CFTR型氯离子通道研究进展

囊性纤维化跨膜传导调节因子（CFTR）是一种重要的氯离子通道,突变易引起囊性纤维化病变,故得名。一系列研究表明,CFTR由5个结构域组成：两个跨膜结构域形成氯离子通道;两个核苷酸结合结构域调节通道的开闭;一个调节结构域主要影响氯通道的活动。这些结构域通过协同作用共同控制了氯离子的跨膜流动,而一些突变可以影响细胞功能而导致囊性纤维化的发生。本文通过介绍CFTR基本结构、调节机制、与囊性纤维化病变的关系及针对CFTR的治疗而对CFTR型氯离子通道有一个的全面的理解。     

Chloride channels and cystic fibrosis of the pancreas

Cystic fibrosis (CF) affects approximately 1 in 2000 people making it one of the commonest fatal, inherited diseases in the Caucasian population. CF is caused by mutations in a cyclic AMP-regulated chloride channel known as CFTR, which is found on the apical plasma membrane of many exocrine epithelial cells. In the CF pancreas, dysfunction of the CFTR reduces the secretory activity of the tubular duct cells, which leads to blockage of the ductal system and eventual fibrosis of the whole gland. One possible approach to treating the disease would be to activate an alternative chloride channel capable of bypassing defective CFTR. A strong candidate for this is a chloride channel regulated by intracellular calcium, which has recently been shown to protect the pancreas in transgenic CF mice. Pharmacological intervention directed at activating this calcium-activated Cl^– conductance might provide a possible therapy to treat the problems of pancreatic dysfunction in CF.     

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-sensitive outwardly rectifying chloride channels are involved in oxidative stress-induced apoptosis of mesangial cells

Volume-sensitive outwardly rectifying (VSOR) Cl- channels have been electrophysiologically identified in human and mouse mesangial cells, but the functional role of VSOR Cl- channels in mesangial cell apoptosis is not clear. The aim of the present study was to demonstrate the role of VSOR Cl- channels in oxidative stress-induced mesangial cell apoptosis. H2O2-induced Cl- currents showed phenotypic properties of VSOR Cl- channels, including outward rectification, voltage-dependent inactivation at more positive potentials, sensitivity to hyperosmolarity, and inhibition by VSOR Cl- channel blockers. Moreover, blockage of VSOR Cl- channels by DIDS (100 microM), NPPB (10 microM) or niflumic acid (10 microM) rescued mesangial cell apoptosis induced by H2O2. Treatment with 150 microM H2O2 for 2h resulted in significant reduction of cell volume, in contrast, nuclear condensation and/or fragmentation were not observed and the caspase-3 activity was also not increased. The early-phase alterations in cell volume were markedly abolished by pretreatment with VSOR Cl- channel blockers. We conclude that VSOR Cl- channels are involved in H2O2-induced apoptosis in cultured mesangial cells and its mechanism is associated with apoptotic volume decrease processes.     

Functional architecture of the CFTR chloride channel

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of membrane transport proteins. CFTR is unique among ABC proteins in that it functions not as an active transporter but as an ATP-gated Cl^? channel. As an ion channel, the function of the CFTR transmembrane channel pore that mediates Cl^? movement has been studied in great detail. On the other hand, only low resolution structural data is available on the transmembrane parts of the protein. The structure of the channel pore has, however, been modeled on the known structure of active transporter ABC proteins. Currently, significant barriers exist to building a unified view of CFTR pore structure and function. Reconciling functional data on the channel with indirect structural data based on other proteins with very different transport functions and substrates has proven problematic. This review summarizes current structural and functional models of the CFTR Cl^? channel pore, including a comprehensive review of previous electrophysiological investigations of channel structure and function. In addition, functional data on the three-dimensional arrangement of pore-lining helices, as well as contemporary hypotheses concerning conformational changes in the pore that occur during channel opening and closing, are discussed. Important similarities and differences between different models of the pore highlight current gaps in our knowledge of CFTR structure and function. In order to fill these gaps, structural and functional models of the membrane-spanning pore need to become better integrated.     

