[Mechanosensitivity of gramicidin A channels in semispherical bilayer membranes at constant tension].

Mechanoelectrical transduction in gramicidin A channels was studied in macroscopic planar lipid bilayer membranes bulged at constant tension. We found a supralinear increase in the single channel activity, which was proportional to the square of membrane radius but could not be accounted for by the increase in membrane surface area or by recruitment of new channels. When being extrapolated to biological membranes, these observations may suggest that the activity of permeability of ion channels can be influenced simply by changing the shape of the membrane, with or without stretching.

Role of membrane curvature in mechanoelectrical transduction: ion carriers nonactin and valinomycin sense changes in integral bending energy.

We describe the phenomenon of mechanoelectrical transduction in macroscopic lipid bilayer membranes modified by two cation-selective ionophores, valinomycin and nonactin. We found that bulging these membranes, while maintaining the membrane tension constant, produced a marked supralinear increase in specific carrier-mediated conductance. Analyses of the mechanisms involved in mechanoelectrical transduction induced by the imposition of a hydrostatic pressure gradient or by an amphipathic compound chlorpromazine reveal similar changes in the charge carrier motility and carrier reaction rates at the interface(s). Furthermore, the relative change in membrane conductance was independent of membrane diameter, but was directly proportional to the square of membrane curvature, thus relating the observed phenomena to the bilayer bending energy. Extrapolated to biological membranes, these findings indicate that ion transport in cells can be influenced simply by changing shape of the membrane, without a change in membrane tension.

Actin modifies Ca2+ block of epithelial Na+ channels in planar lipid bilayers.

The mechanism by which the cytoskeletal protein actin affects the conductance of amiloride-sensitive epithelial sodium channels (ENaC) was studied in planar lipid bilayers. In the presence of monomeric actin, we found a decrease in the single-channel conductance of alpha-ENaC that did not occur when the internal [Ca2+]free was buffered to <10 nM. An analysis of single-channel kinetics demonstrated that Ca2+ induced the appearance of long-lived closed intervals separating bursts of channel activity, both in the presence and in the absence of actin. In the absence of actin, the duration of these bursts and the time spent by the channel in its open, but not in its short-lived closed state, were inversely proportional to [Ca2+]. This, together with a lengthening of the interburst intervals, translated into a dose-dependent decrease in the single-channel open probability. In contrast, a [Ca2+]-dependent decrease in alpha-ENaC conductance in the presence of actin was accompanied by lengthening of the burst intervals with no significant changes in the open or closed (both short- and long-lived) times. We conclude that Ca2+ acts as a "fast-to-intermediate" blocker when monomeric actin is present, producing a subsequent attenuation of the apparent unitary conductance of the channel.

Cystic fibrosis transmembrane conductance regulator facilitates ATP release by stimulating a separate ATP release channel for autocrine control of cell volume regulation.

These studies provide evidence that cystic fibrosis transmembrane conductance regulator (CFTR) potentiates and accelerates regulatory volume decrease (RVD) following hypotonic challenge by an autocrine mechanism involving ATP release and signaling. In wild-type CFTR-expressing cells, CFTR augments constitutive ATP release and enhances ATP release stimulated by hypotonic challenge. CFTR itself does not appear to conduct ATP. Instead, ATP is released by a separate channel, whose activity is potentiated by CFTR. Blockade of ATP release by ion channel blocking drugs, gadolinium chloride (Gd(3+)) and 4,4'-diisothiocyanatostilbene-2,2'disulfonic acid (DIDS), attenuated the effects of CFTR on acceleration and potentiation of RVD. These results support a key role for extracellular ATP and autocrine and paracrine purinergic signaling in the regulation of membrane ion permeability and suggest that CFTR potentiates ATP release by stimulating a separate ATP channel to strengthen autocrine control of cell volume regulation.

Gating of amiloride-sensitive Na(+) channels: subunit-subunit interactions and inhibition by the cystic fibrosis transmembrane conductance regulator.

