Arachidonic acid inhibits Cl secretion in cultures of dog tracheal epithelium

Inhibition of cystic fibrosis transmembrane conductance regulator chloride channel currents by arachidonic acid

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel is inhibited by a number of different classes of organic anions which are able to enter and block the channel pore from its cytoplasmic end. Here I show, using patch clamp recording from CFTR-transfected baby hamster kidney cell lines, that the cis-unsaturated fatty acid arachidonic acid also inhibits CFTR Cl- currents when applied to the cytoplasmic face of excised membrane patches. This inhibition was of a relatively high affinity compared with other known CFTR inhibitors, with an apparent Kd of 6.5 ± 0.9 µM. However, in contrast with known CFTR pore blockers, inhibition by arachidonic acid was only very weakly voltage dependent, and was insensitive to the extracellular Cl- concentration. Arachidonic acid-mediated inhibition of CFTR Cl- currents was not abrogated by inhibitors of lipoxygenases, cyclooxygenases or cytochrome P450, suggesting that arachidonic acid itself, rather than some metabolite, directly affects CFTR. Similar inhibition of CFTR Cl- currents was seen with other fatty acids, with the rank order of potency linoleic [Formula: see text] arachidonic [Formula: see text] oleic > elaidic [Formula: see text] palmitic [Formula: see text] myristic. These results identify fatty acids as novel high affinity modulators of the CFTR Cl- channel.Key words: CFTR, chloride channel, fatty acid, channel block, cystic fibrosis.

Effect of t-butylhydroperoxide on chloride secretion in rat tracheal epithelia

Oxidative stress has been known to play important roles in various inflammatory diseases of lung such as allergic bronchitis, dust particle-induced inflammatory diseases, or chronic bronchitis. However, the effects of oxidants on Cl- secretion in tracheal epithelia have not been determined. To examine the effects of oxidants on Cl- secretion of the airway epithelia rat tracheal epithelial cells were cultured on porous filters and short circuit current (Isc) was measured in an Ussing chamber system. t-Butylhydroperoxide, which was widely used as a model substance to study the mechanism of cell injury resulted from oxidative stress, induced a transient increase in Isc by dose-dependent manner. The response was not observed in Cl(-)-free medium, and inhibited by 100 microM bumetanide. N(-Diphenyl-1,4-phenylene-diamine (DPPD, 5 microM), an inhibitor of lipid peroxidation, blocked the t-butylhydroperoxide response. When t-butylhydroperoxide was added after the administration of forskolin or H-89, a protein kinase A inhibitor, the t-butylhydroperoxide-induce Isc increase was abolished. Pretreatment of indomethacin (10 microM) completely inhibited the t-butylhydroperoxide response, but pretreatment of thapsigargin (1 microM) did not, t-Butylhydroperoxide induced gradual increases in cytosolic Ca2+ level, and increased [3H]arachidonic acid release in the presence of thapsigargin. These results indicate that t-butylhydroperoxide stimulates Cl-secretion via activation of phospholipase A2 and subsequent production of cyclooxygenase metabolities by Ca(2+)-dependent and -independent mechanisms.

Modulation of K+ channels by arachidonic acid in T84 cells. II. Activation of a Ca(2+)-independent K+ channel

We used single-channel recording techniques to identify and characterize a large-conductance, Ca(2+)-independent K+ channel in the colonic secretory cell line T84. In symmetric potassium gluconate, this channel had a linear current-voltage relationship with a single-channel conductance of 161 pS. Channel open probability (Po) was increased at depolarizing potentials. Partial substitution of bath K+ with Na+ indicated a permeability ratio of K+ to Na+ of 25:1. Channel Po was reduced by extracellular Ba2+. Event-duration analysis suggested a linear kinetic model for channel gating having a single open state and three closed states: C3<-->C2<-->C1<-->O. Arachidonic acid (AA) increased the Po of the channel, with an apparent stimulatory constant (Ks) of 1.39 microM. Neither channel open time (O) nor the fast closed time (C1) was affected by AA. In contrast, AA dramatically reduced mean closed time by decreasing both C3 and C2. The cis-unsaturated fatty acid linoleate increased Po also, whereas the saturated fatty acid myristate and the trans-unsaturated fatty acid elaidate did not affect Po. This channel is activated also by negative pressure applied to the pipette during inside-out recording. Thus we determined the effect of the stretch-activated channel blockers amiloride and Gd3+ on the K+ channel after activation by AA. Amiloride (2 mM) on the extracellular side reduced single-channel amplitude in a voltage-dependent manner, whereas Gd3+ (100 microM) had no effect on channel activity. Activation of this K+ channel may be important during stimulation of Cl- secretion by agonists that use AA as a second messenger (e.g., vasoactive intestinal polypeptide, adenosine) or during the volume regulatory response to cell swelling.

