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

K2P TASK-2 and KCNQ1-KCNE3 K+ channels are major players contributing to intestinal anion and fluid secretion

KEY POINTS

K⁺ channels are important in intestinal epithelium as they ensure the ionic homeostasis and electrical potential of epithelial cells during anion and fluid secretion. Intestinal epithelium cAMP-activated anion secretion depends on the activity of the (also cAMP dependent) KCNQ1-KCNE3 K⁺ channel, but the secretory process survives after genetic inactivation of the K⁺ channel in the mouse. Here we use double mutant mice to investigate which alternative K⁺ channels come into action to compensate for the absence of KCNQ1-KCNE3 K⁺ channels. Our data establish that whilst Ca²⁺ -activated K_Ca 3.1 channels are not involved, K_2P two-pore domain TASK-2 K⁺ channels are major players providing an alternative conductance to sustain the intestinal secretory process. Work with double mutant mice lacking both TASK-2 and KCNQ1-KCNE3 channels nevertheless points to yet-unidentified K⁺ channels that contribute to the robustness of the cAMP-activated anion secretion process.

ABSTRACT

Anion and fluid secretion across the intestinal epithelium, a process altered in cystic fibrosis and secretory diarrhoea, is mediated by cAMP-activated CFTR Cl^- channels and requires the simultaneous activity of basolateral K⁺ channels to maintain cellular ionic homeostasis and membrane potential. This function is fulfilled by the cAMP-activated K⁺ channel formed by the association of pore-forming KCNQ1 with its obligatory KCNE3 β-subunit. Studies using mice show sizeable cAMP-activated intestinal anion secretion in the absence of either KCNQ1 or KCNE3 suggesting that an alternative K⁺ conductance must compensate for the loss of KCNQ1-KCNE3 activity. We used double mutant mouse and pharmacological approaches to identify such a conductance. Ca²⁺ -dependent anion secretion can also be supported by Ca²⁺ -dependent K_Ca 3.1 channels after independent CFTR activation, but cAMP-dependent anion secretion is not further decreased in the combined absence of K_Ca 3.1 and KCNQ1-KCNE3 K⁺ channel activity. We show that the K_2P K⁺ channel TASK-2 is expressed in the epithelium of the small and large intestine. Tetrapentylammonium, a TASK-2 inhibitor, abolishes anion secretory current remaining in the absence of KCNQ1-KCNE3 activity. A double mutant mouse lacking both KCNQ1-KCNE3 and TASK-2 showed a much reduced cAMP-mediated anion secretion compared to that observed in the single KCNQ1-KCNE3 deficient mouse. We conclude that KCNQ1-KCNE3 and TASK-2 play major roles in the intestinal anion and fluid secretory phenotype. The persistence of an, admittedly reduced, secretory activity in the absence of these two conductances suggests that further additional K⁺ channel(s) as yet unidentified contribute to the robustness of the intestinal anion secretory process.

Role of the BK channel (KCa1.1) during activation of electrogenic K+ secretion in guinea pig distal colon

Secretagogues acting at a variety of receptor types activate electrogenic K(+) secretion in guinea pig distal colon, often accompanied by Cl(-) secretion. Distinct blockers of K(Ca)1.1 (BK, Kcnma1), iberiotoxin (IbTx), and paxilline inhibited the negative short-circuit current (I(sc)) associated with K(+) secretion. Mucosal addition of IbTx inhibited epinephrine-activated I(sc) ((epi)I(sc)) and transepithelial conductance ((epi)G(t)) consistent with K(+) secretion occurring via apical membrane K(Ca)1.1. The concentration dependence of IbTx inhibition of (epi)I(sc) yielded an IC(50) of 193 nM, with a maximal inhibition of 51%. Similarly, IbTx inhibited (epi)G(t) with an IC(50) of 220 nM and maximal inhibition of 48%. Mucosally added paxilline (10 μM) inhibited (epi)I(sc) and (epi)G(t) by ∼50%. IbTx and paxilline also inhibited I(sc) activated by mucosal ATP, supporting apical K(Ca)1.1 as a requirement for this K(+) secretagogue. Responses to IbTx and paxilline indicated that a component of K(+) secretion occurred during activation of Cl(-) secretion by prostaglandin-E(2) and cholinergic stimulation. Analysis of K(Ca)1.1α mRNA expression in distal colonic epithelial cells indicated the presence of the ZERO splice variant and three splice variants for the COOH terminus. The presence of the regulatory β-subunits K(Ca)β1 and K(Ca)β4 also was demonstrated. Immunolocalization supported the presence of K(Ca)1.1α in apical and basolateral membranes of surface and crypt cells. Together these results support a cellular mechanism for electrogenic K(+) secretion involving apical membrane K(Ca)1.1 during activation by several secretagogue types, but the observed K(+) secretion likely required the activity of additional K(+) channel types in the apical membrane.

