Cl- channels in basolateral renal medullary membrane vesicles: IV. Analogous channel activation by Cl- or cAMP-dependent protein kinase

Chloride transport

Chloride transport along the nephron is one of the key actions of the kidney that regulates extracellular volume and blood pressure. To maintain steady state, the kidney needs to reabsorb the vast majority of the filtered load of chloride. This is accomplished by the integrated function of sequential chloride transport activities along the nephron. The detailed mechanisms of transport in each segment generate unique patterns of interactions between chloride and numerous other individual components that are transported by the kidney. Consequently, chloride transport is inextricably intertwined with that of sodium, potassium, protons, calcium, and water. These interactions not only allow for exquisitely precise regulation but also determine the particular patterns in which the system can fail in disease states.

Function of chloride channels in the kidney

Numerous Cl- channels have been identified in the kidney using physiological approaches and thus are thought to be involved in a range of physiological processes, including vectorial transepithelial Cl- transport, cell volume regulation, and vesicular acidification. In addition, expression of genes from several Cl- channel gene families has also been observed. However, the molecular characteristics of a number of Cl- channels within the kidney are still unknown, and the physiological roles of Cl- channels identified by molecular means remain to be determined. A gene knockout approach using mice might shed further light on the characteristics of these various Cl- channels. In addition, study of diseases involving Cl- channels (channelopathies) might clarify the physiological role of specific Cl- channels. To date, more is known about CLC Cl- channels than any other Cl- channels within the kidney. This review focuses on the physiological roles of CLC Cl- channels within the kidney, particularly kidney-specific ClC-K Cl- channels, as well as the recently identified maxi anion channel in macula densa, which is involved in tubulo-glomerular feedback.

Chloride channels in basolateral TAL membranes. XVIII. Phenylglyoxal induces functional mcCIC-Ka activity in basolateral MTAL membranes

Cultured mouse MTAL cells contain more mRNA encoding the Cl- channel mcCIC-Ka, which mediates CTAL Cl- absorption, than mRNA encoding the Cl- channel mmCIC-Ka, which mediates MTAL Cl- absorption. mmCIC-Ka and mcCIC-Ka have three functional differences: 1) mmCIC-Ka open time probability, Po, increases with increasing cytosolic Cl-, but variations in cytosolic Cl- do not affect Po in mcCIC-Ka; 2) mmCIC-Ka is gated by (ATP + PKA), while (ATP + PKA) have no effect on Po in mcCIC-Ka; and 3) mmCIC-Ka channels have single-ion occupancy, while mcCIC-Ka channels have multi-ion occupancy. Using basolateral vesicles from MTAL cells fused into bilayers, we evaluated the effects of 1 mM cytosolic phenylglyoxal (PGO), which binds covalently to lysine or arginine, on Cl- channels. With PGO pretreatment, Cl- channels were uniformly not gated either with increases in cytosolic-face Cl- or with (ATP + PKA) at 2 mM cytosolic-face Cl-; and they exhibited multi-ion occupancy kinetics typical for mcCIC-Ka channels. Thus, in basolateral MTAL membranes, blockade of Cl- access to arginine or lysine residues on mmCIC-Ka by PGO results in Cl- channels having the functional characteristics of mcCIC-Ka channels.

Cl- channels in basolateral TAL membranes. XIX. Cytosolic Cl- regulates mmCIC-Ka and mcCIC-Ka channels

We evaluated the effects of culturing mouse MTAL cells under conditions that suppressed steady-state cytosolic Cl- on chloride channels fused into bilayers from basolateral vesicles of cultured MTAL cells. We used two agents to suppress Cl- entry: 10(-6) M PGE2 and 10(-4) M bumetanide. Basolateral Cl- channels from control cultured MTAL cells exhibited the signature characteristics of mmCIC-Ka channels: increased open-time probability (Po) either by raising cytosolic-face [Cl-] or, at 2 mM cytosolic Cl-, by adding (ATP + PKA), and first-order conductance kinetics. Either 10(-6) M PGE2 or 10(-4) M bumetanide in culture media reduced steady-state MTAL cytosolic Cl-. Chloride channels from these cells exhibited characteristics unique to CTAL mcCIC-Ka channels, namely: no augmentation of Po either by raising cytosolic Cl- or with cytosolic (ATP + PKA), and multi-ion occupancy. Semi-quantitative RT-PCR and real-time quantitative PCR showed that culturing MTAL cells with 10(-6) M PGE2 or 10(-4) M bumetanide reduced mRNA levels encoding mmCIC-Ka but not mRNA levels encoding mcCIC-Ka. However, when MTAL cells were cultured under control conditions, and then pre-incubated for 60 minutes with 10(-4) M bumetanide, cytosolic Cl- fell acutely but Cl- channels exhibited characteristics of mmCIC-Ka channels. Thus PGE2 and bumetanide, both of which lower steady-state MTAL cytosolic Cl- concentrations, inhibit either the transcriptional and/or the translational processes for mmCIC-Ka synthesis.

