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K Strange

Publications and source records attributed to K Strange.

At least 37 records · Page 2Linked to original sources
PubMed1996

NMDA receptor activation inhibits neuronal volume regulation after swelling induced by veratridine-stimulated Na+ influx in rat cortical cultures.

Neurons and glia experience rapid fluctuations in transmembrane solute and water fluxes during normal brain activity. Cell volume must be regulated under these conditions to maintain optimal neural function. Almost nothing is known, however, about how brain cells respond to volume challenges induced by changes in transmembrane solute flux. As such, we characterized the volume-regulatory mechanisms of cultured cortical neurons swollen by veratridine-stimulated Na+ influx. Exposure of cortical neurons to 100 microM veratridine for 10-15 min caused a 1.8- to 2-fold increase in cell volume that persisted for at least 90 min. This volume increase was blocked by extracellular Na+ removal or by exposure to 5 microM tetrodotoxin, indicating that swelling is a result of Na+ entry via Na+ channels. Treatment of cells with veratridine together with various NMDA receptor antagonists had no effect on the magnitude of swelling. NMDA receptor antagonist-treated cells, however, underwent nearly complete volume recovery within 50-70 min after veratridine exposure. This recovery suggests that NMDA receptor activation disrupts neuronal osmoregulatory pathways. Volume regulation was blocked by Ba2+, quinidine, or 5-nitro-2-(3-phenylpropylamino) benzoic acid, indicating that swelling activates volume regulatory K+ and Cl- channels. Veratridine also caused a rapid, transient increase in intracellular Ca2+. Extracellular Ca2+ removal or intracellular Ca2+ chelation prevented or dramatically reduced veratridine-induced increases in intracellular Ca2+ and completely blocked volume recovery. These findings indicate that increases in Ca2+ during cell swelling induced by Na+ influx are required for activation of neuronal volume-regulatory pathways.

Animals↗
PubMed1996

Single channel properties of a volume sensitive anion channel: lessons from noise analysis.

Swelling activated anion channels have recently been recognized to play an important role in not only volume regulatory electrolyte movement, but also in organic osmolyte transport. A swelling-activated, outwardly rectifying anion channel termed VSOAC (volume-sensitive organic osmolyte/anion channel) is a major pathway for swelling-induced loss of organic osmolytes and organic anions from mammalian cells. VSOAC has been described in numerous cell types. Until recently, however, the unitary conductance and gating kinetics of VSOAC were uncertain. Stationary noise analysis and single channel measurements have produced estimates for the unitary conductance of swelling-activated, outwardly rectifying anion channels that vary by > 15-fold. This review describes our current understanding of the single channel properties of VSOAC.

Animals↗
PubMed1996

Cloning of an aquaporin homologue present in water channel containing endosomes of toad urinary bladder.

Regulation of total body water balance in amphibians by antidiuretic hormone (ADH) contributed to their successful colonization of terrestrial habitats approximately 200-300 million years ago. In the mammalian kidney, ADH modulates epithelial cell apical membrane water permeability (Pf) by fusion and retrieval of cytoplasmic vesicles containing water channel proteins called aquaporins (AQPs). To determine the role of AQPs in ADH-elicited Pf in amphibians, we have identified and characterized a unique AQP from Bufo marinus called AQP toad bladder (AQP-TB). AQP-TB possesses many structural features common to other AQPs, AQP-TB is expressed abundantly in ADH-responsive tissues, including toad urinary bladder and skin as well as lung, skeletal muscle, kidney, and brain. In a manner identical to that reported for the mammalian ADH-elicited water channel AQP2, AQP-TB expression is increased significantly by intervals of dehydration or chronic ADH stimulation. However, expression of AQP-TB protein in Xenopus laevis oocytes does not significantly increase oocyte Pf. The lack of expression of functional AQP-TB water channels in oocytes may result from intracellular sequestration of AQP-TB due to the presence of a YXRF sequence motif present in its carboxyterminal domain.

Amino Acid Sequence↗
PubMed1996

Swelling-activated anion conductance in skate hepatocytes: regulation by cell Cl- and ATP.

