Simultaneous GLP-1 receptor activation and angiotensin receptor blockade increase natriuresis independent of altered arterial pressure in obese OLETF rats.

Obesity is associated with an inappropriately activated renin-angiotensin-aldosterone system, suppressed glucagon-like peptide-1 (GLP-1), increased renal Na+ reabsorption, and hypertension. To assess the link between GLP-1 and angiotensin receptor type 1 (AT1) signaling on obesity-associated impairment of urinary Na+ excretion (UNaV) and elevated arterial pressure, we measured mean arterial pressure (MAP) and heart rate by radiotelemetry and metabolic parameters for 40 days. We tested the hypothesis that stimulation of GLP-1 signaling provides added benefit to blockade of AT1 by increasing UNaV and further reducing arterial pressure in the following groups: (1) untreated Long-Evans Tokushima Otsuka (LETO) rats (n = 7); (2) untreated Otsuka Long-Evans Tokushima Fatty (OLETF) rats (n = 9); (3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day; n = 9); (4) OLETF + GLP-1 receptor agonist (EXE; 10 µg exenatide/kg/day; n = 7); and (5) OLETF + ARB + EXE (Combo; n = 6). On day 2, UNaV was 60% and 62% reduced in the EXE and Combo groups, respectively, compared with that in the OLETF rats. On day 40, UNaV was increased 69% in the Combo group compared with that in the OLETF group. On day 40, urinary angiotensinogen was 4.5-fold greater in the OLETF than in the LETO group and was 56%, 62%, and 58% lower in the ARB, EXE, and Combo groups, respectively, than in the OLETF group. From day 2 to the end of the study, MAP was lower in the ARB and Combo groups than in the OLETF rats. These results suggest that GLP-1 receptor activation may reduce intrarenal angiotensin II activity, and that simultaneous blockade of AT1 increases UNaV in obesity; however, these beneficial effects do not translate to a further reduction in MAP.

Effects of urine composition on epithelial Na+ channel-targeted protease activity.

We examined human urinary proteolytic activity toward the Epithelial Sodium Channel (ENaC). We focused on two sites in each of alpha and gamma ENaC that are targets of endogenous and exogenous proteases. We examined the effects of ionic strength, pH and urinary H(+)-buffers, metabolic intermediates, redox molecules, and large urinary proteins. Monoatomic cations caused the largest effect, with sodium inhibiting activity in the 15-515 mEq range. Multivalent cations zinc and copper inhibited urinary proteolytic activity at concentrations below 100 µmol/L. Similar to sodium, urea caused a 30% inhibition in the 0-500 mmol/L range. This was not observed with acetone and ethanol. Modulating urinary redox status modified activity with H2O2 stimulated and ascorbate inhibited activity. Minimal effects (<10%) were observed with caffeine, glucose, several TCA cycle intermediates, salicylic acid, inorganic phosphate, albumin, creatinine, and Tamm-Horsfall protein. The cumulative activity of ENaC-cleaving proteases was highest at neutral pH, however, alpha and gamma proteases exhibited an inverse dependence with alpha stimulated at acidic and gamma stimulated at alkaline pH. These data indicate that ENaC-targeting urinary proteolytic activity is sensitive to sodium, urea and pH and changes in these components can modify channel cleavage and activation status, and likely downstream sodium absorption unrelated to changes in protein or channel density.

Interacting domains in the epithelial sodium channel that mediate proteolytic activation.

