Transmitter-induced changes of the membrane voltage of HT29 cells

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

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.

Carbachol induces oscillations in membrane potential and intracellular calcium in a colonic tumor cell line, HT-29

The patch-clamp technique was used to study the effects of carbachol (CCh) on HT-29 cells. During CCh exposure, the cells (n = 23) depolarized close to the equilibrium potential for Cl- (E(Cl-); -48 mV) and the membrane potential then started to oscillate (16/23 cells). In voltage-clamp experiments, similar oscillations in whole cell currents could be demonstrated. The whole cell conductance increased from 225 +/- 25 pS in control solution to 6,728 +/- 1,165 pS (means +/- SE, n = 17). In substitution experiments (22 mM Cl- in bath solution, E(Cl-) = 0 mV), the reversal potential changed from -41.6 +/- 2.2 mV (means +/- SE, n = 9) to -3.2 +/- 2.0 mV (means +/- SE, n = 7). When the cells were loaded with the calcium-sensitive fluorescent dye, fluo 3, and simultaneously patch clamped, CCh caused a synchronous oscillating pattern of fluorescence and membrane potential. In cell-attached patches, the CCh-activated currents reversed at a relative membrane potential of 1.9 +/- 3.7 mV (means +/- SE, n = 11) with control solution in the pipette and at 46.2 +/- 5.3 mV (means +/- SE, n = 10) with a 15 mM Cl- solution in the pipette. High K+ (144 mM) did not change the reversal potential significantly (P < or = 0.05, n = 8). In inside-out patches, calcium-dependent Cl- channels could be demonstrated with a conductance of 19 pS (n = 7). It is concluded that CCh causes oscillations in membrane potential that involve calcium-dependent Cl- channels and a K+ permeability.

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.

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)

Activation of Ca(2+)-dependent K+ and Cl- currents by UTP and ATP in CFPAC-1 cells

Activation of Cl- and K+ conductances by nucleotide receptor-operated mobilization of intracellular Ca2+ was investigated in CFPAC-1 cells with the perforated-patch technique. Adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) caused a dose-dependent fast and transient membrane hyperpolarization. UTP was more effective than ATP. In voltage-clamped cells, two currents with different ionic permeability and kinetics were activated by the nucleotides. The first one was carried by Cl- ions, peaked in the first few seconds after addition of nucleotides, and lasted for 1 +/- 0.3 min. Its amplitude was about 2.7 nA at -100 mV with 100 mumol/l of either ATP or UTP. The second current was carried by K+ ions and was blocked by Cs+. This current peaked more slowly and had a mean duration of 4.6 +/- 0.7 min. Its amplitude was 0.9 nA and 0.5 nA at -20 mV with 100 mumol/l UTP and ATP, respectively. Activation of the nucleotide receptor caused a transient increase in intracellular Ca2+ concentration ([Ca2+]i) that was similar in the presence or absence of extracellular Ca2+. The ED50 for UTP was 24 mumol/l and that for ATP was 94 mumol/l. Depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by thapsigargin prevented both the nucleotide-induced [Ca2+]i increase and the activation of membrane currents. Addition of 2 mmol/l Ca2+ to thapsigargin-treated cells produced a sustained increase of Cl- and K+ currents, which was reversed by Ca2+ removal. The present study demonstrates that CFPAC-1 cells respond to nucleotide receptor activation with a transient increase in [Ca2+]i that stimulates Ca(2+)-dependent Cl- and K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolated perfused rabbit colon crypts: stimulation of Cl- secretion by forskolin

The aim of this study was to characterize ion conductances and carrier mechanisms of isolated in vitro perfused rabbit colonic crypts. Crypts were isolated from rabbit colon mucosa and mounted on a pipette system which allowed controlled perfusion of the lumen. In non-stimulated conditions basolateral membrane voltage (Vbl) was -65 +/- 1 mV (n = 240). Bath Ba2+ (1 mmol/l) and verapamil (0.1 mmol/l) depolarized Vbl by 21 +/- 2 mV (n = 7) and 31 +/- 1 (n = 4), respectively. Lowering of bath Cl- concentration hyperpolarized Vbl from -69 +/- 3 to -75 +/- 3 mV (n = 9). Lowering of luminal Cl- concentration did not change Vbl. Basolateral application of loop diuretics (furosemide, piretanide, bumetanide) had no influence on Vbl in non-stimulated crypts. Forskolin (10(-6) mol/l) in the bath depolarized Vbl by 29 +/- 2 mV (n = 54) and decreased luminal membrane resistance. In one-third of the experiments a spontaneous partial repolarization of Vbl was seen in the presence of forskolin. During forskolin-induced depolarization basolateral application of loop diuretics hyperpolarized Vbl significantly and concentration dependently with a potency sequence of bumetanide > piretanide > or = furosemide. Lowering bath Cl- concentration hyperpolarized Vbl. Lowering of luminal Cl- concentration from 120 to 32 mmol/l during forskolin-induced depolarization led to a further depolarization of Vbl by 7 +/- 2 mV (n = 10). We conclude that Vbl of rabbit colonic crypt cells is dominated by a K+ conductance. Stimulation of the cells by forskolin opens a luminal Cl- conductance. Basolateral uptake of Cl- occurs via a basolateral Na+:2Cl-:K+ cotransport system.