Volume regulation of spermatozoa by quinine-sensitive channels

Bovine spermatozoa were shown to exhibit rapid regulatory volume decrease (RVD) when exposed to hypotonic saline media. This quinine- and quinidine-sensitive regulatory volume decrease was coincident with K⁺ release due to stretch-activation of inhibitor-specific presumptive K⁺ channels. The regulatory volume decrease response was much faster than a similar phenomenon observed in human peripheral blood lymphocytes. Studies on volume changes in different electrolyte and nonelectrolyte media suggested that: (1) this inhibitor-specific channel could also be a nonspecific pore in the spermatozoal membrane for nonelectrolytes below 150 daltons; (2) subpopulations (of nearly equal size) of the spermatozoa differ in the expression of the pore; (3) capacitation abolishes this distinction between subpopulations of spermatozoa; and (4) the general case of RVD for other mammalian spermatozoa was also established. Mol. Reprod. Dev. 46:535–550, 1997. © 1997 Wiley-Liss, Inc.     

Outwardly rectifying chloride channels and CF: A divorce and remarriage

Outwardly rectifying Cl^– channels were originally thought to be the central element in cystic fibrosis. The role of these channels in CF was questioned to such an extent that doubts were rasied about the validity of the original experiments. Recent data reestablishes a role for outwardly rectifying Cl^– channels (ORCC) in CF and suggests that the protein encoded by the CF gene, the cystic fibrosis transmembrane regulator (CFTR), can effect the regulation of more than one channel in the airway. This minireview deals with the rise, fall, and resurrection of the role of outwardly rectifying Cl^– channels in CF.     

Trimethyltin chloride induced chloride secretion across rat distal colon

TMT (trimethyltin chloride), an organotin, is ubiquitous in the environment. The consumption of contaminated food may cause exposure of the human diet to this toxic compound. The present study was to investigate the effects of TMT on the regulation of ion transport across the rat distal colon. The rat colonic mucosa was mounted in Ussing chambers. The effects of TMT were assessed using the I_sc (short‐circuit current). Both apical and basolateral TMT induced, dose‐dependently, an increase in I_sc, which was due to a stimulation of Cl^? secretion as measured using ion substitution experiments and pharmacological manoeuvres. The secretion was also inhibited by several K⁺ channel blockers administrated at the basolateral side. When the apical side was permeabilized by nystatin, the TMT‐induced K⁺ conductance was effectively blocked by tetrapentylammonium, a Ca²⁺‐sensitive K⁺ channel blocker. The response of TMT was sensitive to the basolateral Ca²⁺ and the intracellular Ca²⁺ store, which could be disclosed by applying the inhibitors of ryanodine receptors and inositol 1,4,5‐trisphosphate receptors. In conclusion, TMT led to Cl^? secretion, which was essentially regulated by basolateral Ca²⁺‐sensitive K⁺ channels. These results suggest the importance of K⁺ channels in the toxicity hazard of TMT.     

Voltage-gated chloride currents in cultured canine tracheal epithelial cells

Summary Chloride ions (Cl^–) are concentrated in airway epithelial cells and subsequently secreted into the tracheal lumen by downhill flux through apical Cl^– channels. We have studied Cl^– currents in cultured canine tracheal cells using the whole-cell voltage-clamp technique. Ultrastructural techniques demonstrated that the cells used in the electrophysiological experiments possessed apical membrane specializations known to be present in the intact, transporting cell type. Cultured cells 2–6 days old were characterized by an input resistance of 3.4±0.8 G (n=11) and a capacitance of 63.8±10.8 pF (n=26). A comparison of 3 and 4 day-old cells with 5 and 6 day-old cells showed that the input resistance decreased almost 50%, and the cell capacitance and the inward and outward currents increased concomitantly approximately 200%. Cultured cells 3–4 days old held at –40 mV produced currents of 196±22 pA at 50 mV and –246±27 pA at –90 mV (n=212) with pipette and bath solutions containing primarily 140 KCl and 140 NaCl, respectively. The chloride channel blocker diphenylamine-2-carboxylate (DPC, 100 m) suppressed whole-cell currents by 76.8% at 60 mV; however, currents were unaffected by the stilbenes SITS (1mm) and DNDS (1–30 m). Replacement of K⁺ with Cs⁺ in the pipette solution did not affect the outward current, the current reversal potential, or the input resistance of the cells, indicating that the current was not significantly K⁺ dependent when the intrapipette solution was buffered to a Ca²⁺ concentration of 20nm. The Cl^–/Na⁺ permeability ratio was estimated to be greater than 11 as calculated from reversal potential measurements in the presence of an internal to external NaCl concentration ratio of 12. Current equilibrium permeabilities, relative to Cl^– were: I^– (2.9)NO ₃ ^– (1.1)Br^– (1.1)Cl^– (1.0)F^– (0.93)MeSO ₄ ^– (0.19)gluconate (0.18)aspartate (0.14). Depolarizations to potentials greater than 20 mV elicited a time-dependent component in the outward current in 71% of the cells studied. Currents inactivated with a double exponential time course at the most depolarized voltages. Recovery from inactivation was fast, holding potential-dependent, and followed a double exponential time course. Current amplitude was increased via a cAMP-dependent pathway as has been demonstrated for single Cl-selective channels in cell-attached patches from cultured canine and human tracheal epithelial cells. Forskolin, an activator of adenylate cyclase, produced a 260% increase in the outward current at +50 mV. In summary, cultured canine tracheal cells have a single resting conductance that is Cl^– selective, voltage-dependent, and modulated by a cAMP-dependent mechanism. This preparation appears to be appropriate for analysis of cellular modulation of airway Cl^– channels and Cl^– secretion.     

