Cell Volume Regulation: Intracellular Transmission. Advances in Comparative and Environmental Physiology. Berlin: Springer Berlin Heidelberg; 1991. pp. 187-248. [DOI: 10.1007/978-3-642-77124-8_7]

Cell content of phosphatidylinositol (4,5)bisphosphate in Ehrlich mouse ascites tumour cells in response to cell volume perturbations in anisotonic and in isosmotic media

The labelling pattern of cellular phosphoinositides (PtdInsP(n)) was studied in Ehrlich ascites cells labelled in vivo for 24 h with myo-[2-(3)H]- or l-myo-[1-(3)H]inositol and exposed to anisotonic or isosmotic volume perturbations. In parallel experiments the cell volume ([(14)C]3-OMG space) was monitored. In hypotonic media the cells initially swelled osmotically and subsequently as expected showed a regulatory volume decrease (RVD) response. Concurrently, the cell content of PtdInsP(2) showed a marked, transient decrease and the content of PtdInsP a small, transient increase. The changes in PtdInsP(2) and PtdInsP content increased progressively with the extent of hypotonicity (in the range 1.00-0.50 relative osmolarity). No evidence was found for either hydrolysis of PtdInsP(2) or formation of PtdInsP(3). In hypertonic medium (relative osmolarity 1.50), cells initially shrank osmotically and subsequently as expected showed a small regulatory volume increase (RVI) response. Concurrently, the cell content of PtdInsP(2) showed a marked increase and the content of PtdInsP a small decrease, i.e. changes in the opposite direction of those seen in hypotonic media. In isosmotic media with high (100 mm) or low (0.8 mm) K(+) concentration, cells slowly swelled or shrank due to uptake or loss of isosmotic KCl. Under these conditions, with largely unchanged intracellular ionic strength, the cell content of PtdInsP(2) and PtdInsP remained constant. Our results show that PtdInsP(2) is not volume sensitive per se, and moreover that the regulatory volume adjustments in Ehrlich ascites cells are not mediated by PtdInsP(2) hydrolysis and its subsequent production of second messengers. The simplest interpretation of the observed effects would be that PtdInsP(2) is controlled by ionic strength, probably via activation/inhibition of phosphoinositide-specific phosphatases/kinases. In Ehrlich ascites cells, as shown previously, the opposing ion channels and transporters activated during RVD and RVI, respectively, are controlled with tight negative coordination by a common cell volume 'set-point' that is shifted in anisotonic media, but unchanged during cell swelling in isosmotic high K(+) medium. We hypothesize that PtdInsP(2) might orchestrate this 'set-point' shift.

Roles of phospholipase A2 isoforms in swelling- and melittin-induced arachidonic acid release and taurine efflux in NIH3T3 fibroblasts

Osmotic swelling of NIH3T3 mouse fibroblasts activates a bromoenol lactone (BEL)-sensitive taurine efflux, pointing to the involvement of a Ca(2+)-independent phospholipase A(2) (iPLA(2)) (Lambert IH. J Membr Biol 192: 19-32, 2003). We report that taurine efflux from NIH3T3 cells was not only increased by cell swelling but also decreased by cell shrinkage. Arachidonic acid release to the cell exterior was similarly decreased by shrinkage yet not detectably increased by swelling. NIH3T3 cells were found to express cytosolic calcium-dependent cPLA(2)-IVA, cPLA(2)-IVB, cPLA(2)-IVC, iPLA(2)-VIA, iPLA(2)-VIB, and secretory sPLA(2)-V. Arachidonic acid release from swollen cells was partially inhibited by BEL and by the sPLA(2)-inhibitor manoalide. Cell swelling elicited BEL-sensitive arachidonic acid release from the nucleus, to which iPLA(2)-VIA localized. Exposure to the bee venom peptide melittin, to increase PLA(2) substrate availability, potentiated arachidonic acid release and osmolyte efflux in a volume-sensitive, 5-lipoxygenase-dependent, cyclooxygenase-independent manner. Melittin-induced arachidonic acid release was inhibited by manoalide and slightly but significantly by BEL. A BEL-sensitive, melittin-induced PLA(2) activity was also detected in lysates devoid of sPLA(2), indicating that both sPLA(2) and iPLA(2) contribute to arachidonic acid release in vivo. Swelling-induced taurine efflux was inhibited potently by BEL and partially by manoalide, whereas the reverse was true for melittin-induced taurine efflux. It is suggested that in NIH3T3 cells, swelling-induced taurine efflux is dependent at least in part on arachidonic acid release by iPLA(2) and possibly also by sPLA(2), whereas melittin-induced taurine efflux is dependent on arachidonic acid release by sPLA(2) and, to a lesser extent, iPLA(2).

