Regulation of the Na+2Cl–K+ cotransporter in in vitro perfused rectal gland tubules of Squalus acanthias

Previously it has been shown that the Na+2Cl–K+ cotransporter accepts NH4 + at its K+ binding site. This property can be used to estimate its transport rates by adding NH4 + to the bath and measuring the initial furosemide-dependent rates of change in BCECF fluorescence. We have utilized this technique to determine the regulation of the furosemide-inhibitable Na+2Cl–K+ cotransporter in in vitroperfused rectal gland tubules (RGT) of Squalus acanthias. Addition of NH4 + to the bath (20 mmol/l) led to an initial alkalinization, corresponding to NH3 uptake. This was followed by an acidification, corresponding to NH4 + uptake. The rate of this uptake was quantified by exponential curve fitting and is given in arbitrary units (Δfluorescence/time). This acidification could be completely inhibited by furosemide. In the absence of any secretagogue preincubation of RGT in a low Cl– solution (6 mmol/l, low Cl–) for 10 min enhanced the uptake rate significantly from 4.04±0.51 to 12.7±1.30 (n=5). The addition of urea (200 mmol/l) was without effect, but the addition of 300 mmol/l mannitol (+300 mannitol) enhanced the rate significantly from 7.24±1.33 to 14.7±4.6 (n=6). Stimulation of NaCl secretion by a solution maximizing the cytosolic cAMP concentration (Stim) led to a significant increase in NH4 + uptake rate from 5.00±1.33 to 13.3±1.54 (n=6). Similar results were obtained in the additional presence of Ba2+ (1 mmol/l): the uptake rate was increased significantly from 4.23±0.34 to 15.1±1.86 (n=16). In the presence of Stim low Cl– had no additional effect on the uptake rate: 15.1±3.1 versus 15.2±2.8 in high Cl– (n=6). The uptake rate in Stim containing additional +300 mannitol (22.3±4.0, n=5) was not significantly different from that obtained with Stim or +300 mannitol alone. By whatever mechanism the NH4 + uptake rate was increased furosemide (500 µmol/l) always reduced this rate to control values. Hence three manoeuvres enhanced furosemide-inhibitable uptake rates of the Na+2Cl–K+ cotransporter probably independently: (1) lowering of cytosolic Cl– concentration; (2) cell shrinkage; and (3) activation by cAMP.

Does stimulation of NaCl secretion in in vitro perfused rectal gland tubules of Squalus acanthias increase membrane capacitance?

NaCl secretion in rectal gland tubules (RGT) of Squalus acanthias requires the activation of Cl– channels in the luminal membrane. The RGT and its mechanism of activation are an early evolutionary paradigm of exocrine secretion. The respective Cl– channels probably resemble the shark equivalent of the cystic fibrosis transmembrane conductance regulator (CFTR). Activation of these Cl– channels occurs via cAMP. It has been hypothesized that the activation of CFTR occurs via exocytosis or inhibited endocytosis. To examine this question directly by electrical measurements we have performed whole-cell patch-clamp analyses of in vitro perfused RGT. NaCl secretion was stimulated by a solution (Stim) containing forskolin (10 µmol/l), dibutyryl-cAMP (0.5 mmol/l) and adenosine (0.5 mmol/l). This led to the expected strong depolarization and an increase in membrane conductance (G m). The membrane capacitance (C m) was measured by a newly devised two-frequency synchronous detector method. It was increased by Stim significantly from 5.00±0.22 to 5.17±0.21 pF (n=50). The increase in C m correlated with the increase in G m with a slope of 51 fF/nS. Next the effect of furosemide (500 µmol/l) was examined in previously stimulated RGT. Furosemide was supposed to inhibit coupled Na+2Cl–K+ uptake and to reduce cell volume but not membrane trafficking of Cl– channels. Furosemide reduced G m slightly (due to the fall in cytosolic Cl– concentration) and C m to the same extent by which Stim had increased it. Both changes were statistically significant, and the slope of ΔC m/ΔG m was similar to that caused by Stim. Inhibitors of microtubules or actin (colchicine, phalloidin and cytochalasin D added at 10 µmol/l to the pipette solution and dialysed for >10 min) did not alter cell voltage, G m or C m, nor did these inhibitors abolish the stimulatory effect of cAMP. These data suggest that the small C m changes observed with Stim reflect a minor cell volume increase and an ”unfolding” of the plasma membrane. The present data do not support the exocytosis/endocytosis hypothesis of cAMP-mediated activation of Cl– channels in these cells.

