G alpha i-3 regulates epithelial Na+ channels by activation of phospholipase A2 and lipoxygenase pathways

Lipoxygenases modulate the effect of glutoxim on Na+ transport in the frog skin epithelium

Using voltage-clamp technique, the involvement of lipoxygenases in the effect of immunomodulatory drug glutoxim on Na⁺ transport in frog skin was investigated. It was shown for the first time that preincubation of the skin with lipoxygenase inhibitors caffeic acid, baicalein, and nordihydroguaiaretic acid significantly decreases the stimulatory effect of glutoxim on Na⁺ transport. The data suggest the involvement of lipoxygenase oxidation pathway of arachidonic acid metabolism in the glutoxim effect on Na⁺ transport in frog skin epithelium.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Inflammation effects on the electrical properties of atrial tissue and inducibility of postoperative atrial fibrillation

BACKGROUND

Atrial fibrillation is the most common complication after cardiac surgery. Postoperative atrial fibrillation (PAF) has been shown to increase length of stay, morbidity, and mortality. Because the clinical behavior of PAF parallels that of inflammation following surgery, we investigated the effect of the inflammatory mediator arachidonic acid on the electrical behavior of normal atrial tissue in vitro and assessed the efficacy of the topical application of anti-inflammatory drugs at suppressing PAF in an animal model.

METHODS

To study changes in electrical behavior from inflammation, the conduction properties of six normal canine right atrial appendages were quantified as a function of the direction of impulse propagation with and without 80 mum arachidonic acid. To study the effect of topical anti-inflammatory drugs, 24 adult mongrel dogs were prepared according to the model of sterile talc pericarditis. Nine dogs received talc alone (T), seven received talc combined with 600 mg ibuprofen (T + I), and eight received talc combined with 10 mg methylprednisolone (T + M). Three days following preparation, programmed electrical stimulation was performed to quantify conduction characteristics and to attempt the induction of atrial fibrillation (AF).

RESULTS

In vitro, arachidonic acid produced an anisotropic and rapidly reversible 36.1 +/- 3.4% (P = 0.01) decrease in conduction velocity transverse to the long axis only. In vivo, both ibuprofen and methylprednisolone significantly reduced the incidence of sustained AF (from 56 to 0% T + I and 12% T + M, respectively, P = 0.02). No differences in conduction velocities or refractory periods were seen during sinus rhythm among the groups.

CONCLUSIONS

Acute inflammation as mimicked by arachidonic acid slows conduction anisotropically, mainly transverse to the long axis of the atrial myocardial fibers. This may set the stage for reentry. Preventing inflammation in vivo by the topical application of anti-inflammatory drugs supports this hypothesis, suggesting a possible role for inflammation in the genesis of postoperative atrial fibrillation and shedding light on the mechanism underlying PAF.

Efficient Characterization of Use-Dependent Ion Channel Blockers by Real-Time Monitoring of Channel State

Ion channels are important therapeutic targets for the treatment of a variety of conditions. Among ion channel blocking agents, use-dependent inhibitors can be especially effective therapeutic agents. Use dependence allows the selective inhibition of hyperactive neurons or tachycardiac myocytes, while minimizing effects on cells with normal activity. For voltage-gated channels, the use-dependent compounds typically bind to and inhibit a particular kinetic state that is induced by specific voltage changes. Drug discovery programs that focus on this class of drugs need to rank the use dependence of the compounds. A meaningful comparison among different molecules requires voltage clamp-based assays with continuous voltage control and compensation for or elimination of electrode drift-related effects. A method was developed based on automated electrophysiology in which voltage and frequency dependence of voltage-gated ion channel blockers can be compared using a protocol in which voltage error is compensated for in real time.

Characterization of Na+-permeable cation channels in LLC-PK1 renal epithelial cells

In this study, the presence of Na(+)-permeable cation channels was determined and characterized in LLC-PK1 cells, a renal tubular epithelial cell line with proximal tubule characteristics derived from pig kidney. Patch-clamp analysis under cell-attached conditions indicated the presence of spontaneously active Na(+)-permeable cation channels. The channels displayed nonrectifying single channel conductance of 11 pS, substates, and an approximately 3:1 Na(+)/K(+) permeability-selectivity ratio. The Na(+)-permeable cation channels were inhibited by pertussis toxin and reactivated by G protein agonists. Cation channel activity was observed in quiescent cell-attached patches after vasopressin stimulation. The addition of protein kinase A and ATP to excised patches also induced Na(+) channel activity. Spontaneous and vasopressin-induced Na(+) channel activity were inhibited by extracellular amiloride. To begin assessing potential molecular candidates for this cation channel, both reverse transcription-PCR and immunocytochemical analyses were conducted in LLC-PK1 cells. Expression of porcine orthologs of the alphaENaC and ApxL genes were found in LLC-PK1 cells. The expression of both gene products was confirmed by immunocytochemical analysis. Although alphaENaC labeling was mostly intracellular, ApxL labeled to both the apical membrane and cytoplasmic compartments of subconfluent LLC-PK1 cells. Vasopressin stimulation had no effect on alphaENaC immunolabeling but modified the cellular distribution of ApxL, consistent with an increased membrane-associated ApxL. The data indicate that proximal tubular LLC-PK1 renal epithelial cells express amiloride-sensitive, Na(+)-permeable cation channels, which are regulated by the cAMP pathway, and G proteins. This channel activity may implicate previously reported epithelial channel proteins, although this will require further experimentation. The evidence provides new clues as to potentially relevant Na(+) transport mechanisms in the mammalian proximal nephron.

Arachidonic acid regulates surface expression of epithelial sodium channels

Epithelial Na+ channels (ENaCs) are regulated by the phospholipase A2 (PLA2) product arachidonic acid. Pharmacological inhibition of PLA2 with aristolochic acid induced a significant increase in amiloride-sensitive currents in Xenopus oocytes expressing ENaC. Arachidonic acid or 5,8,11,14-eicosatetraynoic acid (ETYA), a non-metabolized analog of arachidonic acid, induced a time-dependent inhibition of Na+ transport. These effects were also observed by co-expression of a calcium-independent or a calcium-dependent PLA2. Channels with a truncated alpha, beta,or gamma C terminus were not inhibited by arachidonic acid or ETYA. Furthermore, mutation of Tyr618 in the PY motif of the beta subunit abrogated the inhibitory effect of ETYA, suggesting that intact PY motifs participate in arachidonic acid-mediated ENaC inhibition. Analyses of channels expressing a series of beta subunit C-terminal truncations revealed a second region N-terminal to the PY motif (spanning residues betaVal580-betaGly599) that allowed for ETYA-mediated ENaC inhibition. Analyses of both ENaC surface expression and ENaC trafficking with mutants that either gate channels open or closed in response to [(2-(trimethylammonium) ethyl] methanethiosulfonate bromide, or with brefeldin A, suggest that ETYA reduces channel surface expression by inhibiting ENaC exocytosis and increasing ENaC endocytosis.

