Channels, carriers, and pumps in the pathogenesis of sodium-sensitive hypertension

Aldosterone Synthase Inhibitors for Cardiorenal Protection: Ready for Prime Time?

BACKGROUND

Aldosterone is the principal mineralocorticoid hormone and the final effector of the renin-angiotensin-aldosterone system. This hormone is primarily synthesized by the CYP11B2 enzyme and produced by the adrenal zona glomerulosa. Through genomic and non-genomic effects, it plays an important role in cardiovascular and renal disease. To counteract aldosterone-mediated damage, steroidal mineralocorticoid receptor antagonists are recommended by international guidelines, but endocrine side effects often limit their use in a substantial proportion of patients. Conversely, nonsteroidal mineralocorticoid receptor antagonists, with an improved selectivity and safety profile, are gaining a prominent position among therapeutic pillars. However, blocking the mineralocorticoid receptors does not completely inhibit aldosterone effects because of escape mechanisms and non-genomic activity. Thus, inhibiting aldosterone synthesis could be a promising strategy to prevent aldosterone-mediated cardiorenal damage. The limited specificity for CYP11B2 and side effects due to off-target activity hampered the development of first-generation aldosterone synthase inhibitors (ASIs).

SUMMARY

The development of highly specific ASIs led to successful clinical trials in patients with resistant and uncontrolled hypertension. Additionally, a recent randomized clinical trial showed a significant benefit of ASIs in patients with chronic kidney disease and albuminuria.

KEY MESSAGES

The strength of the clinical evidence collected so far is still limited, and larger outcome-based clinical trials are needed to confirm the promising role of ASIs in cardiorenal damage.

Sorting nexins: role in the regulation of blood pressure

Sorting nexins (SNXs) are a family of proteins that regulate cellular cargo sorting and trafficking, maintain intracellular protein homeostasis, and participate in intracellular signaling. SNXs are also important in the regulation of blood pressure via several mechanisms. Aberrant expression and dysfunction of SNXs participate in the dysregulation of blood pressure. Genetic studies show a correlation between SNX gene variants and the response to antihypertensive drugs. In this review, we summarize the progress in SNX-mediated regulation of blood pressure, discuss the potential role of SNXs in the pathophysiology and treatment of hypertension, and propose novel strategies for the medical therapy of hypertension.

Urinary proteomics reveals key markers of salt sensitivity in hypertensive patients during saline infusion

BACKGROUND

Hypertension is a complex disease and is the major cause of cardiovascular complications. In the vast majority of individuals, the aetiology of elevated blood pressure (BP) cannot be determined, thus impairing optimized therapies and prognosis for individual patients. A more precise understanding of the molecular pathogenesis of hypertension remains a pressing priority for both basic and translational research. Here we investigated the effect of salt on naive hypertensive patients in order to better understand the salt intake-blood pressure relationship.

METHODS

Patients underwent an acute saline infusion and were defined as salt-sensitive or salt-resistant according to mean blood pressure changes. Urinary proteome changes during the salt load test were analysed by a label-free quantitative proteomics approach.

RESULTS

Our data show that salt-sensitive patients display equal sodium reabsorption as salt-resistant patients, as major sodium transporters show the same behaviour during the salt load. However, salt-sensitive patients regulate the renin angiotensin system (RAS) differently from salt-resistant patients, and upregulate proteins, as epidermal growth factor (EGF) and plasminogen activator, urokinase (PLAU), involved in the regulation of epithelial sodium channel ENaC activity.

CONCLUSIONS

Salt-sensitive and salt-resistant subjects have similar response to a saline/volume infusion as detected by urinary proteome. However, we identified glutamyl aminopeptidase (ENPEP), PLAU, EGF and Xaa-Pro aminopeptidase 2 precursor XPNPEP2 as key molecules of salt-sensitivity, through modulation of ENaC-dependent sodium reabsorption along the distal tubule.

