Reaction of nitric oxide with superoxide inhibits basolateral K+ channels in the rat CCD

Pathophysiology, mechanisms, and managements of tissue hypoxia

Oxygen is needed to generate aerobic adenosine triphosphate and energy that is required to support vital cellular functions. Oxygen delivery (DO₂) to the tissues is determined by convective and diffusive processes. The ability of the body to adjust oxygen extraction (ERO₂) in response to changes in DO₂ is crucial to maintain constant tissue oxygen consumption (VO₂). The capability to increase ERO₂ is the result of the regulation of the circulation and the effects of the simultaneous activation of both central and local factors. The endothelium plays a crucial role in matching tissue oxygen supply to demand in situations of acute drop in tissue oxygenation. Tissue oxygenation is adequate when tissue oxygen demand is met. When DO₂ is severely compromised, a critical DO₂ value is reached below which VO₂ falls and becomes dependent on DO₂, resulting in tissue hypoxia. The different mechanisms of tissue hypoxia are circulatory, anaemic, and hypoxic, characterised by a diminished DO₂ but preserved capacity of increasing ERO₂. Cytopathic hypoxia is another mechanism of tissue hypoxia that is due to impairment in mitochondrial respiration that can be observed in septic conditions with normal overall DO₂. Sepsis induces microcirculatory alterations with decreased functional capillary density, increased number of stopped-flow capillaries, and marked heterogeneity between the areas with large intercapillary distance, resulting in impairment of the tissue to extract oxygen and to satisfy the increased tissue oxygen demand, leading to the development of tissue hypoxia. Different therapeutic approaches exist to increase DO₂ and improve microcirculation, such as fluid therapy, transfusion, vasopressors, inotropes, and vasodilators. However, the effects of these agents on microcirculation are quite variable.

Effects of Reactive Oxygen Species on Tubular Transport along the Nephron

Reactive oxygen species (ROS) are oxygen-containing molecules naturally occurring in both inorganic and biological chemical systems. Due to their high reactivity and potentially damaging effects to biomolecules, cells express a battery of enzymes to rapidly metabolize them to innocuous intermediaries. Initially, ROS were considered by biologists as dangerous byproducts of respiration capable of causing oxidative stress, a condition in which overproduction of ROS leads to a reduction in protective molecules and enzymes and consequent damage to lipids, proteins, and DNA. In fact, ROS are used by immune systems to kill virus and bacteria, causing inflammation and local tissue damage. Today, we know that the functions of ROS are not so limited, and that they also act as signaling molecules mediating processes as diverse as gene expression, mechanosensation, and epithelial transport. In the kidney, ROS such as nitric oxide (NO), superoxide (O₂^-), and their derivative molecules hydrogen peroxide (H₂O₂) and peroxynitrite (ONO₂^-) regulate solute and water reabsorption, which is vital to maintain electrolyte homeostasis and extracellular fluid volume. This article reviews the effects of NO, O₂^-, ONO₂^-, and H₂O₂ on water and electrolyte reabsorption in proximal tubules, thick ascending limbs, and collecting ducts, and the effects of NO and O₂^- in the macula densa on tubuloglomerular feedback.

Endogenous flow-induced nitric oxide reduces superoxide-stimulated Na/H exchange activity via PKG in thick ascending limbs

Luminal flow stimulates endogenous nitric oxide (NO) and superoxide (O2 (-)) production by renal thick ascending limbs (TALs). The delicate balance between these two factors regulates Na transport in TALs; NO enhances natriuresis, whereas O2 (-) augments Na absorption. Endogenous, flow-stimulated O2 (-) enhances Na/H exchange (NHE). Flow-stimulated NO reduces flow-induced O2 (-), a process mediated by cGMP-dependent protein kinase (PKG). However, whether flow-stimulated, endogenously-produced NO diminishes O2 (-)-stimulated NHE activity and the signaling pathway involved are unknown. We hypothesized that flow-induced NO reduces the stimulation of NHE activity caused by flow-induced O2 (-) via PKG in TALs. Intracellular pH recovery after an acid load was measured as an indicator of NHE activity in isolated, perfused rat TALs. l-Arginine, the NO synthase substrate, decreased NHE activity by 34 ± 5% (n = 5; P < 0.04). The O2 (-) scavenger tempol decreased NHE activity by 46 ± 8% (n = 6; P < 0.004) in the absence of NO. In the presence of l-arginine, the inhibitory effect of tempol on NHE activity was reduced to -19 ± 6% (n = 6; P < 0.03). The soluble guanylate cyclase inhibitor LY-83583 blocked the effect of l-arginine thus restoring tempol's effect on NHE activity to -42 ± 4% (n = 6; P < 0.0005). The PKG inhibitor KT-5823 also inhibited l-arginine's effect on tempol-reduced NHE activity (-43 ± 5%; n = 5; P < 0.03). We conclude that flow-induced NO reduces the stimulatory effect of endogenous, flow-induced O2 (-) on NHE activity in TALs via an increase in cGMP and PKG activation.

A pilot study of the effects of eplerenone add-on therapy in patients taking renin-angiotensin system blockers

HYPOTHESIS

This study determined the parameters for predicting the clinical effects of eplerenone (Ep) add-on therapy on blood pressure (BP) and proteinuria in patients taking angiotensin-converting enzyme inhibitors (ACEis) or angiotensin II type I receptor blockers (ARBs).

MATERIALS AND METHODS

Patients were treated with a gradual increase of Ep to a final dose of 50 mg/day for 2 months. In 35 patients, the efficacy of Ep was evaluated by peripheral BP, proteinuria, and the transtubular K gradient (TTKG). Fifteen patients had additional analysis for central BP, plasma renin activity (PRA) and plasma aldosterone concentration (PAC), measured in the supine position, and 24-hour urine collection before and after receiving Ep.