内皮素-1预处理对培养乳鼠心肌细胞低氧损伤的保护作用

实验观察了 0 0 1- 1nmol/L内皮素 1(ET 1)预处理对低氧孵育 ( 3 %O2 5 %CO2 ,12h)的培养乳鼠心肌细胞乳酸脱氢酶 (LDH)释放量、培养液上清超氧化物歧化酶 (SOD)活性以及丙二醛 (MDA)含量的影响。用Fluo 3 /AM负载培养的心肌细胞 ,在激光扫描共聚焦显微镜下监测急性低氧的心肌细胞 [Ca2 +]i 的变化和ET 1预处理对低氧所致 [Ca2 +]i 变化的影响。结果如下 :( 1)心肌细胞低氧孵育 12h后 ,培养液上清LDH活力和MDA含量较常氧对照组明显升高 ,分别为 43 3 3± 1 2 1U/Lvs 19 3 3± 1 0 3U/L和 1 71± 0 0 2nmol/Lvs 0 91± 0 0 3nmol/L (P<0 0 1) ,SOD活性为 16 93± 1 11U/ml明显低于常氧对照组的 3 3 48± 1 15U/ml (P <0 0 1) ;0 0 1- 1nmol/LET 1预处理呈浓度依赖性抑制低氧培养心肌细胞LDH释放 ,减少培养液上清MDA含量、提高SOD活性 (P <0 0 1)。 ( 2 )低氧灌流后 2 9± 1 5s (n =2 3 )心肌细胞自发性钙瞬变完全终止 ,[Ca2 +]i 升高了 10 7± 13 2 % (P <0 0 0 1) ;0 0 1- 1nmol/LET 1能明显加快心肌细胞钙瞬变的频率 (P <0 0 1) ;ET 1预处理后低氧所致钙瞬变终止的时间较单纯低氧组明显推迟 ,[Ca2 +]i过度升高被明显减轻 (P <0 0 1)。上述结果表明 ,0 0 1- 1nmol/LET 1预处理可减轻培     

Growth hormone signalling and apoptosis in neonatal rat cardiomyocytes

Growth hormone (GH) has been reported to be useful to treat heart failure. To elucidate whether GH has direct beneficial effects on the heart, we examined effects of GH on oxidative stress-induced apoptosis in cardiac myocytes. TUNEL staining and DNA ladder analysis revealed that hydrogen peroxide (H₂O₂)-induced apoptosis of cardiomyocytes was significantly suppressed by the pretreatment with GH. GH strongly activated extracellular signal-regulated kinases (ERKs) in cardiac myocytes and the cardioprotective effect of GH was abolished by inhibition of ERKs. Overexpression of dominant negative mutant Ras suppressed GH-stimulated ERK activation. Overexpression of Csk that inactivates Src family tyrosine kinases also inhibited ERK activation evoked by GH. A broad-spectrum inhibitor of protein tyrosine kinases (PTKs), genistein, strongly suppressed GH-induced ERK activation and the cardioprotective effect of GH against apoptotic cell death. GH induced tyrosine phosphorylation of EGF receptor and JAK2 in cardiac myocytes, and an EGF receptor inhibitor tyrphostin AG1478 and a JAK2 inhibitor tyrphostin B42 completely inhibited GH-induced ERK activation. Tyrphostin B42 also suppressed the phosphorylation of EGF receptor stimulated by GH. These findings suggest that GH has a direct protective effect on cardiac myocytes against apoptosis and that the effect of GH is attributed at least in part to the activation of ERKs through Ras and PTKs including JAK2, Src, and EGF receptor tyrosine kinase.     