Metformin increases urinary sodium excretion by reducing phosphorylation of the sodium-chloride cotransporter

OBJECTIVE

Metformin is an antidiabetic drug that is widely used to treat patients with diabetes mellitus. Recent studies have reported that treatment with metformin not only improved blood glucose levels but also reduced blood pressure. However, it remains unclear how metformin reduces blood pressure. We hypothesized that metformin affects sodium reabsorption in the kidneys.

METHODS

Urinary sodium excretion and expression of renal sodium transporters were examined in 8-week-old male C57BL/6 mice with acute and chronic treatment of metformin. In addition, we examined metformin effects using ex vivo preparations of mice kidney slices.

RESULTS

In this study, we demonstrated that metformin increased urinary sodium excretion by reducing phosphorylation of the thiazide-sensitive Na-Cl cotransporter (NCC) in acute and chronic metformin administration. We also confirmed reduction of phosphorylated NCC in an ex vivo study. The activity of other renal sodium transporters, such as NKCC2, ENaC, and NHE3 did not show significant changes. WNK-OSR1/SPAK kinase signals were not involved in this inactivation effect of metformin on NCC.

CONCLUSION

Metformin increased urinary sodium excretion by reducing phosphorylation of NCC, suggesting its role in improving hypertension.

Na+/H+ Exchanger 3 Is Expressed in Two Distinct Types of Ionocyte, and Probably Augments Ammonia Excretion in One of Them, in the Gills of the Climbing Perch Exposed to Seawater

The freshwater climbing perch, Anabas testudineus, is an euryhaline teleost and an obligate air-breather with the ability to actively excrete ammonia. Members of the Na+/H+ exchanger (NHE) family help maintain intracellular pH homeostasis and ionic balance through the electroneutral exchange of Na+ and H+. This study aimed to obtain, from the gills of A. testudineus, the full cDNA coding sequence of nhe3, and to determine the effects of exposure to seawater or 100 mmol l-1 of NH4Cl in fresh water on its mRNA and protein expression levels. Efforts were also made to elucidate the type of ionocyte that Nhe3 was associated with in the branchial epithelium of A. testudineus. The transcript level and protein abundance of nhe3/Nhe3 were very low in the gills of freshwater A. testudineus, but they increased significantly in the gills of fish acclimated to seawater. In the gills of fish exposed to seawater, Nhe3 was expressed in two distinct types of seawater-inducible Na+/K+-ATPase (Nka)-immunoreactive ionocytes. In Nkaα1b-immunoreactive ionocytes, Nhe3 had an apical localization. As these ionocytes also expressed apical Rhcg1 and basolateral Rhcg2, which are known to transport ammonia, they probably participated in proton-facilitated ammonia excretion in A. testudineus during seawater acclimation. In Nkaα1c-immunoreactive ionocytes, Nhe3 was atypically expressed in the basolateral membrane, and its physiological function is uncertain. For A. testudineus exposed to NH4Cl in fresh water, the transcript and protein expression levels of nhe3/Nhe3 remained low. In conclusion, the branchial Nhe3 of A. testudineus plays a greater physiological role in passive ammonia transport and acid-base balance during seawater acclimation than in active ammonia excretion during environmental ammonia exposure.

Rare Renal Diseases Can Be Used as Tools to Investigate Common Kidney Disorders

BACKGROUND

The prevention and slowing of chronic kidney disease still represent major challenges in nephrology. To this end, a major contribution may come from the extensive knowledge on the molecular pathways involved in the pathogenesis of rare kidney diseases, since it is now possible to shed light on several aspects of these pathologies thanks to the introduction of new technologies, including next-generation sequencing.

SUMMARY

In steroid-resistant nephrotic patients, a genetic background has been demonstrated in both children and adults; individualized mutations have been correlated with glomerular filtration barrier alterations. In addition, studies on genetic tubulopathies expressing hypertensive phenotypes can provide useful information for a correct diagnostic and therapeutic approach in patients with essential hypertension and a poor responsiveness to therapy.

KEY MESSAGE

This review deals with the pathogenesis of rare glomerular diseases and tubulopathies associated with hypertension, highlighting the importance of the study of rare diseases to better understand the molecular basis of more common and complex disorders leading to end-stage renal disease.