Epithelial Sodium Channel (ENaC) proteolysis at sites in the extracellular loop of the α and γ subunits leads to marked activation. The mechanism of this effect remains debated, as well as the role of the N- and C-terminal fragments of these subunits created by cleavage. We introduced cysteines at sites bracketing upstream and downstream the cleavage regions in α and γ ENaC to examine the role of these fragments in the activated channel. Using thiol modifying reagents, as well as examining the effects of cleavage by exogenous proteases we constructed a functional model that determines the potential interactions of the termini near the cleavage regions. We report that the N-terminal fragments of both α and γ ENaC interact with the channel complex; with interactions between the N-terminal γ and the C-terminal α fragments being the most critical to channel function and activation by exogenous cleavage by subtilisin. Positive charge modification at a.a.135 in the N-terminal fragment of γ exhibited the largest inhibition of channel function. This region was found to interact with the C-terminal α fragment between a.a. 205 and 221; a tract which was previously identified to be the site of subtilisin's action. These data provide the first evidence for the functional channel rearrangement caused by proteolysis of the α and γ subunit and indicate that the untethered N-terminal fragments of these subunits interact with the channel complex.

A long isoform of the epithelial sodium channel alpha subunit forms a highly active channel.

A long isoform of the human Epithelial Sodium Channel (ENaC) α subunit has been identified, but little data exist regarding the properties or regulation of channels formed by α728. The baseline whole cell conductance of oocytes expressing trimeric α728ßγ channels was 898.1±277.2 and 49.59±13.2 µS in low and high sodium solutions, respectively, and was 11 and 2 fold higher than the conductances of α669ßγ in same solutions. α728ßγ channels were also 2 to 5 fold less sensitive to activation by the serine proteases subtilisin and trypsin than α669ßγ in low and high Na+ conditions. The long isoform exhibited lower levels of full length and cleaved protein at the plasma membrane and a rightward shifted sensitivity to inhibition by increases of [Na+]i. Both channels displayed similar single channel conductances of 4 pS, and both were activated to a similar extent by reducing temperature, altogether indicating that activation of baseline conductance of α728ßγ was likely mediated by enhanced channel activity or open probability. Expression of α728 in native kidneys was validated in human urinary exosomes. These data demonstrate that the long isoform of αENaC forms the structural basis of a channel with different activity and regulation, which may not be easily distinguishable in native tissue, but may underlie sodium hyperabsorption and salt sensitive differences in humans.

Redox artifacts in electrophysiological recordings.

Electrophysiological techniques make use of Ag/AgCl electrodes that are in direct contact with cells or bath. In the bath, electrodes are exposed to numerous experimental conditions and chemical reagents that can modify electrode voltage. We examined voltage offsets created in Ag/AgCl electrodes by exposure to redox reagents used in electrophysiological studies. Voltage offsets were measured in reference to an electrode separated from the solution by an agar bridge. The reducing reagents Tris-2-carboxyethly-phosphine, dithiothreitol (DTT), and glutathione, as well as the oxidizing agent H(2)O(2) used at experimentally relevant concentrations reacted with Ag in the electrodes to produce voltage offsets. Chloride ions and strong acids and bases produced offsets at millimolar concentrations. Electrolytic depletion of the AgCl layer, to replicate voltage clamp and sustained use, resulted in increased sensitivity to flow and DTT. Offsets were sensitive to electrode silver purity and to the amount and method of chloride deposition. For example, exposure to 10 µM DTT produced a voltage offset between 10 and 284 mV depending on the chloride deposition method. Currents generated by these offsets are significant and dependent on membrane conductance and by extension the expression of ion channels and may therefore appear to be biological in origin. These data demonstrate a new source of artifacts in electrophysiological recordings that can affect measurements obtained from a variety of experimental techniques from patch clamp to two-electrode voltage clamp.

Coupling of epithelial Na+ and Cl- channels by direct and indirect activation by serine proteases.

