Activation of ion transport pathways by changes in cell volume

Volume-activated chloride channels in HL-60 cells: potent inhibition by an oxonol dye

When swollen in hypotonic media, HL-60 cells exhibit a regulatory volume decrease (RVD) response as a result of net losses of K+ and Cl-. This is primarily caused by a dramatic increase in Cl- permeability, which may reflect the opening of volume-sensitive channels (11). To test this hypothesis, we measured volume-activated Cl- currents in HL-60 cells using the patch-clamp technique. The whole cell Cl- conductance (in nS/pF at 100 mV) increased from 0.09 +/- 0.06 to 1.15 +/- 0.19 to 1.64 +/- 0.40 as the tonicity (in mosmol/kgH2O) of the external medium was decreased from 334 to 263 to 164, respectively. Cl- currents showed no significant inactivation during 800-ms pulses. Current-voltage curves exhibited outward rectification and were identical at holding potentials of 0 or -50 mV, suggesting that the gating of the channels is voltage independent. The selectivity sequence, based on permeability ratios (PX/PCl) calculated from the shifts of the reversal potentials, was SCN- > I- approximately NO3- > Br- > Cl- >> gluconate. 4-Acetamido-4'- isothiocyanostilbene-2,2'-disulfonic acid (SITS; 0.5 mM) inhibits HL-60 Cl- channels in a voltage-dependent manner, with approximately 10-fold increased affinity at potentials greater than +40 mV. Voltage-dependent blockade by SITS indicates that the binding site is located near the outside, where it senses 20% of the membrane potential. These Cl- channels were also inhibited in a voltage-independent manner by the oxonol dye bis-(1,3-dibutylbarbituric acid)pentamethine oxonol [diBA-(5)-C4] with a concentration that gives half inhibition (IC50) of 1.8 microM at room temperature. A similar apparent IC50 value (1.2 microM) was observed for net 36Cl- efflux into a Cl(-)-free hypotonic medium at 21 degrees C. It seems likely, therefore, that the volume-activated Cl- channels are responsible for the net Cl- efflux during RVD. These Cl- channels have properties similar to the "mini-Cl-" channels described in lymphocytes and neutrophils and are strongly inhibited by low concentrations of diBA-(5)-C4.

Regulatory volume decrease in a renal distal tubular cell line (A6). I. Role of K+ and Cl-

Changes in volume of A6 epithelial cells were monitored by recording cell thickness (Tc). The response of Tc to a reduction of the basolateral osmolality from 260 to 140 mosmol/kg was recorded while transepithelial Na+ transport was inhibited by 20 microM amiloride. With Cl--containing bathing media, this osmotic challenge elicited a rapid rise in Tc followed by a regulatory volume decrease (RVD). Substitution of SO4(2-) or gluconate for Cl- markedly reduced the RVD, whereas cells completely maintained their ability to regulate their volume after replacing Cl- by NO3(-). A conductive pathway for Cl- excretion is suggested, which is insensitive to NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid], an inhibitor of some types of Cl- channels. Ba2+ (5 or 20 mM) reduced the RVD. A more pronounced inhibition of the RVD was obtained with 500 microM quinine, a potent blocker of volume-activated K+ channels. K+-induced depolarization of the basolateral membranes of tissues incubated with SO4(2-)-containing solutions completely abolished the RVD. Noise analysis in the presence of Ba2+ showed the activation of an apical K+ conductive pathway. These results demonstrate that cell volume regulation is controlled by processes involving Cl- and K+ excretion through conductive pathways.

Impaired cell volume regulation in intestinal crypt epithelia of cystic fibrosis mice

Cystic fibrosis is a disease characterized by abnormalities in the epithelia of the lungs, intestine, salivary and sweat glands, liver, and reproductive systems, often as a result of inadequate hydration of their secretions. The primary defect in cystic fibrosis is the altered activity of a cAMP-activated Cl- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) channel. However, it is not clear how a defect in the CFTR Cl- channel function leads to the observed pathological changes. Although much is known about the structural properties and regulation of the CFTR, little is known of its relationship to cellular functions other than the cAMP-dependent Cl- secretion. Here we report that cell volume regulation after hypotonic challenge is also defective in intestinal crypt epithelial cells isolated from CFTR -/- mutant mice. Moreover, the impairment of the regulatory volume decrease in CFTR -/- crypts appears to be related to the inability of a K+ conductance to provide a pathway for the exit of this cation during the volume adjustments. This provides evidence that the lack of CFTR protein may have additional consequences for the cellular function other than the abnormal cAMP-mediated Cl- secretion.

Interstitial space, electrical resistance and ion concentrations during hypotonia of rat hippocampal slices

1. The degree to which mammalian brain cells swell in hypotonic environments has not previously been determined. We exposed hippocampal tissue slices prepared from anaesthetized rats to artificial cerebrospinal fluid from which varying amounts of NaCl had been deleted. Interstitial volume (ISV) change was determined from the volume of dilution of the marker ions tetramethylammonium (TMA+) or tetraethylammonium (TEA+). Tissue electrical resistance was measured as the voltage generated by constant current pulses. 2. ISV decreased as a function of lowered extracellular osmolality (osmotic pressure, pi o), indicating cell swelling. After reaching a minimum, ISV recovered partially, suggesting regulatory volume decrease of cells. After restoring normal pi o the ISV expanded, indicating post-hypotonic cell shrinkage. The electrical resistance of the tissue (Ro) increased when pi o was lowered, due to the reduced ionic strength, as well as restricted ISV. 3. To control for low NaCl concentration, reduced NaCl was replaced by mannitol or fructose. In isosmotic, NaCl-deficient solution, ISV showed inconsistent change, and Ro corrected for ionic strength tended to decrease. 4. Extracellular K+ concentration decreased slightly in low pi o except when spreading depression caused it to increase. Extracellular Ca2+ concentration decreased substantially, consistently and reversibly. Administration of isosmotic low-NaCl concentration solutions caused a similar decrease in extracellular Ca2+ concentrations. We propose that low Na+ concentration in extracellular fluid impaired the extrusion of Ca2+. 5. In severely hypotonic solution, ISV was reduced to 25% of its control volume, corresponding to a mean cell volume increase of at least 11%, probably more. From plotting relative changes in ISV against osmolarity we concluded that, within the range tested, hypotonic cell swelling was not opposed by the close approach of plasma membranes of neighbouring cells.

Modulation of Na(+)-H+ exchange by altered cell volume in perfused rat mandibular salivary gland

1. Intracellular pH (pHi) was measured by spectrofluorometry in perfused mandibular salivary glands isolated from the rat and loaded with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Cell volume changes were estimated from changes in intracellular water content measured by proton NMR spectroscopy. 2. Stimulation with 1 microM acetylcholine (ACh) led to a 15 +/- 2% decrease in cell volume. A transient decrease in pHi was followed by a sustained increase (0.17 +/- 0.03 pH units) that has previously been attributed to the upregulation of the Na(+)-H+ exchanger. 3. Increasing perfusate osmolarity by addition of 60 mM sucrose caused a 19 +/- 2% decrease in cell volume and a sustained increase in pHi (0.12 +/- 0.01 pH units) that was abolished by 1 mM amiloride. Acid loading experiments indicated that the increase in pHi was due to an alkaline shift in the pH dependence of the Na(+)-H+ exchanger. 4. A 20% reduction in perfusate osmolarity prevented the cell shrinkage normally associated with ACh stimulation and largely abolished the ACh-induced increase in pHi. 5. Steady-state Na(+)-H+ exchanger activity, estimated from the initial rate of change in pHi following addition of amiloride, increased 9-fold during stimulation with ACh. When cell shrinkage was prevented by simultaneous exposure to the hypotonic solution, the activity of the exchanger still increased 7-fold in response to ACh. 6. We conclude that, although cell shrinkage leads to upregulation of the Na(+)-H+ exchanger, this factor alone is insufficient to account for the marked increase in exchanger activity that follows muscarinic stimulation.