Small and intermediate conductance chloride channels in HT29 cells

Recently, it has been shown that intermediate conductance outwardly rectifying chloride channels (ICOR) are blocked by cytosolic inhibitor (C. I.) found in the cytosol of human placenta and epithelial cells. C. I. also reduced the baseline current in excised membrane patches of HT29 cells. In the present study, this effect of C. I. was characterized further. Heat treated human placental cytosol was extracted in organic solvents and dissolved in different electrolyte solutions. It is shown that the reduction of baseline conductance (g(o)) is caused by inhibition of small non-resolvable channels, which are impermeable to Na+ and SO4(2-), but permeable to Cl-. The regulation of these small Cl(-)-conducting channels (g(o)) and of ICOR was examined further. First, no activating effects of protein kinase A (PKA) on the open probability (Po) of the ICOR or on the g(o)) were observed. The Po of the ICOR was reduced by 22% in a Ca(2+)-free solution. g(o) was insensitive to changes in the Ca2+ activity. The effects of C. I. from a cystic fibrosis (CF) placenta and the CF pancreatic duct cell line CFPAC-1 were compared with the effects of corresponding control cytosols, and no significant differences between CF and control cytosols were found. We conclude that the excised patches of HT29 cells contain ICOR and small non-resolvable Cl(-)-conducting channels which are similarly inhibited by C. I.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-induced intracellular Ca2+ transients in HT29 cells

In the present study we have investigated the mechanism of intracellular Ca2+ activity ([Ca2+]i) changes in HT29 cells induced by adenosine triphosphate (ATP), carbachol (CCH), and neurotensin (NT). [Ca2+]i was measured with the fluorescent Ca2+ indicator fura-2 at the single-cell level or in small cell plaques with high time resolution (1-40Hz). ATP and CCH induced not only a dose-dependent [Ca2+]i peak response, but also changes of the plateau phase. The [Ca2+]i plateau was inversely dependent on the ATP concentration, whereas the CCH-induced [Ca2+]i plateau increased at higher CCH concentrations. NT showed (from 10(-10) to 10(-7) mol/l) in most cases only a [Ca2+]i spike lasting 2-3 min. The [Ca2+]i plateau induced by ATP (10(-6) mol/l) and CCH (10(-5) mol/l) was abolished by reducing the Ca2+ activity in the bath from 10(-3) to 10(-4) mol/l (n = 7). In Ca(2+)-free bathing solution the [Ca2+]i peak value for all three agonists was not altered. Using fura-2 quenching by Mn2+ as an indicator of Ca2+ influx the [Ca2+]i peak was always reached before Mn2+ influx started. Every agonist showed this delayed stimulation of the Ca2+ influx with a lag time of 23 +/- 1.5 s (n = 15) indicating a similar mechanism in each case. Verapamil (10(-6)-10(-4) mol/l) blocked dose dependently both phases (peak and plateau) of the CCH-induced [Ca2+]i increase. Short pre-incubation with verapamil augmented the effect on the [Ca2+]i peak, whereas no further influence on the plateau was observed. Ni2+ (10(-3) mol/l) reduced the plateau value by 70%.

