Effects of forskolin on crypt cells of rat distal colon. Activation of nonselective cation channels in the crypt base and of a chloride conductance pathway in other parts of the crypt

Purinergic control of apical ion conductance by luminal ATP in rat colonic epithelium

ATP, released e.g. after cell damage or during inflammation, can alter ion transport across the intestinal mucosa via stimulation of purinergic receptors in the basolateral as well as in the apical membrane of epithelial cells. When ATP acts from the serosal side, it induces an increase in short-circuit current (Isc) via Cl- secretion across the colonic epithelium. In contrast, mucosal ATP or its derivative, BzATP, predominantly stimulating ionotropic P2X4 and P2X7 receptors, evoke an increase in Isc, which could not be explained by Cl- secretion. The underlying ion currents after stimulation of apical purinergic receptors in rat distal colon are still unclear and were investigated in the present study. Ussing chamber experiments revealed that the Isc induced by mucosal ATP was dependent on the presence of mucosal Ca2+ and inhibited by the K+ channel blocker, Ba2+, indicating the involvement of Ca2+-dependent K+ channels. Blockade of the transepithelial Isc by lanthanides (La3+, Gd3+) suggests that Ca2+ enters the epithelium via nonselective cation channels. Experiments with basolaterally depolarized epithelia confirmed the activation of apical lanthanide-sensitive Na+- and Ca2+-permeable cation channels by ATP. Putative candidates might be TRP channels, from which several subtypes were detected in colonic tissue in RT-PCR experiments. In addition, the activation of an apical Cl- conductance was observed when suitable Cl- concentration gradients were applied. Consequently, mucosal ATP, acting as 'danger signal', stimulates cation and anion channels in the apical membrane to induce a secretory response as part of the local defence mechanism in the intestinal epithelium.

Melatonin inhibits prostaglandin E2- and sodium nitroprusside-induced ion secretion in rat distal colon

Although the gastrointestinal tract is a rich source of melatonin and possesses numerous melatonin-binding sites, the role of melatonin in this tissue has not yet been fully elucidated. In this work we focused on the role of melatonin in the modulation of ion transport in rat distal colon. Whereas melatonin had no effect on colonic secretion or caused only infrequent and small changes in the short circuit current (Isc) due to its solvent ethanol, this mediator significantly modulated the secretion elicited by some secretagogues. Out of the five substances tested (prostaglandin E(2); 5-hydroxytryptamine; bethanechol; histamine; sodium nitroprusside) melatonin inhibited the effect of prostaglandin E(2) (PGE(2)) and sodium nitroprusside (SNP). Melatonin concentration-dependently decreased PGE(2)-evoked Isc and this inhibitory effect was more obvious from the mucosal side. The basal level of cAMP in colonic mucosa was not influenced by melatonin, but this drug prevented a PGE(2)-induced increase in the level of cAMP. The neurotoxin tetrodotoxin blocked the inhibitory effect of melatonin on SNP-induced Isc. Our data suggests that melatonin takes part in the modulation of colonic ion transport. The modulatory effect of melatonin on PGE(2)-induced Isc occurs directly at the level of the epithelium, whereas the effect on SNP-induced Isc is indirect and located in tetrodotoxin-sensitive enteric neurons.

P2Y6 receptor mediates colonic NaCl secretion via differential activation of cAMP-mediated transport

Extracellular nucleotides are important regulators of epithelial ion transport. Here we investigated nucleotide-mediated effects on colonic NaCl secretion and the signal transduction mechanisms involved. Basolateral UDP induced a sustained activation of Cl(-) secretion, which was completely inhibited by 293B, a specific inhibitor of cAMP-stimulated basolateral KCNQ1/KCNE3 K(+) channels. We therefore speculated that a basolateral P2Y(6) receptor could increase cAMP. Indeed UDP elevated cAMP in isolated crypts. We identified an epithelial P2Y(6) receptor using crypt [Ca(2+)](i) measurements, RT-PCR, and immunohistochemistry. To investigate whether the rat P2Y(6)elevates cAMP, we coexpressed the P2Y(1) or P2Y(6) receptor together with the cAMP-regulated cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel in Xenopus oocytes. A two-electrode voltage clamp was used to monitor nucleotide-induced Cl(-) currents. In oocytes expressing the P2Y(1) receptor, ATP transiently activated the endogenous Ca(2+)-activated Cl(-) current, but not CFTR. In contrast, in oocytes expressing the P2Y(6)receptor, UDP transiently activated the Ca(2+)-activated Cl(-) current and subsequently CFTR. CFTR Cl(-) currents were identified by their halide conductance sequence. In summary we find a basolateral P2Y(6) receptor in colonic epithelial cells stimulating sustained NaCl secretion by way of a synergistic increase of [Ca(2+)](i) and cAMP. In support of these data P2Y(6) receptor stimulation differentially activates CFTR in Xenopus oocytes.

