Membranous glomerulopathy in an adult patient with X-linked agammaglobulinemia receiving intravenous gammaglobulin

BACKGROUND

Immune complex deposition in the subepithelial zone of glomerular capillaries can lead to membranous glomerulopathy.

OBJECTIVE

To present the case of a 23-year-old man with X-linked agammaglobulinemia (XLA) who developed idiopathic membranous glomerulopathy while receiving intravenous immunoglobulin (IVIG).

METHODS

We performed an immunological workup, genetic testing, and a renal biopsy.

RESULTS

XLA was confirmed with less than 0.02% CD19+ cells in the blood after sequence analysis revealed a nonfunctional BTK gene. The patient presented with microhematuria, which persisted for 3 years and spanned treatment with 5 different preparations of intravenous gammaglobulin. Immunohistochemistry revealed membranous glomerulopathy.

CONCLUSION

Although endogenous serum immunoglobulin (Ig) production is severely impaired in XLA, rare B lymphocytes that have managed to mature can produce functional IgG antibodies. The pathogenic immune complexes could reflect IVIG reacting with polymorphic autoantigens, an endogenous IgG-producing clone reacting with a common idiotype present in the IVIG, or both.

Improving outcomes from acute kidney injury (AKI): Report on an initiative

Acute Kidney Injury (AKI) is a complex disorder for which currently there is no accepted definition. We describe an initiative to develop uniform standards for defining and classifying AKI and establish a forum for multidisciplinary interaction to improve care for patients with, or at risk for AKI. Members representing key societies in critical care and nephrology along with additional experts in adult and pediatric AKI participated in a 2-day conference in Amsterdam in September 2005 to draft consensus recommendations for diagnosing and staging AKI. This report describes the proposed diagnostic and staging criteria for AKI and the formation of a multidisciplinary collaborative network.

Liddle syndrome: genetics and mechanisms of Na+ channel defects

Our current understanding of Na+ transport defects has been greatly expanded over the last several years and has provided new insights into unusual clinical syndromes resulting from mutations of specific ion transporters. These genetic disorders affect Na+ balance, with both Na+ retaining and Na+ wasting conditions being the consequence. A major focus of these studies has been the epithelial sodium channel (ENaC), which can be directly affected by mutations (eg, Liddle syndrome, autosomal recessive pseudohypoaldosteronism, type I) or by changes in the response to (autosomal recessive pseudohypoaldosteronism, type I), or production of mineralocorticoids (apparent mineralocorticoid excess syndrome, glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined syndromes in which ENaC activity is "dysregulated" with subsequent development of disorders of systemic blood pressure that can be attributed to a primary renal mechanisms. The focus of the current review is on Liddle syndrome ("pseudoaldosteronism").

Prevention, protection, and the intrarenal renin-angiotensin systems

The use of angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor antagonists, or aldosterone antagonists can have important beneficial effects on the progression of renal disease associated with glomerular and interstitial fibrosis, especially if the adverse side effects (eg, hyperkalemia) can be minimized. Because it appears that chronic renal insufficiency and proteinuria may well be cardiovascular risk factors, it is exciting to note that recent large scale epidemiologic studies (Heart Outcomes and Prevention Evaluation [HOPE]) have shown both cardioprotective and renoprotective effects of ACE inhibition. It appears paradoxic that such renoprotective effects are clearly evident in diabetes mellitus in which the plasma renin activity may be suppressed. Even in this setting, it appears that there is activation of the renal angiotensin system(s), and inhibitions of these intrarenal systems are involved in the renoprotective effects of these agents. Recent studies have identified nearly all of the components needed to generate angiotensin II in the renal luminal compartment, and suggest that there may be a direct effect through AT(1) receptors on NaCl transport in the distal nephron. The possibility that the components of this intraluminal renin-angiotensin system may be acutely regulated by variations in dietary salt intake provides an opportunity to better understand the normal maintenance of salt balance. Whether or not inhibition of these pathways is involved in the renoprotective effects of ACE inhibitors and angiotensin receptor antagonists is an important issue to be addressed.

