with impaired renal autoregulation may be associated with an increase in the serum creatinine concentration. As long as this increase is neither excessive nor progressive, physicians should not be dissuaded from trying to achieve newly established blood pressure goals.

Role of macula densa nitric oxide and cGMP in the regulation of tubuloglomerular feedback

BACKGROUND

Previous studies have suggested that nitric oxide (NO) produced within cells of the macula densa (MD) modulates tubuloglomerular feedback (TGF). We tested the hypothesis that NO produced in the MD acts locally as an autacoid to activate soluble guanylate cyclase and cGMP-dependent protein kinase in the MD itself.

METHODS

Rabbit afferent arterioles (Af-Arts) and attached MD were simultaneously microperfused in vitro. The TGF response was determined by measuring the Af-Art diameter before and after increasing NaCl in the MD perfusate (from 17 mmol/L of Na and 2 of Cl to 65 mmol/L of Na and 50 of Cl). TGF was studied before (control TGF) and after inhibiting components of the NO-cGMP-dependent cascade in the tubular or vascular compartment.

RESULTS

Increasing NaCl concentration in the MD perfusate decreased the Af-Art diameter by 3.2 +/- 0.5 microm (from 18.5 +/- 1.3 to 15.4 +/- 1.3 microm, P < 0.001). Adding a soluble guanylate cyclase inhibitor (LY83583) to the MD increased TGF response to 6.3 +/- 1.1 microm (P < 0.031 vs. control TGF). Similarly, when cGMP-dependent protein kinase was inhibited with KT5823, TGF was augmented from 2.6 +/- 0.3 to 4.0 +/- 0.7 microm (P < 0.023). An analogue of cGMP in the MD reversed the TGF-potentiating effect of both 7-nitroindazole (7NI; an nNOS inhibitor) and LY83583. Inhibition of MD guanylate cyclase did not alter the effect of acetylcholine (a NO-cGMP-dependent vasodilator) on the Af-Art. Perfusing the Af-Art with the guanylate cyclase inhibitor did not potentiate TGF, suggesting that the effect of NO occurred at the MD via a cGMP-dependent mechanism. To determine whether the effect of NO in the MD was entirely mediated by cGMP, TGF was studied after giving (1) LY83583 or (2) LY83583 plus 7NI. Adding the nNOS inhibitor to the MD did not potentiate the TGF response further.

CONCLUSIONS

We concluded the following: (1) NO produced by the MD inhibits TGF via stimulation of soluble guanylate cyclase, generating cGMP and activating cGMP-dependent protein kinase; (2) NO acts on the MD itself rather than by diffusing to the Af-Art; and (3) most, if not all, of the effect of NO in the MD is due to a cGMP-dependent mechanism rather than to other NO mediators.

Constitutive nitric oxide synthesis in the kidney--functions at the juxtaglomerular apparatus

Tubulo-vascular information transfer at the renal juxtaglomerular apparatus (JGA) serves to adjust the biosynthesis and release of renin, the key enzyme of the renin angiotensin system, and to regulate glomerular arteriolar muscle tone. The macula densa serves as a sensor of tubular NaCl. Concentration-dependent salt uptake through the Na-K-2Cl cotransporter located in the apical membrane of macula densa cells triggers a signal transduction cascade that involves the synthesis of nitric oxide (NO) through a type 1 NO synthase (NOS1) which is described with respect to its complex mRNA structure and regulatory aspects. The anatomical and functional targets of the NO-soluble guanylyl cyclase-cGMP pathway at the JGA are reviewed.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Role of cGMP-kinase II in the control of renin secretion and renin expression

To investigate the roles of the cGMP-dependent protein kinases (cGKs) in the control of the renin system, we studied the regulation of renin in cGKI- or cGKII-deficient mice in vivo and in vitro. Renal renin mRNA levels both under stimulatory (low-salt diet plus ramipril) and inhibitory (high-salt diet) conditions were not different between wild-type and cGKI-/- mice, but were significantly elevated in cGKII-/- mice under all experimental conditions. In primary cultures of renal juxtaglomerular cells (JG) established from wild-type, cGKI-/-, and cGKII-/- mice, the adenylate cyclase activator forskolin stimulated renin secretion similarly in all genotypes tested. 8-bromo-cGMP attenuated basal and forskolin-stimulated renin secretion in cultures from wild-type and cGKI-/-, but had no effect in cells isolated from cGKII-/- mice. Activation of cGKs by 8-bromo-cGMP decreased renin secretion from the isolated perfused rat kidney, independent of prestimulation by beta-adrenoreceptor activation, macula densa inhibition, reduced perfusion pressure, or by a nominally calcium-free perfusate. Taken together, these findings suggest that activation of cGKII has a general inhibitory effect on renin secretion from renal JG cells.