Gossypol induces chloride secretion in rat proximal colon

The effect of gossypol on electrolyte transport was investigated in rat colon mounted in Ussing chambers. The addition of gossypol to the mucosal or serosal side led to an increase in mucus secretion, which we did not quantify. Mucosally or serosally added gossypol also induced a rise in short circuit current (I(sc)) and tissue conductance (G(t)). Part of the mucosally added gossypol seemed to be bound to the mucus because the effects on I(sc) and G(t) were smaller when gossypol was added to the mucosal side. Serosally added gossypol had an effect on I(sc) at a concentration of 10 micromol l(-1). Mucus secretion was reduced in low Ca(2+) buffer. The increase in I(sc) was diminished by blockers of Cl- channels, K+ channels, of the Na+/K+ ATPase and of the Na+/K+/2 Cl- cotransporter. Measurements of unidirectional ion fluxes showed that gossypol added to the mucosal side had no effect on net Na+ transport, but increased Cl- secretion. The effect of mucosally added gossypol was significantly reduced by the use of low Cl- buffers and abolished when the buffer was additionally depleted of HCO(3)(-). Calmodulin antagonists inhibited the effect on secretion. These findings indicate that gossypol induces chloride secretion via a calmodulin-dependent mechanism. High concentrations of gossypol induced a strong increase in G(t) that could be blocked by W7, a blocker of calmodulin-dependent myosin light chain kinase. This indicates that the rise in G(t) is not due to an unspecific toxic effect, but instead, to specific opening of tight junctions.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Basolateral outward rectifier chloride channel in isolated crypts of mouse colon

Single channel patch-clamp techniques were used to demonstrate the presence of outwardly rectifying chloride channels in the basolateral membrane of crypt cells from mouse distal colon. These channels were rarely observed in the cell-attached mode and, in the inside-out configuration, only became active after a delay and depolarizing voltage steps. Single channel conductance was 23.4 pS between -100 and -40 mV and increased to 90.2 pS between 40 and 100 mV. The channel permeability sequence for anions was: I(-) > SCN(-) > Br(-) > Cl(-) > NO(3)(-) > F(-)>> SO(4)(2-) approximately gluconate. In inside-out patches, the channel open probability was voltage dependent but insensitive to intracellular Ca(2+) concentration. In cell-attached mode, forskolin, histamine, carbachol, A-23187, and activators of protein kinase C all failed to activate the channel, and activity could not be evoked in inside-out patches by exposure to the purified catalytic subunit of cAMP-dependent protein kinase A. The channel was inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate, 9-anthracenecarboxylic acid, and DIDS. Stimulation of G proteins with guanosine 5'-O-(3-thiotriphosphate) decreased the channel open probability and conductance, whereas subsequent addition of guanosine 5'-O-(2-thiodiphosphate) reactivated the channel.

Basolateral K+ channel involvement in forskolin-activated chloride secretion in human colon