Localization and functional characterization of Na+/H+ exchanger isoform NHE4 in rat thick ascending limbs

The Na+/H+ exchanger NHE4 was cloned from a rat stomach cDNA library and shown to be expressed predominantly in the stomach and less dramatically in the kidney. The role and precise localization of NHE4 in the kidney are still unknown. A polyclonal antibody against a unique NHE4 decapeptide was used for immunohistochemistry in rat kidney. Simultaneous use of antibodies to Tamm-Horsfall glycoprotein and aquaporin-2 or -3 permitted identification of thick ascending limbs and collecting ducts, respectively. The results indicate that NHE4 is highly expressed in basolateral membranes of thick ascending limb and distal convoluted tubule, whereas collecting ducts from cortex to inner medulla and proximal tubules showed weaker basolateral NHE4 expression. Western blot analysis of NHE4 in membrane fractions prepared from the inner stripe of the outer medulla revealed the presence of a 95-kDa protein that was enriched in basolateral membrane vesicles isolated from medullary thick ascending limbs. The inhibition curve of H+-activated (22)Na uptake by 5-(N-ethyl-N-isopropyl)amiloride (EIPA) was consistent with the presence, beyond the EIPA high-affinity NHE1 isoform, of an EIPA low-affinity NHE with apparent half-maximal inhibition of 2.5 microM. Kinetic analyses showed that the extracellular Na+ dependence of NHE4 activity followed a simple hyperbolic relationship, with an apparent affinity constant of 12 mM. Intravesicular H+ activated NHE4 by a positive cooperative mechanism. NHE4 had an unusual low affinity for intravesicular H+ with a half-maximal activation value of pK 6.21. We conclude that NHE4, like NHE1, is expressed on the basolateral membrane of multiple nephron segments. Nevertheless, these two proteins exhibited dramatically different affinities for intracellular H+, suggesting that they may play distinct physiological roles in the kidney.

An Alu cassette in the human epithelial sodium channel

Here, we report the presence of two splice variants of the human epithelial sodium channel alpha subunit (h alpha ENaC) containing Alu cassette, namely h alpha ENaC+22 and h alpha ENaC+Alu, in various tissues. Functional expression of these splice variants with hENaC beta and gamma subunits produced loss-of-channel activity in the Xenopus oocyte expression system. Interestingly, coexpression of h alpha ENaC+22 or h alpha ENaC+Alu, respectively, with wild type hENaC alpha, beta, and gamma subunits enhanced the expression of amiloride-sensitive current in oocytes. The presence of Alu sequences in the 3'-untranslated region of h gamma ENaC was also identified.

Genetic forms of human hypertension

Our basic understanding of sodium mechanisms provides unique insights into epithelial transport processes, and unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect sodium balance, with both sodium-retaining and sodium-wasting conditions being the consequence. A major focus of such studies has been the epithelial sodium channel, which can be activated by mutations in the channel subunits or mineralocorticoid receptor, and changes in the response to or production of mineralocorticoids. As a result, there are now clearly defined Mendelian syndromes in which epithelial sodium channel activity is 'dysregulated', with the subsequent development of systemic hypertension with suppressed plasma renin activity that can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension, and more clearly define the interactions of dietary constituents such as sodium and potassium in the regulation of blood pressure.

Simplified citrate anticoagulation for continuous renal replacement therapy

BACKGROUND

Regional anticoagulation with trisodium citrate is an effective form of anticoagulation for continuous renal replacement therapy (CRRT) for patients with contraindications to heparin. However, because of the metabolic complications of trisodium citrate, it is a complicated technique requiring specialized dialysis solutions. We have designed a simplified protocol for citrate regional anticoagulation for CRRT.

METHODS

Two percent trisodium citrate was delivered at 250 mL/h via the prefilter port of a COBE PRISMA device, with the rate adjusted to maintain a postfilter ionized calcium (iCa++) <0.5 mmol/L. A central calcium gluconate infusion was used to maintain a systemic iCa++ at 1.1 mmol/L. A standard dialysate solution consisting of 0.9% saline, KCl 3 mmol/L, and MgSO4 1 mmol/L was delivered at 1000 mL/h. We retrospectively reviewed the outcomes and complications associated with this protoco1 in 29 patients treated from July 1999 to October 1999, evaluating the frequency of clotting of the dialyzer, bleeding complications, citrate toxicity, and patient mortality.

RESULTS

The Kaplan-Meier curve for dialyzer survival demonstrated a 61% survival rate at 48 hours. There were no episodes of significant bleeding or citrate toxicity. Seventy-two percent of patients died for reasons unrelated to CRRT.