Up-Regulation of Intestinal Phosphate Transporter NaPi-IIb (SLC34A2) by the Kinases SPAK and OSR1

BACKGROUND/AIMS

SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1), kinases controlled by WNK (with-no-K[Lys] kinase), are powerful regulators of cellular ion transport and blood pressure. Observations in gene-targeted mice disclosed an impact of SPAK/OSR1 on phosphate metabolism. The present study thus tested whether SPAK and/or OSR1 contributes to the regulation of the intestinal Na(+)-coupled phosphate co-transporter NaPi-IIb (SLC34A2).

METHODS

cRNA encoding NaPi-IIb was injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 or catalytically inactive (D164A)OSR1. The phosphate (1 mM)-induced inward current (I(Pi)) was taken as measure of phosphate transport.

RESULTS

I(Pi) was observed in NaPi-IIb expressing oocytes but not in water injected oocytes, and was significantly increased by co-expression of SPAK, (T233E)SPAK, OSR1, (T185E)OSR1 or SPAK+OSR1, but not by co-expression of (T233A)SPAK, (D212A)SPAK, (T185A)OSR1, or (D164A)OSR1. SPAK and OSR1 both increased the maximal transport rate of the carrier.

CONCLUSIONS

SPAK and OSR1 are powerful stimulators of the intestinal Na+-coupled phosphate co-transporter NaPi-IIb.

Down-Regulation of Excitatory Amino Acid Transporters EAAT1 and EAAT2 by the Kinases SPAK and OSR1

SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1) are cell volume-sensitive kinases regulated by WNK (with-no-K[Lys]) kinases. SPAK/OSR1 regulate several channels and carriers. SPAK/OSR1 sensitive functions include neuronal excitability. Orchestration of neuronal excitation involves the excitatory glutamate transporters EAAT1 and EAAT2. Sensitivity of those carriers to SPAK/OSR1 has never been shown. The present study thus explored whether SPAK and/or OSR1 contribute to the regulation of EAAT1 and/or EAAT2. To this end, cRNA encoding EAAT1 or EAAT2 was injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type SPAK or wild-type OSR1, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 or catalytically inactive (D164A)OSR1. The glutamate (2 mM)-induced inward current (I Glu) was taken as a measure of glutamate transport. As a result, I Glu was observed in EAAT1- and in EAAT2-expressing oocytes but not in water-injected oocytes, and was significantly decreased by coexpression of SPAK and OSR1. As shown for EAAT2, SPAK, and OSR1 decreased significantly the maximal transport rate but significantly enhanced the affinity of the carrier. The effect of wild-type SPAK/OSR1 on EAAT1 and EAAT2 was mimicked by (T233E)SPAK and (T185E)OSR1, but not by (T233A)SPAK, (D212A)SPAK, (T185A)OSR1, or (D164A)OSR1. Coexpression of either SPAK or OSR1 decreased the EAAT2 protein abundance in the cell membrane of EAAT2-expressing oocytes. In conclusion, SPAK and OSR1 are powerful negative regulators of the excitatory glutamate transporters EAAT1 and EAAT2.

SPAK and OSR1 Sensitivity of Excitatory Amino Acid Transporter EAAT3

BACKGROUND/AIMS

Kinases involved in the regulation of epithelial transport include SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1). SPAK and OSR1 are both regulated by WNK (with-no-K(Lys)) kinases. The present study explored whether SPAK and/or OSR1 influence the excitatory amino acid transporter EAAT3, which accomplishes glutamate and aspartate transport in kidney, intestine and brain.

METHODS

cRNA encoding EAAT3 was injected into Xenopus laevis oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 and catalytically inactive (D164A)OSR1. Glutamate-induced current was taken as measure of electrogenic glutamate transport and was quantified utilizing dual electrode voltage clamp. Furthermore, Ussing chamber was employed to determine glutamate transport in the intestine from gene-targeted mice carrying WNK insensitive SPAK (spak(tg/tg)) and from corresponding wild-type mice (spak(+/+)).