Renal expression of transforming growth factor-beta inducible gene-h3 (beta ig-h3) in normal and diabetic rats

BACKGROUND

Transforming growth factor-beta (TGF-beta) has been implicated in the pathogenesis of a number of kidney diseases characterized by glomerulosclerosis and tubulointerstitial fibrosis. TGF-beta is secreted in a latent form requiring extracellular modification to become biologically active. TGF-beta inducible gene-h3 (beta ig-h3) is a recently identified TGF-beta-induced gene product. The present study sought to examine beta ig-h3 expression in normal and diabetic rats.

METHODS

Beta ig-h3, TGF-beta1 and alpha1 (IV) collagen gene expression were assessed by Northern blot analysis and in situ hybridization in 20 Sprague Dawley rats, randomly assigned to receive streptozotocin (diabetic, N = 11) or citrate buffer alone (control, N = 9) and sacrificed eight months later. The effect of exogenous TGF-beta1 on beta ig-h3 expression was also assessed in cultured proximal tubular cells.

RESULTS

In situ hybridization localized beta ig-h3 gene expression to the juxtaglomerular apparatus and the pars recta (S3 segment) of proximal tubules in both control and diabetic animals. Kidney TGF-beta 1, beta ig-h3 and alpha1 (IV) collagen mRNA from diabetic rats were increased two- to threefold compared with controls (P < 0.01). There was a significant correlation between TGF-beta1 and beta ig-h3 gene expression in kidneys from diabetic rats (r = 0.73, P = 0.01). In addition, beta ig-h3 mRNA increased in response to exogenous TGF-beta1 in a dose-dependent fashion in cultured proximal tubular cells.

CONCLUSION

These findings support the hypothesis that biologically active TGF-beta plays a pathogenetic role in diabetic kidney disease and suggest that beta ig-h3 may be a useful index of TGF-beta1 bioactivity in the kidney.

Alpha-adducin polymorphisms and renal sodium handling in essential hypertensive patients

The relationship between blood pressure and sodium (Na) excretion is less steep in hypertension caused by increased renal tubular reabsorption. We recently demonstrated that one mutation in rat alpha-adducin gene: (1) is responsible for approximately 50% of the hypertension of MHS rats, and (2) stimulates tubular Na-K pump activity when transfected in renal epithelial cell, suggesting that its pressor effect may occur because an increased tubular reabsorption. Linkage and association studies demonstrated that the alpha-adducin locus is relevant for human hypertension. A point mutation (G460W) was found in human alpha-adducin gene, the 460W variant (G/W) is more frequent in hypertensives than in normotensives. The aim of this study was to test whether acute changes in body Na may differently affect blood pressure in humans as a function of alpha-adducin genotype. The pressure-natriuresis relationship was analyzed in 108 hypertensive using two different acute maneuvers: Na removal (furosemide 25 mg p.o.) and, two days later, Na load (310 mmoles i.v. in 2 hr). We found that 80 patients were wild-type homozygous (G/G), 26 were G/W heterozygous, and 2 were W/W homozygous with similar blood pressure, age body mass index, gender, plasma and urinary sodium and potassium. In basal condition G/W-W/W patients showed a lower plasma renin activity and fractional excretion of Na. In either case the pressure-natriuresis relationship was less sleep in G/W-W/W than in G/G patients, obviously negative for Na depletion with furosemide (-0.011 +/- 0.004 vs. -0.002 +/- 0.002 mm Hg/mumol/min, P < 0.03), and positive for Na load (0.086 +/- 0.02 vs. 0.027 +/- 0.007 mm Hg/mumol/min, P < 0.001). The finding of reduced slope after Na depletion or Na load supports the hypothesis that, as MHS rats, humans bearing one W alpha-adducin variant display an increased of renal tubular sodium reabsorption.

Expression of neuronal type nitric oxide synthase and renin in the juxtaglomerular apparatus of angiotensin type-1a receptor gene-knockout mice

Angiotensin type-1a (AT1a) receptor gene-knockout (AT1a-/-) mice exhibit chronic hypotension and renin overproduction. In the kidneys of AT1a-/- mice, the activity of neuronal type nitric oxide synthase (N-NOS) was histochemically detected by nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase (NADPHd) reaction combined with N-NOS immunohistochemistry. The localization of renin was detected by immunohistochemistry and the results were analyzed morphometrically. The levels of N-NOS and renin mRNA in the renal cortical tissue were determined by reverse transcription-PCR and Northern blot analysis, respectively. In the renal sections from wild-type mice, NADPHd activity and N-NOS immunoreactivity were localized to the discrete region of the macula densa in contact with the parent glomerulus. In contrast, N-NOS-positive macula densa cells were distributed beyond the original location of the macula densa, occasionally extending to the opposite side of the distal tubules. The mean number of N-NOS positive macula densa cells was significantly increased in AT1a-/- mice (186 per 100 glomeruli) compared with wild-type mice (65 per 100 glomeruli). AT1a-/- mice showed 1.4-times higher N-NOS mRNA levels in the renal cortical tissues than wild-type mice. The plasma renin activity was significantly higher in AT1a-/- mice (205.5 +/- 26.1 ng/ml/hr) than in wild-type mice (8.0 +/- 0.2 ng/ml/hr). The renin-positive areas per glomerulus and renal renin gene expression were 12-times and 2.6-times higher in AT1a-/- mice than in wild-type mice, respectively. These abnormalities, however, were less remarkable in AT1a-/- mice compared with angiotensinogen-knockout mice. When AT1a-/- mice were fed a high-salt diet, the signal intensity of the NADPHd reaction and the number of positively-stained macula densa cells were significantly decreased. The levels of renal cortical N-NOS mRNA were also suppressed by the treatment. Dietary salt loading produced a parallel decrease in plasma renin activity, renal renin-immunoreactive areas, and the levels of renin mRNA without affecting systemic blood pressure. These results provide evidence for the possible involvement of N-NOS at the macula densa in the increased renin production in AT1a-/- mice.

