Cholinergic stimulation produces oscillations of cytosolic Ca2+ in a secretory epithelial cell line, T84

Ca2+- and PKC-dependent stimulation of PGE2 synthesis by deoxycholic acid in human colonic fibroblasts

We investigated prostanoid biogenesis by human colonic fibroblasts (CCD-18Co cells and nine primary fibroblast cultures) exposed to a primary (cholic, CA) or a secondary (deoxycholic, DCA) bile acid. Basal PGE2 levels in CCD-18Co cultures and fibroblast strains initiated from normal and adenocarcinomatous colon, respectively, were 1.7 +/- 0.3, 4.0 +/- 2.0, and 15.0 +/- 4.8 ng/mg protein. Peak levels 24 h after exposure to DCA (300 microM) rose, respectively, seven-, six- and sevenfold, but CA elicited no such responses. Increases in PGE2 synthesis were preceded by sequential increases in PGH synthase-2 mRNA and protein expression and were fully prevented by a nonselective (indomethacin) or a selective (celecoxib) nonsteroidal anti-inflammatory drug. DCA, but not CA, caused abrupt, transient increases in fibroblast intracellular Ca2+ concentration ([Ca2+]i) approximately 1 min after exposure. Increased [Ca2+]i was required for DCA-mediated induction of PGE2 synthesis, and protein kinase C was a further essential component of this signaling pathway. Colonic fibroblasts may be a major target for prostanoid biogenesis induced by fecal bile acids and, potentially, other noxious actions of these agents.

Splice variants of a ClC-2 chloride channel with differing functional characteristics

We identified two ClC-2 clones in a guinea pig intestinal epithelial cDNA library, one of which carries a 30-bp deletion in the NH(2) terminus. PCR using primers encompassing the deletion gave two products that furthermore were amplified with specific primers confirming their authenticity. The corresponding genomic DNA sequence gave a structure of three exons and two introns. An internal donor site occurring within one of the exons accounts for the deletion, consistent with alternative splicing. Expression of the variants gpClC-2 and gpClC-2Delta77-86 in HEK-293 cells generated inwardly rectifying chloride currents with similar activation characteristics. Deactivation, however, occurred with faster kinetics in gpClC-2Delta77-86. Site-directed mutagenesis suggests that a protein kinase C-mediated phosphorylation consensus site lost in gpClC-2Delta77-86 is not responsible for the observed change. The deletion-carrying variant is found in most tissues examined, and it appears more abundant in proximal colon, kidney, and testis. The presence of a splice variant of ClC-2 modified in its NH(2)-terminal domain could have functional consequences in tissues where their relative expression levels are different.

Pharmacological activation of cloned intermediate- and small-conductance Ca(2+)-activated K(+) channels

We previously characterized 1-ethyl-2-benzimidazolinone (1-EBIO), as well as the clinically useful benzoxazoles, chlorzoxazone (CZ), and zoxazolamine (ZOX), as pharmacological activators of the intermediate-conductance Ca(2+)-activated K(+) channel, hIK1. The mechanism of activation of hIK1, as well as the highly homologous small-conductance, Ca(2+)-dependent K(+) channel, rSK2, was determined following heterologous expression in Xenopus oocytes using two-electrode voltage clamp (TEVC) and excised, inside-out patch-clamp techniques. 1-EBIO, CZ, and ZOX activated both hIK1 and rSK2 in TEVC and excised inside-out patch-clamp experiments. In excised, inside-out patches, 1-EBIO and CZ induced a concentration-dependent activation of hIK1, with half-maximal (K(1/2)) values of 84 microM and 98 microM, respectively. Similarly, CZ activated rSK2 with a K(1/2) of 87 microM. In the absence of CZ, the Ca(2+)-dependent activation of hIK1 was best fit with a K(1/2) of 700 nM and a Hill coefficient (n) of 2.0. rSK2 was activated by Ca(2+) with a K(1/2) of 700 nM and an n of 2.5. Addition of CZ had no effect on either the K(1/2) or n for Ca(2+)-dependent activation of either hIK1 or rSK2. Rather, CZ increased channel activity at all Ca(2+) concentrations (V(max)). Event-duration analysis revealed hIK1 was minimally described by two open and three closed times. Activation by 1-EBIO had no effect on tau(o1), tau(o2), or tau(c1), whereas tau(c2) and tau(c3) were reduced from 9.0 and 92.6 ms to 5.0 and 44.1 ms, respectively. In conclusion, we define 1-EBIO, CZ, and ZOX as the first known activators of hIK1 and rSK2. Openers of IK and SK channels may be therapeutically beneficial in cystic fibrosis and vascular diseases.