RESULTS

EAAT3 activity was significantly decreased by wild-type SPAK and (T233E)SPAK, but not by (T233A)SPAK and (D212A)SPAK. SPAK decreased maximal transport rate without affecting significantly affinity of the carrier. Similarly, EAAT3 activity was significantly downregulated by wild-type OSR1 and (T185E)OSR1, but not by (T185A)OSR1 and (D164A)OSR1. Again OSR1 decreased maximal transport rate without affecting significantly affinity of the carrier. Intestinal electrogenic glutamate transport was significantly lower in spak(+/+) than in spak(tg/tg) mice.

CONCLUSION

Both, SPAK and OSR1 are negative regulators of EAAT3 activity.

Downregulation of peptide transporters PEPT1 and PEPT2 by oxidative stress responsive kinase OSR1

BACKGROUND/AIMS

OSR1 (oxidative-stress-responsive kinase 1) participates in the regulation of renal tubular ion transport, cell volume and blood pressure. Whether OSR1 contributes to the regulation of organic solute transport remained; however, elusive. The present study thus explored the OSR1 sensitivity of the peptide transporters PEPT1 and PEPT2.

METHODS

cRNA encoding PEPT1 or PEPT2 were injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type OSR1, WNK1 insensitive inactive (T185A)OSR1, constitutively active (T185E)OSR1, and catalytically inactive (D164A)OSR1. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp, the abundance of hemagglutinin-tagged PEPT2 (PEPT2-HA) by chemiluminescence.

RESULTS

In Xenopus oocytes injected with cRNA encoding PEPT1 or PEPT2, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an appreciable inward current (I(gly-gly)). Coexpression of OSR1 significantly decreased Igly-gly in both PEPT1 and PEPT2 expressing oocytes. The effect of OSR1 coexpression on Igly-gly in PEPT1 expressing oocytes was mimicked by coexpression of (T185E)OSR1, but not of (D164A)OSR1 or (T185A)OSR1. Kinetic analysis revealed that coexpression of OSR1 decreased maximal Igly-gly. OSR1 further decreased the PEPT2-HA protein abundance in the cell membrane.

CONCLUSION

OSR1 has the capacity to downregulate the peptide transporters PEPT1 and PEPT2 by decreasing the carrier protein abundance in the cell membrane.

Negative regulation of the creatine transporter SLC6A8 by SPAK and OSR1

BACKGROUND/AIMS

Transport regulation involves several kinases including SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1), which are under control of WNK (with-no-K[Lys]) kinases. The present study explored whether SPAK and/or OSR1 participate in the regulation of the creatine transporter CreaT (SLC6A8), which accomplishes Na+ coupled cellular uptake of creatine in several tissues including kidney, intestine, heart, skeletal muscle and brain.

METHODS

cRNA encoding SLC6A8 was injected into Xenopus laevis oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 and catalytically inactive (D164A)OSR1. Transporter activity was determined from creatine (1 mM) induced current utilizing dual electrode voltage clamp.

RESULTS

Coexpression of wild-type SPAK and of (T233E)SPAK, but not of (T233A)SPAK or of (D212A)SPAK was followed by a significant decrease of creatine induced current in SLC6A8 expressing oocytes. Coexpression of SPAK significantly decreased maximal transport rate. Coexpression of wild-type OSR1, (T185E)OSR1 and (T185A)OSR1 but not of (D164A)OSR1 significantly negatively regulated SLC6A8 activity. OSR1 again decreased significantly maximal transport rate.

CONCLUSIONS

Both, SPAK and OSR1, are negative regulators of the creatine transporter SLC6A8.

SPAK dependent regulation of peptide transporters PEPT1 and PEPT2

BACKGROUND/AIMS

SPAK (STE20-related proline/alanine-rich kinase) is a powerful regulator of renal tubular ion transport and blood pressure. Moreover, SPAK contributes to the regulation of cell volume. Little is known, however, about a role of SPAK in the regulation or organic solutes. The present study thus addressed the influence of SPAK on the peptide transporters PEPT1 and PEPT2.

