Nitric oxide modulates ionic transport in the isolated intestine of the eel, Anguilla anguilla

We investigated the role of NO (nitric oxide) in the isolated intestine of the sea water adapted eel, by testing the effect of various donors on I(sc) (short-circuit current), due to net Cl(-) absorption in the control conditions. We found that the endogenous NO-synthase substrate l-arginine as well as two different NO donors, SNP (sodium nitroprusside) and SIN-1 (3-morpholinosydnonimine), produced a slow and gradual decrease of I(sc). The effect of SNP was reduced by the pretreatment with ODQ (1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one), a specific inhibitor of the soluble guanylyl cyclase, suggesting the involvement of cGMP (cyclic GMP) in some physiological actions of NO. The effect of the NO donors on I(sc) was similar to that observed when the tissues were perfused with solution in which the HCO(3)(-) buffer was substituted with Hepes buffer. In addition the NO donors produced a negligible effect on I(sc) when the tissues were perfused with Hepes buffer or in the presence of bilateral SITS(4-Acetoamido-4'-iso-thiocyanatostilbene-2,2'disulphonic acid), an inhibitor of the HCO(3)(-) transport mechanisms, operating on both cell membranes of the eel enterocyte and responsible for HCO(3)(-) uptake by the cell. Based on these observations we suggest that NO regulates I(sc) and hence the transepithelial ion transport indirectly by modulating the endocellular concentration of HCO(3)(-) and/or H(+). In addition it is likely that NO modulates the permeability of the paracellular pathway since SNP produced also an increase of the tissue conductance and a decrease of the magnitude of the dilution potential.     

Effect of the flavonol quercetin on ion transport in the isolated intestine of the eel, Anguilla anguilla

Flavonoids are phenolic compounds used in fish diet formulations for the control of sex differentiation. Consequently it is of interest to know their effects on fish gastrointestinal mucosa that is the first target of these substances after food intake. We studied the effects of the flavonol quercetin on the transepithelial electrical parameters of the isolated intestine of the eel, Anguilla anguilla, by employing Ussing chamber technique. We showed that luminal quercetin (10(-4) M) reduced the serosa negative I(sc) (short circuit current) due to a net Cl- absorption in the control conditions. The reduction of I(sc) was blocked by luminal glybenclamide (10(-4) M). The I(sc) response to the flavonoid was also observed when HCO3- was omitted from the perfusion solutions. The effect of quercetin was nullified by TMB8, a blocker of Ca2+ release from intracellular stores, and was strongly reduced by trifluoroperazine, an inhibitor of calmodulin actions, while it was unmodified by experimental manoeuvres able to alter cytosolic cAMP concentrations. These results suggest that mucosal quercetin stimulated Cl- but not HCO3- secretion; the secretory activity appears to be dependent on the Ca2+-calmodulin system, independent of the cAMP pathway. In addition, this study showed that flavonol mimics the effect of carbachol on the tissue. This conclusion is supported by the following observations: 1) noradrenalin reversed the reduction of I(sc) produced by both carbachol and quercetin; 2) the flavonol was ineffective in tissues in which the muscarinic agonist had already elicited its maximal reduction of I(sc).     

Apical trypsin increases ion transport and resistance by a phospholipase C-dependent rise of Ca2+

We investigated the mechanisms by which serine proteases alter lung fluid clearance in rat lungs and vectorial ion transport in airway and alveolar epithelial cells. Inhibition of endogenous protease activity by intratracheal instillation of soybean trypsin inhibitor (SBTI) or alpha(1)-antitrypsin decreased amiloride-sensitive lung fluid clearance across rat fluid-filled lungs; instillation of trypsin partially restored this effect. Gelatin zymography demonstrated SBTI-inhibitable trypsin-like activity in rat lung lavage fluid. Apical trypsin and human neutrophil elastase, but not agonists of protease activated receptors, increased Na(+) and Cl(-) short-circuit currents (I(sc)) and transepithelial resistance (R(TE)) across human bronchial and nasal epithelial cells and rat alveolar type II cells, mounted in Ussing chambers, for at least 2 h. The increase in I(sc) was fully reversed by amiloride and glibenclamide. The increase in R(TE) was not prevented by ouabain, suggesting that trypsin decreased paracellular conductance. Apical trypsin also induced a transient increase in intracellular Ca(2+) in human airway cells; treatment of these cells with BAPTA-AM mitigated the trypsin-induced increases of intracellular Ca(2+) and of I(sc) and R(TE). Increasing intracellular Ca(2+) in airway cells with either ionomycin or thapsigargin reproduced the increase in I(sc), whereas inhibitors of phospholipase C (PLC) prevented the increases in both Ca(2+) and I(sc). These data indicate trypsin-like proteases and elastase, either present in lung cells or released by inflammatory cells into the alveolar space, play an important role in the clearance of alveolar fluid by increasing ion transport and paracellular resistance via a PLC-initiated rise of intracellular Ca(2+).     