RESULTS

Ep add-on therapy reduced the mean arterial pressure (p=0.0005) and central BP (p=0.009) independently to the baseline PAC. Ep induced PRA, but failed to show effects on PAC, TTKG, or albuminuria. Correlation analysis showed inverse relationships between the percent reduction in albuminuria and baseline PAC.

CONCLUSIONS

Ep add-on therapy in patients taking renin-angiotensin system blockers is expected to reduce BP, even in patients with low PAC. In contrast, the anti-proteinuric action of Ep is dependent on baseline plasma aldosterone levels. TTKG is not appropriate for evaluating the efficacy of Ep.

Delayed and acute effects of interferon-gamma on activity of an inwardly rectifying K+ channel in cultured human proximal tubule cells

The activity of an inwardly rectifying K(+) channel in cultured human renal proximal tubule cells (RPTECs) is stimulated and inhibited by nitric oxide (NO) at low and high concentrations, respectively. In this study, we investigated the effects of IFN-gamma, one of the cytokines which affect the expression of inducible NO synthase (iNOS), on intracellular NO and channel activity of RPTECs, using RT-PCR, NO imaging, and the cell-attached mode of the patch-clamp technique. Prolonged incubation (24 h) of cells with IFN-gamma (20 ng/ml) enhanced iNOS mRNA expression and NO production. In these cells, a NOS inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME; 100 microM), elevated channel activity, suggesting that NO production was so high as to suppress the channel. This indicated that IFN-gamma would chronically suppress channel activity by enhancing NO production. Acute effects of IFN-gamma was also examined in control cells. Simple addition of IFN-gamma (20 ng/ml) to the bath acutely stimulated channel activity, which was abolished by inhibitors of IFN-gamma receptor-associated Janus-activated kinase [P6 (1 microM) and AG490 (10 microM)]. However, l-NAME did not block the acute effect of IFN-gamma. Indeed, IFN-gamma did not acutely affect NO production. Moreover, the acute effect was not blocked by inhibition of PKA, PKG, and phosphatidylinositol 3-kinase (PI3K). We conclude that IFN-gamma exerted a delayed suppressive effect on K(+) channel activity by enhancing iNOS expression and an acute stimulatory effect, which was independent of either NO pathways or phosphorylation processes mediated by PKA, PKG, and PI3K in RPTECs.

Tempol improves renal hemodynamics and pressure natriuresis in hyperthyroid rats

Hyperthyroidism in rats is associated with increased oxidative stress. These animals also show abnormal renal hemodynamics and an attenuated pressure-diuresis-natriuresis (PDN) response. We analyzed the role of oxidative stress as a mediator of these alterations by examining acute effects of tempol, a superoxide dismutase mimetic. The effects of increasing bolus doses of tempol (25-150 micromol/kg) on mean arterial pressure (MAP), renal vascular resistance (RVR), and cortical (CBF) and medullary (MBF) blood flow were studied in control and thyroxine (T4)-treated rats. In another experiment, tempol was infused at 150 micromol.kg(-1).h(-1) to analyze its effects on the glomerular filtration rate (GFR) and on PDN response in these animals. Tempol dose dependently decreased MAP and RVR and increased CBF and MBF in control and T4-treated rats, but the T4 group showed a greater responsiveness to tempol in all of these variables. The highest dose of tempol decreased RVR by 13.5 +/- 2.1 and 5.5 +/- 1.2 mmHg.ml(-1).min(-1) in hyperthyroid (P < 0.01) and control rats, respectively. GFR was not changed by tempol in controls but was significantly increased in the hyperthyroid group. Tempol did not change the absolute or fractional PDN responses of controls but significantly improved those of hyperthyroid rats, although without attaining normal values. Tempol increased the slopes of the relationship between renal perfusion pressure and natriuresis (T4+tempol: 0.17 +/- 0.05; T4: 0.09 +/- 0.03 microeq.min(-1).g(-1).mmHg(-1); P < 0.05) and reduced 8-isoprostane excretion in hyperthyroid rats. These results show that antioxidant treatment with tempol improves renal hemodynamic variables and PDN response in hyperthyroid rats, indicating the participation of an increased oxidative stress in these mechanisms.

Effect of hydrogen peroxide on ROMK channels in the cortical collecting duct

We used the patch-clamp technique to study the effect of H(2)O(2) on the apical ROMK-like small-conductance K (SK) channel in the cortical collecting duct (CCD). The addition of H(2)O(2) decreased the activity of the SK channels and the inhibitory effect of H(2)O(2) was larger in the CCD from rats on a K-deficient diet than that from rats on a normal-K or a high-K diet. However, application of H(2)O(2) did not inhibit the SK channels in inside-out patches. This suggests that the H(2)O(2)-mediated inhibition of SK channels was not due to direct oxidation of the SK channel protein. Because a previous study showed that H(2)O(2) stimulated the expression of Src family protein tyrosine kinase (PTK) which inhibited SK channels (3), we explored the role of PTK in mediating the effect of H(2)O(2) on SK channels. The application of H(2)O(2) stimulated the activity of endogenous PTK in M-1 cells and increased tyrosine phosphorylation of ROMK in HEK293 cells transfected with GFP-ROMK1 and c-Src. However, blockade of PTK only attenuated but did not completely abolish the inhibitory effect of H(2)O(2) on SK channels. Since H(2)O(2) has also been demonstrated to activate mitogen-activated protein kinase, P38, and ERK (3), we examined the role of P38 and ERK in mediating the effect of H(2)O(2) on SK channels. Similar to blockade of PTK, suppression of P38 and ERK did not completely abolish the H(2)O(2)-induced inhibition of SK channels. However, combined use of ERK, P38, and PTK inhibitors completely abolished the effect of H(2)O(2) on SK channels. Also, treatment of the CCDs with concanavalin A, an agent which has been shown to inhibit endocytosis (19), abolished the inhibitory effect of H(2)O(2). We conclude that addition of H(2)O(2) inhibited SK channels by stimulating PTK activity, P38, and ERK in the CCD and that H(2)O(2) enhances the internalization of the SK channels.