Restoration by VIP of the carbachol-stimulated Cl- secretion in TTX-treated guinea pig distal colon

To determine if vasoactive intestinal peptide (VIP) restores neural activity from tetrodotoxin (TTX) blockade, we studied the effects of VIP and related agents on carbachol (Cch)-induced Cl(-) secretion in control-isolated guinea pig distal colon and in that treated with TTX. The short circuit current (I(sc)) increased dose-dependently after serosal applications of Cch (10(-6) - 2 x 10(-5) M) and VIP (5 x 10(-9) - 10(-7) M). But no additive or synergistic increase in I(sc) was observed. Cch- and VIP-induced I(sc) was completely abolished by a serosal application of TTX (10(-6) M). However, a serosal application, not mucosal, of VIP (10(-7) M) and 8-bromo-cAMP (10(-3) M) restored the Cch-stimulated, TTX-inhibited I(sc) by 113% and 75.8%, respectively. Furthermore, mucosal and serosal applications of forskolin (aden late cyclase activator) restored the I(sc) by 43.9% and 65.3%, respectively. The restored I(sc) was completely abolished by atropine (muscarinic receptor antagonist). These results suggest that VIP may restore the cholinergic activity by increasing the level of intracellular cAMP, and that cholinergic neuron is very likely to be responsible for the regulation of Cl(-) secretion at neuroepithelial junctions. The exact mechanism of VIP's effect on the TTX-inhibited epithelial Cl(-) secretion, and its possible usefulness in the treatment of TTX-induced pathophysiological conditions, remain to be determined.

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

Secretory modulation of basolateral membrane inwardly rectified K(+) channel in guinea pig distal colonic crypts

Cell-attached recordings revealed K(+) channel activity in basolateral membranes of guinea pig distal colonic crypts. Inwardly rectified currents were apparent with a pipette solution containing 140 mM K(+). Single-channel conductance (gamma) was 9 pS at the resting membrane potential. Another inward rectifier with gamma of 19 pS was observed occasionally. At a holding potential of -80 mV, gamma was 21 and 41 pS, respectively. Identity as K(+) channels was confirmed after patch excision by changing the bath ion composition. From reversal potentials, relative permeability of Na(+) over K(+) (P(Na)/P(K)) was 0.02 +/- 0.02, with P(Rb)/P(K) = 1.1 and P(Cl)/P(K) < 0.03. Spontaneous open probability (P(o)) of the 9-pS inward rectifier ((gp)K(ir)) was voltage independent in cell-attached patches. Both a low (P(o) = 0.09 +/- 0.01) and a moderate (P(o) = 0.41 +/- 0.01) activity mode were observed. Excision moved (gp)K(ir) to the medium activity mode; P(o) of (gp)K(ir) was independent of bath Ca(2+) activity and bath acidification. Addition of Cl(-) and K(+) secretagogues altered P(o) of (gp)K(ir). Forskolin or carbachol (10 microM) activated the small-conductance (gp)K(ir) in quiescent patches and increased P(o) in low-activity patches. K(+) secretagogues, either epinephrine (5 microM) or prostaglandin E(2) (100 nM), decreased P(o) of (gp)K(ir) in active patches. This (gp)K(ir) may be involved in electrogenic secretion of Cl(minus sign) and K(+) across the colonic epithelium, which requires a large basolateral membrane K(+) conductance during maximal Cl(-) secretion and, presumably, a lower K(+) conductance during primary electrogenic K(+) secretion.

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.

