Effect of extracellular adenosine triphosphate on electrical properties of subconfluent Madin-Darby canine kidney cells

ATP stimulates Cl- secretion and reduces amiloride-sensitive Na+ absorption in M-1 mouse cortical collecting duct cells

1. Using equivalent short circuit current (ISC) measurements we examined the effect of extracellular ATP on transepithelial ion transport in M-1 mouse cortical collecting duct cells. Apical addition of ATP produced a rapid transient peak increase in ISC. This was followed by a fall below basal ISC due to a reduction in the amiloride-sensitive ISC component. 2. The ATP-induced ISC increase was preserved in the presence of apical amiloride while it was reduced in the absence of extracellular Cl- and in the presence of the apical Cl- channel blockers diphenylamine-2-carboxylic acid (DPC, 1 mM), DIDS (300 microM) and niflumic acid (100 microM). 3. The stimulatory effect of apical ATP on ISC was concentration dependent with an EC50 of about 0.6 microM. Basolateral ATP elicited a similar ISC response. Experiments using the ATP scavenger hexokinase demonstrated that the ATP effects were elicited via separate apical and basolateral receptors. 4. ATP and UTP applied to either the apical or the basolateral bath equi-potently stimulated ISC while 'purified' ADP and UDP had no effect consistent with P2Y2 purinoceptors, the expression of which was confirmed using RT-PCR. 5. Intracellular calcium concentration ([Ca2+]i) measurements using fura-2 demonstrated that ATP and UTP elicited a rise in [Ca2+]i with EC50 values of 1.1 and 0.6 microM, respectively. The shape and time course of the calcium response were similar to those of the ISC response. The peak ISC response was preserved in the nominal absence of extracellular calcium but was significantly reduced in cells pre-incubated with the calcium chelator BAPTA AM. 6. We conclude that in M-1 cells extracellular ATP reduces amiloride-sensitive Na+ absorption and stimulates Cl- secretion via calcium-activated Cl- channels through activation of P2Y2 purinoreceptors located in the apical and basolateral membrane.

Follicle-stimulating hormone activates a cAMP-dependent chloride conductance in TM4 Sertoli cells

The effect of follicle-stimulating hormone (FSH) on the electrical properties of TM4-Sertoli cells was investigated. Addition of 5 IU/ml FSH caused a dose-dependent and reversible depolarization of the resting membrane potential by +15.3 +/- 1.0 mV accompanied by a decrease of the input resistance. The depolarization was completely abolished in chloride-free solutions. The reversal potential of the effect was close to the calculated reversal potential for chloride. We conclude that FSH activates a chloride conductance in cultured TM4 Sertoli cells.

Properties and regulation of ion channels in MDCK cells

The MDCK cell has proven to be a useful model cell line for the study of properties and regulation of renal epithelial ion channels. Patch clamp studies disclosed the existence of several K+ channels and of a Cl- channel, and their regulation by hormones, cell volume, trace elements and drugs. Most hormones affect K+ channels at least in part by increasing cytosolic Ca2+. However, indirect evidence points to additional mechanisms contributing to K+ channel activation. Cell swelling activates both K+ channels and unselective anion channels. ICln, a protein cloned from MDCK cells, is either a Cl- channel or a regulator of thereof. ICln is up-regulated by cellular acidification and is crucial for rapid regulatory cell volume decrease.

Synthesis of charybdotoxin and of two N-terminal truncated analogues. Structural and functional characterisation

Charybdotoxin and two N-terminal truncated peptides, corresponding to the 2-37 and 7-37 sequences, were obtained by stepwise solid-phase synthesis using N alpha-t-butyloxycarbonyl and benzyltype side-chain protection. While this strategy was generally useful, the S-acetamidomethyl protecting group used for the six cysteines was not completely stable under HF treatment and its subsequent removal by mercury(II) treatment was neither complete nor devoid of side reactions. The completely deprotected native and truncated sequences were folded efficiently in the presence of glutathione and were finally purified by high-pressure liquid chromatography with overall yields of 4.0-5.0%. Each protein was characterised chemically, structurally and functionally. 1H-NMR spectroscopy was used and a complete assignment of all the protons of the three synthetic proteins was achieved. NMR data show that synthetic charybdotoxin is indistinguishable from the natural protein. The two truncated proteins contain the same elements of secondary structure and a similar overall three-dimensional structure, in agreement with circular dichroic measurements. The shortest analogue, however, may have local structural perturbations and/or higher flexibility. Biological activity on dog epithelial Ca(2+)-activated K+ channels and on rat brain synaptosomal voltage-dependent K+ channels show that synthetic charybdotoxin was as potent as the natural toxin on both channels. For both channels, deletion of the first amino acid, 5-oxoproline (pyroglutamic acid) decreased only slightly the potency of the inhibitor, while deletion of the entire 1-6 segment reduced potency much more. We conclude that the N-terminal region of charybdotoxin plays a functional role in tuning the toxin's biological activity but is not essential for the folding and stability of the structure. The structure of the shortest analogue represents an interesting example of how a well organised and stable alpha/beta fold can be engineered with only 31 amino acid residues.