CONCLUSIONS

A CRRT protocol using regional 2% trisodium citrate anticoagulation is not associated with significant bleeding complications or citrate toxicity, and represents a simplified approach compared with previous applications using 4% trisodium citrate.

Genetic renal tubular disorders of renal ion channels and transporters

This perspective on genetic renal tubular transport disorders selectively reviews the pathophysiology of renal apical Na(+) transport systems. These transporters play an essential role in the control of extracellular fluid volume and blood pressure. Significant advancements in the understanding of the role of these genes in Mendelian forms of extracellular volume homeostatic disorders have been achieved in the recent years. Of even greater importance will be the ongoing definition of the various factors that regulate the expression and activity of the Na(+) transport systems. These regulatory pathways, and the responses to environmental factors such as dietary salt, stress, and so on, may determine the appearance, severity and complexity of the clinical phenotypes that result from genetic disorders of the renal apical Na(+) transporters.

Disorders of the epithelial Na(+) channel in Liddle's syndrome and autosomal recessive pseudohypoaldosteronism type 1

The epithelial Na(+) channel (ENaC) is the key step in many Na(+)-absorptive epithelia, such as kidney and distal colon, that controls the overall rate of transepithelial Na(+) transport. ENaC is composed of three homologous subunits, alpha, beta, and gamma. The alpha subunit is the key subunit for the formation of a functional ion channel, while the beta and gamma subunits can greatly potentiate the level of expressed Na(+) currents. ENaCs belong to the recently identified DEG/ENaC supergene family, sharing the same basic structure with cytoplasmic amino and carboxy termini, two transmembrane regions, and a large extracellular loop. The human ENaC genes have been cloned, and using genetic linkage analysis the involvement of ENaC gene mutations in two distinct human diseases, Liddle's syndrome and autosomal recessive pseudohypoaldosteronism type 1 (PHA-1), has been demonstrated. In Liddle's syndrome, gain-of-function mutations in the beta or gamma ENaC subunits have been found; all identified mutations so far reside in the carboxy terminus of the protein, either deleting or modifying the functionally important PY motif. In PHA-1, loss-of-function mutations in the alpha, beta, or gamma subunits have been found; these mutations either truncate a significant portion of the structure or modify an amino acid that plays an important role in channel function. In this review, our current understanding about ENaC and the pathophysiology of Liddle's syndrome and PHA-1 caused by ENaC mutations will be discussed.

Interaction of syntaxins with epithelial ion channels

Recent evidence suggests that some of the syntaxin isoforms may physically interact with and regulate the transport activity of a defined set of membrane transport proteins. This review examines recent studies of the cystic fibrosis transmembrane conductance regulator and the epithelial sodium channel which define distinct roles of syntaxin 1A and syntaxin 3 in the regulation of surface expression as well as intrinsic properties of these epithelial ion transporters.

Aldosterone-related genetic effects in hypertension

Our basic understanding of Na(+) transport mechanisms provides unique insights into epithelial transport processes. Unusual clinical syndromes can arise from mutations of these ion transporters. These genetic disorders affect Na(+) balance, resulting in both N(a+) retaining and Na(+) wasting conditions. A major focus has been the epithelial sodium channel (ENaC), which can be activated by mutations (eg, Liddle's syndrome), changes in the response to mineralocorticoids (apparent mineralocorticoid excess syndrome), or production of mineralocorticoids (glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined Mendelian syndromes in which ENaC activity is "dysregulated." This dysregulation leads to systemic hypertension associated with suppressed plasma renin activity, which can be attributed to a primary renal mechanism. Applying these insights to the far more common disorder of low-renin hypertension may shed new light on the underlying pathophysiology of this common form of human hypertension.

Macula densa Na(+)/H(+) exchange activities mediated by apical NHE2 and basolateral NHE4 isoforms

