Intestinal secretagogues increase cytosolic free Ca2+ concentration and K+ conductance in a human intestinal epithelial cell line

Cholinergic regulation of epithelial ion transport in the mammalian intestine

Acetylcholine (ACh) is critical in controlling epithelial ion transport and hence water movements for gut hydration. Here we review the mechanism of cholinergic control of epithelial ion transport across the mammalian intestine. The cholinergic nervous system affects basal ion flux and can evoke increased active ion transport events. Most studies rely on measuring increases in short-circuit current (ISC = active ion transport) evoked by adding ACh or cholinomimetics to intestinal tissue mounted in Ussing chambers. Despite subtle species and gut regional differences, most data indicate that, under normal circumstances, the effect of ACh on intestinal ion transport is mainly an increase in Cl- secretion due to interaction with epithelial M3 muscarinic ACh receptors (mAChRs) and, to a lesser extent, neuronal M1 mAChRs; however, AChR pharmacology has been plagued by a lack of good receptor subtype-selective compounds. Mice lacking M3 mAChRs display intact cholinergically-mediated intestinal ion transport, suggesting a possible compensatory mechanism. Inflamed tissues often display perturbations in the enteric cholinergic system and reduced intestinal ion transport responses to cholinomimetics. The mechanism(s) underlying this hyporesponsiveness are not fully defined. Inflammation-evoked loss of mAChR-mediated control of epithelial ion transport in the mouse reveals a role for neuronal nicotinic AChRs, representing a hitherto unappreciated braking system to limit ACh-evoked Cl- secretion. We suggest that: i) pharmacological analyses should be supported by the use of more selective compounds and supplemented with molecular biology techniques targeting specific ACh receptors and signalling molecules, and ii) assessment of ion transport in normal tissue must be complemented with investigations of tissues from patients or animals with intestinal disease to reveal control mechanisms that may go undetected by focusing on healthy tissue only.

The protective effects of Aspergillus candidus metabolites against hydrogen peroxide-induced oxidative damage to Int 407 cells

The protective effects of 3,3"-di-OH-terphenyllin (3,3"-DHT) and 3-OH-terphenyllin (3-HT), two secondary metabolites produced by Aspergillus candidus (CCRC31543), against hydrogen peroxide-induced oxidative damage to Int 407 cells, were evaluated. The results showed that H2O2 caused an increase in lactate dehydrogenase (LDH) leakage and DNA damage in Int 407 cells; however, the addition of 3,3"-DHT and 3-HT significantly reduced this effect (P<0.05). Intracellular reactive oxygen species (ROS) formation in Int 407 cells pre-incubated with 3,3"-DHT and 3-HT was decreased by 30 and 35%, respectively. The activity of glutathione peroxidase and catalase in Int 407 cells pre-incubated with 3,3"-DHT was increased 25 and 33%, respectively; however, the activity of catalase was increased 30% in Int 407 cells pre-incubated with 3-HT. The activity of glutathione reductase in Int 407 cells pre-incubated with 3,3"-DHT and 3-HT was decreased 36 and 21%, respectively. The intracellular glutathione level did not vary (P>0.05), but oxidized glutathione levels increased when Int 407 cells were pre-incubated with these two compounds. These findings suggest that 3,3"-DHT and 3-HT have the ability to protect against oxidative damage to Int 407 cells and that this protective effect may be associated with the ability to reduce ROS formation and increase catalase activity.

Receptor-mediated facilitation of cell volume regulation by swelling-induced ATP release in human epithelial cells

Osmotic swelling induces the release of intracellular ATP in a number of cell types. In the immediate vicinity of the cell surface, released ATP has been shown to reach a concentration high enough to stimulate P2-purinergic receptors in a human epithelial cell line, Intestine 407. The role of released ATP in the regulatory volume decrease (RVD) after cell swelling was thus studied in Intestine 407 cells. The RVD was suppressed by an ATP hydrolyzing enzyme, apyrase, or by a purinergic receptor antagonist, suramin. Extracellular application of ATP accelerated the RVD rate in a concentration-dependent manner. An increase in the cytosolic free-Ca(2+) concentration was induced by a hypotonic challenge, and the swelling-induced Ca(2+) response was partially suppressed by apyrase or suramin. A rise in cytosolic Ca(2+) was also induced by extracellular application of ATP or UTP, but not ADP, 2-methylthio-ATP or alpha,beta-methylene ATP. The ATP-induced Ca(2+) response was blocked by suramin. Therefore, it is concluded that RVD is facilitated by ATP, which is released upon cell swelling, by augmenting intracellular Ca(2+) rise via the stimulation of purinergic (P2Y(2)) receptors in the human epithelial cell.