Hypothesis: the role of acquired tubulointerstitial disease in the pathogenesis of salt-dependent hypertension

We present a new hypothesis to explain the development of salt-dependent hypertension in humans. We propose that hypertension has two phases: an early phase in which elevations in blood pressure (BP) are mainly episodic and are mediated by a hyperactive sympathetic nervous or renin-angiotensin system, and a second phase in which BP is persistently elevated and that is primarily mediated by an impaired ability of the kidney to excrete salt (NaCl). We propose that the transition from the first phase to the second occurs as a consequence of catecholamine-induced elevations in BP that preferentially damage regions of the kidney (juxtamedullary and medullary regions) that do not autoregulate well to changes in renal perfusion pressure. The catecholamine response is associated with both an increase in peritubular capillary pressure and a reduction in peritubular capillary plasma flow, resulting in injury to the peritubular capillaries with ischemia to the tubules and interstitium. The local injury triggers the release or activation (angiotensin II, adenosine, renal sympathetic nerves) or inhibition (nitric oxide, prostaglandins, dopamine) of vasoactive mediators that further augment ischemia and result in abnormal tubuloglomerular feedback and enhanced NaCl reabsorption. The peritubular capillary injury with rarefaction simultaneously blunts the pressure natriuresis mechanism. The combined effect of enhanced tubuloglomerular feedback and impaired pressure natriuresis results in a defect in NaCl excretion which, on the exposure to salt, results in the development of persistent hypertension. Evidence is provided to suggest that this may be the major mechanism for the development of salt-dependent hypertension, and particularly for the hypertension associated with blacks, aging and obesity. Thus, essential hypertension may be a type of acquired tubulointerstitial renal disease.

Ontogeny of angiotensin II receptors, types 1 and 2, in ovine mesonephros and metanephros

By RNAse protection assay, hybridization histochemistry, and in vitro autoradiography it was shown that both mRNA and protein for AT1 and AT2 receptors were present in ovine fetal meso- and metanephroi at 40 days of gestation (term approximately 150 days). AT1 mRNA was localized to presumptive mesangial cells of glomeruli at 40-, 75-, 131-gestational-day-old fetuses and two-day-old lambs, in addition to being widely present in interstitial cells of the cortex and medulla, once these zones formed (60 days). By two days after birth the medullary AT1 distribution was confined to the inner stripe of the outer medulla. AT2 mRNA was present in peripheral interstitial/tissue of the mesonephros, and interstitial tissue surrounding developing glomeruli, but not the outermost nephrogenic mesenchyme in the metanephros from 40 to approximately 131 days (the period of active nephrogenesis). In addition, AT2 mRNA was localized to epithelial cells of the macula densa in metanephroi (40 to 131 gestational days) during, but not after completion, of nephrogenesis. These studies suggest that angiotensin II (Ang II) could have differentiating effects, via AT1 receptors, from very early in development. The unique epithelial site of AT2 expression in the macula densa raises the possibility that Ang II may play a role in the invariant positioning of the macula densa at the pole of its glomerulus, via this receptor.

Renin-1 is essential for normal renal juxtaglomerular cell granulation and macula densa morphology

The secretion of renin from granules stored in renal juxtaglomerular cells plays a key role in blood pressure homeostasis. The synthesis and release of renin and the extent of granulation is regulated by several mechanisms including signaling from the macula densa, neuronal input, and blood pressure. Through the use of a gene-targeting vector containing homology arms generated using the polymerase chain reaction, we have inactivated the Ren-1(d) gene, one of two mouse genes encoding renin, and report that lack of renin-1(d) results in altered morphology of the macula densa of the kidney distal tubule and complete absence of juxtaglomerular cell granulation. Furthermore, Ren-1(d-/-) mice exhibit sexually dimorphic hypotension. The altered growth morphology of the macula densa in Ren-1(d)-null mice should provide a tool for the investigation of the JG cell-macula densa signaling. Furthermore, the current data indicate that expression of the Ren-1(d) gene is a prerequisite for the formation of storage granules, even though the related protein renin-2 is present in these mice, suggesting that renin-1(d) and renin-2 are secreted by distinct pathways in vivo.