Contrasting effects of cPLA2 on epithelial Na+ transport

Activity of the epithelial Na+ channel (ENaC) is the limiting step for discretionary Na+ reabsorption in the cortical collecting duct. Xenopus laevis kidney A6 cells were used to investigate the effects of cytosolic phospholipase A2 (cPLA2) activity on Na+ transport. Application of aristolochic acid, a cPLA2 inhibitor, to the apical membrane of monolayers produced a decrease in apical [3H]arachidonic acid (AA) release and led to an approximate twofold increase in transepithelial Na+ current. Increased current was abolished by the nonmetabolized AA analog 5,8,11,14-eicosatetraynoic acid (ETYA), suggesting that AA, rather than one of its metabolic products, affected current. In single channel studies, ETYA produced a decrease in ENaC open probability. This suggests that cPLA2 is tonically active in A6 cells and that the end effect of liberated AA at the apical membrane is to reduce Na+ transport via actions on ENaC. In contrast, aristolochic acid applied basolaterally inhibited current, and the effect was not reversed by ETYA. Basolateral application of the cyclooxygenase inhibitor ibuprofen also inhibited current. Both effects were reversed by prostaglandin E2 (PGE2). This suggests that cPLA2 activity and free AA, which is metabolized to PGE2, are necessary to support transport. This study supports the fine-tuning of Na+ transport and reabsorption through the regulation of free AA and AA metabolism.

Increase of [Ca(2+)]i and release of arachidonic acid via activation of M2 receptor coupled to Gi and rho proteins in oesophageal muscle

We have previously shown that acetylcholine-induced contraction of oesophageal circular muscle depends on activation of phosphatidylcholine selective phospholipase C and D, which result in formation of diacylglycerol, and of phospholipase 2 which produces arachidonic acid. Diacylglycerol and arachidonic acid interact synergistically to activate protein kinase C. We have therefore investigated the relationship between cytosolic Ca(2+) and activation of phospholipase A(2) in response to acetylcholine-induced stimulation, by measuring the intracellular free Ca(2+) ([Ca(2+)]i), muscle tension, and [3H] arachidonic acid release. Acetylcholine-induced contraction was associated with increased [Ca(2+)]i and arachidonic acid release in a dose-dependent manner. In Ca(2+)-free medium, acetylcholine did not produce contraction, [Ca(2+)]i increase, and arachidonic acid release. In contrast, after depletion of Ca(2+) stores by thapsigargin (3 microM), acetylcholine caused a normal contraction, [Ca(2+)]i increase and arachidonic acid release. The increase in [Ca(2+)]i and arachidonic acid release were attenuated by the M2 receptor antagonist methoctramine, but not by the M3 receptor antagonist p-fluoro-hexahydro siladifenidol. Increase in [Ca(2+)]i and arachidonic acid release by acetylcholine were inhibited by pertussis toxin and C3 toxin. These findings indicate that contraction and arachidonic acid release are mediated through muscarinic M2 coupled to Gi or rho protein activation and Ca(2+) influx. Acetylcholine-induced contraction and the associated increase in [Ca(2+)]i and release of arachidonic acid were completely reduced by the combination treatment with a phospholipase A(2) inhibitor dimethyleicosadienoic acid and a phospholipase D inhibitor pCMB. They increased by the action of the inhibitor of diacylglycerol kinase R59949, whereas they decreased by a protein kinase C inhibitor chelerythrine. These data suggest that in oesophageal circular muscle acetylcholine-induced [Ca(2+)]i increase and arachidonic acid release are mediated through activation of M2 receptor coupled to Gi or rho protein, resulting in the activation of phospholipase A(2) and phospholipase D to activate protein kinase C.

Permissive effect of thyroid hormones on induction of rat colonic Na+ transport by aldosterone is not localised at the level of Na+ channel transcription

The interrelationship between thyroid hormones and aldosterone has been examined in the regulation of rat colonic amiloride-sensitive Na+ transport which translocates Na+ through apical amiloride-sensitive Na+ channels and basolateral Na+, K+-ATPase. Electrogenic Na+ transport was measured in an Ussing chamber by the short-circuit current and identified by Na+ channel blocker amiloride. Na+-pumping activity of the basolateral Na+,K+-ATPase was investigated in nystatin-treated epithelium by measuring the equivalent short-circuit current after addition of mucosal Na+. The abundance of mRNA coding for alpha, beta and gamma subunits of the Na+ channel (rENaC) was estimated using Northern blot analysis. Hyperaldosteronism was induced by a low-salt diet and hypothyroidism by methimazole. The low-Na+ diet induced electrogenic Na+ transport in euthyroid rats but its effect was almost completely inhibited in hypothyroid animals even if the plasma concentration of aldosterone was high enough to stimulate this transport pathway both in euthyroid and hypothyroid rats. A kinetic study of the basolateral Na+,K+-ATPase revealed a decrease of Na+ transport capacity in hypothyroid rats kept on the low-Na+ diet in comparison with euthyroid animals fed the same diet. No significant differences in steady-state levels of alpha, beta and gamma rENaC mRNA were detected between euthyroid and hypothyroid rats. These data suggest that hypothyroidism decreases the efficacy of the basolateral Na+ pump but fails to inhibit it completely even though it inhibits the transepithelial electrogenic Na+ transport in response to aldosterone. We conclude that the permissive effect of thyroid hormones on the induction of electrogenic Na+ transport by aldosterone is localised beyond the transcriptional step of Na+ channel regulation.