Cl- channels in basolateral TAL membranes. XVI. MTAL and CTAL cells each contain the mRNAs encoding mmClC-Ka and mcClC-Ka

BACKGROUND

Our prior data indicate that two separate but homologous basolateral chloride (Cl-) channels, mmClC-Ka and mcClC-Ka, are the principal mediators of net Cl- absorption in mouse medullary thick ascending limb (MTAL) and cortical thick ascending limb (CTAL) cells, respectively. In the present studies, we evaluated the possibility that there might be translational or post-translational suppression of mmClC-Ka and mcClC-Ka activity in CTAL and MTAL cells, respectively.

METHODS

Polymerase chain reaction (PCR) fragments were prepared that were highly specific for either mmClC-Ka or mcClC-Ka, the cDNAs encoding mmClC-Ka and mcClC-Ka, respectively.

RESULTS

Using reverse transcription (RT)-PCR with these highly specific products, mRNAs specific for non-homologous channel sequences in either mmClC-Ka or mcClC-Ka were present in both MTAL and CTAL cells.

CONCLUSIONS

Both mouse MTAL and CTAL cells contain the mRNAs encoding mmClC-Ka and mcClC-Ka. There may be translational or post-translational suppression of mmClC-Ka activity in CTAL cells, and of mcClC-Ka activity in MTAL cells.

Chloride channels in the loop of Henle

Cl- transport in the loop of Henle is responsible for reclamation of 25-40% of the filtered NaCl load and for the formation of dilute urine. Our understanding of the physiologic and molecular mechanisms responsible for Cl- reabsorption in both the thin ascending limb and thick ascending limb of Henle's loop has increased greatly over the last decade. Plasma membrane Cl- channels are known to play an integral role in transcellular Cl- transport in both the thin and thick ascending limbs. This review focuses on the functional characteristics and molecular identities of these Cl- channels, as well as the role of these channels in the pathophysiology of disease.

Localization of rat CLC-K2 chloride channel mRNA in the kidney

To gain insight into the physiological role of a kidney-specific chloride channel, CLC-K2, the exact intrarenal localization was determined by in situ hybridization. In contrast to the inner medullary localization of CLC-K1, the signal of CLC-K2 in our in situ hybridization study was highly evident in the superficial cortex, moderate in the outer medulla, and absent in the inner medulla. To identify the nephron segments where CLC-K2 mRNA was expressed, we performed in situ hybridization of CLC-K2 and immunohistochemistry of marker proteins (Na+/Ca2+ exchanger, Na+-Cl- cotransporter, aquaporin-2 water channel, and Tamm-Horsfall glycoprotein) in sequential sections of a rat kidney. Among the tubules of the superficial cortex, CLC-K2 mRNA was highly expressed in the distal convoluted tubules, connecting tubules, and cortical collecting ducts. The expression of CLC-K2 in the outer and inner medullary collecting ducts was almost absent. In contrast, a moderate signal of CLC-K2 mRNA was observed in the medullary thick ascending limb of Henle's loop, but the signal in the cortical thick ascending limb of Henle's loop was low. These results clearly demonstrated that CLC-K2 was not colocalized with CLC-K1 and that its localization along the nephron segments was relatively broad compared with that of CLC-K1.

