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PubMed2002

Factors affecting ammonium uptake by C11 clone of MDCK cells.

In several tissues ammonium ions are able to use the transport pathways of other ions, particularly of K+. We investigated this possibility in the C11 clone of MDCK cells, thought to represent intercalated cells, in control and 0 Cl- conditions. Cell pH was measured by ratiometric fluorescence microscopy using the pH indicator BCECF. After preincubating the cells for 10 min in control or 0 Cl- (substituted by gluconate) Ringer, an ammonium pulse was applied to induce cell acidification. The magnitude of the initial alkalinization (DeltapH) was 0.24+/-0.03 ( n=28) pH units in controls, which fell to 0.023+/-0.01 ( n=12) in 0 Cl-, suggesting uptake of NH4+ balancing the alkalinization by NH3. Addition of 10(-3) M bumetanide or furosemide to the 0 Cl- medium, or 10(-4 )M hexamethylene amiloride, did not alter DeltapH. However, with 5 mM Ba+, DeltapH increased to 38% of control. When 2.5x10(-4) M ouabain, an inhibitor of Na+-K+ ATPase, was used, DeltapH increased to 46% of control. Inhibition of H+-K+ ATPase by SCH28080 or by omeprazol caused significant increase in DeltapH. In 0 Cl- solution, these cells underwent a mean volume reduction (-d V) of -10.24+/-1.96% per 10 min as measured by confocal microscopy. To investigate if NH4+ influx was regulated by cell volume or by cell Cl-, volume reduction was avoided by two procedures. When preincubating with NPPB, a Cl- channel blocker, in 0 Cl-, volume reduction was inhibited (d V=-2.12% per 10 min), and DeltapH was 0.24+/-0.04 ( n=5). When the cells were preincubated in hypotonic 0 Cl- (260 mosmol/l), cell volume reduction was abolished (d V=+2.6% per 10 min) and DeltapH was 0.52+/-0.07 ( n=7). Thus, activation of NH4+ influx by several transporters was due to volume reduction rather than to [Cl-] alteration.

Animals↗
PubMed2004

A potential role for astrocytes in mediating the antiepileptic actions of furosemide in vitro.

Epileptic seizures are characterized by abnormal electrical discharge. In previous studies we established a powerful antiepileptic action for a commonly used diuretic (furosemide). However, it remains unclear precisely how furosemide terminates abnormal electrical discharges. To address this issue, we performed in vitro experiments to examine conditions where furosemide exerts antiepileptic activity and patch-clamp studies to analyze the effect of furosemide on neuronal membrane properties, synaptic function and inward potassium current. Furosemide was not found to alter synaptic field responses, excitatory postsynaptic currents or intrinsic membrane properties of principal hippocampal neurons. Our in vitro studies indicate that furosemide does not abolish spontaneous epileptiform bursting during co-application of Ba2+ or Cs+ ions (to block inwardly rectifying potassium channels). Our patch-clamp data indicate that furosemide enhances the function of astrocytic, but not neuronal, inward potassium channels and that this modulation may be required for its antiepileptic activity. Although a variety of antiepileptic drugs are already available, none of these compounds selectively target astrocytes while preserving synaptic/neuronal function. Thus, furosemide-mediated modulation of inward potassium current (on astrocytes) represents a new target for control of abnormal electrical discharge in the CNS.

4-Aminopyridine↗
PubMed1996

Technical report: spiral CT pneumocolon for suspected colonic neoplasms.

Abdominal spiral computed tomography (CT) pneumocolon is performed after cleansing the colon, using a smooth muscle relaxant and rectal air insufflation. Rapid dynamic scanning is undertaken with intravenous contrast to give optimal hepatic and vascular enhancement, and 5-10 mm contiguous slices. The area of interest in the colon is reconstructed every 2.5 mm. This technique was performed in four patients with colorectal cancer and the CT findings were compared with the barium enema and pathology following surgical resection. Spiral CT pneumocolon clearly showed the primary tumour in all cases as an enhancing soft tissue mass, and was able to detect local extension and lymphadenopathy as well as assess the liver, peritoneum and remaining abdomen. CT depicted the morphology of the primary tumour more clearly than barium enema, and in one case also detected a 1 cm polyp which was not seen on the barium study because the patient was incontinent of barium and views were limited. There was good correlation between the CT and pathological findings. Compared to a barium enema, spiral CT pneumocolon is quick, with minimal patient discomfort, no risk of barium incontinence, and there is good assessment of the local and distant abdominal disease. Multiplanar formatting is possible and 3-D reconstruction gives intra- and extra-luminal views.