Beta-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

Distal lung epithelial cells isolated from fetal rats were cultured (48 h) on permeable supports so that transepithelial ion transport could be quantified electrometrically. Unstimulated cells generated a short-circuit current (I(sc)) that was inhibited (~80%) by apical amiloride. The current is thus due, predominantly, to the absorption of Na(+) from the apical solution. Isoprenaline increased the amiloride-sensitive I(sc) about twofold. Experiments in which apical membrane Na(+) currents were monitored in basolaterally permeabilized cells showed that this was accompanied by a rise in apical Na(+) conductance (G(Na(+))). Isoprenaline also increased apical Cl- conductance (G(Cl-)) by activating an anion channel species sensitive to glibenclamide but unaffected by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). The isoprenaline-evoked changes in G(Na(+)) and G(Cl(minus sign)) could account for the changes in I(sc) observed in intact cells. Glibenclamide had no effect upon the isoprenaline-evoked stimulation of I(sc) or G(Na(+)) demonstrating that the rise in G(Cl-) is not essential to the stimulation of Na(+) transport.     

Extracellular UTP stimulates electrogenic bicarbonate secretion across CFTR knockout gallbladder epithelium

The loss of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial HCO(3)(-) secretion contributes to the pathogenesis of pancreatic and biliary disease in cystic fibrosis (CF) patients. Recent studies have investigated P2Y(2) nucleotide receptor agonists, e.g., UTP, as a means to bypass the CFTR defect by stimulating Ca(2+)-activated Cl(-) secretion. However, the value of this treatment in facilitating transepithelial HCO(3)(-) secretion is unknown. Gallbladder mucosae from CFTR knockout mice were used to isolate the Ca(2+)-dependent anion conductance during activation of luminal P2Y(2) receptors. In Ussing chamber studies, UTP stimulated a transient peak in short-circuit current (I(sc)) that declined to a stable plateau phase lasting 30-60 min. The plateau I(sc) after UTP was Cl(-) independent, HCO(3)(-) dependent, insensitive to bumetanide, and blocked by luminal DIDS. In pH stat studies, luminal UTP increased both I(sc) and serosal-to-mucosal HCO(3)(-) flux (J(s-->m)) during a 30-min period. Substitution of Cl(-) with gluconate in the luminal bath to inhibit Cl(-)/HCO(3)(-) exchange did not prevent the increase in J(s-->m) and I(sc) during UTP. In contrast, luminal DIDS completely inhibited UTP-stimulated increases in J(s-->m) and I(sc). We conclude that P2Y(2) receptor activation results in a sustained (30-60 min) increase in electrogenic HCO(3)(-) secretion that is mediated via an intracellular Ca(2+)-dependent anion conductance in CF gallbladder.     

Gossypol mediated DNA degradation

Gossypol, a polyphenolic compound isolated from cotton plant was found to degrade pBR322 DNA in vitro in a reaction which required the presence of a metal ion, a reducing agent (2-mercaptoethanol) and oxygen as revealed after agarose gel electrophoresis. Fe3+ and Co2+ showed maximum degradation whereas addition of Ca2+ and Mg2+ prevented the gossypol mediated DNA damage. Gossypol caused degradation of rat liver DNA incubated in vitro even in the absence of added metal ions and 2-mercaptoethanol. Incubation of intact rat liver nuclei with gossypol revealed DNA degradation and nuclei isolated from rats treated with gossypol in vivo showed higher susceptibility to DNA fragmentation when incubated with gossypol in vitro than control nuclei. EcoR1 and AIuI digestion of DNA isolated from gossypol treated rats gave clear cut evidence for DNA degradation. These observations indicate that gossypol is genotoxic and considerable care has to be exercised in its use.     