Regulation of Na(+) reabsorption by the aldosterone-induced small G protein K-Ras2A

Xenopus laevis A6 cells were used as model epithelia to test the hypothesis that K-Ras2A is an aldosterone-induced protein necessary for steroid-regulated Na(+) transport. The possibility that increased K-Ras2A alone is sufficient to mimic aldosterone action on Na(+) transport also was tested. Aldosterone treatment increased K-Ras2A protein expression 2.8-fold within 4 h. Active Ras is membrane associated. After aldosterone treatment, 75% of K-Ras was localized to the plasma membrane compared with 25% in the absence of steroid. Aldosterone also increased the amount of active (phosphorylated) mitogen-activated protein kinase kinase likely through K-Ras2A signaling. Steroid-induced K-Ras2A protein levels and Na(+) transport were decreased with antisense K-ras2A oligonucleotides, showing that K-Ras2A is necessary for the natriferic actions of aldosterone. Aldosterone-induced Na(+) channel activity, was decreased from 0.40 to 0.09 by pretreatment with antisense ras oligonucleotide, implicating the luminal Na(+) channel as one final effector of Ras signaling. Overexpression of K-Ras2A increased Na(+) transport approximately 2.2-fold in the absence of aldosterone. These results suggest that aldosterone signals to the luminal Na(+) channel via multiple pathways and that K-Ras2A levels are limiting for a portion of the aldosterone-sensitive Na(+) transport.

Vasopressin stimulates sodium transport in A6 cells via a phosphatidylinositide 3-kinase-dependent pathway

The enzyme phosphatidylinositide 3-kinase (PI3K) phosphorylates the D-3 position of the inositol ring of inositol phospholipids and produces 3-phosphorylated inositides. These novel second messengers are thought to mediate diverse cellular signaling functions. The fungal metabolite wortmannin covalently binds to PI3K and selectively inhibits its activity. The role of PI3K in basal and hormone-stimulated transepithelial sodium transport was examined using this specific inhibitor. Wortmannin, 50 nM, did not affect basal, aldosterone-stimulated, or insulin-stimulated transport in A6 cells. Wortmannin completely inhibits vasopressin stimulation of transport in these cells. Vasopressin stimulates PI3K activity in A6 cells. Vasopressin stimulation of transport is also blocked by 5 microM LY-294002, a second inhibitor of PI3K. One-hour preincubation with wortmannin blocked vasopressin stimulation of protein kinase A activity in the cells. Sodium transport responses to exogenous cAMP and forskolin, which directly activates adenylate cyclase, were not affected by wortmannin. These results indicate that wortmannin inhibits vasopressin stimulation of Na(+) transport at a site proximal to activation of adenylate cyclase. The results suggest that PI3K may be involved in receptor activation by vasopressin.

Perinatal PTX-sensitive G-protein expression and regulation of conductive 22Na+ transport in lung apical membrane vesicles

Using apical membrane vesicles (AMV) prepared from mature foetal and early neonatal guinea pig lung we show that pertussis toxin (PTX)-sensitive G-protein regulation of conductive 22Na+ uptake undergoes rapid changes following birth. Thus, G-protein activation by intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl, which in late gestation stimulated uptake, consistently induced inhibition of conductive Na+ uptake into AMV prepared from neonatal lung at 4 days of age (N4) (52+/-9%, n=8, P<0.05). This response was not significantly different in the presence of the relatively impermeant anion isethionate (Ise-) (69+/-9%, n=7, P<0.05). Changes in the regulation of uptake were already detectable on the day of birth (N0) in AMV resuspended in NaCl, with GTPgammaS inducing both stimulatory and inhibitory responses. These data indicate that the processes by which 22Na+ uptake into AMV is regulated by G-proteins undergoes a change at birth and by 4 days of age, G-protein regulation of uptake occurs predominantly via modulation of co-localised Na+ channels. Intravesicular incorporation of GDPbetaS or pre-treatment with PTX did not significantly alter conductive 22Na+ uptake in the presence of NaCl or NaIse suggesting that constitutively active G-proteins are not involved in this process. Pre-treatment of AMV with PTX prevented the inhibition of conductive 22Na+ uptake by GTPgammaS (105+/-16% n=7) indicating that a PTX-sensitive G-protein mediates the inhibition of channels in neonatal AMV. Western blotting demonstrated enrichment of Gialpha1, Gialpha2, Gialpha3 and Goalpha in the apical membrane preparations. We also show that there is a significant rise in the levels of Gialpha3 during the early neonatal period providing a potential candidate for the G-protein mediated changes in regulation of conductive 22Na+ uptake in neonatal AMV.

Omega-3 polyunsaturated fatty acids--modulation of voltage-dependent L-type Ca2+ current in guinea-pig tracheal smooth muscle cells

Omega-3 polyunsaturated fatty acids have been reported to be associated with favorable changes in the respiratory system. To determine one of the mechanisms for this effect, membrane currents were recorded in guinea-pig tracheal myocytes by using the whole-cell voltage clamp technique. Without EGTA in the patch pipette containing the Cs-internal solution, command voltage pulses positive to +0 mV from a holding potential of -60 mV elicited a voltage-dependent L-type Ca2+ current (I(Ca x L)) and a subsequent outward current. Upon repolarization, slowly decaying inward tail currents were recorded. The outward currents and the inward tail current were enhanced by methyl-1,4,-dihydro-2,6-dimethyl-3-nitro-4-(2-trigluromethylphenyl )-pyridine-5-carboxylate, and blocked by Cd2+ or nifedipine. Inclusion of EGTA (5 mM) in the patch pipette also abolished these currents, indicating that they were Ca2+-dependent. When [Cl-]o or [Cl-]i was changed, the reversal potential of these currents shifted, thus behaving like a Cl(-)-sensitive ion channel. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic acid. a Cl- channel blocker, inhibited the currents. The omega-3 polyunsaturated fatty acids eicosapentaenoic acid (3-30 microM) and docosahexaenoic acid (30 microM) suppressed I(Ca x L) and then inhibited I(Ca x Cl) in a reversible manner. Similar inhibitory effects of eicosapentaenoic acid on I(Ca x L) were observed with 5 mM EGTA in the patch pipette. Neurokinin A (1 microM) and caffeine (10 mM) also transiently activated I(Cl x Ca), probably due to Ca2+ release from Ca2+ storage sites. Pretreatment of the cells with eicosapentaenoic acid markedly suppressed the activation of I(Cl x Ca) by neurokinin A or caffeine. These results suggest that omega-3 polyunsaturated fatty acids inhibit voltage-dependent L-type Ca2+ currents and also Ca2+-activated Cl- currents in tracheal smooth muscle cells from the guinea-pig, which may play a role in modulation of tracheal smooth muscle tone.