METHODS

To this end, cRNA encoding PEPT1 or PEPT2 were injected into Xenopus laevis oocytes without or with additional injection of cRNA encoding wild-type, SPAK, WNK1 insensitive inactive (T233A)SPAK, constitutively active (T233E)SPAK, and catalytically inactive (D212A)SPAK. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp and PEPT2 protein abundance in the cell membrane by chemiluminescence. Intestinal electrogenic peptide transport was estimated from peptide induced current in Ussing chamber experiments of jejunal segments isolated from gene targeted mice expressing SPAK resistant to WNK-dependent activation (spak(tg/tg)) and respective wild-type mice (spak(+/+)).

RESULTS

In PEPT1 and in PEPT2 expressing oocytes, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an inward current, which was significantly decreased following coexpression of SPAK. The effect of SPAK on PEPT1 was mimicked by (T233E)SPAK, but not by (D212A)SPAK or (T233A)SPAK. SPAK decreased maximal peptide induced current of PEPT1. Moreover, SPAK decreased carrier protein abundance in the cell membrane of PEPT2 expressing oocytes. In intestinal segments gly-gly generated a current, which was significantly higher in spak(tg/tg) than in spak(+/+) mice.

CONCLUSION

SPAK is a powerful regulator of peptide transporters PEPT1 and PEPT2.

SPAK and OSR1 sensitivity of voltage-gated K+ channel Kv1.5

SPS1-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1) are potent regulators of several transporters and ion channels. The kinases are under regulation of with-no-K(Lys) (WNK) kinases. The present study explored whether SPAK and/or OSR1 modify the expression and/or activity of the voltage-gated K(+) channel Kv1.5, which participates in the regulation of diverse functions including atrial cardiac action potential and tumor cell proliferation. cRNA encoding Kv1.5 was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild-type SPAK, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1, and catalytically inactive (D164A)OSR1. Voltage-gated K(+) channel activity was quantified utilizing dual electrode voltage clamp and Kv1.5 channel protein abundance in the cell membrane utilizing chemiluminescence of Kv1.5 containing an extracellular hemagglutinin epitope (Kv1.5-HA). Kv1.5 activity and Kv1.5-HA protein abundance were significantly decreased by wild-type SPAK and (T233E)SPAK, but not by (T233A)SPAK and (D212A)SPAK. Similarly, Kv1.5 activity and Kv1.5-HA protein abundance were significantly down-regulated by wild-type OSR1 and (T185E)OSR1, but not by (T185A)OSR1 and (D164A)OSR1. Both, SPAK and OSR1 decrease cell membrane Kv1.5 protein abundance and activity.

SPAK and OSR1 dependent down-regulation of murine renal outer medullary K channel ROMK1

BACKGROUND/AIMS

The kinases SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1) participate in the regulation of the NaCl cotransporter NCC and the Na+, K+, 2Cl- cotransporter NKCC2. The kinases are regulated by WNK (with-no-K[Lys]) kinases. Mutations of genes encoding WNK kinases underly Gordon's syndrome, a monogenic disease leading to hypertension and hyperkalemia. WNK kinases further regulate the renal outer medullary K+ channel ROMK1. The present study explored, whether SPAK and/or OSR1 have similarly the potential to modify the activity of ROMK1.

METHODS

ROMK1 was expressed in Xenopus oocytes with or without additional expression of wild-type SPAK, constitutively active (T233E)SPAK, catalytically inactive (D212A)SPAK, wild-type OSR1, constitutively active (T185E)OSR1 and catalytically inactive (D164A)OSR1. Channel activity was determined utilizing dual electrode voltage clamp and ROMK1 protein abundance in the cell membrane utilizing chemiluminescence of ROMK1 containing an extracellular hemagglutinin epitope (ROMK1-HA).

RESULTS

ROMK1 activity and ROMK1-HA protein abundance were significantly down-regulated by wild-type SPAK and (T233E)SPAK, but not by (D212A)SPAK. Similarly, ROMK1 activity and ROMK1-HA protein abundance were significantly down-regulated by wild-type OSR1 and (T185E)OSR1, but not by (D164A)OSR1.

CONCLUSION

ROMK1 protein abundance and activity are down-regulated by SPAK and OSR1.

