cAMP-stimulated Cl- secretion is increased by glucocorticoids and inhibited by bumetanide in semicircular canal duct epithelium
© Pondugula et al.; licensee BioMed Central Ltd. 2013
Received: 14 December 2012
Accepted: 11 March 2013
Published: 27 March 2013
The vestibular system controls the ion composition of its luminal fluid through several epithelial cell transport mechanisms under hormonal regulation. The semicircular canal duct (SCCD) epithelium has been shown to secrete Cl- under β2-adrenergic stimulation. In the current study, we sought to determine the ion transporters involved in Cl- secretion and whether secretion is regulated by PKA and glucocorticoids.
Short circuit current (I sc ) from rat SCCD epithelia demonstrated stimulation by forskolin (EC50: 0.8 μM), 8-Br-cAMP (EC50: 180 μM), 8-pCPT-cAMP (100 μM), IBMX (250 μM), and RO-20-1724 (100 μM). The PKA activator N6-BNZ-cAMP (0.1, 0.3 & 1 mM) also stimulated I sc . Partial inhibition of stimulated I sc individually by bumetanide (10 & 50 μM), and [(dihydroindenyl)oxy]alkanoic acid (DIOA, 100 μM) were additive and complete. Stimulated I sc was also partially inhibited by CFTRinh-172 (5 & 30 μM), flufenamic acid (5 μM) and diphenylamine-2,2′-dicarboxylic acid (DPC; 1 mM). Native canals of CFTR+/− mice showed a stimulation of Isc from isoproterenol and forskolin+IBMX but not in the presence of both bumetanide and DIOA, while canals from CFTR−/− mice had no responses. Nonetheless, CFTR−/− mice showed no difference from CFTR+/− mice in their ability to balance (rota-rod). Stimulated I sc was greater after chronic incubation (24 hr) with the glucocorticoids dexamethasone (0.1 & 0.3 μM), prednisolone (0.3, 1 & 3 μM), hydrocortisone (0.01, 0.1 & 1 μM), and corticosterone (0.1 & 1 μM) and mineralocorticoid aldosterone (1 μM). Steroid action was blocked by mifepristone but not by spironolactone, indicating all the steroids activated the glucocorticoid, but not mineralocorticoid, receptor. Expression of transcripts for CFTR; for KCC1, KCC3a, KCC3b and KCC4, but not KCC2; for NKCC1 but not NKCC2 and for WNK1 but only very low WNK4 was determined.
These results are consistent with a model of Cl- secretion whereby Cl- is taken up across the basolateral membrane by a Na+-K+-2Cl- cotransporter (NKCC) and potentially another transporter, is secreted across the apical membrane via a Cl- channel, likely CFTR, and demonstrate the regulation of Cl- secretion by protein kinase A and glucocorticoids.
The inner ear controls the ion composition of its luminal fluid, endolymph, through a multiplicity of transepithelial transport mechanisms in different cell types bounding the lumen. The high-K+, low Na+, low Ca2+ endolymph composition is needed for proper auditory and vestibular function [1–3]. K+ secretion by both strial marginal cells and vestibular dark cells is stimulated by β-adrenergic receptors via cytosolic cAMP as second messenger [4, 5]. Secretion of the primary anion, Cl-, is known to also be under adrenergic control in semicircular canal duct (SCCD) epithelium .
Cl- transport by several epithelia has been shown to be under control of a cAMP signal pathway that is mediated by apical CFTR Cl- channels via protein kinase A (PKA) . Vectorial transport in those epithelia depends also on basolateral Na+-K+-ATPase and K+ channels as well as basolateral Cl transporters such as the Na+-K+-2Cl- cotransporter (NKCC1/Slc12a2) [6, 8–10]. Cellular cAMP levels and cAMP-mediated processes, including ion transport, depend on cAMP metabolism regulated by the enzymatic activity of anabolic adenylyl cyclase and catabolic phosphodiesterase [11–13].
Glucocorticoids can modify cellular responses via genomic and non-genomic pathways, including regulation of ion transport processes [14–17]. Na+ absorption by SCCD has already been demonstrated to be stimulated by glucocorticoids [17, 18]. There is considerable evidence in various tissues, including epithelia, that glucocorticoids have long-term (genomic) effects on cAMP metabolism and potentiate cAMP-mediated responses, including ion transport by activation of the glucocorticoid receptor [19–21].