Swelling-induced, CFTR-independent ATP release from a human epithelial cell line: lack of correlation with volume-sensitive cl(-) channels

To examine a possible relation between the swelling-induced ATP release pathway and the volume-sensitive Cl(-) channel, we measured the extracellular concentration of ATP released upon osmotic swelling and whole-cell volume-sensitive Cl(-) currents in a human epithelial cell line, Intestine 407, which lacks expression of cystic fibrosis transmembrane conductance regulator (CFTR). Significant release of ATP was observed within several minutes after a hypotonic challenge (56-80% osmolality) by the luciferin/luciferase assay. A carboxylate analogue Cl(-) channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate, suppressed ATP release in a concentration-dependent manner with a half-maximal inhibition concentration of 6.3 microM. However, swelling-induced ATP release was not affected by a stilbene-derivative Cl(-) channel blocker, 4-acetamido-4'-isothiocyanostilbene at 100 microM. Glibenclamide (500 microM) and arachidonic acid (100 microM), which are known to block volume-sensitive outwardly rectifying (VSOR) Cl(-) channels, were also ineffective in inhibiting the swelling-induced ATP release. Gd(3+), a putative blocker of stretch-activated channels, inhibited swelling-induced ATP release in a concentration-dependent manner, whereas the trivalent lanthanide failed to inhibit VSOR Cl(-) currents. Upon osmotic swelling, the local ATP concentration in the immediate vicinity of the cell surface was found to reach approximately 13 microM by a biosensor technique using P2X(2) receptors expressed in PC12 cells. We have raised antibodies that inhibit swelling-induced ATP release from Intestine 407 cells. Earlier treatment with the antibodies almost completely suppressed swelling-induced ATP release, whereas the activity of VSOR Cl(-) channel was not affected by pretreatment with the antibodies. Taking the above results together, the following conclusions were reached: first, in a CFTR-lacking human epithelial cell line, osmotic swelling induces ATP release and increases the cell surface ATP concentration over 10 microM, which is high enough to stimulate purinergic receptors; second, the pathway of ATP release is distinct from the pore of the volume-sensitive outwardly rectifying Cl(-) channel; and third, the ATP release is not a prerequisite to activation of the Cl(-) channel.

The action of 5-hydroxytryptamine on normal and cystic fibrosis mouse colon: effects on secretion and intracellular calcium

The ability of mouse colon to generate a secretory response to stimulation by 5-hydroxytryptamine (5-HT) was investigated in intact colonic sheets mounted in Ussing chambers. A preparation of intact isolated crypts was used to determine whether 5-HT action was associated with an elevation of cytosolic calcium levels, measured using the calcium-sensitive fluorescent dye, fura-2. 5-HT increased the short-circuit current, an effect that was inhibited by 55% in the absence of chloride and by 83% in the presence of serosal frusemide, consistent with the stimulation of electrogenic chloride secretion. This was confirmed by the observation that colonic tissue from transgenic cystic fibrosis mice (n = 4) failed to respond to 5-HT, although wild-type tissues generated an increased short-circuit current of 52.4+/-1.1 microAcm(-2) (n = 9). The electrical response to 5-HT was calcium-dependent. 5-HT action was unaffected by tetrodotoxin and was not mimicked by the 5-HT3 agonist 1-phenylbiguanide, indicating that neural mechanisms are not involved. The cyclooxygenase inhibitor indomethacin, however, reduced the 5-HT-induced rise in short-circuit current by 73%, suggesting that prostaglandin production contributes to the response. Stimulation of crypts with acetylcholine elicited an increase in cytosolic calcium levels, but no such response was detected on application of 5-HT (10(-6) to 10(-4) M), suggesting that 5-HT does not directly modulate intracellular calcium in colonic crypt cells. It is concluded that mouse colon responds to 5-HT challenge with a stimulation of electrogenic chloride secretion and that this effect is mediated by indirect mechanisms that might involve immune elements within the colonic wall.