Epithelial cell dynamics in rabbit cecum and proximal colon P1

The large intestine of mammals has long been viewed as an osmoregulatory organ, and evidence suggests that fluid and solute transport mechanisms within the intestine are heterogeneous, varying depending on the particular segment involved. Variations in function are often matched by morphological correlates, but despite the widespread use of rabbit large intestine as an experimental model, there is a lack of knowledge about the cellular makeup and dynamics in the colonic mucosal epithelium. The presence of mitotic figures and immunohistochemical localization of proliferating cell nuclear antigen (PCNA) were used to identify the proliferative zone(s). Cellular migration patterns were determined through the use of the thymidine analog 5-bromo-2-deoxyuridine (BrdU) over a 24-, 48-, and 72-hr period. Apoptotic nuclei were identified utilizing terminal deoxynucleotidyl transferase d-UTP nick-end labeling (TUNEL). Both cecum and the initial portion of the proximal colon (P1) exhibited a proliferative zone at or near the crypt base, and migration proceeded upwards toward the surface epithelium lining the intestinal lumen, where apoptosis occurred Turnover time of crypt columnar cells was determined to be about 3 days; that of mucous cells was estimated to be about 5 weeks. Rabbit cecum and proximal colon P1 are similar in their cellular morphology and epithelial cell kinetics. In both, the major proliferative zone is located at or near the crypt base, from which crypt columnar cells migrate toward the lumenal surface epithelium over a period of 3 days. Goblet cell turnover rate is much slower than that of columnar cells.

Prostanoids stimulate K secretion and Cl secretion in guinea pig distal colon via distinct pathways

Short-circuit current (I(sc)) and transepithelial conductance (Gt) were measured in guinea pig distal colonic mucosa isolated from submucosa and underlying muscle layers. Indomethacin (2 microM) and NS-398 (2 microM) were added to suppress endogenous production of prostanoids. Serosal addition of PGE2 (10 nM) stimulated negative I(sc) consistent with K secretion, and concentrations >30 nM stimulated positive I(sc) consistent with Cl secretion. PGE2 also stimulated Gt at low and high concentrations. Dose responses to prostanoids specific for EP prostanoid receptors were consistent with stimulating K secretion through EP2 receptors, based on a rank order potency (from EC50 values) of PGE2 (1.9 nM) > 11-deoxy-PGE1 (8.3 nM) > 19(R)-hydroxy-PGE2 (13.9 nM) > butaprost (67 nM) > 17-phenyl-trinor-PGE2 (307 nM) >> sulprostone (>10 microM). An isoprostane, 8-iso-PGE2, stimulated K secretion with an EC50 of 33 nM. Cl secretory response was stimulated by PGD2 and BW-245C, a DP prostanoid receptor-specific agonist: BW-245C (15 nM) > PGD2 (30 nM) > PGE2 (203 nM). Agonists specific for FP, IP, and TP prostanoid receptors were ineffective in stimulating I(sc) and Gt at concentrations <1 microM. These results indicate that PGE2 stimulated electrogenic K secretion through activation of EP2 receptors and electrogenic KCl secretion through activation of DP receptors. Thus stimulation of Cl secretion in vivo would occur either via physiological concentrations of PGD2 (<100 nM) or pathophysiological concentrations of PGE2 (>100 nM) that could occur during inflammatory conditions.

Effect of alpha1-adrenergic stimulation of Cl- secretion and signal transduction in exocrine glands (Rana esculenta)

In the present work, the effect of stimulation of alpha-adrenergic receptors on Cl- secretion via exocrine frog skin glands was investigated. The alpha-adrenergic stimulation was performed by addition of the adrenergic agonist noradrenaline in the presence of the beta-adrenergic antagonist propranolol. In the presence of propranolol, noradrenaline had no effect on the cellular cAMP content. The Cl- secretion was measured as the amiloride-insensitive short circuit current (ISC). Addition of noradrenaline induced a biphasic increase in the ISC. The increase in ISC coincided with an increase in the net 36Cl- secretion. The noradrenaline-induced increase in ISC was dose-dependent with an EC50 of 13 +/- 0.3 microM. Epifluorescence microscopic measurements of isolated, fura-2-loaded frog skin gland acini were used to characterize the intracellular calcium ([Ca2+]i) response. Application of noradrenaline induced a biphasic [Ca2+]i response, which was dose-dependent with an EC50 of 11 +/- 6 microM. The Ca2+ plateau unlike the peak-response was sensitive to removal of Ca2+ from the extracellular medium. The noradrenaline-induced increase in the Cl- secretion as well as in [Ca2+]i was sensitive to the alpha1-adrenergic antagonist prazosine. Ryanodine and caffeine had no effect on [Ca2+]i indicating that the release was independent of ryanodine-sensitive Ca2+ stores. Noradrenaline mediated a significant increase in the cellular inositol 1,4,5-trisphosphate (IP3) content suggesting that the signal transduction pathway leading to the noradrenaline-induced increase in Ca2+ involved IP3 and a release of Ca2+ from IP3-sensitive stores.

