Responses of endothelial cells to hypotonic solutions: lack of regulatory volume decrease

Regulation of Ca2+-dependent K+ current by alphavbeta3 integrin engagement in vascular endothelium

Interactions between endothelial cells and extracellular matrix proteins are important determinants of endothelial cell signaling. Endothelial adhesion to fibronectin through alpha(v)beta(3) integrins or the engagement and aggregation of luminal alpha(v)beta(3) receptors by vitronectin triggers Ca2+ influx. However, the underlying signaling mechanisms are unknown. The electrophysiological basis of alpha(v)beta(3) integrin-mediated changes in endothelial cell Ca2+ signaling was studied using whole cell patch clamp and microfluorimetry. The resting membrane potential of bovine pulmonary artery endothelial cells averaged -60 +/- 3 mV. In the absence of intracellular Ca2+ buffering, the application of soluble vitronectin (200 microg/ml) resulted in activation of an outwardly rectifying K+ current at holding potentials from -50 to +50 mV. Neither a significant shift in reversal potential (in voltage clamp mode) nor a change in membrane potential (in current clamp mode) occurred in response to vitronectin. Vitronectin-activated current was significantly inhibited by pretreatment with the alpha(v)beta(3) integrin antibody LM609 by exchanging extracellular K+ with Cs+ or by the application of iberiotoxin, a selective inhibitor of large-conductance, Ca2+-activated K+ channels. With intracellular Ca2+ buffered by EGTA in the recording pipette, vitronectin-activated K+ current was abolished. Fura-2 microfluorimetry revealed that vitronectin induced a significant and sustained increase in intracellular Ca2+ concentration, although vitronectin-induced Ca2+ current could not be detected. This is the first report to show that an endothelial cell ion channel is regulated by integrin activation, and this K+ current likely plays a crucial role in maintaining membrane potential and a Ca2+ driving force during engagement and activation of endothelial cell alpha(v)beta(3) integrin.

Cell volume response to hyposmotic shock and elevated cAMP in bovine trabecular meshwork cells

PURPOSE

Hyposmolar perfusion of intact trabecular meshwork (TM) induces a decrease in its hydraulic conductivity (Lp). However, exposure to agents that elevate intracellular cAMP in TM cells increases Lp. Since volume of TM cells could directly influence porosity of the TM and hence Lp, this study has investigated changes in volume in response to acute hyposmotic shock (i.e. regulatory volume decrease or RVD) and elevated cAMP in cultured TM cells.

METHODS

Bovine trabecular meshwork cells (BTMC), grown on glass coverslips and loaded with the fluorescent dye MQAE, were used to measure rapid changes in cell volume using the principle of dynamic fluorescence quenching. Activation of volume-regulated anion channels (VRAC) was assessed by measuring volume-sensitive Cl(-) currents (I(Cl,swell)) in the whole cell configuration of the patch clamp technique and by determining the swelling-induced enhancement in I(-) permeability using the halide-sensitivity of MQAE. Expressions of ClC (chloride channels of the ClC gene family), P-glycoprotein (Pgp), and cystic fibrosis transmembrane regulator (CFTR) Cl(-) channels were examined by RT-PCR. Elevation of cAMP in response to forskolin was confirmed by determining the phosphorylation of cAMP response element-binding protein and activating transcription factor-1 (CREB, ATF-1), which form the downstream targets of protein kinase A.

RESULTS

As a response to hyposmotic shock, there was an acute increase in cell volume but there was no robust RVD. Patch clamp experiments showed activation of a characteristic Cl(-) current in response to cell swelling. This Cl(-) current was inhibited by NPPB (100microM) and fluoxetine (50microM), both of which are known blockers of VRAC. Experiments, which used the halide-sensitivity of MQAE, also indicated a 9-fold increase in I(-) influx upon cell swelling (8.9+/-4.6; n=9), consistent with activation of a VRAC-like Cl(-) current. To examine whether RVD is limited by K(+) conductance, the swollen cells were exposed to gramicidin, which is known to induce cation channel activity. Such a maneuver led to secondary swelling with [Na(+)](o)=140mM but a rapid shrinkage [Na(+)](o)=8mM indicating that the RVD is limited by cationic conductance necessary for K(+) efflux. Exposure to forskolin, which resulted in CREB and ATF-1 phosphorylation, caused a reversible decrease in cell volume (14.5+/-5%; n=20) under isosmotic and hyposmotic conditions. RT-PCR analysis confirmed expression of ClC-2, ClC-5, and Pgp Cl(-) channels in bovine TM cells. However, ClC-3 and CFTR were not expressed.