A Kir2.3-like K+ Conductance in Mouse Cortical Collecting Duct Principal Cells

K(+) channels play an important role in renal collecting duct cell function. The current study examined barium (Ba(2+))-sensitive whole-cell K(+) currents (IKBa) in mouse isolated collecting duct principal cells. IKBa demonstrated strong inward rectification and was inhibited by Ba(2+) in a dose- and voltage-dependent fashion, with the K (d) decreasing with hyperpolarization. The electrical distance of block by Ba(2+) was around 8.5%. As expected for voltage-dependent inhibition, the association constant increased with hyperpolarization, suggesting that the rate of Ba(2+) entry was increased at negative potentials. The dissociation constant also increased with hyperpolarization, consistent with the movement of Ba(2+) ions into the intracellular compartment at negative potentials. These properties are not consistent with ROMK but are consistent with the properties of Kir2.3. Kir2.3 is thought to be the dominant basolateral K(+) channel in principal cells. This study provides functional evidence for the expression of Kir2.3 in mouse cortical collecting ducts and confirms the expression of Kir2.3 in this segment of the renal tubule using reverse-transcriptase polymerase chain reaction. The conductance described here is the first report of a macroscopic K(+) conductance in mouse principal cells that shares the biophysical profile of Kir2.3. The properties and dominant nature of the conductance suggest that it plays an important role in K(+) handling in the principal cells of the cortical collecting duct.

Effect of aging on corticosterone secretion in diestrous rats

The roles of age and prolactin (PRL) in regulating glucocorticoid secretion in diestrous rats were investigated. Adrenal zona fasciculata-reticularis (ZFR) cells from young, adult, middle (mid)-aged, and old female rats were isolated. Estrous cycle stage was determined by light microscopy after vaginal smears. Blood samples were collected from right jugular vein at 0, 30, 60, and 120 min after challenge with adrenocorticotropin (ACTH). During the diestrous phase, plasma levels of estradiol and progesterone were lower in mid-aged and old rats than in either young or adult rats. Age-dependent increases of the basal levels of plasma PRL and corticosterone were observed. No difference of ACTH-increased plasma concentrations of corticosterone was observed among young, adult, mid-aged, and old rats. Aging increased the basal, ACTH-, PRL-, forskolin (an adenylate cyclase activator)-, and 3-isobutyl-l-methylxanthine (IBMX, a non-selective phosphodiesterase inhibitor)-stimulated release of corticosterone and production of adenosine 3', 5'-cyclic monophosphate (cAMP) in ZFR cells. However, the 8-Br-cAMP (a membrane-permeable cAMP)-stimulated release of corticosterone was not affected by age. Taken together, these data indicated that aging increased corticosterone secretion in female rats during diestrous phase, which is in part due to an increase in cAMP accumulation. In conclusion, aging and PRL play a stimulatory role in the co-regulation of corticosterone secretion.

Effect of Peroxynitrite on Passive K+ Transport in Human Red Blood Cells

Peroxynitrite is generated in vivo by the reaction between nitric oxide, from endothelial and other cells, and the superoxide anion. It is therefore pertinent to examine its effects on the membrane permeability of red blood cells. Treatment of human red blood cells with peroxynitrite (nominally 1 mM) markedly stimulated passive K+ permeability. The main effect was on a Cl(-)-independent K+ pathway, which remains unidentified. Although K+-Cl- cotransport (KCC) was stimulated, this was dependent on saline composition, being inhibited by physiological levels of glucose (IC50 4 mM), and also by sucrose and MOPS. Effects on the Cl(-)-independent K+ pathway were less dependent on saline composition, and were not inhibited by amiloride, ethylisopropylamiloride, dimethylamiloride or gadolinium. Na+-K+-2Cl- cotransporter was inhibited whilst there was little effect on the Gardos channel (Ca2+-activated K+ channel). Peroxynitrite was markedly more effective in oxygenated cells than deoxygenated ones. Treatment with peroxynitrite per se did not affect initial cell volume. Anisotonic swelling modestly increased the Cl(-)-independent K+ influx, but did not affect peroxynitrite-stimulated KCC. Decreasing extracellular pH from 7.4 to 7.2 or 7.0 increased KCC stimulation, whilst the Cl(-)-independent component of K+ transport was lowest at pH 7.2. Finally, protein phosphatase inhibition with calyculin A (100 nM) inhibited KCC, implying that, as with other KCC stimuli, peroxynitrite acts via decreased protein phosphorylation; pre-treatment with calyculin A also inhibited the Cl(-)-independent component of K+ transport. These findings are relevant to the actions of peroxynitrite in vivo.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Regulation of an inwardly rectifying K+ channel by nitric oxide in cultured human proximal tubule cells

We investigated the effects of nitric oxide (NO) on activity of the inwardly rectifying K(+) channel in cultured human proximal tubule cells, using the cell-attached mode of the patch-clamp technique. An inhibitor of NO synthases, N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 microM), reduced channel activity, which was restored by an NO donor, sodium nitroprusside (SNP; 10 microM) or 8-bromo-cGMP (8-BrcGMP; 100 microM). However, SNP failed to activate the channel in the presence of an inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM). Similarly, the SNP effect was abolished by a protein kinase G (PKG)-specific inhibitor, KT-5823 (1 microM), but not by a protein kinase A-specific inhibitor, KT-5720 (500 nM). Another NO donor, S-nitroso-N-acetyl-D,L-penicillamine (10 microM), mimicked the SNP-induced channel activation. In contrast to the stimulatory effect of SNP at a low dose (10 microM), a higher dose of SNP (1 mM) reduced channel activity, which was not restored by 8-BrcGMP. Recordings of membrane potential with the slow whole cell configuration demonstrated that l-NAME (100 microM) and the high dose of SNP (1 mM) depolarized the cell by 10.1 +/- 2.6 and 9.2 +/- 1.0 mV, respectively, whereas the low dose of SNP (10 microM) hyperpolarized it by 7.1 +/- 0.7 mV. These results suggested that the endogenous NO would contribute to the maintenance of basal activity of this K(+) channel and hence the potential formation via a cGMP/PKG-dependent mechanism, whereas a high dose of NO impaired channel activity independent of cGMP/PKG-mediated processes.