Glutathione transport system in human small intestine epithelial cells

The present study characterizes for the first time a GSH specific transporter in a human intestinal epithelial cell line (I407). GSH metabolism is very important for the antioxidant and detoxifying action of intestine and for the maintenance of the luminal thiol-disulfide ratio involved in regulation mechanisms of the protein activity of epithelial cells. GSH level decreases have been related to physio-pathological alterations either of intestine or other organs. GSH specific transport systems have been identified in membranes of various cell types of rat, mice and rabbit. The presence of a Na+-independent transport system of GSH is confirmed by the similar behaviour of GSH uptake time-courses when Na+ in extracellular uptake medium was replaced with choline+ or K+ as well as by kinetic saturation and by the trans-stimulation effect on GSH uptake in GSH preloaded cells. Moreover, this transporter is activated when cations are present in extracellular medium and it is affected by membrane potential changes with an increase in GSH uptake values when membrane depolarization occurs. The present results also show a remarkable affinity and specificity of this transporter for GSH; in fact, Km value is very low (90 +/- 20 microM) and only compounds strictly related to GSH structure, such as GSH S-conjugates and GSH-ethyl ester, inhibit GSH uptake in 1407 cells. Finally, a possible hormonal control and modulation by the thiol-disulfide status of GSH transporter activity is suggested.

Identification and function of type-2 and type-3 ryanodine receptors in gut epithelial cells

Reverse transcription-PCR (RT-PCR) techniques were used to identify the expression of ryanodine receptor (RyR) isoforms in gut epithelial cells. Restriction digest and sequence analysis of the PCR product showed the presence of RyR 2 and RyR 3. [3H]Ry binding studies on a microsome preparation, in a high-salt buffer, showed specific binding with an EC50 of 15 microM. In order to determine a potential functional role for these RyRs, we first characterized the response of the cells to acetylcholine. At all concentrations used acetylcholine induced sinusoidal cytosolic Ca2+ concentration ([Ca2+]i) oscillations. In response to 10(-4) M acetylcholine, levels of inositol 1,4,5-trisphosphate (InsP3) showed a peak of six times the basal level, at 30 s after stimulation. Application of caffeine alone failed to elicit a rise in cytosolic Ca2+. However, caffeine (5-50 mM) did rapidly and reversibly inhibit the acetylcholine-induced [Ca2+]i oscillations. The effects of Ry were more complex. Applied alone, Ry had no effect on the [Ca2+]i signal. When applied during agonist-evoked [Ca2+]i oscillations, Ry (10 microM) slowly blocked the response. In the continuous presence of Ry (10 microM) a short application of acetylcholine elicited a [Ca2+]i response that continued as oscillations even when the agonist was removed. The oscillations, in the presence of Ry (10 microM) but absence of agonist, were blocked either by removal of extracellular Ca2+ or by an application of a higher concentration of Ry (100 microM). These effects are consistent with the known use-dependence and dose-dependence for Ry action at the RyR. We conclude that the RyR 2 and RyR 3, identified by RT-PCR, play a central role in [Ca2+]i oscillations in gut epithelial cells.

Mucosal protection through active intestinal secretion: neural and paracrine modulation by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is widely distributed within enteroendocrine cells and neurons of the digestive tract. It stimulates active anion secretion in the intestinal epithelium, an effect which promotes the dilution and elimination of luminal pathogens. The intestinal secretory effects of 5-HT appear to be mediated in part by epithelial 5-HT2 like receptors that are linked to phosphatidylinositol turnover. In addition, 5-HT appears to interact with 5-HT3 or 5-HT4 receptors on submucosal neurons to elicit neuronal depolarization and an associated increase in intestinal secretion. Although the precise cellular mechanisms by which 5-HT modulates mucosal ion transport are incompletely understood, it is clear that 5-HT plays an important role as a intestinal secretagogue in certain diarrheal states, in laxation, and in intestinal hypersensitivity reactions.