Differential regulation of Na+ and Cl- conductances by PTX-sensitive G proteins in fetal lung apical membrane vesicles

In apical membrane vesicles (AMV) prepared from late gestation fetal guinea pig lung we show that conductive 22Na+ uptake is modulated by at least two pathways involving pertussis toxin (PTX)-sensitive G proteins. Intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl caused a significant stimulation (P<0. 05) of conductive Na+ uptake in AMV to 150+/-10% (n=10) of control, whereas GDPbetaS reduced uptake to 65+/-9% (n=4) of control. This contrasting response to GTPgammaS and GDPbetaS is characteristic of a G protein mediated pathway. GTPgammaS induced a significantly smaller stimulation, 125+/-8% (n=5) of control, in the presence of the relatively impermeant anion isethionate (Ise-). Taken together, these data indicate modulation of both Na+ and Cl- channels in the apical membrane by co-localised G protein(s). Treatment with PTX stimulated conductive 22Na+ uptake to 171+/-20% (n=13) of control in AMV resuspended in NaCl, but did not have a significant effect, 94+/-19% of control, in the presence of NaIse indicating the existence of tonic activation of Cl- channels in these AMV under resting conditions. As the combined effects of PTX and GTPgammaS diminished uptake, we propose that the G protein(s) responsible for Na+ channel activation in response to GTPgammaS is PTX-sensitive and that additional PTX-insensitive G proteins might also modulate 22Na+ uptake in these AMV. The presence of Gialpha1, Gialpha2, Gialpha3 and Goalpha in this apical membrane preparation was confirmed by PTX catalysed [32P]ADP-dependent ribosylation and Western blotting. Incubation of AMV with 200 microM DTT caused an inhibition of conductive Na+ uptake in AMV resuspended in NaCl or NaIse to 66+/-8% (n=11) and 64+/-8% (n=6) of control respectively. Pre-treatment with DTT did not affect the ability of GTPgammaS to stimulate conductive Na+ uptake suggesting that the regulation of 22Na+ uptake in late gestation guinea pig fetal lung AMV is unlikely to involve an associated regulatory protein.

Changes in G protein expression account for impaired modulation of hepatic cAMP formation after BDL

The regulation of cAMP synthesis by hormones and bile acids is altered in isolated hamster hepatocytes 2 days after bile duct ligation (BDL) [Y. Matsuzaki, B. Bouscarel, M. Le, S. Ceryak, T. W. Gettys, J. Shoda, and H. Fromm. Am. J. Physiol. 273 (Gastrointest. Liver Physiol. 36): G164-G174, 1997]. Therefore, studies were undertaken to elucidate the mechanism(s) responsible for this impaired modulation of cAMP formation. Hepatocytes were isolated 48 h after either a sham operation or BDL. Both preparations were equally devoid of cholangiocyte contamination. Although the basal cAMP level was not affected after BDL, the ability of glucagon to maximally stimulate cAMP synthesis was decreased by approximately 40%. This decreased glucagon effect after BDL was not due to alteration of the total glucagon receptor expression. However, this effect was associated with a parallel 50% decreased expression of the small stimulatory G protein alpha-subunit (GsalphaS). The expression of either the large subunit (GsalphaL) or the common beta-subunit remained unchanged. The expression of Gialpha2 and Gialpha3 was also decreased by 25 and 46%, respectively, and was associated with the failure of ANG II to inhibit stimulated cAMP formation. Therefore, alterations of the expression of GsalphaS and Galphai are, at least in part, responsible for the attenuated hormonal regulation of cAMP synthesis. Because cAMP has been reported to stimulate both bile acid uptake and secretion, impairment of cAMP synthesis and bile acid uptake may represent an initial hepatocellular defense mechanism during cholestasis.

G-protein-mediated signaling in cholesterol-enriched arterial smooth muscle cells. 2. Role of protein kinase C-delta in the regulation of eicosanoid production

PGI2 generation by the vessel wall is an agonist for cyclic-AMP-dependent cholesteryl ester hydrolysis. The process of enhanced PGI2 synthesis is stimulated, in part, by G-protein-coupled receptor ligands. Cellular cholesterol enrichment has been hypothesized to alter G-protein-mediated PGI2 synthesis. In the studies reported herein, cells generated PGI2 in response to AlF4-, GTPgammaS, and ATP in a dose-dependent manner. G-protein agonists stimulated eicosanoid production principally by activating phospholipase A2, but not phospholipase C. This is in contrast to PDGF, which stimulated phospholipase A2 and PLCgamma activities. Galphai subunits mediate G-protein agonist-induced PGI2 synthesis, since ATP- and PDGF-induced PGI2 synthesis was inhibited by pertussis toxin. Although cholesterol enrichment reduced arachidonic acid- and PDGF-induced PGI2 synthesis, cholesterol enrichment enhanced PGI2 release in response to AlF4-, GTPgammaS, and ATP. The enhancement of PGI2 release in cholesterol-enriched cells was augmented by mevalonate, which inhibits the ability of cholesterol enrichment to reduce membrane-associated G-protein subunits. Since cholesterol enrichment inhibited PDGF and AlF4--induced MAP kinase activity [Pomerantz, K., Lander, H. M., Summers, B., Robishaw, J. D., Balcueva, E. A., & Hajjar, D. P. (1997) Biochemistry 36, 9523-9531] (the major mechanism by which phospholipase A2 is activated), these results suggest that cholesterol enrichment induces other alternative signaling pathways leading to phospholipase A2 activation. A PKC-dependent pathway is described herein that is involved in enhanced eicosanoid production in cholesterol-enriched cells. This conclusion is supported by two observations: (1) G-protein-linked PGI2 production is inhibited by calphostin, and (2) cholesterol enrichment augments the specific translocation of the delta-isoform of PKC from the cytosol to the plasma membrane following treatment of cells with phorbol ester. These data support the concept that, in cells possessing normal levels of cholesterol, MAP-kinase-dependent pathways mediate eicosanoid synthesis in response to G-protein activation; however, under conditions of high cellular cholesterol levels, augmented G-protein-linked eicosanoid production results from enhanced PKCdelta activity.

Substrate-dependent expression of Na+ transport and shunt conductance in A6 epithelia

A6 epithelia grown in tissue culture vary enormously in their baseline rates of Na+ transport due to differences in growth media, serum, and other unknown factors. To evaluate the effect(s) of substrates on expression of Na+ transport, we determined short-circuit currents, open-circuit voltages, and electrical resistances of mature confluent A6 epithelia grown on a variety of commercially available permeable supports. Because the cells, growth conditions, and all other factors were the same, differences in transport could be attributed alone to the substrate on which the cells were grown. Tissues were grown on both large- and small-diameter inserts of the same type with differing ratios of edge length to area so that the contribution of the edge and tight junction conductances to the combined shunt conductance of the inserts could be evaluated. Shunt and cellular conductances and the cellular Thévenin electromotive force were determined after aldosterone stimulation and amiloride inhibition of Na+ transport. Marked and extreme differences were observed not only for expression of Na+ transport (controls, 0.09-3.94 microA/cm2; aldosterone, 1.53-28.2 microA/cm2) due to changes of apical membrane conductance but also for the development of junctional conductances (3,250 to < infinity omega.cm2) and edge conductances (13,175 to < infinity omega.cm) among substrates.