The amiloride-inhibitable Na+ conductance is reduced by the cystic fibrosis transmembrane conductance regulator in normal but not in cystic fibrosis airways

Cystic fibrosis (CF) airway cells, besides their well-known defect in cAMP-dependent Cl- conductance, are characterized by an enhanced Na+ conductance. In this study we have examined the Na+ conductance in human respiratory tract by measuring transepithelial voltage and resistance (Vte, Rte) and by assessing membrane voltages (Vm) of freshly isolated airway epithelial cells from CF and non-CF patients. Basal amiloride inhibitable (10 micromol/liter) equivalent short circuit current (Isc = Vte/Rte) was significantly increased in CF compared with non-CF tissues. After stimulation by forskolin (10 micromol/liter) a significant depolarization of Vm corresponding to the cAMP-dependent activation of a Cl- conductance was observed in non-CF but not in CF airway cells. In non-CF tissue but not in CF tissue the effects of amiloride and N-methyl-D-glucamine on Vm were attenuated in the presence of forskolin. Also the amiloride-inhibitable Isc was significantly reduced by forskolin (1 micromol/liter) and isobutylmethylxanthine (IBMX; 100 micromol/liter) only in non-CF tissue. We conclude that cystic fibrosis transmembrane conductance regulator acts as a downregulator of epithelial Na+ channels in human airways. This downregulation of epithelial Na+ channels is absent in CF airways, leading to hyperabsorption and to the characteristic increase in mucus viscosity.

Cloning of sgk serine-threonine protein kinase from shark rectal gland – a gene induced by hypertonicity and secretagogues

Recently, the cell-volume-regulated serine-threonine protein kinase h-sgk was cloned from a human hepatoma cell line. The sgk gene was shown to be induced by cell shrinkage in many different mammalian cell lines. In this study, two highly conserved serine-threonine protein kinases, sgk-1 and sgk-2, were cloned from rectal gland tissue of the spiny dogfish (Squalus acanthias). Both kinases showed a distinct pattern of tissue specificity, with high expression levels in kidney, intestine, liver and heart. In rectal gland slices sgk-1 transcription was induced by exposure to hypertonic solution, reduction of the extracellular urea concentration, and addition of the secretagogues vasoactive intestinal polypeptide (VIP) and carbachol. The shark sgk-1 serine-threonine protein kinase may therefore provide a link between cell volume, Cl–secretion and protein phosphorylation state in shark rectal gland cells.

No evidence for cell-to-cell coupling in rat colonic crypts: studies with Lucifer Yellow and with photobleaching

Epithelial cells of exocrine glands (pancreas, lacrimal glands, salivary glands, sweat glands and gastric glands) are intimately linked together by gap junctions. Due to this close junctional coupling exocrine secretion occurs as the well concerted effort of a cell population. Colonic crypts have, on the one hand, anatomical and functional properties resembling those of exocrine glands (mostly crypt base cells) and, on the other hand, properties of absorbing cells (mostly surface cells). In the mid-crypt, depending on the functional status, absorption and secretion can occur. The present study was aimed at examining whether rat distal colonic crypt cells co-ordinate their functional status by cell-to-cell coupling. Two types of measurements were performed: as an independent assessment of cell viability the membrane voltage (Vm) was measured with the fast whole-cell patch-clamp technique; to investigate cellular coupling simultaneously Lucifer Yellow (LY) (mol. wt. 443) distribution was visualized using digital video imaging. LY (500 micromol/l) was included into the patch pipette filling solution. The recorded Vm was -73.4+/-2.3 mV in crypt base cells (n=15), -63.7+/-2.1 mV in mid-crypt cells (n=17) and -52.3+/-2. 9 mV in crypt surface cells. All cells tested reversibly responded to carbachol (100 micromol/l) with a persistent hyperpolarization, as previously shown. Activation of Cl- secretion by elevation of the cAMP concentration with forskolin (5 micromol/l) led to a reversible depolarization. Throughout the duration of each individual experiment [mean experimental time in basal cells: 18.3+/-2.5 min (n=15), in mid-crypt cells: 19.6+/-3.4 min (n=17) and in crypt surface cells: 11.7+/-3.4 min (n=13)] LY dye distribution was solely confined to the patched cell. In addition bleaching of calcein fluorescence in laser scan microscopy was not followed by dye back diffusion, whereas this was clearly the case in pancreatic acini (n=5). These data indicate that colonic crypt cells are not coupled by gap junctions under resting conditions or in the presence of secretagogues.