NG-nitro-L-arginine methyl ester modulates intestinal secretion and motility produced by carbachol

The effects of the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine methyl ester, on carbachol-induced diarrhoea, fluid accumulation and motility changes were studied. Pretreatment of mice with NG-nitro-L-arginine methyl ester (1-25 mg/kg i.p.) and NG-nitro-L-arginine (2.5-50 mg/kg i.p.) but not NG-nitro-D-arginine methyl ester (25 mg/kg i.p.) prevented in a dose-related manner the carbachol (0.5 mg/kg i.p.)-induced diarrhoea in mice. L-Arginine (150-1500 mg/kg i.p.) administered to mice pretreated with NG-nitro-L-arginine methyl ester counteracted the antidiarrhoeal activity of NG-nitro-L-arginine methyl ester in a dose-related manner. Pretreatment of rats with NG-nitro-L-arginine methyl ester (2.5-25 mg/kg i.p.) decreased the intestinal fluid accumulation induced by carbachol in rats. NG-Nitro-D-arginine methyl ester was without effect. Intraperitoneal pretreatment of rats with NG-nitro-L-arginine methyl ester (2.5-25 mg/kg) reduced the increase in small intestinal transit induced by carbachol. NG-nitro-L-arginine methyl ester had no effect. These results provide evidence that nitric oxide may play a role in diarrhoea, intraluminal fluid accumulation and motility changes induced by carbachol.

Histamine modulates three types of K+ current in a human intestinal epithelial cell line

K+ conductance species in a human intestinal epithelial cell line (Intestine 407) were studied in connection with their sensitivities to an intestinal secretagogue, histamine, using the tight-seal whole-cell patch-clamp technique. Applications of positive command pulses rapidly induced outward K+ currents. The conductance became progressively larger with increasing command voltages, exhibiting an outwardly rectifying current voltage relation. Inward K+ currents were also rapidly activated upon applications of hyperpolarizing pulses at potentials negative to the equilibrium potential of K+ (EK), and the conductance inwardly rectified. Application of a Ca2+ ionophore, ionomycin, brought about activation of additional K+ currents. An inhibitor of protein kinase C, polymyxin B, did not affect the ionomycin-induced response. Histamine (10-200 microM) also activated a similar K+ current which was abolished by cytosolic Ca2+ chelation. Under conditions where Ca2+ mobilization was minimized, histamine was found to significantly augment inwardly rectifying K+, but suppress outwardly rectifying K+, currents. Polymyxin B blocked these effects of histamine. An activator of protein kinase C, 1-oleoyl-2-acetylglycerol, mimicked the histamine effects. It is concluded that the intestinal epithelial cell has three distinct types of K+ conductance and that histamine modulates not only Ca(2+)-activated K+ conductance via Ca2+ mobilization, but also inward- and outward-rectifier K+ conductances via activation of protein kinase C.

Muscarinic stimulation of gallbladder epithelium. II. Fluid transport, cell volume, and ion permeabilities

Activation of muscarinic receptors in the fluid-absorptive epithelium of the Necturus gallbladder elevates cytosolic Ca2+ concentration, transiently hyperpolarizes the cell membrane voltages, and decreases the apparent fractional resistance of the apical membrane [G. A. Altenberg, M. Subramanyam, J. S. Bergmann, K. M. Johnson, and L. Reuss. Am. J. Physiol. 265 (Cell Physiol. 34): C1604-C1612, 1993]. In these studies, we show that at the peak of the hyperpolarization both apical and basolateral membrane resistances (Ra and Rb, respectively) decreased, but in 2-3 min Ra returned to control values while Rb rose to a level approximately 60% higher than control. The acetylcholine (ACh)-induced decrease in Ra is caused by activation of apical membrane maxi K+ channels secondary to elevation of cytosolic Ca2+ concentration. The increase in Rb is due to decreases in K+ and Cl- conductances. ACh had no effects on cell KCl content or water volume, although K+ conductance transiently increased. These results can be explained by the changes in basolateral membrane conductances. ACh did not alter fluid absorption. In conclusion, ACh has complex time-dependent effects on K+ and Cl- electrodiffusive permeabilities without measurable changes in cell volume or in the rate of transepithelial fluid transport.