Activation of extracellular signal-regulated kinases Erk-1 and Erk-2 by cell swelling in H4IIE hepatoma cells

Hepatic metabolism and gene expression are among the factors controlled by the cellular hydration state, which changes within minutes in response to aniso-osmotic environments, cumulative substrate uptake, oxidative stress and under the influence of hormones such as insulin. The signalling events coupling cell-volume changes to altered cell function were studied in H4IIE rat hepatoma cells. Hypo-osmotic cell swelling resulted within 1 min in a tyrosine kinase-mediated activation of the extracellular signal-regulated protein kinases Erk-1 and Erk-2, which was independent of protein kinase C and cytosolic calcium. Activation of mitogen-activated protein kinases was followed by an increased phosphorylation of c-Jun, which may explain our recently reported finding of an about 5-fold increase in c-jun mRNA level in response to cell swelling. Pretreatment of cells with pertussis or cholera toxin abolished the swelling-induced activation of Erk-1 and Erk-2, suggesting the involvement of G-proteins. Thus, a signal-transduction pathway resembling growth factor signalling is activated already by osmotic water shifts across the plasma membrane, thereby providing a new perspective for adaption of cell function to alterations of the environment.

Characterization of the swelling-induced alkalinization of endocytotic vesicles in fluorescein isothiocyanate-dextran-loaded rat hepatocytes

Short-term cultivated rat hepatocytes were allowed to endocytose fluorescein isothiocyanate (FITC)-coupled dextran and the apparent vesicular pH (pHves) was measured by single-cell fluorescence. After 2 h of exposure to FITC-dextran, the apparent pH in the vesicular compartments accessible to endocytosed FITC-dextran was 6.01 +/- 0.05 (n = 39) in normo-osmotic media. Hypo-osmotic exposure increased, whereas hyper-osmotic exposure decreased apparent pHves. by 0.18 +/- 0.02 (n = 26) and 0.12 +/- 0.01 (n = 23) respectively. Incubation of the cells with unlabelled dextran for 2h before a 2-h FITC-dextran exposure had no effect on apparent pHves and its osmosensitivity. When, however, hepatocytes were exposed to unlabelled dextran for 5 h after a 2 h exposure to FITC-dextran, in order to allow transport of endocytosed FITC-dextran to late endocytotic/lysosomal compartments, apparent pHves. decreased to 5.38 +/- 0.04 (n = 12) and the apparent pH in the vesicular compartment containing the dye was no longer sensitive to aniso-osmotic exposure. These findings indicate that the osomosensitivity of pHves. is apparently restricted to early endocytotic compartments. Aniso-osmotic regulation of apparent pHves. in freshly FITC-loaded hepatocytes was not accompanied by aniso-osmolarity-induced changes of the cytosolic free calcium concentration, and neither vasopressin nor extracellular ATP, which provoked a marked Ca2+ signal, affected apparent pHves. Dibutyryl-cyclic AMP (cAMP) or vanadate (0.5 mmol/l) were without effect on apparent pHves. and its osmosensitivity. However, pertussis toxin-treatment or genistein (but not daidzein) or the erbstatin analogue methyl 2,5-dihydroxycinnamate fully abolished the osmo-sensitivity of apparent pHves., but did not affect apparent pHves. It is concluded that regulation of pHves. by cell volume occurs in early endocytotic compartments, but probably not in lysosomes, and is mediated by a G-protein and tyrosine kinase-dependent, but Ca2+- and cAMP-independent mechanism.

Effects of ionic strength on the regulation of Na/H exchange and K-Cl cotransport in dog red blood cells

Dog red cell membranes contain two distinct volume-sensitive transporters: swelling-activated K-Cl cotransport and shrinkage-activated Na/H exchange. Cells were prepared with intracellular salt concentration and weight percentage of cell water (%cw) varied independently by transient permeabilization of the cell membrane to cations. The dependence of transporter-mediated Na and K influxes upon %cw and upon extracellular salt concentration (c(ext)) was measured in cells so prepared. It was found that the critical value of %cw at which transporters are activated, called the set point, is similar for the two transporters, and that the set points for the two transporters decrease similarly with increasing extracellular salt concentration. These findings suggest a common mechanism of regulation of these two transporters. Cellular Na, K, and Cl concentrations were measured as functions of %cw and c(ext). Using these data together with data from the literature for other solute concentrations, empirical expressions were developed to describe the dependence of the intracellular concentrations of all significant small molecule electrolytes, and therefore the intracellular ionic strength, upon %cw and c(ext). A mechanistic model for the dependence of the set point of an individual transporter upon intracellular ionic strength is proposed. According to this model, the set point represents a critical extent of association between the transporter and a postulated soluble regulatory protein, called regulator. Model functions are presented for the calculation of the thermodynamic activity of regulator, and hence extent of regulator-transporter association, as a function of total intracellular protein concentration (or %cw) and ionic strength. The experimentally observed dependence of set point %cw on c(ext) are simulated using these functions and the empirical expressions described above, together with reasonable but not uniquely determined values of model parameters.

Chondrocyte deformation and local tissue strain in articular cartilage: a confocal microscopy study

It is well accepted that mechanical forces can modulate the metabolic activity of chondrocytes, although the specific mechanisms of mechanical signal transduction in articular cartilage are still unknown. One proposed pathway through which chondrocytes may perceive changes in their mechanical environment is directly through cellular deformation. An important step toward understanding the role of chondrocyte deformation in signal transduction is to determine the changes in the shape and volume of chondrocytes during applied compression of the tissue. Recently, a technique was developed for quantitative morphometry of viable chondrocytes within the extracellular matrix using three-dimensional confocal scanning laser microscopy. In the present study, this method was used to quantify changes in chondrocyte morphology and local tissue deformation in the surface, middle, and deep zones in explants of canine articular cartilage subjected to physiological levels of matrix deformation. The results indicated that at 15% surface-to-surface equilibrium strain in the tissue, a similar magnitude of local tissue strain occurs in the middle and deep zones. In the surface zone, local strains of 19% were observed, indicating that the compressive stiffness of the surface zone is significantly less than that of the middle and deep zones. With this degree of tissue deformation, significant decreases in cellular height of 26, 19, and 20% and in cell volume of 22, 16, and 17% were observed in the surface, middle, and deep zones, respectively. The deformation of chondrocytes in the surface zone was anisotropic, with significant lateral expansion occurring in the direction perpendicular to the local split-line pattern. When compression was removed, there was complete recovery of cellular morphology in all cases. These observations support the hypothesis that deformation of chondrocytes or a change in their volume may occur during in vivo joint loading and may have a role in the mechanical signal transduction pathway of articular cartilage.