Activation of K+ and Cl− channels in MDCK cells during volume regulation in hypotonic media

Single-channel patch-clamp experiments were performed on MDCK cells in order to characterize the ionic channels participating in regulatory volume decrease (RVD). Subconfluent layers of cultured cells were exposed to a hypotonic medium (150 mOsm), and the membrane currents at the single-channel level were measured in cell-attached experiments. The results indicate that MDCK cells respond to a hypotonic swelling by activating several different ionic conductances. In particular, a potassium and a chloride channel appeared in the recordings more frequently than other channels, and this allowed a more detailed study of their properties in the inside-out configuration of the patch-clamp technique. The potassium channel had a linear I/V curve with a unitary conductance of 24 +/- 4 pS in symmetrical K+ concentrations (145 mM). It was highly selective for K+ ions vs. Na+ ions: PNa/PK less than 0.04. The time course of its open probability (P0) showed that the cells responded to the hypotonic shock with a rapid activation of this channel. This state of high activity was maintained during the first minute of hypotonicity. The chloride channel participating in RVD was an outward-rectifying channel: outward slope conductance of 63.3 +/- 4.7 pS and inward slope conductance of 26.1 +/- 4.9 pS. It was permeable to both Cl- and NO3- and its maximal activation after the hypotonic shock was reached after several seconds (between 30 and 100 sec). The activity of this anionic channel did not depend on cytoplasmic calcium concentration. Quinine acted as a rapid blocker of both channels when applied to the cytoplasmic side of the membrane. In both cases, 1 mM quinine reversibly reduced single-channel current amplitudes by 20 to 30%. These results indicate that MDCK cells responded to a hypotonic swelling by an early activation of highly selective potassium conductances and a delayed activation of anionic conductances. These data are in good agreement with the changes of membrane potential measured during RVD.     

Defective regulation of apical membrane chloride transport and exocytosis in cystic fibrosis

A biochemical link is proposed between recent observations on defective regulation of Cl^– transport in CF respiratory epithelial cells and studies showing altered biological activity of calmodulin in exocrine glands from CF patients. A consensus is emerging that defective -adrenergic secretory responsiveness in CF cells is caused by a defect in a regulator protein at a site distal to cyclic AMP formation. Our results indicate that this protein might be a specific calmodulin acceptor protein which modifies the activity of calmodulin in epithelial cells. Alteration in Ca²⁺/calmodulin dependent regulation of Cl^– transport and protein secretion could explain (i) alterations in Ca²⁺ homeostasis seen in CF, (ii) defective -adrenergic responses of CF cells, and (iii) the observed inability of cyclic AMP (acting via its specific protein kinase, A-kinase) to open apical membrane Cl^– channels in CF epithelial cells. Most of the physiological abnormalities of CF including elevated sweat electrolytes and hyperviscous mucus can be explained on this basis.Abbreviations -adrenergic agonist acting at its receptor cAMP cyclic AMP - PDE phosphodiesterase - CaM calmodulin - Pase phosphatase     

Mechanical effects of ET-1 in cardiomyocytes isolated from normal and heart-failed rabbits