Role of endogenous channels in red blood cells response to their exposure to the pore forming toxin Sticholysin II

Sticholysin II (St II) is a highly hemolytic cytolysin isolated from the sea anemone Stichodactyla heliantus. The toxin hemolytic action takes place through the formation of channels that provoke an electrolyte unbalance leading to osmotic shock. The lytic event must involve the exchange of electrolytes and the entrance of water, leading to red blood cell disruption. These processes can occur through St II pores and/or the endogenous red blood cells transporters. In order to evaluate the contribution of these channels to water, anion and cation transport, we have measured the hemolysis and K+ efflux rates in the presence of several specific inhibitors. The results obtained in the presence of Hg, an AQP1 blocker, indicate that water transport through these channels is not essential for the occurrence of the lytic process induced by St II. The data also support a partial role of K+ and anion transporters. In particular, they are compatible with a preferential K+ efflux though the K(+)/Cl- co-transport as a response to the promoted swelling. Furthermore, they suggest that chloride influx, a process that can regulate both K+ efflux and lysis, is partially mediated by the endogenous cell transporters, in particular, band-3 anion exchange system being relevant at early stages of the lytic process.

Contribution of the IsK (MinK) potassium channel subunit to regulatory volume decrease in murine tracheal epithelial cells

The cell volume regulatory response following hypotonic shocks is often achieved by the coordinated activation of K(+) and Cl(-) channels. In this study, we investigate the identity of the K(+) and Cl(-) channels that mediate the regulatory volume decrease (RVD) in ciliated epithelial cells from murine trachea. RVD was inhibited by tamoxifen and 1,9-dideoxyforskolin, two agents that block swelling-activated Cl(-) channels. These data suggest that swelling-activated Cl(-) channels play an important role in cell volume regulation in murine tracheal epithelial cells. Ba(2+) and apamin, inhibitors of K(+) channels, were without effect on RVD, while tetraethylammoniun had little effect on RVD. In contrast, clofilium, an inhibitor of the KvLQT/IsK potassium channel complex potently inhibited RVD, suggesting a role for the KvLQT/IsK channel complex in cell volume regulation by tracheal epithelial cells. To investigate further the role of KvLQT/IsK channels in RVD, we used IsK knock-out mice. When exposed to hypotonic solutions, tracheal cells from IsK(+/+) mice underwent RVD, whereas cells from IsK(-/-) failed to recover their normal size. These data suggest that the IsK potassium subunit plays an important role in RVD in murine tracheal epithelial cells.

Cell volume measurement using scanning ion conductance microscopy

We report a novel scanning ion conductance microscopy (SICM) technique for assessing the volume of living cells, which allows quantitative, high-resolution characterization of dynamic changes in cell volume while retaining the cell functionality. The technique can measure a wide range of volumes from 10(-19) to 10(-9) liter. The cell volume, as well as the volume of small cellular structures such as lamelopodia, dendrites, processes, or microvilli, can be measured with the 2.5 x 10(-20) liter resolution. The sample does not require any preliminary preparation before cell volume measurement. Both cell volume and surface characteristics can be simultaneously and continuously assessed during relatively long experiments. The SICM method can also be used for rapid estimation of the changes in cell volume. These are important when monitoring the cell responses to different physiological stimuli.

Thrombin-, bradykinin-, and arachidonic acid-induced Ca2+ signaling in Ehrlich ascites tumor cells

Stimulation of single Ehrlich ascites tumor cells with agonists (bradykinin, thrombin) and with arachidonic acid (AA) induces increases in the free intracellular Ca2+ concentration ([Ca2+]i) in the presence and absence of extracellular Ca2+, measured using the Ca2+-sensitive probe fura 2. Sequential stimulation with two agonists elicits sequential increases in [Ca2+]i, unlike addition of the same agonist twice. Bradykinin and thrombin have additive effects on [Ca2+]i in Ca2+-free medium. The phosphoinositidase C inhibitor U-73122 inhibits the agonist-induced increases in [Ca2+]i, whereas ryanodine has no effect. Pretreatment of cells in Ca2+-free medium with thapsigargin abolishes the bradykinin-induced increase in [Ca2+]i but not the response to thrombin. The AA-induced response is not inhibited by U-73122 and cannot be mimicked by the inactive structural analog trifluoromethylarachidonyl ketone. Pretreatment of the cells with 50 microM AA (but not with 10 microM AA) abolishes the agonist-induced increase in [Ca2+]i. Thus bradykinin, thrombin, and AA induce increases in [Ca2+]i in Ehrlich cells due to Ca2+ entry and release from intracellular stores. Thrombin causes release of Ca2+ from an intracellular store that is insensitive to bradykinin and is not depleted by thapsigargin but is depleted by AA.