Functional and immunohistochemical studies were performed to localize and identify Na(+)/H(+) exchanger (NHE) isoforms in macula densa cells. By using the isolated perfused thick ascending limb with attached glomerulus preparation dissected from rabbit kidney, intracellular pH (pH(i)) was measured with fluorescence microscopy by using 2',7'-bis-(2-carboxyethyl)-5-(and -6) carboxyfluorescein. NHE activity was assayed by measuring the initial rate of Na(+)-dependent pH(i) recovery from an acid load imposed by prior lumen and bath Na(+) removal. Removal of Na(+) from the bath resulted in a significant, DIDS-insensitive, ethylisopropyl amiloride (EIPA)-inhibitable decrease in pH(i). This basolateral transporter showed very low affinity for EIPA and Hoechst 694 (IC(50) = 9.0 and 247 microM, respectively, consistent with NHE4). The recently reported apical NHE was more sensitive to inhibition by these drugs (IC(50) = 0.86 and 7.6 microM, respectively, consistent with NHE2). Increasing osmolality, a known activator of NHE4, greatly stimulated basolateral NHE. Immunohistochemical studies using antibodies against NHE1-4 peptides demonstrated expression of NHE2 along the apical and NHE4 along the basolateral, membrane, whereas NHE1 and NHE3 were not detected. These results suggest that macula densa cells functionally and immunologically express NHE2 at the apical membrane and NHE4 at the basolateral membrane. These two isoforms likely participate in Na(+) transport, pH(i), and cell volume regulation and may be involved in tubuloglomerular feedback signaling by these cells.

Low renin hypertension in the next millennium

With the expression cloning of the subunits of the epithelial sodium channel, a new era has evolved in our basic understanding of the low-renin forms of human hypertension. The monogenic hypertensive syndromes manifest dysregulation of the epithelial sodium channel in the cortical collecting tubule. These rare syndromes provide a schema for organizing our thinking about the more common form(s) of low renin hypertension, and raise the possibility that dysregulation of sodium channel activity and consequent salt retention and volume expansion provide a basic pathophysiological mechanism for low-renin hypertension. What are needed are more specific agents to interrupt the mineralocorticoid response pathways, and clinically relevant approaches to measuring sodium channel activity at the level of the collecting tubule in the individual patient. The combined use of aldosterone antagonists and angiotensin-converting enzyme inhibitors or angiotensin receptor antagonists could have a beneficial effect on "progression" of renal disease associated with glomerular and interstitial fibrosis, especially if the effects of hyperkalemia on the heart and aldosterone secretion can be minimized.

Regulation by PKC isoforms of Na(+)/H(+) exchanger in luminal membrane vesicles isolated from cortical tubules

The present study was designed to determine the Na/H exchanger isoforms present in luminal membrane vesicles (LMV) isolated from rat kidney cortical tubule suspensions, as well as the effects of acute phorbol ester (phorbol myristate acetate, PMA) and angiotensin II (ANG II) pretreatment of suspensions on NHE activity and protein kinase C (PKC) isoform abundance. In LMV, both NHE3 and NHE2 proteins were found by Western blot analysis, but only ethylisopropylamiloride-sensitive and almost completely Hoe-694-resistant Na/H exchange activity was observed from (22)Na uptake and thus attributed to NHE3. PMA pretreatment increased Na/H exchange activity and PKC isoforms alpha, delta, and epsilon abundance in LMV, and these effects were prevented by PKC inhibition. Low-dose ANG II (10(-11) M) pretreatment increased Na/H exchange activity and only PKC-zeta abundance in LMV, and these effects were also prevented by PKC inhibition. After high-dose ANG II (10(-7) M), Na/H exchange activity was decreased in LMV. PKC inhibition did not prevent this effect. In conclusion, the stimulating effects of PMA and low-dose ANG II are explained by the translocation of different isoforms of PKC in LMV, whereas the inhibitory effect of high-dose ANG II is not PKC dependent.

Interaction of syntaxins with the amiloride-sensitive epithelial sodium channel

Amiloride-sensitive sodium channels mediate sodium entry across the apical membrane of epithelial cells in variety of tissues. The rate of Na(+) entry is controlled by the regulation of the epithelial sodium channel (ENaC) complex. Insertion/retrieval of the ENaC complex into the apical membrane as well as direct kinetic effects at the single channel level are recognized mechanisms of regulation. Recent data suggest that the syntaxin family of targeting proteins interact with and functionally regulate a number of ion channels and pumps. To evaluate the role of these proteins in regulating ENaC activity, we co-expressed rat ENaC cRNA (alpha, beta, gamma subunits) with syntaxin 1A or 3 cRNAs in Xenopus oocytes. Basal ENaC currents were inhibited by syntaxin 1A and stimulated by syntaxin 3. Both syntaxin 1A and syntaxin 3 could be co-immunoprecipitated with ENaC subunit proteins, suggesting physical interaction. Interestingly, immunofluorescence data suggest that with either syntaxin isoform the ENaC-associated epifluorescence on the oocyte surface is enhanced. These data indicate that (i) both syntaxin isoforms increase the net externalization of the ENaC channel complex, (ii) that the functional regulation is isoform specific, and (iii) suggest that ENaC may be regulated through mechanisms involving protein-protein interactions.