A primary culture of guinea pig gallbladder epithelial cells that is responsive to secretagogues

We have developed a cell culture of guinea pig gallbladder epithelial cells with which to study ion transport. When grown on permeable supports, the cultured epithelia developed a transepithelial resistance (R(t)) of approximately 500 Omega. cm(2). The epithelial cell origin of the cell culture was further confirmed by immunocytochemical localization of cytokeratin. Ionomycin and forskolin increased transepithelial voltage and short-circuit current (I(sc)) and decreased R(t). The response to ionomycin was transient, whereas that to forskolin was sustained. Both were attenuated by replacement of Cl(-) and/or HCO(3)(-). Mucosal addition of the anion transport inhibitors DIDS or diphenylamine-2-carboxylic acid (DPC) blocked the response to ionomycin. The response to forskolin was blocked by DPC but not by DIDS. Ionomycin, but not forskolin, increased intracellular Ca(2+) concentration in fura 2-loaded cells. PGE(2), histamine, vasoactive intestinal polypeptide, and secretin elicited a sustained increase in I(sc). Responses to ATP and CCK were transient. Thus cultured guinea pig gallbladder epithelia display the range of responses observed in the native tissue and are an appropriate model for studies of ion transport in gallbladder and intestinal epithelia.     

Lateral intercellular space volume as a determinant of CFTR-mediated anion secretion across small intestinal mucosa

Studies of full-thickness, small intestinal preparations have shown that maximal anion secretion [indexed by short-circuit current (I(sc))] during intracellular cAMP (cAMP(i)) stimulation is transient and followed by a decline toward baseline. Declining I(sc) is preceded by decreases in transepithelial conductance (G(t)), which in the small intestine reflects the lateral intercellular space (LIS) volume of the paracellular pathway. We hypothesized that decreases in LIS volume limit the magnitude and duration of cAMP(i)-stimulated anion secretion. Experimental manipulations to increase the patency of the LIS (assessed by G(t) and electron microscopy) were investigated for an effect on the magnitude of cAMP(i)-stimulated anion secretion (assessed by the I(sc) and isotopic fluxes) across murine small intestine. In control studies, changes of G(t) after cAMP(i) stimulation were associated with a morphological "collapse" of the LIS, which did not occur in intestine of CFTR-null mice. Removal of the outer intestinal musculature, exposure to a serosal hypertonic solution, or increased serosal hydrostatic pressure minimized reductions in G(t) and increased the cAMP(i)-stimulated I(sc) response. Increased I(sc) primarily resulted from increased Cl(-) secretion that was largely bumetanide sensitive. However, bumetanide-insensitive I(sc) was also increased, and similar increases occurred in the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1)-null intestine, indicating that activities of non-NKCC1 anion uptake proteins are also affected by LIS volume. Thus LIS patency is an important determinant of the magnitude and duration of CFTR-mediated anion secretion in murine small intestine. Decreases in LIS volume may limit the pool of available anions to basolateral transporters involved in transepithelial secretion.     

A calmodulin dependent Ca2+-activated K+ channel in the adipocyte plasma membrane

Increased membrane permeability (conductance) that is specific for K+ and directly activated by Ca2+ ions, has been identified in isolated adipocyte plasma membranes using the K+ analogue, 86Rb+. Activation of these K+ conductance pathways (channels) by free Ca2+ was concentration dependent with a half-maximal effect occurring at 32 +/- 4 nM free Ca2+ (n = 7). Addition of calmodulin further enhanced the Ca2+ activating effect on 86Rb+ uptake (K+ channel activity). Ca2+-dependent 86Rb+ uptake was inhibited by tetraethylammonium ion and low pH. It is concluded that the adipocyte plasma membrane possesses K+ channels that are activated by Ca2+ and amplified by calmodulin.     

The action of epinephrine on Madin-Darby canine kidney cells

We have used cultured monolayers of Madin-Darby canine kidney (MDCK) cells, which form epithelial layers of high transepithelial resistance, grown on Millipore filters, for transport studies. In the absence of hormones net ion transport is of small magnitude and is consistent with a net absorptive flow (apical to basal) of Na+. Epinephrine, effective only from the basolateral cell surface, stimulates a net secretion (basal to apical) of Cl-. A substantial portion of net Cl- secretion is inhibited by loop diuretics such as furosemide applied to the basolateral cell aspects. The participation of a diuretic-sensitive cotransport system for Na+, K+, and Cl-, similar to that found in other cells, in transepithelial Cl- flux is postulated. The action of catecholamines on MDCK cell adenylate cyclase and on a Ca2+-activated K+ conductance is described.     