The role of cytosolic Ca2+ in the secretion of NaCl in isolated in vitro perfused rectal gland tubules of Squalus acanthias

In many exocrine glands cytosolic Ca2+ ([Ca2+]i) plays a pivotal role in stimulation-secretion coupling. In the rectal gland of the dogfish Squalus acanthias this appears not to be the case and it is believed that secretion is mainly controlled by the Cl- conductance of the luminal membrane. We have examined this question in a study of isolated in vitro perfused rectal gland tubules (RGT). Three types of measurements were performed: (1) measurements of [Ca2+]i by the fura-2 technique; (2) measurements of transepithelial electrical parameters, i.e. transepithelial voltage (Vte), transepithelial resistance (Rte), the equivalent short-circuit current (Isc) and the voltage across the basolateral membrane (Vbl), and (3) whole-cell patch-clamp measurements of cellular voltage (Vm), conductance (Gm) and membrane capacitance (Cm). The data indicates that carbachol (CCH) increases [Ca2+]i by increasing store release and Ca2+ influx. Other agonists, producing cytosolic cAMP, also increased [Ca2+] by enhancing Ca2+ influx. CCH hyperpolarized these cells and enhanced Gm significantly. The effect of CCH on Vte and Isc was most marked under control conditions and disappeared in RGT otherwise stimulated by agonists that lead to cAMP production. It is concluded that [Ca2+]i plays a major role in the stimulation of NaCl secretion in RGT by enhancing the basolateral K+ conductance. cAMP-producing agonists enhance [Ca2+]i by increased Ca2+ influx. CCH releases Ca2+ from respective stores. CCH, unlike the cAMP-producing agonists, only increases basolateral K+ conductance. It modulates secretion especially under conditions in which the cAMP pathway is not fully activated.

Cl- transport by cystic fibrosis transmembrane conductance regulator (CFTR) contributes to the inhibition of epithelial Na+ channels (ENaCs) in Xenopus oocytes co-expressing CFTR and ENaC

1. Epithelial Na+ channels (ENaCs) are inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) when CFTR is activated by protein kinase A. Since cAMP-dependent activation of CFTR Cl- conductance is defective in cystic fibrosis (CF), ENaC currents are not inhibited by CFTR. This could explain the enhanced Na+ conductance found in CF. In the present study, we examined possible mechanisms of interaction between CFTR and ENaC co-expressed in Xenopus oocytes. 2. The magnitude of CFTR Cl- currents activated by 3-isobutyl-1-methylxanthine (IBMX) in oocytes co-expressing either wild-type or mutant CFTR and ENaC determined the degree of downregulation of ENaC currents. 3. The ability of CFTR to inhibit ENaC currents was significantly reduced either when extracellular Cl- was replaced by poorly conductive anions, e.g. SCN- or gluconate, or when CFTR was inhibited by diphenylamine-carboxylate (DPC, 1 mmol l-1). 4. Downregulation of ENaC was more pronounced at positive when compared with negative clamp voltages. This suggests that outward currents, i.e. influx of Cl- through activated CFTR most effectively downregulated ENaC. 5. Activation of endogenous Ca2+-activated Cl- currents by 1 micromol l-1 ionomycin did not inhibit ENaC current. This suggests that inhibition of ENaC mediated by Cl- currents may be specific to CFTR. 6. The present findings indicate that downregulation of ENaC by CFTR is correlated to the ability of CFTR to conduct Cl-. The data have implications for how epithelia switch from NaCl absorption to NaCl secretion when CFTR is activated by secretagogues.

Catecholamines modulate podocyte function

The aim of this study was to investigate the influence of adrenoceptor agonists on the intracellular calcium activity ([Ca2+]i), membrane voltage (Vm), and ion conductances (Gm) in differentiated mouse podocytes. [Ca2+]i was measured by the Fura-2 fluorescence method in single podocytes. Noradrenaline and the alpha 1-adrenoceptor agonist phenylephrine induced a reversible and concentration-dependent biphasic increase of [Ca2+]i in podocytes (EC50 approximately 0.1 microM for peak and plateau), whereas the alpha 2-adrenoceptor agonist UK 14.304 did not influence [Ca2+]i. The [Ca2+]i response induced by noradrenaline was completely inhibited by the alpha 1-adrenoceptor antagonist prazosin (10 nM). In a solution with a high extracellular K+ (72.5 mM), [Ca2+]i was unchanged and the [Ca2+]i increase induced by noradrenaline was not inhibited by the L-type Ca2+ channel blocker nicardipine (1 microM). Vm and Gm were examined with the patch-clamp technique in the slow whole-cell configuration. Isoproterenol, phenylephrine, and noradrenaline depolarized podocytes and increased Gm. The order of potency for the adrenoceptor agonists was isoproterenol (EC50 approximately 1 nM) > noradrenaline (EC50 approximately 0.3 microM) > phenylephrine (EC50 approximately 0.5 microM). The beta 2-adrenoceptor antagonist ICI 118.551 (5 to 100 nM) inhibited the effect of isoproterenol on Vm. Stimulation of adenylate cyclase by forskolin mimicked the effect of isoproterenol on Vm and Gm (EC50 approximately 40 nM). Isoproterenol induced a time- and concentration-dependent increase of cAMP in podocytes. The effect of isoproterenol was unchanged in the absence of Na+ or in an extracellular solution with a reduced Ca2+ concentration, whereas it was significantly increased in an extracellular solution with a reduced Cl- concentration (from 145 to 32 mM). The data indicate that adrenoceptor agonists regulate podocyte function: They increase [Ca2+]i via an alpha 1-adrenoceptor and induce a depolarization via a beta 2-adrenoceptor. The depolarization is probably due to an opening of a cAMP-dependent Cl- conductance.