We therefore sought to determine in SCCD a) whether there is a basal constitutive adenylyl cyclase activity driving cAMP-mediated Cl- secretion, b) whether Cl- secretion is enhanced by glucocorticoid hormones via activation of glucocorticoid receptors, c) whether Cl- secretion is mediated through PKA action and d) whether Cl- secretion is mediated by a basolateral NKCC and/or KCC and an apical CFTR Cl- channel. Our findings establish that the SCCD is a site in the inner ear for PKA-mediated Cl- transport, that this transport depends on NKCC1 and another Cl- uptake process (perhaps KCC), that apical CFTR is likely involved and that it is enhanced by glucocorticoid-receptor activation.
Adenylyl cyclase activator, cAMP analogs, and phosphodiesterase inhibitors stimulate I sc
The lipid-soluble drugs forskolin, 8-pCPT-cAMP, RO-20-1724, 3-isobutyl-1-methylxanthine (IBMX), were added to both the apical and basolateral baths. Amiloride was added to only the apical side and bumetanide to the basolateral side. Amiloride had no significant effect on I sc , whereas subsequent addition of both forskolin (Figure 1) and 8-Br-cAMP (Figure 2A) increased I sc in a concentration dependent manner with an EC50 of about 0.8 μM and 180 μM respectively. Forskolin showed no additional effect after prior stimulation by either 8-pCPT-cAMP (100 μM) (Figure 2B) or by RO-20-1724 (100 μM) (Figure 2D), demonstrating constitutive activity of adenylyl cyclase in SCCD epithelium.
Glucocorticoids increase forskolin-stimulated I sc
The natural and synthetic glucocorticoids stimulated within their respective physiologic and therapeutic ranges , while aldosterone was only effective at concentrations much higher than found under normal physiologic conditions .
Glucocorticoids increase forskolin-stimulated I sc by activation of glucocorticoid receptor
We investigated whether dexamethasone, hydrocortisone, and aldosterone increased FSK-stimulated I sc by activation of glucocorticoid receptors and/or mineralocorticoid receptors. SCCD epithelia were incubated in the presence of dexamethasone (100 nM), hydrocortisone (1 μM) or aldosterone (1 μM) alone or in the presence of receptor antagonists. Mifepristone significantly reduced the effects of dexamethasone, hydrocortisone and aldosterone (Figure 4A,D,E), consistent with action of all of these corticosteroids at the glucocorticoid receptor. Mifepristone is also known to be an antagonist of the progesterone receptor; however, progesterone (10–1000 nM) had no effect on forskolin stimulation (Figure 4F). The mineralocorticoid receptor antagonist spironolactone had no significant effect on dexamethasone-, hydrocortisone- or aldosterone-treated epithelia (Figure 4A,D,E), consistent with a lack of involvement of mineralocorticoid receptor in the increase of the FSK-stimulated I sc in these cells. These findings suggest that corticosteroids increase forskolin-stimulated I sc (cAMP-mediated Cl- secretion) solely by activation of glucocorticoid receptors in SCCD epithelia.
PKA activator stimulates I sc
Blockers of basolateral Na+-K+-2Cl-and K+-Cl-cotransporters inhibit forskolin and/or forskolin + IBMX-stimulated I sc
Expression of KCC and WNK isoforms
RT-PCR demonstration of the presence of KCC1, KCC3a, KCC3b, KCC4 and absence of KCC2 in rat SCCD primary cultures
RT-PCR demonstration of the presence of WNK1 and absence of WNK4 in rat SCCD primary cultures
Blockers of apical Cl- transport inhibit forskolin and/or forskolin + IBMX-stimulated Isc
Sensory transduction of acceleration in the vestibular labyrinth is mediated by modulation of K+ currents through sensory cells, where the K+ originates from the high-[K+] luminal fluid. Secretion of K+ is controlled by β-adrenergic stimulation (among other signal pathways) [30, 31] and it is to be expected that secretion of the primary anion would also be regulated by the same agonists. Movements of anions would be expected to be transcellular since the paracellular pathway must be extremely tight in this epithelium to support the large ion concentration gradients. Cl- secretion by the semicircular canal duct (SCCD) in the vestibular labyrinth is stimulated by cAMP as second messenger  and similar mechanisms have been proposed in the cochlea [32, 33], although the cell types responsible in the cochlea have not yet been unambiguously determined (see ).
The SCCD is an epithelial domain that has a high ratio of surface area to endolymph volume and would therefore be a strong candidate for a site of effective ion homeostasis. Indeed, it was recently shown that these cells also absorb Na+ via the epithelial sodium channel (ENaC) under glucocorticoid receptor control [17, 18] and absorb Ca2+ via an epithelial Ca2+ channel , in addition to their role in Cl- secretion.