DNA microarray analysis of neonatal mouse lung connects regulation of KDR with dexamethasone-induced inhibition of alveolar formation

Treating H441 cells with dexamethasone raised the abundance of mRNA encoding the epithelial Na(+) channel alpha- and beta-subunits and increased transepithelial ion transport (measured as short-circuit current, I(sc)) from <4 microA.cm(-2) to 10-20 microA.cm(-2). This dexamethasone-stimulated ion transport was blocked by amiloride analogs with a rank order of potency of benzamil >or= amiloride > EIPA and can thus be attributed to active Na(+) absorption. Studies of apically permeabilized cells showed that this increased transport activity did not reflect a rise in Na(+) pump capacity, whereas studies of basolateral permeabilized cells demonstrated that dexamethasone increased apical Na(+) conductance (G(Na)) from a negligible value to 100-200 microS.cm(-2). Experiments that explored the ionic selectivity of this dexamethasone-induced conductance showed that it was equally permeable to Na(+) and Li(+) and that the permeability to these cations was approximately fourfold greater than to K(+). There was also a small permeability to N-methyl-d-glucammonium, a nominally impermeant cation. Forskolin, an agent that increases cellular cAMP content, caused an approximately 60% increase in I(sc), and measurements made after these cells had been basolaterally permeabilized demonstrated that this response was associated with a rise in G(Na). This cAMP-dependent control over G(Na) was disrupted by brefeldin A, an inhibitor of vesicular trafficking. Dexamethasone thus stimulates Na(+) transport in H441 cells by evoking expression of an amiloride-sensitive apical conductance that displays moderate ionic selectivity and is subject to acute control via a cAMP-dependent pathway.     

Imidazoline Receptor Contributes to Ion and Water Transport across the Intestine of the Eel Acclimated to Sea Water

Guanabenz, an I2-imidazoline-related compound with high affinity for intestinal membrane of the eel (), enhanced the transepithelial potential difference (PD) and short-circuit current (Isc) from serosa to mucosa after pretreatment with isobutylmethylxanthine (IBMX), serotonin (5-HT) and methacholine (MCh). The mucosal effect of guanabenz was not mimicked by adrenaline, indicating that the mucosal guanabenz binding site is not adrenoceptors. The mucosal guanabenz enhanced the Isc in a concentration-dependent manner. Similar enhancement in the Isc was also obtained after addition of other imidazoline derivatives such as ST93, clonidine, ST91, naphazoline and UK14,304 into the mucosal fluid. On the other hand, the effect of guanabenz was completely blocked by mucosal RX821002 or efaroxan, another imidazoline derivatives. Since some imidazoline derivatives act as agonists and others as antagonist, there must exist imidazoline receptor on the mucosal side of the eel intestine. Accompanied by an increase in the PD, NaCl and water absorption across the intestine was also enhanced by mucosal guanabenz. To search for endogenous ligands for the imidazoline receptor, luminal fluid in the intestine of the seawater eels was collected. However, most luminal fluid was ineffective. Only one among 10 samples showed guanabenz-like activity, suggesting that the endogenous ligands is secreted into the lumen under restricted condition alone.     

Gd3+-sensitive Na+ transport across the integument of Hirudo medicinalis

Leeches Hirudo medicinalis were exposed to either artificial pond water (APW; 1 mM NaCl) or to high-salinity conditions (HS; 200 mM NaCl) for several days. The aim of the study was to assess whether transepithelial ion conductances in their dorsal integuments were affected by this long-term acclimation. In voltage-clamp experiments using Ussing-type chambers, the transepithelial potential V(T) was clamped to 0 mV, and amiloride-sensitive currents (I(ami)) and total Na(+) transport (I(Na)) were determined. Apical Ca(2+)-free conditions strongly increased I(ami) to a similar magnitude in both differently acclimated integuments. Apical application of the lanthanide gadolinium <0.1 mM decreased the short-circuit current (I(sc)). In contrast, higher concentrations up to 10 mM Gd(3+) upregulated I(sc) by an additional 90% in APW integuments and by an additional 300% in HS integuments. This Gd(3+) effect was due to a doubling of I(Na) in APW and a more than sixfold increase of I(Na) in HS integuments. In summary, the macroscopic electrophysiological variables, including I(Na), were generally not affected by long-term exposure to high salinity. However, the presence of Gd(3+)-sensitive Na(+) conductances or regulating structures were greatly upregulated during HS acclimation.     