1. In this study we investigated the role of basolateral potassium transport in maintaining cAMP-activated chloride secretion in human colonic epithelium. 2. Ion transport was quantified in isolated human colonic epithelium using the short-circuit current technique. Basolateral potassium transport was studied using nystatin permeabilization. Intracellular calcium measurements were obtained from isolated human colonic crypts using fura-2 spectrofluorescence imaging. 3. In intact isolated colonic strips, forskolin and prostaglandin E2 (PGE2) activated an inward transmembrane current (ISC) consistent with anion secretion (for forskolin DeltaISC = 63.8+/-6.2 microA cm(-2), n = 6; for PGE2 DeltaISC = 34.3+/-5.2 microA cm(-2), n = 6). This current was inhibited in chloride-free Krebs solution or by inhibiting basolateral chloride uptake with bumetanide and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid DIDS). 4. The forskolin- and PGE2-induced chloride secretion was inhibited by basolateral exposure to barium (5 mM), tetrapentylammonium (10 microM) and tetraethylammonium (10 mM). 5. The transepithelial current produced under an apical to serosal K+ gradient in nystatin-perforated colon is generated at the basolateral membrane by K+ transport. Forskolin failed to activate this current under conditions of high or low calcium and failed to increase the levels of intracellular calcium in isolated crypts 6. In conclusion, we propose that potassium recycling through basolateral K+ channels is essential for cAMP-activated chloride secretion.

Volume regulation following hypotonic shock in isolated crypts of mouse distal colon

1. A video-imaging technique of morphometry was used to measure the diameter as an index of cell volume in intact mouse distal colon crypts submitted to hypotonic shock. 2. Transition from isotonic (310 mosmol l-1) to hypotonic (240 mosmol l-1) saline caused a pronounced increase in crypt diameter immediately followed by regulatory volume decrease (RVD). 3. Exposure of crypts to Cl--free hyposmotic medium increased the rapidity of both cell swelling and RVD. Exposure of crypts to Na+-free hyposmotic medium reduced the total duration of swelling. Return to initial diameter was followed by further shrinkage of the crypt cells. 4. The chloride channel inhibitor NPPB (50 microM) delayed the swelling phase and prevented the subsequent normal decrease in diameter. 5. The K+ channel blockers barium (10 mM), charybdotoxin (10 nM) and TEA (5 mM) inhibited RVD by 51, 44 and 32 %, respectively. 6. Intracellular [Ca2+] rose from a baseline of 174 +/- 17 nM (n = 8) to 448 +/- 45 nM (n = 8) during the initial swelling phase 7. The Ca2+ channel blockers verapamil (50 microM) and nifedipine (10 microM), the chelator of intracellular Ca2+ BAPTA AM (30 microM), or the inhibitor of Ca2+ release TMB-8 (10 microM), dramatically reduced volume recovery, leading to 51 % (n = 9), 25 % (n = 7), 37 % (n = 6), 32 % (n = 8) inhibition of RVD, respectively. TFP (50 microM), an antagonist of the Ca2+-calmodulin complex, significantly slowed RVD. The Ca2+ ionophore A23187 (2 microM) provoked a dramatic reduction of the duration and amplitude of cell swelling followed by extensive shrinkage. The release of Ca2+ from intracellular stores using bradykinin (1 microM) or blockade of reabsorption with thapsigargin (1 microM) decreased the duration of RVD. 8. Prostaglandin E2 (PGE2, 5 microM) slightly delayed RVD, whereas leukotriene D4 (LTD4, 100 nM) and arachidonic acid (10 microM) reduced the duration of RVD. Blockade of phospholipase A2 by quinacrine (10 microM) inhibited RVD by 53 %. Common inhibition of PGE2 and LTD4 synthesis by ETYA (50 microM) or separate blockade of PGE2 synthesis by 1 microM indomethacin reduced the duration of RVD. Blockade of LTD4 synthesis by nordihydroguaiaretic acid (NDGA) did not produce any significant effect on cell swelling or subsequent RVD. 9. Staurosporine (1 microM), an inhibitor of protein kinases, inhibited RVD by 58 %. Taken together the experiments demonstrate that the RVD process is under the control of conductive pathways, extra- and intracellular Ca2+ ions, protein kinases, prostaglandins and leukotrienes.

Inhibition of a K+ conductance by the phosphatase inhibitor calyculin A in rat distal colon

Basal membrane permeability of epithelial cells from the lower third and the middle of rat colonic crypts is dominated by a K+ conductance as shown by ion replacement experiments. Calyculin A, an inhibitor of protein phosphatases, induced a depolarization of these cells. The depolarization was concomitant with an inhibition of membrane current. The current inhibited by calyculin A had a reversal potential identical with the theoretical K+ equilibrium potential indicating that the drug inhibits a basal K+ conductance. The efficiency of calyculin A was comparable with that of other well-known K+ channel blockers such as Ba2+, tetraethylammonium or quinine. In the intact tissue, calyculin A exerted an inhibitory action on forskolin-induced anion secretion, an effect which may be explained by the decrease in the driving force for Cl- exit after inhibition of cellular K+ conductance. Together with previous results, these data suggest an inhibition of epithelial K+ conductance by phosphorylation.