Aged↗
PubMed2001

Sustained stimulation of exocytosis triggers continuous membrane retrieval in rat pituitary somatotrophs.

We studied the relationship between exocytosis and endocytosis in rat pituitary somatotrophs using patch-clamp capacitance, FM1-43 fluorescence imaging and amperometry. Stimulation of exocytosis through voltage-dependent Ca2+ channels by depolarizations (1-5 s) increased the capacitance by 4.3 +/- 0.9 % and the fluorescence by 6.6 +/- 1.1 % (10 cells). The correlation between the capacitance and fluorescence changes indicated that the cell membrane and granule membrane added via exocytosis were stained with the membrane-bound fluorescent dye FM1-43 in a quantitatively similar manner. Intracellular dialysis (0.5-4.5 min) with elevated Ca2+ (1.5-100 microM) evoked continuous exocytosis that was detected with a carbon fibre electrode from dopamine-loaded cells (10 cells) or as an increase in FM1-43 fluorescence (56 +/- 10 %; 21 cells). Interestingly during Ca2+ dialysis the capacitance did not significantly change (2 +/- 1 %; 31 cells), indicating that endocytosis efficiently retrieved increased cell membrane. Sustained endocytosis was not blocked when the intracellular GTP (300 microM) was replaced with GTP[gamma]S. Replacing intracellular Ca2+ (100 microM) with Ba2+ (300 microM) or Sr2+ (200 microM), or reducing the pH of the intracellular solution from 7.2 to 6.2 did not block sustained endocytosis. Our results suggest that pituitary somatotrophs have the ability to undergo continuous exocytosis and membrane retrieval that persist in whole-cell recordings.

Animals↗
PubMed1997

Isolated bovine retinal pigment epithelial cells express delayed rectifier type and M-type K+ currents.

Outwardly rectifying K+ currents in freshly isolated bovine retinal pigment epithelial (RPE) cells were characterized using the whole cell and perforated-patch configurations of the patch-clamp technique. All cells exhibited a delayed rectifier type K+ current. This current had an activation threshold voltage of approximately -40 mV, activated with a sigmoidal trajectory, and inactivated completely over a period of several seconds. External tetraethylammonium (TEA) was an effective blocker of the delayed rectifier current [apparent dissociation constant (Kd) = 5.1 mM], but external Ba2+ was relatively ineffective. Approximately 24% of the cells also exhibited a sustained outwardly rectifying K+ current that became activated at voltages positive to approximately -80 mV. This current resembled the neuronal M-current. External Ba2+ was a potent blocker of this current (apparent Kd = 1.1 mM), but external TEA and Cs+ were relatively ineffective. These results indicate that freshly isolated bovine RPE cells express K+ currents of both the delayed rectifier and M types. The latter may contribute to the resting K+ conductances of the apical and basolateral membranes.

Amphotericin B↗
PubMed2000

Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca(2+) oscillations and prolactin release through K(IR) channel activation.

Opioid receptors can couple to K(+) and Ca(2+) channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca(2+) signaling and basal prolactin release. The objective of this study was to examine a possible role for K(+) channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) inhibited [Ca(2+)](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca(2+)](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca(2+) stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH(3)MORDOR cells with an IC(50) of 4 nM. DPDPE increased inward K(+) current recorded from GH(3)MORDOR cells but had no significant effect on K(+) currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K(+) currents recorded from GH(3)MOR and GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K(+) current was inwardly rectifying and was inhibited by Ba(2+) but not TEA. DPDPE had no effect on delayed rectifier-, Ca(2+)-, and voltage-activated or A-type K(+) currents, recorded from GH(3)MORDOR cells. Ba(2+) attenuated the inhibition of [Ca(2+)](i) and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K(IR) channels in these actions of the opioid.