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.

Psoralens: novel modulators of Cl- secretion

We evaluated effects of psoralens on Cl- secretion (short-circuit current, I(sc)) across T84 monolayers. Methoxsalen failed to increase I(sc). Several observations suggest that psoralens open cystic fibrosis transmembrane conductance regulator Cl- channels. 1) After activation of the Ca2+-dependent basolateral membrane K+ channel (K(Ca)) by 1-ethyl-2-benzimidazolinone or thapsigargin, methoxsalen (10 microM) further increased I(sc). 2) When added before carbachol (CCh), methoxsalen potentiated the I(sc) response to CCh, as predicted, if it increased apical Cl- conductance. 3) After establishment of a mucosal-to-serosal Cl- gradient and permeabilization of basolateral membrane with nystatin, psoralens increased Cl- current, which was inhibited by glibenclamide. In contrast, neither TS-TM calix[4]arene nor Cd2+, inhibitors of outwardly rectifying Cl- channels and the ClC-2 Cl-channel, respectively, inhibited psoralen-induced Cl- current. In contrast to their effects on Cl- conductance, psoralens failed to significantly affect basolateral membrane K+ conductance; subsequent addition of 1-ethyl-2-benzimidazolinone induced a large increase in K+ conductance. Also, in excised patches, methoxsalen failed to activate K(Ca). In addition to potentiating the peak response to CCh, psoralens induced a secondary, sustained response. Indeed, when added up to 60 min after return of CCh-induced I(sc) to baseline, psoralens induced a sustained I(sc). This sustained response was inhibited by atropine, demonstrating the requirement for continuous muscarinic receptor activation by CCh. This sustained response was inhibited also by verapamil, removal of bath Ca2+, and charybdotoxin. These results suggest that return of I(sc) to baseline after CCh stimulation is not due to downregulation of Ca2+ influx or K(Ca). Finally, we obtained similar results with psoralens in rat colon and primary cultures of murine tracheal epithelium. On the basis of these observations, we conclude that psoralens represent a novel class of Cl- channel openers that can be used to probe mechanisms underlying Ca2+-mediated Cl- secretion.

Metabotropic glutamate receptor activation in cerebellar Purkinje cells as substrate for adaptive timing of the classically conditioned eye-blink response

To understand how the cerebellum adaptively times the classically conditioned nictitating membrane response (NMR), a model of the metabotropic glutamate receptor (mGluR) second messenger system in cerebellar Purkinje cells is constructed. In the model, slow responses, generated postsynaptically by mGluR-mediated phosphoinositide hydrolysis and calcium release from intracellular stores, bridge the interstimulus interval (ISI) between the onset of parallel fiber activity associated with the conditioned stimulus (CS) and climbing fiber activity associated with unconditioned stimulus (US) onset. Temporal correlation of metabotropic responses and climbing fiber signals produces persistent phosphorylation of both AMPA receptors and Ca(2+)-dependent K+ channels. This is responsible for long-term depression (LTD) of AMPA receptors. The phosphorylation of Ca(2+)-dependent K+ channels leads to a reduction in baseline membrane potential and a reduction of Purkinje cell population firing during the CS-US interval. The Purkinje cell firing decrease disinhibits cerebellar nuclear cells, which then produce an excitatory response corresponding to the learned movement. Purkinje cell learning times the response, whereas nuclear cell learning can calibrate it. The model reproduces key features of the conditioned rabbit NMR: Purkinje cell population response is timed properly; delay conditioning occurs for ISIs of up to 4 sec, whereas trace conditioning occurs only at shorter ISIs; mixed training at two different ISIs produces a double-peaked response; and ISIs of 200-400 msec produce maximal responding. Biochemical similarities between timed cerebellar learning and photoreceptor transduction, and circuit similarities between the timed cerebellar circuit and a timed dentate-CA3 hippocampal circuit, are noted.

