Osmotic regulation of Na+ transport across A6 epithelium: interactions with prostaglandin E2 and cyclic AMP

Prostaglandin E2 stimulates the epithelial sodium channel (ENaC) in cultured mouse cortical collecting duct cells in an autocrine manner

Prostaglandin E₂ (PGE₂) is the most abundant prostanoid in the kidney, affecting a wide range of renal functions. Conflicting data have been reported regarding the effects of PGE₂ on tubular water and ion transport. The amiloride-sensitive epithelial sodium channel (ENaC) is rate limiting for transepithelial sodium transport in the aldosterone-sensitive distal nephron. The aim of the present study was to explore a potential role of PGE₂ in regulating ENaC in cortical collecting duct (CCD) cells. Short-circuit current (I_SC) measurements were performed using the murine mCCD_cl1 cell line known to express characteristic properties of CCD principal cells and to be responsive to physiological concentrations of aldosterone and vasopressin. PGE₂ stimulated amiloride-sensitive I_SC via basolateral prostaglandin E receptors type 4 (EP₄) with an EC₅₀ of ∼7.1 nM. The rapid stimulatory effect of PGE₂ on I_SC resembled that of vasopressin. A maximum response was reached within minutes, coinciding with an increased abundance of β-ENaC at the apical plasma membrane and elevated cytosolic cAMP levels. The effects of PGE₂ and vasopressin were nonadditive, indicating similar signaling cascades. Exposing mCCD_cl1 cells to aldosterone caused a much slower (∼2 h) increase of the amiloride-sensitive I_SC. Interestingly, the rapid effect of PGE₂ was preserved even after aldosterone stimulation. Furthermore, application of arachidonic acid also increased the amiloride-sensitive I_SC involving basolateral EP₄ receptors. Exposure to arachidonic acid resulted in elevated PGE₂ in the basolateral medium in a cyclooxygenase 1 (COX-1)–dependent manner. These data suggest that in the cortical collecting duct, locally produced and secreted PGE₂ can stimulate ENaC-mediated transepithelial sodium transport.

Effects of cytochrome P-450 metabolites of arachidonic acid on the epithelial sodium channel (ENaC)

Sodium reabsorption via the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron plays a central role in the regulation of body fluid volume. Previous studies have indicated that arachidonic acid (AA) and its metabolite 11,12-EET but not other regioisomers of EETs inhibit ENaC activity in the collecting duct. The goal of this study was to investigate the endogenous metabolism of AA in cultured mpkCCD(c14) principal cells and the effects of these metabolites on ENaC activity. Liquid chromatography/mass spectrometry analysis of the mpkCCD(c14) cells indicated that these cells produce prostaglandins, 8,9-EET, 11,12-EET, 14,15-EET, 5-HETE, 12/8-HETE, and 15-HETE, but not 20-HETE. Single-channel patch-clamp experiments revealed that 8,9-EET, 14,15-EET, and 11,12-EET all decrease ENaC activity. Neither 5-, 12-, nor 15-HETE had any effect on ENaC activity. Diclofenac and ibuprofen, inhibitors of cyclooxygenase, decreased transepithelial Na(+) transport in the mpkCCD(c14) cells. Inhibition of cytochrome P-450 (CYP450) with MS-PPOH activated ENaC-mediated sodium transport when cells were pretreated with AA and diclofenac. Coexpression of CYP2C8, but not CYP4A10, with ENaC in Chinese hamster ovary cells significantly decreased ENaC activity in whole-cell experiments, whereas 11,12-EET mimicked this effect. Thus both endogenously formed EETs and their exogenous application decrease ENaC activity. Downregulation of ENaC activity by overexpression of CYP2C8 was PKA dependent and was prevented by myristoylated PKI treatment. Biotinylation experiments and single-channel analysis revealed that long-term treatment with 11,12-EET and overexpression of CYP2C8 decreased the number of channels in the membrane. In contrast, the acute inhibitory effects are mediated by a decrease in the open probability of the ENaC. We conclude that 11,12-EET, 8,9-EET, and 14,15-EET are endogenously formed eicosanoids that modulate ENaC activity in the collecting duct.