We determined in the present study that cAMP acts via PKA, whose target may be an apical CFTR Cl channel . Evidence supporting the involvement of CFTR include inhibition of Cl- secretion (cAMP-stimulated I sc ) by the poorly-specific inhibitors flufenamic acid and DPC and by the specific inhibitor CFTRinh-172. Partial inhibition in rSCCD by CFTRinh-172 is consistent with reports of significant but only partial inhibition in avian proximal tubule at 20 μM . In addition, mRNA message for CFTR was found to be present in the rat primary cultures of SCCD (GEO database, Accession GSE6196, ). The observation of cAMP-stimulated I sc in mouse canals extends the findings in gerbil and rat  to another rodent species. The absence of cAMP-stimulated I sc in CFTR knockout mice is consistent with an essential role of CFTR in canal Cl- secretion, although there is no strict proof ruling out the unlikely occurrence of dissection damage to only the knockout mouse canals.
Interestingly, there is no correlation of deafness in persons with dysfunctional CFTR (cystic fibrosis) [37, 38] and no reports of vertigo in this population. Our results with vestibular tests of CFTR knockout mice are consistent with that observation. It may be, however, that this anion transport system is not by itself essential for normal inner ear function, but may be necessary in times of systemic stress when β-adrenergic agonist levels are elevated, but this proposition remains to be tested.
The results here demonstrated increased I sc by stimulation of the cAMP signal pathway in several ways: through increased cAMP production (forskolin stimulation of adenylyl cyclase), addition of exogenous cAMP analogs (8-Br-cAMP and 8-pCPT-cAMP), and inhibition of cAMP catabolism (IBMX and RO-20-1724 inhibition of phosphodiesterase). The action of all of these agents has been well-documented in other cells [11–13, 19–21]. Increased Cl- secretion by inhibition of phosphodiesterase implies that the adenylyl cyclase is constitutively active in the SCCD epithelium in the absence of β-adrenergic agonists, and is therefore consistent with the earlier observation of constitutive low-level Cl- secretion . Stimulation of Cl- secretion has been reported in mouse jejunum, guinea pig distal colon, T84 monolayers and human colonic epithelial cells by phosphodiesterase inhibition [39–42]. Of particular interest is the finding that phosphodiesterase inhibition did not elevate Cl- secretion in the absence of exogenous stimulation of adenylyl cyclase in T84 epithelial monolayers . This is in contradistinction to our observations in rat semicircular canal primary cultures (vide infra and ).
The responsiveness of adenylyl cyclase and phosphodiesterase to multiple physiological challenges is related to the presence of multiple families of isoforms with tissue-specific localization [11–13]. Ten isoforms of adenylyl cyclase (AC; membranous AC1-9 and soluble AC) have been identified in mammalian cells . In SCCD, the non-selective adenylyl cyclase activator forskolin, which is known to activate all identified adenylyl cyclase isoforms with varying sensitivity [11, 43, 44], stimulated Cl- secretion. Gene array results showed expression of transcripts for AC2 and AC4 in SCCD among the genes that were tested (AC2, AC3, AC4, AC5, AC6, and AC8) using Affymetrix Rat Genome 230 2.0 Array chips . However, it is not known whether other isoforms that were not represented on the gene array are expressed in SCCD.
Eleven families of phosphodiesterases (PDE1 to PDE11) have been identified in mammalian cells, with each family having several isoforms . Phosphodiesterases show three types of substrate specificity . The PDE4, PDE7, and PDE8 families hydrolyze cAMP specifically. PDE5, PDE6, and PDE9 are cGMP specific and PDE1, PDE2, PDE3, PDE10, and PDE11 are dual substrate phosphodiesterases i.e. they hydrolyze both cAMP and cGMP. In SCCD, both the non-selective phosphodiesterase inhibitor IBMX and the selective PDE4 inhibitor RO-20-1724  stimulated Cl- secretion. Gene array results showed expression of the transcripts for PDE4 and PDE7 isoforms but not PDE8 in SCCD . Expression of PDE4 at the transcript level is consistent with RO-20-1724 stimulation of I sc .