Cross-talk between calcium and cAMP-dependent intracellular signaling pathways. Implications for synergistic secretion in T84 colonic epithelial cells and rat pancreatic acinar cells

Treatment of various cells with combinations of agents that increase either cAMP or cytosolic calcium can lead to synergistic responses. This study examined interactions, or cross-talk, between these two intracellular messengers and its implication for signaling in two secretory cell types, T84 human colonic epithelial cells and rat pancreatic acinar cells. T84 cell chloride secretion was measured in Ussing chambers. Acinar cell activation was monitored as amylase secretion. Cytosolic calcium was assessed via fura-2 microfluorimetry. A cell-permeant analogue of cAMP synergistically enhanced secretory responses to calcium-mobilizing hormones in both cell types, but paradoxically reduced overall calcium mobilization. The reduction in calcium mobilization could be attributed to an inhibition of calcium influx in T84 cells, although a different mechanism likely operates in acinar cells. The effects of the cAMP analogue were reproduced by other agents that increase cAMP. Furthermore, econazole, an inhibitor of calcium influx, potentiated secretory responses to calcium-dependent stimulation in T84 cells without itself inducing secretion. We conclude that there is cross-talk between calcium and cAMP-dependent signaling pathways at the level of second messenger generation in two secretory cell types. This cross-talk appears to regulate the extent of secretory responses.

ATP-sensitive K+ channels regulated by intracellular Ca2+ and phosphorylation in normal (T84) and cystic fibrosis (CFPAC-1) epithelial cells

The elementary K+ conductance activated by the cAMP or the Ca2+ second messenger pathways was investigated in the model salt-secreting epithelium, the human T84 cell line. Under Cl(-)-free conditions, an inwardly rectifying whole-cell K+ current was evoked by either forskolin 10 (mumol/l) or acetylcholine 1 (mumol/l) and blocked by extracellular charybdotoxin 10 (nmol/l). In the cell-attached mode, both secretory agonists induced the opening of a channel showing inward rectification with a unitary chord conductance of 36.8 +/- 2.5 pS (n = 26) for inward currents. In inside-out patches, a 35-pS inwardly rectifying K+ channel that corresponded to the channel recorded in the cell-attached configuration was recorded in the presence of 0.3 mumol/l free Ca2+ at the inner side of the membrane. This channel was blocked by Ba2+ (5 mumol/l) and by charybdotoxin (50 nmol/l). Its open probability was enhanced by intracellular Ca2+ with and EC50 of 0.25 mumol/l and strongly reduced by intracellular MgATP with an IC50 of 600 mumol/l. In the continuous presence of ATP, the channel activity was consistently increased by 125 kU/l catalytic subunit of cAMP-dependent protein kinase. In the cystic fibrosis pancreatic duct cell line CFPAC-1, a K+ channel was also recorded, with similar characteristics and regulation as the 35-pS channel in T84 cells. We conclude that an ATP-sensitive K+ channel regulated by intracellular Ca2+ and phosphorylation supports the main K+ current activated by secretory agonists in normal cystic fibrosis cell lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurodeoxycholate activates potassium and chloride conductances via an IP3-mediated release of calcium from intracellular stores in a colonic cell line (T84)