The basolateral Ca2+-dependent K+ channel in rat colonic crypt cells

Previous studies have indicated that a 16-pS K+ channel (KCca) in the basolateral membrane is responsible for the acetylcholine-induced whole-cell K+ conductance in these cells. In the present study we have examined this channel in excised inside-out patches of the basolateral membrane. Over a wide voltage range this channel showed inward rectification. The Ca2+ sensitivity was very marked, with a Hill coefficient of three and with half-maximal activation at 330 nmol/l. After several minutes most channels showed a slow run-down. Channel activity could be refreshed by addition of ATP (1 mmol/l) to the bath solution. The non-metabolizable derivative 5'-adenylylimidodiphosphate (AMP-PNP) had no such effect. In contrast, it inhibited channel activity by some 50%. ATP and its derivatives had no effect on the Ca2+ sensitivity. Channels activated by ATP were subsequently studied in the presence of alkaline (10 kU/l) or acidic (1 kU/l) phosphatase. Both phosphatases reduced channel activity significantly. These data suggest that the 16-pS K+ channel is directly controlled by cytosolic Ca2+. This regulatory step is probably distal to an activation produced by protein-kinase-C-dependent phosphorylation. As is the case for several other K+ channels, high concentrations of non-metabolizable ATP analogues inhibit this channel.

The cystic fibrosis transmembrane conductance regulator attenuates the endogenous Ca2+ activated Cl- conductance of Xenopus oocytes

Oocytes from Xenopus laevis activate a Ca2+ dependent Cl- conductance when exposed to the Ca2+ ionophore ionomycin. This Ca2+ activated Cl- conductance (CaCC) is strongly outwardly rectifying and has a halide conductivity ratio (GI- / GCl-) of about 4.4. This is in contrast to the cystic fibrosis transmembrane conductance regulator (CFTR)-Cl- conductance, which produces more linear I/V curves with a GI- / GCl- ratio of about 0.52. Ionomycin enhanced CaCC (DeltaG) in water injected and CFTR expressing ooyctes in the absence of 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/l) by (microS) 23 +/- 1.9 (n=9) and 23.6 +/- 2.3 (n=11). Stimulation by IBMX did not change CaCC in water injected oocytes. CaCC was inhibited in CFTR-expressing ooyctes after stimulation with IBMX or a membrane permeable form of cAMP and was only 5.1 +/- 0.48 microS (n=18) and 6. 9 +/- 0.6 (n=3), respectively. Inhibition of CaCC was correlated to the amount of CFTR-current activated by IBMX. DeltaF508-CFTR which demonstrates only a small residual function in activating a cAMP dependent Cl- channel in oocytes inhibited CaCC to a lesser degree (DeltaG=12.1 +/- 1.1 microS; n=7). Changes of CFTR and CaCC-Cl- whole cell conductances were also measured when extracellular Cl- was replaced by I-. The results confirmed the reduced activation of CaCC in the presence of activated CFTR. No evidence was found for inhibition of CFTR-currents by increase of intracellular Ca2+. Moreover, intracellular cAMP was not changed by ionomycin and stimulation by IBMX did not change the ionomycin induced Ca2+ increase in Xenopus oocytes. Taken together, these results suggest that activation of CFTR-Cl- currents is paralleled by an inhibition of Ca2+ activated Cl- currents in ooyctes of Xenopus laevis. These results provide another example for CFTR-dependent regulation of membrane conductances other than cAMP-dependent Cl- conductance. They might explain previous findings in epithelial tissues of CF-knockout mice.