Cell swelling activates an outwardly rectifying anion conductance in mammalian cells. The channel responsible for this conductance mediates volume-regulatory efflux of organic osmolytes such as taurine. We observed a similar conductance in hepatocytes from the skate Raja erinacea. Whole cell Cl- conductance was increased > 100-fold by a 2-fold increase in hepatocyte volume. The conductance was outwardly rectifying and had a relative cation permeability of approximately 0.2. Cation permeability was increased by reductions in patch pipette CsCl concentration, suggesting that the channel pore contains saturable anion and cation binding sites with different anion and cation affinities. The conductance had a broad anion selectivity and a relative taurine permeability of 0.17. Activation of the conductance required intracellular ATP or a nonhydrolyzable ATP analogue. Elevation of intracellular Cl- from 20 to 155 mM reduced current activation while the rate and extent of cell swelling were unaffected. Reduction of intracellular Cl- concentration to 5-10 mM caused spontaneous current activation without cell swelling. These results suggest that increases in cell Cl- levels increase the volume set point of the channel. We propose that the main function of the outwardly rectifying anion channel is nonselective transport of organic solutes.

Adenosine Triphosphate↗
PubMed1996

Cellular and molecular physiology of volume-sensitive anion channels.

Maintenance of a constant cell volume in the face of osmotic stress is an evolutionarily ancient homeostatic process. Over the last two decades physiologists have gained an impressive understanding of the "volume-sensitive" channels, cotransporters, exchangers, metabolic pathways, and genes that are responsible for modulating intracellular solute content and cell volume. This review focuses on one part of this story, the characteristics and osmoregulatory functions of volume-sensitive anion channels. Three distinct types of swelling-activated anion channels have been observed and studied extensively in animal cells. These channels include 1) ClC-2, which is a member of the ClC family of voltage-gated anion channels, 2) an outwardly rectifying intermediate conductance channel, and 3) a large-conductance or "maxi" channel. In addition to these three channels, several other less well-characterized anion channels have been observed. This review discusses the electrophysiological and molecular biological characteristics and regulation of these channels. The possible roles different types of anion channels might play in cell volume homeostasis are also discussed.

Amino Acid Sequence↗
PubMed1996

In situ localization and osmotic regulation of the Na(+)-myo-inositol cotransporter in rat brain.

Organic osmolytes accumulate in the mammalian brain when plasma osmolality is elevated chronically. An understanding of organic osmolyte regulation is important for elucidating the pathophysiology of numerous disease states, particularly those in which plasma osmolality is altered or manipulated. To this end, we have previously characterized the mechanisms of osmoregulatory myo-inositol accumulation and loss in rat brain glial cells. The validity of cell culture models of physiological functions, however, requires demonstration of similar behavior in vivo. We therefore examined the effect of serum hypertonicity and its correction on expression of the Na(+)-myo-inositol cotransporter (SMIT) in rat brain. Northern analysis revealed that chronic serum hypertonicity increased brain SMIT mRNA levels four- to sevenfold. Rapid reduction of serum osmolality caused a rapid fall in SMIT mRNA levels. In situ hybridization revealed a widespread distribution of cells expressing SMIT with striking regional variability in the number of and intensity of cells labeled. These results confirm and extend our studies of cultured glial cells and indicate that myo-inositol accumulation in vivo is due to increased expression of SMIT. Regional variability in the distribution of SMIT may be a factor in the differential vulnerability of certain brain regions to shifts in plasma osmolality.

Animals↗
PubMed1995

Single-channel properties of a volume-sensitive anion conductance. Current activation occurs by abrupt switching of closed channels to an open state.

Swelling-induced loss of organic osmolytes from cells is mediated by an outwardly rectified, volume-sensitive anion channel termed VSOAC (Volume-Sensitive Organic osmolyte/Anion Channel). Similar swelling-activated anion channels have been described in numerous cell types. The unitary conductance and gating kinetics of VSOAC have been uncertain, however. Stationary noise analysis and single-channel measurements have produced estimates for the unitary conductance of swelling-activated, outwardly rectified anion channels that vary by > 15-fold. We used a combination of stationary and nonstationary noise analyses and single-channel measurements to estimate the unitary properties of VSOAC. Current noise was analyzed initially by assuming that graded changes in macroscopic current were due to graded changes in channel open probability. Stationary noise analysis predicts that the unitary conductance of VSOAC is approximately 1 pS at 0 mV. In sharp contrast, nonstationary noise analysis demonstrates that VSOAC is a 40-50 pS channel at +120 mV (approximately 15 pS at 0 mV). Measurement of single-channel events in whole-cell currents and outside-out membrane patches confirmed the nonstationary noise analysis results. The discrepancy between stationary and nonstationary noise analyses and single-channel measurements indicates that swelling-induced current activation is not mediated by a graded increase in channel open probability as assumed initially. Instead, activation of VSOAC appears to involve an abrupt switching of single channels from an OFF state, where channel open probability is zero, to an ON state, where open probability is near unity.

Animals↗
PubMed1995

Characterization of the voltage-dependent properties of a volume-sensitive anion conductance.