CONCLUSIONS

TM cells respond to acute hyposmotic shock in an osmometric manner, but their RVD is limited by K(+) conductance. The lack of CFTR expression and decrease in cell volume in response to forskolin concomitant with hyposmolarity suggest that elevated cAMP activates a K(+) conductance. Thus, the altered resistance to aqueous outflow in response to hyposmotic perfusion of the TM and elevated cAMP may be attributed to persistent cell swelling and cell shrinkage, respectively.

A slow outward current and a hypoosmolality induced anion conductance in embryonic chicken osteoclasts

In this paper we report on a hypoosmolality induced current, I(osmo), in embryonic chicken osteoclasts, which could only be studied when blocking a simultaneously active, unidentified slow outward current, I(slo). I(slo) was observed in all of the examined cells when both the intracellular and extracellular solutions contained sodium as the major cation and no potassium. The current was outwardly rectifying and activated at membrane potentials more positive than -44 +/- 12 mV (n = 31). The time to half activation of the current was also voltage dependent and was 350 ms at Vm = +80 mV, and 78 ms at Vm = +120 mV. The current did not inactivate during periods up to 5 s. Extracellular 4-AP (5 mM), TEA (5 mM) and Ba2+ (1 mM), blockers of K+ conductances in chicken osteoclasts, did not influence I(slo). However, I(slo) was inhibited by 50 microM extracellular verapamil, which allowed us to study I(osmo) in isolation. Exposure of the osteoclasts to hypotonic solution resulted in the development of a depolarization activated I(osmo). It developed after a 1-min delay and reached its maximum within 10 minutes. Half-maximal activation occurred after 4.4 +/- 0.9 min (n = 9). The current activated within a few ms upon depolarization and did not inactivate during at least 5 sec. I(osmo) reversed around the calculated Nernst potential for Cl- (E(Cl) = +7.3 mV and V(rev) = +5.4 +/- 3.6 mV, n = 9). The underlying conductance, G(osmo) exhibited moderate outward rectification around 0 mV in symmetrical Cl- solutions. Ion substitution experiments showed that G(osmo) is an anion conductance with P(Cl) approximately = P(F) > P(gluc) >> P(Na). I(osmo) was blocked by 0.5 mM SITS but 50 microM verapamil, 5 mM TEA, 5 mM 4-AP, 1 mM Ba2+, 50 microM cytochalasin D and 0.5 mM alendronate did not have any effect on the current. Cl- currents have been implicated in charge neutralization during osteoclastic acid secretion for bone resorption. The present results imply that osmolality may be a factor controlling this charge neutralization.

Inhibition of volume-regulated anion channels by expression of the cystic fibrosis transmembrane conductance regulator

1. To investigate whether the cystic fibrosis transmembrane conductance regulator (CFTR) interacts with volume regulated anion channels (VRACs), we measured the volume-activated chloride current (ICl,swell) using the whole-cell patch-clamp technique in calf pulmonary artery endothelial (CPAE) cells and in COS cells transiently transfected with wild-type (WT) CFTR and the deletion mutant DeltaF508 CFTR. 2. ICl,swell was significantly reduced in CPAE cells expressing WT CFTR to 66.5 +/- 8.8 % (n = 13; mean +/- s. e.m.) of the control value (n = 11). This reduction was independent of activation of the CFTR channel. 3. Expression of DeltaF508 CFTR resulted in two groups of CPAE cells. In the first group IBMX and forskolin could activate a Cl- current. In these cells ICl,swell was reduced to 52.7 +/- 18.8 % (n = 5) of the control value (n = 21). In the second group IBMX and forskolin could not activate a current. The amplitude of ICl,swell in these cells was not significantly different from the control value (112.4 +/- 13.7 %, n = 11; 21 control cells). 4. Using the same method we showed that expression of WT CFTR in COS cells reduced ICl,swell to 62.1 +/- 11.9 % (n = 14) of the control value (n = 12) without any changes in the kinetics of the current. Non-stationary noise analysis suggested that there is no significant difference in the single channel conductance of VRAC between CFTR expressing and non-expressing COS cells. 5. We conclude that expression of WT CFTR down-regulates ICl, swell in CPAE and COS cells, suggesting an interaction between CFTR and VRAC independent of activation of CFTR.