The mammalian collecting duct (CD) is continuously exposed to urinary proteases. The CD expresses an epithelial Na(+) channel (ENaC) that is activated after cleavage by serine proteases. ENaC also exists at the plasma membrane in the uncleaved form, rendering activation by extracellular proteases an important mechanism for regulating Na(+) transport. Many exogenous and a small number of endogenous extracellular serine proteases have been shown to activate the channel. Recently, kallikrein 1 (KLK1) was shown to increase γENaC cleavage in the native CD indicating a possible direct role of this endogenous protease in Na(+) homeostasis. To explore this process, we examined the coordinated effect of this protease on Na(+) and Cl(-) transport in a polarized renal epithelial cell line (Madin-Darby canine kidney). We also examined the role of native urinary proteases in this process. Short-circuit current (I(sc)) was used to measure transport of these ions. The I(sc) exhibited an ENaC-dependent Na(+) component that was amiloride blockable and a cystic fibrosis transmembrane conductance regulator (CFTR)-dependent Cl(-) component that was blocked by inhibitor 172. Apical application of trypsin, an exogenous S1 serine protease, activated I(ENaC) but was without effects on I(CFTR). Subtilisin an exogenous S8 protease that mimics endogenous furin-type proteases activated both currents. A similar activation was also observed with KLK1 and native rat urinary proteases. Activation with urinary proteases occurred within minutes and at protease concentrations similar to those in the CD indicating physiological significance of this process. ENaC activation was irreversible and mediated by enhanced cleavage of γENaC. The activation of CFTR was indirect and likely dependent on activation of an endogenous apical membrane protease receptor. Collectively, these data demonstrate coordinated stimulation of separate Na(+) and Cl(-) transport pathways in renal epithelia by extracellular luminal proteases. They also indicate that baseline urinary proteolytic activity is sufficient to modify Na(+) and Cl(-) transport in these epithelia.

Methods for stable recording of short-circuit current in a Na+-transporting epithelium.

Epithelial Na(+) transport as measured by a variety of techniques, including the short-circuit current technique, has been described to exhibit a "rundown" phenomenon. This phenomenon manifests as time-dependent decrease of current and resistance and precludes the ability to carry out prolonged experiments aimed at examining the regulation of this transport. We developed methods for prolonged stable recordings of epithelial Na(+) transport using modifications of the short-circuit current technique and commercial Ussing-type chambers. We utilize the polarized MDCK cell line expressing the epithelial Na(+) channel (ENaC) to describe these methods. Briefly, existing commercial chambers were modified to allow continuous flow of Ringer solution and precise control of such flow. Chamber manifolds and associated plumbing were modified to allow precise temperature clamp preventing temperature oscillations. Recording electrodes were modified to eliminate the use of KCl and prevent membrane depolarization from KCl leakage. Solutions utilized standard bicarbonate-based buffers, but all gasses were prehydrated to clamp buffer osmolarity. We demonstrate that these modifications result in measurements of current and resistance that are stable for at least 2 h. We further demonstrate that drifts in osmolarity similar to those obtained before prior to our modifications can lead to a decrease of current and resistance similar to those attributed to rundown.

Acute cholesterol-induced anti-natriuretic effects: role of epithelial Na+ channel activity, protein levels, and processing.

The epithelial Na(+) channel (ENaC) is modulated by membrane lipid composition. However, the effect of an in vivo change of membrane composition is unknown. We examined the effect of a 70-day enhanced cholesterol diet (ECD) on ENaC and renal Na(+) handling. Rats were fed a standard chow or one supplemented with 1% cholesterol and 0.5% cholic acid (ECD). ECD animals exhibited marked anti-diuresis and anti-natriuresis (40 and 47%), which peaked at 1-3 weeks. Secondary compensation returned urine output and urinary Na(+) excretion to control levels by week 10. During these initial changes, there were no accompanying effects on systolic blood pressure, serum creatinine, or urinary creatinine excretion, indicating that the these effects of ECD preceded those which modify renal filtration and blood pressure. The effects of ECD on ENaC were evaluated by measuring the relative protein content of α, ß, and γ subunits. α and γ blots were further examined for subunit cleavage (a process that activates ENaC). No significant changes were observed in α and ß levels throughout the study. However, levels of cleaved γ were elevated, suggesting that ENaC was activated. The changes of γ persisted at week 10 and were accompanied by additional subunit fragments, indicating potential changes of γ-cleaving proteases. Enhanced protease activity, and specifically that which could act on the second identified cleavage site in γ, was verified in a newly developed urinary protease assay. These results predict enhanced ENaC activity, an effect that was confirmed in patch clamp experiments of principal cells of split open collecting ducts, where ENaC open probability was increased by 40% in the ECD group. These data demonstrate a complex series of events and a new regulatory paradigm that is initiated by ECD prior to the onset of elevated blood pressure. These events lead to changes of renal Na(+) handling, which occur in part by effects on extracellular γ-ENaC cleavage.