Hypertension

The monogenic forms of human hypertension have yielded to the power of modern genetic techniques in the last several years. With the successful expression cloning of the subunits of the epithelial sodium channel, a whole era has evolved in our basic understanding of the low renin forms of human hypertension. Of note, all of these hypertensive syndromes (Liddle's syndrome, glucocorticoid-remediable aldosteronism, and the apparent mineralocorticoid excess syndrome) share an underlying dysregulation of the activity of the epithelial sodium channel in the cortical collecting tubule. Loss of function defects due to mutations in the channel subunits themselves, or in the mineralocorticoid receptor (pseudohypoaldosteronism, type I) also affect blood pressure regulation consequent to renal salt wasting and dysregulation of the epithelial sodium channel in the cortical collecting tubule.

The epithelial sodium channel in hypertension

Our understanding of Na(+) transport defects has exploded in the past several years, and has provided unique insights into epithelial transport processes, and unusual clinical syndromes resulting from mutations of specific ion transporters. These genetic disorders affect Na(+) balance, with both Na(+) retaining and Na(+) wasting conditions being the consequence. A major focus of these studies has been the epithelial sodium channel (ENaC), which can be directly affected by mutations (eg, Liddle syndrome, autosomal recessive pseudohypoaldosteronism, type I) or by changes in the response to (autosomal recessive pseudohypoaldosteronism, type I), or production of mineralocorticoids (apparent mineralocorticoid excess syndrome, glucocorticoid-remediable aldosteronism). As a result, we now have clearly defined syndromes in which ENaC activity is dysregulated with subsequent development of disorders of systemic blood pressure that can be attributed to a primary renal mechanism.

Expression of transfected human Na+/H+ exchanger (NHE-1) in the the basolateral membrane of opossum kidney cells

Some epithelial cells have Na+/H+ exchanger (NHE) activity in both apical and basolateral membranes. Amiloride-sensitive NHE-1 is generally identified in the basolateral membrane. The renal cell line, OK7a, targets amiloride-resistant NHE predominantly to the apical membrane. It is controversial whether the transfected NHE-1 is targeted preferentially to the basolateral membrane in OK7a cells, when human NHE-1 is chronically expressed under control of constitutively active promoters. We tried to identify the membranes in which the transfected human NHE-1 could be detected following acute expression in OK7a cells. We have always observed small Na(+)-dependent pH recovery in the basolateral membrane in OK7a cells. It is, however, controversial whether or not OK7a cells express NHE activity in the basolateral membrane. We also characterized Na(+)-dependent pH recovery in the basolateral membrane. It was not inhibited by [4,4'diisothiocyanatostilbene-2,2'-disulfonic acid] (DIDS), [4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid] (SITS), or contralateral amiloride. Li+ but not K+, chol+, or NMG+ could replace Na+. These results are consistent with the presence of the NHE in the basolateral membrane. NHE activities were predominant in the apical membrane and those in both membranes were resistant to amiloride analogs. After stable transfection with human NHE-1 in a vector utilizing the metallothionein promoter, overnight induction with Zn(2+)increased the NHE activity and its sensitivity to amiloride only in the basolateral membrane in OK7a cells. We conclude that the transfected human NHE-1 is exclusively targeted to the basolateral membrane of OK7a cells during acute induction.

Immunolocalization of the Na+/H+ exchanger isoform NHE2 in rat kidney

Four Na+/H+ exchangers (NHE1 to NHE4) have been detected in the kidney. Renal NHE2 expression sites have not been fully established. We have raised rabbit antisera against an oligopeptide related to the amino acids 652 to 661 of rat NHE2. Western blot analysis of plasma membrane fractions isolated from rat renal cortex showed that affinity-purified anti-NHE2 antibody detected an 85-kDa protein in apical but not in basolateral membranes. The labeling of this 85-kDa protein was specifically blocked by preincubation of the antibody with its monomeric peptide, indicating specific recognition. Indirect immunolabeling was performed on sections of paraformaldehyde-fixed rat kidney embedded in paraffin. Strong staining was seen in the apical membrane of cortical thick ascending limbs, distal convoluted tubules, and connecting tubules. Much weaker apical staining was found in medullary thick ascending limbs of Henle. In the inner medulla, some thin limbs were intensively labeled by the anti-NHE2 antibody. No staining could be detected in any segments of the proximal tubule and collecting duct.