In search of the structural basis for gating of amiloride-sensitive Na(+) channels, kinetic properties of single homo and heterooligomeric ENaCs formed by the subunits with individual truncated cytoplasmic domains were studied in a cell-free planar lipid bilayer reconstitution system. Our results identify the N-terminus of the alpha-subunit as a major determinant of kinetic behavior of both homooligomeric and heterooligomeric ENaCs, although the carboxy-terminal domains of beta- and gamma-ENaC subunits play important role(s) in modulation of the kinetics of heterooligomeric channels. We also found that the cystic fibrosis transmembrane conductance regulator (CFTR) inhibits amiloride-sensitive channels, at least in part, by modulating their gating. Comparison of these data suggests that the modulatory effects of the beta- and gamma-ENaC subunits, and of the CFTR, may involve the same, or closely related, mechanism(s); namely, "locking" the heterooligomeric channels in their closed state. These mechanisms, however, do not completely override the gating mechanism of the alpha-channel.

Intracellular H+ regulates the alpha-subunit of ENaC, the epithelial Na+ channel.

Protons regulate electrogenic sodium absorption in a variety of epithelia, including the cortical collecting duct, frog skin, and urinary bladder. Recently, three subunits (alpha, beta, gamma) coding for the epithelial sodium channel (ENaC) were cloned. However, it is not known whether pH regulates Na+ channels directly by interacting with one of the three ENaC subunits or indirectly by interacting with a regulatory protein. As a first step to identifying the molecular mechanisms of proton-mediated regulation of apical membrane Na+ permeability in epithelia, we examined the effect of pH on the biophysical properties of ENaC. To this end, we expressed various combinations of alpha-, beta-, and gamma-subunits of ENaC in Xenopus oocytes and studied ENaC currents by the two-electrode voltage-clamp and patch-clamp techniques. In addition, the effect of pH on the alpha-ENaC subunit was examined in planar lipid bilayers. We report that alpha,beta,gamma-ENaC currents were regulated by changes in intracellular pH (pHi) but not by changes in extracellular pH (pHo). Acidification reduced and alkalization increased channel activity by a voltage-independent mechanism. Moreover, a reduction of pHi reduced single-channel open probability, reduced single-channel open time, and increased single-channel closed time without altering single-channel conductance. Acidification of the cytoplasmic solution also inhibited alpha, beta-ENaC, alpha,gamma-ENaC, and alpha-ENaC currents. We conclude that pHi but not pHo regulates ENaC and that the alpha-ENaC subunit is regulated directly by pHi.

Peptide inhibition of ENaC.

Liddle's disease is an autosomal dominant form of human hypertension resulting from a basal activation of amiloride-sensitive Na+ channels (ENaC). This channel activation is produced by mutations in the beta- and/or gamma-carboxy-terminal cytoplasmic tails, in many cases causing a truncation of the last 45-76 amino acids. In this study, we tested two hypotheses; first, beta- and gamma-ENaC C-terminal truncation mutants (beta DeltaC and gamma DeltaC), in combination with the wild-type alpha-ENaC subunit, reproduce the Liddle's phenotype at the single channel level, i.e., an increase in open probability (Po), and second, these C-terminal regions of beta- and gamma-ENaC act as intrinsic blockers of this channel. Our results indicate that alpha beta DeltaC gamma DeltaC-rENaC, incorporated into planar lipid bilayers, has a significantly higher single channel Po compared to the wild-type channel (0.85 vs 0.60, respectively), and that 30-mer synthetic peptides corresponding to the C-terminal region of either beta- or gamma-ENaC block the basal-activated channel in a concentration-dependent fashion. Moreover, there was a synergy between the peptides for channel inhibition when added together. We conclude that the increase in macroscopic Na+ reabsorption that occurs in Liddle's disease is at least in part due to an increase in single channel Po and that the cytoplasmic tails of the beta- and gamma-ENaC subunits are important in the modulation of ENaC activity.

Subunit stoichiometry of a core conduction element in a cloned epithelial amiloride-sensitive Na+ channel.