Intracellular Ca2+ transients in HT29 cells induced by hypotonic cell swelling

Cell swelling induced by hypotonic solution led to an osmolality-dependent increase in intracellular Ca2+ activity ([Ca2+]i) in HT29 cells. At moderate reductions in osmolality from 290 to 240 or 225 mosmol/l in most cases only a small monophasic increase of [Ca2+]i to a stable plateau of 10-20 nmol/l above resting [Ca2+]i was observed. Lower osmolalities resulted in a triphasic increase of [Ca2+]i to a peak value. In a first phase after the volume change, lasting 20-40 s, [Ca2+]i increased slowly by about 30 nmol/l. Thereafter [Ca2+]i increased more rapidly within 20-30 s to a peak value. This peak was 189 +/- 45 nmol/l (190 mosmol/l, n = 9) and 243 +/- 41 nmol/l (160 mosmol/l, n = 20) above resting [Ca2+]i. The peak was then followed by a decline of [Ca2+]i over the next 2-3 min to a stable plateau value of 28 +/- 6 (n = 6) and 32 +/- 11 nmol/l (n = 11) above resting [Ca2+]i at 190 and 160 mosmol/l, respectively. The plateau lasted as long as the hypotonic solution was present. Under Ca(2+)-free bath conditions the peak value for the cell-swelling-induced [Ca2+]i transient was reached significantly later (60-100 s, compared to 40-60 s under control conditions). The peak values under Ca(2+)-free conditions were not significantly lower. This indicates that the [Ca2+]i peak was mostly of intracellular origin. No [Ca2+]i plateau phase was observed under Ca(2+)-free bath conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-dependent activation of small-conductance Cl- channels in HT29 colon carcinoma cells

The present study was performed to examine the conductance properties in the colon carcinoma cell line HT29 and the activation of Cl- channels by cAMP. A modified cell-attached nystatin patch-clamp technique was used, allowing for the simultaneous recording of the cell membrane potential (PD) and the conductance properties of the cell-attached membrane. In resting cells, PD was -56 +/- 0.4 mV (n = 294). Changing the respective ion concentrations in the bath indicate that these cells possess a dominating K+ conductance and a smaller Cl- conductance. A significant non-selective cation conductance, which could not be inhibited by amiloride, was only observed in cells examined early after plating. The K+ conductance was reversibly inhibited by 1 - 5 mmol/l Ba2+. Stimulation of the cells by the secretagogues isoproterenol and vasointestinal polypeptide (VIP) depolarized PD and induced a Cl- conductance. Similar results were obtained with compounds increasing cytosolic cAMP: forskolin, 3-isobutyl-1-methylxanthine, cholera toxin and 8-bromoadenosine cyclic 3',5'-monophosphate (8-Br-cAMP). VIP (1 nmol/l, n = 10) and isoproterenol (1 mumol/l, n = 12) depolarized the cells dose-dependently and reversibly by 12 +/- 2 mV and 13 +/- 2 mV. The maximal depolarization was reached after some 20 s. The depolarization was due to increases in the fractional Cl- conductance. Simultaneously the conductance of the cell-attached membrane increased from 155 +/- 31 pS to 253 +/- 40 pS (VIP, n = 4) and from 170 +/- 43 pS to 268 +/- 56 pS (isoproterenol, n = 11), reflecting the gating of Cl- channels in the cell-attached membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Small-conductance Cl- channels in HT29 cells: activation by Ca2+, hypotonic cell swelling and 8-Br-cGMP

The present study demonstrates the activation of Cl- channels in HT29 cells by agonist (ATP, neurotensin, carbachol) increasing cytosolic Ca2+, by hypotonic cell swelling and by cGMP. Cell-attached nystatin patch-clamp (CAN) as well as slow and fast whole-cell recordings were used. The cell membrane potential was depolarized in a dose-dependent manner with half-maximal effects at 0.4 mumol/l for ATP, 60 pmol/l for neurotensin and 0.8 mumol/l for carbachol. The depolarization, which was caused by Cl- conductances increases, occurred within 1 s and was accompanied by a simultaneous and reversible increase of the input conductance of the cell-attached membrane from 295 +/- 32 pS to 1180 +/- 271 pS (ATP; 10 mumol/l, n = 21) and 192 +/- 37 pS to 443 +/- 128 pS (neurotensin; 1 nmol/l, n = 8). The effects of the agonists could be mimicked by ionomycin (0.2 mumol/l), suggesting that an increase in intracellular Ca2+ was responsible for the activation of Cl- channels. The depolarization was followed by a secondary hyperpolarization. Hypotonic cell swelling also depolarized the cells and induced an increase in the membrane conductance. With 120 mmol/l NaCl the depolarization was 10 +/- 0.8 mV and the cell-attached conductance increased from 228 +/- 29 pS to 410 +/- 65 (n = 26) pS. NaCl at 90 mmol/l and 72.5 mmol/l had even stronger effects. Comparable conductance increases were also obtained when the different agonists or hypotonic cell swelling were examined in whole cell experiments. 5-Nitro-2-(3-phenylpropylamino)-benzoate (1 mumol/l) did not prevent the effects of Ca(2+)-increasing hormones and of hypotonic solutions.(ABSTRACT TRUNCATED AT 250 WORDS)