Adenylate Cyclase Toxin↗
PubMed2000

Reverse microemulsion synthesis of nanostructured complex oxides for catalytic combustion

Catalysts play an important role in many industrial processes, but their use in high-temperature applications-such as energy generation through natural gas combustion, steam reforming and the partial oxidation of hydrocarbons to produce feedstock chemicals--is problematic. The need for catalytic materials that remain stable and active over long periods at high operation temperatures, often in the presence of deactivating or even poisoning compounds, presents a challenge. For example, catalytic methane combustion, which generates power with reduced greenhouse-gas and nitrogen-oxide emissions, is limited by the availability of catalysts that are sufficiently active at low temperatures for start-up and are then able to sustain activity and mechanical integrity at flame temperatures as high as 1,300 degrees C. Here we use sol-gel processing in reverse microemulsions to produce discrete barium hexa-aluminate nanoparticles that display excellent methane combustion activity, owing to their high surface area, high thermal stability and the ultrahigh dispersion of cerium oxide on the their surfaces. Our synthesis method provides a general route to the production of a wide range of thermally stable nanostructured composite materials with large surface-to-volume ratios and an ultrahigh component dispersion that gives rise to synergistic chemical and electronic effects, thus paving the way to the development of catalysts suitable for high-temperature industrial applications.

Journal Article↗
PubMed1991

A new site for the activation of cardiac calcium channels defined by the nondihydropyridine FPL 64176.

We examined the effects of a new ligand, FPLnM-64176, on L-type Ca++ channels in cardiac tissue. FPL 64176 (10-1 microM) enhanced Ca++ influx into neonatal rat ventricular myocytes, a response which was blocked by nifedipine. FPL 64176 had no effect on [3H]PN200-110 binding in rat ventricular membranes, but dramatically increased L-type Ca++ channel current amplitude. FPL 64176 (1 microM) slowed both the activation and the inactivation kinetics of the L-channel in neonatal rat ventricular cells. We also noted a hyperpolarizing shift in the threshold and peak potential of the Ca++ channel current-voltage relationship in response to the compound. Additionally, the binding site for FPL 64176 appeared to be located on the extracellular face of the channel. We conclude that FPL 64176 is a potent new activator of L-type Ca++ channels with a novel mechanism and site of action.

Animals↗
PubMed1996

K+ channels and the intracellular calcium signal in human melanoma cell proliferation.

K+ channels, membrane voltage, and intracellular free Ca2+ are involved in regulating proliferation in a human melanoma cell line (SK MEL 28). Using patch-clamp techniques, we found an inwardly rectifying K+ channel and a calcium-activated K+ channel. The inwardly rectifying K+ channel was calcium independent, insensitive to charybdotoxin, and carried the major part of the whole-cell current. The K+ channel blockers quinidine, tetraethylammonium chloride and Ba2+ and elevated extracellular K+ caused a dose-dependent membrane depolarization. This depolarization was correlated to an inhibition of cell proliferation. Charybdotoxin affected neither membrane voltage nor proliferation. Basic fibroblast growth factor and fetal calf serum induced a transient peak in intracellular Ca2+ followed by a long-lasting Ca2+ influx. Depolarization by voltage clamp decreased and hyperpolarization increased intracellular Ca2+, illustrating a transmembrane flux of Ca2+ following its electrochemical gradient. We conclude that K+ channel blockers inhibit cell-cycle progression by membrane depolarization. This in turn reduces the driving force for the influx of Ca2+, a messenger in the mitogenic signal cascade of human melanoma cells.

Animals↗
PubMed1993

The S4-S5 loop contributes to the ion-selective pore of potassium channels.