The role of exocytosis in the activation of the chloride conductance in Chinese hamster ovary cells (CHO) stably expressing CFTR

The aim of this study was to examine the question of whether activation of wt-CFTR (wild-type cystic fibrosis transmembrane conductance regulator) by cAMP and the opening of a Cl- conductance is paralleled by exocytosis and corresponding increases in membrane capacitance. To this end three types of Chinese hamster ovary (CHO) cells were examined: a control group of CHO cells; a group of CHO cells stably expressing wt-CFTR at high levels (also called BQ2-CHO); and a group of CHO cells stably expressing the frequent mutation DeltaF508-CFTR. Whole-cell patch-clamp studies were performed to measure the membrane voltage (Vm), the membrane conductance (Gm) and the membrane capacitance (Cm). Cm was assessed by a two-frequency lock-in amplifier method. Forskolin (Fsk, 0.1 micromol/l) and isobutylmethylxanthine (IBMX, 0.1 mmol/l) were used to increase cytosolic cAMP. It is shown that Fsk and IBMX had no effect on Vm and Gm in control CHO and DeltaF508-CFTR-CHO cells. Fsk and IBMX depolarized wt-CFTR-expressing CHO cells significantly (from -40 +/- 1.5 to -32 +/- 1.6 mV, n = 41) and enhanced Gm strongly from 5.0 +/- 0.9 to 36 +/- 3.9 nS (n = 65). The conductance increase was mostly for Cl-, because under stimulated conditions a reduction in bath Cl- concentration depolarized these cells further and significantly from -26 +/- 1.8 to -10 +/- 1.2 mV (n = 16). This conductance had the characteristic wt-CFTR selectivity of Br- > Cl- > I- (n = 16). Despite this large increase in the Fsk- and IBMX-induced conductance Cm was not altered significantly (15.5 versus 15.7 pF, n = 50). These data indicate that stable overexpression of wt-CFTR but not of DeltaF508-CFTR in CHO cells induces a cAMP-activated Cl- conductance. The activation of this large conductance obviously proceeds with little if any exocytosis.

Characterization of sodium and chloride conductances in preneoplastic and neoplastic murine colonocytes

Glucocorticoids, such as dexamethasone, induce amiloride-sensitive Na+ conductances in rat distal colon epithelium. The activity of these conductances diminishes from the surface to the base of the crypt whereas cAMP-stimulated Cl- secretion decreases from the crypt base to the surface. These gradients are likely to be perturbed during carcinogenesis. We therefore determined the magnitude of Na+ and Cl- conductances in colonocytes isolated from normal and carcinogen-treated rats. Colon carcinogenesis was induced by injection of dimethylhydrazine (DMH) (18 mg/kg) for 5 weeks. Before sacrifice animals were treated for 3 days with dexamethasone. Colonocyte populations from the surface to the crypt base (C1-C5) were harvested from the distal colon by a Ca2+-chelating procedure. The activity of Na+ conductances was determined by uptake of 22Na+ by surface and crypt colonocyte populations and by membrane vesicles in the presence and absence of 10 microM amiloride. In control rats Na+ conductance was highest in surface colonocytes and absent in the crypt base. As early as 2 weeks after initiation of DMH treatment amiloride-inhibited Na+ uptake was virtually absent in the upper crypt. Transcriptional assessment of the alpha-, beta- and gamma-subunits that constitute the epithelial Na+ channel revealed that DMH treatment reduces the expression of beta-subunit mRNA. We then examined 36Cl- efflux from isolated colonocytes of normal and carcinogen-treated rats in response to forskolin (0.01 mM). Forskolin induced a marked rise in cAMP in lower crypt cells concomitant with a significant stimulation of 36Cl- efflux. Intracellular cAMP increased in upper crypt cells in response to forskolin without an increase in 36Cl- efflux. By contrast, upper crypt colonocytes from DMH-treated rats showed forskolin-stimulated efflux beginning 4 weeks after initiation of treatment. We conclude that induction of Na+ conductances by glucocorticoids is inhibited during the early stages of chemical carcinogenesis due to lack of induction of the beta-subunit of the channel. By contrast, Cl- transport is stimulated both in surface and lower crypt cell compartments during different stages of chemical carcinogenesis.

Cystic fibrosis transmembrane conductance regulator activates water conductance in Xenopus oocytes