Effects of lipids on ENaC activity in cultured mouse cortical collecting duct cells

Direct effects on epithelial Na+ channels (ENaC) activity by lipids, e.g., arachidonic acid (AA), eicosatetraynoic acid (ETYA), linoleic acid (LA), stearic acid (SA), hydroxyeicosatetraenoic acid (HETE), 11,12-epoxyeicosatrienoic acid (EET), (PGF2), and (PGE2), in cultured mouse cortical collecting duct (M1) cells were clarified by using single-channel recordings in this study. In a cell-attached recording, a bath application of 10 microM AA significantly reduced the ENaC open probability (NPo), whereas 10 microM ETYA or 5 microM LA only induced a slight inhibition. The inside-out recording as a standard protocol was thereafter performed to examine effects of these lipids on ENaC activity. Within 10 min after the formation of the inside-out configuration, the NPo of ENaC in cultured mouse cortical collecting duct (M1) cells remained relatively constant. Application of ETYA or LA or SA exhibited a similar inhibition on the channel NPo when applied to the extracellular side, suggesting that fatty acids could exert a nonspecific inhibition on ENaC activity. 11,12-EET, a metabolite of AA via the cytochrome P450 epoxygenase pathway, significantly inhibited the ENaC NPo, whereas 20-HETE, a metabolite of AA via the hydroxylase pathway, only caused a small inhibition of the ENaC NPo, to a similar degree as that seen with ETYA and LA. However, both PGE2 and PGF2alpha significantly enhanced the ENaC NPo. These results suggest that fatty acids exert a nonspecific effect on ENaC activity due to the interaction between the channel proximity and the lipid. The opposite effects of 11,12-EET and prostaglandin (PG) implicate different mechanisms in regulation of ENaC activity by activation of epoxygenase and cyclooxygenase.

Initial and extended inflammatory messages of the nongenomic signaling pathway of the TCDD-activated Ah receptor in U937 macrophages

Using 2,3,7,8-tetrachlorodibenzo(p)dioxin (TCDD) we have investigated the mechanisms through which the AhR elicits inflammation through the nongenomic pathway. This AhR signaling depends on the initial action of TCDD to rapidly increase the intracellular concentration of free Ca(2+), which subsequently activates cPLA2 and additional inflammatory markers (e.g. COX-2 mRNA expression) lasting up to 72h. Inhibition of cPLA2 activity resulted in attenuation of these inflammatory responses. We have hypothesized that specific protein kinases are responsible for further propagation of the initial transient nongenomic signaling into long-lasting cellular effects, and found protein kinase C (PKC) is activated at an early stage, followed by activation of cAMP-dependent protein kinase (PKA) at later stages. We clearly established in U937 macrophages cPLA2 activation is an essential initial step to activate the nongenomic inflammatory pathway of ligand-activated AhR. Furthermore, this pathway does not require the participation of ARNT, thus distinguishing itself from the classical genomic pathway.

Intracellular calcium plays a role as the second messenger of hypotonic stress in gene regulation of SGK1 and ENaC in renal epithelial A6 cells

In A6 cells, a renal cell line derived from Xenopus laevis, hypotonic stress stimulates the amiloride-sensitive Na(+) transport. Hypotonic action on Na(+) transport consists of two phases, a nongenomic early phase and a genomic delayed phase. Although it has been reported that, during the genomic phase, hypotonic stress stimulates transcription of Na(+) transport-related genes, such as serum- and glucocorticoid-inducible kinase 1 (SGK1) and subunits of the epithelial Na(+) channel (ENaC), increasing Na(+) transport, the mechanism remains unknown. We focused the present study on the role of intracellular Ca(2+) in hypotonicity-induced SGK1 and ENaC subunit transcription. Since hypotonic stress raises intracellular Ca(2+) concentration in A6 cells, we hypothesized that Ca(2+)-dependent signals participate in the genomic action. Using real-time quantitative RT-PCR and Western blot techniques and measuring short-circuit currents, we observed that 1) BAPTA-AM and W7 blunted the hypotonicity-induced expression of SGK1 mRNA and protein, 2) ionomycin dose dependently stimulated expression of SGK1 mRNA and protein under an isotonic condition and the time course of the stimulatory effect of ionomycin on SGK1 mRNA was remarkably similar to that of hypotonic action on SGK1 mRNA, 3) hypotonic stress stimulated transcription of three ENaC subunits in an intracellular Ca(2+)-dependent manner, and 4) BAPTA-AM retarded the delayed phase of hypotonic stress-induced Na(+) transport but had no effect on the early phase. These observations indicate for the first time that intracellular Ca(2+) plays a role as the second messenger in hypotonic stress-induced Na(+) transport by stimulating transcription of SGK1 and ENaC subunits.