Volume-activated chloride channels in rat parotid acinar cells

1. Rat parotid acinar cells undergo a regulatory volume decrease in response to hypotonically induced cell swelling that is sensitive to K+ and Cl- gradients. To investigate the potential mechanisms involved, the whole-cell patch-clamp technique was used to characterize a volume-sensitive Cl- channel in rat parotid acinar cells. 2. Exposure of cells to a hyposmotic gradient induced large Cl- currents that exhibited outward rectification and were not affected by membrane potential or the absence of intracellular Ca2+. Low external pH increased the currents at all potentials without affecting current kinetics. These currents were nearly abolished when the cells were in hypertonic conditions. This decrease in the current amplitude was correlated with a decrease in the cell size. 3. The volume-sensitive currents displayed little or no time dependence, whereas Ca(2+)-activated Cl- channels, present in the same cells, displayed slow activation kinetics and large, time-dependent tail currents upon repolarization to the holding potential. 4. The reversal potential of the osmotically activated channels was close to the predicted chloride equilibrium potential and was sensitive to the physiological extracellular Cl- concentration ([Cl-]o). The relationship between reversal potential and [Cl-]o was fitted to a modified Nernst equation with a slope of 51 mV per decade, consistent with a Cl- selective conductance. 5. The anion permeability sequence of the channel, obtained from the shifts of the reversal potentials of the volume-sensitive Cl- current, was: SCN- > I- > NO-3 > Br- > Cl- > formate > propionate = methanesulphonate = acetate > or = F- > or = butyrate > valerate > gluconate = glucuronate = glutamate. 6. The current through the volume-sensitive channels was inhibited by the Cl- channel blocker SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) in a voltage-dependent manner. 7. We conclude that rat parotid acinar cells express an outwardly rectifying Cl- current that can be activated by swelling under hypotonic conditions. This Cl- conductance may be an element of the cellular mechanisms of volume regulation in exocrine glands.

Cell volume and shape oscillations in rat type-II somatotrophs at hypotonic conditions

The size and shape of growth hormone (GH)-producing rat type-II somatotrophs was studied during osmotic manipulation. When somatotrophs were exposed to large osmotic stress (200 and 225 mOsm), the peak projected cell area (PCA) was 132.9% +/- 12.6% and 116.8% +/- 2.8% (P < 0.01) and triggered a regulatory volume decrease (RVD) to avoid lysis. At lower osmotic stress (250 mOsm), the rate of swelling was slower, and the volume reached a steady state at 109.4% +/- 2.4% (P < 0.05) and was without RVD. At 275 and 287 mOsm, the swelling was delayed [PCA peak at 3-4 min; 105.8% +/- 1.5% (P < 0.05) and 104.2% +/- 1.7%] and then showed repeated synchronized cycles of swelling and shrink-age (P < 0.05). The data suggest that somatotrophs may have more than one mechanism for volume regulation. One mechanism is for large swelling (classic RVD response), whereas the other represents more physiological mechanisms for regulating the cell volume within a more limited geometry range. For low osmotic stress (250-287 mOsm), the somatotrophs became less spherical during swelling and, thus, were without membrane dilation. Therefore, this type of volume regulation must work independently from membrane stress. Related volume regulation mechanisms may underlie the previously observed volume fluctuations in somatotrophs seen during secretory stimulation with GH-releasing hormone.

Synergistic action of vasopressin and aldosterone on basolateral Na(+)-K(+)-ATPase in the cortical collecting duct

The respective effects of aldosterone and arginine vasopressin (AVP) were examined on the number of active Na(+)-K(+)-ATPase and their pumping activity in nonperfused microdissected mouse cortical collecting tubules (CCD) by measuring specific 3H-ouabain binding and ouabain-sensitive 86Rb uptake. In adrenalectomized (ADX) animals, incubation of CCD with AVP (10(-8) M for 5 min) had no effect on the number of pumps. In contrast, in ADX animals replete with aldosterone, AVP induced a approximately equal to 40% increase in the number of pumps. This was accompanied by a approximately equal to 60-65% increase in ouabain-sensitive Rb uptake. AVP effect was dose-dependent (10(-10)-10(-8) M) and was reproduced by dDAVP, forskolin and 8-Br cAMP, indicating a V2 pathway. It was inhibited by amiloride 10(-5) M, and did not occur in CCD incubated in hyperosmotic solution, suggesting that the signal was transmitted via apical sodium entry and cell swelling. Finally, the AVP-dependent increase in the number of pumps was rapid (within 5 min) and transient (< 25 min). These results demonstrate that, in the CCD, aldosterone and AVP act synergistically to increase not only the apical sodium entry but also the basolateral Na(+)-K(+)-ATPase transport capacity: AVP allows a rapid recruitment and/or activation of an aldosterone-dependent pool of latent Na(+)-K(+)-ATPase.

Volume-activated amino acid efflux from term human placental tissue: stimulation of efflux via a pathway sensitive to anion transport inhibitors

The effect of a hyposmotic challenge and hence cell-swelling upon the efflux of a variety of solutes from isolated human placental tissue has been examined. A hyposmotic shock increased the fractional release of taurine, the most abundant free amino acid in placental tissue, via a pathway sensitive to niflumic acid, DIDS (4,4'-Diisothiocyanatostilbene-2',2'-disulphonic acid,) NPPB (5-Nitro-2(3-phenylpropylamino)benzoic acid) and DIOA (R(+)[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden -5-y) oxy] acetic acid). In contrast, tamoxifen was without effect. The cell-swelling induced efflux of taurine was attenuated (40 per cent) by replacing external Cl- with NO3-. The efflux of glutamic acid was also markedly increased by a hyposmotic challenge. Niflumic acid inhibited both basal and volume-activated glutamic acid efflux. A hyposmotic shock also increased alpha-aminoisobutyric acid efflux but not that of 3-O-methylglucose and SO4(2)-. The results suggest that the human placenta can respond to cell-swelling by releasing organic osmolytes such as amino acids via a pathway which is sensitive to anion transport inhibitors. However, it appears that the volume-activated amino acid transport system is independent from the placental anion-exchange pathways. The efflux of these compounds may act with K+ and Cl- efflux to effect a regulatory volume decrease in placental tissue. In addition, volume-activated transport may play a role in transplacental amino acid transfer.

Cloning of a swelling-induced chloride current related protein from rabbit heart

Recently, pIcln has been reported to be a regulator of a swelling-induced chloride conductance. We have cloned a cDNA RCL-H1 from rabbit heart, of which primary structure is highly homologous to that of pIcln. Outwardly rectifying currents were recorded in oocytes expressing RCL-H1, which is consistent with the result of pIcln. RNA blot analysis revealed the widespread expression of RCL-H1 mRNA in rabbit tissues. RCL-H1 may play an important role in regulating cell volume and give a clue to revealing molecular structure of swelling-induced chloride channel(s).

Mechanism of hyperosmolality stimulation of ANP secretion: its dependency on calcium and sodium

The calcium dependency of hyperosmolality stimulation of atrial natriuretic peptide (ANP) secretion was determined using isolated superfused nonbeating rat left atrium. Increasing osmolality by 65, 85, and 100 mosmol/kgH2O by superfusion with sucrose produced a peak rise in ANP secretion of 1.8-, 2.0-, and 2.7-fold. To determine whether calcium influx played a role in osmolality (osm)-stimulated ANP secretion, atria were superfused with 2 mM lanthanum, a calcium antagonist. Lanthanum inhibited by 85% the response to a 100 mosmol/kgH2O increase in osm. The voltage-dependent calcium channel blocker isradipine had no effect on osm-stimulated ANP secretion, suggesting that calcium influx via voltage-dependent calcium channels was not playing a significant role. Likewise, depleting sarcoplasmic reticulum calcium with 1 microM ryanodine did not block the response to osm, suggesting that calcium influx was not adequate to induce consequential release of calcium from the sarcoplasmic reticulum. To determine whether calcium influx was via Na(+)-Ca2+ exchange, we determined the sodium dependency of osm-stimulated ANP secretion. Replacement of sodium with lithium or choline blocked the secretory response to 100 mosmol/kgH2O. We conclude that osm-stimulated ANP secretion is calcium and sodium dependent. Calcium influx via Na(+)-Ca2+ exchange is highly implicated as the mechanism of cellular calcium entry.