Alternative mechanism of activation of the epithelial na+ channel by cleavage.

We examined activation of the human epithelial sodium channel (ENaC) by cleavage. We focused on cleavage of alphaENaC using the serine protease subtilisin. Trimeric channels formed with alphaFM, a construct with point mutations in both furin cleavage sites (R178A/R204A), exhibited marked reduction in spontaneous cleavage and an approximately 10-fold decrease in amiloride-sensitive whole cell conductance as compared with alphaWT (2.2 versus 21.2 microsiemens (microS)). Both alphaWT and alphaFM were activated to similar levels by subtilisin cleavage. Channels formed with alphaFD, a construct that deleted the segment between the two furin sites (Delta175-204), exhibited an intermediate conductance of 13.2 microS. More importantly, alphaFD retained the ability to be activated by subtilisin to 108.8 +/- 20.9 microS, a level not significantly different from that of subtilisin activated alphaWT (125.6 +/- 23.9). Therefore, removal of the tract between the two furin sites is not the main mechanism of channel activation. In these experiments the levels of the cleaved 22-kDa N-terminal fragment of alpha was low and did not match those of the C-terminal 65-kDa fragment. This indicated that cleavage may activate ENaC by the loss of the smaller fragment and the first transmembrane domain. This was confirmed in channels formed with alphaLD, a construct that extended the deleted sequence of alphaFD by 17 amino acids (Delta175-221). Channels with alphaLD were uncleaved, exhibited low baseline activity (4.1 microS), and were insensitive to subtilisin. Collectively, these data support an alternative hypothesis of ENaC activation by cleavage that may involve the loss of the first transmembrane domain from the channel complex.

Cholesterol induces renal vasoconstriction and anti-natriuresis by inhibiting nitric oxide production in anesthetized rats.

Although hypercholesterolemia is implicated in the pathophysiology of many renal disorders as well as hypertension, its direct actions in the kidney are not yet clearly understood. In the present study, we evaluated renal responses to administration of cholesterol (8 microg x min(-1).100 g body wt(-1); bound by polyethylene glycol) into the renal artery of anesthetized male Sprague-Dawley rats. Total renal blood flow (RBF) was measured by a Transonic flow probe, and glomerular filtration rate (GFR) was determined by Inulin clearance. In control rats (n = 8), cholesterol induced reductions of 10 +/- 2% in RBF [baseline (b) 7.6 +/- 0.3 microg x min(-1).100 g(-1)], 17 +/- 3% in urine flow (b, 10.6 +/- 0.9 microg x min(-1).100 g(-1)), 29 +/- 3% in sodium excretion (b, 0.96 +/- 0.05 mumol.min(-1).100 g(-1)) and 24 +/- 2% in nitrite/nitrate excretion (b, 0.22 +/- 0.01 nmol.min(-1).100 g(-1)) without an appreciable change in GFR (b, 0.87 +/- 0.03 ml.min(-1).100 g(-1)). These renal vasoconstrictor and anti-natriuretic responses to cholesterol were absent in rats pretreated with nitric oxide (NO) synthase inhibitor, nitro-l-arginine methylester (0.5 microg x min(-1).100 g(-1); n = 6). In rats pretreated with superoxide (O(2)(-)) scavenger tempol (50 microg x min(-1).100 g(-1); n = 6), the cholesterol-induced renal responses remained mostly unchanged, although there was a slight attenuation in anti-natriuretic response. This anti-natriuretic response to cholesterol was abolished in furosemide-pretreated rats (0.3 microg x min(-1).100 g(-1); n = 6) but remained unchanged in amiloride-pretreated rats (0.2 microg x min(-1).100 g(-1); n = 5), indicating that Na(+)/K(+)/2Cl(-) cotransport is the dominant mediator of this effect. These data demonstrate that cholesterol-induced acute renal vasoconstrictor and antinatriuretic responses are mediated by a decrease in NO production. These data also indicate that tubular effect of cholesterol on sodium reabsorption is mediated by the furosemide sensitive Na(+)/K(+)/2Cl(-) cotransporter.