Cl- channels in basolateral TAL membranes. XIV. Kinetic properties of a basolateral MTAL Cl- channel

BACKGROUND

This article reports studies on the kinetics of chloride (Cl-) conductance in Cl- channels fused into bilayers from basolaterally enriched vesicles from rabbit outer medulla. A considerable body of evidence indicates that these channels represent rbClC-Ka, a 77 kDa kidney-specific protein of the ClC family of Cl- channels. rbClC-Ka, a candidate channel for mediating net Cl- absorption in the medullary thick ascending limb (MTAL), has been cloned from rabbit outer medulla and localized by immunofluorescence to basolateral membranes of the MTAL. Thus, this is the first account, to our knowledge, of the kinetics of ion permeation through a renal Cl- channel mediating net basolateral Cl- absorption in the thick ascending limb of Henle (TALH), and this channel may represent rbClC-Ka.

METHODS

The electrophysiological properties of these channels were studied by fusing basolaterally enriched MTAL vesicles into planar bilayer membranes.

RESULTS

Cl- conductance through these channels was concentration dependent and saturable. The relationship between gCl (pS) and symmetrical aqueous Cl- concentrations could be expressed in terms of the Michaelis equation with a limiting conductance (GClmax, pS) of 114 pS at infinitely high aqueous Cl- concentrations and a K1/2 of 163 mM Cl-. A log-log plot of the conductance-Cl- concentration relations, in the nonsaturating Cl- concentration range, had a slope of 0.91, that is, virtually unity. The relatively impermeant anion I- produced a voltage-dependent conductance blockade that could be overcome at high electric field strengths.

CONCLUSIONS

The experimental data described earlier here fulfill the traditional criteria for a first-order process with a single Cl- ion occupying these channels at a given time. Although the channels may contain multiple ion binding sites, the latter function, in integral kinetic terms, as a single rate-limiting locus.

Cl- channels in basolateral TAL membranes: XIII. Heterogeneity between basolateral MTAL and CTAL Cl- channels

BACKGROUND

Antidiuretic hormone (ADH) or adenosine 3', 5'-cyclic phosphate (cAMP) analogues augment net NaCl absorption in microperfused mouse medullary thick ascending limb (MTAL) segments but not in cortical thick ascending limb (CTAL) segments. This ADH-dependent MTAL effect is due to increased apical Na+/K+/2Cl- admittance and apical K+ recycling accompanied by a rise in calculated intracellular Cl- concentrations and by a threefold rise in basolateral Cl- conductance. rbClC-Ka, a 75.2 member of the ClC family of Cl- channels, mediates net Cl- absorption in the MTAL. The gating characteristics of rbClC-Ka channels from their intracellular surfaces are, to our knowledge, unique among Cl- channels. The channels are activated by small increases in intracellular Cl- (K1/2 = 10 mM Cl-). Adenosine triphosphate plus the catalytic subunit of protein kinase A (ATP + PKA) gate rbClC-Ka when cytosolic Cl- concentrations are 25 mM. Thus, in mouse MTAL segments, ADH-dependent rises in cytosolic Cl- are primarily responsible for basolateral Cl- conductance increases.

METHODS

These experiments compared the properties of Cl- channels fused into bilayers from basolaterally enriched vesicles from cultured mouse CTAL cells with rbClC-Ka channels.

RESULTS

The key findings were that anti-rbClC-Ka, antibody that recognizes and blocks rbClC-Ka, recognized and blocked basolateral Cl- channels in CTAL cells, that the extracellular faces of the CTAL channels were, like rbClC-Ka, substrate gated with a K1/2 of approximately 170 mM Cl-, and that, unlike rbClC-Ka channels, cytosolic faces of basolateral CTAL Cl- channels were not gated by either increasing cytosolic Cl- concentrations or cytosolic (ATP + PKA). This failure of activation of basolateral CTAL Cl- channels was confirmed using excised patch clamp studies. Finally, on Western blots, anti-rbClC-Ka recognized a 74 kDa band on basolateral CTAL vesicles.

CONCLUSIONS

Basolateral CTAL Cl- channels probably share a high degree of structural homology and possibly molecular mass with rbClC-Ka channels. However, significant differences between rbClC-Ka channels and CTAL Cl- channels account for the inability of increasing either cytosolic Cl- or (PKA + ATP) to raise Po in CTAL basolateral Cl- channels.

ClC and CFTR chloride channel gating

Chloride channels are widely expressed and play important roles in cell volume regulation, transepithelial transport, intracellular pH regulation, and membrane excitability. Most chloride channels have yet to be identified at a molecular level. The ClC gene family and the cystic fibrosis transmembrane conductance regulator (CFTR) are distinct chloride channels expressed in many cell types, and mutations in their genes are the cause of several diseases including myotonias, cystic fibrosis, and kidney stones. Because of their molecular definition and roles in disease, these channels have been studied intensively over the past several years. The focus of this review is on recent studies that have provided new insights into the mechanisms governing the opening and closing, i.e. gating, of the ClC and CFTR chloride channels.