P-glycoprotein and cell volume-activated chloride channels

The multidrug resistance P-glycoprotein (P-gp) is an active drug transporter which can expel hydrophobic compounds from cells. Expression of P-gp has many effects on cells and tissues and the physiological function, or functions, of P-gp are still unclear. Recently, expression of P-gp has been associated with altered activity of chloride channels which play a role in regulating cell volume of response to osmotic shock or nutrient uptake. The nature and physiological role of this association has been a subject of some debate. In this article, mechanisms by which P-gp might influence cell volume-activated chloride currents is discussed, and the potential physiological role of this regulation considered.

Volume-activated chloride currents associated with the multidrug resistance P-glycoprotein

The ability to regulate volume is an important property of most, if not all cells. In epithelial cells, amongst others, cell volume-activated chloride channels are central to this response. The molecular identities of these channels are not yet known. Expression of the human multidrug resistance P-glycoprotein (P-gp) has been associated with cell volume-regulated chloride currents, although the nature of this association is the subject of debate. Recent data indicate that P-gp acts by regulating the activation of an endogenous channel protein. In this review, evidence associating P-gp with cell volume-activated chloride currents, and the possible mechanisms by which this might be achieved, are discussed.

Visipaque is isotonic to human and rat blood plasma

Even at its highest concentration, 320 mg I/ml, Visipaque - based on the nonionic dimer iodixanol - is isoosmotic to blood plasma, whereas Omnipaque (300 mg I/ml) - based on the nonionic monomer iohexol - has an osmolality of about twice that of the plasma. However, the fact that the solution is isoosmotic to plasma does not necessarily mean that it is isotonic to plasma. An isoosmotic solution can still cause a net movement of water over the plasma membranes of, for example, erythrocytes and endothelial cells. Determination of the tonicity of Visipaque 320 mg I/ml and comparison with that of Omnipaque 300 mg I/ml and hypertonic NaCl have been performed. No change in the water content of human erythrocytes was seen after mixing whole blood 10:1 with either Visipaque 320 mg I/ml or 155 mM NaCl, whereas a significant decrease in water content occurred with Omnipaque 300 mg I/ml and 620 mM NaCl. No difference in the water content of rat erythrocytes was evident after mixing whole blood with Visipaque 320 mg I/ml or isotonic NaCl. However, as with human erythrocytes, a significant decrease in water content occurred after rat blood was mixed with Omnipaque 300 mg I/ml. In conclusion, Visipaque 320 mg I/ml does not cause any net movement of water over the human or the rat erythrocyte plasma membrane, i.e., Visipaque is isotonic to human and rat blood plasma.

The Ernest Witebsky memorial lecture. Red but not dead: not a hapless sac of hemoglobin

The purpose of this presentation was to demonstrate that the red blood cell is not a hapless sac of hemoglobin and that much research is still needed for better understanding of its complexities. After a brief historical introduction the following subjects are presented: 1). Phosphofructokinase is the rate limiting step in the anaerobic glycolytic pathway. Ribose-5-phosphate, a metabolite of the oxidative pentose phosphate pathway is essential for the generation of phosphoribosylpyrophosphate which in turn is needed for the synthesis of adenosine monophosphate from adenine by the action of adenine phosphoribosyl transferase. 2). There are at least 17 blood group systems with more than 400 epitopes expressed on the red cell membrane. The Rh null and the McLeod phenotypes associated with abnormally shaped red cells and hemolytic anemia are briefly described as is the present understanding of the nature of the Rh complex. 3). The structure of the cytoskeleton and the composition and behavior of the lipid bilayer are presented with some discussion of the MN and Ss sialoglycoproteins and the Leach phenotype. 4). Touched upon is the role of phosphoinositides with some emphasis on recent discoveries relating to the glycophosphoinositide protein anchor. 5). The intricacies of the many faceted transport mechanisms are introduced. Briefly mentioned are the mechanisms activated when regulatory volume adjustments occur in fine tuning red cell volume after exposure respectively to hypotonic or hypertonic stress. Sufficient evidence is presented to convince that a cell doesn't have to have a nucleus to be respected even though it is just a corpuscle.

Sodium-calcium relationships and cardiac function during coronary bolus perfusion

The present review deals with the side-effects of contrast media (CM) on cardiac function during coronary angiography. A physiological approach is used to redefine existing concepts of CM osmotoxicity and chemotoxicity in terms of osmolal, ionic and molecular effects. The main idea conveyed is that purely ionic effects are of central importance during and immediately following the transit of a brief coronary bolus. Ionic effects result largely from rapid transient washout of normal extracellular ions, but are also influenced by ions present in the CM. In particular, the calcium (Ca) and sodium (Na) ions controlling cardiac function are easily affected. The myocardial Na-Ca exchange, which is mainly a physiological mechanism for cellular Ca efflux during cardiac relaxation, is therefore highlighted in detail. The importance of avoiding a potential Na-Ca mismatch is shown by examples from basic physiology, cardiac surgery and coronary angiography and by results of experiments with Visipaque. In the isomolal and isotonic CM Visipaque, which is based on the dimer isodixanol (320 mg I/ml), an available osmolal space is filled with an appropriately balanced supplement consisting of NaCl (19mM) and CaCl2 (0.3 mM).

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Effects of extracellular bicarbonate ions and pH on volume-regulatory taurine efflux from rat cerebral cortical slices in vitro: evidence for separate neutral and anionic transport mechanisms

Net efflux of [3H]taurine from cells in pre-loaded slices of rat cerebral cortex has previously been shown to occur via an anionic pathway, believed to consist of exchange of anionic taurine for extracellular Cl-, and operating under both isomotic and hypoosmotic conditions, and a calmodulin-dependent mechanism, activated by hypoosmotic stress: the latter may comprise conductive channels (Law, R.O. (1994) Biochim. Biophys. Acta 1221, 21-28). Experiments have now been performed to examine two inter-related problems; firstly, how anion/anion exchange (assuming 1:1 stochiometry) could contribute to the regulation of brain cell volume, and secondly, whether the hypoosmotically-activated component of efflux represents a second anionic transport process or loss of neutral taurine. The former process has been shown to be strongly dependent upon extracellular pH and bicarbonate concentration, being accelerated by low pH (7.0) and high (60 mmol/l) bicarbonate, and retarded by alkalinization (pH 7.8) and low (2.5 mmol/l) bicarbonate. Taurine efflux is inhibited by acetazolamide, with accompanying cell swelling in both isoosmotic and hypoosmotic media. It is hypothesized that inwardly directed bicarbonate transport, in exchange for intracellular Cl-, operates in parallel with efflux of anionic taurine in exchange for extracellular Cl-, and it is the subsequent conversion of bicarbonate to CO2 and water, under the influence of carbonic anhydrase, that effects a volume-regulatory decrease in internal osmotic potential. The dependence of taurine efflux upon pH and bicarbonate persists in the presence of trifluoperazine (an inhibitor of calmodulin activation) but is abolished in hypoosmotic media by the anion transport inhibitor niflumic acid. Cell depolarization in high K+ has no effect on taurine efflux, which is envisaged as involving parallel electroneutral anion exchange processes (taurine/Cl- and Cl-/bicarbonate) augmented, in hypoosmotic media, by diffusive efflux of neutral taurine.