The role of reactive oxygen species in the regulation of tubular function

The phrase reactive oxygen species covers a number of molecules and atoms, including the quintessential member of the group, O2-; singlet oxygen; H2O2; organic peroxides; and OONO-. While nitric oxide (NO) is also technically a member of the reactive oxygen species family, it is generally considered with a different class of compounds and will not be considered here. To our knowledge, there are currently no published data reporting the effects of reactive oxygen species on net transepithelial flux in the proximal nephron. However, there is evidence that OONO- regulates Na+/K+ adenosine triphosphatase (ATPase) activity as well as paracellular permeability. While it is easy to speculate that such an effect on the pump would decrease net transepithelial solute and water reabsorption, one cannot do so without knowing how other transporters are affected. O2- stimulates NaCl absorption by the thick ascending limb by activating protein kinase C and blunting the effects of NO. The effects of O2- on thick ascending limb NaCl absorption may be important for the initiation of salt-sensitive hypertension. To our knowledge, there are no published data concerning the role of reactive oxygen species in the regulation of solute absorption in either the distal convoluted tubule or the collecting duct. However, OONO- inhibits basolateral K+ channels in the cortical collecting duct, although the net effect of such inhibition is unknown.

CONCLUSION

While the regulation of tubular transport by reactive oxygen species is important to overall salt and water balance, we know very little about where and how these regulators act along the nephron.

Activity of the basolateral K+ channels is coupled to the Na+-K+-ATPase in the cortical collecting duct

Microelectrode and patch-clamp techniques were used in the isolated cortical collecting duct to study the effects of stimulating Na+-K+-ATPase by raising bath K+ (Fujii Y and Katz AI. Am J Physiol Renal Fluid Electrolyte Physiol 257: F595-F601, 1989 and Muto S, Asano Y, Seldin D, and Giebisch. Am J Physiol Renal Physiol 276: F143-F158, 1999) on the transepithelial (VT) and basolateral membrane (VB) voltages and basolateral K+ channel activity. Increasing bath K+ from 2.5 to 8.5 mM resulted in an initial hyperpolarization of both VT and VB followed by a delayed depolarization. The effects of raising bath K+ on VT and VB were attenuated by decreasing luminal Na+ from 146.8 to 14.0 mM and were abolished by removal of luminal Na+, whereas those were magnified in desoxycorticosterone acetate (DOCA)-treated rabbits. Increasing bath K+ also led to a significant reduction of the intracellular Na+ and Ca2+ concentrations. The transepithelial conductance (GT) or fractional apical membrane resistance (fRA) were unaltered during the initial hyperpolarization phase, whereas, in the late depolarization phase, there were an increase in GT and a decrease in fRA, both of which were attenuated in the presence of low luminal Na+ (14.0 mM). In tubules from DOCA-treated animals, bath Ba2+ not only caused a significantly larger initial hyperpolarization of VT and VB but also blunted the late depolarization by high bath K+. Nomega-nitro-l-arginine methyl ester (l-NAME) partially mimicked the effect of Ba2+ and decreased the amplitude of the late depolarization. Patch-clamp experiments showed that raising bath K+ from 2.5 to 8.5 mM resulted in an increased activity of the basolateral K+ channel, which was absent in the presence of l-NAME. We conclude that stimulation of Na+-K+-ATPase increases the basolateral K+ conductance and that this effect involves suppression of nitric oxide-dependent inhibition of K+ channels.

Cyclic AMP and cyclic GMP independent stimulation of ventricular calcium current by peroxynitrite donors in guinea pig myocytes

We investigated the potential involvement of peroxynitrite (ONOO(-)) in the modulation of calcium current (I(Ca)) in guinea pig ventricular myocytes with the whole-cell patch clamp technique and with cyclic AMP (cAMP) measurements. Because of the short half-life of ONOO(-) at physiological pH, we induced an increase in its intracellular levels by using donors of the precursors, nitric oxide (NO) and superoxide anion (O(2) (-)). High concentrations of NO donors, SpermineNONOate (sp/NO, 300 microM) or SNAP (300 microM) increased basal I(Ca) (50.3 +/- 4.6%, n = 7 and 46.2 +/- 5.0%, n = 13). The superoxide anion donor Pyrogallol (100 microM) also stimulated basal I(Ca) (44.6 +/- 2.8%, n = 11). At lower concentration sp/NO (10 nM) and Pyrogallol (1 microM), although separately ineffective on I(Ca), enhanced the current if applied together (33.5 +/- 0.7%, n = 7). The simultaneous donor of O(2) (-) and NO, SIN-1 (500 microM), also stimulated basal I(Ca) (22.8 +/- 2.1%, n = 13). In the presence of saturating cyclic GMP (cGMP, 50 microM) in the patch pipette or of extracellular dibutyryl cGMP (dbcGMP, 100 microM), I(Ca) was still increased by SIN-1 (32.0 +/- 6.1%, n = 4 and 30.0 +/- 5.4%, n = 8). Both Manganese(III)tetrakis(4-benzoic acid) porphyrin chloride (MnTBAP, 100 microM) a ONOO(-) scavenger, and superoxide dismutase (SOD) (150 U/ml) reversed the stimulatory effect of SIN-1 on I(Ca) (respectively -0.6 +/- 4.1%, n = 4 and 3.6 +/- 4.3%, n = 4). Intracellular cAMP level was unaltered by SIN-1, while it was enhanced by blocking the NO-cGMP pathway with the NO synthase inhibitor L-NMMA. These results suggest that peroxynitrite donors increase cardiac calcium current without the involvement of cAMP and cGMP.