Multiple properties have been attributed to the cystic fibrosis transmembrane conductance regulator (CFTR), the gene product which is mutated in cystic fibrosis (CF). In this context it has been reported that CFTR transports water. In the present study we demonstrate that expression of wild-type CFTR (wtCFTR) in Xenopus oocytes and then stimulation by 3-isobutyl-1-methylxanthine (IBMX, 1 mmol/l) activates a Cl- conductance and, in parallel, a water conductance, as measured by a volume increase gravimetrically. In water-injected control oocytes or oocytes expressing a mutant form of CFTR (G551D-CFTR) IBMX had very little effect on Cl- conductance and no effect on water conductance. Phloretin (350 micro;mol/l) and p-chloromercuri-benzene sulphonate (pCMBS, 1 mmol/l) inhibited water transport but did not inhibit Cl- currents when measured in double-electrode voltage-clamp experiments. In contrast, glibenclamide (100 micro;mol/l) inhibited wtCFTR Cl- conductance but did not inhibit water conductance in IBMX-stimulated oocytes. Moreover, gravimetric and [14C]glycerol uptake measurements indicated enhanced glycerol uptake by wtCFTR-expressing oocytes after stimulation with IBMX. Enhanced glycerol uptake could be inhibited by phloretin and pCMBS but not by glibenclamide. Taken together, the data suggest that activation of wtCFTR by an increase of intracellular cAMP is paralleled by the activation of a glycerol-permeable water conductance. Both water and Cl- conductive pathways can be inhibited differentially. Thus, water permeation through wtCFTR probably occurs at a site of CFTR which is spatially apart from the domain responsible for Cl- conductance, or CFTR might be a regulator of an endogenous water channel in oocytes.

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.

Intracellular pH in rat pancreatic ducts

In order to study the mechanism of H+ and HCO3- transport in a HCO3- secreting epithelium, pancreatic ducts, we have measured the intracellular pH (pHi) in this tissue using the pH sensitive probe BCECF. We found that exposures of ducts to solutions containing acetate/acetic acid or NH4+/NH3 buffers (20 mmol/l) led to pHi changes in accordance with entry of lipid-soluble forms of the buffers, followed by back-regulation of pHi by duct cells. In another type of experiment, changes in extracellular pH of solutions containing HEPES or HCO3-/CO2 buffers led to significant changes in pHi that did not seem to be back-regulated efficiently by duct cells. The sensitivity of pHi to the inhibitor HOE 694 and to changes in Na+ gradients, indicate that the Na+/H+ exchanger is present in this epithelium. Similarly, the sensitivity to Cl- and HCO3- gradients indicated the presence of the Cl-/HCO3- exchanger. Under some conditions, these exchangers can be invoked to regulate cell pH.

Adrenaline inhibits depolarization-induced increases in capacitance the presence of elevated [Ca2+]i in insulin secreting cells

Cell capacitance (Cm), cell conductance (Gm), access conductance (Ga) and membrane voltage (Vm) were measured simultaneously in insulin secreting cells using the dual frequency method. Depolarization and stimulation of the cells with secretagogues increased Cm. EGTA abolished the increase in [Ca2+]i and prevented the rise of Cm. Adrenaline inhibited the augmentation of Cm without lowering [Ca2+]i. In pertussis toxin pretreated cells adrenaline had no effect. Thus, stimulation of insulin secretion is accompanied by an increase in Cm. Inhibition of exocytosis by adrenaline occurs even in the presence of elevated [Ca2+]i, i.e. at a more distal step of exocytosis.

Angiotensin II increases the cytosolic calcium activity in rat podocytes in culture

In the glomerulus, angiotensin II (Ang II) reduces the ultrafiltration coefficient and enhances the filtration of macromolecules. During glomerular injury, inhibition of the renin-angiotensin system by angiotensin-converting-enzyme inhibitors reduces proteinuria and retards the progression to end-stage renal insufficiency. The mechanisms by which Ang II modulates glomerular function are still a matter of investigation. To study whether Ang II may regulate the cytosolic calcium activity ([Ca2+]i) in podocytes, these cells were propagated in short-term culture and the effect of Ang II was examined with the Fura-2 microfluorescence technique in single podocytes. The cellular identity of cultured podocytes was proven by the expression of WT-1 and pp44, specific antibodies against podocytes in vivo. Ang II led to a concentration-dependent, reversible and slow increase of [Ca2+]i with an EC50 of 3 nmol/liter Ang II (N = 229). Ten nmol/liter Ang II increased [Ca2+]i from 41 +/- 9 to 260 +/- 34 nmol/liter (N = 210). In a solution with an extracellular reduced Ca2+ concentration of 10 micromol/liter, Ang II-mediated [Ca2+]i increase was significantly reduced by 60 +/- 20% (N = 12), indicating that the [Ca2+]i increase was due to a Ca2+ influx from the extracellular space and a release of Ca2+ from intracellular stores. Flufenamate, an inhibitor of non-selective ion channels, significantly inhibited Ang II-mediated increase of [Ca2+]i (IC50 = 20 micromol/liter, N = 29), whereas the L-type Ca2+ channel blocker nicardipine even in high concentrations of > 1 micromol/liter had only a small inhibitory effect. The AT1 receptor antagonist losartan inhibited Ang II-mediated [Ca2+]i increase with an IC50 of about 0.3 nmol/liter (N = 35). The data suggest that Ang II increases [Ca2+]i in podocytes by an influx of Ca2+ through non-selective channels and by a release of Ca2+ from intracellular stores. The effect of Ang II is mediated via an AT1 receptor.

