Shrinkage-induced activation of Na+/H+ exchange in primary rat astrocytes: role of myosin light-chain kinase

ATP-mediated increase in H+ efflux from retinal Müller cells of the axolotl

Previous work has shown that activation of tiger salamander retinal radial glial cells by extracellular ATP induces a pronounced extracellular acidification, which has been proposed to be a potent modulator of neurotransmitter release. This study demonstrates that low micromolar concentrations of extracellular ATP similarly induce significant H+ effluxes from Müller cells isolated from the axolotl retina. Müller cells were enzymatically isolated from axolotl retina and H+ fluxes were measured from individual cells using self-referencing H+-selective microelectrodes. The increased H+ efflux from axolotl Müller cells induced by extracellular ATP required activation of metabotropic purinergic receptors and was dependent upon calcium released from internal stores. We further found that the ATP-evoked increase in H+ efflux from Müller cells of both tiger salamander and axolotl were sensitive to pharmacological agents known to interrupt calmodulin and protein kinase C (PKC) activity: chlorpromazine (CLP), trifluoperazine (TFP), and W-7 (all calmodulin inhibitors) and chelerythrine, a PKC inhibitor, all attenuated ATP-elicited increases in H+ efflux. ATP-initiated H+ fluxes of axolotl Müller cells were also significantly reduced by amiloride, suggesting a significant contribution by sodium-hydrogen exchangers (NHEs). In addition, α-cyano-4-hydroxycinnamate (4-cin), a monocarboxylate transport (MCT) inhibitor, also reduced the ATP-induced increase in H+ efflux in both axolotl and tiger salamander Müller cells, and when combined with amiloride, abolished ATP-evoked increase in H+ efflux. These data suggest that axolotl Müller cells are likely to be an excellent model system to understand the cell-signaling pathways regulating H+ release from glia and the role this may play in modulating neuronal signaling.NEW & NOTEWORTHY Glial cells are a key structural part of the tripartite synapse and have been suggested to regulate synaptic transmission, but the regulatory mechanisms remain unclear. We show that extracellular ATP, a potent glial cell activator, induces H+ efflux from axolotl retinal Müller (glial) cells through a calcium-dependent pathway that is likely to involve calmodulin, PKC, Na+/H+ exchange, and monocarboxylate transport, and suggest that such H+ release may play a key role in modulating neuronal transmission.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Combination treatment of nocturnal enuresis with desmopressin and indomethacin

BACKGROUND

We investigated the effect of combining indomethacin and desmopressin in treating children with monosymptomatic nocturnal enuresis (MNE) and desmopressin-resistant nocturnal polyuria.

METHODS

Twenty-three children with MNE, nocturnal polyuria, and partial or no response to desmopressin were recruited from incontinence clinics of our tertiary referral center. We used a randomized single-arm crossover placebo-controlled study design consisting of two 3-week treatment periods with a combination of desmopressin (0.4 mg) and indomethacin (50 mg) or desmopressin and placebo at bedtime. Home recordings at baseline and for the final 2 weeks of each treatment period were performed and included nocturnal urine output measurements. The number of dry nights achieved and reduction in the nocturnal urine output were the main effect parameters. Student's t test and Pearson's correlation coefficient were used for statistical analysis.

RESULTS

The addition of indomethacin to desmopressin significantly reduced nocturnal urine output (from 324 ± 14 ml to 258 ± 13 ml, p < 0.001). This did not lead to more dry nights in all children, and we found no statistically significant reduction in enuresis frequency (from 68 % ± 0.1 to 56 % ± 0.1, p = 0.24).

CONCLUSIONS

Addition of indomethacin to desmopressin can further reduce nocturnal urine output in children with MNE and desmopressin-resistant nocturnal polyuria. The combination treatment does not, however, improve outcome in terms of frequency of nights with enuresis. The dissociation of antidiuretic and antienuretic effect may reflect nocturnal bladder reservoir dysfunction in children who present with normal daytime bladder function.