Swelling-activated K+ fluxes in vascular endothelial cells: role of intracellular Ca2+

Swelling of bovine aortic endothelial cells activates Ca(2+)-dependent K+ channels. To determine the role of Ca2+ in this response, we examined the effect of cell swelling on intracellular Ca2+ concentration ([Ca2+]i), and the role of [Ca2+]i in swelling-activated K+ efflux. Basal [Ca2+]i, measured by fura 2 fluorescence, was 62 nM and increased by 36 nM in hypotonic medium (220 mosmol/l) compared with a 277 nM increase in response to extracellular ATP. In cells loaded with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA), the increases induced by swelling and by ATP were reduced to 13 and 20 nM, respectively. Exposure to hypotonic medium (220 mosmol/kg) or to the Ca2+ ionophore A-23187 stimulated a furosemide-insensitive 86Rb efflux consistent with activation of K+ channels. The swelling-activated efflux was inhibited 16% by 5 mM tetraethylammonium and 24% by 23 mM tetrabutylammonium, but not by 100 microM quinidine, a pattern similar to that previously observed for swelling-activated K+ channels in cell-attached patches. The effects of A-23187 and hypotonic swelling on 86Rb efflux were completely additive, suggesting Ca(2+)-independent activation by cell swelling. Removal of Ca2+ from the external medium or loading of cells with BAPTA to buffer intracellular Ca2+ blocked the activation of 86Rb efflux by A-23187, but not by hypotonic swelling. Hypertonic medium (440 mosmol/kg by the addition of sucrose) attenuated the increased 86Rb efflux in response to A-23187. We conclude that the activation of K+ efflux in swollen endothelial cells occurs independently of changes in [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of protein phosphorylation in control of K flux pathways of trout red blood cells

The role of protein phosphatases in the regulation of K flux pathways of the trout red blood cell has been investigated using the phosphatase inhibitors calyculin A and okadaic acid. Both inhibitors completely blocked an oxygenation-activated Cl-dependent K flux with a 50% inhibitory concentration of 17 and 675 nmol/l, respectively, but not the hypotonically activated Cl-independent K uptake. N-ethylmaleimide (NEM) and staurosporine caused an increase in the Cl-dependent flux. In both cases preincubation with calyculin A blocked activation but, when added during activation, it prevented any further increase with NEM but abolished the staurosporine-induced uptake. K uptake that was activated by NEM and "clamped" by calyculin A was volume sensitive, indicating a dual influence on this pathway. Chelerythrine, a protein kinase inhibitor, activated a Cl-independent K uptake that was unaffected by calyculin A. It is concluded that activation and deactivation of both Cl-dependent and Cl-independent pathways require changes in the phosphorylation of an as yet unidentified target protein(s), although with different sets of protein kinase and/or phosphatases. These observations also suggest a complex model of kinase-phosphatase regulation and provide drugs for the pharmacological definition and manipulation of Cl-dependent and Cl-independent K flux pathways.

Regulatory volume decrease in HL-60 cells: importance of rapid changes in permeability of Cl- and organic solutes

Results obtained through the use of inhibitors and isotope flux and equilibration techniques indicate that the regulatory volume decrease (RVD) response of human promyelocytic leukemic HL-60 cells occurs largely through the efflux of K+ and Cl- through separate conductive membrane pathways. These "channels" differ pharmacologically and in their modes of activation from those described in lymphocytes and Ehrlich ascites tumor cells. With use of measured 86Rb+ and 36Cl- fluxes, together with a diffusion kinetic model, the membrane potential (Em) and apparent K+ and Cl- permeabilities (PK and PCl) were estimated under various isotonic and hypotonic conditions. Under isotonic (300 mosM) conditions, Em is close to the Nernst potential for K+ and PCl is < 0.1 PK. Rapid and steeply graded increases in the measured Cl- efflux rate and calculated PCl occur with decreasing tonicity, with the largest increases at tonicities < 80% of isotonic. K+ efflux and the apparent PK increase only modestly with decreasing tonicity. At 50% tonicity, PCl rises to nearly 10 times PK, which should cause substantial membrane depolarization, with Em approaching the Nernst potential for Cl-. Gramicidin treatment markedly accelerates the rate of RVD and net 36Cl- efflux in hypotonic Na(+)-and Cl(-)-free media, providing further evidence that PK is rate limiting during RVD. K+ loss exceeds Cl- loss during RVD, and the total loss of K+ and Cl- is insufficient to account for the observed degree of volume recovery in 50% tonicity media, indicating that other (organic) osmolytes must take part in the HL-60 cell RVD response.

X-ray microanalysis of ion contents in vacuoles and cytoplasm of the growing tips of a hydroid polyp as related to osmotic changes and growth pulsations

Growth pulsations (GP) in hydroid polyps are associated with changes in vacuolar patterns which can be imitated by altering external osmolarity. With the use of X-ray spectroscopy we measured the elemental contents in the vacuoles and cytoplasm of the growing tips of a hydroid polyp, Podocoryne carnea, under various tonicity conditions. Under hypertonic condition which arrested the samples at the retraction phase of normal GP, the elemental content within the vacuolar compartment appeared to be similar to that of the external medium, confirming our previous conclusion about the dehermetization of the vacuolar compartment under these conditions. Under hypotonical condition which arrested samples at the extension GP phase (vacuoles isolated) element ratio data displayed an obvious bimodality. At least one of the data groups could be characterized by a significant increase in the concentrations of sodium and potassium, as related to Cl, Ca and Mg, and in comparison to the same ratios in hypotonical samples and those in the external medium. We suggest that under hypotonical conditions the isolated vacuolar compartment is formed by influx of sodium and potassium ions. These cations are accompanied by anions other than chloride. Potassium appears to be transferred into the vacuoles from the cytoplasm while the sodium derives from the external environment.

Regulation of cell function by the cellular hydration state

Cellular hydration can change within minutes under the influence of hormones, nutrients, and oxidative stress. Such short-term modulation of cell volume within a narrow range acts per se as a potent signal which modifies cellular metabolism and gene expression. It appears that cell swelling and cell shrinkage lead to certain opposite patterns of cellular metabolic function. Apparently, hormones and amino acids can trigger those patterns simply by altering cell volume. Thus alterations of cellular hydration may represent another important mechanism for metabolic control and act as another second or third messenger linking cell function to hormonal and environmental alterations.

ATP is not required for anion current activated by cell swelling in multidrug-resistant lung cancer cells

During whole cell recording with 4 mM ATP and 0.1 mM GTP in the pipette, outwardly rectifying Cl- currents (155 +/- 20.5 pA/pF) were repetitively activated on reduction of bath solution osmolarity from 290 mosM (control) to 210 mosM. These currents were sensitive to 0.1-1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Omission of ATP from the pipette solution reduced the current magnitude to 42.7 +/- 9.5 pA/pF and prevented repetitive activation. More hyposmotic solutions (160 mosM) usually elicited current repetitively despite an ATP-free pipette solution. In cells depleted of ATP (to < 5% of control) by preincubation with 2-deoxyglucose (10 mM) and rotenone (100 nM), hyposmotic solutions failed to activate significant current. Cell volume increased to 230 +/- 18% of control (19.1 +/- 1.2 microns) in 210 mosM bath (normal cells) but only to 114 +/- 13% of control in ATP-depleted cells exposed to 160 mosM solution. This failure of ATP-depleted cells to swell in hypotonic external solutions was reversed by overnight pretreatment with cytochalasin D (2 micrograms/ml; n = 6) but not by colchicine (250 microM; n = 8). In outside-out patches of membrane dialyzed with zero ATP and excised from swollen cells, we observed sustained activation of a 53-pS outwardly rectifying channel (chord conductance, +100 mV; open probability approximately 1.0). In cell-attached patches from normal and ATP-depleted cells, we activated similar channels by suction. ATP does not appear to be an absolute requirement for the activation of this Cl- channel in H69AR cells but may be essential for the normal volume response and channel activation mediated through cytoskeletal elements within cells.