Protein kinase A phosphorylation and G protein regulation of type II pneumocyte Na+ channels in lipid bilayers

Protein kinase A (PKA)- and G protein-mediated regulation of immunopurified adult rabbit alveolar epithelial type II (ATII) cell proteins that exhibit amiloride-sensitive Na+ channel activity was studied in planar lipid bilayers and freshly isolated ATII cells. Addition of the catalytic subunit of PKA + ATP increased single channel open probability from 0.42 +/- 0.05 to 0.82 +/- 0.07 in a voltage-independent manner, without affecting unitary conductance. This increase in open probability of the channels was mainly due to a decrease in the time spent by the channel in its closed state. The apparent inhibition constant for amiloride increased from 8.0 +/- 1.8 microM under control conditions to 15 +/- 3 microM after PKA-induced phosphorylation; that for ethylisopropylamiloride increased from 1.0 +/- 0.4 to 2.0 +/- 0.5 microM. Neither pertussis toxin (PTX) nor guanosine 5'-O-(3-thiotriphosphate) affected ATII Na+ channel activity in bilayers. Moreover, PTX failed to affect amiloride-inhibitable 22Na+ uptake in freshly isolated ATII cells. In vitro, ADP ribosylation induced by PTX revealed the presence of a specifically ribosylated band at 40-45 kDa in the total solubilized ATII cell protein fraction, but not in the immunopurified fraction. Moreover, the immunopurified channel was downregulated in response to guanosine 5'-O-(3-thiotriphosphate)-mediated activation of the exogenous G alpha(i-2), but not G(oA), G alpha(i-1), or G alpha(i-3), protein added to the channel. This effect occurred only in the presence of actin. These results suggest that amiloride-sensitive Na+ channels in adult alveolar epithelia regulated by PKA-mediated phosphorylation also retain the ability to be regulated by G alpha([i-2), but not G alpha([i-1) or G alpha(i-3), protein.

The amiloride-sensitive epithelial Na+ channel: binding sites and channel densities

The amiloride-sensitive Na+ channel found in many transporting epithelia plays a key role in regulating salt and water homeostasis. Both biochemical and biophysical approaches have been used to identify, characterize, and quantitate this important channel. Among biophysical methods, there is agreement as to the single-channel conductance and gating kinetics of the highly selective Na+ channel found in native epithelia. Amiloride and its analogs inhibit transport through the channel by binding to high-affinity ligand-binding sites. This characteristic of high-affinity binding has been used biochemically to quantitate channel densities and to isolate presumptive channel proteins. Although the goals of biophysical and biochemical experiments are the same in elucidating mechanisms underlying regulation of Na+ transport, our review highlights a major quantitative discrepancy between methods in estimation of channel densities involved in transport. Because the density of binding sites measured biochemically is three to four orders of magnitude in excess of channel densities measured biophysically, it is unlikely that high-affinity ligand binding can be used physiologically to quantitate channel densities and characterize the channel proteins.

Inhibitory effects of omega-3 polyunsaturated fatty acids on receptor-mediated non-selective cation currents in rat A7r5 vascular smooth muscle cells

1. The effects of omega-3 polyunsaturated fatty acids on receptor-mediated non-selective cation current (Icat) and K+ current were investigated in aortic smooth muscle cells from foetal rat aorta (A7r5 cells). The whole-cell voltage clamp technique was employed. 2. With a K(+)-containing solution, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA, 30 microM) produced an outward current at a holding potential of -40 mV. This response was inhibited by tetraethylammonium (20 mM) or Cs+ in the patch pipette solution, and the reversal potential of the EPA-induced current followed the K+ equilibrium potential in a near Nernstian manner. 3. Under conditions with a Cs(+)-containing pipette solution, both vasopressin and endothelin-1 (100 nM) induced a long-lasting inward current at a holding potential of -60 mV. The reversal potential of these agonist-induced currents was about +0 mV, and was not significantly altered by the replacement of the extracellular or intracellular Cl+ concentration, suggesting that the induced current was a cation-selective current (Icat). 4. La3+ and Cd2+ (1 mM) completely abolished these agonist-induced Icat, but nifedipine (10 microM) failed to inhibit it significantly. 5. omega-3 polyunsaturated fatty acids (3-100 microM), EPA, DHA and docosapentaenoic acids (DPA), inhibited the agonist-induced Icat in a concentration-dependent manner. The potency of the inhibitory effect was EPA > DHA > DPA, and the half maximal inhibitory concentration (IC50) of EPA was about 7 microM. 6. Arachidonic and linoleic acids (10, 30 microM) showed a smaller inhibitory effect compared to omega-3 fatty acids. Also, oleic and stearic acids (30 microM) did not show a significant inhibitory effect on Icat. 7. A similar inhibitory action of EPA was observed when Icat was activated by intracellularly applied GTP gamma S in the absence of agonists, suggesting that the site of action of omega-3 fatty acids is not located on the receptor. 8. These results demonstrate that omega-3 polyunsaturated fatty acids can activate a K+ current and also effectively inhibit receptor-mediated non-selective cation currents in rat A7r5 vascular smooth muscle cells. Thus, the data suggest that omega-3 fatty acids may play an important role in the regulation of vascular tone.

Renal epithelial protein (Apx) is an actin cytoskeleton-regulated Na+ channel

Apx, the amphibian protein associated with renal amiloride-sensitive Na+ channel activity and with properties consistent with the pore-forming 150-kDa subunit of an epithelial Na+ channel complex initially purified by Benos et al. (Benos, D. J., Saccomani, G., and Sariban-Sohraby, S.(1987) J. Biol. Chem. 262, 10613-10618), has previously failed to generate amiloride-sensitive Na+ currents (Staub, O., Verrey, F., Kleyman, T. R., Benos, D. J., Rossier, B. C., and Kraehenbuhl, J.-P.(1992) J. Cell Biol. 119, 1497-1506). Renal epithelial Na+ channel activity is tonically inhibited by endogenous actin filaments (Cantiello, H. F., Stow, J., Prat, A. G., and Ausiello, D. A.(1991) Am. J. Physiol. 261, C882-C888). Thus, Apx was expressed and its function examined in human melanoma cells with a defective actin-based cytoskeleton. Apx-transfection was associated with a 60-900% increase in amiloride-sensitive (Ki = 3 microM) Na+ currents. Single channel Na+ currents had a similar functional fingerprint to the vasopressin-sensitive, and actin-regulated epithelial Na+ channel of A6 cells, including a 6-7 pS single channel conductance and a perm-selectivity of Na+:K+ of 4:1. Na+ channel activity was either spontaneous, or induced by addition of actin or protein kinase A plus ATP to the bathing solution of excised inside-out patches. Therefore, Apx may be responsible for the ionic conductance involved in the vasopressin-activated Na+ reabsorption in the amphibian kidney.