Renal β-intercalated cells maintain body fluid and electrolyte balance

Inactivation of the B1 proton pump subunit (ATP6V1B1) in intercalated cells (ICs) leads to type I distal renal tubular acidosis (dRTA), a disease associated with salt- and potassium-losing nephropathy. Here we show that mice deficient in ATP6V1B1 (Atp6v1b1-/- mice) displayed renal loss of NaCl, K+, and water, causing hypovolemia, hypokalemia, and polyuria. We demonstrated that NaCl loss originated from the cortical collecting duct, where activity of both the epithelial sodium channel (ENaC) and the pendrin/Na(+)-driven chloride/bicarbonate exchanger (pendrin/NDCBE) transport system was impaired. ENaC was appropriately increased in the medullary collecting duct, suggesting a localized inhibition in the cortex. We detected high urinary prostaglandin E2 (PGE2) and ATP levels in Atp6v1b1-/- mice. Inhibition of PGE2 synthesis in vivo restored ENaC protein levels specifically in the cortex. It also normalized protein levels of the large conductance calcium-activated potassium channel and the water channel aquaporin 2, and improved polyuria and hypokalemia in mutant mice. Furthermore, pharmacological inactivation of the proton pump in β-ICs induced release of PGE2 through activation of calcium-coupled purinergic receptors. In the present study, we identified ATP-triggered PGE2 paracrine signaling originating from β-ICs as a mechanism in the development of the hydroelectrolytic imbalance associated with dRTA. Our data indicate that in addition to principal cells, ICs are also critical in maintaining sodium balance and, hence, normal vascular volume and blood pressure.

Distinct roles of central and peripheral prostaglandin E2 and EP subtypes in blood pressure regulation

Prostaglandin E(2) (PGE(2)) is a major prostanoid with a wide variety of biological activities. PGE(2) can influence blood pressure (BP) both positively and negatively. In particular, centrally administered PGE(2) induces hypertension whereas systemic administration of PGE(2) produces a hypotensive effect. These physiologically opposing effects are generated by the existence of multiple EP receptors, namely EP(1-4), which are G protein-coupled receptors with distinct signaling properties. This review highlights the distinct roles of PGE(2) in BP regulation and the involvement of specific EP receptor subtypes.

Expression of calmodulin and myosin light chain kinase during larval settlement of the Barnacle Balanus amphitrite

Barnacles are one of the most common organisms in intertidal areas. Their life cycle includes seven free-swimming larval stages and sessile juvenile and adult stages. The transition from the swimming to the sessile stages, referred to as larval settlement, is crucial for their survivor success and subsequent population distribution. In this study, we focused on the involvement of calmodulin (CaM) and its binding proteins in the larval settlement of the barnacle, Balanus ( = Amphibalanus) amphitrite. The full length of CaM gene was cloned from stage II nauplii of B. amphitrite (referred to as Ba-CaM), encoding 149 amino acid residues that share a high similarity with published CaMs in other organisms. Quantitative real-time PCR showed that Ba-CaM was highly expressed in cyprids, the stage at which swimming larvae are competent to attach and undergo metamorphosis. In situ hybridization revealed that the expressed Ba-CaM gene was localized in compound eyes, posterior ganglion and cement glands, all of which may have essential functions during larval settlement. Larval settlement assays showed that both the CaM inhibitor compound 48/80 and the CaM-dependent myosin light chain kinase (MLCK) inhibitor ML-7 effectively blocked barnacle larval settlement, whereas Ca(2+)/CaM-dependent kinase II (CaMKII) inhibitors did not show any clear effects. The subsequent real-time PCR assay showed a higher expression level of Ba-MLCK gene in larval stages than in adults, suggesting an important role of Ba-MLCK gene in larval development and competency. Overall, the results suggest that CaM and CaM-dependent MLCK function during larval settlement of B. amphitrite.

Regulatory function of hyperosmotic stress-induced signaling cascades in the expression of transcription factors and osmolyte transporters in freshwater Japanese eel primary gill cell culture