Ion transport by sheep distal airways in a miniature chamber

Ion transport and the electric profile of distal airways of sheep lungs were studied in a miniature polypropylene chamber with a 1-mm aperture. Small airways with an inner diameter < 1 mm were isolated, opened longitudinally, and then mounted as a flat sheet onto the 1-mm aperture where it was glued and secured with an O-ring. Both sides of the tissue were bathed with identical physiological solutions at 37 degrees C and oxygenated. Pooled data from 27 distal airways showed an inner airway diameter of 854 +/- 22 (SE) microm and a transepithelial potential difference (PD) of 1.86 +/- 0.29 mV, lumen negative. Short-circuit current (I(sc)) was 25 +/- 3.5 microA/cm(2), tissue resistance was 96 +/- 14 Omega, and conductance was 15.2 +/- 1.7 mS/cm(2). At baseline, amiloride-sensitive Na transport accounted for 51% of I(sc) (change in I(sc) = 9.7 +/- 2.6 microA/cm(2); n = 8 airways), corresponding to 0.36 microeq. cm(-2). h(-1). Treatment with 0.1 mM bumetanide did not reduce the I(sc) (n = 5 airways). Exposure to 1 microM Ca ionophore A-23187 raised the I(sc) by 9 microA/cm(2) (47%; P < 0.03; n = 6 airways). The latter effect was blunted by bumetanide. Carbachol at 1 microM provoked a biphasic response, an initial rapid rise in I(sc) followed by a decline (n = 3 airways). There was no significant increase in PD or I(sc) in response to isoproterenol or dibutyryl cAMP. The data suggest that Na absorption constitutes at least 50% of baseline transport activity. Cl or other anion secretion such as HCO(3) appears to be present and could be stimulated by raising intracellular Ca.     

Regulation of goby intestinal ion absorption by the calcium messenger system

The role of the calcium messenger system in the regulation of ion absorption across the teleost intestine was studied using pharmacological intervention. Radiochloride transport was independent of external Ca2+ over the range 10 microM to 2.5 mM. Treatment with the Ca2+ ionophore A23187 (to hyperpolarization of the apical membrane potential of intestinal epithelial cells. The Ca2+-calmodulin antagonists trifluoperazine (TFP) and calmidazolium (R24571) produced opposite effects, i.e., stimulation of Cl- absorption and cellular depolarization. Treatment with TFP or R24571 will block or override the inhibitory action of A23187. These data suggest a regulatory role for Ca2+ in the control of intestinal NaCl absorption and mediation via calmodulin.     

Molecular identification of the apical Ca2+ channel in 1, 25-dihydroxyvitamin D3-responsive epithelia

In mammals, the extracellular calcium concentration is maintained within a narrow range despite large variations in daily dietary input and body demand. The small intestine and kidney constitute the influx pathways into the extracellular Ca2+ pool and, therefore, play a primary role in Ca2+ homeostasis. We identified an apical Ca2+ influx channel, which is expressed in proximal small intestine, the distal part of the nephron and placenta. This novel epithelial Ca2+ channel (ECaC) of 730 amino acids contains six putative membrane-spanning domains with an additional hydrophobic stretch predicted to be the pore region. ECaC resembles the recently cloned capsaicin receptor and the transient receptor potential-related ion channels with respect to its predicted topology but shares less than 30% sequence homology with these channels. In kidney, ECaC is abundantly present in the apical membrane of Ca2+ transporting cells and colocalizes with 1,25-dihydroxyvitamin D3-dependent calbindin-D28K. ECaC expression in Xenopus oocytes confers Ca2+ influx with properties identical to those observed in distal renal cells. Thus, ECaC has the expected properties for being the gatekeeper of 1,25-dihydroxyvitamin D3-dependent active transepithelial Ca2+ transport.     

Ion channels and transporters in the electroreceptive ampullary epithelium from skates.

Two ampullary epithelial properties necessary for electroreception were used to identify the types of ion channels and transporters found in apical and basal membranes of ampullary receptor cells of skates and to assess their individual role under voltage-clamp conditions. The two essential properties are (1) a steady-state negative conductance generated in apical membranes and (2) a small, spontaneous current oscillation originating in basal membranes (Lu and Fishman, 1995). The effects of pharmacological agents and ion substitutions on these properties were evaluated from transorgan or transepithelial complex admittance determinations in the frequency range 0.125 to 50 Hz measured in individual, isolated ampullary organs. In apical membranes, L-type Ca channels were found to be responsible for generation of the steady-state negative conductance. In basal membranes, K and Ca-dependent Cl (Cl(Ca)) channels were demonstrated to contribute to a net positive membrane conductance. L-type Ca channels were also evident in basal membranes and are thought to function in synaptic transmission from the electroreceptive epithelium to the primary afferent nerve. In addition to ion channels in basal membranes, two transporters (Na+/K+ pump and Na(+)-Ca+ exchanger) were apparent. Rapid (minutes) cessation of the current oscillation after blockage of any of the basal ion channels (Ca, Cl(Ca), K) suggests critical involvement of each of these channel types in the generation of the oscillation. Suppression of either Na+/K+ transport or Na(+)-Ca2+ exchange also eliminated the oscillation but at a slower rate, indicating an indirect effect.     