Diversity and regulation of amiloride-sensitive Na+ channels

Amiloride-sensitive Na+ channels play a vital role in many important physiological processes such as delineation of the final urine composition, sensory transduction, and whole-body Na+ homeostasis. These channels display a wide range of biophysical properties, and are regulated by cAMP-mediated second messenger systems. The first of these channels has recently been cloned. This cloned amiloride-sensitive Na+ channel is termined ENaC (Epithelial Na+ Channel) and, in heterologous cellular expression systems, displays a single channel conductance of 4 to 7 pS, a high PNa/PK (> 10), a high amiloride sensitivity (Ki(amil) = 150 nM), and relatively long open and closed times. ENaC may form the core conduction element of many of these functionally diverse forms of Na+ channel. The kinetic and regulatory differences between these channels may be due, in large measure, to unique polypeptides that associate with the core element, forming a functional channel unit.

Blockade of swelling-induced chloride channels by phenol derivatives

1. In NIH3T3 fibroblasts, the chloride channel involved in regulatory volume decrease (RVD) was identified as ICln, a protein isolated from a cDNA library derived from Madin Darby canine Kidney (MDCK) cells. ICln expressed in Xenopus laevis oocytes gives rise to an outwardly rectifying chloride current, sensitive to the extracellular addition of nucleotides and the known chloride channel blockers, DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and NPPB (5-nitro-2-(3-phenylpropylamino)-benzoic acid). We set out to study whether substances structurally similar to NPPB are able to interfere with RVD. 2. RVD in NIH3T3 fibroblasts and MDCK cells is temperature-dependent. 3. RVD, the swelling-dependent chloride current and the depolarization seen after reducing extracellular osmolarity can be blocked by gossypol and NDGA (nordihydroguaiaretic acid), both structurally related to NPPB. 4. The cyclic AMP-dependent chloride current elicited in CaCo cells is less sensitive to the two substances tested while the calcium-activated chloride current in fibroblasts is insensitive. 5. The binding site for the two phenol derivatives onto ICln seems to be distinct but closely related to the nucleotide binding site identified as G x G x G, a glycine repeat located at the predicted outer mouth of the ICln channel protein.

Regulation of a sodium channel-associated G-protein by aldosterone

The action of aldosterone to increase apical membrane permeability in responsive epithelia is thought to be due to activation of sodium channels. This channel is regulated, in part, by G-proteins, but it is not known if this mechanism is regulated by aldosterone. We report that aldosterone stimulates the expression of the 41-kDa alphai3 subunit of the heterotrimeric GTP-binding proteins in A-6 cells. Both mRNA and the total amount of this protein are increased by aldosterone. The G-protein is palmitoylated in response to the steroid, and the newly synthesized subunit is found to co-localize with the sodium channel. Aldosterone stimulation of sodium transport is significantly inhibited by inhibition of palmitoylation. These results suggest that aldosterone regulates sodium channel activity in epithelia through stimulation of the expression and post-translational targeting of a channel regulatory G-protein subunit.

Lidocaine action and conformational changes in cytoskeletal protein network in human red blood cells

The mechanism of action of lidocaine, which is commonly used clinically as a local anesthetic, was studied in human red blood cells. The influx of [14C]lidocaine through the cell membrane induced reversible transformation of human red blood cells from discocytes to stomatocytes. This change in shape depended on the lidocaine concentration and required both ATP and carbonic anhydrase. The lidocaine-induced shape change occurred as a result of spectrin aggregation, which altered the intracellular environment of the human red blood cells, mediated by carbonic anhydrase and activation of vacuolar type H(+)-ATPase (V-ATPase). Lidocaine controlled the influx of 22Na into the human red blood cells in a concentration-dependent manner. When incubated in media containing 6-chloro-9-[(4-diethylamino)-1-methyl-butyl]amino-2-methoxyacridine (mepacrine), an inhibitor of Na+ channels, human red blood cells changed shape from discocytes to stomatocytes and the intracellular pH decreased. This phenomenon was very similar to the shape change induced by lidocaine. These results suggest that the mode of action of lidocaine is related to a conformational change in the cytoskeletal protein network.

Effects of phosphorylation on ion channel function

A fundamental property of ion channels is their ability to be modulated by intracellular second messenger systems acting via covalent modifications of the channel protein itself. One such important biochemical reaction is phosphorylation on serine, threonine, and tyrosine residues. Ion channels in the kidney are no exception. Moreover, many ion channels, including many amiloride-sensitive epithelial Na+ channels, are subject to modulation by a multiplicity of inputs. For example, renal Na+ channels are not gated by voltage in their unphosphorylated state. However, upon phosphorylation by PKA plus ATP, these channels become voltage-dependent as well as having their open probability increased. Phosphorylation by PKC inhibits channel activity regardless of whether the channel was previously phosphorylated by PKA. Likewise, Na+ channel ADP-ribosylation by PTX overrides the actions of cAMP-dependent phosphorylation. Consistent with this idea is the fact that the phosphorylation sites for PKA and PKC and the ADP-ribosylation sites occur on different polypeptides comprising the channel complex. Epithelial Na+ channel activity is also regulated by methylation, arachidonic acid metabolites, and by interactions with cytoskeletal components. An exciting new age in understanding renal Na+ channel function has begun. Canessa and collaborators [103, 104] and Lingueglia et al [105] have, for the first time, identified by expression cloning an amiloride-sensitive Na+ channel from rat distal colon. The messenger RNA encoding the subunits comprising this channel are expressed in the distal tubule and cortical collecting tubule of the kidney (Rossier, unpublished observations). In addition, our laboratory has successfully cloned a mammalian homologue of this same channel from bovine renal papillary collecting ducts [106].(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the actin cytoskeleton on epithelial Na+ channel regulation

The regulatory role of actin filament organization on epithelial Na+ channel activity is reviewed in this report. The actin cytoskeleton, consisting of actin filaments and associated actin-binding proteins, is essential to various cellular events including the maintenance of cell shape, the onset of cell motility, and the distribution and stability of integral membrane proteins. Functional interactions between the actin cytoskeleton and specific membrane transport proteins are, however, not as well understood. Recent studies from our laboratory have determined that dynamic changes in the actin cytoskeletal organization may represent a novel signaling mechanism in the regulation of ion transport in epithelia. This report summarizes work conducted in our laboratory leading to an understanding of the molecular steps associated with the regulatory role of the actin-based cytoskeleton on epithelial Na+ channel function. The basis of this interaction lies on the regulation by actin-binding proteins and adjacent structures, of actin filament organization which in turn, modulates ion channel activity. The scope of this interaction may extend to such relevant cellular events as the vasopressin response in the kidney.