Glucocorticoids are known to affect cAMP metabolism and potentiate cAMP-mediated responses, including salt and water transport by activation of the glucocorticoid receptor [16, 19–21]. It is known that glucocorticoids stimulate anion transport, including Cl- in mammalian ileal mucosa, by increasing the concentration of cyclic nucleotides . Glucocorticoids elevate cellular cAMP levels by regulating the activity of both adenylyl cyclases and phosphodiesterases, predominantly by suppressing the activity of phosphodiesterases [19, 20]. It is not known if glucocorticoids increase cAMP-mediated Cl- secretion in SCCD epithelium by a similar mechanism.
Glucocorticoids increase cAMP-mediated Cl- secretion in SCCD epithelium by activation of exclusively the glucocorticoid receptor. However, the exact mechanism of potentiation of cAMP response is not known. The stimulation of cAMP-mediated I sc could not have been a result of the increase in glucocorticoid-activated ENaC function , since increased Na+ conductance would depolarize the cell, which in turn would reduce the outward driving force for Cl- across the apical membrane, in contrast to observations. In addition, Na+ absorption was blocked by amiloride in these experiments. Another putative mechanism of action of glucocorticoids in SCCD is regulation of NKCC expression, as observed in vascular smooth muscle and trabecular meshwork cells [45, 46]. However, there was no change in expression of NKCC1 in SCCD after incubation with 100 nM dexamethasone for 24 h . The molecular mechanism for glucocorticoid-increased cAMP-mediated Cl- secretion in SCCD epithelium has not yet been resolved.
One pathway of corticosteroid action is via the kinases SGK1 and WNK1  among other kinases involved in regulation of ion transport such as SPAK and OSR1 . Our gene array of rat SCCD cultures  showed SPAK or Stk39 to be present (1387059_at based on Genbank NM_019362) and minimally down-regulated 1.2-fold by dexamethasone. There were no probe sets identified on the chips for OSR1. WNK1 is downstream from SGK1 and is implicated in the control of both NKCC and KCC transporters and thereby their roles in Cl- transport . The expression of WNK1 in the SCCD may therefore be involved in the regulation of Cl- secretion in SCCD through these two transporters. The transcript expression for WNK4 was nearly absent.
Complete additive inhibition of the cAMP-stimulated I sc by bumetanide and DIOA is consistent with the participation of both Na+-K+-2Cl- and K+-Cl- cotransporter in Cl- cotransport. The concentration of bumetanide used fully inhibits NKCC1 but has no effect on KCC1 and KCC4 [50, 51]. Similarly, DIOA is a potent well-established KCC transport inhibitor (IC50 = 10–40 μM) that is without effect on NKCC [52, 53] and has been used as a specific KCC inhibitor in the range 30 – 100 μM [53–55]. However, DIOA has also been observed in several epithelia to have diverse effects [56–59].
The canonical interpretation that the effect of DIOA is a result of inhibition of a KCC, however, is problematic. Foremost, the KCC would be operating “backwards” from its most-commonly observed mode and if this were indeed the case it would necessitate an extremely low intracellular [Cl-]. A low intracellular concentration of the secreted ion species (Cl-) would not favor a strong efflux across the apical membrane. Although most cells operate KCC transporters as cellular efflux pathways, Payne calculates that some neuronal cells have sufficiently-low intracellular [Cl-] to thermodynamically drive influx of KCl . The membrane potential, intracellular [K+] and [Cl-] of SCCD epithelial cells is not known, so it is not possible at this time to evaluate whether these cells are able to support concurrent basolateral KCC electroneutral influx and apical electrogenic Cl- secretion. The discussion by Payne  shows that at the normal extracellular [K+] of about 4 mM, the intracellular [Cl-] would need to be about 5 mM or less; a highly unlikely constellation. An elevation of extracellular [K+] at the canal epithelium due to hair cell stimulation is also highly unlikely since the hair cells are located remotely from the canals and excess K+ would be removed from perilymph by vestibular dark cells recycling the K+ back into the lumen.
The observation that basolateral bumetanide causes only a partial inhibition of cAMP-stimulated Cl- secretion at 50 μM suggests the existence of a parallel Cl- uptake mechanism. Block of the remaining secretion by DIOA points to a KCC, subject to the caveats given above. An alternative explanation is that DIOA is not a specific inhibitor for KCC in SCCD and either blocks another Cl- uptake mechanism that operates under thermodynamically-favorable conditions or acts broadly on active transport through inhibition of metabolic processes. This last suggestion has been observed with another lipid-soluble weak acid, the Cl- channel blocker NPPB, restricting its use to <100 μM . In addition, DIOA has been shown to interfere with ATP recovery after metabolic insult and to release cytochrome c from isolated mitochondria at the concentration used here . Nonetheless, if this is the basis for action of DIOA in the semicircular canal epithelium, the metabolic interference is not complete (Figure 6) and the likely involvement of a parallel Cl- uptake pathway in rat canal remains.