Nucleotides mobilize intracellular calcium stores of renal proximal cells in primary culture: existence of a suramin-sensitive mechanism

Changes of intracellular calcium concentrations [Ca2+]i were measured in primary cultured rabbit proximal convoluted tubules (PCT). A dual-excitation, digital-imaging inverted microscope was used to monitor the fura-2 fluorescence. The basal calcium level was 106 +/- 11 nM (n = 36). The stimulatory effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine were studied. ATP and ADP induced transient increases of [Ca2+]i (1059 +/- 115% of the resting level (n = 29), and 659 +/- 134% (n = 10), respectively) by releasing calcium from cytoplasmic stores. Adenosine had less effect (279 +/- 48% of the resting level, n = 3). In the same conditions the ATP antagonist suramin (100 microM) inhibited the action of ATP and ADP to 231 +/- 52% (n = 3), and 308 +/- 29% (n = 4) of the resting level, respectively, but did not modify that of adenosine (281 +/- 72%, n = 3). A pretreatment (500 ng/ml for 2 h at 37 degrees C) of the culture with the toxin of Bordetella pertussis completely blocked the ATP response. Our results are evidence for the presence of a functional suramin-sensitive ATP and ADP puriceptor in cultured renal proximal cells. A pertussis-toxin-sensitive G protein is linked to the transduction mechanism. This receptor is distinct from an adenosine puriceptor also found in the proximal monolayer.

Toxin pharmacology of the ATP-induced hyperpolarization in Madin-Darby canine kidney cells

The effects of Leiurus quinquestriatus hebraeus (LQH) venom, mamba venom, Buthus tamulus (BT) venom, purified apamin and synthetic charybdotoxin on the membrane hyperpolarization induced by extracellular ATP were examined in Madin-Darby canine kidney cells. For this we used a membrane potential probe (bisoxonol) to determine the potential variations. The relation between bisoxonal fluorescence and membrane potential was established by treating Madin-Darby canine kidney cells suspended in solutions containing various external sodium concentrations with gramicidin. Extracellular ATP induced a rapid hyperpolarization that was blocked by LQH venom and synthetic charybdotoxin. BT venom also blocked the response but at a much higher concentration than that of LQH. Mamba venom (Dendroaspis polylepis) and apamin did not modify the ATP-induced hyperpolarization. We concluded that the ATP induced hyperpolarization was due to the augmentation of the potassium conductance probably through Ca(2+)-activated K+ channels sensitive to charybdotoxin but not to mamba venom. The interaction previously described between charybdotoxin and dendrotoxin (the main toxin of mamba venom) was not observed in our case.

Evidence that plasma membrane electrical potential is required for vesicular stomatitis virus infection of MDCK cells: a study using fluorescence measurements through polycarbonate supports

We used fluorescence microscopy of Madin-Darby Canine Kidney (MDCK) cells grown on polycarbonate filters to study a possible link between plasma membrane electrical potential (delta psi pm) and infectivity of vesicular stomatitis virus (VSV). Complete substitution of K+ for extracellular Na+ blocks VSV infection of MDCK cells as well as baby hamster kidney (BHK) cells. When we independently perfused the apical and basal-lateral surfaces of high resistance monolayers, high K+ inhibited VSV infection of MDCK cells only when applied to the basal-lateral side; high K+ applied apically had no effect on VSV infection. This morphological specificity correlates with a large decrease in delta psi pm of MDCK cells when high K+ buffer is perfused across the basal-lateral surface. Depolarization of the plasma membrane by 130 mM basal K+ causes a sustained increase of cytosol pH in MDCK cells from 7.3 to 7.5 as reported by the fluorescent dye BCECF. Depolarization also causes a transient increase of cytosol Ca2+ from 70 to 300 nM as reported by the dye Fura-2. Neither increase could explain the block of VSV infectivity by plasma membrane depolarization. One alternative hypothesis is that delta psi pm facilitates membrane translocation of viral macromolecules as previously described for colicins, mitochondrial import proteins, and proteins secreted by Escherichia coli.