Indirect activation of the epithelial Na+ channel by trypsin.

We tested the hypothesis that the serine protease trypsin can indirectly activate the epithelial Na(+) channel (ENaC). Experiments were carried out in Xenopus oocytes and examined the effects on the channel formed by all three human ENaC subunits and that formed by Xenopus epsilon and human beta and gamma subunits (epsilonbetagammaENaC). Low levels of trypsin (1-10 ng/ml) were without effects on the oocyte endogenous conductances and were specifically used to test the effects on ENaC. Addition of 1 ng/ml trypsin for 60 min stimulated the amiloride-sensitive human ENaC conductance (g(Na)) by approximately 6-fold. This effect on the g(Na) was [Na(+)]-independent, thereby ruling out an interaction with channel feedback inhibition by Na(+). The indirect nature of this activation was confirmed in cell-attached patch clamp experiments with trypsin added to the outside of the pipette. Trypsin was comparatively ineffective at activating epsilonbetagammaENaC, a channel that exhibited a high spontaneous open probability. These observations, in combination with surface binding experiments, indicated that trypsin indirectly activated membrane-resident channels. Activation by trypsin was also dependent on catalytic activity of this protease but was not accompanied by channel subunit proteolysis. Channel activation was dependent on downstream activation of G-proteins and was blocked by G-protein inhibition by injection of guanyl-5'-yl thiophosphate and by pre-stimulation of phospholipase C. These data indicate a receptor-mediated activation of ENaC by trypsin. This trypsin-activated receptor is distinct from that of protease-activated receptor-2, because the response to trypsin was unaffected by protease-activated receptor-2 overexpression or knockdown.

Aldosterone receptor antagonism exacerbates intrarenal angiotensin II augmentation in ANG II-dependent hypertension.

Effects of aldosterone receptor (AR) blockade with eplerenone (epl) on renal Na(+) excretion, arterial blood pressure, intra-adrenal and renal ANG II, and plasma aldosterone levels during ANG II-dependent hypertension were evaluated. Rats from one cohort (n = 10/group) 1) control, 2) control + epl (25 mg/day), 3) ANG II (60 ng/min), and 4) ANG II + epl were maintained in metabolic cages for 28 days for daily urine collections. Systolic blood pressure (SBP) was measured weekly by tail-cuff. In a second cohort (n = 12/group), daily SBP was measured by telemetry (n = 6 rats/group) 1) control, 2) ANG II, and 3) ANG II + epl. A diet containing epl (0.1%) was provided after 1 wk of ANG II infusion. Direct monitoring of BP by telemetry showed that epl delayed the onset of the increase in SBP by 2 days and slightly reduced SBP (186 +/- 6 vs. 177 +/- 8 mmHg). Epl transiently increased Na(+) excretion within 24 h of treatment in both normo- and hypertensive rats; however, balance was reestablished within 5 days suggesting that alternative mechanisms for conserving Na(+) are activated. Cortical alpha-epithelial Na(+) channel content (alpha-ENaC) was not altered after 21 days of epl treatment. Epl exacerbated the ANG II-mediated increases in intrarenal ANG II (226 +/- 16 vs. 365 +/- 38 fmol/g) and further increased intra-adrenal ANG II (3.9 +/- 0.3 vs. 8.2 +/- 0.9 fmol/mg) and aldosterone (255 +/- 55 vs. 710 +/- 87 pmol/mg) content. Exacerbation of intrarenal ANG II levels likely contributes to the maintenance of alpha-ENaC protein content and thus Na(+) reabsorption, which helps explain the ineffectiveness of AR blockade in reducing SBP in ANG II-infused models of hypertension.