P2 receptor-mediated signal transduction in Ehrlich ascites tumor cells

The mechanisms, by which the P2 receptor agonists adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) evoke an increase in the free cytosolic calcium concentration ([Ca2+]i) and in intracellular pH (pHi), have been investigated in Ehrlich ascites tumor cells. The increase in [Ca2+]i evoked by ATP or UTP is abolished after depletion of intracellular Ca2+ stores with thapsigargin in Ca2+-free medium, and is inhibited by U73122, an inhibitor of phospholipase C (PLC), indicating that the increase in [Ca2+]i is primarily due to release from intracellular, Ins(1,4,5)P3-sensitive Ca2+ stores. ATP also activates a capacitative Ca2+-entry pathway. ATP as well as UTP evokes a biphasic change in pHi, consisting of an initial acidification followed by alkalinization. Suramin and 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS) inhibit the biphasic change in pHi, apparently by acting as antagonists at P2 receptors. The alkalinization evoked by the P2 receptor agonists is found to be due to activation of a 5'-(N-ethyl-N-isopropyl)amiloride (EIPA)-sensitive Na+/H+ exchanger. ATP and UTP elicit rapid cell shrinkage, presumably due to activation of Ca2+ sensitive K+ and Cl- efflux pathways. Preventing cell shrinkage, either by incubating the cells at high extracellular K+ concentration, or by adding the K+-channel blocker, charybdotoxin, does not affect the increase in [Ca2+]i, but abolishes the activation of the Na+/H+ exchanger, indicating that activation of the Na+/H+ exchanger is secondary to the Ca2+-induced cell shrinkage.

Osmosensitive C1- currents and their relevance to regulatory volume decrease in human intestinal T84 cells: outwardly vs. inwardly rectifying currents

1. The swelling-activated outwardly rectifying Cl- current (ICl(swell)) recorded in T84 human intestinal cells was completely blocked by 10 microM tamoxifen, while 300 microM Cd2+ had no effect. 2. A ClC-2-like, inwardly rectifying Cl- current was activated after strong hyperpolarization in T84 cells. This current was completely inhibited by 300 microM Cd2+, unaffected by 10 microM tamoxifen, and its magnitude increased slightly in response to cell swelling under hyposmotic conditions. However, the swelling-dependent modulation occurred only after prior activation by hyperpolarizing voltages. 3. T84 cells behaved initially close to perfect osmometers in response to changes in external osmolalities between +20 and -30 %. The cells underwent full regulatory volume decrease (RVD) within 16 min when exposed to 30 or 10 % hyposmotic shocks. 4. Pharmacological tools were used to determine the anionic pathway(s) involved in RVD in T84 cells. Tamoxifen (10 microM), 1,9-dideoxyforskolin (DDFSK; 100 microM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 microM) blocked RVD while 300 microM Cd2+ had no effect upon RVD following a 30 % hyposmotic shock. The RVD response was similarly unaffected by Cd2+ when cells were exposed to a smaller (10 %) hyposmotic shock. 5. In conclusion, these data show that the anionic pathway primarily activated by cell swelling and relevant to RVD in T84 cells is the tamoxifen-, DDFSK- and DIDS-sensitive ICl(swell) and not the hyperpolarization-activated, Cd2+-sensitive Cl- current associated with the ClC-2 Cl- channel.