Expression of NOS1 and soluble guanylyl cyclase by human kidney epithelial cells: morphological evidence for an autocrine/paracrine action of nitric oxide

BACKGROUND

Nitric oxide plays an important role in the kidney through effects on both renal hemodynamics and tubular functions. Tubular epithelial cells are thus a target for nitric oxide. However, as to whether tubular epithelial cells endogeneously produce nitric oxide under physiologic conditions in human kidney is currently unknown. The aim of the present study was to characterize and localize in situ the nitric oxide synthase (NOS) isoforms (NOS1, NOS2, and NOS3) expressed in human normal kidney, and soluble guanylyl cyclase, the well-known target for nitric oxide.

METHODS

Five complementary experimental approaches were used: (1) detection of NOS reductase activity by nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, (2) immunolocalization of the NOS isoforms (NOS1, NOS2, NOS3), (3) immunoblot analysis, (4) quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis of NOS mRNA, and (5) measurement of NOS activity as the conversion rate of l-[14C]-arginine to l-[14C]-citrulline. In addition, in situ detection of soluble guanylyl cyclase was assessed by immunohistochemistry.

RESULTS

All these techniques led to consistent results showing that epithelial cells of most tubules along the human nephron exhibit functional NOS1, with a corticomedullary gradient observed both at the protein and mRNA levels. Moreover, epithelial cells expressing NOS1 also express soluble guanylyl cyclase, indicating that these cells possess the machinery for autocrine/paracrine effect of nitric oxide.

CONCLUSION

The present study demonstrates that NOS1 is strongly expressed in most tubules of the human nephron and therefore invites to consider epithelial cells as one of the major source of nitric oxide in the human kidney under physiologic conditions.

Tiron exerts effects unrelated to its role as a scavenger of superoxide anion: effects on calcium binding and vascular responses

The effect of tiron (4,5-dihydroxy-1,3-benzene disulfonic acid) on the binding of Ca2+ and its effect on vascular responses of the rat perfused mesenteric bed was studied at concentrations of tiron that are used widely to scavenge superoxide anion. In competition assays in buffered solutions with no tissue present, tiron decreased the fluorescence ratio of fura-FF, a measure of [Ca2+]: the inhibition constant (Ki) of tiron with Ca2+ was 0.692 ± 0.036 mM. In the mesenteric bed perfused at constant flow and preconstricted with 90 mM KCl, tiron evoked decreases in perfusion pressure of the mesenteric bed in a concentration-dependent manner (Rmax = 43.58 ± 2.6 mmHg; EC50 = 1.46 ± 0.33 mM). This vasodilator effect of tiron was similar in the presence of the superoxide anion scavenger, tempol (Rmax = 46.12 ± 1.87 mmHg; EC50 = 1.34 ± 0.27 mM). In the presence of 90 mM KCl, increasing concentrations of Ca2+ increased perfusion pressure and tiron shifted the concentration-response curve to Ca2+ to the right. In freshly drawn blood from rats, tiron increased clotting time. The data indicate that tiron binds Ca2+ at concentrations at or below those commonly used to scavenge superoxide anion, an action that may be responsible for a variety of biological responses. The interpretation of effects of tiron in previous work on the role of superoxide anion may need to be re-evaluated.Key words: tiron, calcium, mesenteric vascular bed, superoxide anion, tempol.

Role of nitric oxide in the regulation of nephron transport

Nitric oxide (NO) plays an important role in various physiological processes in the kidney. In vivo experiments first suggested that the natriuretic and diuretic effects caused by NO may be due to decreased NaCl and fluid absorption by the nephron. In the last 10 years, several reports have directly demonstrated a role for NO in modulating transport in different tubule segments. The effects of NO on proximal tubule transport are still controversial. Both stimulation and inhibition of net fluid and bicarbonate have been reported in this segment, whereas only inhibitory effects of NO have been found in Na/H exchanger and Na/K-ATPase activity. The effects of NO in the thick ascending limb are more homogeneous than in the proximal tubule. In this segment, NO decreases net Cl and bicarbonate absorption. A direct inhibitory effect of NO on the Na-K-2Cl cotransporter and the Na/H exchanger has been reported, while NO was found to stimulate apical K channels in this segment. In the collecting duct, NO inhibits Na absorption and vasopressin-stimulated osmotic water permeability. An inhibitory effect of NO on H-ATPase has also been reported in intercalated cells of the collecting duct. Overall, the reported effects of NO in the different nephron segments mostly agree with the natriuretic and diuretic effects observed in vivo. However, the net effect of NO on transport is still controversial in some segments, and in cases like the distal tubule, it has not been studied.