Swelling-induced K+ influx in cultured primary astrocytes

The effect of swelling of cultured primary astrocytes from rat brain in hypotonic medium on K+ influx has been studied. A decrease in osmolality from 310 to 180 mOsm increased the activity of sodium pump (ouabain-inhibited 86Rb+ influx) and Na+,K+,2Cl- cotransport (ouabain-insensitive bumetanide-inhibited 86Rb+ influx) by 70 and 35%, respectively. It is suggested that activation of these transport systems makes it possible to retain a high potassium concentration in the cells under regulatory volume decrease.

Selection for MDR1/P-glycoprotein enhances swelling-activated K+ and Cl- currents in NIH/3T3 cells

The relationship between multidrug resistance (MDR) P-glycoprotein expression and swelling-activated Cl- and K+ conductance was investigated in mouse NIH/3T3 fibroblasts and their colchicine-selected counterparts (COL1000, high P-glycoprotein). Whole cell patch-clamp and isotopic flux experiments confirmed that swelling-activated Cl- currents were induced by 20-30% bath dilution only in the MDR-expressing cell line. However, at bath dilutions > 30%, both cell lines developed Cl- currents that reached similar large magnitudes at higher dilution levels. Thus the apparent absolute difference in cell lines at lower dilutions is due to a shift in the response curve relating hypotonicity to Cl- conductance. At all dilutions and in both cell lines, the swelling-activated Cl- currents were outwardly rectifying, active at negative cell voltages, and inactivated at positive voltages. Verapamil (100 microM) and 1,9-dideoxyforskolin (100 microM), which inhibit P-glycoprotein drug transport, did not significantly inhibit the swelling-activated Cl- conductance efflux in the COL1000 cells also showed a leftward shift in the response curve to hypotonicity. These results indicate that response curve to hypotonicity. These results indicate that colchicine-selection for increased P-glycoprotein expression did not lead to the expression of swelling-activated Cl- channels, but instead enhanced a step in the pathway from bath dilution to regulatory volume decrease that is common to both K+ and Cl- channels.

Na+ transport pathways in secretory acinar cells: membrane cross talk mediated by [Cl-]i

Fluid secretion by epithelial cells can be modulated by agents that activate Cl- channels in the apical membrane. To sustain secretion, Cl- influx across the basolateral membrane must also be accelerated. To examine the cellular mechanisms that couple Cl- efflux across the apical membrane to Na(+)-coupled Cl- entry across the basolateral membrane, we employed optical imaging techniques, utilizing single rat salivary acinar cells. Na+ influx was negligible in resting cells but was rapidly increased by carbachol due to activation of a Na(+)-H+ exchanger, a Na(+)-K(+)-2Cl- cotransporter, and, most likely, a nonselective cation channel. Receptor stimulation was not necessary, since elevation of intracellular Ca2+ concentration ([Ca2+]i) by thapsigargin activated the Na+ transporters at equivalent rates. Cell acidification, activation of protein kinase C, cell shrinkage, and other events associated with the rise of [Ca2+]i had little effect on Na+ transport in resting cells. Nevertheless, stimulation of cells in a medium that prevented normal Ca(2+)-induced cell shrinkage prevented activation of all three transport pathways. The block of the activation was not overcome by osmotic shrinkage but was relieved when [Cl-]i was allowed to fall, including conditions in which [Cl-]i fell in the absence of cell shrinkage. Activation of a Na(+)-H+ exchanger, Na(+)-K(+)-2Cl- cotransporter, and nonselective cation channel therefore exhibits a requirement for agonist-induced fall in [Cl-]i. Low [Cl-]i may create a permissive environment for Ca(2+)-dependent activation of multiple Na(+)-transport pathways, providing a mechanism for cross talk that coordinates transport activities of the apical and basolateral membranes in secretory epithelial cells.

Characterization of regulatory volume decrease in the THP-1 and HL-60 human myelocytic cell lines

Exposure to hypotonic stress produces a transient increase in cell volume followed by a regulatory volume decrease (RVD) in both THP-1 and HL-60 cells. In contrast, cells exposed to hypotonic stress in a high K/low Na Hanks' solution not only failed to volume regulate, but displayed a secondary swelling. Thus, while an outward K gradient was required for RVD, the secondary swelling indicated that hypotonic stress increased permeability in the absence of a negative membrane potential. The K channel blocker quinine (1-4 mM) blocked RVD in both cell types. Gramicidin's ability to overcome the quinine block of RVD indicated that RVD is mediated by a quinine-sensitive cation transport mechanism that is independent of the swelling-induced anion transport mechanism. Barium (1-4 mM), another K channel blocker, slowed the rate of RVD, while 4-aminopyridine, charybdotoxin, tetraethylammonium chloride, tetrabutylammonium chloride, and gadolinium had no effect on RVD. Furthermore, RVD was not mediated by calcium-activated conductances, since it occurred normally in Ca-free medium, in medium containing cadmium, and in BAPTA-loaded cells. Gramicidin produced little or no volume change in isotonic medium, suggesting that basal C1 permeability of both THP-1 and HL-60 cells is low. However, swelling induced an anion efflux pathway that is permeable to both chloride and bromide, but is impermeable to methanesulfonate and glutamate. The anion channel blocker 3,5-diiodosalicylic acid (DISA) antagonized RVD in both cell types. In conclusion, RVD in THP-1 and HL-60 cells is mediated by independent anion and cation transport mechanisms that involve both a DISA-sensitive anion pathway and a quinine-inhibitable K efflux pathway, neither of which requires increases in intracellular calcium to be activated.

Role of cell volume variations in Na(+)-K(+)-ATPase recruitment and/or activation in cortical collecting duct

The aim of this study was to examine whether cell volume variations could play a role in the previously reported Na(+)-K(+)-ATPase pump recruitment and/or activation induced by an increase in intracellular Na concentration (Nai) in cortical collecting ducts (CCD). Isolated CCD from kidneys of aldosterone-repleted mice were incubated in hyper-, hypo-, or isosmotic solutions with and without Na to modify Nai and cell volume independently. Nai, cell volume, and the number of basolateral pumps were measured using 22Na, image analysis, and specific [3H]ouabain binding, respectively. Ouabain-sensitive 86Rb uptake was also measured. In CCD with high Nai, pump recruitment and/or activation was observed only when an increase in tubular volume was associated with Na load. Pump recruitment and/or activation was also induced by cell swelling in the absence of Na load. Recruited and/or activated pumps display an affinity for ouabain and a specific activity (ouabain-sensitive Rb uptake per pump unit) similar to basal pumps. We conclude that 1) cell swelling is implied in the process of Nai-dependent pump recruitment and/or activation, 2) cell swelling can promote pump recruitment and/or activation independently of Na load, 3) basal and recruited and/or activated pumps probably correspond to the same Na(+)-K(+)-ATPase isoform.