Cl- channels in basolateral renal medullary membranes. XII. Anti-rbClC-Ka antibody blocks MTAL Cl- channels

Cl- channels in the medullary thick ascending limb (MTAL) studied by either patch-clamp technique or reconstitution into lipid bilayers are activated by increases in intracellular Cl- concentrations. rbClC-Ka, a ClC Cl- channel, may represent this channel. We therefore evaluated the role of rbClC-Ka in transcellular MTAL Cl- transport in two separate ways. First, an antibody was raised against a fusion protein containing a 153-amino acid fragment of rbClC-Ka. Immunostaining of rabbit kidney sections with the antibody was localized to basolateral regions of MTAL and cortical thick ascending limb (CTAL) segments and also to the cytoplasm of intercalated cells in the cortical collecting duct. Second, Cl- uptake and efflux were measured in suspensions of mouse MTAL segments. Cl- uptake was bumetanide sensitive and was stimulated by treatment with a combination of vasopressin + forskolin + dibutyryl adenosine 3',5-cyclic monophosphate (DBcAMP). Cl- efflux was also increased significantly by vasopressin + forskolin + DBcAMP from 114 +/- 20 to 196 +/- 36 nmol.mg protein-1.45 s-1 (P = 0.003). Cl- efflux was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate (154 +/- 26 vs. 70 +/- 21 nmol.mg protein-1.45 s-1, P = 0.003). An anti-rbClC-Ka antibody, which inhibits the activity of MTAL Cl- channels in lipid bilayers, reduced Cl- efflux from intact MTAL segments (154 +/- 28 vs. 53 +/- 14 nmol.mg protein-1.45 s-1, P = 0.02). These results support the view that rbClC-Ka is the basolateral membrane Cl- channel that mediates vasopressin-stimulated net Cl- transport in the MTAL segment.

Cloning and functional expression of rat CLC-5, a chloride channel related to kidney disease

We have cloned a novel member of the CLC chloride channel family from rat brain, rCLC-5. The cDNA predicts a 83-kDa protein belonging to the branch including CLC-3 and CLC-4, with which it shares approximately 80% identity. Expression of rCLC-5 in Xenopus oocytes elicits novel anion currents. They are strongly outwardly rectifying and have a conductivity sequence of NO3- > Cl- > Br- > I- >> glutamate-. Although CLC-5 has consensus sites for phosphorylation by protein kinase A, raising the intracellular cAMP concentration had no effect on these currents. Currents were also unchanged when rCLC-5 was coexpressed with rCLC-3 and rCLC-4, either singly or in combination. rCLC-5 is expressed predominantly in kidney and also in brain, lung, and liver. Along the nephron, rCLC-5 message is detectable in all tubule segments investigated, but expression in the glomerulus and the S2 segment of the proximal tubule is low.

Cloning, tissue distribution, and intrarenal localization of ClC chloride channels in human kidney

Two kidney-specific chloride channels, ClC-K1 and ClC-K2, have been isolated from rat kidney. In the present study, we sought to isolate human homologue of rat ClC-K2 chloride channel that was present in the thick ascending limb of Henle's loop and collecting ducts. Human kidney cDNA library was screened with the whole rat ClC-K2 cDNA probe. Two highly homologous but not identical cDNAs were isolated and sequenced. Northern analysis showed that both clones were expressed only in kidney among various human tissues, demonstrating that kidney-specific ClC family members were also present in human kidney. Because both clones had almost the same nucleotide identity (approximately 80%) with rat ClC-K2, we could not determine by sequence alone which human clone corresponded to rat ClC-K2. Accordingly, we performed reverse transcription PCR using dissected human nephron segments and identified the site of expression of each clone in human nephron segments. One clone was only expressed in the thin limb of Henle's loop and the other was expressed in glomeruli, proximal tubules, and collecting ducts. We identified the latter clone as human ClC-K2 based on the localization of rat ClC-K1 and ClC-K2. Identification of human ClC-K2 clone will be of help in understanding the genetic involvement of chloride channel in disorders of chloride transport such as Bartter's syndrome.