The molecular composition of a core conduction element formed by the alpha-subunit of cloned epithelial Na+ channels (ENaC) was studied in planar lipid bilayers. Two pairs of in vitro translated proteins were employed in combinatorial experiments: 1) wild-type (WT) and an N-terminally truncated alphaDeltaN-rENaC that displays accelerated kinetics (tauo = 32 +/- 13 ms, tauc = 42 +/- 11 ms), as compared with the WT channel (tauc1 = 18 +/- 8 ms, tauc2 = 252 +/- 31 ms, and tauo = 157 +/- 43 ms); and 2) WT and an amiloride binding mutant, alphaDelta278-283-rENaC. The channels that formed in a alphaWT:alphaDeltaN mixture fell into two groups: one with tauo and tauc that corresponded to those exhibited by the alphaDeltaN-rENaC alone, and another with a double-exponentially distributed closed time and a single-exponentially distributed open time that corresponded to the alphaWT-rENaC alone. Five channel subtypes with distinct sensitivities to amiloride were found in a 1alphaWT:1alphaDelta278-283 protein mixture. Statistical analyses of the distributions of channel phenotypes observed for either set of the WT:mutant combinations suggest a tetrameric organization of alpha-subunits as a minimal model for the core conduction element in ENaCs.

Purification and reconstitution of an outwardly rectified Cl- channel from tracheal epithelia.

We reported the identification of three outwardly rectified Cl- channel (ORCC) candidate proteins (115, 85, and 52 kDa) from bovine tracheal epithelia. We have raised polyclonal antibodies against these isolated proteins. Incorporation into planar lipid bilayers of material partly purified from bovine tracheal apical membranes with one of these antibodies as a ligand (anti-p115) resulted in the incorporation of an ORCC identical in biophysical characteristics to one we previously described. We developed a new purification procedure to increase the yield and purity of this polypeptide. The purification scheme that gave the best results in terms of overall protein yield and purity was a combination of anion- and cation-exchange chromatography followed by immunopurification. By use of this purification scheme, 7 microg of the 115-kDa protein were purified from 20 mg of tracheal apical membrane proteins. Incorporation of this highly purified material into planar lipid bilayers revealed a DIDS-inhibitable channel with the following properties: linear conductance of 87 +/- 9 pS in symmetrical Cl- solutions, halide selectivity sequence of I- > Cl- > Br-, and lack of sensitivity to protein kinase A, Ca2+, or dithiothreitol. Using anti-Galphai antibodies to precipitate Galphai protein(s) from the partly purified preparations, we demonstrated that the loss of rectification of the ORCC was due to uncoupling of Galphai protein(s) from the ORCC protein and that the 115-kDa polypeptide is an ORCC.

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells.

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.

Cation permeability of a cloned rat epithelial amiloride-sensitive Na+ channel.

1. Conductance of heterotrimeric rat epithelial Na+ channels (alpha, beta, gamma-rENaCs) for Li+ and Na+ in planar lipid bilayers was a non-linear function of ion concentration, with a maximum of 30.4 +/- 2.9 pS and 18.5 +/- 1.9 pS at 1 M Li+ and Na+, respectively. 2. The alpha, beta, gamma-rENaC conductance measured in symmetrical mixtures of Na(+)-Li+ (1 M) exhibited an anomalous mole fraction dependence, with a minimum at 4:1 Li+ to Na+ molar ratio. 3. Permeability ratios PK/PNa and PLi/PNa of the channel calculated from the bionic reversal potentials were dependent on ion concentration: PK/PNa was 0.11 +/- 0.01, and PLi/PNa was 1.6 +/- 0.3 at 50 mM; PK/PNa was 0.04 +/- 0.01 and PLi/PNa was 2.5 +/- 0.4 at 3 M, but differed from the ratios of single-channel conductances in symmetrical Li+, Na+ or K+ solutions. The permeability sequence determined by either method was Li+ > Na+ > K+ >> Rb+ Cs+. 4. Predictions of a model featuring two binding sites and three energy barriers (2S3B), and allowing double occupancy, developed on the basis of single ion current-voltage relationships, are in agreement with the observed conductance maximum in single ion experiments, conductance minimum in the mole fraction experiments, non-linearity of the current-voltage curves in bionic experiments, and the concentration dependence of permeability ratios. 5. Computer simulations using the 2S3B model recreate the ion concentration dependencies of single-channel conductance observed for the immunopurified bovine renal amiloride-sensitive Na+ channel, and short-circuit current in frog skin, thus supporting the hypothesis that ENaCs form a core conduction unit of epithelial Na+ channels.