Oxidative stress and potassium channel function

1. Modulation of K+ channel activities by cellular oxidative stress has emerged as a significant determinant of vasomotor function in multiple disease states. 2. Evidence from in vitro and in vivo studies suggest that superoxide (O2-) and hydrogen peroxide (H2O2) enhance BKCa channel activity in rat and cat cerebral arterioles; however, activity is decreased by peroxynitrite (ONOO-) in rat cerebral arteries. The mechanisms of changes in BKCa channel properties are not fully understood and may involve oxidation of cysteine residues that are located in the cell membranes. 3. Studies further suggest that O2- increases KATP channel activity in guinea-pig cardiac myocytes, but decreases opening in cerebral vasculature. Both H2O2 and ONOO- enhance KATP channel activity in the myocardium and in coronary, renal, mesenteric and cerebral vascular beds. Alteration of KATP channels by free radicals may be due to oxidation of SH groups or changes in the cytosolic concentration of ATP. 4. It does appear that O2- produced by either reaction of xanthine and xanthine oxidase or elevated levels of glucose reduces Kv channel activity and the impairments can be partially restored by free radical scavengers, superoxide dismutase and catalase. 5. Thus, redox modulation of potassium channel activity is an important mechanism regulating cell vascular smooth muscle membrane potential.

Interaction of O(2)(-) and NO in the thick ascending limb

Nitric oxide (NO) is an important regulator of NaCl absorption by the thick ascending limb of the loop of Henle (THAL). The free radical superoxide (O(2)(-)) reacts with NO, decreasing its bioavailability. O(2)(-) is produced by mitochondria and various oxidases, some of which are present in the THAL. However, the ability of the THAL to produce O(2)(-) and its interaction with NO have not been studied. We hypothesized that NO bioavailability is decreased by O(2)(-). THALs were isolated and perfused and NO production was measured with an NO-selective microelectrode. Addition of L-Arg (250 micromol/L), but not D-arginine, to the bath increased NO release by 34.8 +/- 11.8 pA (n=7). The response to L-Arg was completely abolished by the NO synthase inhibitor L-NAME (n=7). Scavenging THAL O(2)(-) with the superoxide dismutase (SOD) mimetic Tempol (50 micromol/L) increased L-Arg-induced NO release. At all concentrations of L-Arg tested (50, 100, 250, 500, and 750 micromol/L), further addition of Tempol to the bath significantly increased NO release by THALs. Addition of SOD (300 U/mL) to the bath increased L-Arg-induced NO levels by 82% (n=5; P<0.02). Pretreatment of THALs with the SOD inhibitor diethyl-dithiocarbamate (250 micromol/L) blunted L-Arg-induced NO release by 63% compared with untreated tubules (n=5; P<0.05). Finally, we tested the effect of Tempol on NO-induced inhibition of THAL chloride transport. Addition of L-Arg decreased THAL Cl(-) absorption by 35%. Subsequent addition of Tempol (50 micromol/L) to the bath further decreased Cl(-) absorption by 35% (n=6; P<0.05). We conclude that NO bioavailability in the THAL is decreased by O(2)(-). In addition, we believe our studies are the first to show that endogenous O(2)(-) may act as a physiological regulator of nephron NaCl transport.

The coronary circulation in diabetes: influence of reactive oxygen species on K+ channel-mediated vasodilation

Enhanced oxidative stress, particularly an excess production of superoxide, has been implicated in the altered vasomotor responsiveness observed in diabetes mellitus (DM). Recent evidence suggests that an altered regulation of K+ channel activity by enhanced oxidative stress may participate in the abnormal vascular responses. This review examines the mechanism of hyperglycemia-induced superoxide production and describes the consequences on hyperpolarization-mediated vasodilation. Several pathways have been proposed as mechanisms for hyperglycemia-induced superoxide overproduction, including increased flux through the polyol pathway, depletion of nicotinamide adenine dinucleotide phosphate (NADPH), altered endogenous antioxidant enzymes, and reduced availability of tetrahydrobiopterin, an essential cofactor for nitric oxide synthase (NOS). The resulting excess production of superoxide has been implicated in the impaired dilator responses to ATP-sensitive K+ (KATP) channel openers in aorta and in mesenteric and cerebral arteries of streptozotocin-induced diabetic rats. This may have important implications for ischemia-mediated vasodilation. Potential alterations in voltage-sensitive K+ (KV) channel regulation also have been implicated in the vascular pathogenesis of DM. For example, incubation of small rat coronary arteries in high glucose for 24 h greatly reduces KV channel activity and functional responses, both of which can be partially restored by antioxidant treatment. However, not all K+ channels are adversely affected by reactive oxygen species (ROS). For example, high-conductance Ca(2+)-activated K+ (BKCa) channels may compensate for the loss of other vasodilator mechanisms in disease states such as atherosclerosis where ROS generation is increased. Therefore, BKCa channels may be refractory to superoxide, providing a compensatory mechanism for partially reversing the reduced dilator responses attributed to the dysfunction of other K+ channel types. In summary, determining the effect of ROS on K+ channel-mediated dilation will be important for understanding the pathophysiology of diabetic vascular dysfunction and for developing therapies to improve tissue perfusion in this disease.

Ca2+ mediates the effect of inhibition of Na+-K+-ATPase on the basolateral K+ channels in the rat CCD

We investigated the effect of inhibiting Na+-K+-ATPase on the basolateral 18-pS K+ channel in the cortical collecting duct (CCD) of the rat kidney. Inhibiting Na+-K+-ATPase with strophanthidin decreased the activity of the 18-pS K+ channel and increased the intracellular Ca2+ to 420 nM. Removal of extracellular Ca2+ abolished the effect of strophanthidin. When intracellular Ca2+ was raised with 5 microM ionomycin or A-23187 to 300, 400, and 500 nM, the activity of the 18-pS K+ channel in cell-attached patches fell by 40, 85, and 96%, respectively. To explore the mechanism of Ca2+-induced inhibition, the effect of 400 nM Ca2+ on channel activity was studied in the presence of calphostin C, an inhibitor of protein kinase C, or KN-93 and KN-62, inhibitors of calmodulin-dependent kinase II. Addition of calphostin C or KN-93 or KN-62 failed to block the inhibitory effect of high concentrations of Ca2+ . This suggested that the inhibitory effect of high concentrations of Ca2+ was not mediated by protein kinase C or calmodulin-dependent kinase II pathways. To examine the possibility that the inhibitory effect of high concentrations of Ca2+ was mediated by the interaction of nitric oxide with superoxide, we investigated the effect of 400 nM Ca2+ on channel activity in the presence of 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron) or N(omega)-nitro-L-arginine methyl ester. Pretreatment of the tubules with 4,5-dihydroxy-1,3-benzenedisulfonic acid or N(omega)-nitro-L-arginine methyl ester completely abolished the inhibitory effect of 400 nM Ca2+ on channel activity. Moreover, application of 4,5-dihydroxy-1,3-benzenedisulfonic acid reversed the inhibitory effect of strophanthidin. We conclude that the effect of inhibiting Na+-K+-ATPase is mediated by intracellular Ca2+ and the inhibitory effect of high concentrations of Ca2+ is the result of interaction of nitric oxide with superoxide.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Autocrine effects of nitric oxide on HCO(3)(-) transport by rat thick ascending limb