Serotonin activates electrolyte transport via 5-HT2A receptor in rat colonic crypt cells

The aim of this study was to demonstrate that 5-HT activates electrolyte transport directly via 5-HT2A receptor in rat colonic crypt cells. Patch-clamp whole cell recording was performed in isolated crypts to measure the 5-HT-induced changes in electrogenic K+ and Cl- currents. Superfusing 5-HT (10 microM) in the bath solution increased both K+ and Cl- currents, which were antagonized by the presence of ketanserin (1 microM), a selective 5-HT2A antagonist, in the bath solution. Mesulergine (1 microM) a 5-HT2A and 5-HT2C antagonist, had no inhibitory effect. Strong chelation of the intracellular Ca2+ by 5 mM BAPTA inhibited 5-HT-induced currents. 5-HT also failed to activate K+ and C1- currents in the presence of GDPbetaS (0.5 mM) in the pipette solution. Intracellular administration of GTPgammaS (0.1 mM) mimicked the stimulatory effect of 5-HT, that was inhibited by 5 mM BAPTA. H-7 (0.05 mM), an inhibitor of protein kinase C, A, and G, did not affect the currents. These data indicate that a G protein-coupled pathway is involved in the activation of electrolyte secretion via 5-HT2A receptor.

Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium

1. Anion secretion induced by capacitative Ca2+ entry through apical and basolateral membranes of cultured equine sweat gland epithelium was studied using the short-circuit current (Isc) technique. 2. Thapsigargin induced an increase in Isc that could be inhibited when external Ca2+ was chelated by EGTA. 3. The inhibition of the thapsigargin-induced Isc could be reversed by re-addition of Ca2+ to apical or basolateral solutions. The magnitude of the reactivated Isc depended predominantly on basolateral Ca2+ concentration. 4. The magnitude of the reactivated Isc upon basolateral Ca2+ addition increased with the thapsigargin concentration, indicating its dependence on the emptied state of the Ca2+ store induced by thapsigargin. 5. The thapsigargin-induced Isc, as well as the Ca(2+)-dependent reactivation of Isc in EGTA-treated epithelia, was inhibitable by apical, but not basolateral, addition of flufenamate, and by basolateral addition of La3+. Other Ca2+ channel blockers, verapamil and nifedipine, had no effect when applied to either membrane. 6. The results suggest that thapsigargin-induced anion secretion by the equine sweat gland epithelial cells is crucially dependent upon the Ca2+ influx occurring primarily through the basolateral membrane, and that apical and basolateral membranes may possess different pathways for Ca2+ entry.

Ca2+ regulated K+ and non-selective cation channels in the basolateral membrane of rat colonic crypt base cells

We have previously shown that a new type of K+ channel, present in the basolateral membrane of the colonic crypt base (blm), is necessary for cAMP-activated Cl- secretion. Under basal conditions, and when stimulated by carbachol (CCH) alone, this channel is absent. In the present patch clamp-study we examined the ion channels present in the blm under cell-attached and in cell-excised conditions. In cell-attached recordings with NaCl-type solution in the pipette we measured activity of a K+ channel of 16 +/- 0.3 pS (n = 168). The activity of this channel was sharply increased by CCH (0. 1 mmol/l, n = 26). Reduction of extracellular Ca2+ to 0.1 mmol/l (n = 34) led to a reversible reduction of activity of this small channel (SKCa). It was also inactivated by forskolin (5 micromol/l, n = 38), whilst the K+ channel noise caused by the very small K+ channel increased. Activity of non-selective cation channels (NScat) was rarely observed immediately prior to the loss of attached basolateral patches and routinely in excised patches. The NScat, with a mean conductance of 49 +/- 1.0 pS (n = 96), was Ca2+ activated and required >10 micromol/l Ca2+ (cytosolic side = cs). It was reversibly inhibited by ATP (<1 mmol/l, n = 13) and by 3',5-dichloro-diphenylamine-2-carboxylate (10-100 micromol/l, n = 5). SKCa was also Ca2+ dependent in excised inside-out basolateral patches. Its activity stayed almost unaltered down to 1 micromol/l (cs) and then fell sharply to almost zero at 0.1 micromol/l Ca2+ (cs, n = 12). SKCa was inhibited by Ba2+ (n = 31) and was charybdotoxin sensitive (1 nmol/l) in outside-out basolateral patches (n = 3). Measurements of the Ca2+ activity ([Ca2+]i) in these cells using fura-2 indicated that forskolin and depolarization, induced by an increase in bath K+ concentration to 30 mmol/l, reduced [Ca2+]i markedly (n = 8-10). Hyperpolarization had the opposite effect. The present data indicate that the blm of these cells contains a small-conductance Ca2+-sensitive K+ channel. This channel is activated promptly by very small increments in [Ca2+]i and is inactivated by a fall in [Ca2+]i induced by forskolin.