Antagonism of kinin effects on epithelial by Hoe 140: apparently competitive and non-competitive interactions

1. Hoe-140, a potent kinin receptor antagonist, was investigated for its ability to inhibit the effects of lysylbradykinin (kallidin) on a cultured colonic epithelium, HCA-7 Colony 29, derived from a human adenocarcinoma. 2. Measurements of electrogenic chloride secretion (as short circuit current), and of intracellular Ca2+ (from Fura-2 fluorescence) were used to assess the action of lysylbradykinin in the absence and presence of Hoe 140. 3. From short circuit current data, Hoe 140 appeared to be a competitive antagonist with a Ki value of 5 nM. However, with measurements of intracellular Ca2+ Hoe 140 was apparently a non-competitive antagonist with a Ki of between 4-6 nM. 4. Because of the unexpected finding of non-competitive antagonism, measurements were made with a second antagonist pair, histamine and mepyramine. Mepyramine behaved as a competitive antagonist against responses to histamine with a Ki value of approximately 5 nM when short circuit current measurements were evaluated. However, when intracellular Ca2+ concentration was used as a measure mepyramine, 30 nM, produced a near parallel shift in the response curve, but at 100 nM the maximal response was depressed. 5. The reasons why the apparent type of antagonism depends upon the method of measurement is discussed, bearing in mind that the increase in intracellular Ca2+ is a signal which precedes the increase in short circuit current.

Volume-regulatory Cl- channel currents in cultured human epithelial cells

1. During osmotic swelling, cultured human small intestinal epithelial cells (Intestine 407) exhibited activation of large Cl- currents under the patch-clamp whole-cell configuration. The volume-sensitive Cl- conductance was independent of intracellular Ca2+ and cyclic AMP. 2. The anion permeability sequence of the current was SCN- > I- > Br- > Cl- > F- > gluconate-, corresponding to Eisenman's sequence I. 3. Cl- currents were instantaneously activated by command pulses in a range of -120 to +45 mV. At potentials more positive than +50 mV the current showed a time-dependent inactivation. This inactivation was accelerated by increased depolarization. The instantaneous current-voltage relationship rectified in the outward direction. 4. A stilbene-derivative Cl- channel blocker, 4-acetamido-4'-isothiocyanostilbene (SITS), inhibited the Cl- current at micromolar concentrations. SITS facilitated inactivation at positive potentials. Outward currents were more prominently suppressed by SITS than inward currents. The concentrations required for 50% inhibition (IC50) of outward and inward currents were 1.5 and 6 microM, respectively. The outward and inward currents were equally inhibited by a carboxylate analogue Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) or diphenylamine-2-carboxylate (DPC) at higher doses (IC50 = 25 for NPPB or 350 microM for DPC). Inactivation kinetics at large depolarizations was not affected by NPPB or DPC. 5. The Cl- current was blocked by an unsaturated fatty acid, arachidonic acid (IC50 = 8 microM). Arachidonic acid was still effective in the presence of inhibitors of lipoxygenase (nordihydroguaiaretic acid, 10 microM), cyclo-oxygenase (indomethacin, 10 microM) and protein kinase C (polymyxin B, 30 microM). The Cl- current was also sensitive to another cis unsaturated fatty acid, oleic acid, which is not a substrate for oxygenases. A trans isomer of oleate, elaidic acid, and a saturated fatty acid, palmitic acid, were ineffective. 6. Single Intestine 407 cells exposed to a hypotonic solution showed a regulatory volume decrease after initial osmotic swelling. The volume regulation was abolished by SITS, NPPB, arachidonate and oleate, but not by elaidate and palmitate. 7. It is concluded that outwardly rectifying Cl- channels, which are sensitive to arachidonic acid, are activated upon osmotic swelling and involved in the subsequent cell volume regulation.

Membrane conductance and cell volume changes evoked by vasoactive intestinal polypeptide and carbachol in small intestinal crypts

We have used the perforated-patch whole-cell recording mode of the patch-clamp technique to monitor membrane potential and measured cell volume changes by image analysis, to determine the nature of the response to secretagogues of isolated whole guinea-pig small-intestinal crypts. Vasoactive intestinal polypeptide (VIP) produced a dose-dependent depolarisation (EC50 = 30 nM) and an increase in membrane conductance that could be potentiated by carbachol. Similar depolarisations were observed with forskolin. The depolarisation induced by 100 nM VIP was smaller when pipette [Cl-] was 60 mM than when it was 145 mM, suggesting an effect through Cl- conductance activation. Carbachol alone produced a hyperpolarisation (EC50 = 2 microM). The Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) produced a small hyperpolarization. When VIP was added in the presence of NPPB, the depolarisation was observed instead, consistent with the parallel activation of a K+ conductance. Both carbachol (100 microM) and VIP (100 nM) induced a 25%-30% shrinkage of crypts, which was maximal 8 min after addition of the secretagogue. The induced shrinkage was sustained in the continued presence of agonist and was reversed upon washout. Shrinkage induced by the agonists was abolished by increasing extracellular K+ from 6 mM to 20 mM and was inhibited partially in the presence of 100 microM anthracene-9-carboxylic acid in the bath. The decrease in volume induced by 100 nM VIP was totally abolished in the presence of 100 microM NPPB. The results are consistent with the view that both VIP and carbachol induce secretion in small-intestinal crypts.