Aldosterone-induced modification of osmoregulated ENaC trafficking

Aldosterone and osmotic stress are well known to regulate the epithelial Na(+) channel (ENaC)-mediated Na(+) transport in renal epithelial cells. However, we have no information on how aldosterone and osmotic stress interact on stimulation of ENaC-mediated Na(+) transport in renal epithelium. In the present report, we studied how application of aldosterone (1 microM for 1 day) modifies the action of hypotonic stress on the ENaC-mediated Na(+) transport in renal A6 epithelial cells by measuring the benzamil (a specific inhibitor for ENaC)-sensitive short-circuit current. The present study suggests that: (1) most ENaCs in cells without aldosterone treatment are translocated to Golgi apparatus, (2) major parts of aldosterone-generated ENaCs are located at the endoplasmic reticulum, (3) aldosterone diminishes the endocytosis rate of ENaCs from the apical membrane without any significant changes in the insertion rate of ENaCs into the apical membrane, and (4) application of sucrose after hypotonic stress stimulates the endocytosis of ENaCs, and elongates the functional life time of ENaCs by enhancing recycle of ENaCs into the endoplasmic reticulum in a retrograde manner.

Hypotonicity stimulates renal epithelial sodium transport by activating JNK via receptor tyrosine kinases

We previously reported that hypotonic stress stimulated transepithelial Na(+) transport via a pathway dependent on protein tyrosine kinase (PTK; Niisato N, Van Driessche W, Liu M, Marunaka Y. J Membr Biol 175: 63-77, 2000). However, it is still unknown what type of PTK mediates this stimulation. In the present study, we investigated the role of receptor tyrosine kinase (RTK) in the hypotonic stimulation of Na(+) transport. In renal epithelial A6 cells, we observed inhibitory effects of AG1478 [an inhibitor of the EGF receptor (EGFR)] and AG1296 [an inhibitor of the PDGF receptor (PDGFR)] on both the hypotonic stress-induced stimulation of Na(+) transport and the hypotonic stress-induced ligand-independent activation of EGFR. We further studied whether hypotonic stress activates members of the MAP kinase family, ERK1/2, p38 MAPK, and JNK/SAPK, via an RTK-dependent pathway. The present study indicates that hypotonic stress induced phosphorylation of ERK1/2 and JNK/SAPK, but not p38 MAPK, that the hypotonic stress-induced phosphorylation of ERK1/2 and JNK/SAPK was diminished by coapplication of AG1478 and AG1296, and that only JNK/SAPK was involved in the hypotonic stimulation of Na(+) transport. A further study using cyclohexamide (a protein synthesis inhibitor) suggests that both RTK and JNK/SAPK contributed to the protein synthesis-independent early phase in hypotonic stress-induced Na(+) transport, but not to the protein synthesis-dependent late phase. The present study also suggests involvement of phosphatidylinositol 3-kinase (PI3-kinase) in RTK-JNK/SAPK cascade-mediated Na(+) transport. These observations indicate that 1) hypotonic stress activates JNK/SAPK via RTKs in a ligand-independent pathway, 2) the RTK-JNK/SAPK cascade acts as a mediator of hypotonic stress for stimulation of Na(+) transport, and 3) PI3-kinase is involved in the RTK-JNK/SAPK cascade for the hypotonic stress-induced stimulation of Na(+) transport.