Outwardly rectified, swelling-activated anion conductances have been described in numerous cell types. The major functional variable observed amongst these conductances is the extent and rate of depolarization-induced inactivation. In general, the conductances can be divided into two broad classes, those that show rapid inactivation in response to strong depolarization and those that show little or no voltage dependence. The swelling-activated anion conductance in rat C6 glioma cells is inactivated nearly completely by membrane depolarization above +90 mV and reactivated by membrane hyperpolarization. The kinetics of inactivation and reactivation are fit by single and double exponentials, respectively. Voltage-dependent behavior is well described by a simple linear kinetic model in which the channel exists in an open or one of three inactivated states. pH-induced changes in voltage-dependent gating suggest that the voltage sensor contains critical basic amino acid residues. Extracellular ATP blocks the channel in a voltage-dependent manner. The block is sensitive to the direction of net Cl- movement and increases open channel noise indicating that ATP interacts with the channel pore. Blockage of the channel with ATP dramatically slows depolarization-induced inactivation.

Adenosine Triphosphate↗
PubMed1995

ATP depletion and inactivation of an ATP-sensitive taurine channel by classic ion channel blockers.

Cell volume regulation in different cell types is mediated in part by plasma membrane channel(s) that allow taurine and other important intracellular organic osmolytes to efflux from the cell. Previous studies have demonstrated that intracellular ATP is required for activation of a volume-sensitive taurine-permeable channel. The present study examined the relation between cellular ATP and ADP concentrations and swelling-induced [14C]taurine efflux and anion current (whole-cell patch-clamp) after exposure of isolated skate (Raja erinacea) hepatocytes to metabolic poisons and a series of ion channel blockers. When intracellular ATP content was lowered with gradually increasing concentrations of 2,4-dinitrophenol, a sigmoidal relation between ATP content and volume-activated [14C]taurine efflux was observed. Taurine efflux was progressively inhibited over a relatively narrow range of intracellular ATP levels, indicating that physiologic alterations in cellular nucleotides may modulate the opening of the channel. Surprisingly, the inhibition of [14C]taurine efflux by a number of ion channel blockers [glibenclamide, 5-nitro-2-(3-phenylpropylamino)benzoate, diphenylamine-2-carboxylate, ketoconazole, gossypol, niflumic acid, and quinine] was related to a decrease in cellular ATP concentrations and ATP/ADP ratios, rather than to a direct interaction with the channel. In contrast, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and pyridoxal-5-phosphate inhibited volume-activated anion channels but had no effect on cellular ATP levels. These findings suggest multiple sites for regulation of volume-sensitive osmolyte channels and indicate that some putative ion channel blockers may actually alter the activity of ATP-regulated transporters by depleting cellular ATP.

2,4-Dinitrophenol↗
PubMed1994

Osmoregulatory changes in myo-inositol content and Na+/myo-inositol cotransport in rat cortical astrocytes.

Exposure of cortical astrocytes to 325, 350, or 390 mosM culture media for 48 h caused a 1.4-, 2.1-, and 3.5-fold increase, respectively, in cellular content of the compatible osmolyte myo-inositol. Elevated myo-inositol levels accounted for approximately 56-100% of the solute needed by the cells for complete volume regulation under hypertonic conditions. Myo-inositol accumulation was associated with 4-5-fold (peak rate) and 1.8-2-fold (steady-state rate) increases in the rate of Na(+)-dependent myo-inositol uptake when cells were acclimated to 390 mosM culture medium for 12 h or 24-96 h, respectively. When medium osmolality was elevated by 25 mosM, peak and steady-state increases in myo-inositol uptake of 1.7-fold and 1.3-fold, respectively, were observed. Exposure to 390 mosM medium for 12-48 h induced a 3-8-fold increase in cotransporter mRNA levels suggesting that the increase in myo-inositol uptake is brought about by increased cotransporter gene expression. Abrupt return of hypertonic cells to an isotonic medium induced a rapid increase in myo-inositol efflux and a return of cotransporter mRNA to control values in < 2 h. In contrast, the cotransporter remained fully activated at hypertonic levels for 16 h. Between 16-24 h after the transfer, the rate of myo-inositol uptake returned to control values. The remarkable sensitivity of the cotransporter to hypertonic stress indicates that upregulation of myo-inositol transport in glial cells is likely to occur in a variety of disease states that cause an elevation of plasma osmolality. Slow downregulation of the cotransporter may be responsible in part for the slow loss of myo-inositol and cerebral edema that occurs with too rapid correction of chronic plasma hypertonicity.

Animals↗
PubMed1994

Permeability properties of rat renal lysosomes.