Synergism between hypotonically induced calcium release and fatty acyl-CoA esters induced calcium release from intracellular stores

The non-mitochondrial Ca2+ stores in permeabilized A7r5 cells responded to a decrease in Mg-ATP concentration with a pronounced Ca2+ release if 20 microM CoA was present. This release was rather specific for the preincubation or removal of ATP. ATP gamma S was much less effective and AMP-PNP, GTP, ITP, CTP, UTP, ADP, AMP, adenosine and adenine had no effect. CoA activated with an EC50 of 6 microM. Dephospho-CoA was a less effective cofactor and desulfo-CoA was ineffective. The release induced by Mg-ATP removal did not occur in the presence of 2% fatty acid-free bovine serum albumin and did not develop at 4 degrees C. All these findings suggest that CoA had to be acylated by endogenous fatty-acyl-CoA synthetase to become effective. Myristoyl- and palmitoyl-CoA esters were identified as the most effective cofactors for the release. Ca2+ release induced by removing Mg-ATP did not occur if the osmolality of the medium was kept constant by addition of mannitol, sucrose, KCl, MgCl2 or Mg-GTP, indicating that the decrease in tonicity was the trigger for the release. Mg-ATP plus CoA also synergized with Ca2+ release induced by a hypotonic shock imposed by diluting the medium with H2O. Osmolality changes induced by decreasing the Mg-ATP concentration were more effective in releasing Ca2+ than equal decreases in concentration of all solutes. We conclude that fatty acyl-CoA esters sensitize the hypotonically induced Ca2+ release from the non-mitochondrial Ca2+ stores.

Role for anions in pulmonary endothelial permeability

beta-Adrenergic stimulation reduces albumin permeation across pulmonary artery endothelial monolayers and induces changes in cell morphology that are mediated by Cl- flux. We tested the hypothesis that anion-mediated changes in endothelial cells result in changes in endothelial permeability. We measured permeation of radiolabeled albumin across bovine pulmonary arterial endothelial monolayers when the extracellular anion was Cl-, Br-, I-, F-, acetate (Ac-), gluconate (G-), and propionate (Pr-). Permeability to albumin (Palbumin) was calculated before and after addition of 0.2 mM of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), which reduces permeability. In Cl-, the Palbumin was 3.05 +/- 0.86 x 10(-6) cm/s and fell by 70% with the addition of IBMX. The initial Palbumin was lowest for Pr- and Ac-. Initial Palbumin was higher in Br-, I-, G-, and F- than in Cl-. A permeability ratio was calculated to examine the IBMX effect. The greatest IBMX effect was seen when Cl- was the extracellular anion, and the order among halide anions was Cl- > Br- > I- > F-. Although the level of extracellular Ca2+ concentration ([Ca2+]o) varied over a wide range in the anion solutions, [Ca2+]o did not systematically affect endothelial permeability in this system. When Cl- was the extracellular anion, varying [Ca2+]o from 0.2 to 2.8 mM caused a change in initial Palbumin but no change in the IBMX effect. The anion channel blockers 4-acetamido-4'-isothiocyanotostilbene-2, 2'-disulfonic acid (0.25 mM) and anthracene-9-carboxylic acid (0.5 mM) significantly altered initial Palbumin and the IBMX effect. The anion transport blockers bumetanide (0.2 mM) and furosemide (1 mM) had no such effects. We conclude that extracellular anions influence bovine pulmonary arterial endothelial permeability and that the pharmacological profile fits better with the activity of anion channels than with other anion transport processes.