Whole cell sodium conductance of principal cells freshly isolated from rat cortical collecting duct

Cortical collecting duct fragments were manually dissected from 6-wk-old Sprague-Dawley rats. The fragments were enzymatically digested (collagenase A) into single cells, washed, and resuspended in serum-free RPMI 1640. Individual cells were examined electrophysiologically using the whole cell patch-clamp technique. Two morphologically distinct cell types were present in the cell suspension. Small round cells that had a capacitance of 7 pF and larger oval cells with a capacitance of 29 pF were consistently observed. Whole cell electrophysiological examination revealed that the small round cells had virtually no plasma membrane ionic conductance, whereas both inward and outward currents were observed in the larger oval-type cells. Also, superfusion of 250 pM arginine vasopressin specifically increased the inward conductance of only the larger cells. The effect could be completely inhibited by 2 microM amiloride or 100 mumol of the Rp diastereomer of 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (a specific adenosine 3',5'-cyclic monophosphate inhibitor). These findings are consistent with the hypothesis that the larger cells are principal cells and the smaller cells are intercalated cells and directly demonstrate that an amiloride-sensitive whole cell conductance is readily observable in freshly isolated cortical collecting duct cells. Thus the whole cell configuration of the patch-clamp technique appears to be well suited for assessing cellular mechanisms that regulate the ionic conductances of cortical collecting duct cells.

G-protein regulation of ion channels

Even though the vast majority of ion channels are regulated by voltage, extracellular ligands, phosphorylation, intracellular ions, or a combination of these influences, probably only a handful of ion channels are regulated by direct interaction with activated G proteins. Although results from electrophysiological studies of some channels are consistent with the hypothesis of regulation via direct physical interactions with G proteins, strong biochemical evidence for such interactions is still lacking. In most cases, such evidence has been difficult to obtain because ion channels are present at very low abundances in cell membranes, or because the molecular identity of the channel is unknown. The recent cloning of members of the inwardly rectifying K+ channel and voltage-gated Ca2+ channel families should facilitate the rigorous study of the putative interactions between G proteins and ion channels.

Amiloride-sensitive and HCO3(-)-dependent ion transport activated by aldosterone and vasotocin in A6 cells

We studied the effects of aldosterone (Aldo) and arginine vasotocin (AVT) on ion transport of renal epithelial cell line (A6) by measuring short-circuit current (Isc). AVT induced a rapid, transient increase in Isc, followed by a decrease toward the baseline in cells untreated with Aldo. In cells treated with Aldo, Isc showed a biphasic response to AVT, i.e., both transient and sustained increases over 40 min after addition of AVT. The transient increase was composed only of amiloride-insensitive Isc regardless of Aldo treatment, whereas the sustained increase contained both amiloride-sensitive and amiloride-insensitive components. The main part of the amiloride-insensitive, sustained Isc depended on HCO3(-). In cells treated with Aldo for 1 day, removal of HCO3(-) in the bathing solution enhanced the amiloride-sensitive component and decreased the amiloride-insensitive one. These data suggest that 1) Aldo treatment is necessary for an AVT-induced sustained increase of Isc and 2) a HCO3(-)-dependent Isc mainly contributes to the sustained increase in amiloride-insensitive Isc.

Role of lipoxygenase metabolites in ischemic preconditioning

Preconditioning with brief intermittent periods of ischemia before a sustained period of ischemia has been shown to reduce infarct size and improve recovery of function in rat hearts. The mediators of this protective response are unknown in rats. We tested the hypothesis that a lipoxygenase metabolite might be involved in preconditioning, since lipoxygenase metabolites such as 12-hydroperoxyeicosatetraenoic acid have been shown to increase K+ channel activity and to decrease Ca2+ channel activity, which could have a protective effect on ischemic injury. In support of this hypothesis, we report that the lipoxygenase inhibitors nordihydroguaiaretic acid (NDGA, 5 mumol/L) and eicosatetraynoic acid (7 mumol/L) added just before and during preconditioning blocked the protective effects of preconditioning on recovery of function during reflow after 30 minutes of global ischemia. In addition, these lipoxygenase inhibitors partially blocked the ability of preconditioning to attenuate the rise in cytosolic free calcium during sustained ischemia. We also investigated the effects of preconditioning on eicosanoid metabolism by using high-performance liquid chromatography and found that 12-hydroxyeicosatetraenoic acid (12-HETE), the stable product of the lipoxygenase pathway, was made during the preconditioning protocol and that 12-HETE accumulation was blocked by NDGA. Thus, there is a correlation between functional recovery after ischemia and stimulation of the lipoxygenase pathway of arachidonic acid metabolism before the sustained period of ischemia; inhibition of the lipoxygenase pathway eliminates the protective effect of preconditioning on recovery of function after ischemia.

Antidiuretic hormone-responding nonselective cation channel in distal nephron epithelium (A6)

Arginine vasotocin (AVT, 70 mU/ml) added from the basolateral side transiently activated a nonselective cation (NSC) channel with a single-channel conductance of 28.5 pS and almost identical selectivity for Na+ and K+ in the apical membrane of distal nephron cells (A6) cultured on permeable supports for 10-12 days in media containing 10% fetal bovine serum without supplemental aldosterone. The open probability (Po) of the NSC channel at the apical resting membrane potential in cell-attached patches was approximately 0.09 and increased when the apical membrane depolarized. The Po of the NSC channel was decreased by a rise in cytosolic Ca2+ concentration within a range of 30 nM-1 microM but not affected by cytosolic pH within a range of 6-8. The channel was activated by the application of negative pressure (10-60 cmH2O) into the patch pipette. Gadolinium (2 microM), an inhibitor of stretch-activated channels, decreased the Po by 40%. This blocking action of gadolinium was more effective after the channel was activated by stretch, i.e., 2 microM gadolinium decreased the Po by 70% when a negative pressure (60 cmH2O) was applied into the patch pipette. Amiloride (10 and 100 microM) showed a blocking action on the channel only when the NSC channel was activated by stretch.