Crypt base cells show forskolin-induced Cl- secretion but no cation inward conductance

Whole-cell patch-clamp studies in base cells of isolated colonic crypts of rats pretreated with dexamethasone were performed to examine the effects of stimulation by forskolin (10 micromol/l). The experiments were designed in order to distinguish between two postulated effector mechanisms: the activation of a non-selective cation channel and the activation of Cl- channels. As shown in an accompanying report, forskolin depolarizes the membrane voltage (Vm) by some 40-50 mV and enhances the whole-cell membrane conductance (Gm) substantially in these cells. In this report all experiments were performed in the presence of forskolin. A reduction of the bath Na+ concentration from 145 to 2 mmol/l led to a hyperpolarization of Vm by some 20-30 mV. This hyperpolarization occurred very slowly suggesting that the hyperpolarization produced by the low-Na+ solution was caused indirectly and not by a change in the equilibrium potential for Na+, ENa+. A complete kinetic analysis of the effect on voltage of bath Na+ revealed a saturation-type relation with a high apparent affinity for Na+ of around 5-10 mmol/l. A reduction in bath Cl- concentration from 145 to 32 mmol/l caused a depolarization of Vm from -34 +/- 3 to -20 +/- 4 mV (n = 13) in the presence of a high bath Na+ concentration, but had the opposite effect at low (5 mmol/l) Na+ concentrations: Vm was hyperpolarized from -46 +/- 4 to -62 +/- 6 mV (n = 13). If the effect of Na+ on Vm was caused by a non-selective cation channel the opposite would have been expected. To test directly whether the Na+2Cl-K+ cotransporter was responsible for the effects of changes in bath Na+ on Vm, the effects of increasing concentrations of several loop diuretics were examined. Furosemide, piretanide, torasemide and bumetanide (up to 0.1-0.5 mmol/l) all hyperpolarized Vm, albeit only by less than 10 mV. Another subclass of loop diuretics containing a tetrazolate in position 1 [e.g. azosemide, no. 19A and no. 20A from Schlatter E, Greger R, Weidtke C (1983) Pflüger Arch 396: 210-217] were much more effective. Azosemide hyperpolarized Vm from -46 +/- 3 to -74 +/- 2 mV (n = 18) and reduced Gm from 11 +/- 1 to 4 +/- 1 nS (n = 14). These data indicate that forskolin stimulates Cl- secretion in these cells by a mechanism fully compatible with the current scheme for exocrine secretion involving the Na+2Cl-K+ cotransporter.