The rat semicircular canal duct epithelium expresses three major ion transport systems, including a β2-adrenergic receptor-stimulated Cl- secretion . We have demonstrated here that Cl- secretion is under control of a PKA-mediated signal pathway that depends on basolateral Cl- uptake via NKCC and (KCC?) transporters and that it is enhanced by activation of glucocorticoid receptors. Interestingly, NKCC does not appear to contribute to ENaC-mediated Na+ absorption  in the same cells. The role of the β2-adrenergic receptor/cAMP pathway in inner ear function remains unknown but is likely activated under stress.
All experiments were performed on primary cultures of rat SCCD, except the measurements of short circuit current from native canals and vestibular function that were performed on CFTR mutant mice.
Cell cultures of rat SCCD epithelium
Primary cultures of neonatal Wistar rat SCCD epithelium were produced as described previously . Briefly, animals were anesthetized and sacrificed according to a protocol approved by the Kansas State University Institutional Animal Care and Use Committee. SCCD epithelial cells were microdissected from isolated temporal bones. Isolated cells, exclusive of connective tissue and common crus, were seeded on 6.5 mm diameter permeable (4 μm) Costar (#3470) Transwell supports and cultured in DMEM/F12 medium (Invitrogen 12500–062; http://www.invitrogen.com/site/us/en/home/support/Product-Technical-Resources/media_formulation.60.html) supplemented with 5% fetal bovine serum, 26.8 mM NaHCO3-, penicillin (100 U/ml) and streptomycin (100 μg/ml). Cultures treated with steroids, in the presence and absence of antagonists, were exposed for 24 h unless otherwise stated.
Amiloride (#A-7410, Sigma), forskolin (#F-6886, Sigma), spironolactone (#S-3378, Sigma), mifepristone (#M-8046, Sigma), corticosterone (#C-2505, Sigma), prednisolone (#6004, Sigma), bumetanide (#B-3023, Sigma), RO-20-1724 (#557502, Calbiochem), 3-isobutyl-1-methylxanthine (IBMX; #I-7018, Sigma), flufenamic acid (#F-9005, Sigma), CFTR inh-172 ((gift from Dr. Bruce Schultz), and [(dihydroindenyl)oxy]alkanoic acid (DIOA; # D-129, Sigma) were dissolved in DMSO. Progesterone (#P-8783, Sigma), and aldosterone (#215360050, Acros Organics, New Jersey) were dissolved in absolute ethanol. DMSO or ethanol alone had no effect on electrical parameters in Ussing chamber experiments.
Cyclodextrin-encapsulated dexamethasone (#D-2915, Sigma), hydrocortisone (#H-0396, Sigma), isoproterenol (#I-6504, Sigma), 8-Br-cAMP (#203800, Calbiochem), 8-pCPT-cAMP (#C-3912, Sigma), and N6-Benzoyl-Adenosine 3′,5′-cyclic Monophosphate, Sodium Salt (N6-BNZ-cAMP; #116802, Calbiochem) were dissolved in water.
Mice breeding and maintenance
CFTR heterozygous parents in a C57BL/6 background were bred and the offspring were genotyped for CFTR+/+, CFTR+/− or CFTR−/− All the animals were maintained on Colyte in water to avoid intestinal impaction .
Dissection of mouse SCCD
CFTR+/− and CFTR−/− mice (34–39 days old)  were anesthetized with sodium pentobarbital (50–100 mg/kg; i.p.) and sacrificed under a protocol approved by the Institutional Animal Care and Use Committee of Kansas State University. The temporal bones were removed and SCCD without common crus were dissected in HEPES-buffered saline. The SCCD were mounted in a 200 μl perfusion chamber on the stage of an inverted microscope (Nikon ECLIPSE TE 300) and continuously perfused at 37°C with HEPES-buffed solution at an exchange rate of 180 μl/sec.