Cl- regulation of a Ca(2+)-activated nonselective cation channel in beta-agonist-treated fetal distal lung epithelium

Nonselective cation (NSC) channels have been identified in the apical membrane of fetal distal lung epithelium (FDLE). However, their physiological role in Na+ transport is uncertain. Because terbutaline, a beta 2-agonist, increases Na+ transport by FDLE, we studied its effect and selected signal transduction mechanisms on NSC channel activity. Using patch-clamp and single-cell imaging techniques, we found that terbutaline activated the NSC channel by 1) increasing its sensitivity to cytosolic Ca2+ concentration ([Ca2+]c) by 100- to 1,000-fold, 2) increasing [Ca2+]c from 35 nM to 3.3 microM, 3) producing a dependency of the NSC channel activity on the cytosolic Cl- concentration ([Cl-]c) at a physiological [Ca2+]c, and 4) inducing a reduction in the [Cl-]c from 45 to 25 mM, which directly activates the beta 2-treated NSC channel. These observations indicate that a beta 2-agonist physiologically activates an amiloride-blockable NSC channel in FDLE through an increase in its sensitivity to [Ca2+]c, resulting in the development of a [Cl-]c dependency at a physiological [Ca2+]c associated with both an increase in [Ca2+]c and a reduction in [Cl-]c. A development of the [Cl-]c dependency and a reduction in [Cl-]c act as a second messenger of the beta-agonist signal transduction pathway in this Na(+)-transporting epithelium.

Na+/H+ exchange and PLA2 activity act interdependently to mediate the rapid effects of 1 alpha, 25-dihydroxyvitamin D3

1 alpha, 25-Dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3) has been shown to rapidly increase cytosolic calcium in freshly isolated and cultured rat hepatocytes. The rise in cytosolic calcium is dependent on phospholipase A2 (PLA2) activation and cell alkalinization through the Na+/H+ antiport system. To further characterize the rapid effects of 1 alpha, 25-(OH)2D3, cultured hepatocytes were treated with inhibitors of PLA2 and the Na+/H+ antiport system. 1 alpha, 25-(OH)2D3 treatment caused a 31-66% increase in [32P]lysophosphatidylinositol (LPI) and a 0.04 increase in pH within 5 minutes. Inhibition of the Na+/H+ antiport system with amiloride or removal of extracellular sodium abolished the 1 alpha, 25-(OH)2D3 rise in LPI. Inhibition of PLA2 with bromophenacylbromide also blocked the 1 alpha, 25-(OH)2D3-induced rise in [32P]LPI and cytosolic alkalinization in response to 1 alpha, 25-(OH)2D3. The data indicate that 1 alpha, 25-(OH)2D3 rapidly increases the activity of PLA2 and the Na+/H+ antiport system. The production of LPI is dependent on PLA2 activation and cell alkalinization through the Na+/H+ antiport system. It appears that the two events are interdependent in hepatocytes.

Vasopressin and protein kinase A activate G protein-sensitive epithelial Na+ channels

To determine the molecular steps involved in the vasopressin-induced renal Na+ reabsorption, the patch-clamp technique was utilized to study the role of this hormone in the regulation of apical Na+ channels in renal epithelial A6 cells. Addition of arginine vasopressin (AVP) induced and/or enhanced Na+ channel activity within 5 min of addition under cell-attached conditions. The AVP-induced channel activity was a reflection of both an increase in the average apparent channel number (0.2-1.7) and the percent open time (2-56%). Addition of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, the adenosine 3',5'-cyclic monophosphate (cAMP) analogues, 8-(4-chlorophenylthio)-cAMP and 8-bromo-cAMP, or forskolin elicited a comparable effect to that of AVP. The induced channels had similar properties to Na+ channels previously reported, including a channel conductance of 9 pS, Na(+)-to-K+ selectivity of 3-5:1, and high amiloride sensitivity. The cAMP-dependent protein kinase A (PKA) in the presence of ATP induced and/or enhanced Na+ channel activity in excised inside-out patches with a change in average apparent channel number and percent open probability similar to those observed with either AVP or cAMP analogues in intact cells. Addition of activated pertussis toxin (100 ng/ml) completely blocked the AVP- or PKA-induced Na+ channel activity in excised inside-out patches, whereas incubation of intact cells with the toxin completely prevented the effect of both activators. The data indicate that AVP mediates its effect through a cAMP-dependent pathway involving PKA activation whose target is the G protein pathway that regulates apical epithelial Na+ channel activity.

Activation of epithelial Na+ channels by protein kinase A requires actin filaments

We have recently demonstrated a novel role for "short" actin filaments, a distinct species of polymerized actin different from either monomeric (G-actin) or long actin filaments (F-actin), in the activation of epithelial Na+ channels. In the present study, the role of actin in the activation of apical Na+ channels by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase A (PKA) was investigated by patch-clamp techniques in A6 epithelial cells. In excised inside-out patches, addition of deoxyribonuclease I, which prevents actin polymerization, inhibited Na+ channel activation mediated by PKA. Disruption of endogenous actin filament organization with cytochalasin D for at least 1 h prevented the PKA-mediated activation of Na+ channels but not activation following the addition of actin to the cytosolic side of the patch. To assess the role of PKA on actin filament organization, actin was used as a substrate for the specific phosphorylation by the PKA. Actin was phosphorylated by PKA with an equilibrium stoichiometry of 2:1 mol PO4-actin monomer. Actin was phosphorylated in its monomeric form, but only poorly once polymerized. Furthermore, phosphorylated actin reduced the rate of actin polymerization. Thus actin allowed to polymerize for at least 1 h in the presence of PKA and ATP to obtain phosphorylated actin filaments induced Na+ channel activity in excised inside-out patches, in contrast to actin polymerized either in the absence of PKA or in the presence of PKA plus a PKA inhibitor (nonphosphorylated actin filaments). This was also confirmed by using purified phosphorylated G-actin incubated in a polymerizing buffer for at least 1 h at 37 degrees C. These data suggest that the form of actin required for Na+ channel activation (i.e., "short" actin filaments) may be favored by the phosphorylation of G-actin and may thus mediate or facilitate the activation of Na+ channels by PKA.