In the present study, we investigated the early activation of osmotic stress-related protein kinases, with the aim of characterizing their functional links with downstream effectors (i.e. transcription factors and osmolyte transporters). Freshwater eel primary gill cells were cultured in hypertonic medium (500 mosmol l(-1)) for 6 h. Protein lysates and total RNA were collected for western blotting and quantitative real-time PCR assays. In this study, the osmotic challenge stimulated histone H3 phosphorylation, various signaling pathways (i.e. ERK1/2, p38 MAPK, JNK, CREB, MARCKS and MLCK) and expression of some downstream effectors (i.e. Na(+)/K(+)-ATPase, TauT and Ostf). Increased phosphorylation of acetylated histone is known to promote chromatin relaxation for global gene transcription, probably leading to the activation of downstream effectors for osmotic responses. In addition, the importance of the p38 MAPK and MLCK pathways in the regulation of the expression of Na(+)/K(+)-ATPase and TauT was demonstrated. Inhibition of the p38 MAPK pathway by SB202190 reduced histone H3 phosphorylation and TauT mRNA expression. Moreover, inhibition of the MLCK pathway by ML-7 decreased the expression level of Na(+)/K(+)-ATPase but increased the transcript level of TauT. Collectively, the present study reveals possible functional links of osmosensing signaling cascades to the regulation of downstream effectors.

Wortmannin inhibits the increase in myofilament Ca(2+) sensitivity induced by cross-talk of endothelin-1 with norepinephrine in canine ventricular myocardium

Endothelin-1 (ET-1) modulates cardiac contractility by cross-talk with norepinephrine (NE) in canine ventricular myocardium. The present experiments were performed to investigate the influence of wortmannin that has inhibitory action on phosphatidylinositol 3-kinase (PI3-K) (IC50 = 3 nM) and myosin light chain kinase (MLCK) (IC50 = 200 nM) on Ca(2+) signaling and the inotropic effects of ET-1 induced by cross-talk with NE. Experiments were carried out in isolated canine ventricular trabeculae and indo-1/AM-loaded single ventricular cardiomyocytes. ET-1 alone elicited a transient small negative inotropic effect (NIE). In the presence of NE at low (1-10 nM) and high (100 nM) concentrations, ET-1 induced a long-lasting positive inotropic effect (PIE) or a marked sustained NIE, respectively. Wortmannin up to 300 nM did not affect the contractility; and at 1 microM and higher, it decreased the basal contraction without suppressing Ca(2+) transients. Wortmannin (1 microM) inhibited the long-lasting PIE of ET-1 without affecting the ET-1-induced increase in Ca(2+) transients. Wortmannin at the same concentration did not affect the ET-1-induced transient and sustained NIE and the PIE mediated by beta-adrenoceptor stimulation. These results imply that wortmannin exerts selective inhibitory action on the increase in myofilament Ca(2+) sensitivity induced by cross-talk of ET-1 with NE probably through an inhibition of MLCK in canine ventricular myocardium.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Mechanisms of hypotonic inhibition of the sodium, proton exchanger type 1 (NHE1) in a biliary epithelial cell line (Mz-Cha-1)

AIM

To elucidate the cellular events that results in inhibition of Na(+), H(+) exchanger type 1 (NHE1) by hypotonicity.

METHODS

Intracellular pH (pH(i)) was measured in biliary epithelial cells, with the pH-sensitive fluorochrome 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) using a spectrophotometer. Regulatory volume decrease (RVD) was analysed from confocal images. Changes in NHE1 membrane content were visualized by confocal laser scanning microscopy after transfection of Mz-Cha-1 cells with a NHE1-cMyc fusion protein.

RESULTS

In Mz-Cha-1 cells hypotonicity (-80 mmol L(-1) NaCl) inhibited endogenous Na(+), H(+) exchange. Tyrosine and serine kinase inhibitors were incapable to prevent inhibition. As several signalling pathways influence Na(+), H(+) exchange, we tested the effect of the Ca(++), Calmodulin, protein kinase C or the cAMP, protein kinase A system on inhibition of Na(+), H(+) exchange by hypotonic challenge, but neither system was involved. In contrast, cytoskeleton did influence the effect of hypotonicity. Inhibition of microtubule polymerization by colchicine prevented inhibition of NHE1, and also restored Na(+), H(+) exchange kinetics. Specific inhibition of Src kinases with PP2, attenuated pH(i) recovery rate from 1.93 +/- 0.16 pH units min(-1) (normotonic environment) to 1.02 +/- 0.50 pH units min(-1) (hypotonic environment). Membrane staining of NHE1-cMyc fusion protein was maintained after hypotonic exposure in colchicine pre-treated cells as was RVD. Microfilament inhibition by cytochalasin preserved NHE1 activity. Inhibition of phosphatidylinositol-3'-kinase was unable to restore Na(+), H(+) exchange activity.

CONCLUSION

We conclude that regulation of Na(+), H(+) exchange during RVD is mediated by cytoskeletal elements. This receptor independent pathway is regulated by Src.