The role of the IsK protein in the specific pharmacological properties of the IKs channel complex

IKs channels are composed of IsK and KvLQT1 subunits and underly the slowly activating, voltage-dependent IKs conductance in heart. Although it appears clear that the IsK protein affects both the biophysical properties and regulation of IKs channels, its role in channel pharmacology is unclear. In the present study we demonstrate that KvLQT1 homopolymeric K+ channels are inhibited by the IKs blockers 293B, azimilide and 17-beta-oestradiol. However, IKs channels induced by the coexpression of IsK and KvLQT1 subunits have a 6-100 fold higher affinity for these blockers. Moreover, the IKs activators mefenamic acid and DIDS had little effect on KvLQT1 homopolymeric channels, although they dramatically enhanced steady-state currents through heteropolymeric IKs channels by arresting them in an open state. In summary, the IsK protein modulates the effects of both blockers and activators of IKs channels. This finding is important for the action and specificity of these drugs as IsK protein expression in heart and other tissues is regulated during development and by hormones.

Effect of intracellular pH on agonist-induced [Ca2+]i transients in HT29 cells

In this study we examined the influence of intracellular pH (pHi) on agonist-induced changes of intracellular Ca2+ activity ([Ca2+]i) in HT29 cells. pHi and [Ca2+]i were measured microspectrofluorimetrically using BCECF and fura-2, respectively. Buffers containing trimethylamine (TriMA), NH3/NH4+ and acetate were used to clamp pHi to defined values. The magnitudes of the peak and plateau of [Ca2+]i transients induced by carbachol (CCH, 10(-6) mol/l) were greatly enhanced by an acidic pHi and nearly abolished by an alkaline pHi. The relationship between pHi and the [Ca2+]i peak was nearly linear from pHi 7.0 to 7.8. This effect of pHi was also observed at higher CCH concentrations (10(-4 )and 10(-5) mol/l), at which the inhibitory effect of an alkaline pHi was more pronounced than the stimulatory effect of an acidic pHi. An acidic pHi shifted the CCH concentration/response curve to the left, whereas an alkaline pHi led to a rightward shift. The influence of pHi on [Ca2+]i transients induced by neurotensin (10(-8) mol/l) or ATP (5 x 10(-7) mol/l) was similar to its influence on those induced by CCH, but generally not as pronounced. Measurements of cellular inositol 1,4,5-trisphosphate (InsP3) showed no changes in response to acidification with acetate (20 mmol/l) or alkalinization with TriMA (20 mmol/l). The InsP3 increase induced by CCH was unaltered at an acidic pHi, but was augmented at an alkaline pHi. Confocal measurements of cell volume showed no significant changes induced by TriMA or acetate. Slow-whole-cell patch-clamp experiments showed no additional effect of CCH on the membrane voltage (Vm) measured after TriMA or acetate application. We conclude that pHi is a physiological modulator of hormonal effects in HT29 cells, as the [Ca2+]i responses to agonists were significantly changed at already slightly altered pHi. The measurements of InsP3, cell volume and Vm show that pHi must act distally to the InsP3 production, and not via changes of cell volume or Vm.

KVLQT channels are inhibited by the K+ channel blocker 293B

Previous data have indicated that the chromanol 293B blocks a cAMP activated K+ conductance in the colonic crypt, a K+ conductance in pig cardiac myocytes and the K+ conductance induced by IsK protein expression in Xenopus oocytes. We have also shown that cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) up-regulates, apart from the typical Cl- current, a 293B- inhibitable K+ current. Very recently it has been shown that the IsK protein interacts with KVLQT subunits to produce a K+ channel. These data have prompted us to ask the following questions: Is the 293B-inhibitable current in oocytes expressing CFTR and activated by cAMP caused by an endogenous Xenopus KVLQT (XKVLQT), and is mouse KVLQT (mKVLQT) expressed in oocytes inhibited by 293B? Antisense and sense probes for XKVLQT were coinjected with CFTR cRNA into oocytes. After 3-4 days the oocytes were examined by two electrode voltage clamp. It was found that in control oocytes expressing CFTR and stimulated by isobutylmethylxanthine (IBMX, 1 mmol/l) 293B (10 micromol/l) reduced the conductance (Gm). In oocytes coinjected with the sense probe for XKVLQT and pretreated with IBMX 293B still reduced Gm, whilst the 293B-inhibitable Gm was almost completely absent in oocytes coinjected with XKVLQT antisense. In another series a full length clone for mKVLQT was generated by PCR techniques and the cRNA was injected into oocytes. After several days these oocytes, unlike water injected ones, were found to be strongly hyperpolarized and their Gm was increased significantly. The oocytes were depolarized significantly and their Gm was reduced reversibly by 10 micromol/l 293B. These data indicate that CFTR activation by IBMX indeed co-activates an endogenous oocyte XKVLQT channel and that this channel is inhibited by a new class of channel blockers, of which 293B is the prototype.