Feed forward cycle of hypotonic stress-induced ATP release, purinergic receptor activation, and growth stimulation of prostate cancer cells

ATP is released in many cell types upon mechanical strain, the physiological function of extracellular ATP is largely unknown, however. Here we report that ATP released upon hypotonic stress stimulated prostate cancer cell proliferation, activated purinergic receptors, increased intracellular [Ca(2+)](i), and initiated downstream signaling cascades that involved MAPKs ERK1/2 and p38 as well as phosphatidylinositol 3-kinase (PI3K). MAPK activation, the calcium response as well as induction of cell proliferation upon hypotonic stress were inhibited by preincubation with the ATP scavenger apyrase, indicating that hypotonic stress-induced signaling pathways are elicited by released ATP. Hypotonic stress increased prostaglandin E(2) (PGE(2)) synthesis. Consequently, ATP release was inhibited by antagonists of PI3K (LY294002 and wortmannin), phospholipase A(2) (methyl arachidonyl fluorophosphonate (MAFP)), cyclooxygenase-2 (COX-2) (indomethacin, etodolac, NS398) and 5,8,11,14-eicosatetraynoic acid (ETYA), which are involved in arachidonic acid metabolism. Furthermore, ATP release was abolished in the presence of the adenylate cyclase (AC) inhibitor MDL-12,330A, indicating regulation of ATP-release by cAMP. The hypotonic stress-induced ATP release was significantly blunted when the ATP-mediated signal transduction cascade was inhibited on different levels, i.e. purinergic receptors were blocked by suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), the Ca(2+) response was inhibited upon chelation of intracellular Ca(2+) by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), and ERK1,2 as well as p38 were inhibited by UO126 and SB203580, respectively. In summary our data demonstrate that hypotonic stress initiates a feed forward cycle of ATP release and purinergic receptor signaling resulting in proliferation of prostate cancer cells.

Extracellular Ca2+regulates the stimulation of Na+transport in A6 renal epithelia

We investigated the involvement of intracellular and extracellular Ca2+in the stimulation of Na+transport during hyposmotic treatment of A6 renal epithelia. A sudden osmotic decrease elicits a biphasic stimulation of Na+transport, recorded as increase in amiloride-sensitive short-circuit current ( Isc) from 3.4 ± 0.4 to 24.0 ± 1.3 μA/cm2( n = 6). Changes in intracellular Ca2+concentration ([Ca2+]i) were prevented by blocking basolateral Ca2+entry with Mg2+and emptying the intracellular Ca2+stores before the hyposmotic challenge. This treatment did not noticeably affect the hypotonicity-induced stimulation of Isc. However, the absence of extracellular Ca2+severely attenuated Na+transport stimulation by the hyposmotic shock, and Iscmerely increased from 2.2 ± 0.3 to 4.8 ± 0.7 μA/cm2. Interestingly, several agonists of the Ca2+-sensing receptor, Mg2+(2 mM), Gd3+(0.1 mM), neomycin (0.1 mM), and spermine (1 mM) were able to substitute for extracellular Ca2+. When added to the basolateral solution, these agents restored the stimulatory effect of the hyposmotic solutions on Iscin the absence of extracellular Ca2+to levels that were comparable to control conditions. None of the above-mentioned agonists induced a change in [Ca2+]i. Quinacrine, an inhibitor of PLA2, overruled the effect of the agonists on Na+transport. In conclusion, we suggest that a Ca2+-sensing receptor in A6 epithelia mediates the stimulation of Na+transport without the interference of changes in [Ca2+]i.

Prostaglandin E2 stimulates sodium reabsorption in MDCK C7 cells, a renal collecting duct principal cell model

We examined the direct epithelial effects of the major product of arachidonic acid metabolism in the kidney, prostaglandin E(2) (PGE(2)), on ion transport and signal transduction in the hormone-sensitive Madin-Darby canine kidney (MDCK) C7 subclone as a model of renal collecting duct principal cells. MDCK C7 cells were grown on microporous permeable filter supports and mounted in Ussing-type chambers. Reverse transcriptase (RT)-PCR and sequencing were used to determine E-prostanoid (EP) receptor expression. Basolateral and, about 14-fold less potent, apical addition of PGE(2) increased short-circuit current (I(sc)) in a concentration-dependent manner. This ion transport was biphasic with a rapid peak not detectable under chloride-free conditions. The remaining, stably elevated current was unaffected by furosemide, hydrochlorothiazide, ethylisopropanol amiloride, and 5-nitro-2-(3-phenyl-propyl-amino)benzoic acid (NPPB). In contrast, apical amiloride (10 microM) significantly decreased I(sc), indicating sodium reabsorption. The effect of PGE(2) was attenuated in the presence of vasopressin. Agonists acting by cAMP elevation like dibutyryl-cAMP and theophylline also induced an amiloride-sensitive ion transport with similar kinetics as PGE(2). Moreover, PGE(2) rapidly increased intracellular cAMP levels. RT-PCR demonstrated mRNA expression of the epithelial sodium channel (ENaC), and of the EP2 receptor in MDCK C7 cells. Accordingly, EP2 receptor agonist butaprost mimicked PGE(2) epithelial action. In conclusion, PGE(2) induces amiloride-sensitive sodium reabsorption in MDCK C7 monolayers. This ion transport is most likely mediated by EP2 receptor activation leading to increased intracellular cAMP levels. Therefore, PGE(2) might also contribute to Na(+) reabsorption in the mammalian collecting duct.