Chloride currents across the membrane of mammalian skeletal muscle fibres

1. Chloride currents through the membrane of rat psoas muscle fibre segments were investigated with a double Vaseline gap under conditions minimizing the currents of other ion species. 2. In Cl(-)-free solutions a time- and voltage-independent conductance of 1.1 +/- 0.4 microS was observed. 3. As with intact fibres, the steady-state Cl- conductance was 2.5 +/- 0.9 mS cm-2; the halide selectivity was Cl- > Br- > I-, and Cl- currents were completely blocked by 0.1 mM 9-anthracene carboxylic acid (9-AC). 4. Voltage steps from -85 mV to between -125 and +55 mV elicited currents with deactivation upon hyperpolarization and activation upon depolarization. Activation was fitted with two exponentials, the smaller time constant increasing from 37.5 ms at +55 mV to 67.0 ms at -5 mV, the larger time constant (450 ms) being independent of potentials more positive than -5 mV. The two deactivation time constants ranged between 30.6 (-105 mV) and 99.3 ms (-35 mV), and 139.4 (-105 mV) and 738.5 ms (-35 mV). 5. The activation curve was fitted with a Boltzmann distribution (half-maximum, -39 mV; slope at inflexion point, 1/17.2 mV). Deactivation was incomplete. At very negative potentials about one-quarter of the maximum number of channels were open. 6. When tested with 5 and 61 mM intracellular Cl- concentration ([Cl-]i) the kinetic parameters were not different. 7. During depolarizations lasting > 5 s, activation was followed by a decline. With progressively longer prepulses going positive to the reversal potential and test pulses going negative, the responses to test and prepulses decreased with similar time constants, suggesting a real inactivation process.

Cl- channels in basolateral renal medullary vesicles. VIII. Partial purification and functional reconstitution of basolateral mTAL Cl- channels

Cl- channels fused from basolateral mTAL membranes into planar bilayers have distinctive functional characteristics which, when taken together, are unique among Cl- channels. The properties of these 50 to 60 pS channels can account for the characteristics of basolateral Cl- conductances in microperfused mTAL segments and thus may mediate net basolateral Cl- absorption in the intact mTAL. In the present studies, we solubilized basolateral membranes from rabbit mTAL. Since basolateral mTAL Cl- channels contain arginine- and lysine-rich domains, we exposed these solubilized membranes to sequential cation- and anion-exchange chromatography. The bound and unbound eluates from cation- and anion-exchange chromatography were reconstituted into proteoliposomes which, when fused into bilayers, yielded Cl- channels whose properties were virtually identical to those described above for native basolateral mTAL channels fused into bilayers. As judged by valinomycin-sensitive conductive 36Cl- uptake, proteoliposomes reconstituted from the unbound eluates after anion-exchange chromatography were enriched at least 30-fold in Cl- channel activity and had about 30% of the total Cl- channel activity solubilized in native vesicles.

Cl- channels in basolateral renal medullary membranes: VII. Characterization of the intracellular anion binding sites

A unique property of basolateral membrane Cl- channels from the mTAL is that the Cl- concentration facing the intracellular aspects of these channels is a determinant of channel open time probability (Po). The K1/2 for maximal activation of Po by Cl- facing intracellular domains of these channels is 10 mM Cl-. The present experiments evaluated the nature of these Cl(-)-interactive sites. First, we found that the impermeant anion isethionate, when exposed to intracellular Cl- channel faces, could augment Po with a K1/2 in the range of 10 mM isethionate without affecting conductance (gCl, pS). Second, pretreatment of the solutions facing the intracellular aspects of the channels with either 1 mM phenylglyoxal (PGO), an arginine-specific reagent, or the lysine/terminal amine reagent trinitrobenzene sulfonic acid (TNBS, 1 mM), prevented the activation of Po usually seen when the Cl- concentration of solutions facing intracellular channel domains was raised from 2 to 50 mM. However, when the Cl- channel activity was increased by first raising the Cl- concentration bathing intracellular channel faces from 2 to 50 mM, subsequent addition of either PGO or TNBS to solutions bathing intracellular Cl- channel faces had no effect on Po. We conclude that the intracellular aspects of these Cl- channels contain Cl(-)-interactive loci (termed [Cl]i) which are accessible to impermeant anions in intracellular fluids and which contain arginine- and lysine-rich domains which can be inactivated, at low ambient Cl- or isethionate concentrations, by interactions with PGO or TNBS.