Effect of ibuprofen and rofecoxib transport parameters in the frog corneal epithelium

Effects of cyclooxygenase (COX) inhibitors on transport parameters of the frog corneal epithelium were studied. Epithelial cells of the intact cornea were impaled with microelectrodes. Under short-circuit current (I(sc)) conditions, 10(-4) M ibuprofen (IBU) (non-specific COX inhibitor) or 5 x 10(-5) M rofecoxib (COX-2 inhibitor) were added to the tear solution. With ibuprofen, I(sc) decreased by 1.0 from 3.1 microA/cm2; intracellular potential, V(o), depolarized by 14.2 from -56.9 mV; IBU did not affect the transepithelial conductance, g(t), or the apical membrane fractional resistance, fR(o). With rofecoxib, I(sc) decreased by 0.9 from 4.3 microA/cm2; V(o) depolarized by 18 from -62.4 mV; g(t) significantly increased by 0.03 from 0.37 ms/cm2; and fR(o) decreased by 12 from 50. Basolateral membrane K+ and apical membrane Cl- partial conductances were studied by the ion substitution method. Depolarization of V(o) by an increase in stromal K+ from 4 to 79 mM was smaller with IBU (17.5 mV) or rofecoxib (19.2 mV) than without the inhibitors (29.1 and 29.3 mV, respectively). Depolarization of V(o), by a decrease in tear Cl- from 81 to 8.1 mM, was abolished by the COX inhibitors. Decrease in I(sc) and V(o) can be explained by a decrease in the K+ and Cl-? conductances. Experiments with amphotericin B ruled out a major effect of the inhibitors on the Na+/K+ ATPase pump.     

Protease-activated receptor-2-mediated inhibition of ion transport in human bronchial epithelial cells

A cytoprotective role for protease-activated receptor-2 (PAR2) has been suggested in a number of systems including the airway, and to this end, we have studied the role that PARs play in the regulation of airway ion transport, using cultures of normal human bronchial epithelial cells. PAR2 activators, added to the basolateral membrane, caused a transient, Ca2+-dependent increase in short-circuit current (I(sc)), followed by a sustained inhibition of amiloride-sensitive I(sc). These phases corresponded with a transient increase in intracellular Ca2+ concentration and then a transient increase, followed by decrease, in basolateral K+ permeability. After PAR2 activation and the addition of amiloride, the forskolin-stimulated increase in I(sc) was also attenuated. By contrast, PAR2 activators added to the apical surface of the epithelia or PAR1 activators added to both the apical and basolateral surfaces were without effect. PAR2 may, therefore, play a role in the airway, regulating Na+ absorption and anion secretion, processes that are central to the control of airway surface liquid volume and composition.     

Short-chain fatty acids enhance diffusional ca transport in the epithelium of the rat cecum and colon

We examined the effect of short-chain fatty acids (SCFAs) on Ca absorption from the large intestine in rats in vitro. An Ussing-type chamber technique was used to determine the net transport of Ca from the luminal side to the basolateral side of isolated epithelium in cecum and colon preparations. The concentration of Ca in the serosal and mucosal Tris buffer solution was 1.25 mM and 10 mM, respectively. Both solutions were warmed at 37 degrees C and bubbled with 95% O2 and 5% CO2. During and after the incubation period (30 min or 60 min), the Ca concentration in the serosal medium was determined and the net transepithelial Ca transport was evaluated. The addition of 80 mM acetic acid, 40 mM propionic acid and 10 mM butyric acid to the mucosal medium increased net Ca absorption (about 300%) in the cecum and colon. An individual application of acetic, propionic or butyric acid (0.01 to 100 mM) to the mucosal medium also increased net Ca absorption at doses of 10 mM and /or 100 mM in the cecum and colon. An increase in solute concentration in the mucosal medium by addition of glycerol or PGE400, or a decrease in pH (7.0-3.0) by addition of HCl did not affect transepithelial Ca transport. We concluded that SCFAs affect the epithelial tissue and promote Ca absorption from the large intestine in vitro. The enhancement of Ca transport induced by SCFAs might be involved in the paracellular transport mechanism.