Prostaglandin E2 induces upregulation of Na+ transport across Xenopus lung epithelium

The apical mucus on pulmonary epithelia is not only critical for physiological functions such as gas exchange or inflammatory processes, but also contains surfactants and multiple molecules that mediate cellular responses. A tight control of transepithelial ion transport maintains viscosity of this layer and, e.g., the amiloride-sensitive sodium channels (ENaCs) in lung epithelia of vertebrates are the most important regulatory sites for transcellular sodium uptake. Dysfunction of this sodium transport results in reduced liquid absorption and causes massive problems with gas exchange. We used dissected lungs of Xenopus laevis in Ussing chambers to investigate the influence of prostaglandin E2 (PGE2) on the regulation of short-circuit current (ISC) and amiloride-sensitive sodium absorption (Iami). Apical application of PGE2 (1 microM) increased ISC by 38% and Iami by approximately 60%. In contrast, a different prostaglandin, PGI2, neither affected ISC nor Iami. Forskolin increased current to a similar magnitude and preincubation of the lung with an RP-isomer of cyclic AMP, an inhibitor of protein kinase A (PKA), abolished the effects of both PGE2 and forskolin. Transepithelial Na+ uptake was also upregulated by the prostaglandin receptor agonists misoprostol and sulprostone. The Iami in Xenopus oocytes that heterologously expressed ENaCs was not affected by PGE2.

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.

Use of hollow fiber membrane filtration for the removal of DMSO from platelet concentrates

It has been hypothesized that, in addition to freezing injury, some damage to platelets may result from the cell packing that occurs during removal of the cryoprotectant. This study examined DMSO removal by fluid exchange across hollow-fiber (HF) filters as an alternative to centrifugation. The DMSO solution with or without cell suspension was passed once through the filter. The optimum exchange during unloading of DMSO was determined by varying the flow rates in the external and internal compartments of the HF filter. Initially, buffered solutions of a 5% DMSO solution in the absence of platelets were pumped into the fibers and exchanged against PBS. The residual DMSO was determined by osmometry. The exchange of DMSO across the membrane was flow dependent and also influenced by the chemical nature of the HF fibers. No protocol using a reasonable rate flow through the fibers removed more than 95% of the DMSO in a single pass. The optimum protocol was achieved with polysynthane fibers with an internal flow rate of approximately 20 mi/min and an external flow rate of 100 ml/min. Subsequently, frozen/thawed platelet concentrates in DMSO were washed using centrifugation and compared to the HF filtration method. Platelet quality was assayed by flow cytometry, cell count, morphology and osmotic stress test. Both filtration and centrifugal washing techniques resulted in comparable morphological scores and numbers of discoid cells. When agents reducing platelet activation were added, platelet quality was improved after washing by either technique. The lower platelet osmotic response with HF filtration than with centrifugation while using activation inhibitors was attributed to the remaining amount of the inhibitors. All other parameters tested were similar. The expression of CD62P was equivalent with both techniques, and centrifugation did not activate platelets more than filtration contrary to what was originally anticipated. In conclusion, platelet quality was comparable after washing by either technique but hollow fiber filtration does remove cryoprotectant more rapidly than does centrifugation.