SPAK-sensitive regulation of glucose transporter SGLT1

The WNK-dependent STE20/SPS1-related proline/alanine-rich kinase SPAK is a powerful regulator of ion transport. The study explored whether SPAK similarly regulates nutrient transporters, such as the Na(+)-coupled glucose transporter SGLT1 (SLC5A1). To this end, SGLT1 was expressed in Xenopus oocytes with or without additional expression of wild-type SPAK, constitutively active (T233E)SPAK, WNK-insensitive (T233A)SPAK or catalytically inactive (D212A)SPAK, and electrogenic glucose transport determined by dual-electrode voltage-clamp experiments. Moreover, Ussing chamber was employed to determine the electrogenic glucose transport in intestine from wild-type mice (spak(wt/wt)) and from gene-targeted mice carrying WNK-insensitive SPAK (spak(tg/tg)). In SGLT1-expressing oocytes, but not in water-injected oocytes, the glucose-dependent current (I(g)) was significantly decreased following coexpression of wild-type SPAK and (T233E)SPAK, but not by coexpression of (T233A)SPAK or (D212A)SPAK. Kinetic analysis revealed that SPAK decreased maximal I(g) without significantly modifying the glucose concentration required for halfmaximal I(g) (K(m)). According to the chemiluminescence experiments, wild-type SPAK but not (D212A)SPAK decreased SGLT1 protein abundance in the cell membrane. Inhibition of SGLT1 insertion by brefeldin A (5 μM) resulted in a decline of I(g), which was similar in the absence and presence of SPAK, suggesting that SPAK did not accelerate the retrieval of SGLT1 protein from the cell membrane but rather down-regulated carrier insertion into the cell membrane. Intestinal electrogenic glucose transport was significantly lower in spak(wt/wt) than in spak(tg/tg) mice. In conclusion, SPAK is a powerful negative regulator of SGLT1 protein abundance in the cell membrane and thus of electrogenic glucose transport.

A role of intestine in hypertension: mechanism of suppression of intestinal Na-H exchanger isoform-3 in spontaneously hypertensive rats

The main objective of this study was to investigate the role and the underlying mechanism of Na-H exchanger-3 (NHE-3) expression in spontaneously hypertensive rat (SHR) intestine. Expression of colonic and ileal NHE-3 isoform, its regulatory factor-1 (NHERF-1) and cyclic GMP kinase II (cGKII) were examined using western blot analysis. Since NHE-3 activity is regulated by its abundance on the plasma membrane, its levels were also examined in lipid rafts-enriched membrane fractions. The lipid rafts fractions were characterized by examining the concentration of flotillin-1 and caveolin-1, total protein, and cholesterol. Twelve-weeks-old SHR used in this study developed significant hypertension, proteinuria, and renal and cardiac hypertrophy. These changes were significantly reversed by captopril treatment. There was a significant decrease in the levels of NHE-3 and NHERF-1 proteins, and sodium pump activity, but an increase in the cGKII levels in both tissues from SHR. Reduction in NHERF-1 levels was reversed by captopril but not of the other proteins. Cholesterol profile was significantly different in SHR colon as compared to normo-tensive Wistar Kyoto rats. These findings suggest that suppression of NHE-3 in intestine is a counteracting mechanism of hypertension and is regulated by NHERF-1 through cGKII activation in SHR. NHE-3 suppression together with decrease in the sodium pump activity would accumulate intracellular Na(+) and may contribute to the reported hypertension-induced tissue damage in the GI-tract.

Proteases, cystic fibrosis and the epithelial sodium channel (ENaC)

Proteases perform a diverse array of biological functions. From simple peptide digestion for nutrient absorption to complex signaling cascades, proteases are found in organisms from prokaryotes to humans. In the human airway, proteases are associated with the regulation of the airway surface liquid layer, tissue remodeling, host defense and pathogenic infection and inflammation. A number of proteases are released in the airways under both physiological and pathophysiological states by both the host and invading pathogens. In airway diseases such as cystic fibrosis, proteases have been shown to be associated with increased morbidity and airway disease progression. In this review, we focus on the regulation of proteases and discuss specifically those proteases found in human airways. Attention then shifts to the epithelial sodium channel (ENaC), which is regulated by proteolytic cleavage and that is considered to be an important component of cystic fibrosis disease. Finally, we discuss bacterial proteases, in particular, those of the most prevalent bacterial pathogen found in cystic fibrosis, Pseudomonas aeruginosa.