BACKGROUND

In vivo and in vitro studies have shown that nitric oxide (NO) is an important modulator of transport processes along the nephron. The thick ascending limb (TAL) plays a significant role in the urine-concentrating mechanism and in the maintenance of acid/base balance.

METHODS

TALs from male Sprague-Dawley rats were isolated and perfused, and net bicarbonate flux (J(HCO3)(-) was determined.

RESULTS

In perfused TALs, 0.5 mmol/L L-arginine (L-Arg), the substrate for NO synthase, significantly lowered J(HCO3)(-) from 35.4 +/- 4.6 to 23.2 +/- 2.9 pmol. mm(-1). min(-1), a decrease of 36.9 +/- 11.6% (P < 0.025). D-Arg (0.5 mmol/L) had no effect on J(HCO3)(-) (N = 7). In the presence of 5 mmol/L L-NAME, an NO synthase (NOS) inhibitor, the addition of L-Arg did not affect TAL J(HCO3)(-) (43.4 +/- 4.4 vs. 44.6 +/- 5.0 pmol. mm(-1). min(-1)). L-NAME alone (5 mmol/L) did not affect TAL J(HCO3)(-). After removing L-Arg from the bath, J(HCO3)(-) increased from 26.2 +/- 3.9 to 34.8 +/- 3.2 pmol. mm(-1). min(-1) (P < 0.01), indicating no cytotoxicity of NO. We next investigated the effect of cGMP analogues on TAL J(HCO3)(-). 8-Br-cGMP (50 micromol/L) and db-cGMP (50 micromol/L) significantly decreased J(HCO3)(-) by 26.3 +/- 9.1% and 35.1 +/- 11.6%, respectively. In the presence of cGMP (50 micromol/L), the addition of L-Arg had no effect on J(HCO3)(-). In the presence of KT-5823 (2 mircromol/L), a protein kinase G inhibitor, the addition of L-Arg did not change TAL J(HCO3)(-) (N = 5).

CONCLUSIONS

We conclude that (1) endogenously produced NO inhibits TAL J(HCO3)(-) in an autocrine manner, (2) cGMP mediates all the effects of NO, and (3) this effect is mediated by protein kinase G activation.

Nitric oxide and atrial natriuretic factor stimulate cGMP-dependent membrane insertion of aquaporin 2 in renal epithelial cells

In collecting duct principal cells, aquaporin 2 (AQP2) is shuttled from intracellular vesicles to the plasma membrane upon vasopressin (VP) stimulation. VP activates adenylyl cyclase, increases intracellular cAMP, activating protein kinase A (PKA) to phosphorylate AQP2 on the COOH-terminal residue, serine 256. Using rat kidney slices and LLC-PK1 cells stably expressing AQP2 (LLC-AQP2 cells), we now show that AQP2 trafficking can be stimulated by cAMP-independent pathways. In these systems, the nitric oxide (NO) donors sodium nitroprusside (SNP) and NONOate and the NO synthase substrate L-arginine mimicked the effect of VP, stimulating relocation of AQP2 from cytoplasmic vesicles to the plasma membrane. Unlike VP, these other agents did not increase intracellular cAMP. However, SNP increased intracellular cGMP, and exogenous cGMP stimulated AQP2-membrane insertion. Atrial natriuretic factor, which signals via cGMP, also stimulated AQP2 translocation. The VP and SNP effects were blocked by the kinase inhibitor H89. SNP did not stimulate membrane insertion of AQP2 in LLC-PK1 cells expressing the phosphorylation-deficient mutant 256SerAla-AQP2, indicating that phosphorylation of Ser256 is required for signaling. Both PKA and cGMP-dependent protein kinase G phosphorylated AQP2 on this COOH-terminal residue in vitro. These results demonstrate a novel, cAMP-independent and cGMP-dependent pathway for AQP2 membrane insertion in renal epithelial cells.

The role of nitric oxide in the regulation of ion channels in airway epithelium: implications for diseases of the lung

The human respiratory tract is covered with airway surface liquid (ASL) that is essential for lung defense and normal airway function. The quantity and composition of ASL is regulated by active ion transport across the airway epithelium. Abnormal electrolyte transport produces changes in ASL volume and composition, inhibits mucociliary clearance and leads to chronic infection of airway surfaces, as is evident in cystic fibrosis. Agonists that induce intracellular increases in cAMP or Ca2+ are generally associated with electrolyte secretion. While these mechanisms have been studied in detail for many years, modulation of ion channels by nitric oxide (NO) has emerged only recently as a significant determinant of ion channel function. NO is a physiological regulator of transepithelial ion movement and alterations of its generation and action may play an important role in the pathogenesis of lung disorders characterized by hypersecretion of ASL. This review presents the current understanding of regulation of airway epithelial ion channels by NO and attempts to highlight the importance of this regulation for lung defense.