Protein kinase C phosphorylation disengages human and mouse-1a P-glycoproteins from influencing the rate of activation of swelling-activated chloride currents

1. Whole-cell, swelling-activated Cl- currents, ICl(swell), were characterized in Chinese hamster ovary (CHO) cells and found to exhibit time-dependent inactivation at depolarizing potentials, tamoxifen and dideoxyforskolin sensitivity, and an anion permeability sequence: SCN- > I- > Br- > Cl- > F- > gluconate-. 2. CHO cells permanently transfected with either the human MDR1 or mouse mdr1a cDNAs demonstrated an increased rate of activation of ICl(swell) compared with parental cells or those permanently transfected with the mouse mdr1b cDNA. However, no differences in the magnitude of the currents were observed at steady state. 3. Pretreatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) did not affect ICl(swell) in MDR1 or mdr1a permanently transfected CHO cells. In contrast, pretreatment with TPA reduced ICl(swell) in MDR1(G185V)-expressing transfected NIH3T3 fibroblasts. Subsequently, the CHO cell lines were shown to contain significantly reduced levels of protein kinase C (PKC), suggesting that PKC concentrations might be limiting in these cell lines, at least under whole-cell patch clamp conditions. 4. Addition of purified PKC to the pipette solution, followed by a pretreatment with TPA, reduced the rate of ICl(swell) activation in human Pgp- and mouse Pgp1a-expressing CHO cells to the levels observed in parental and mouse Pgp1b-expressing cells. This confirms that PKC is limiting in these cells under whole-cell, patch clamp conditions. Furthermore, these results suggest that PKC-mediated phosphorylation of human Pgp and mouse Pgp1a disengages the influence which these Pgps have on ICl(swell). 5. These studies also demonstrate a functional distinction between the two mouse homologues, Pgp1a and Pgp1b. Although both can function as drug efflux pumps, only Pgp1a can act like human Pgp to influence ICl(swell).

Expansion of the cellular content of ribonucleoside triphosphates induces cell shrinkage and KCl loss in Ehrlich ascites tumor cells and in Chinese hamster ovary cells

The conversion to corresponding triphosphate derivatives of various ribonucleosides has been studied in Ehrlich ascites tumor cells and in Chinese hamster ovary cells under conditions that are optimal for cellular uptake of orthophosphate. The initial cellular uptake of orthophosphate is followed by a cellular loss of Cl- which might be consistent with a H2PO4-/Cl- exchange mechanism. Subsequent addition of ribonucleosides to the medium leads to cellular accumulation of the corresponding triphosphate and to a concomitant loss of KCl and to sustained cell volume reduction. The latter two events are quite unspecific with regard to the nucleobase moiety of the ribonucleoside triphosphate accumulated (adenine, guanine and purine being almost equally effective) and they depend in a rather simple way on the increase of the cellular content of these compounds. The KCl loss seems to depend on opening of the separate K+ and Cl- channels. The pharmacological profile of the putative ion channels could not be identified in spite of experiments with conventional blockers. In the case of purine riboside the accumulation of the corresponding triphosphate and concomitant loss of KCl and cell water may be followed by a regain of cell volume due to a continued purine riboside triphosphate accumulation, which apparently depends on the uptake of orthophosphate by cotransport with Na+ and which for osmotic reasons is accompanied by the uptake of water and hence volume increase. The possibility that the nucleoside triphosphate induced opening of a putative Cl- channel may be due to a direct effect of triphosphate on a channel protein is discussed.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.

Stretch-activated channels in airway epithelial cells

Two type of stretch-activated (SA) ion channels were identified in the basolateral membrane of isolated rabbit airway epithelial cells by patch-clamp techniques. Pressure activation and deactivation of one channel, which had a conductance of 29 pS, occurred after a delay of approximately 20-30 s. The open probability of this delayed stretch-activated (DSA) channel was increased from < 0.01 to 0.45 at 50 mmHg of suction. The reversal potential of the DSA channel, calculated from the pipette potential at which membrane currents reversed [-31.3 +/- 3.6 (SD) mV] and the resting membrane potential (-27.8 +/- 3.3 mV) was +3.5 +/- 3.3 mV. None of the equilibrium potentials of the ions used were similar to the calculated reversal potential of the DSA channel, suggesting that this channel is nonselective for cations. The DSA channel gating behavior was characterized by bursts of rapid transitions between open and closed states. The distribution of the open and closed times revealed that this gating behavior could be fitted with two open states and two closed states. Only the slow time constant of the closed state was decreased by suction. The second SA channel was selective for K+ and had a conductance of 65 pS but a long delay was not associated with the pressure sensitivity of this channel. The open probability of the K(+)-selective SA channel was increased from < 0.01 to 0.30 by 50 mmHg of suction. The K(+)-selective SA channel was distinct from the well-characterized basolateral K+ channel.