A role for benzamil-sensitive proteins of the central nervous system in the pathogenesis of salt-dependent hypertension

1. Although increasing evidence suggests that salt-sensitive hypertension is a disorder of the central nervous system (CNS), little is known about the critical proteins (e.g. ion channels or exchangers) that play a role in the pathogenesis of the disease. 2. Central pathways involved in the regulation of arterial pressure have been investigated. In addition, systems such as the renin-angiotensin-aldosterone axis, initially characterized in the periphery, are present in the CNS and seem to play a role in the regulation of arterial pressure. 3. Central administration of amiloride, or its analogue benzamil hydrochloride, has been shown to attenuate several forms of salt-sensitive hypertension. In addition, intracerebroventricular (i.c.v.) benzamil effectively blocks pressor responses to acute osmotic stimuli, such as i.c.v. hypertonic saline. Amiloride or its analogues have been shown to interact with the brain renin-angiotensin-aldosterone system (RAAS) and to effect the expression of endogenous ouabain-like compounds. Alterations of brain RAAS function and/or endobain expression could play a role in the interaction between amiloride compounds and arterial pressure. Peripheral treatments with benzamil, even at higher doses than those given centrally, have little or no effect on arterial pressure. These data provide strong evidence that benzamil-sensitive proteins (BSPs) of the CNS play a role in cardiovascular responsiveness to sodium. 4. Mineralocorticoids have been linked to human hypertension; many patients with essential hypertension respond well to pharmacological agents antagonizing the mineralocorticoid receptor and certain genetic forms of hypertension are caused by chronically elevated levels of aldosterone. The deoxycorticosterone acetate (DOCA)-salt model of hypertension is a benzamil-sensitive model that incorporates several factors implicated in the aetiology of human disease, including mineralocorticoid action and increased dietary sodium. The DOCA-salt model is ideal for investigating the role of BSPs in the pathogenesis of hypertension, because mineralocorticoid action has been shown to modulate the activity of at least one benzamil-sensitive protein, namely the epithelial sodium channel. 5. Characterizing the BSPs involved in the pathogenesis of hypertension may provide a novel clinical target. Further studies are necessary to determine which BSPs are involved and where, in the nervous system, they are located.

Salt-sensitive men show reduced heart rate variability, lower norepinephrine and enhanced cortisol during mental stress

Salt sensitivity (SS) represents a risk factor for essential hypertension, which has been related to enhanced cardiovascular stress reactivity possibly mediated by increased noradrenergic susceptibility. We investigated biophysiological responses to mental stress in salt-sensitive (ss) and salt-resistant (sr) subjects, hypothesizing lower heart rate variability (HRV) and higher cortisol in the ss. A total of 48 healthy normotensive Caucasian men (age 25.6+/-2.6, body mass index 22.9+/-2.3) were phenotyped for SS (defined as significant drop in mean arterial pressure>3 mm Hg under the low-salt diet) by a 2-week high- versus low-salt diet. Subjects underwent a standardized mental stress task with continuous cardiovascular monitoring before, during and after the test (Finapres; Ohmeda, Louisville, CO, USA). Blood samples were drawn to examine cortisol and catecholamines before, after and 20 min after stress. The task elicited significant increases of systolic blood pressure (SBP), diastolic BP (DBP) and heart rate (HR) and a significant decrease of HRV (all time effects P<0.0001). The ss subjects showed lower norepinephrine (NE) and higher cortisol, indicated by significant group effects (P=0.009 and 0.025, respectively). HR increased and HRV decreased more in the ss under the stress, shown by significant time by group interactions (P=0.045 and 0.003, respectively). The observation of a more pronounced HR rise coupled with a greater decrease of HRV in healthy ss men under the influence of brief mental stress confirms their enhanced physiological stress reactivity. The lower peripheral NE may represent an effort to compensate for increased noradrenergic receptor sensitivity. The enhanced cortisol levels are backed by recent genetic findings on HSD11B2 polymorphisms and may promote hypertension.