Inverse relationship of urinary cyclic GMP to blood pressure reactivity in the CARDIA study: vasodilatory regulation of sympathetic vasoconstriction. Coronary Artery Risk Development in Young Adults

OBJECTIVE

To determine whether urinary cyclic GMP (cGMP), which mediates the actions of the vasodilators nitric oxide and atrial natriuretic factor, is inversely related to blood pressure (BP) reactivity. In previous work, we found that urinary cGMP was inversely related to diastolic BP, but cGMP levels were higher among individuals presumed to have increased adrenergic activity, increased reactivity, and increased risk of hypertension (blacks, individuals with a family history of hypertension).

METHOD

We measured 24-hour urinary cGMP levels in a substudy of 529 individuals in the Coronary Artery Risk Development in Young Adults (CARDIA) study; the sample was 23 to 35 years of age and approximately balanced for race (black/white) and gender. BP reactivity to stressors (video game, star-tracing, cold pressor) was tested 3 years earlier. Baseline BP was included as a covariate in all analyses.

RESULTS

Diastolic BP reactivity to cold pressor was inversely related to cGMP excretion (p < .05); the relationship was strongest among black women with a family history of hypertension (partial r = -.33, p < .01). Systolic BP reactivity to star-tracing was also inversely related to cGMP (p < .01); the relationship for both star-tracing and video game stressors was strongest among black men (partial r values = -.25 and -.24, respectively; p values < .01).

CONCLUSIONS

The results indicate that vasodilatory activity may impact the BP response to stress through modulation of adrenergic activation, particularly among blacks.

Cloning and functional studies of splice variants of the alpha-subunit of the amiloride-sensitive Na+ channel

The alpha-subunit of the amiloride-sensitive epithelial Na+ channel (alpha ENaC) is critical in forming an ion conductive pore in the membrane. We have identified the wild-type and three splice variants of the human alpha ENaC (h alpha ENaC) from the human lung cell line H441, using RT-PCR. These splice variants contain various structures in the extracellular domain, resulting in premature truncation (h alpha ENaCx), 19-amino acid deletion (h alpha ENaC-19), and 22-amino acid insertion (h alpha ENaC + 22). Wild-type h alpha ENaC and splice variants were functionally characterized in Xenopus oocytes by coexpression with hENaC beta- and gamma-subunits. Unlike wild-type h alpha ENaC, undetectable or substantially reduced amiloride-sensitive currents were observed in oocytes expressing these splice variants. Wild-type h alpha ENaC was the most abundantly expressed h alpha ENaC mRNA species in all tissues in which its expression was detected. These findings indicate that the extracellular domain is important to generate structural and functional diversity of h alpha ENaC and that alternative splicing may play a role in regulating hENaC activity.

Evidence for an amiloride-insensitive Na+/H+ exchanger in rat renal cortical tubules

We have characterized the Na+/H+ exchanger (NHE) isoforms expressed in rat renal cortical tubule fragments. Amiloride sensitivity of the Na(+)-dependent intracellular pH (pHi) recovery in suspended tubules that had been acid loaded by an NH4+ prepulse was determined in nominally CO2/HCO3(-)-free solution, using the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In the presence of 140 mM extracellular Na+, 800 microM amiloride inhibited the rate of Na(+)-dependent pHi recovery by only 65%, demonstrating the presence of a Na(+)-dependent amiloride-insensitive H+ extrusion system. This system was not affected by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but was activated by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Lowering extracellular Na+ concentration permitted 300 microM amiloride to completely inhibit Na(+)-dependent pHi recovery. These results can be explained by the expression of a Na+/H+ exchange with the pharmacological properties of NHE4. Using reverse transcriptase-polymerase chain reaction, we found specific mRNA for NHE1, NHE2, NHE3, and NHE4 isoforms in the renal cortex. Immunohistochemical studies using polyclonal antibodies against rat NHE4 peptide demonstrated that NHE4 is heterogeneously expressed on basolateral membrane domains of cortical tubules. These results strongly suggest that amiloride-insensitive Na+/H+ exchange expressed in renal cortical tubule suspensions is mediated by NHE4.