The ion conductances of colonic crypts from dexamethasone-treated rats

Whole-cell patch-clamp studies were performed in isolated colonic crypts of rats pretreated with dexamethasone (6 mg/kg subcutaneously on 3 days consecutively prior to the experiment). The cells were divided into three categories according to their position along the crypt axis: surface cells (s.c.); mid-crypt cells (m.c.) and crypt base cells (b.c.). The zero-current membrane voltage (Vm) was -56 +/- 2 mV in s.c (n = 34); -76 +/- 2 mV in m.c. (n = 47); and -87 +/- 1 mV in b.c. (n = 87). The whole-cell conductance (Gm) was similar (8-12 nS) in all three types of cells. A fractional K+ conductance accounting for 29-67% of Gm was present in all cell types. A Na+ conductance was demonstrable in s.c. by the hyperpolarizing effect on Vm of a low-Na+ (5 mmol/l) solution. In m.c. and b.c. the hyperpolarizing effect was much smaller, albeit significant. Amiloride had a concentration-dependent hyperpolarizing effect on Vm in m.c. and even more so in s.c.. It reduced Gm by approximately 12%. The dissociation constant (KD) was around 0.2 micromol/l. Triamterene had a comparable but not additive effect (KD = 30 micromol/l, n = 14). Forskolin (10 micromol/l, in order to enhance cytosolic adenosine 3', 5'-cyclic monophosphate or cAMP) depolarized Vm in all three types of cells. The strongest effect was seen in b.c.. Gm was enhanced significantly in b.c. by 83% (forskolin) to 121% [8-(4-chlorophenylthio)cAMP]. The depolarization of Vm and increase in Gm was caused to large extent by an increase in Cl-conductance as shown by the effect of a reduction in bath Cl-concentration from 145 to 32 mmol/l. This manoeuvre hyperpolarized Vm under control conditions significantly by 6-9 mV in all three types of cells, whilst it depolarized Vm in the presence of forskolin in m.c. and in b.c.. These data indicate that s.c. of dexamethasone-treated rats possess mostly a K+ conductance and an amiloride- and triamterene-inhibitable Na+ conductance. m.c. and b.c. possess little or no Na+ conductance; their Vm is largely determined by a K+ conductance. Forskolin (via cAMP) augments the Cl- conductance of m.c. and b.c. but has only a slight effect on s.c.

Apical polarity in human colon carcinoma cell lines

The non-polar human adenocarcinoma cells (HT29) when grown as monolayers or aggregates, have no tight junctions and no brush border. When these cells are treated with forskolin (15-100 microM) or cholera toxin (1 nM) intercellular lumina appear between the cells and about 30% of the cells facing the medium or the lumina are fully covered with a brush border. Aggregates embedded in collagen type I and treated with forskolin form a brush border only on cells facing the intercellular lumina. Monolayers of the polar human colon adenocarcinoma cell line Caco-2 express spontaneously tight junctions and a brush border in all the cells. When grown in aggregates the inner cells lose their polarity and only the cells facing the medium are polar. This polarity was not found when the aggregates were embedded in collagen gels. Aggregates embedded in collagen and treated with forskolin form bubble-like structures with a single layer of polar cells facing a central lumen. The data indicate that cell polarity is probably controlled by both internal factors such as cAMP and external factors such as cell-cell and cell-substratum molecules.

A new class of inhibitors of cAMP-mediated Cl- secretion in rabbit colon, acting by the reduction of cAMP-activated K+ conductance

Previously we have shown that arylaminobenzoates like 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB), which are very potent inhibitors of NaCl absorption in the thick ascending limb of the loop of Henle, are only poor inhibitors of the cAMP-mediated secretion of NaCl in rat colon. This has prompted our search for more potent inhibitors of NaCl secretion in the latter system. The chromanole compound 293 B inhibited the equivalent short-circuit current (Isc) induced by prostaglandin E2 (n = 7), vasoactive intestinal polypeptide (VIP, n = 5), adenosine (n = 3), cholera toxin (n = 4) and cAMP (n = 6), but not by ionomycin (n = 5) in distal rabbit colon half maximally (IC50) at 2 mumol/l from the mucosal and at 0.7 mumol/l from the serosal side. The inhibition was reversible and paralleled by a significant increase in transepithelial membrane resistance [e.g. in the VIP series from 116 +/- 16 omega.cm2 to 136 +/- 21 omega.cm2 (n = 5)]. A total of 25 derivatives of 293 B were examined and structure activity relations were obtained. It was shown that the racemate 293 B was the most potent compound within this group and that its effect was due to the enantiomer 434 B which acted half maximally at 0.25 mumol/l. Further studies in isolated in vitro perfused colonic crypts revealed that 10 mumol/l 293 B had no effect on the membrane voltage across the basolateral membrane (Vbl) in non-stimulated crypt cells: -69 +/- 3 mV versus -67 +/- 3 mV (n = 10), whilst in the same cells 1 mmol/l Ba2+ depolarised Vbl significantly. However, 293 B depolarised Vbl significantly in the presence of 1 mumol/l forskolin: -45 +/- 4 mV versus -39 +/- 5 mV (n = 7). Similar results were obtained with 0.1 mmol/l adenosine. 293 B depolarised Vbl from -40 +/- 5 mV to -30 +/- 4 mV (n = 19). This was paralleled by an increase in the fractional resistance of the basolateral membrane. VIP had a comparable effect. The hyperpolarisation induced by 0.1 mmol ATP was not influenced by 10 mumol/l 293 B: -75 +/- 6 mV versus -75 +/- 6 mV (n = 6). Also 293 B had no effect on basal K+ conductance (n = 4). Hence, we conclude that 293 B inhibits the K+ conductance induced by cAMP. This conductance is apparently relevant for Cl- secretion and the basal K+ conductance is insufficient to support secretion.