Endothelin (ET-1) is found at elevated concentrations in the plasma of patients with heart failure and in animal models of cardiomyopathy. The peptide is a potent positive inotropic agent, the effects of which are mediated by increases in cytosolic Ca²⁺ in cardiomyocytes. The object of this study was to investigate at the cellular level, the actions of ET-1 on contractile function and on Ca²⁺ currents in heart-failed ventricular myocardium. Male New Zealand White rabbits (8 wks) were treated with twice weekly injections of epirubicin (4 mg/kg/wk, n=7) or with saline (n=7) for 6 wks, followed by a washout period of 2 wks. Ventricular cardiomyocytes were isolated from rabbit hearts using Langendorff perfusion with collagenase; contractile function was examined using a video microscopy method, and L-type Ca²⁺ currents were recorded using a whole-cell patch-clamp technique. ET-1 produced a concentration-dependent increase in contractile response (% increase from basal value) to a maximum at 1 nM ET-1 of 69 ± 11% (mean ± S.D.) in control cardiomyocytes and 33 ± 6% in heart-failed cells. However, there was no significant change in the EC₅₀ obtained with ET-1 for healthy (0.31 ± 0.1 nM) and for failed cardiomyocytes (0.24 ± 0.1 nM). The effects of ET-1 on L-type Ca²⁺ channels were similar with a peak amplitude at 1 nM ET-1 of –3.26 ± 0.8 in control cardiomyocytes and –3.32 ± 0.9 nA in heart-failed cells. The attenuation of the contractile response to ET-1 in heart-failed cells may reflect a desensitization of ET receptors as a consequence of elevated circulating levels of ET and was not reflected by alteration of transmembrane Ca²⁺ conductance. It is probable, therefore, that multiple signalling pathways are involved in the actions of ET on ventricular myocardium.Recipient of Servier Investigator Award     

Role of TASK2 potassium channels regarding volume regulation in primary cultures of mouse proximal tubules

Several papers reported the role of TASK2 channels in cell volume regulation and regulatory volume decrease (RVD). To check the possibility that the TASK2 channel modulates the RVD process in kidney, we performed primary cultures of proximal convoluted tubules (PCT) and distal convoluted tubules (DCT) from wild-type and TASK2 knockout (KO) mice. In KO mice, the TASK2 coding sequence was in part replaced by the lac-Z gene. This allows for the precise localization of TASK2 in kidney sections using beta-galactosidase staining. TASK2 was only localized in PCT cells. K+ currents were analyzed by the whole-cell clamp technique with 125 mM K-gluconate in the pipette and 140 mM Na-gluconate in the bath. In PCT cells from wild-type mice, hypotonicity induced swelling-activated K+ currents insensitive to 1 mM tetraethylammonium, 10 nM charybdotoxin, and 10 microM 293B, but blocked by 500 microM quinidine and 10 microM clofilium. These currents were increased in alkaline pH and decreased in acidic pH. In PCT cells from TASK2 KO, swelling-activated K+ currents were completely impaired. In conclusion, the TASK2 channel is expressed in kidney proximal cells and could be the swelling-activated K+ channel responsible for the cell volume regulation process during osmolyte absorptions in the proximal tubules.     

Reconstitution and regulation of an epithelial chloride channel

We have used a monoclonal antibody (MAb E12), one of several such antibodies raised against theophylline-treated Necturus gallbladder epithelial cells, to isolate a chloride channel protein by the use of an immunoaffinity column and FPLC. This protein (M_r 219,000) has been reconstituted into a planar lipid bilayer, where it behaves as a chloride-selective channel (P_Cl/P_Na = 20.2; P_Na/P_K = 1) whose unit conductance is 62.4 ± 4.6 pS. Antibody added to the trans side (there is no effect from the cis side) causes channel open probability to drop to virtually zero, but has no effect on the conductance or the selectivity of single channels. To test the role of phosphorylation in the activity of the native channel, we studied the effects of the protein phosphatase inhibitor okadaic acid (OA) on intact gallbladders, and showed that channels opened by theophylline treatment and closed by antibody are reopened reversibly by OA (0.01–1.0 M). Addition of the catalytic subunit of protein phosphatase 2A (PP-2A) to the cis side of a bilayer containing reconstituted chloride channels caused closure of the channels after a delay, and subsequent addition of ATP and the catalytic subunit of cAMP-dependent protein kinase (PKA) caused immediate reopening. These data indicate that (a) this chloride channel protein inserts in a directed way into the bilayer such that the cis side is intracellular, (b) the purified channel protein is phosphorylated, and (c) gating from the cellular side is controlled by the direct phosphorylation and dephosphorylation of the channel protein.     