Effect of bradykinin, ATP and adrenaline on cell membrane resistances of Madin-Darby canine kidney cells

1. Previous studies have shown that bradykinin, ATP and adrenaline hyperpolarize the cell membrane of Madin-Darby canine kidney (MDCK) cells by activation of calcium-sensitive K+ channels. The present study has been performed to determine the effect of these hormones on the resistance of the cell membrane and the cellular coupling. To this end, cellular cable analysis has been performed. 2. As a result, all three hormones lead to the expected, marked decrease of cell membrane resistance. 3. However, the bradykinin-induced reduction of cell membrane resistance was sustained, contrasting with only transient hyperpolarization induced by bradykinin and only transient activation of the K+ channels. Thus, the cable analysis reveals the sustained activation of an additional conductance. 4. ATP, but not the other two hormones, leads to a delayed increase of the intercellular coupling resistances. 5. Prolonged exposure of the cells to adrenaline leads to oscillations of the cell membrane potential, apparently by oscillatory activation of the K+ channels.

Inhibition of ion conductances by osmotic shrinkage of Madin-Darby canine kidney cells

Osmotic swelling of Madin-Darby canine kidney (MDCK) cells enhances the ion conductances of the cell membrane, which allows release of cellular ions and subsequent regulatory cell volume decrease. The present study has been performed to test whether cell shrinkage similarly affects the ion conductances of MDCK cell membranes. Increase of extracellular osmolarity by addition of 50 mM NaCl or 100 mM mannitol leads within 3 min to a hyperpolarization of the cell membrane, a marked increase of cell membrane resistance [by 223 +/- 38% (n = 8) and 228 +/- 21% (n = 5), respectively], as well as a moderate increase of the K+ selectivity of the cell membrane (by 37 +/- 13%, n = 9). Thus exposure to hypertonic extracellular fluid decreases the cell membrane conductances including the K+ conductance. Cell volume measurements reveal a regulatory cell volume increase, which is sensitive to both furosemide and dimethylamiloride. Extracellular ATP (10 microM), which activates calcium-sensitive K+ channels, hyperpolarizes the cell membrane close to the K+ equilibrium potential. The respective values are -69.9 +/- 3.1 mV (n = 9) in isotonic fluid, -79.4 +/- 1.8 mV (n = 9) within 3 min, and -76.4 +/- 1.8 mV (n = 7) within 16-h exposure to hypertonic extracellular fluid. This observation points to a sustained increase of intracellular K+ activity after exposure to hypertonic extracellular fluid.

Effect of trifluoperazine on renal epithelioid Madin-Darby canine kidney cells

Following exposure to a number of hormones, the cell membrane in Madin-Darby Canine Kidney (MDCK) cells is hyperpolarized by increase of intracellular calcium activity. The present study has been performed to elucidate the possible role of calmodulin in the regulation of intracellular calcium activity and cell membrane potential. To this end trifluoperazine has been added during continuous recording of cell membrane potential or intracellular calcium. Trifluoperazine leads to a transient increase of intracellular calcium as well as a sustained hyperpolarization of the cell membrane by activation of calcium sensitive K+ channels. Half-maximal effects are observed between 1 and 10 mumol/L trifluoperazine. A further calmodulin antagonist, chlorpromazine, (50 mumol/L), similarly hyperpolarizes the cell membrane. The effects of trifluoperazine are virtually abolished in the absence of extracellular calcium. Pretreatment of the cells with either pertussis toxin or phorbol-ester TPA does not interfere with the hyperpolarizing effect of trifluoperazine. In conclusion, calmodulin is apparently involved in the regulation of calcium transfer across the cell membrane but not in the stimulation of K+ channels by intracellular calcium.

Further analysis of ATP-mediated activation of K+ channels in renal epithelioid Madin Darby canine kidney (MDCK) cells

ATP activates K+ channels by increasing intracellular calcium activity in Madin Darby canine kidney (MDCK) cells. The present study has been performed to test for the involvement of G-proteins and of protein kinase C in the intracellular transmission of these effects. To this end, the effect of ATP on intracellular calcium and K+ channel activity has been studied in cells pretreated with the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and/or pertussis toxin. The ATP-induced increase of intracellular calcium is not significantly affected by pretreatment with pertussis toxin, is significantly blunted by pretreatment with TPA and is abolished by pretreatment with both pertussis toxin and the phorbol ester. The ATP activation of K+ channels is similarly blunted by pretreatment with TPA, but is not abolished by pretreatment with both the phorbol ester and pertussis toxin. Furthermore, the ATP-induced hyperpolarization is not abolished in cells pretreated with both pertussis toxin and TPA. In those cells, ATP may activate K+ channels by calcium-dependent mechanisms or lead to localized increases of intracellular calcium sufficient to activate the K+ channels but escaping detection with fura-2 fluorescence.