Identification of an amiloride binding domain within the alpha-subunit of the epithelial Na+ channel.

Limited information is available regarding domains within the epithelial Na+ channel (ENaC) which participate in amiloride binding. We previously utilized the anti-amiloride antibody (BA7.1) as a surrogate amiloride receptor to delineate amino acid residues that contact amiloride, and identified a putative amiloride binding domain WYRFHY (residues 278-283) within the extracellular domain of alpharENaC. Mutations were generated to examine the role of this sequence in amiloride binding. Functional analyses of wild type (wt) and mutant alpharENaCs were performed by cRNA expression in Xenopus oocytes and by reconstitution into planar lipid bilayers. Wild type alpharENaC was inhibited by amiloride with a Ki of 169 nM. Deletion of the entire WYRFHY tract (alpharENaC Delta278-283) resulted in a loss of sensitivity of the channel to submicromolar concentrations of amiloride (Ki = 26.5 microM). Similar results were obtained when either alpharENaC or alpharENaC Delta278-283 were co-expressed with wt beta- and gammarENaC (Ki values of 155 nM and 22.8 microM, respectively). Moreover, alpharENaC H282D was insensitive to submicromolar concentrations of amiloride (Ki = 6.52 microM), whereas alpharENaC H282R was inhibited by amiloride with a Ki of 29 nM. These mutations do not alter ENaC Na+:K+ selectivity nor single-channel conductance. These data suggest that residues within the tract WYRFHY participate in amiloride binding. Our results, in conjunction with recent studies demonstrating that mutations within the membrane-spanning domains of alpharENaC and mutations preceding the second membrane-spanning domains of alpha-, beta-, and gammarENaC alters amiloride's Ki, suggest that selected regions of the extracellular loop of alpharENaC may be in close proximity to residues within the channel pore.

Streaming potential measurements in alphabetagamma-rat epithelial Na+ channel in planar lipid bilayers.

Streaming potentials across cloned epithelial Na+ channels (ENaC) incorporated into planar lipid bilayers were measured. We found that the establishment of an osmotic pressure gradient (Deltapi) across a channel-containing membrane mimicked the activation effects of a hydrostatic pressure differential (DeltaP) on alphabetagamma-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Deltapi negates channel activation by DeltaP when the Deltapi was directed against DeltaP. A streaming potential of 2.0 +/- 0.7 mV was measured across alphabetagamma-rat ENaC (rENaC)-containing bilayers at 100 mM symmetrical [Na+] in the presence of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that between two and three water molecules are translocated together with a single Na+ ion. A minimal effective pore diameter of 3 A that could accommodate two water molecules even in single file is in contrast with the 2-A diameter predicted from the selectivity properties of alphabetagamma-rENaC. The fact that activation of alphabetagamma-rENaC by DeltaP can be reproduced by the imposition of Deltapi suggests that water movement through the channel is also an important determinant of channel activity.

Point mutations in alpha bENaC regulate channel gating, ion selectivity, and sensitivity to amiloride.

We have generated two site-directed mutants, K504E and K515E, in the alpha subunit of an amiloride-sensitive bovine epithelial Na+ channel, alpha bENaC. The region in which these mutations lie is in the large extracellular loop immediately before the second membrane-spanning domain (M2) of the protein. We have found that when membrane vesicles prepared from Xenopus oocytes expressing either K504E or K515E alpha bENaC are incorporated into planar lipid bilayers, the gating pattern, cation selectivity, and amiloride sensitivity of the resultant channel are all altered as compared to the wild-type protein. The mutated channels exhibit either a reduction or a complete lack of its characteristic burst-type behavior, significantly reduced Na+:K+ selectivity, and an approximately 10-fold decrease in the apparent inhibitory equilibrium dissociation constant (Ki) for amiloride. Single-channel conductance for Na+ was not affected by either mutation. On the other hand, both K504E and K515E alpha bENaC mutants were significantly more permeable to K+, as compared to wild type. These observations identify a lysine-rich region between amino acid residues 495 and 516 of alpha bENaC as being important to the regulation of fundamental channel properties.

Role of actin in regulation of epithelial sodium channels by CFTR.