Secretagogue response of goblet cells and columnar cells in human colonic crypts

Crypts of Lieberkühn were isolated from human colon, and differential interference contrast microscopy distinguished goblet and columnar cells. Activation with carbachol (CCh, 100 microM) or histamine (10 microM) released contents from goblet granules. Stimulation with prostaglandin E(2) (PGE(2), 5 microM) or adenosine (10 microM) did not release goblet granules but caused the apical margin of columnar cells to recede. Goblet volume was lost during stimulation with CCh or histamine ( approximately 160 fl/cell), but not with PGE(2) or adenosine. Three-quarters of goblet cells were responsive to CCh but released only 30% of goblet volume. Half-time for goblet volume release was 3.7 min. PGE(2) stimulated a prolonged fluid secretion that attained a rate of approximately 350 pl/min. Columnar cells lost approximately 50% of apical volume during maximal PGE(2) stimulation, with a half-time of 3.3 min. In crypts from individuals with ulcerative colitis, goblet cells were hypersensitive to CCh for release of goblet volume. These results support separate regulation for mucus secretions from goblet cells and from columnar cells, with control mechanisms restricting total release of mucus stores.

Secretagogue response of goblet cells and columnar cells in human colonic crypts

Crypts of Lieberkühn were isolated from human colon, and differential interference contrast microscopy distinguished goblet and columnar cells. Activation with carbachol (CCh, 100 microM) or histamine (10 microM) released contents from goblet granules. Stimulation with prostaglandin E2 (PGE2, 5 microM) or adenosine (10 microM) did not release goblet granules but caused the apical margin of columnar cells to recede. Goblet volume was lost during stimulation with CCh or histamine (approximately 160 fl/cell), but not with PGE2 or adenosine. Three-quarters of goblet cells were responsive to CCh but released only 30% of goblet volume. Half-time for goblet volume release was 3.7 min. PGE2 stimulated a prolonged fluid secretion that attained a rate of approximately 350 pl/min. Columnar cells lost approximately 50% of apical volume during maximal PGE2 stimulation, with a half-time of 3.3 min. In crypts from individuals with ulcerative colitis, goblet cells were hypersensitive to CCh for release of goblet volume. These results support separate regulation for mucus secretions from goblet cells and from columnar cells, with control mechanisms restricting total release of mucus stores.

The role of K+ channels in colonic Cl- secretion

Cl- secretion in the rat colonic crypt base cell (bc) requires the coordinated (a) opening of Cl- channels in the luminal membrane; (b) activation of the Na+2Cl-K+ cotransporter; (c) enhanced conductive K+ exit from the cell; and (d) increased pumping by the (Na+ + K+)-ATPase. In this study we focus on the importance of conductive K+ exit. After stimulation with the cholinergic agonist carbachol (CCH, 0.1-10 mumol/l) bc respond with a marked increase in whole cell (wc) conductance and a hyperpolarization of the membrane voltage (Vm). This is paralleled by a marked increase in the (Cl- secretory) short-circuit current (Isc) in Ussing chamber studies of the intact distal colon. Current evidence favors the view that CCH, via IP3, enhances cytosolic Ca2+ activity, and that Ca2+ increases the open probability of Cl- channels indirectly and that of K+ channels directly. After stimulation with PGE2 bc also enhance the wc conductance, but this is paralleled by a marked depolarization of Vm. Again these effects correspond to a marked increase in (Cl- secretory) Isc. The depolarization and enhanced wc conductance is partly due to the activation of Cl- channels. However, current evidence suggests that these effects on Cl- channels are paralleled by an activation of K+ channels. The chromanol 293B, by inhibiting these K+ channels specifically, abolishes PGE2-induced Cl- secretion completely, but has no effect on basal K+ conductance or on CCH-induced Cl- secretion. CCH apparently activates a Ca(2+)-dependent K+ channel with a conductance of 10-20 pS, whilst PGE2 (or cAMP) activate a much smaller K+ channel. Only the latter K+ channel can be inhibited by 293B in excised patches. Noise analysis suggests that this K+ channel has a conductance of < 3 pS and fast kinetics. The complete 293B induced inhibition of Cl- secretion caused by PGE2 can be explained by the fact that PGE2 induces a marked depolarization and that this depolarization reduces the basal K+ conductance. Current evidence suggests that this inhibition of the basal K+ conductance is caused by a depolarization induced inhibition of Ca2+ entry.