Carbachol-activated calcium entry into HT-29 cells is regulated by both membrane potential and cell volume

Intracellular Ca2+ ([Ca2+]i) was measured in single Cl(-)-secretory HT-29/B6 colonic carcinoma cells with the Ca2+ probe fura-2 and digital imaging microscopy. Resting [Ca2+]i was 63 +/- 3 nM (n = 62). During treatment with the muscarinic agonist carbachol, [Ca2+]i rapidly increased to 901 +/- 119 nM and subsequently reached a stable level of 309 +/- 23 nM, which depended on Ca2+ entry into the cells from the extracellular solution. The goal of this study was to characterize the Ca2+ entry pathway across the cell membrane with respect to its dependence on membrane potential and cell volume. Under resting conditions [Ca2+]i showed no apparent dependence on either potential or cell volume. After stimulating Ca2+ entry with carbachol (100 microM), [Ca2+]i increased with hyperpolarization (low-K+ or valinomycin treatment) and decreased with depolarization (high-K+ or gramicidin treatment) of the cell, as expected from changes in driving force for Ca2+ entry. In stimulated cells, hypotonic solutions caused [Ca2+]i to increase, whereas hypertonic solutions blocked Ca2+ entry. The shrinkage-induced decreases in [Ca2+]i were only slightly affected when the membrane potential was increased with valinomycin, suggesting that shrinkage directly affects the carbachol-activated Ca2+ conductance. In contrast, the swelling-induced increase in [Ca2+]i was significantly reduced in valinomycin-treated cells, suggesting an indirect dependence on a swelling-activated K+ conductance. Thus, carbachol-stimulated Ca2+ entry is under the dual control of membrane potential and cell volume. This mechanism may serve as a regulatory influence that determines the extent of Ca2+ influx during cholinergic stimulation.

A basolateral K+ conductance modulated by carbachol dominates the membrane potential of small intestinal crypts

The contribution of possible ionic conductances to the membrane potential (Em) of cells in guinea-pig small intestinal crypts has been studied using the nystatin "perforated-patch" approach in current-clamp experiments. Changes in extracellular K+ produced shifts in Em, with a 37 mV change in potential per ten-fold increase in extracellular K+ concentration. Reduction of extracellular Cl- by 130 mM led to a 7 mV hyperpolarisation while Na+ replacement was without effect on Em. The muscarinic agonist carbachol produced a hyperpolarisation which could be ascribed to an increase in basolateral K+ conductance. This effect was sustained in the presence of extracellular Ca2+ but was transient in its absence. We conclude that the conductance of the basolateral membrane of small intestinal crypts is mainly K+ selective and can be reversibly increased by muscarinic activation.

Synchronous transporting activity in epithelial cells in relation to intracellular calcium concentration

Cultured monolayers of human sweat-gland epithelia have been used to measure electrogenic sodium transport, as short-circuit current, and intracellular Ca2+ concentration ([Ca]i) from Fura-2 fluorescence. The sodium currents in response to the agonists lysylbradykinin, histamine and carbachol show oscillatory behaviour in the 1-2 per minute frequency range. The oscillations can be terminated either by using specific antagonists or with amiloride, which prevents sodium entry into the epithelium. Oscillatory behaviour is also seen when [Ca]i is measured and occurs in the same frequency range. Sodium transport in these cultured epithelia is thought to result from an increase in [Ca]i, which in turn activates calcium-sensitive potassium channels, so increasing the electrochemical gradient for sodium entry. The oscillatory behaviour implies that the epithelial cells behave in synchrony to increase [Ca]i, so inducing synchronous changes in sodium current. It is shown that the behaviour is not unique to sodium-absorbing epithelia, and the possible utility of synchronous behaviour in epithelial tissues is discussed.