Cystic fibrosis (CF) airway epithelia exhibit enhanced Na+ reabsorption in parallel with diminished Cl- secretion. We tested the hypothesis that actin plays a role in the regulation of a cloned epithelial Na+ channel (ENaC) by the cystic fibrosis transmembrane conductance regulator (CFTR). We found that immunopurified bovine tracheal CFTR coreconstituted into a planar lipid bilayer with alpha,beta,gamma-rat ENaC (rENaC) decreased single-channel open probability (Po) of rENaC in the presence of actin by over 60%, a significantly greater effect than was observed in the absence of actin (approximately 20%). In the presence of actin, protein kinase A plus ATP activated both CFTR and rENaC, but CFTR was activated in a sustained manner, whereas the activation of rENaC was transitory. ATP alone could also activate ENaC transiently in the presence ofactin but had no effect on CFTR. Stabilizing short actin filaments at a fixed length with gelsolin (at a ratio to actin of 2:1) produced a sustained activation of alpha,beta,gamma-rENaC in both the presence or absence of CFTR. Gelsolin alone (i.e., in the absence of actin) had no effect on the conductance or Po of either CFTR or rENaC. We have also found that short actin filaments produced their modulatory action on alpha-rENaC independent of the presence of the beta- or gamma-rENaC subunits. In contrast, CFTR did not affect any properties of the channel formed by alpha-rENaC alone, i.e., in the absence of beta- or gamma-rENaC. These results indicate that CFTR can directly downregulate single Na+ channel activity, which may account for the observed differences between Na+ transport in normal and CF-affected airway epithelia. Moreover, the presence of actin confers an enhanced modulatory ability of CFTR on Na+ channels.

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.

Mechanosensitivity of an epithelial Na+ channel in planar lipid bilayers: release from Ca2+ block.

A family of novel epithelial Na+ channels (ENaCs) have recently been cloned from several different tissues. Three homologous subunits (alpha, beta, gamma-ENaCs) from the core conductive unit of Na(+)-selective, amiloride-sensitive channels that are found in epithelia. We here report the results of a study assessing the regulation of alpha,beta,gamma-rENaC by Ca2+ in planar lipid bilayers. Buffering of the bilayer bathing solutions to [Ca2+] < 1 nM increased single-channel open probability by fivefold. Further investigation of this phenomenon revealed that Ca2+ ions produced a voltage-dependent block, affecting open probability but not the unitary conductance of ENaC. Imposing a hydrostatic pressure gradient across bilayers containing alpha,beta,gamma-rENaC markedly reduced the sensitivity of these channels to inhibition by [Ca2+]. Conversely, in the nominal absence of Ca2+, the channels lost their sensitivity to mechanical stimulation. These results suggest that the previously observed mechanical activation of ENaCs reflects a release of the channels from block by Ca2+.

Kinetic interconversion of rat and bovine homologs of the alpha subunit of an amiloride-sensitive Na+ channel by C-terminal truncation of the bovine subunit.

We have recently cloned the alpha subunit of a bovine amiloride-sensitive Na+ channel (alphabENaC). This subunit shares extensive homology with both rat and human alphaENaC subunits but shows marked divergence at the C terminus beginning at amino acid 584 of the 697-residue sequence. When incorporated into planar lipid bilayers, alphabENaC almost exclusively exhibits a main transition to 39 picosiemens (pS) with very rare 13 pS step transitions to one of two subconductance states (26 and 13 pS). In contrast, the alpha subunit of the rat renal homolog of ENaC (alpharENaC) has a main transition step to 13 pS that is almost constituitively open, with a second stepwise transition of 26 to 39 pS. A deletion mutant of alphabENaC, encompassing the entire C-terminal region (R567X), converts the kinetic behavior of alphabENaC to that of alpharENaC, i. e. a transition to 13 pS followed by a second 26 pS transition to 39 pS. Chemical cross-linking of R567X restores the wild-type alphabENaC gating pattern, whereas treatment with the reducing agent dithiothreitol produced only 13 pS transitions. In contrast, an equivalent C-terminal truncation of alpharENaC (R613X) had no effect on the gating pattern of alpharENaC. These results are consistent with the hypothesis that interactions between the C termini of alphabENaC account for the different kinetic behavior of this member of the ENaC family of Na+ channels.