Mutagenesis experiments on voltage-gated K+ channels have suggested that the ion-selective pore is comprised mostly of H5 segments. To see whether regions outside of the H5 segment might also contribute to the pore structure, we have studied the effect of single amino acid substitutions in the segment that connects the S4 and S5 putative transmembrane segments (S4-S5 loop) on various permeation properties of Shaker K+ channels. Mutations in the S4-S5 loop alter the Rb+ selectivity, the single-channel K+ and Rb+ conductances, and the sensitivity to open channel block produced by intracellular tetraethylammonium ion, Ba2+, and Mg2+. The block of Shaker K+ channels by intracellular Mg2+ is surprising, but is reminiscent of the internal Mg2+ blockade of inward rectifier K+ channels. The results suggest that the S4-S5 loop constitutes part of the ion-selective pore. Thus, the S4-S5 loop and the H5 segment are likely to contribute to the long pore characteristic of voltage-gated K+ channels.

Amino Acid Sequence↗
PubMed1997

Arachidonic acid activation of potassium channels in rat visual cortex neurons.

We investigated the effects of arachidonic acid on K+ channels in freshly dissociated neurons of 10- to 20-day-old rat visual cortex, using a perforated and conventional whole-cell patch-clamp and inside-out excised patch configurations. Arachidonic acid at 5-30 microM induced an outward current in 88.1% of the neurons in whole-cell mode, and evoked channel opening with a conductance of 170-238 pS in 90.5% of neurons under inside-out patch recording. Arachidonic acid-activated K+ channels were partially blocked by extracellular administration of 1 mM tetraethylammonium and 100 nM charybdotoxin. However, Ba2+ completely blocked the channel in all cases. None of the other K+ channel blockers, including 4-aminopyridine, quinidine, apamin and glibenclamide, inhibited the arachidonic acid-activated channels. Intracellular perfusion with Ca2+-free and 5 mM BAPTA in Ca2+-free extracellular perfusate containing 2 mM EGTA in conventional whole-cell recording did not inhibit the K+ channel, implying that the channel is not Ca2+ dependent. Neither guanosine 5'-O-(2-thiodiphosphate) nor staurosporine applied in inside-out mode affected the arachidonic acid-activated channels, indicating that G-protein and protein kinase C are not involved in this phenomenon. In addition, neither indomethacin nor nordihydroguaiaretic acid blocked the channel currents, demonstrating that it is arachidonic acid itself but not its metabolites that induced the effect. Among the fatty acids tested, only cis-unsaturated fatty acids, having more than two double bonds, such as arachidonic acid, docosahexaenoic acid and linolenic acid, activated the K+ channels. These findings suggest that there exists a novel type of K+ channel activated by arachidonic acid which may play a critical role in modulating neuronal excitability in cortical neurons.

4-Aminopyridine↗
PubMed1997

Characterization of maxi-K-channels in bovine trabecular meshwork and their activation by cyclic guanosine monophosphate.

PURPOSE: Electrophysiological characterization of trabecular meshwork cells and investigation of their response to elevation of cytosolic cyclic guanosine monophosphate (cGMP). METHODS: Bovine trabecular meshwork cells were cultured according to established methods and were studied, using the whole-cell and single-channel configurations of the patch-clamp technique. RESULTS: In single-channel experiments, cells expressed a channel with characteristics typical of maxi-K-channels. The channel was densely distributed in the membrane and had a high conductance of 326 +/- 4 pS (Pico Siemens) (symmetrical 150 mmol/l KCl; 37 degrees C) for potassium and negligible conductance for sodium (0.9 +/- 1 pS). The open probability could be elevated by depolarization, increasing cytosolic calcium, or adding adenosine triphosphate (1 mmol/ l). The channel could be blocked by external charybdotoxin (10(-8) mol/1), external TEA+ tetraethyl ammonium chloride (1 mmol/l) and by internal Ba2+ (10 mmol/l), whereas external Ba2+ and internal TEA+ (10 mmol/l) had no effect. In whole-cell experiments, trabecular meshwork cells displayed a strong outward conductance. Part of this conductance (35 +/- 5%) could be blocked by charybdotoxin and stimulated by ionomycin (10(-5) mol/1). Addition of 8-bromo-cGMP (10(-3) mol/1) stimulated the current to 290 +/- 57% (n = 4) of the original level, charybdotoxin led to a reduction of this current to 156 +/- 28% of the initial value. CONCLUSIONS: Trabecular meshwork cells express maxi-K-channels. These channels can be stimulated by raising internal cGMP levels and are known for their importance in smooth muscle relaxation. The results in this study supply further evidence that trabecular meshwork displays smooth muscle-like properties and contributes to the clarification of the mechanism leading to the relaxation of trabecular meshwork by nitrate and nonnitrate vasodilatators.