Gene expression of Na+/H+ exchanger in zebrafish H+ -ATPase-rich cells during acclimation to low-Na+ and acidic environments

In mammalian nephrons, most of the Na(+) and HCO(3)(-) is reabsorbed by proximal tubular cells in which the Na(+)/H(+) exchanger 3 (NHE3) is the major player. The roles of NHEs in Na(+) uptake/acid-base regulation in freshwater (FW) fish gills are still being debated. In the present study, functional genomic approaches were used to clone and sequence the full-length cDNAs of the nhe family from zebrafish (Danio rerio). A phylogenetic tree analysis of the deduced amino acid sequences showed that zNHE1-8 are homologous to their mammalian counterparts. By RT-PCR analysis and double/triple in situ hybridization/immunocytochemistry, only zebrafish NHE3b was expressed in zebrafish gills and was colocalized with V-H(+)-ATPase but not with Na(+)-K(+)-ATPase, indicating that H(+)-ATPase-rich (HR) cells specifically express NHE3b. A subsequent quantitative RT-PCR analysis demonstrated that acclimation to low-Na(+) FW caused upregulation and downregulation of the expressions of znhe3b and zatp6v0c (H(+)-ATPase C-subunit), respectively, in gill HR cells, whereas acclimation to acidic FW showed reversed effects on the expressions of these two genes. In conclusion, both NHE3b and H(+)-ATPase are probably involved in Na(+) uptake/acid-base regulation in zebrafish gills, like mammalian kidneys, but the partitioning of these two transporters may be differentially regulated depending on the environmental situation in which fish are acclimatized.

Advances in genetic hypertension

PURPOSE OF REVIEW

Mendelian forms of hypertension are rare genetic disorders that cause severe hypertension. This review will explore the recently identified molecular mechanisms and pathogenesis of genetic disorders that cause hypertension in children.

RECENT FINDINGS

Hypertension is now believed to be a polygenic disorder resulting from the interaction of multiple genes and the environment. A few forms of severe hypertension have been linked to single genes. The genes responsible for these disorders have all been cloned and all participate in pathways involved in heightened renal sodium reabsorption. The increased sodium reabsorption arises in the distal nephron and leads to volume expansion and hypertension.

SUMMARY

Investigating forms of monogenic hypertension has advanced the understanding of sodium transport and volume control by the kidney. Future studies will identify novel genes, pathways and treatment targets important in the fight against primary hypertension.

Molecular mechanisms and regulation of furosemide-sensitive Na-K-Cl cotransporters

PURPOSE OF REVIEW

Relevant advances towards understanding how furosemide-sensitive Na-K-Cl cotransporters (NKCC) are regulated by alternative splicing, phosphorylation and membrane expression have been made, which are critical to comprehending the role of NKCCs in blood pressure homeostasis.

RECENT FINDINGS

A major breakthrough has been the description of a macromolecular complex responsible for the regulatory phosphorylation of NKCCs, involving members of two families of novel serine-threonine kinases: WNK kinases and Ste-20-related kinases SPAK and OSR1. A new regulatory pathway has been defined, with WNK lying upstream of SPAK-OSR1 and the latter kinases directly phosphorylating NKCC. New evidence has arisen supporting regulation of NKCC membrane expression, possibly through the same mechanisms regulating phosphorylation. Alternative splicing of kidney-specific NKCC2 also appears to be a regulated process. Renal roles for NKCC1 have been described, including an unexpected role in controlling renin secretion.

SUMMARY

We now begin to understand the biochemical pathways mediating NKCC regulatory phosphorylation, which are governed by kinases that, like NKCCs, have been linked to the genesis of hypertension. Complementary long-term regulation of NKCC membrane expression, alternative splicing or gene transcription, however, should not be overlooked. Deciphering the relationships between these processes will enhance our understanding of the pathogenesis of hypertension.