Two point mutations within the adducin genes are involved in blood pressure variation

The Milan hypertensive strain of rats (MHS) develops a genetic form of renal hypertension that, when compared to its normotensive control (MNS), shows renal dysfunction similar to that of a subset of human patients with primary hypertension. MHS and MNS were shown to be homozygous by multilocus minisatellite analysis and monolocus microsatellite markers. We show here that one point mutation in each of two genes coding for the membrane skeleton protein adducin is associated with blood pressure in the Milan strain of rats. Adducin is a heterodimer formed by alpha and beta subunits that promotes the assembly of actin with spectrin. MHS and MNS differ, respectively, by the amino acids Y and F at position 316 of the alpha subunit. In the beta-adducin locus, MHS is always homozygous for R at position 529 while in MNS either R or Q occurs in that position. The R/Q heterozygotes showed lower blood pressure than any of the homozygotes. In vitro phosphorylation studies suggest that both of these amino acid substitutions occur within protein kinase recognition sites. Analysis of an F2 generation demonstrated that Y alleles segregated with a significant increment in blood pressure. This effect is modulated by the presence of the R allele of the beta subunit. Taken together, these findings strongly support a role for adducin polymorphisms in causing variation of blood pressure in the Milan strain of rats.

Osmotic regulation of sodium pump in rat brain synaptosomes: the role of cytoplasmic sodium

The effect of hypoosmolality of incubation medium on the rat of ouabain-sensitive 86Rb+ transport in rat brain synaptosomes was studied. A decreased osmolality from 310 to 250 mOsm increased the rate of 86Rb+ uptake from 3.72 to 6.23 nmol/mg of protein min. To evaluate the involvement of cytoplasmic sodium in sodium pump stimulation inhibitors of ion channels and transport pathways able to increase [Na+]in were used. Tetrodotoxin (1 microM), amiloride (0.5 mM) and verapamil (0.1 mM) had no influence on the osmotic response of the sodium pump. The decrease of sodium concentration in incubation medium to 15 mM, leading to a practical loss of its transmembrane gradient, did not abolish stimulation of pump. No increase in 22Na+ influx or intrasynaptosomal sodium content was registered at hypotonic conditions. It is suggested that osmotic regulation of Na+,K(+)-ATPase is not connected with an increase of internal sodium through opening of sodium channels, or with activation of other membrane sodium-transporting systems.

Soluble polycations and cationic amphiphiles inhibit volume-sensitive K-Cl cotransport in human red cell ghosts

We have measured the effect of soluble polycations (spermine and methylglyoxal) and cationic amphiphiles (sphingosine and tetracaine) on K-Cl cotransport in shrunken and swollen red cell ghosts. All substances inhibited cotransport, and for each agent, the concentration at which inhibition was half-maximal was about the same for swollen and shrunken ghosts. Acetylspermine was a much less effective inhibitor than spermine, which demonstrates that inhibition depends on the cationic groups of spermine. Spermine was a more effective inhibitor in ATP-free ghosts than in ghosts containing ATP, which eliminates the possibility that inhibition of cotransport activity results from inhibition of protein kinase activity. Inhibition by spermine is as effective in K-free ghosts as in high-K ghosts; spermine does not inhibit cotransport by reducing the effective K concentration at the inner membrane surface. We conclude that regulation of K-Cl cotransport involves negative charges (phosphatidylserine or phosphatidylinositides) at the inner membrane surface and suggest a model that accounts for our findings.

Calcium-dependent chloride current induced by axotomy in rat sympathetic neurons

1. Seven to ten days after sectioning their axons, rat sympathetic neurons were studied using intracellular recording techniques in an in vitro preparation of the superior cervical ganglion. 2. In 75% of axotomized cells, an after-depolarization (ADP) was observed following spike firing or depolarization with intracellular current pulses. Discontinuous single-electrode voltage-clamp techniques were employed to study the ADP. When the membrane potential was clamped at the resting level just after an action potential, a slow inward current was recorded in cells that showed an ADP. 3. In the presence of TTX and TEA, inward peaks and outward currents were recorded during depolarizing voltage jumps, followed by slowly decaying inward tail currents accompanied by large increases in membrane conductance. The inward peak and tail currents activated between -10 and -20 mV and reached maximum amplitudes around 0 mV. With depolarizing jumps to between +40 and +50 mV, net outward currents were recorded during the depolarizing jumps but inward tail currents were still activated. 4. In the presence of the Ca2+ channel blocker cadmium, or when Ca2+ was substituted by Mg2+, the ADP disappeared. In voltage-clamped cells, cadmium blocked the inward tail currents. The reversal potential for the inward tail current was approximately -15 mV. Substitution of the extracellular NaCl by sucrose or sodium isethionate increased the amplitude of the inward tail current, and displaced its equilibrium potential to more positive values. Changes in extracellular [K+] did not appreciably affect the inward tail current amplitude or equilibrium potential. Niflumic acid, a blocker of chloride channels activated by Ca2+, almost completely blocked the tail current. 5. No ADPs were observed in non-axotomized neurons, and when depolarizing pulses were applied while in voltage clamp no inward tail currents were evoked in these normal cells. 6. It is concluded that axotomy of sympathetic ganglion cells produces the appearance of a Ca(2+)-dependent chloride current responsible for the ADP observed following spike firing.

Taurine efflux and the regulation of cell volume in incubated slices of rat cerebral cortex

The kinetics of efflux [3H]taurine have been examined in pre-loaded slices of rat cerebral cortex incubated in media of variable osmolality. Alterations in the rate of the slowest phase of efflux, considered to represent cellular loss, have been correlated with cell volumes, provisionally identified as the slice non-inulin space. Efflux was stimulated by reduction in medium osmolality, and impaired in hyperosmotic media; these variations were accompanied by moderate (non-osmometric) cell swelling and shrinkage, respectively. The rates of taurine efflux into media in which NaCl was partly replaced by sucrose, and measurement of the corresponding cell volumes, suggest that ionic strength, rather than osmolality or cell volume per se, may be a significant controlling factor. In both isosmolal and hyposmolal media efflux was significantly impaired by the anion transport inhibitor niflumic acid, with accompanying cell swelling, or by replacement of chloride by gluconate. In hyposmotic, but not isosmotic, media efflux was impaired, and cell volumes increased, in the presence of trifluoperazine or TMB-8, a reported blocker of intracellular calcium release, and the effects of niflumic acid and trifluoperazine on both variables were strongly additive. It is suggested that in both isosmotic and hyposmotic media taurine, efflux occurs by anionic transport, mainly through exchange with external chloride, whereas in hyposmotic media a second pathway is present, probably a volume-activated calmodulin-dependent channel.

Volume and surface area measurement of viable chondrocytes in situ using geometric modelling of serial confocal sections

This study describes a technique for noninvasive determination of the surface area and volume of chondrocytes using the confocal scanning laser microscope, and the fundamental limitations associated with its application. Using geometric modelling principles, an isointensity surface contour was formed from a series of optical sections recorded with the confocal microscope. Using a combined surface- and volume-based algorithm, the surface area, volume and other morphometric descriptions were calculated from a polygonal description of the cell surface. The high image contrast required for repeatable identification of the cell border was achieved through the use of a fluorescent dye, which was excluded from cells by an intact membrane. Calibration results indicated that the theoretical modelling algorithm is relatively precise when applied to simulated convex (ellipsoidal) cells, with overall errors of less than 0.5% in surface area and volume measurements. When applied to low-noise, high-contrast volume data recorded on the confocal microscope, typical coefficients of variation of 2-4% were determined for length measurements, 2-5% for volume measurements and 3-6% for surface area measurements either for latex microspheres or for chondrocytes. While the precision of the method is comparable to standard histological techniques, its accuracy is difficult to assess, as systematic errors are unpredictable and may be introduced from several sources.