Extracellular ATP inhibits the small-conductance K channel on the apical membrane of the cortical collecting duct from mouse kidney

We have used the patch-clamp technique to study the effects of changing extracellular ATP concentration on the activity of the small-conductance potassium channel (SK) on the apical membrane of the mouse cortical collecting duct. In cell-attached patches, the channel conductance and kinetics were similar to its rat homologue. Addition of ATP to the bathing solution of split-open single cortical collecting ducts inhibited SK activity. The inhibition of the channel by ATP was reversible, concentration dependent (K(i) = 64 microM), and could be completely prevented by pretreatment with suramin, a specific purinergic receptor (P(2)) blocker. Ranking of the inhibitory potency of several nucleotides showed strong inhibition by ATP, UTP, and ATP-gamma-S, whereas alpha, beta-Me ATP, and 2-Mes ATP failed to affect channel activity. This nucleotide sensitivity is consistent with P(2)Y(2) purinergic receptors mediating the inhibition of SK by ATP. Single channel analysis further demonstrated that the inhibitory effects of ATP could be elicited through activation of apical receptors. Moreover, the observation that fluoride mimicked the inhibitory action of ATP suggests the activation of G proteins during purinergic receptor stimulation. Channel inhibition by ATP was not affected by blocking phospholipase C and protein kinase C. However, whereas cAMP prevented channel blocking by ATP, blocking protein kinase A failed to abolish the inhibitory effects of ATP. The reduction of K channel activity by ATP could be prevented by okadaic acid, an inhibitor of protein phosphatases, and KT5823, an agent that blocks protein kinase G. Moreover, the effect of ATP was mimicked by cGMP and blocked by L-NAME (N(G)-nitro-l-arginine methyl ester). We conclude that the inhibitory effect of ATP on the apical K channel is mediated by stimulation of P(2)Y(2) receptors and results from increasing dephosphorylation by enhancing PKG-sensitive phosphatase activity.

The cGMP-dependent protein kinase stimulates the basolateral 18-pS K channel of the rat CCD

We used the patch-clamp technique to study the effect of cGMP on the 18-pS K channel in the basolateral membrane of the rat cortical collecting duct. Addition of 100 microM 8-bromoguanosine 3', 5'-cyclic monophosphate (8-Br-cGMP) increased the activity of the 18-pS K channel, defined by NP(o), by 95%. In contrast, applying 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) has no effect on channel activity. The effect of 8-Br-cGMP was observed only in cell-attached but not in inside-out patches. Application of 1 microM KT-5823, an inhibitor of the cGMP-dependent protein kinase (PKG), not only reduced the channel activity, but also completely abolished the stimulatory effect of 8-Br-cGMP, suggesting that the 18-pS K channel is not a cGMP-gated K channel. Addition of H-89, an agent that also blocks the PKG, mimicked the effect of KT-5823. To examine the possibility that the effect of 8-Br-cGMP is the result of inhibiting cGMP-dependent phosphodiesterase (PDE) and, accordingly, increasing cAMP or cGMP levels, we explored the effect on the 18-pS K channel of IBMX, an agent that inhibits the PDE. The addition of 100 microM IBMX had no significant effect on channel activity in cell-attached patches. Moreover, in the presence of IBMX, 8-Br-cGMP increased the channel activity to the same extent as that observed in the absence of IBMX, suggesting that the effect of cGMP is not mediated by inhibiting the cGMP-dependent PDE. That the effect of cGMP is mediated by stimulating PKG was further indicated by experiments in which application of exogenous PKG restored the channel activity when it decreased after the excision of the patches. In contrast, adding exogenous cAMP-dependent protein kinase catalytic subunit failed to reactivate the run-down channels. We conclude that cGMP stimulates the 18-pS channel, and the effect of cGMP is mediated by PKG.

Nitric oxide inhibits sodium/hydrogen exchange activity in the thick ascending limb

Nitric oxide (NO) inhibits transport in various nephron segments, and the thick ascending limb (TAL) expresses nitric oxide synthase (NOS). However, the effects of NO on TAL transport have not been extensively studied. We tested the hypothesis that NO inhibits apical and basolateral Na(+)/H(+) exchange by the TAL by measuring intracellular pH (pH(i)) of isolated, perfused rat TALs using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). The NO donor spermine NONOate (SPM, 10 microM) decreased steady-state pHi in medullary TALs from 7.18 +/- 0.13 to 7.13 +/- 0.14 (P < 0.02), whereas controls did not decrease significantly. We next measured the buffering capacity of medullary TALs and the rate at which they recovered from acid loads to investigate the mechanism whereby NO reduces steady-state pHi. SPM decreased H+ flux (JH) from 2.41 +/- 0.66 to 0.97 +/- 0.19 pmol. min(-1). mm(-1), 55%. To assure that the decrease in JH was due to NO, another donor, nitroglycerin (NTG; 10 microM), was used. NTG decreased J(H) from 1.65 +/- 0.11 to 1.07 +/- 0.24 pmol. min(-1). mm(-1), 37%. To determine the relative contributions of the apical and basolateral Na+/H+ exchangers, 5-(N,N-dimethyl)amiloride (DMA; 100 microM) was added to either bath or lumen. With DMA added to the bath, SPM decreased J(H) from 4.78 +/- 1.08 to 2.74 +/- 0.54 pmol. min(-1). mm(-1), an inhibition of 41%; and with DMA added to the lumen, SPM decreased J(H) from 2.31 +/- 0.29 to 1.74 +/- 0.27 pmol. min(-1). mm(-1), a reduction of 26%. Addition of DMA alone to both bath and lumen resulted in an 87% inhibition of JH. We conclude that NO inhibits both apical and basolateral Na+/H+ exchangers and consequently may play an important role in regulating pHi and may alter acid/base balance by directly affecting bicarbonate absorption in the TAL.