Ca2+-Calmodulin and Janus Kinase 2 Are Required for Activation of Sodium-Proton Exchange by the Gi-Coupled 5-Hydroxytryptamine1aReceptor

The type 1 sodium-proton exchanger (NHE-1) is expressed ubiquitously and regulates key cellular functions, including mitogenesis, cell volume, and intracellular pH. Despite its importance, the signaling pathways that regulate NHE-1 remain incompletely defined. In this work, we present evidence that stimulation of the 5-hydroxytryptamine 1A (5-HT1A) receptor results in the formation of a signaling complex that includes activated Janus kinase 2 (Jak2), Ca2+/calmodulin (CaM), and NHE-1, and which involves tyrosine phosphorylation of CaM. The signaling pathway also involves rapid agonist-induced association of CaM and NHE-1 as assessed by coimmunoprecipitation studies and by bioluminescence resonance energy transfer studies in living cells. We propose that NHE-1 is activated through this pathway: 5-HT1A receptor --> G(i2)alpha and/or G(i3)alpha --> Jak2 activation --> tyrosine phosphorylation of CaM --> increased binding of CaM to NHE-1 --> induction of a conformational change in NHE-1 that unmasks an obscured proton-sensing and/or proton-transporting region of NHE-1 --> activation of NHE-1. The G(i/o)-coupled 5-HT1A receptor now joins a handful of Gq-coupled receptors and hypertonic shock as upstream activators of this emerging pathway. In the course of this work, we have presented clear evidence that CaM can be activated through tyrosine phosphorylation in the absence of a significant role for elevated intracellular Ca2+. We have also shown for the first time that the association of CaM with NHE-1 in living cells is a dynamic process.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

Osmotic stress and the cytoskeleton: the R(h)ole of Rho GTPases

Hyperosmotic stress initiates a variety of compensatory and adaptive responses, which either serve to restore near-normal volume or remodel and reinforce the cell structure to withstand the physical challenge. The latter response is brought about by the reorganization of the cytoskeleton; however, the underlying mechanisms are not well understood. Recent research has provided major breakthroughs in our knowledge about the link between message and structure, i.e. between signalling and cytoskeletal remodelling, predominantly in the context of cell migration. The major components of this progress are the in-depth characterization of Rho family small GTPases, master regulators of the cytoskeleton, and the discovery of the actin-related protein 2/3 complex, a signalling-sensitive structural element of the actin polymerization machinery. The primary aim of this review is to find the place of these novel and crucial players in osmotically induced (volume-dependent) remodelling of the cytoskeleton. We aim to address three questions: (1) What are the major structural changes in the cytoskeleton under hyperosmotic conditions? (2) Are the Rho family small GTPases (Rho, Rac and Cdc42) regulated by osmotic stress, and if so, by what mechanisms? (3) Are Rho GTPases involved, as mediators, in major adaptive responses, including cytoskeleton rearrangement, changes in ion transport and genetic reprogramming? Our answers will show how fragmentary our current knowledge is in these areas. Therefore, this overview has been written with the hardly disguised intention that it might foster further research in this field by highlighting some intriguing questions.

The Na+/H+ exchanger NHE1 in stress-induced signal transduction: implications for cell proliferation and cell death

The ubiquitous plasma membrane Na+/H+ exchanger NHE1 is highly conserved across vertebrate species and is extensively characterized as a major membrane transport mechanism in the regulation of cellular pH and volume. In recent years, the understanding of the role of NHE1 in regulating cell function has expanded from one of a household protein involved in ion homeostasis to that of a multifaceted regulator and/or modulator of a wide variety of cell functions. NHE1 plays pivotal roles in response to a number of important physiological stress conditions which, in addition to cell shrinkage and acidification, include hypoxia and mechanical stimuli, such as cell stretch. It has recently become apparent that NHE1-mediated modulation of, e.g., cell migration, morphology, proliferation, and death results not only from NHE1-mediated changes in pHi, cell volume, and/or [Na+]i, but also from direct protein-protein interactions with, e.g., ezrin/radixin/moesin (ERM) proteins and regulation of cellular signaling events, including the activity of mitogen-activated protein kinases (MAPKs) and Akt/protein kinase B (PKB). The aim of this review is to present and discuss new findings implicating NHE1 activation as a central signaling event activated by stress conditions and modulating cell proliferation and death. The pathophysiological importance of NHE1 in modulating the balance between cell proliferation and cell death in cancer and in ischemia/severe hypoxia will also be briefly addressed.