Relationship of diastolic blood pressure with cyclic GMP excretion among young adults (the CARDIA Study): influence of a family history of hypertension. Coronary Artery Risk Development in Young Adults

BACKGROUND

Guanosine 3':5'-cyclic monophosphate (cGMP) is the second messenger of nitric oxide and atrial natriuretic factor, and mediates local vasodilatation. These vasodilatory factors are important in blood pressure regulation and possibly in the etiology of hypertension. Urinary cGMP levels among normotensive young adults have not previously been studied.

SUBJECTS

A subset of normotensive participants from the CARDIA study (n = 563), aged 23-35 years, was studied. The sample was approximately balanced for sex and race (black/white).

METHODS

Twenty-four-hour urinary cGMP levels were measured using an enzyme immunoassay; levels were adjusted for creatinine excretion. The blood pressure, smoking status, and risk factors for hypertension [including a family history of hypertension (FHH), the body mass index, education, alcohol intake, and sodium excretion] were also measured.

RESULTS

Women excreted more cGMP than did men, and blacks excreted more cGMP than did whites (both P < 0.0001). Excretion of cGMP was also greater among smokers (P < 0.001) and those with an FHH (P = 0.05), and was related directly and independently to sodium excretion (P < 0.02). The diastolic blood pressure (DBP) was related inversely to the excretion of cGMP among individuals without an FHH (r = -0.36, P < 0.001), but not among individuals with an FHH. In multiple regression analysis, the excretion of cGMP remained related significantly to the DBP and accounted for more variance in DBP than did any other variable among those without an FHH (delta R2 = 0.08, P < 0.001).

CONCLUSIONS

Urinary cGMP excretion is related inversely and independently to the DBP among those without an FHH but not among those with an FHH, suggesting that cGMP-related vasodilatation is impaired in those with an FHH. Sex differences in urinary excretion of cGMP are consistent with results from studies showing that estrogen increases the endothelial production of nitric oxide.

Sodium transporters in health and disease

Our understanding of sodium transport defects has exploded in the past 18 months, and has provided unique insights into epithelial transport processes and unusual clinical syndromes resulting from mutations of specific transporters. These genetic disorders disturb sodium balance, with both sodium retaining and sodium wasting disorders as the consequence.

Acute renal failure in the 21st century: recommendations for management and outcomes assessment

Acute renal failure (ARF) remains a common and potentially devastating disorder affecting as many of 5% of all hospitalized patients, with a higher prevalence in patients in critical care units. ARF is more frequently observed in the setting of multiorgan dysfunction syndrome (MODS) and in elderly patients with complex disease, where mortality is high. Numerous technical advances have not yet impacted favorably on this high mortality rate. This report summarizes recommendations from participants at the National Institutes of Health Conference: "Acute Renal Failure in the 21st Century," May 6 to 8, 1996, in Bethesda, MD. The focus is on categorizing recent clinically relevant developments in the field and on identification of new research initiatives to transfer a new body of knowledge derived from fundamental studies and laboratory investigation to the management of ARF in the new millennium. The development of a multicenter database through cooperative multicenter studies is advocated. Future studies should define the appropriate outcome measures to assess and emphasize the impact of hemodynamic monitoring, adjunctive agents, and adequacy and modality of renal replacement therapy on outcomes in ARF.

Expression of the amiloride-sensitive sodium channel beta subunit gene in human B lymphocytes

The amiloride-sensitive, epithelial sodium channel (ENaC) is composed of at least three subunits (alpha, beta, and gamma). This study demonstrates that the ENaC beta subunit gene is expressed in human B cell lines, peripheral blood lymphocytes, and lymph node at the mRNA level. Further, this study shows that both wild-type and mutated alleles of the ENaC beta subunit gene are transcribed in human B lymphocytes derived from an individual affected with Liddle's syndrome, an autosomal dominant form of human hypertension.