W-7 modulates Kv4.3: pore block and Ca2+-calmodulin inhibition.

Ca(+)-calmodulin (Ca(2+)-CaM)-dependent protein kinase II (Ca(2+)/CaMKII) is an important regulator of cardiac ion channels, and its inhibition may be an approach for treatment of ventricular arrhythmias. Using the two-electrode voltage-clamp technique, we investigated the role of W-7, an inhibitor of Ca(2+)-occupied CaM, and KN-93, an inhibitor of Ca(2+)/CaMKII, on the K(v)4.3 channel in Xenopus laevis oocytes. W-7 caused a voltage- and concentration-dependent decrease in peak current, with IC(50) of 92.4 muM. The block was voltage dependent, with an effective electrical distance of 0.18 +/- 0.05, and use dependence was observed, suggesting that a component of W-7 inhibition of K(v)4.3 current was due to open-channel block. W-7 made recovery from open-state inactivation a biexponential process, also suggesting open-channel block. We compared the effects of W-7 with those of KN-93 after washout of 500 muM BAPTA-AM. KN-93 reduced peak current without evidence of voltage or use dependence. Both W-7 and KN-93 accelerated all components of inactivation. We used wild-type and mutated K(v)4.3 channels with mutant CaMKII consensus phosphorylation sites to examine the effects of W-7 and KN-93. In contrast to W-7, KN-93 at 35 muM selectively accelerated open-state inactivation in the wild-type vs. the mutant channel. W-7 had a significantly greater effect on recovery from inactivation in wild-type than in mutant channels. We conclude that, at certain concentrations, KN-93 selectively inhibits Ca(2+)/CaMKII activity in Xenopus oocytes and that the effects of W-7 are mediated by direct interaction with the channel pore and inhibition of Ca(2+)-CaM, as well as a change in activity of Ca(2+)-CaM-dependent enzymes, including Ca(2+)/CaMKII.

The A-kinase anchoring protein 15 regulates feedback inhibition of the epithelial Na+ channel.

Protein kinase A anchoring proteins or AKAPs regulate the activity of many ion channels. Protein kinase A (PKA) is a well-recognized target of AKAPs, with other kinases now emerging as additional targets. We examined the roles of epithelial-expressed AKAPs in regulating the epithelial Na+ channel (ENaC). Experiments used heterologous expression with AKAP15, AKAP-KL, and AKAP79 in Xenopus oocytes. Experiments were carried out under high and low Na+ conditions, as Na+ loading is known to affect the baseline activity of ENaC in a PKC-dependent mechanism. ENaC activity was unaffected by AKAP79 and AKAP-KL expression. However, oocytes coexpressing AKAP15 exhibited an 80% and 91% reduction in the amiloride-sensitive, whole-cell conductance in high and low Na+ conditions, respectively. The reduced channel activity was unaffected by PKA activation or inhibition, indicating a PKA-independent mechanism. Expression with a membrane-targeting domain, mutant form of AKAP15 (AKAP15m) prevented the decrease of ENaC activity, but only under low Na+ conditions. In high sodium conditions, coexpression with AKAP15m led to an increase of ENaC activity to levels similar to those observed under low Na+. These results indicate that membrane-associated AKAP15 reduces ENaC activity whereas the cytoplasmically associated one may participate in the channel's feedback inhibition by intracellular Na+, a process known to involve PKC. This hypothesis was further confirmed in coexpression experiments, which demonstrated functional and physical interaction between AKAP15 and PKCalpha. We propose that AKAP15 regulates ENaC via a novel PKA-independent pathway.

A simple in vivo method for assessing changes of membrane-bound ion channel density in Xenopus oocytes.