Isc from Primary cultures of rat SCCD
SCCD epithelia were bathed in HEPES-buffered solution equilibrated with air. The composition was (in mM) 150 NaCl, 3.6 KCl, 1 MgCl2, 0.7 CaCl2, 5 glucose, and 10 HEPES, pH 7.4. Transepithelial voltage (VT) and resistance (RT) were measured from confluent monolayers of SCCD in an Ussing chamber under current clamp (ΔI=1 μA) mode as described previously [17, 18]. The equivalent I sc was calculated from I sc = VT/RT. I sc was measured either in the absence of steroids (Figure 1, 2, 7B left) or after 24 h incubation with steroid. Steroid-stimulated Na+ current was blocked with amiloride before measurement of cAMP-stimulated I sc . Cytosolic cAMP levels were raised by stimulation of adenylyl cyclase with forskolin, block of phosphodiesterase with IBMX or RO-20-1724 or a combination of forskolin and IBMX, as indicated. The lipid-soluble drugs dexamethasone, hydrocortisone, aldosterone, forskolin, 8-pCPT-cAMP, N6-BNZ-cAMP, spironolactone, mifepristone, corticosterone, prednisolone, progesterone and [(dihydroindenyl)oxy]alkanoic acid (DIOA) were added to both the apical and basolateral baths. Amiloride was added to only the apical side and bumetanide to the basolateral side.
Isc from mouse native canal ducts; Voltage-sensitive vibrating probe recordings
The vibrating probe technique was identical to that previously described [34, 64]. Briefly, the current density was monitored from mouse SCCD by vibrating (200–400 Hz) a platinum-iridium wire microelectrode with a platinum-black tip positioned 20 μm from the basolateral surface of the SCCD with computer-controlled stepper-motor manipulators (Applicable Electronics, Forest Dale, MA) and probe software (ASET version 2.00, Science Wares, East Falmouth, MA). The bath references were 26-gauge Pt-black electrodes. The phase-locked signal from the electrode was detected by phase-sensitive amplifiers, digitized y (0.5 Hz, 16 bit), and the output was expressed as current density at the electrode. HEPES-buffered solution was used as the bath.
Vestibular function test
4 sets of littermate-matched CFTR+/− and CFTR−/− mice (mean age = 56 d) were tested 4 times/day (with a five minute rest between tests) over five days. The RotaRod (Series 8, IITC, Woodland Hills, CA) used a 32 mm diameter rod and was programmed to start at 4 RPM, increasing to 40 RPM over 1 minute. All mice were placed on the rod before rotation was initiated and the time until falling was recorded. The instrument was modified by placing a landing cushion on the platform of the timer stop switch. The daily mean time for each mouse was used to calculate the mean and standard error of each group. After the last run of each day, all homozygous mice were given a 1 ml saline injection to prevent dehydration.
RNA isolation and RT-PCR
Total RNA was extracted from SCCD primary cultures using RNeasy Micro Kit and the quality and quantity were determined as described previously . RT-PCR was performed on total RNA as described previously using a One Step RT-PCR Kit following the manufacturer’s protocol (#210210, Qiagen) .
GenBank # (species)
Amplicon size (bp)
S: GAG GTT CGA AGA CGA TCA GA
AS: TCG CTC CAC CAA CTA AGA AC
S: GTT CGC CTC ACT CTT CCT GGC
AS: TGG GCC ACC ACA TAC AGG GA
S: CAT CAC AGA TGA ATC TCG GG
AS: TTC TCT GGG TCT GTC TCC C
S: CCT CGC CTC CTC ACC TTT GC
AS: TCA CTC TGA CGC CAG CCA TTG
S: AGT AAA AGC CCG GAT TCA GG
AS: ATG AAA GTA CCC ATT TGG GG
S: AGG AAG CTG CTG AGC GCA C
AS: CAG CAT TGT ACA GGT GCA GC
S: ACC AGA AAG CCT CAT GTA AGC C
AS: GTC CGC AGG GAA CGT CAT TG
S: CAC CTC CCG CCG CAA CAG
AS: TCC ACA CAG CAA AGA GCA CCC
Electrophysiology data are presented as original recordings and as mean values ± SE from n observations. Student’s t-test was used to determine statistical significance of paired and unpaired samples. The Hill equation was fitted to concentration-response curves by using individual data points in order to retain appropriate weighting and presented here plotted with the mean and SEM. Differences were considered significant for P < 0.05.
Current address for SRP: 112 Greene Hall, Department of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849.
This work was supported by NIH-NIDCD grant R01-DC00212 and NIH-NCRR grant P20-RR017686 to DCM and by the Cystic Fibrosis Foundation to RCD. Publication of this article was funded in part by the Kansas State University Open Access Publishing Fund. We thank Dr. Bruce Schultz for the gift of CFTRinh-172.
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