Heterologous expression of rat NHE4: a highly amiloride-resistant Na+/H+ exchanger isoform

Molecular cloning and expression have previously defined three members of the Na+/H+ exchanger (NHE) gene family NHE1 and NHE2 are sensitive to inhibition by amiloride and its 5'-amino alkyl-substituted analogues, whereas NHE3 is quite resistant to amiloride inhibition. Each of these exchangers has narrowly defined cation specificities for Na+ and Li+. Expression studies with NHE4 have not been as successful, with only a description of modest expression of activity (C. Bookstein, M. W. Musch, A. DePaoli, Y. Xie, M. Villereal, M. C. Rao, and E. B. Chang. J. Biol. Chem. 269: 29704-29709, 1994). We now report that NHE4 activity in stably transfected fibroblasts is activated by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), permitting functional characterization of this NHE isoform. The activating effect of DIDS was unique among the disulfonic stilbenes, and competition studies suggested a cross-linking mechanism. NHE4 is extremely resistant to amiloride and ethylisopropylamiloride inhibition and, unlike other NHE isoforms, affects K+/H+ exchange as well as Na+/H+ and Li+/H+ exchange. These findings demonstrate that NHE4 is a functionally distinct member of the NHE gene family and suggest a unique physiological role for this cation/H+ exchanger.

Polymorphism in the beta subunit and Na+ transport

Primary dysregulation of the epithelial sodium channel, manifested by continuing Na+ channel activity despite dietary salt excess, would cause inappropriate renal sodium reabsorption, blunted sodium excretion, and low-renin hypertension. There are now well-characterized genetic causes of hypertension in human pedigrees, which are explained by inappropriate and/or constitutive activation of the epithelial Na+ channel. Although Liddle's syndrome has been the most thoroughly investigated, the incidence of such activating mutations in the subunits of the Na+ channel appears to be relatively rare. Of continuing interest is the possibility that polymorphisms in the channel subunits could result in activation of the channel in response to normal regulatory influences. One such example is provided by a provocative report by Su et al. In this issue of the journal (J Am Soc Nephrol 1996;7:2543-2549). Despite several important technical limitations of the findings presented in the article, the suggestion that polymorphisms found in defined human populations would affect the regulation of sodium channel activity in response to environmental variables is worthy of serious consideration, and serves as a further stimulus to defining the functional significance of the various polymorphisms described in the subunits of the amiloride-sensitive sodium channel.

Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis

Pages F1004–F1014: Y. Zhang, A. K. Mircheff, C. B. Hensley, C. E. Magyar, D. G. Warnock, R. Chambrey, K.-P. Yip, D. J. Marsh, N.-H. Holstein-Rathlou, and A. A. McDonough. “Rapid redistribution and inhibition of renal sodium transporters during acute pressure natriuresis.” The immunoblot panels in Figures 2 and 5–7 were inadvertently printed from low-resolution copies of the original artwork; in addition, the panels in Fig. 6 were incorrectly labeled. The correct figures are reproduced on the following pages.

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Genetic association of 11 beta-hydroxysteroid dehydrogenase type 2 (HSD11B2) flanking microsatellites with essential hypertension in blacks

11 beta-Hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) specifically modulates access of the mineralocorticoid aldosterone to the kidney mineralocorticoid type 1 receptors in a physiological environment in which there is a molar excess of cortisol. Cortisol and aldosterone have similar affinities for mineralocorticoid receptors. Mechanistically, 11 beta-HSD2 converts cortisol to cortisone. The other known isoform, 11 beta-HSD1, not only catalyzes the cortisol to cortisone reaction but also the reverse reaction, making it unlikely to play an important role in modulating the access of aldosterone to mineralocorticoid receptors. Mutations in the HSD11B2 gene (both exonic and intronic) have been demonstrated to cause reduced activity of this enzyme in the syndrome of apparent mineralocorticoid excess, a rare autosomal recessive disorder. We hypothesized that this locus is also involved in the etiology of essential hypertension. To test this locus and flanking chromosomal regions for allelic association and genetic linkage to essential hypertension, it is necessary to have informative genetic markers. To this end, we have refined the localization of 11 beta-HSD2 to 16q22.1. We genotyped subjects using the nearest flanking microsatellites (D16S301 and D16S496). We conducted an association study using black subjects with hypertensive end-stage renal disease, black normotensive control subjects, and black and white individuals from the general population. We used chi 2 analysis and Fisher's exact test to test for association with these candidate gene markers. No significant association was found between D16S301 and hypertension. However, a positive association with hypertension was found at the D16S496 microsatellite locus (chi 2 = 6.98, df = 1, P < or = .008). Our data suggest that HSD11B2 is associated with hypertension in our black subjects with hypertensive end-stage renal disease. The 16q22.1 chromosome region potentially harbors a candidate gene for essential hypertension. Confirmation of our findings in another independently ascertained group of hypertensive subjects will provide a basis for proceeding with sib-pair linkage analyses.