Heterologous expression systems, such as Xenopus oocytes, are widely used to study the regulation and the structure function relationship of ion channels and transporters. In the case of ion channels, activity can be easily measured by conventional two-electrode voltage clamping. However, this method only measures the sum of the activity of all plasma membrane-bound channels. Therefore, this measurement cannot discriminate between effects on channel density and individual channel activity. To address this shortcoming, we have developed a simple assay to detect changes of membrane-bound channel density in intact oocytes. This nonradioactive assay relies on specific antibody binding in whole live cells utilizing a simple spectrophotometric measurement. This assay is linear over a wide range of channel expression levels and provides a simple cost-effective way of monitoring changes of membrane-bound channel density. Moreover, when the heterologous proteins poorly express at the plasma membrane, this method becomes advantageous to complex biochemical cell fractionation.

Calcium and chloride channel activation by angiotensin II-AT1 receptors in preglomerular vascular smooth muscle cells.

The pathways responsible for the rapid and sustained increases in [Ca(2+)](i) following activation of ANG II receptors (AT(1)) in renal vascular smooth muscle cells were evaluated using fluorescence microscopy. Resting intracellular calcium concentration [Ca(2+)](i) averaged 75 +/- 9 nM. The response to ANG II (100 nM) was characterized by a rapid initial increase of [Ca(2+)](i) by 74 +/- 6 nM (n = 35) followed by a decrease to a sustained level of 12 +/- 2 nM above baseline. The average time from peak to 50% reduction from the peak value (50% time point) was 32 +/- 4 s. AT(1) receptor blockade with 1 microM candesartan (n = 5) prevented the responses to ANG II. In nominally calcium-free conditions (n = 8), the peak increase in [Ca(2+)](i) averaged 42 +/- 7 nM but the sustained phase was absent and the 50% time point was reduced to 11 +/- 4 s. L-type calcium channel blockade with diltiazem reduced the peak [Ca(2+)](i) to 24 +/- 8 nM and the sustained level to 4 +/- 2 nM (n = 10). In cells preincubated in low Cl(-) (3.0 mM), the peak response to ANG II was suppressed as was the sustained response. Blockade of chloride channels with DIDS eliminated both the peak and sustained responses (n = 11); chloride channel blockade with DPC (n = 17) suppressed the peak increase in [Ca(2+)](i) to 18 +/- 5 and also prevented the sustained response. IP3 receptor blockade by 10 microM TMB-8 (n = 6) reduced the peak to 22 +/- 8 and prevented the sustained response. Exposure to 10 microM TMB-8 in the presence of Ca(2+)-free medium prevented the ANG II response (n = 9). In the presence of 100 microM DPC and 10 microM TMB-8 (n = 7), the ANG II response was also prevented. Thus the rapid initial increase in [Ca(2+)](i) is due not only to release from intracellular stores, but also to Ca(2+) influx from the extracellular fluid. Although Ca(2+) entry via L-type calcium channels is responsible for the major portion of the sustained response, other entry pathways participate. The finding that chloride channel blockers markedly attenuate both rapid and sustained responses indicates that chloride channel activation contributes to, rather than being the consequence of, the initial rapid response.

Bisphosphonates stimulate an endogenous nonselective cation channel in Xenopus oocytes: potential mechanism of action.

The mechanisms of action of bisphosphonates (BPs) have been poorly determined. Besides their actions on osteoclasts, these agents exhibit gastrointestinal complications. They have also recently been described as affecting various preparations that express an epithelial Na(+) channel (ENaC). To understand the effects of BP on ion channels and the ENaC in particular, we used the Xenopus oocyte expression system. Alendronate, and similarly risedronate, two aminobisphosphonates, caused a large stimulation of an endogenous nonselective cation conductance (NSCC). This stimulation averaged 63 +/- 12 muS (n = 18) 60 min after the addition of 2 mM alendronate. The effects on the endogenous NSCC were blocked by extracellular acidification to pH 6.4. On the other hand, alendronate caused a small inhibition of ENaC conductance at pH 7.4 and 6.4, but the effects at pH 6.4 were more readily observed in the absence of changes of the endogenous conductance. The effects on membrane capacitance were also markedly different, with a clear decrease at pH 6.4 and no consistent changes at pH 7.4. The effects on the endogenous channel were further augmented by genistein and were inhibited by a tyrosine phosphatase inhibitor, indicating the involvement of the tyrosine kinase pathway. Stimulation of NSCC with BP is expected to cause membrane depolarization and may explain, in part, its mechanisms of action in inhibiting osteoclasts.