Although lysosomes maintain large pH gradients and may be subjected to significant osmotic gradients in vivo, little is known about their passive permeability properties. In recent studies, vacuolar H(+)-adenosine-triphosphatases (ATPases), such as those found in lysosomes, have been suggested to act as water channels. In addition, the erythrocyte and proximal tubule water channel CHIP28 is present on the plasma membrane of proximal tubule cells and may undergo endocytosis so that it is incorporated in lysosomes. We therefore examined water, proton, and small nonelectrolyte permeabilities in freshly purified lysosomes from rat renal proximal tubule. Lysosomes were purified by differential and Percoll gradient centrifugation. The preparation contained only lysosomes when examined by electron microscopy. Moreover, analysis by flow cytometry showed virtually all particles to be positive for acid phosphatase and cathepsin B activities. Permeabilities were measured on a stopped-flow fluorimeter by monitoring the self-quenching or pH-sensitive quenching of entrapped fluorescein derivatives. Osmotic water permeability (Pf) averaged 0.011 +/- 0.003 cm/s (n = 6), a value similar to that of biological membranes containing water channels. However, Pf was insensitive to the organic mercurial reagent p-chloromercuribenzene-sulfonate and to HgCl2 and exhibited an activation energy of 10.8 +/- 0.8 kcal/mol. These results indicate that water flux in lysosomes occurred via the lipid bilayer, and not via water channels. Addition of ATP led to lysosomal acidification (proton flux = 4.6 +/- 0.8 x 10(-11) mmol H+.s-1.cm-2), which was completely inhibited by 0.1 microM bafilomycin. Pf was insensitive to this agent as was the passive proton permeability (0.36 +/- 0.18 cm/s, n = 4). Permeabilities to small nonelectrolytes varied in proportion to the oil-water partition coefficient, confirming the applicability of Overton's rule to lysosomes. We conclude that proximal tubular lysosomes exhibit high Pf, which occurs via the lipid bilayer and not via vacuolar H(+)-ATPase.

Acids↗
PubMed1994

Ketoconazole blocks organic osmolyte efflux independently of its effect on arachidonic acid conversion.

Ketoconazole, cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate, and gossypol are reported inhibitors of the lipoxygenase (LO) and cytochrome P-450 enzyme systems and are potent blockers of swelling-activated efflux of organic osmolytes and volume-sensitive anion channels in C6 glioma cells. To directly test the hypothesis that LO- or cytochrome P-450-derived products of arachidonic acid (AA) participate in the regulation of these volume-sensitive transport pathways, we incubated C6 cells with [1-14C]AA and observed the extent and profile of its conversion under basal conditions and after acute swelling. High-performance liquid chromatographic analysis revealed that most (70-80%) of the labeled AA remained unchanged with only 6-8% and 10-20% of label converted to LO- [12(S)- and 15(S)-hydroxyeicosatetraenoic acid (12- and 15-HETE)] and cyclooxygenase- [prostaglandin (PG) E2 and PGF2a] derived products, respectively. Leukotrienes and epoxyeicosatrienoic acid compounds were not produced. The conversion profile of [1-14C]AA was not altered substantially by cell swelling. Treatment of cells with the LO-derived products 5-, 12-, and 15-HETE or their immediate metabolic precursors, 5(S)-, 12(S)-, and 15(S)-hydroxyperoxyeicosatetraenoic acid, at 5 microM concentrations did not stimulate efflux of [3H]inositol. In addition, treatment with HETEs did not override the inhibition of efflux observed with the LO-cytochrome P-450 blocker ketoconazole. Whole cell patch-clamp experiments demonstrated that volume-sensitive anion channels, the postulated pathway for organic osmolyte efflux in C6 cells, are rapidly and reversibly blocked by ketoconazole in a fashion suggestive of direct inhibition rather than via interruption of a second messenger pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

The volume-sensitive organic osmolyte-anion channel VSOAC is regulated by nonhydrolytic ATP binding.