Blockade of swelling-induced chloride channels by phenol derivatives

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

The effect of non-steroidal anti-inflammatory drugs on the electrical properties of cultured dog tracheal epithelial cells

The effect of non-steroidal anti-inflammatory drugs (NSAIDs) on the electrical properties of primary cultures of dog tracheal epithelium has been studied. The cells used were grown with an air interface in a serum-free medium on membranes coated with human placental collagen. When mounted in Ussing chambers at 37 degrees C, mean values for the baseline short circuit current (Isc) and the transepithelial resistance of 65 tissue specimens from 18 dogs were 24.0 +/- 3.2 microA/cm2 and 458 +/- 128 omega.cm2, respectively. These tissues had been pretreated with amiloride to abolish active Na+ absorption. Under these conditions, the Isc value serves as a measure of active Cl- secretion. The results of this study revealed that the Isc across a cultured monolayer of trachea was attenuated by the tested NSAIDs, indomethacin, fulfenamic acid, mefenamic acid, aspirin, and acetaminophen, with Ki's that ranged from 6.0 x 10(-5) to 2.51 x 10(-3) M. Salicylic acid had no effect on baseline Isc. The Isc sensitivity sequence to the Cl- channel inhibitors tested was: fulfenamic acid >> indomethacin > mefenamic acid >> aspirin > acetaminophen > salicylic acid. The NSAIDs also significantly inhibited both the transient, Ca(2+)-dependent and the sustained, cAMP-dependent increases in Isc elicited by isoproterenol. Thus, the tested NSAIDs appeared to have an effect on the electrical properties of the cells. A similar effect of NSAIDs on ion transport across the human airway epithelium may help to reduce airway fluid secretion.

Inhibition by fatty acids of cyclic AMP-dependent protein kinase activity in brush border membranes isolated from human placental vesicles

1. The inhibitory effects of arachidonic acid (AA) and a number of structurally related fatty acids on cyclic AMP-dependent protein kinase activity have been investigated in brush border membranes (BBM) prepared from human placental vesicles. 2. BBM vesicles were characterized by electron microscopy and displayed enrichment of the appropriate marker enzymes, alkaline phosphatase and gamma-glutamyltranspeptidase; BBM were prepared by vesicles lysis in hypotonic medium. 3. Cyclic AMP-dependent protein kinase (PKA) activity was measured in BBM. At 1 microM, cyclic AMP stimulated a 4.2 +/- 0.06 fold increase over basal levels of [32P]-phosphate incorporation into the synthetic substrate kemptide and this effect was abolished by a selective PKA inhibitor. By use of synergistic pairs of site-selective cyclic AMP analogues, the kinase was identified as the type II enzyme. 4. Cyclic AMP-stimulated PKA activity was inhibited by 10 microM AA and this effect was significantly enhanced by nordihydroguaiaretic acid (NDGA) + indomethacin (Indo), inhibitors of the lipoxygenase and cyclo-oxygenase pathways of AA metabolism respectively. 5. Oleic acid, elaidic acid, but not caprylic or palmitic acids, also significantly inhibited PKA activity and this effect was again enhanced by NDGA + Indo. While arachidonyl alcohol alone was not inhibitory, in the presence of the metabolic inhibitors a significant reduction in stimulated activity was observed. 6. The commercially available PKA type II holoenzyme (activated by cyclic AMP), but not the free catalytic subunit, was inhibitable by AA, oleic or elaidic acids. 7. These results suggest that PKA localized to the brush border membrane of human placental vesicles is inhibited by fatty acids which may compete with cyclic AMP for binding to the kinase regulatory subunit. The reported inhibition by fatty acids of cyclic AMP-dependent Cl- secretion in epithelial cells may therefore be due in part to negative regulation of a Cl- channel-associated PKA.

Modulatory action of arachidonic acid on GABAA/chloride channel receptor function in adult and aged brain cortex membranes

Effect of arachidonic acid on binding parameters of two binding sites on the GABAA receptor and on GABA activated Cl- uptake was investigated in synaptic plasma membrane and in synaptoneurosomes from brain cortex of adult (4-months old) and aged (27-months old) rats. The ligands used were [3H]muscimol, a GABA agonist and [35S]-t-butylbicyclophosphorothionate ([35S]TBPS), a convulsant that binds to the site near the chloride channel. Arachidonic acid increases significantly GABAA agonist binding and concomitantly decreases [35S]TBPS binding in a concentration dependent manner. The analysis of binding parameters in adult brain showed a significant decrease by AA of KD value for low but not for high affinity of [3H]muscimol binding. Concomitantly, AA enhances Bmax values for high affinity binding and has no effect on Bmax value for low affinity binding in synaptic plasma membrane (SPM) from adult brain. In synaptic plasma membrane from aged brain AA increases low and high affinity binding of agonist to GABAA receptor, modified significantly KD value by about 30 and 66%, respectively. On the other hand, AA significantly decreases of [35S]TBPS binding to chloride channel recognition site. Scatchard's analysis indicates that this inhibition results from a decrease of total number of binding sites. Moreover, the affinity of [35S]TBPS binding was increased (KD = 37.0 nM for AA-treated membrane, as compared to 69.3 nM KD in control membrane). GABA-dependent chloride uptake into synaptoneurosomes is also inhibited by AA in a concentration dependent manner in adult brain. In aged brain synaptoneurosomes AA has similar inhibitory effect on GABA-activated chloride uptake.(ABSTRACT TRUNCATED AT 250 WORDS)