The effect of secretagogues on ion conductances of in vitro perfused, isolated rabbit colonic crypts

Several secretagogues were used in this study, including those which enhance intracellular cyclic adenosine monophosphate (cAMP) production, as well as others which elevate intracellular Ca2+ activity and are known to increase Cl- secretion in the intact colon and in colonic carcinoma cell lines. They were examined with respect to their effects on electrophysiological properties in isolated rabbit distal colonic crypts. Crypts were dissected manually and perfused in vitro. Transepithelial voltage (Vte), transepithelial resistance (Rte), membrane voltage across the basolateral membrane (Vbl), and fractional basolateral membrane resistance (FRbl), were estimated. Basolateral prostaglandin E2 (PGE2, > or = 0.1 mumol/l), vasoactive intestinal peptide (VIP, 1 nmol/l) and adenosine (0.1 mmol/l) induced an initial depolarisation and a secondary partial repolarisation of Vbl. In the case of adenosine, the initial depolarization of Vbl was by 31 +/- 2 mV (n = 47). Rte fell significantly from 16.4 +/- 3.6 to 14.2 +/- 3.7 omega.cm2 (n = 6), and FRbl increased significantly from 0.11 +/- 0.02 to 0.51 +/- 0.10 (n = 6). In the second phase the repolarisation of Vbl amounted 11 +/- 2 mV (n = 47) and a steady-state (Vbl) of -51 +/- 2 mV (n = 47) was reached. Rte fell further and significantly to a steady-state value of 12.4 +/- 3.8 omega.cm2 (n = 6) and FRbl fell significantly to 0.42 +/- 0.13 (n = 6). In 30% of the experiments, a transient hyperpolarisation of Vbl by 8 +/- 2 mV (n = 14) was seen during wash out of adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Correction of lethal intestinal defect in a mouse model of cystic fibrosis by human CFTR

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR). A potential animal model of CF, the CFTR-/- mouse, has had limited utility because most mice die from intestinal obstruction during the first month of life. Human CFTR (hCFTR) was expressed in CFTR-/- mice under the control of the rat intestinal fatty acid-binding protein gene promoter. The mice survived and showed functional correction of ileal goblet cell and crypt cell hyperplasia and cyclic adenosine monophosphate-stimulated chloride secretion. These results support the concept that transfer of the hCFTR gene may be a useful strategy for correcting physiologic defects in patients with CF.

Flufenamate and Gd3+ inhibit stimulated Ca2+ influx in the epithelial cell line CFPAC-1

The relevant influx pathway for stimulated Ca2+ entry into epithelial cells is largely unknown. Using flufenamate (Flu) and Gd3+, both known pharmacological blockers of non-selective cation currents in other epithelial preparations, we tested whether the stimulated Ca2+ entry in CFPAC-1 cells was inhibited by these agents. Transmembraneous Ca2+ influx into CFPAC-1 cells was stimulated by either ATP (10(-4) and 10(-5) mol/l), carbachol (CCH, 10(-4) mol/l) or thapsigargin (TG, 10(-8) mol/l). Three different experimental approaches were used. (1) Because the plateau phase of an agonist-induced [Ca2+]i transient reflects Ca2+ influx into these cells, we investigated the influence of Flu and Gd3+ on the level of the stimulated [Ca2+]i plateau. (2) The fura-2 Mn(2+)-quenching technique was used to visualise divalent cation entry and monitor its inhibition. (3) During the "refilling period" after agonist-induced discharge of the intracellular pools the putative influx inhibitors Flu and Gd3+ were given and subsequently the filling state of the agonist-sensitive intracellular stores tested. The results from the first experimental approach showed that both Flu and Gd3+ were potent inhibitors of the stimulated Ca2+ entry in CFPAC-1 cells. Flu reversibly decreased the ATP-induced [Ca2+]i plateau in a concentration dependent manner, with an IC50 value of 33 mumol/l (n = 6). Similar results were obtained for the CCH- (n = 5) and the TG-induced (n = 5) [Ca2+]i plateau. Gd3+ concentration dependently inhibited the stimulated Ca2+ plateau. A complete block of the ATP-induced [Ca2+]i plateau was seen at 0.5 mumol/l (ATP 10(-5) mol/l, n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)