A 133Cs nuclear magnetic resonance study of endothelial Na(+)-K(+)-ATPase activity: can actin regulate its activity?

Using (133)Cs+ NMR, we developed a technique to repetitively measure, in vivo, Na(+)-K(+)-ATPase activity in endothelial cells. The measurements were made without the use of an exogenous shift reagent, because of the large chemical shift of 1.36 +/- 0.13 ppm between intra- and extracellular Cs+. Intracellularly we obtained a spin lattice relaxation time (T1) of 2.0 +/- 0.3 s, and extracellular T1 was 7.9 +/- 0.4 s. Na(+)-K+ pump activity in endothelial cells was determined at 12 +/- 3 nmol Cs+ x min(-1) x (mg Prot)[-1] under control conditions. When intracellular ATP was depleted by the addition of 5 mM 2-deoxy-D-glucose (DOG) and NaCN to about 5% of control, the pump rate decreased by 33%. After 80 min of perfusion with 5 mM DOG and NaCN, reperfusion with control medium rapidly reestablished the endothelial membrane Cs+ gradient. Using (133)Cs+ NMR as a convenient tool, we further addressed the proposed role of actin as a regulator of Na(+)-K+ pump activity in intact cells. Two models of actin rearrangement were tested. DOG caused a rearrangement of F-actin and an increase in G-actin, with a simultaneous decrease in ATP concentration. Cytochalasin D, however, caused an F-actin rearrangement different from that observed for DOG and an increase in G-actin, and cellular ATP levels remained unchanged. In both models, the Na(+)-K(+)-pump activity remained unchanged, as measured with (133)Cs NMR. Our results demonstrate that (133)Cs NMR can be used to repetitively measure Na(+)-K(+)-ATPase activity in endothelial cells. No evidence for a regulatory role of actin on Na(+)-K(+)-ATPase was found.

Ion channels in vascular endothelium

The functional impact of ion channels in vascular endothelial cells (ECs) is still a matter of controversy. This review describes different types of ion channels in ECs and their role in electrogenesis, Ca2+ signaling, vessel permeability, cell-cell communication, mechano-sensor functions, and pH and volume regulation. One major function of ion channels in ECs is the control of Ca2+ influx either by a direct modulation of the Ca2+ influx pathway or by indirect modulation of K+ and Cl- channels, thereby clamping the membrane at a sufficiently negative potential to provide the necessary driving force for a sustained Ca2+ influx. We discuss various mechanisms of Ca2+ influx stimulation: those that activate nonselective, Ca(2+)-permeable cation channels or those that activate Ca(2+)-selective channels, exclusively or partially operated by the filling state of intracellular Ca2+ stores. We also describe the role of various Ca(2+)- and shear stress-activated K+ channels and different types of Cl- channels for the regulation of the membrane potential.

Hypotonically induced calcium release from intracellular calcium stores

Osmotic cell swelling induced by hypotonic stress is associated with a rise in intracellular Ca2+ concentration, which is at least partly due to a release of Ca2+ from internal stores. Since osmotic influx of water dilutes the cytoplasmic milieu, we have investigated how nonmitochondrial Ca2+ stores in permeabilized A7r5 cells respond to a reduction in cytoplasmic tonicity. We now present experimental evidence for a direct Ca2+ release from the stores when exposed to a hypotonic medium. The release is graded, but does not occur through the inositol trisphosphate or the ryanodine receptor. Ca2+ seems to be released through the passive leak pathway, and this phenomenon can be partially inhibited by divalent cations in the following order of potency: Ni2+ = Co2+ > Mn2+ > Mg2+ > Ba2+. This release also occurs in intact A7r5 cells. This novel mechanism of hypotonically induced Ca2+ release is therefore an inherent property of the stores, which can occur in the absence of second messengers. Intracellular stores can therefore act as osmosensors.