Gene targeting of a CFTR allele in HT29 human epithelial cells

HT29 cells endogenously express the cystic fibrosis transmembrane conductance regulator (CFTR) and have been used previously as a model to examine cellular regulation of CFTR expression and chloride secretory function. Homologous recombination has been used to specifically disrupt CFTR transcription in the HT29-18-C1 subclone. Experiments demonstrate successful disruption of a CFTR allele by DNA constructs, which target insertion of the neomycin phosphotransferase gene into CFTR exon 1 via homologous recombination. The mutation of one allele is a partial knockout because this cell line has multiple CFTR alleles. The mutation is confirmed by polymerase chain reaction (PCR) and genomic Southern blot analysis. A 52-68% reduction in CFTR mRNA levels is observed in the mutant cell line by both Northern and PCR analysis. However, Western blots show no decrease in total CFTR protein levels. Consistent with the lack of reduction in CFTR protein, the partial knockout mutant does not demonstrate alterations in cyclic AMP or calcium stimulation of chloride efflux or net osmolyte loss. Results suggest that posttranscriptional regulation of CFTR levels may contribute to maintenance of cellular chloride transport function.

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.

Specific blockade of slowly activating I(sK) channels by chromanols -- impact on the role of I(sK) channels in epithelia

Chromanols, which were recently shown to inhibit cAMP-mediated Cl- secretion in colon crypts via a blockade of a cAMP-activated K+ conductance, were analyzed for their effects on distinct cloned K+ channels expressed in Xenopus oocytes. The lead chromanol 293B specifically inhibited I(sK) channels with an IC50 of 7 micromol/l without affecting the delayed rectifier Kv1.1 or the inward rectifier Kir2.1. Moreover, several other chromanols displayed the same rank order of potency for I(sK) inhibition as demonstrated in colon crypts. Finally, we tested the effects of the previously described I(sK) blocker azimilide on cAMP mediated Cl- secretion in rat colon crypts. Similar to 293B azimilide inhibited the forskolin induced Cl- secretion. These data suggest that I(sK) protein induced K+ conductances are the targets for the chromanol 293B and its analogues, and azimilide.

Active potassium transport across guinea-pig distal colon: action of secretagogues

1. Adrenaline (5 microM) stimulated a K+ secretory current by 2.2 mu equiv h-1 cm-2 in isolated guinea-pig distal colonic epithelium. This secretory activity was inhibited entirely by addition of the loop diuretic bumetanide to the serosal solution. On-going K+ uptake via the absorptive pathway was unaltered by these changes. 2. Prostaglandin E2 (PGE2, 2 microM) stimulated electrogenic K+ secretion and Cl- secretion by 3.0 and 3.6 mu equiv h-1 cm-2, respectively. Serosal addition of bumetanide completely inhibited this K+ secretion but blocked only approximately 70% of Cl- secretion. The bumetanide-insensitive Cl- secretory current was dependent on the presence of Cl- and HCO3- in the bathing solutions. 3. Stimulation of electrogenic K+ secretion by PGE2 occurred with a half-maximal concentration of 4 nM, an affinity approximately 300 times higher than that for stimulation of Cl- secretion by PGE2. 4. Forskolin (10 microM) stimulated Cl- secretion by 4.9 mu equiv h-1 cm-2. The apparent K+ secretory rate was increased by only 1.5 mu equiv h-1 cm-2. A bumetanide-insensitive short-circuit current (ISC) was apparent and of the same size as that stimulated by PGE2. 5. Addition of the Ca2+ ionophore A23187 (10 microM), in the presence of indomethacin (1 microM) to reduce prostaglandin production, inhibited the K+ absorptive pathway by 40% and concurrently stimulated a small rate of electrogenic K+ secretion. 6. Active K+ absorption was inhibited by the addition of ouabain, omeprazole or SCH28080 to the mucosal solution. Both omeprazole and SCH28080 also stimulated a small negative ISC, consistent with electrogenic K+ secretion. 7. Association of K+ absorption, K+ secretion and Cl- secretion is indicated by similarities in transport mechanism and by secretagogue regulation. In particular, maximal rates of K+ secretory current require uptake via apical membrane K+ pumps. Such interrelations support a common cellular locus for these ion transport pathways.