PGE(2) activation of apical membrane Cl(-) channels in A6 epithelia: impedance analysis

Measurements of transepithelial electrical impedance of continuously short-circuited A6 epithelia were made at audio frequencies (0.244 Hz to 10.45 kHz) to investigate the time course and extent to which prostaglandin E(2) (PGE(2)) modulates Cl(-) transport and apical membrane capacitance in this cell-cultured model epithelium. Apical and basolateral membrane resistances were determined by nonlinear curve-fitting of the impedance vectors at relatively low frequencies (<50 Hz) to equations (Păunescu, T. G., and S. I. Helman. 2001. Biophys. J. 81:838--851) where depressed Nyquist impedance semicircles were characteristic of the membrane impedances under control Na(+)-transporting and amiloride-inhibited conditions. In all tissues (control, amiloride-blocked, and amiloride-blocked and furosemide-pretreated), PGE(2) caused relatively small (< approximately 3 microA/cm(2)) and rapid (<60 s) maximal increase of chloride current due to activation of a rather large increase of apical membrane conductance that preceded significant activation of Na(+) transport through amiloride-sensitive epithelial Na(+) channels (ENaCs). Apical membrane capacitance was frequency-dependent with a Cole-Cole dielectric dispersion whose relaxation frequency was near 150 Hz. Analysis of the time-dependent changes of the complex frequency-dependent equivalent capacitance of the cells at frequencies >1.5 kHz revealed that the mean 9.8% increase of capacitance caused by PGE(2) was not correlated in time with activation of chloride conductance, but rather correlated with activation of apical membrane Na(+) transport.

cAmp activation of apical membrane Cl(-) channels: theoretical considerations for impedance analysis

Transepithelial electrical impedance analysis provides a sensitive method to evaluate the conductances and capacitances of apical and basolateral plasma membranes of epithelial cells. Impedance analysis is complicated, due not only to the anatomical arrangement of the cells and their paracellular shunt pathways, but also in particular to the existence of audio frequency-dependent capacitances or dispersions. In this paper we explore implications and consequences of anatomically related Maxwell-Wagner and Cole-Cole dielectric dispersions that impose limitations, approximations, and pitfalls of impedance analysis when tissues are studied under widely ranging spontaneous rates of transport, and in particular when apical membrane sodium and chloride channels are activated by adenosine 3',5'-cyclic monophosphate (cAMP) in A6 epithelia. We develop the thesis that capacitive relaxation processes of any origin lead not only to dependence on frequency of the impedance locus, but also to the appearance of depressed semicircles in Nyquist transepithelial impedance plots, regardless of the tightness or leakiness of the paracellular shunt pathways. Frequency dependence of capacitance precludes analysis of data in traditional ways, where capacitance is assumed constant, and is especially important when apical and/or basolateral membranes exhibit one or more dielectric dispersions.

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.

Blocking action of cytochalasin D on protein kinase A stimulation of a stretch-activated cation channel in renal epithelial A6 cells

We have shown that the apical membrane of renal epithelial A6 cells has a 29-pS stretch-activated nonselective cation (NSC) channel, which is activated by cytosolic cyclic AMP (cAMP) (J Gen Physiol 1997;110:327-36). In general, downstream signalings of cAMP are mediated through a cAMP-activated protein kinase (protein kinase A, PKA)-dependent pathway. Therefore, to study if the channel is activated by a PKA-dependent pathway, we applied a PKA catalytic subunit directly to the channel from the cytosolic surface in cytosol-free excised inside-out patches, using the single channel recording (patch clamp) technique. Application of PKA catalytic subunit with 2 mM ATP increased the open probability (P(o)) of the channel from 0.11 +/- 0.04 to 0.58 +/- 0.10 (mean +/- SD, N = 11, P < 0.001). The channel has a gating kinetics "C(L) <--> C(S) <--> O, " where C(L,) C(S,) and O are the long closed state, the short closed state, and the open state, respectively. PKA influenced the communication of the channel between C(L) and C(S) without affecting the communication between C(S) and O, leading the channel to only stay in C(S) and O. The PKA-induced increase in P(o) was attributable to the interruption of communication between C(L) and C(S) or to the reduction of time the channel stays in C(L.) Pretreatment with cytochalasin D (Cyt-D), an inhibitor of the polymerization of actin filaments, blocked the stimulatory effect of PKA on the channel. These observations suggest that phosphorylation of polymerized actin filaments regulates the gating kinetics of a stretch-activated channel in renal epithelium.