Cell-volume-dependent vascular smooth muscle contraction: role of Na+, K+, 2Cl- cotransport, intracellular Cl- and L-type Ca2+ channels

This study elucidates the role of cell volume in contractions of endothelium-denuded vascular smooth muscle rings (VSMR) from the rat aorta. We observed that hyposmotic swelling as well as hyper- and isosmotic shrinkage led to VSMR contractions. Swelling-induced contractions were accompanied by activation of Ca2+ influx and were abolished by nifedipine and verapamil. In contrast, contractions of shrunken cells were insensitive to the presence of L-type channel inhibitors and occurred in the absence of Ca2+ o. Thirty minutes preincubation with bumetanide, a potent Na+, K+, CI- cotransport (NKCC) inhibitor, decreased Cl(-)i content, nifedipine-sensitive 45Ca uptake and contractions triggered by modest depolarization ([K+]o = 36 mM). Elevation of [K+]o to 66 mM completely abolished the effect of bumetanide on these parameters. Bumetanide almost completely abrogated phenylephrine-induced contraction, partially suppressed contractions triggered by hyperosmotic shrinkage, but potentiated contractions of isosmotically shrunken VSMR. Our results suggest that bumetanide suppresses contraction of modestly depolarized cells via NKCC inhibition and Cl(-)i-mediated membrane hyperpolarization, whereas augmented contraction of isosmotically shrunken VSMR by bumetanide is a consequence of suppression of NKCC-mediated regulatory volume increase. The mechanism of bumetanide inhibition of contraction of phenylephrine-treated and hyperosmotically shrunken VSMR should be examined further.

Prostaglandin Signaling in the Renal Collecting Duct

Prostaglandins mediate autacrine and paracrine signaling over short distances. We used the renal collecting duct as a model system to test the hypothesis that local control of prostaglandin signaling is achieved by expressing inactivation in the same cell as synthesis. Immunocytochemical studies demonstrated that renal collecting ducts in situ express the prostaglandin (PG) synthesis enzyme, cyclooxygenase-1 (COX-1), as well as both components of prostaglandin metabolic inactivation, i.e. the prostaglandin uptake carrier prostaglandin transporter (PGT) and the enzyme 15-hydroxyprostaglandin dehydrogenase. We characterized this system further using the collecting duct cell line Madin-Darby canine kidney (MDCK), which retains COX-2 and prostaglandin dehydrogenase expression but which has lost PGT expression. When we reintroduced PGT, it was correctly sorted to the apical membrane where it altered the sidedness of prostaglandin E2 (PGE2) release, a process we call "vectorial release via sided reuptake." Importantly, although COX-2 and prostaglandin dehydrogenase are expressed in the same MDCK cell, they must be compartmentalized because even in the presence of excess dehydrogenase newly synthesized PGE2 is released largely un-oxidized. However, when PGE2 undergoes first release and then PGT-mediated reuptake, significant oxidation takes place, suggesting that PGT imports PGE2 into the prostaglandin dehydrogenase compartment. Our data are consistent with a new model that offers significant new mechanisms for the fine control of eicosanoid signaling.

Jak2 and Ca2+/calmodulin are key intermediates for bradykinin B2 receptor-mediated activation of Na+/H+ exchange in KNRK and CHO cells

Na(+)/H(+) exchangers are ubiquitous in mammalian cells, carrying out key functions, such as cell volume defense, acid-base homeostasis, and regulation of the cytoskeleton. We used two screening technologies (FLIPR and microphysiometry) to characterize the signal transduction pathway used by the bradykinin B(2) receptor to activate Na(+)/H(+) exchange in two cell lines, KNRK and CHO. In both cell types, B(2) receptor activation resulted in rapid increases in the rate of proton extrusion that were sodium-dependent and could be blocked by the Na(+)/H(+) exchange inhibitors EIPA and MIA or by replacing extracellular sodium with TMA. Activation of Na(+)/H(+) exchange by bradykinin was concentration-dependent and could be blocked by the selective B(2) receptor antagonist HOE140, but not by the B(1) receptor antagonist des-Arg10-HOE140. Inhibitors of Jak2 tyrosine kinase (genistein and AG490) and of CAM (W-7 and calmidazolium) attenuated bradykinin-induced activation of Na(+)/H(+) exchange. Bradykinin induced formation of a complex between CAM and Jak2, supporting a regulatory role for Jak2 and CAM in the activation of Na(+)/H(+) exchange in KNRK and CHO cells. We propose that this pathway (B(2) receptor --> Jak2 --> CAM --> Na(+)/H(+) exchanger) is a fundamental regulator of Na(+)/H(+) exchange activity.