The cAMP-regulated and 293B-inhibited K+ conductance of rat colonic crypt base cells

We have shown previously that secretagogues acting via the second messenger adenosine 3',5'-cyclic monophosphate (cAMP) activate, besides their marked effect on the luminal Cl- conductance, a K+ conductance in the basolateral membrane of colonic crypt cells. This conductance is blocked by the chromanol 293B. This K+ conductance is examined here in more detail in cell-attached (c.a.) and cell-excised (c.e.) patch- clamp studies. Addition of forskolin (5 micromol/l) to the bath led to the activation of very small-conductance (probably < 3 pS) K+ channels in c.a. patches (n = 54). These channels were reversibly inhibited by the addition of 0.1 mmol/l of 293B to the bath (n = 21). Noise analysis revealed that these channels had fast kinetics and produced a Lorentzian noise component with a corner frequency (fc) of 308 +/- 10 Hz (n = 30). The current/voltage curves of this noise indicated that the underlying ion channels were K+ selective. 293B reduced the power density of the noise (So) to 46 +/- 8.7% of its control value and shifted fc from 291 +/- 26 to 468 +/- 54 Hz (n = 8). In c.e. patches from cells previously stimulated by forskolin, the same type of current persisted in 3 out of 18 experiments when the bath solution was a cytosolic-type solution without adenosine 5'-triphosphate (ATP) (CYT). In 15 experiments the addition of ATP (1 mmol/l) to CYT solution was necessary to induce or augment channel activity. In six experiments excision was performed into CYT + ATP solution and channel activity persisted. 293B exerted a reversible inhibitory effect. The channel activity was reduced by 5 mmol/l Ba2+ and was completely absent when K+ in the bath was replaced by Na+. These data suggest that forskolin activates a K+ channel of very small conductance which can be inhibited directly and reversibly by 293B.

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.

Ca2+ influx induced by store release and cytosolic Ca2+ chelation in Ht29 colonic carcinoma cells

Cl- secretion in HT29 cells is regulated by agonists such as carbachol, neurotensin and adenosine 5'-triphosphate (ATP). These agonists induce Ca2+ store release as well as Ca2+ influx from the extracellular space. The increase in cytosolic Ca2+ enhances the Cl- and K+ conductances of these cells. Removal of extracellular Ca2+ strongly attenuates the secretory response to the above-mentioned agonists. The present study utilises patch-clamp methods to characterise the Ca2+ influx pathway. Inhibitors which have been shown previously to inhibit non-selective cation channels, such as flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n=6) inhibited ATP (0.1 mmol.l-1) induced increases in whole-cell conductance (Gm). When Cl- and K+ currents were inhibited by the presence of Cs2SO4 in the patch pipette and gluconate in the bath, ATP (0.1 mmol.l-1) still induced a significant increase in Gm from 1.2 +/- 0.3 nS to 4.7 +/- 1 nS (n = 24). This suggests that ATP induces a cation influx with a conductance of approximately 3-4 nS. This cation influx was inhibited by flufenamate (0.1 mmol.l-1, n = 6) and Gd3+ (10 micromol.l-1, n = 9). When Ba2+ (5 mmol.l-1) and 4,4'-diisothiocyanato-stilbene-2-2'-disulphonic acid (DIDS, 0.1 mmol.l-1) were added to the KCl/K-gluconate pipette solution to inhibit K+ and Cl- currents and the cells were clamped to depolarised voltages, ATP (0.1 mmol.l-1) reduced the membrane current (Im) significantly from 86 +/- 14 pA to 54 +/- 11 pA (n = 13), unmasking a cation inward current. In another series, the cation inward current was activated by dialysing the cell with a KCl/K-gluconate solution containing 5-10 mmol.l-1 1,2-bis-(2-aminoethoxy)ethane-N,N,N',N'-tetraacetic acid (EGTA) or 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA). The zero-current membrane voltage (Vm) and Im (at a clamp voltage of +10 mV) were monitored as a function of time. A new steady-state was reached 30-120 s after membrane rupture. Vm depolarised significantly from -33 +/- 2 mV to -12 +/- 1 mV, and Im fell significantly from 17 +/- 2 pA to 8.9 +/- 1.0 pA (n = 71). This negative current, representing a cation inward current, was activated when Ca2+ stores were emptied and was reduced significantly ( Im) when Ca2+ and/or Na+ were removed from the bathing solution: removal of Ca2+ in the absence of Na+ caused a Im of 5.0 +/- 1.2 pA (n = 12); removal of Na+ in the absence of Ca2+ caused a Im of 12.8 +/- 3.5 pA (n = 4). The cation inward current was also reduced significantly by La3+, Gd3+, and flufenamate. We conclude that store depletion induces a Ca2+/Na+ influx current in these cells. With 145 mmol.l-1 Na+ and 1 mmol.l-1 Ca2+, both ions contribute to this cation inward current. This current is an important component in the agonist-regulated secretory response.