Nonselective cation transport in native esophageal epithelia.

Rabbit esophageal epithelia actively transport Na(+) in a manner similar to that observed in classic electrically tight Na(+)-absorbing epithelia, such as frog skin. However, the nature of the apical entry step is poorly understood. To address this issue, we examined the electrophysiological and biochemical nature of this channel. Western blotting experiments with epithelial Na(+) channel (ENaC) subunit-specific antibodies revealed the presence of all three ENaC subunits in both native and immortalized esophageal epithelial cells. The amino acid sequence of the rabbit alpha-ENaC cloned from native rabbit esophageal epithelia was not significantly different from that of other published alpha-ENaC homologs. To characterize the electrophysiological properties of this native apical channel, we utilized nystatin permeabilization to eliminate the electrical contribution of the basolateral membrane in isolated native epithelia mounted in Ussing-type chambers. We find that the previously described apical Na(+) channel is nonselective for monovalent cations (Li(+), Na(+), and K(+)). Moreover, this channel was not blocked by millimolar concentrations of amiloride. These findings document the presence of a nonselective cation channel in a native Na(+) transporting epithelia, a finding that hereto has been thought to be limited to artificial culture conditions. Moreover, our data are consistent with a potential role of ENaC subunits in the formation of a native nonselective cation channel.

ENaC-membrane interactions: regulation of channel activity by membrane order.

Recently, it was reported that the epithelial Na+ channel (ENaC) is regulated by temperature (Askwith, C.C., C.J. Benson, M.J. Welsh, and P.M. Snyder. 2001. Proc. Natl. Acad. Sci. USA. 98:6459-6463). As these changes of temperature affect membrane lipid order and lipid-protein interactions, we tested the hypothesis that ENaC activity can be modulated by membrane lipid interactions. Two approaches were used to modulate membrane anisotropy, a lipid order-dependent parameter. The nonpharmacological approach used temperature changes, while the pharmacological one used chlorpromazine (CPZ), an agent known to decrease membrane order, and Gd+3. Experiments used Xenopus oocytes expressing human ENaC. Methods of impedance analysis were used to determine whether the effects of changing lipid order indirectly altered ENaC conductance via changes of membrane area. These data were further corroborated with quantitative morphology on micrographs from oocytes membranes studied via electron microscopy. We report biphasic effects of cooling (stimulation followed by inhibition) on hENaC conductance. These effects were relatively slow (minutes) and were delayed from the actual bath temperature changes. Peak stimulation occurred at a calculated Tmax of 15.2. At temperatures below Tmax, ENaC conductance was inhibited with cooling. The effects of temperature on gNa were distinct from those observed on ion channels endogenous to Xenopus oocytes, where the membrane conductance decreased monoexponentially with temperature (t = 6.2 degrees C). Similar effects were also observed in oocytes with reduced intra- and extracellular [Na+], thereby ruling out effects of self or feedback inhibition. Addition of CPZ or the mechanosensitive channel blocker, Gd+3, caused inhibition of ENaC. The effects of Gd+3 were also attributed to its ability to partition into the outer membrane leaflet and to decrease anisotropy. None of the effects of temperature, CPZ, or Gd+3 were accompanied by changes of membrane area, indicating the likely absence of effects on channel trafficking. However, CPZ and Gd+3 altered membrane capacitance in an opposite manner to temperature, consistent with effects on the membrane-dielectric properties. The reversible effects of both Gd+3 and CPZ could also be blocked by cooling and trapping these agents in the rigidified membrane, providing further evidence for their mechanism of action. Our findings demonstrate a novel regulatory mechanism of ENaC.