Animals↗
PubMedDate not supplied

Metalloestrogens: an emerging class of inorganic xenoestrogens with potential to add to the oestrogenic burden of the human breast.

Many compounds in the environment have been shown capable of binding to cellular oestrogen receptors and then mimicking the actions of physiological oestrogens. The widespread origin and diversity in chemical structure of these environmental oestrogens is extensive but to date such compounds have been organic and in particular phenolic or carbon ring structures of varying structural complexity. Recent reports of the ability of certain metal ions to also bind to oestrogen receptors and to give rise to oestrogen agonist responses in vitro and in vivo has resulted in the realisation that environmental oestrogens can also be inorganic and such xenoestrogens have been termed metalloestrogens. This report highlights studies which show metalloestrogens to include aluminium, antimony, arsenite, barium, cadmium, chromium (Cr(II)), cobalt, copper, lead, mercury, nickel, selenite, tin and vanadate. The potential for these metal ions to add to the burden of aberrant oestrogen signalling within the human breast is discussed.

Animals↗
PubMed1989

Electrophysiological properties of hypoglossal motoneurons of guinea-pigs studied in vitro.

Intracellular recordings were made from the hypoglossal nuclear complex in brain slices from guinea-pigs. Retrograde transport of horseradish peroxidase from the tongue confirmed the identity of the visually identified hypoglossal nucleus. Eighteen neurons were stained by intracellular electrophoresis of Lucifer Yellow through the recording pipette. Two types of neurons were encountered, motoneurons with maximal discharge rates of 90 Hz and another type with maximal discharge rates of 250 Hz. Motoneurons were prevalent in the hypoglossal nucleus and the other type prevailed in the adjoining nucleus prepositus hypoglossi. In both nuclei the two types were mixed. Antidromic spikes elicited from hypoglossal root fibres had initial segment and somatodendritic components. Electrical stimulation of the reticular matter dorsolateral to the hypoglossal nucleus elicited excitatory postsynaptic potentials and strychnine sensitive inhibitory postsynaptic potentials. Motoneurons responded to depolarizing current pulses with a train of spikes. The initial spike interval was much shorter than the rest and fast adaptation occurred over three to four intervals. Slow adaptation was most prominent when the neuron was depolarized and discharged at a high rate. High threshold calcium spikes were evoked by depolarizing pulses when sodium spikes were blocked by tetrodotoxin and the potassium conductance reduced by tetraethylammonium bromide. Motoneurons discharged in a single range, inflections on the frequency-current plot being absent. Spikes and spike trains evoked by depolarizing pulses were followed by afterhyperpolarizations with fast and slow parts. The fast phase was eliminated by tetraethylammonium bromide, possibly because the delayed rectifier was involved. A calcium dependent potassium conductance was probably involved in the slow phase, because it was sensitive to inorganic calcium blockers. The amplitude of the afterhyperpolarization following trains of spikes depended on the frequency of the preceding spikes. At constant frequency, the amplitude depended, in addition, on the strength of stimuli arising from different hyperpolarized potentials. Afterdepolarizing potentials were absent. Lissajous plots of double ramp current stimulation showed anomalous rectification between resting potential and spike threshold. The rectification was sensitive to inorganic calcium blockers. Subthreshold responses showed initial sags and rebound responses in all healthy cells and these were eliminated by caesium. Barium, substituted for calcium, unleashed a depolarizing plateau potential sensitive to tetrodotoxin, indicating the presence of a persistent sodium conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