Whole-cell patch-clamp techniques and fluorescence measurements of intracellular Ca2+ concentration, (Ca2+)i, were used to investigate the mechanism of taurodeoxycholate (TDC) stimulation of Cl- secretion in the T84 colonic cell line. During perforated whole-cell recordings, the cell membrane voltage was alternately clamped to EK and ECl. Initially, TDC (0.75 mM) stimulated inward nonselective cation currents that were composed of discrete large conductance single-channel events. This initial response was followed by activation of K+ and Cl- currents with peak values of 385 +/- 41 pA and 98 +/- 28 pA, respectively (n = 12). The K+ and Cl- currents oscillated while TDC was present and returned to baseline levels upon its removal. The threshold for activation of the oscillatory currents was 0.1 mM TDC. Taurocholate, a bile acid that does not stimulate colonic Cl- secretion, induced no current response. The TDC-induced currents could be activated in Ca(2+)-free bathing solutions. Preincubation of cells with the Ca2+ chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethy)-ester (20 microM), (BAPTA-AM), eliminated the K+ and Cl- current responses, although the nonselective cation channel events were still present. Replacement of bath Na+ with NMDG+ inhibited the TDC-induced nonselective cation current but did not affect the K+ or Cl- currents. TDC induced a transient (Ca2+)i rise of 575 +/- 70 nM from a baseline of 71 +/- 5 nM (n = 15); thereafter, (Ca2+)i either plateaued or oscillated. TDC-induced (Ca2+)i oscillations were observed in the absence of bath Ca2+; however, removal of bath Ca2+ during the TDC response caused (Ca2+)i to return to near baseline values. Simultaneous K+ current and (Ca2+)i measurements confirmed that the initial nonselective cation current was independent of (Ca2+)i, while K+ current oscillations were in phase with the (Ca2+)i oscillations. TDC induced inositol monophosphate (IP) accumulation, reflecting production of inositol 1,4,5-trisphosphate (IP3) during TDC stimulation. The response to TDC during standard whole-cell patch-clamp was similar to that observed with perforated whole-cell recordings, except the nonselective cation current was prolonged. When heparin (1 mg/ml) was added to the pipette under these conditions, the Ca(2+)-activated currents were inhibited, but the nonselective cation currents were unaffected. These data suggest that TDC induces a Ca(2+)-independent nonselective cation conductance, perhaps by directly permeabilizing the plasma membrane. TDC stimulates Cl- secretion by activating K+ and Cl- conductances via an IP3-mediated release of Ca2+ from intracellular stores.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Intracellular calcium increases with hyperactivation in intact, moving hamster sperm and oscillates with the flagellar beat cycle

At some time before fertilization, mammalian sperm undergo a change in movement pattern, termed hyperactivation. There is evidence that hyperactivation offers an advantage to sperm for detaching from the oviductal mucosa, for penetrating viscoelastic substances in the oviduct, and for penetrating the zona pellucida. Hyperactivation is known to require extracellular calcium, but little else is known about the mechanisms by which calcium affects sperm movement. The calcium-sensitive fluorescent dye indo-1 was used to follow intracellular calcium levels ([Ca2+]i) in individual moving sperm. Sperm were loaded with 10 microM of the acetoxymethyl ester form of the dye and then rinsed. The dye was excited at 340 nm by using a filtered xenon stroboscope, and images at the 405-nm and 490-nm excitation maxima were simultaneously digitized at 30 per sec for 2.1 sec. [Ca2+]i was significantly higher in the acrosomal and postacrosomal regions of the head and in the flagellar midpiece (the principal piece could not be measured) in hyperactivated than in nonhyperactivated sperm (P < 0.0001). [Ca2+]i oscillations were detected in the proximal half of the midpiece that were identical in frequency to the flagellar-beat-cycle frequency in 12 of 17 hyperactivated sperm (median, 3.5 Hz). Rapid [Ca2+]i oscillations were also detected in the acrosomal and postacrosomal regions, as well as in the distal midpiece. Oscillations were not eliminated by dampening the flagellar bending with methyl cellulose. The [Ca2+]i oscillations detected in sperm are significantly more rapid than oscillations detected in other cell types.

The inward rectifier K+ current underlies oscillatory membrane potential behaviour in bovine pigmented ciliary epithelial cells

1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using whole-cell patch-clamp techniques. At the holding potential of -70 mV, and with EK (potassium equilibrium potential) set at -84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0.5 mM K+ and progressively more inward in 20 and 50 mM K+. 2. The inward rectifier showed V-EK dependence; increasing [K+]o increased the inward conductance from 1.28 nS in 5 mM K+ to 7.42 nS in 50 mM K+. The conductance at a given V-EK was proportional to the square root of [K+]o. 3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2+ (85% with Ki (concentration giving half-maximal inhibition) = 3.1 x 10(-5) M) and TEA+ (74% with Ki = 2.9 x 10(-4) M). 4. The activation time constant was voltage dependent, decreasing from 5 ms to 0.7 ms over the voltage range -90 to -170 mV. With increasing hyperpolarization the current exhibited time-dependent decay. The time constant for this process was voltage sensitive but the steady-state inactivation was independent of external [K+]. 5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]o. 7. In current clamp the cells exhibited an unstable region at a potential of around -70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0.5 mM K+) active inward rectification. 8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.