Regulation of an epithelial chloride channel by direct phosphorylation and dephosphorylation

A native chloride channel in Necturus gallbladder epithelial cells is opened by a theophylline-induced rise in cellular cyclic AMP and is closed by removal of theophylline or by addition of specific antibody; however, it does not close if okadaic acid, an inhibitor of protein phosphatases 1 and 2A, is added. The purified channel reconstituted into lipid bilayers closes upon the addition of protein phosphatase 2A and is reopened by the addition of Mg-ATP and the catalytic subunit of cyclic AMP-dependent protein kinase. These results indicate that the channel protein is purified in a phosphorylated state and that its functional characteristics are at least partly controlled by direct phosphorylation and dephosphorylation.

Single-channel recordings of chloride currents in cultured human skeletal muscle

The Cl- channels in human myoballs were investigated with several recording techniques. Three types of channels were found and dubbed "small", "intermediate", and "large", according to their different conductance. The intermediate Cl- channel was observed most frequently. It was active at the resting potential immediately after seal formation in cell-attached as well as in excised patches. Its Cl- selectivity was rather high (PCl/PNa = 9.46; PCl/PMeSO4 = 7.85 where P denotes permeability) and the slope conductance at the reversal potential with [Cl-]o/[Cl-]i equal to 160 mM/42 mM was 31 pS. The channel showed an open-channel substructure with two subconductance levels having equal amplitudes. It can conduct two kinetically different currents that correspond to the activating and the inactivating Cl- current components described by Zachar et al. (1992). The small Cl- channel had a conductance of 10 pS at the reversal potential, a PCl/PNa of 2.7, and a PCl/PMeSO4 of 22.6. Its open probability was biggest negative to -85 mV, resulting in an inactivating whole-cell Cl- current component. Because of the small channel density and conductance the contribution of this channel type to the whole-cell current seems to be small. Patches with only one small channel were never observed which suggests that this channel type occurs in clusters. A third type of channel with very large conductance (250 pS) was seen only four times.

Cl- channels in basolateral renal medullary vesicles: V. Comparison of basolateral mTALH Cl- channels with apical Cl- channels from jejunum and trachea

Cl- channels from basolaterally-enriched rabbit outer renal medullary membranes are activated either by increases in intracellular Cl- activity or by intracellular protein kinase A (PKA). Phosphorylation by PKA, however, is not obligatory for channel activity since channels can be activated by intracellular Cl- in the absence of PKA. The PKA requirement for activation of Cl- channels in certain secretory epithelia is, in contrast, obligatory. In the present studies, we examined the effects of PKA and intracellular Cl- concentrations on the properties of Cl- channels obtained either from basolaterally-enriched vesicles derived from highly purified suspensions of mouse medullary thick ascending limb (mTALH) segments, or from apical membrane vesicles obtained from two secretory epithelia, bovine trachea and rabbit small intestine. Our results indicate that the Cl- channels from mTALH suspensions were virtually identical to those previously described from rabbit outer renal medulla. In particular, an increase in intracellular (trans) Cl- concentration from 2 to 50 mM increased both channel activity (Po) and channel conductance (gCl, pS). Likewise, trans PKA increased mTALH Cl- channel activity by increasing the activity of individual channels when the trans solutions were 2 mM Cl. Under the latter circumstance, PKA did not activate quiescent channels, nor did it affect gCl. Moreover, when mTALH Cl- channels were inactivated by reducing cis Cl- concentrations to 50 mM, cis PKA addition did not affect Po. These results are consistent with the view that these Cl- channels originated from basolateral membranes of the mTALH. Cl- channels from apical vesicles from trachea and small intestine were completely insensitive to alterations in trans Cl- concentrations and demonstrated markedly different responses to PKA. In the absence of PKA, tracheal Cl- channels inactivated spontaneously after a mean time of 8 min; addition of PKA to trans solutions reactivated these channels. The intestinal Cl- channels did not inactivate with time. Trans PKA addition activated new channels with no effect on basal channel activity. Thus the regulation of Cl- channel activity by both intracellular Cl- and by PKA differ in basolateral mTALH Cl- channels compared to apical Cl- channels from either the tracheal or small intestine.