The role of anion channels and Ca2+ in addition to K+ channels in the physiological volume regulation of murine spermatozoa

Studies in the human, transgenic mice, and cattle indicate that sperm cell volume regulation plays an important role in male fertility as spermatozoa encounter a hypo-osmotic challenge upon ejaculation into the female tract. Physiological regulatory volume decrease (RVD) was examined using flow cytometry in murine sperm released into incubation medium mimicking uterine osmolality and including putative channel inhibitors. The involvement of K+ channels was indicated by the recovery of volume regulation by the K+ ionophore valinomycin in defective sperm from infertile transgenic mice, and from blockage of RVD by quinine in normal sperm. However, in neither case was the recovery complete. The involvement of volume-sensitive osmolyte and anion channels (VSOAC) were investigated using blockers effective in other cell types. NPPB (5-nitro-2(3-phenylpropylamino) benzoic acid) and tamoxifen inhibited RVD but SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonic acid) at 0.4 and 1 mM had no effect whereas DIDS (di-isothiocyanato-stilbene-2,2'-disulphonic acid) at 1 mM enhanced RVD. Verapamil, but not another P-glycoprotein antagonist cyclosporin, caused sperm swelling which persisted in the presence of valinomycin, in Ca2+-free medium and in the presence of thapsigargin, but swelling was abolished by the Ca2+ ionophore A23187. Nifedipine was slightly effective in blocking RVD. Analysis by Western blotting failed to reveal ClC-2 and ClC-3 members of the chloride channel family in murine or rat sperm proteins despite signal bands in positive tissue controls. These findings implicate the involvement of some unidentified VSOAC in sperm volume regulation, which is probably Ca+-dependent.     

Permeabilization by streptolysin-o reveals a role for calcium-dependent protein kinase c isoforms alpha and beta in the response of cultured cardiomyocytes to hyposmotic challenge

Immunocytochemical techniques indicate that the uninhibited activity of protein kinase C alpha and protein kinase C beta are necessary for a normal regulatory volume decrease (RVD) response of cultured chick embryo cardiomyocytes subjected to a hyposmotic environment. Antibodies against protein kinase C isoforms alpha, beta, gamma and epsilon were introduced into the cultured myocytes using a developed streptolysin-O (SLO) permeabilization technique that allows the targeted cells to accumulate large biomolecules without perturbing their normal physiological state. The loaded cells were then tested for their ability to RVD when submitted to hypo-osmotic stimulus. Results show that exposing the cultured cells to SLO in the presence of antibodies against protein kinase C alpha and beta, prior to volume challenge, significantly slows the RVD rate. Additional experiments that combined anti-alpha and anti-beta antibodies in the same exposure media did not result in a significantly different rate than the anti-alpha or anti-beta rates alone. The evidence gained in this study is in agreement with previous work in the cultured chick embryo cardiomyocyte that report the involvement of a calcium dependent protein kinase C in the signal transduction pathway of the RVD.     

Contribution of organic and inorganic osmolytes to volume regulation in rat brain cells in culture

In this work we examined the time course and the amount released, by hyposmolarity, for the most abundant free amino acids (FAA) in rat brain cortex astrocytes and neurons in culture. The aim was to evaluate their contribution to the process of cell volume regulation. Taurine, glutamate, andd-aspartate in the two types of cells, -alanine in astrocytes and GABA in neurons were promptly released by hyposmolarity, reaching a maximum within 1–2 min. after an osmolarity change. A substantial amount of the intracellular pool of these amino acids was mobilized in response to hyposmolarity. The amount released in media with osmolarity reduced from 300 mOsm to 150 mOsm or 210 mOsm, represented 50%–65% and 13%–31%, respectively, of the total amino acid content in cells. In both astrocytes and neurons, the efflux of glutamine and alanine was higher under isosmotic conditions and increased only marginally during hyposmotic conditions.⁸⁶Rb⁺, used as tracer for K⁺, was released from astrocytes, 30% and 11%, respectively, in hyposmotic media of 150 mOsm or 210 mOsm but was not transported in neurons. From these results it was calculated that FAA contribute 54% and inorganic ions 46% to the process of volume regulation in astrocytes exposed to a 150 mOsm hyposmotic medium. This contribution was 55% for FAA and 45% for K⁺ and Cl^– in cells exposed to 210 mOsm hyposmotic solutions. These results indicate that the contribution of FAA to the process of cell volume regulation is higher in astrocytes than in other cell types including renal and blood cells.Special issue dedicated to Dr. Claude Baxter.