Efflux of intracellular organic osmolytes to the external medium is a ubiquitous response to cell swelling. Accumulating evidence indicates that volume regulatory loss of structurally unrelated organic osmolytes from cells is mediated by a relatively nonselective volume-sensitive anion channel. In C6 cells, we have termed this channel VSOAC for volume-sensitive organic osmolyte-anion channel. Swelling-induced activation of VSOAC required the presence of ATP or nonhydrolyzable ATP analogues [adenosine 5'-O-(3-thiotriphosphate), adenylylmethyl-enediphosphonate (AMP-PCP), or 5'-adenylylimidodiphosphate] in the patch pipette. Sustained activation of VSOAC also required ATP. Channel rundown was observed when cellular ATP levels were lowered by intracellular dialysis with the patch pipette solution. Rundown was prevented by the ATP analogue AMP-PCP. Passive swelling-induced myo-[3H]inositol and [3H]taurine efflux was blocked by metabolic inhibitors that decreased cellular ATP levels. Titration of cellular ATP levels with azide demonstrated that the apparent dissociation constant (Kd) for ATP of both myo-inositol and taurine efflux was approximately 1.7 mM. The high Kd for ATP indicates that cellular metabolic state plays an important role in modulating organic osmolyte loss. Regulation of VSOAC activity by ATP prevents depletion of metabolically expensive organic osmolytes when cellular energy production is reduced. In addition, ATP-dependent regulation provides essential feedback to minimize the loss of energy-producing carbon sources such as pyruvate, short-chain fatty acids, ketone bodies, and amino acids, which readily permeate this channel.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1994

Acidification of vasopressin-induced endosomes in toad urinary bladder.

It is well established that water channels (WC) are removed from the apical membrane of vasopressin-sensitive epithelia by endocytosis. The processing and the ultimate fate of endocytosed WC is, however, incompletely understood. In many cells, endosome acidification plays an important role in the processing and sorting of endocytosed proteins. Endosome acidification in the toad urinary bladder was therefore examined in vivo by fluorescence ratio video microscopy after induction of endocytosis by vasopressin removal and transepithelial water flow in the presence of the pH-sensitive fluid phase marker 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-dextran. Fifteen minutes after induction of endocytosis, the majority of endosomes had a neutral or slightly acidic pH. The number of acidic endosomes increased progressively with time. Two hours after endocytosis began, 98% of the endosomes had a pH < 6.0. Bafilomycin completely blocked endosome acidification, indicating that H+ transport is mediated by a vacuolar H(+)-adenosinetriphosphatase. Bafilomycin had no effect on transepithelial water flow in bladders repetitively stimulated by vasopressin. These findings, as well as the work of other investigators, suggest that if WC recycling occurs, it is not dependent on acidification of the endosomal compartment. Acidification of vasopressin-induced endosomes most likely represents a terminal event in the endocytic pathway.

Acids↗
PubMed1993

Maintenance of cell volume in the central nervous system.

Maintenance of the ionic and osmotic composition and volume of intra- and extracellular fluids in the brain is crucial for normal functioning of the central nervous system (CNS). Osmoregulation in the CNS is mediated by solute and water transport across the blood-brain barrier, choroid plexus and plasma membrane of glial cells and neurons. Despite its clinical and physiological significance, however, little is known about the underlying cellular and molecular mechanisms by which CNS osmotic and ionic balance is maintained. In this review, I will discuss our current understanding of cell volume regulation in the CNS and how it relates to various disease processes, such as hyponatremia, renal failure and hypernatremia. A detailed understanding of brain osmoregulatory processes represents a fundamental physiological problem and is required for the treatment of numerous disease states, particularly those encountered in the practice of nephrology.

Animals↗
PubMed1993

Acute volume regulation of brain cells in response to hypertonic challenge.

BACKGROUND: Hypertonic dehydration of the brain through administration of osmotic agents, either alone or in combination with "loop" diuretics, has been a mainstay in the treatment of increased intracranial pressure for decades. Controversy exists, however, as to the mechanism and long-term value of such therapy. Although many cell types possess volume regulatory mechanisms capable of opposing hypertonic dehydration, such behavior in the brain is poorly understood. METHODS: As a model for the mammalian central nervous system, the real-time volume behavior of rat C6 glioma cells was observed by laser light scattering during hypertonic challenge. Cells were allowed to equilibrate in isotonic balanced salt solutions at physiologic pH and temperature, and then rapidly exposed to hypertonic solutions. Experiments were conducted in the presence and absence of sodium, chloride, and the loop diuretic bumetanide to assess their roles in volume regulation. RESULTS: In response to acute, large (70 mOsm) hypertonic exposures, cells immediately shrank and then rapidly regulated their volume completely back to control within minutes. In the presence of the loop diuretic bumetanide, the volume regulatory process was significantly inhibited with only 54% recovery observed at concentrations of 10(-4) M. Volume regulation was also significantly inhibited by removal of extracellular sodium and chloride. CONCLUSIONS: Brain cells possess powerful, electrolyte-dependent and bumetanide-sensitive volume-regulatory mechanisms that directly oppose attempted osmotic shrinkage. These observations suggest a possible new mechanism for the clinically observed synergistic effects of loop and osmotic diuretics in reduction of brain volume.

Animals↗