HCO3- secretion by rat distal colon: effects of inhibitors and extracellular Na+

BACKGROUND/AIMS

The large intestine secretes HCO3- via a Cl-/HCO3- exchange mechanism located in the apical membrane of colonocytes. However, an additional transport system(s) must facilitate HCO3- (OH-) entry or H+ exit across the basolateral cell surface. The aim of this study was to determine that mechanism(s).

METHODS

A modified Ussing apparatus was used to measure net HCO3- secretion in segments of rat distal colon.

RESULTS

When added to the serosal solution, 10 mmol/L 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), 1 mmol/L SITS and 0.1 mmol/L diisothiocyanostilbene-2,2'-disulfonic acid, inhibited HCO3- secretion by 88%, 51%, and 30%, respectively. However, the Na+/H+ exchange inhibitors, amiloride (1 mmol/L), dimethylamiloride (0.1 mmol/L), ethylisopropylamiloride (0.1 mmol/L), failed to affect HCO3- secretion. Acetazolamide (1 mmol/L) blocked HCO3- secretion by approximately 60% when in the serosal solution but had little effect when in the mucosal solution. Ion substitution studies showed that HCO3- secretion required Na+ in the serosal solution (K0.5 approximately 12 mmol/L). HCO3- secretion was unaffected by depolarizing the basolateral membrane potential with K(+)-rich medium.

CONCLUSIONS

These data are consistent with Na+ linked HCO3- transport across the colonocyte basolateral membrane, which appears to be electroneutral.

Effect of acetyl choline on ion transport in sheep tracheal epithelium

The action of acetylcholine (ACh) on sheep tracheal epithelium has been investigated. ACh increases transiently the short-circuit current (ISC). The same response is obtained in tissues in which the apical membrane has been permeabilized with amphotericin B in the presence of a potassium gradient. Microelectrode studies show that the majority of tracheal epithelial cells depolarize as the apical fractional resistance decreases on application of ACh. These results, together with the finding that bumetanide decreases the initial ACh-induced ISC increase, are consistent with an initial activation by ACh of apical Cl- channels and basolateral K+ channels. Following the initial increase, ISC declines to values lower than in control conditions both in untreated and in amphotericin-permeabilized tissues, suggesting that the basolateral K+ conductance falls during this phase. The late decrease in ISC induced by ACh is significantly reduced in tissues pretreated with amiloride, suggesting that the apical Na+ channels are also involved in this response. ACh abolishes the net Na+ absorption by decreasing the mucosal to serosal Na+ flux. This effect is possibly a result of a down-regulation of apical Na+ channels and basolateral K+ channels.

Segmental differences along the crypt axis in the response of cell volume to secretagogues or hypotonic medium in the rat colon

VIP caused a decrease in the diameter of rat colonic crypts. This decrease was followed by a volume increase at the middle and the upper third of the crypt, which finally led to an increase of crypt diameter above the initial control values. At the fundus only a cell shrinkage was observed. The volume increase at the upper parts of the crypt was suppressed by the inhibitor of the Na(+)-K(+)-Cl(-)-cotransporter, furosemide. When crypts were exposed to a hypertonic medium, cell shrinkage was followed by a regulatory volume increase at the middle and the upper third of the crypt but not at the fundus region. These results suggest a gradient in the distribution of the Na(+)-K(+)-Cl(-)-cotransporter along the crypt axis.