Volume regulation in a toad epithelial cell line: role of coactivation of K+ and Cl- channels

1. We have measured changes in cell volume, membrane potential and ionic currents in distal nephron A6 cells following a challenge with hypotonic solutions (HTS). 2. The volume increase induced by HTS is compensated by a regulatory volume decrease (RVD), which is inhibited by both 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and quinine. Quinine (500 microM) completely blocked RVD, whereas 100 microM NPPB delayed and attenuated RVD. 3. The resting potential in A6 cells was -52.3 +/- 4.8 mV (n = 53), and shifted to -35.1 +/- 2.2 mV (n = 33) during HTS. 4. Resting membrane current in A6 cells was 0.35 +/- 0.12 pA pF-1 at -80 mV and 0.51 +/- 0.16 pA pF-1 at +80 mV (n = 5). During cell swelling these values increased to 11.5 +/- 1.1 and 29.3 +/- 2.8 pA pF-1 (n = 29), respectively. 5. Quinine (500 microM) completely blocked the HTS-activated current at -15 mV, the reversal potential for Cl- currents, but exerted only a small block at -100 mV (K+ equilibrium potential). NPPB (100 microM) inhibited the current at both potentials almost to the same extent. The HTS-induced net current reversed at -41 +/- 2.5 mV (n = 15), which is close to the measured resting potential during HTS. 6. The quinine-insensitive current reversed near the Cl- equilibrium potential. The quinine-sensitive current reversed near the K+ equilibrium potential. The respective conductances activated by HTS at the zero-current potential were 2.1 +/- 0.7 nS for K+ and 5.2 +/- 1.3 nS for Cl- (n = 15). 7. Single channel analysis unveiled activation of at least two different channels during HTS. A 36 pS channel reversing at the Cl- equilibrium potential showed increased open probability at depolarized potentials. HTS also activated a K+ channel with a 29 pS conductance in high-K+ extracellular solutions (130 mM) or 12 pS in 2.5 mM K+. 8. This coactivation of K+ and Cl- channels shifts the membrane potential towards a value between EK and ECl (the reversal potentials for K+ and Cl-), where a net efflux of Cl- (Cl- inward current) and K+ (K+ outward current) under zero-current conditions occurs. Block of either the K+ or the Cl- conductance will shift the zero-current potential towards the equilibrium potential of the unblocked channel, preventing net efflux of osmolytes and RVD. This coactivation of K+ and Cl- currents causes a shift of osmolytes out of the cells, which almost completely accounts for the observed RVD.

Cytoskeletal modulation of the response to mechanical stimulation in human vascular endothelial cells

Possible interactions of cytoskeletal elements with mechanically induced membrane currents and Ca2+ signals were studied in human endothelial cells by using a combined patch-clamp and Fura II technique. For mechanical stimulation, cells were exposed to hypotonic solution (HTS). The concomitant cell swelling activates a Cl- current, releases Ca2+ from intracellular stores and activates Ca2+ influx. To interfere with the cytoskeleton, cells were loaded either with the F-actin-stabilizing agent phalloidin (10 mumol/l), or the F-actin-depolymerizing substance cytochalasin B (50 mumol/l). These were administered either in the bath or the pipette solutions. The tubulin structure of the endothelial cells was modulated by taxol (50 mumol/l), which supports polymerization of tubulin, or by the depolymerizing agent colcemid (10 mumol/l) both applied to the bath. Immunofluorescence experiments show that under the chosen experimental conditions the cytoskeletal modifiers employed disintegrate the F-actin and microtubuli cytoskeleton. Neither of these cytoskeletal modifiers influenced the HTS-induced Cl- current. Ca2+ release was not affected by cytochalasin B, taxol or colcemid, but was suppressed if the cells were loaded with phalloidin. Depletion of intracellular Ca2+ stores by thapsigargin renders the intracellular [Ca2+] sensitive to the extracellular [Ca2+], which is indicative of a Ca2+ entry pathway activated by store depletion. Neither cytochalasin B nor phalloidin affected this Ca2+ entry. We conclude that F-actin turnover or depolymerization is necessary for Ca2+ release by mechanical activation. The tubulin network is not involved. The Ca2+ release- activated Ca2+ entry is not modulated by the F-actin cytoskeleton.