Capacitative Ca2+ entry (CCE) induced by luminal and basolateral ATP in polarised MDCK-C7 cells is restricted to the basolateral membrane

In previous studies we have characterised various properties of capacitative Ca2+ entry (CCE) in different epithelia. After Ca2+ store depletion with PLC/InsP3-coupled agonists or by inhibition of store Ca2+ uptake, with for example thapsigargin, Ca2+ influx is activated. This leads to a sustained cellular response (e.g. NaCl secretion). In the present study, we have investigated CCE in polarised MDCK-C7 cells grown on permeable supports in a chamber allowing for separate luminal and basolateral perfusion. The transepithelial resistance (Rte) and voltage (Vte) were measured simultaneously to verify the tightness of the epithelial monolayers. MDCK-C7 cells grew to very tight monolayers (Rto > 3000 omega.cm2). Apical ATP (100 mumol/l) led to a biphasic [Ca2+]i increase. Removal of apical Ca2+ in the continuous presence of ATP did not reduce the stimulated plateau. However, removal of Ca2+ from the basolateral side rapidly and completely interrupted the [Ca2+]i plateau to below basal values ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 213 +/- 15 nmol/l, n = 9). Furthermore, MDCK-C7 responded to basolateral ATP (100 mumol/l) with a biphasic [Ca2+]i transient. Again the plateau phase of the ATP-induced [Ca2+]i effect was fully dependent on the presence of basolateral but not apical Ca2+ ([Ca2+]i decrease during plateau phase after removal of basolateral Ca2+ = 196 +/- 5 nmol/l, n = 10). Receptor-independent depletion of cytosolic Ca2+ stores with thapsigargin from both sides led to a rise in [Ca2+]i, which was also exclusively dependent on the presence of basolateral Ca2+ (n = 8). These data indicate that MDCK-C7 cells express luminal and basolateral P2-receptors coupled to PLC/InsP3/Ca2+. ATP applied from both sides induced a sustained [Ca2+]i plateau which was due to transmembrane Ca2+ influx. The ATP- and thapsigargin-induced Ca2+ influx pathway was exclusively located in the basolateral membrane.

Agonist-induced activation of a non-selective ion current in glomerular endothelial cells

The control of intracellular calcium activity ([Ca2+]i) and membrane voltage (Vm) play an important role in regulating functions of glomerular endothelial cells (GEC). We investigated the effect of extracellular ATP on the intracellular [Ca2+]i, Vm and ion conductances in GEC. ATP (100 mumol/liter) induced a rapid increase of [Ca2+]i in GEC from 20 +/- 6 to 442 +/- 84 nmol/liter, which was followed by a sustained Ca2+ plateau of 112 +/- 29 nmol/liter. In a bath solution with a low extracellular Ca2+ concentration the ATP-induced [Ca2+]i peak was still present, but the [Ca2+]i plateau was completely prevented. In 186 experiments with the patch clamp technique the addition of ATP (1 to 100 mumol/liter) to GEC induced a transient small hyperpolarization, which was followed by a depolarization. During the ATP-induced depolarization an increase of the whole cell conductance was found. The Ca2+ ionophore A23187 (10 mumol/liter) mimicked the effect of ATP on Vm. Reduction of the extracellular Ca2+ to 1 mumol/liter itself depolarized GEC reversibly from -88 +/- 2 to -60 +/- 12 mV and increased the ATP-induced depolarization to -18 +/- 3 mV. In the absence of Na+ in the bathing solution (replacement by NMDG+) ATP induced only an attenuated depolarization and no inward current was activated. Flufenamate (100 mumol/liter), a blocker of non-selective ion channels inhibited the ATP-induced depolarization of Vm significantly by 58 +/- 13%, whereas nicardipine (10 mumol/liter) or amiloride (10 mumol/liter) had no effect. Our data indicate that the resting Vm of GEC cells is almost completely dominated by K+ conductances and that ATP activates a Ca2+ dependent non-selective ion conductance in GEC.

New types of K+ channels in the colon

The mechanism of epithelial Cl- secretion requires K+ recycling via basolateral ion channels. Two types of K+ channels were identified. A Ca(2+)-activated, and a very small cAMP-regulated K+ channel. Increase of cAMP inhibits the Ca(2+)-activated K+ channel and thus makes small K+ channel the limiting step for cAMP-dependent Cl- secretion.