Molecular cloning of NHE1 from winter flounder RBCs: activation by osmotic shrinkage, cAMP, and calyculin A

In this report, we describe the cloning, cellular localization, and functional characteristics of Na(+)/H(+) exchanger 1 (NHE1) from red blood cells of the winter flounder Pseudopleuronectes americanus (paNHE1). The paNHE1 protein localizes primarily to the marginal band and exhibits a 74% similarity to the trout beta-NHE, and 65% to the human NHE1 (hNHE1). Functionally, paNHE1 shares characteristics of both beta-NHE and hNHE1 in that it is activated both by manipulations that increase cAMP and by cell shrinkage, respectively. In accordance, the paNHE1 protein exhibits both protein kinase A consensus sites as in beta-NHE and a region of high homology to that required for shrinkage-dependent activation of hNHE1. After shrinkage-dependent activation of paNHE1 and resulting activation of a Cl(-)/HCO(3)(-) exchanger, their parallel operation results in net uptake of NaCl and osmotically obliged water. Activation of paNHE1 by cAMP is at least additive to that elicited by osmotic shrinkage, suggesting that these stimuli regulate paNHE1 by distinct mechanisms. Finally, exposure to the serine/threonine phosphatase inhibitor calyculin A potently activates paNHE1, and this activation is also additive to that induced by shrinkage or cAMP.

Hypertonicity activates Na+/H+ exchange through Janus kinase 2 and calmodulin

The type 1 sodium-hydrogen exchanger (NHE-1) is a ubiquitous electroneutral membrane transporter that is activated by hypertonicity in many cells. NHE-1 may be an important pathway for Na(+) entry during volume restoration, yet the molecular mechanisms underlying the osmotic regulation of NHE-1 are poorly understood. In the present study we conducted a screen for important signaling molecules that could be involved in hypertonicity-induced activation of NHE-1 in CHO-K1 cells. Hypertonicity rapidly activated NHE-1 in a concentration-dependent manner as assessed by proton microphysiometry and by measurements of intracellular pH on a FLIPR (fluorometric imaging plate reader). Inhibitors of Ca(2+)/calmodulin (CaM) and Janus kinase 2 (Jak2) attenuated this activation, whereas neither calcium chelation nor inhibitors of protein kinase C, the Ras-ERK1/2 pathway, Src kinase, and Ca(2+)/calmodulin-dependent enzymes had significant effects. Hypertonicity also resulted in the rapid tyrosine phosphorylation of Jak2 and STAT3 (the major substrate of Jak2) and CaM. Phosphorylation of Jak2 and CaM were blocked by AG490, an inhibitor of Jak2. Immunoprecipitation studies showed that hypertonicity stimulates the assembly of a signaling complex that includes CaM, Jak2, and NHE-1. Formation of the complex could be blocked by AG490. Thus, we propose that hypertonicity induces activation of NHE-1 in CHO-K1 cells in large part through the following pathway: hypertonicity --> Jak2 phosphorylation and activation --> tyrosine phosphorylation of CaM --> association of CaM with NHE-1 --> NHE-1 activation.