Relation of anion secretory activity to intracellular Ca2+ in response to lysylbradykinin and histamine in a cultured human colonic epithelium

A cultured human epithelial cell line, Colony 29, has been used to investigate the relation between anion secretion and intracellular Ca2+ concentration (Cai) in response to the secretagogues, lysylbradykinin (LBk) and histamine. Anion secretion was measured as short-circuit current (SCC) responses in epithelia cultured on previous supports. Cai was measured both in cell suspensions and epithelial monolayers using Fura-2 fluorescence. While it is concluded that raised Cai is responsible for anion secretion the relationship is complex. For both secretagogues there is a receptor reserve, that is the maximal Cai increase is greater than that required to cause a maximal secretory response. By examining the interactions between maximally effective concentrations of LBk and histamine it was shown that neither the SCC nor Cai responses behaved additively. From observations in the absence of external Ca2+ it was concluded that both secretagogues cause Ca2+ release from the same intracellular source, but that in normal conditions Ca2+ derived from intracellular and extracellular sources is responsible for the full effect.

Modulation of Na+, Cl- and HCO3- transport by carbachol in pig distal jejunum

Acetylcholine and cholinomimetics such as carbachol are potent stimulants of epithelial Cl- secretion in the small and large intestines of several mammalian species. In this study, the effects of carbachol were characterized in vitro on active ion transport in sheets of submucosa-mucosa from the distal jejunum of swine. Carbachol (10 microM) produced an increase in serosa-positive short-circuit current (Isc) in this tissue after its serosal, but not luminal administration. The Na(+)-K(+)-Cl- transport blocker bumetanide (10 microM) produced a 50% decrease in the carbachol-induced Isc elevation after its serosal administration. Peak increases in Isc evoked by carbachol were significantly reduced by 60-85% in tissues bathed in media lacking Cl-, HCO3-, or both anions. The initial drug-induced increase in net charge transfer from serosa to lumen was dependent upon both HCO3- and Cl-, whereas sustained elevations in charge transfer were dependent upon extracellular Cl- only. Radiotracer flux analyses revealed that the drug decreased net Na+ absorption and increased Cl- secretion. In the absence of HCO3-, carbachol decreased Cl- absorption. The effects of carbachol on HCO3- transport were examined by pH-stat titration. The drug rapidly alkalinized the luminal medium immediately after its serosal administration. These results suggest that carbachol stimulates electrogenic anion secretion in the mucosa of the porcine distal jejunum. Furthermore, the ability of carbachol to inhibit spontaneous Na+ absorption is dependent upon extracellular HCO3-.

Biphasic rises in cytosolic free Ca2+ in association with activation of K+ and Cl- conductance during the regulatory volume decrease in cultured human epithelial cells

During exposure to a hypotonic solution (55% osmolarity), cultured human epithelial (Intestine 407) cells exhibit a regulatory volume decrease after osmotic swelling. This process is known to involve parallel activation of volume-regulatory K+ and Cl- conductances. Biphasic increase in the cytosolic free Ca2+ concentration ([Ca2+]i) were observed by microspectrofluorometry, in fura-2-loaded cells upon hypotonic stress. Electrophysiological studies with Ca2(+)-selective and conventional microelectrodes indicated that a biphasic [Ca2+]i increase was associated with a biphasic hyperpolarization, whereas an interposing [Ca2+]i decrease coincided with a transient depolarization. A Ca2+ ionophore, ionomycin, produced a sustained Ca2+ increase and a prolonged hyperpolarization which was sensitive to the K+ channel blocker, quinine. A subsequent hypotonic challenge gave rise to a depolarization, which was sensitive to a stilbene-derivative Cl- channel blocker, without inducing further changes in [Ca2+]i. Normal cell volume regulation in a hypo-osmotic medium could take place even in the presence of ionomycin. It is concluded that a biphasic [Ca2+]i increase is closely associated with activation of the volume-regulatory K+ conductance, and that the interposing [Ca2+]i decrease is neither a causative factor for activation of the volume-regulatory Cl- conductance nor a prerequisite for regulatory volume decrease in epithelial cells exposed to a hypotonic solution.