Molecular characterization of a swelling-induced chloride conductance regulatory protein, pICln

Cells maintain control of their volume by the passage of KCl and water across their membranes, but the regulatory proteins are unknown. Expression in Xenopus oocytes of a novel protein, pICln, activated a chloride conductance. We have cloned analogs of pICln from rat heart and Xenopus ovary. pICln was identified as an abundant soluble cytosolic protein (approximately 40 kd) that does not immunolocalize with the plasma membrane. pICln was found in epithelial and cardiac cells, brain, and Xenopus oocytes, forming complexes with soluble actin and other cytosolic proteins. Monoclonal antibodies recognizing pICln blocked activation of a native hypotonicity-induced chloride conductance (ICl.swell) in Xenopus oocytes, suggesting that pICln may link actin-bound cytoskeletal elements to an unidentified volume-sensitive chloride channel. The high degree of sequence conservation and widespread expression of pICln suggest that it is an important element in cellular volume regulation.

Regulatory volume decrease of cultured human fibroblasts involves changes in intracellular amino-acid pool

Regulatory volume decrease (RVD) has been studied in cultured human fibroblasts incubated in a complete growth medium at low osmolality (215 mosmolal). After the initial swelling induced by hypotonic treatment, cells recover their volume almost completely within about 60 min. This RVD is associated with comparable losses of cell potassium and amino acids. After an initial increase, cell content of sodium is kept at values close to control. Chromatographic analysis of intracellular amino-acid pool has shown that RVD-associated decrease in cell amino acids is due for the most part to changes in the intracellular concentration of L-glutamine. RVD-exerting cells undergo a rapid and marked depolarization that is maintained after cell volume recovery. This change in membrane potential has been detected with measurements of both the transmembrane distribution ratios of L-arginine and of fluorescence of potential-sensitive dye bis-oxonol. Due to depolarization, the trans-membrane gradient of sodium electrochemical potential is lowered. It is proposed that cell depolarization concurs to keep the intracellular concentration of amino acids low by inhibiting sodium-coupled uptake through system A.

Regulatory volume decrease in cultured astrocytes. I. Potassium- and chloride-activated permeability

Regulatory volume decrease (RVD) in detached cerebellar astrocytes in culture after acute exposure to hyposmolarity was characterized in this and the accompanying paper [H. Pasantes-Morales, R. A. Murray, R. Sanches-Olea, and J. Moran. Am. J. Physiol. 266 (Cell Physiol. 35): C172-C178, 1994]. RVD was independent of extracellular calcium, was accelerated at pH 8-9 and retarded at pH 6, and was reduced at temperatures < 18 degrees C. The cationic pathway activated by hyposmolarity was specific for K+ and Rb+, since RVD was abolished and secondary swelling occurred when these ions replaced Na+. However, Li+, choline, tris(hydroxymethyl)aminomethane, and glucosamine, all as Cl- salts, did not affect RVD. The anion pathway was unselective, since RVD was inhibited when NaCl was replaced by anion K+ salts with a permeability rank of SCN- = I- > NO3- > Cl- > benzoate > acetate >> SO3- > gluconate. RVD was unaffected by bumetanide (50 microM) and weakly inhibited by furosemide (2 mM). Quinidine but not other K+ channel blockers inhibited RVD, and its effect was reversed by gramicidin. RVD was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and dipyridamole but not by diphenylamine-2-carboxylate or anthracene-9-carboxylate. These results suggest that diffusion possibly via channels rather than cotransporters is involved in the swelling-activated K+ and Cl- fluxes. Gramicidin did not change astrocyte volume in isosmotic conditions, but greatly accelerated RVD, suggesting that low Cl- permeability in isosmotic conditions markedly increases by swelling, thus making K+ permeability the rate-limiting step for RVD.

A chloride current associated with swelling of cultured chick heart cells

1. Cultured chick heart cells challenged by hyposmotic stress underwent regulatory volume decrease (RVD) that was attenuated by prior depletion of intracellular chloride. 2. During hyposmotic swelling, cell aggregates experienced an initial increase in spontaneous contractile activity followed by eventual quiescence. Conventional microelectrode studies revealed an underlying increase in spontaneous electrical activity, followed by a sustained depolarization beyond threshold. 3. Whole-cell patch clamp studies, with K+ currents blocked, indicated that exposure of cells to hyposmotic solution (NaCl reduction) resulted in a rapid osmotic swelling followed by a substantial increase in whole-cell conductance which persisted for the duration of hyposmotic exposure and was almost completely reversed on return to isosmotic bath solution. 4. For a variety of Cl- concentrations, the reversal potentials (Erev) of the measured swelling-activated current closely followed the calculated Cl- equilibrium potential (ECl) with a linear regression slope of 0.82. When estimated by the Nernst equation, the relationship between Erev and the [Cl-]i/[Cl-]o ratio fitted well with a slope of 51 mV per decade change in the concentration ratio, consistent with a Cl(-)-selective conductance. 5. The permeability ratios of this swelling-activated conductance to chloride, methanesulphonate (MSA) and aspartate (Asp) were calculated as PCl:PMSA:PASP = 1:0.36:0.02, with the ion selectivity sequence of Cl- > MSA- >> Asp-, which suggests the swelling-activated conductance is slightly permeable to other anions. 6. Application of a Cl- channel blocker, diphenylamine-2-carboxylate (DPC, 200 microM), substantially suppressed the swelling-activated current without shifting the Erev of this current. The effect of DPC was independent of membrane potential. 7. This evidence demonstrates that hyposmotic swelling of cultured chick heart cells activates a channel-mediated Cl- conductance which may be associated with the integrated response of volume-regulatory mechanisms.

Kinetics and peculiarities of thermal inactivation of volume-induced Na+/H+ exchange, Na+,K+,2Cl- cotransport and K+,Cl- cotransport in rat erythrocytes

The kinetics of the volume-dependent activation of Na+/H+ exchange, Na+,K+,2Cl(-)-cotransport and K+,Cl(-)-cotransport in rat erythrocytes was studied. The significant increase in the rate of Na+/H+ exchange is observed within 15 min after hypertonic shrinkage while the maximum transport rate is reached by 20 min. A delay of about 5 min was found in activation of Na+,K+,2Cl(-)-cotransport, the maximum transport rate being reached 10 min after shrinkage. Activation of K+,Cl(-)-cotransport by hypotonic swelling was registered within 10 min after cell swelling, with a simultaneous achievement of the constant transport rate. Preincubation of cells at 49 degrees C has no effect on the basal Na+/H+ exchange and Na+,K+,2Cl(-)-cotransport but suppresses the activation of these systems by osmotic shrinkage. On the contrary, the rate of K+,Cl(-)-cotransport in isosmotic medium is raised 10-fold after preincubation at 49 degrees C. The thermal treatment at 49 degrees C blocks the activation of K+,Cl(-)-cotransport by swelling. On the basis of the data on thermal denaturation of spectrin at the same temperature it was suggested that the cytoskeleton of erythrocyte membrane is involved in volume regulation of the ion-transporting systems and that the molecular mechanisms which underlie the activation of Na+/H+ exchange, Na+,K+,2Cl(-)-cotransport and K+,Cl(-)-cotransport are essentially different.