Electrophysiological effects of extracellular ATP on Necturus gallbladder epithelium

The effects of addition of ATP to the mucosal bathing solution on transepithelial, apical, and basolateral membrane voltages and resistances in Necturus gallbladder epithelium were determined. Mucosal ATP (100 microM) caused a rapid hyperpolarization of both apical (Vmc) and basolateral (Vcs) cell membrane voltages (delta Vm = 18 +/- 1 mV), a fall in transepithelial resistance (Rt) from 142 +/- 8 to 122 +/- 7 omega.cm2, and a decrease in fractional apical membrane resistance (fRa) from 0.93 +/- 0.02 to 0.83 +/- 0.03. The rapid initial hyperpolarization of Vmc and Vcs was followed by a slower depolarization of cell membrane voltages and a lumen-negative change in transepithelial voltage (Vms). This phase also included an additional decrease in fRa. Removal of the ATP caused a further depolarization of membrane voltages followed by a hyperpolarization and then a return to control values. fRa fell to a minimum after removal of ATP and then returned to control values as the cell membrane voltages repolarized. Similar responses could be elicited by ADP but not by adenosine. The results of two-point cable experiments revealed that ATP induced an initial increase in cell membrane conductance followed by a decrease. Transient elevations of mucosal solution [K+] induced a larger depolarization of Vmc and Vcs during exposure to ATP than under control conditions. Reduction of mucosal solution [Cl-] induced a slow hyperpolarization of Vmc and Vcs before exposure to ATP and a rapid depolarization during exposure to ATP. We conclude that ATP4- is the active agent and that it causes a concentration-dependent increase in apical and basolateral membrane K+ permeability. In addition, an apical membrane electrodiffusive Cl- permeability is activated by ATP4-.

Cellular mechanisms of ATP-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have shown that ATP enhances intracellular calcium concentration and activates potassium channels in Madin Darby canine kidney (MDCK)-cells, thus leading to hyperpolarization of the cell membrane. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of ATP on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoyl phorbol 13-acetate diester (TPA). In untreated cells, ATP leads to a sustained hyperpolarization and an increase of inositol 1,4,5-trisphosphate (IP3), inositol 1,3,4,5-tetrakisphosphate (IP4), and Cai. In the absence of extracellular calcium, the effect of ATP on PD and Cai is only transient. In cells pretreated with pertussis toxin, the effect of ATP on inositol trisphosphate is almost abolished, but ATP still leads to an increase of PD and Cai, which is sustained in the presence, and transient in the absence, of extracellular calcium. In cells pretreated with TPA, the effect of ATP on inositol trisphosphate is reduced and the effect on Cai blunted; but ATP still leads to a hyperpolarization of the cell membrane, which is sustained in the presence, and transient in the absence, of extracellular calcium. The observations indicate that ATP activates phospholipase C by a phorbol ester and pertussis toxin sensitive mechanism. In addition, ATP enhances Cai by pertussis toxin insensitive mechanisms allowing recruitment of calcium from both, extracellular fluid and intracellular stores. Calcium then activates the potassium channels and thus leads to the hyperpolarization of the cell membrane.

Determination of cell membrane resistance in cultured renal epithelioid (MDCK) cells: effects of cadmium and mercury ions