A biologic function for an "orphan" messenger: D-myo-inositol 3,4,5,6-tetrakisphosphate selectively blocks epithelial calcium-activated chloride channels.

Inositol phosphates are a family of water-soluble intracellular signaling molecules derived from membrane inositol phospholipids. They undergo a variety of complex interconversion pathways, and their levels are dynamically regulated within the cytosol in response to a variety of agonists. Relatively little is known about the biological function of most members of this family, with the exception of inositol 1,4,5-trisphosphate. Specifically, the biological functions of inositol tetrakisphosphates are largely obscure. In this paper, we report that D-myo-inositol 3,4,5,6-tetrakisphosphate (D-Ins(3,4,5,6)P4) has a direct biphasic (activation/inhibition) effect on an epithelial Ca(2+)-activated chloride channel. The effect of D-Ins(3,4,5,6)P4 is not mimicked by other inositol tetrakisphosphate isomers, is dependent on the prevailing calcium concentration, and is influenced when channels are phosphorylated by calmodulin kinase II. The predominant effect of D-Ins(3,4,5,6)P4 on phosphorylated channels is inhibitory at levels of intracellular calcium observed in stimulated cells. Our findings indicate the biological function of a molecule hitherto considered as an "orphan" messenger. They suggest that the molecular target for D-Ins(3,4,5,6)P4 is a plasma membrane Ca(2+)-activated chloride channel. Regulation of this channel by D-Ins(3,4,5,6)P4 and Ca2+ may have therapeutic implications for the disease states of both diabetic nephropathy and cystic fibrosis.

Protein kinase regulation of a cloned epithelial Na+ channel.

We examined the regulation of a cloned epithelial Na+ channel (alpha beta gamma-rENaC) by protein kinase A (PKA) and protein kinase C (PKC). Experiments were performed in Xenopus oocytes and in planar lipid bilayers. At a holding potential of -100 mV, amiloride-sensitive current averaged -1,279 +/- 111 nA (n = 7) in alpha beta gamma-rENaC-expressing oocytes. Currents in water-injected oocytes were essentially unresponsive to 10 microM amiloride. A 1-h stimulation of PKC with 100 nM of PMA inhibited whole-cell currents in Xenopus oocytes to 17.1 +/- 1.8, and 22.1 +/- 2.6% of control (n = 7), at holding potentials of -100 and +40 mV, respectively. Direct injection of purified PKC resulted in similar inhibition to that observed with PMA. Additionally, the inactive phorbol ester, phorbol-12-myristate-13-acetate, 4-O-methyl, was without effect on alpha beta gamma-rENaC currents. Pretreatment with the microtubule inhibitor colchicine (100 microM) did not modify the inhibitory effect of PMA; however, pretreatment with 20 microM cytochalasin B decreased the inhibitory action of PMA to < 20% of that previously observed. In vitro-synthesized alpha beta gamma-rENaC formed an amiloride-sensitive Na(+)-selective channel when incorporated into planar lipid bilayers. Addition of PKC, diacyl-glycerol, and Mg-ATP to the side opposite that which amiloride blocked, decreased the channel's open probability (Po) from 0.44 +/- 0.06 to 0.13 +/- 0.03 (n = 9). To study the effects of PKA on alpha beta gamma-rENaC expressed in Xenopus oocytes, cAMP levels were elevated with 10 microM forskolin and 1 mM isobutyl-methyl-xanthine. This cAMP-elevating cocktail did not cause any stimulation of alpha beta gamma-rENaC currents in either the inward or outward directions. This lack of activation was also observed in oocytes preinhibited with PMA and in oocytes pretreated with cytochalasin B and PMA. Neither alpha-rENaC nor alpha beta gamma-rENaC incorporated into planar lipid bilayers could be activated with PKA and Mg-ATP added to either side of the membrane, as Po remained at 0.63 +/- 0.06 (n = 7) and 0.45 +/- 0.05 (n = 9), respectively. We conclude that: alpha beta gamma-rENaC is inhibited by PKC, and that alpha beta gamma-rENaC is not activated by PKA.