Previous studies have indicated that the cell membrane of Madin Darby Canine Kidney (MDCK) cells is hyperpolarized by a number of hormones and trace elements, in parallel with an enhancement of potassium selectivity. Without knowledge of the cell membrane resistance (Rm), however, any translation of potassium selectivity into potassium conductance remains equivocal. The present study was performed to determine the Rm of MDCK cells by cellular cable analysis. To this end, three microelectrodes were impaled into three different cells of a cell cluster; current was injected via one microelectrode and the corresponding voltage deflections measured by the other two microelectrodes. In order to extract the required specific resistances, the experimental data were analysed mathematically in terms of an electrodynamical model derived from Maxwell's equations. As a result, a mean Rm of 2.0 +/- 0.2 k omega cm2 and an intercellular coupling resistance (Rc) of 6.1 +/- 0.8 M omega were obtained at a mean potential difference across the cell membrane of -47.0 +/- 0.6 mV. An increase of the extracellular K+ concentration from 5.4 to 20 mmol/l depolarized the cell membrane by 16.2 +/- 0.5 mV and decreased Rm by 30.6 +/- 3.0%; 1 mmol/l barium depolarized the cell membrane by 20.1 +/- 1.1 mV and increased Rm by 75.9 +/- 14.3%. Omission of extracellular bicarbonate and carbon dioxide at constant extracellular pH caused a transient hyperpolarization (up to -60.4 +/- 1.4 mV), a decrease of Rm (by 75 +/- 4.5%) and a decrease of Rc (by 23.1 +/- 8.4%).(ABSTRACT TRUNCATED AT 250 WORDS)

Cobalt activates potassium conductance in the plasma membrane of cultured renal epithelioid (MDCK)-cells

Cobalt has been shown to stimulate sodium transport across the distal nephron of the newt kidney. The mechanism of this action remained elusive. The present study has been performed to test for effects of cobalt on electrical properties of cultured subconfluent kidney (MDCK)-cells: cobalt (10 microM) leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cobalt increases the potassium conductance of the cell membrane. The half-maximal effect is elicited by approx. 1 microM. At extracellular calcium concentration reduced to less than 0.1 microM, cobalt (10 microM) leads to a transient hyperpolarization, which can be elicited only once. Thus, cobalt enhances the potassium conductance in a calcium dependent way. At higher concentrations (100 microM) cobalt hyperpolarizes the cell membrane only transiently even in the presence of extracellular calcium. Furthermore 100 microM cobalt interferes with ATP-induced hyperpolarization, which is known to result from calcium mediated activation of K+ channels. Thus, 100 microM cobalt may inhibit ATP-stimulated calcium entry into the cell.

Cadmium enhances potassium conductance in cultured renal epitheloid (MDCK) cells

The kidney is a main target organ for cadmium toxicity. The present study has been performed to test for effects of cadmium on electrical properties of cultured subconfluent kidney (MDCK) cells. Cadmium leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cadmium increases the potassium conductance of the cell membrane. The half maximal effect is elicited congruent to 0.2 microM, a concentration encountered during chronic cadmium intoxication. At extracellular calcium concentration reduced to less than 0.1 microM, 5 microM cadmium leads to a transient hyperpolarization, which can be elicited only once. High concentrations (50 microM) of cadmium lead to a sustained hyperpolarization even at extracellular calcium concentrations of less than 0.1 microM. According to fluorescence measurements cadmium leads to an increase of intracellular calcium activity, which is sustained at 1 mM and transient at less than 1 microM extracellular calcium activity. In conclusion, cadmium at low concentrations enhances the potassium conductance in a calcium dependent way. The observations suggest that cadmium enhances intracellular calcium both by recruitment from intracellular stores and by modification of calcium transport across the cell membrane. At high concentrations cadmium enhances the potassium conductance independently from enhanced intracellular calcium activity.

Activation of potassium channels in renal epithelioid cells (MDCK) by extracellular ATP

Extracellular ATP has been shown to stimulate transepithelial chloride transport in confluent Madin-Darby canine kidney (MDCK) cell layers and to enhance potassium conductance in subconfluent MDCK cells. The present study has been performed to test for the effect of extracellular ATP on channel activity in patches from subconfluent MDCK cells. Within 8 s, addition of extracellular ATP (10 mumol/l) leads to a sustained, but fully reversible, appearance of potassium-selective channels in cell-attached patches [increase of open probability from 0.03 +/- 0.02 (n = 10) to 0.50 +/- 0.07 (n = 6)]. With the use of pipettes filled with 145 mmol/l KCl, inwardly rectifying property of the channels is disclosed with a single-channel conductance of 65.7 +/- 3.1 pS (n = 9) at zero potential difference between pipette and bath and with a reversal potential of 75.4 +/- 2.0 mV (n = 5; pipette negative vs. reference in the bath). The open probability of the channels is not significantly modified by altering pipette potential from -50 mV, pipette positive, to 50 mV, pipette negative. At extracellular calcium activities of less than 10 nmol/l, ATP leads to a transient activation of channels. In conclusion, extracellular ATP activates inwardly rectifying potassium channels in the cell membrane of subconfluent MDCK cells. A sustained activation of the channels requires the presence of extracellular calcium and is probably mediated by increases in intracellular calcium.