Evidence for involvement of the putative first extracellular loop in differential volume sensitivity of the Na+/H+ exchangers NHE1 and NHE2

We studied hyperosmolarity-induced changes in cell volume and cytoplasmic pH in PS120 cells expressing Na(+)/H(+) exchanger (NHE) isoforms and their mutants. Change in cell volume was estimated by measuring change in cell height by means of confocal microscopy. Regulatory volume increase (RVI) and cytoplasmic alkalinization were observed in cells expressing NHE1 but not in cells expressing NHE2 or NHE3. Studies using chimeric exchangers revealed that the membrane domain of the exchanger is responsible for the difference in volume sensitivity between NHE1 and NHE2. Although deletion or point mutation within the first extracellular loop of NHE1 did not affect RVI and alkalinization, point mutations within the corresponding region of NHE2, particularly a region containing aa 41-53, as well as replacement of the N-terminus of NHE2 with the corresponding region of NHE1, rendered NHE2 responsive to the activating effect of cell shrinkage. Thus, the membrane domain plays an important role in the response of the exchanger to cell shrinkage. The data suggest that the putative first extracellular loop of NHE2, but not that of NHE1, may exert an inhibitory influence on hyperosmolarity-induced activation of the exchanger and thereby block RVI.

Inhibition of Rho-kinase reduces renal Na-H exchanger activity and causes natriuresis in rat

Rho-kinase regulates the actin cytoskeleton and therefore modulates transport. The role of Rho-kinase in Na-H exchanger (NHE) activity of rat proximal convoluted tubules (PCTs) was investigated using (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632), a specific inhibitor of Rho-kinase. In spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) rats, apical and basolateral NHE activities were determined by measuring cell pH recovery following luminal NH4+ prepulse and basolateral sodium removal, respectively. Apical NHE activity was greater in 8 to 9 week old hypertensive SHR compared with WKY. Although Y-27632 suppressed pH(i) recovery in both strains, sensitivity was 50-fold higher in adult SHR. Y-27632 suppressed basolateral NHE in both strains with similar sensitivity. Apical NHE activity was not greater in 5-week-old, not yet hypertensive, SHR rats compared with WKY. In clearance studies, Na excretion was less in SHR than in WKY rats. Y-27632 increased Na excretion and fractional excretion Na in both strains but more so in SHR. (22)Na uptake of the brush border membrane vesicle taken from Y-27632-treated rats decreased more than that from vehicle-treated animals in both adult SHR and WKY. We conclude that apical NHE activity is increased in SHR PCT compared with controls and that inhibition of Rho-kinase reduces PCT NHE activities and causes natriuresis.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

Rho family GTP binding proteins are involved in the regulatory volume decrease process in NIH3T3 mouse fibroblasts

The role of Rho GTPases in the regulatory volume decrease (RVD) process following osmotic cell swelling is controversial and has so far only been investigated for the swelling-activated Cl- efflux. We investigated the involvement of RhoA in the RVD process in NIH3T3 mouse fibroblasts, using wild-type cells and three clones expressing constitutively active RhoA (RhoAV14). RhoAV14 expression resulted in an up to fourfold increase in the rate of RVD, measured by large-angle light scattering. The increase in RVD rate correlated with RhoAV14 expression. RVD in wild-type cells was unaffected by the Rho kinase inhibitor Y-27632 and the phosphatidyl-inositol 3 kinase (PI3K) inhibitor wortmannin. The maximal rates of swelling-activated K+ (86 Rb+ as tracer) and taurine ([3H]taurine as tracer) efflux after a 30 % reduction in extracellular osmolarity were increased about twofold in cells with maximal RhoAV14 expression compared to wild-type cells, but were unaffected by Y-27632. The volume set points for activation of release of both osmolytes appeared to be reduced by RhoAV14 expression. The maximal taurine efflux rate constant was potentiated by the tyrosine phosphatase inhibitor Na(3)VO(4), and inhibited by the tyrosine kinase inhibitor genistein. The magnitude of the swelling-activated Cl- current (I(Cl,swell) ) was higher in RhoAV14 than in wild-type cells after a 7.5 % reduction in extracellular osmolarity, but, in contrast to 86Rb+ and [3H]taurine efflux, similar in both strains after a 30 % reduction in extracellular osmolarity. I(Cl,swell) was inhibited by Y-27632 and strongly potentiated by the myosin light chain kinase inhibitors ML-7 and AV25. It is suggested that RhoA, although not the volume sensor per se, is an important upstream modulator shared by multiple swelling-activated channels on which RhoA exerts its effects via divergent signalling pathways.

Low tonicity mediates a downregulation of cyclooxygenase-1 expression by furosemide in the rat renal papilla

It is well known that loop diuretics enhance the renal excretion of prostanoids; therefore, this study aimed to characterize the influence of loop diuretics on the intrarenal expression of cyclooxygenases, which are the key enzymes for prostanoid formation. Male Sprague-Dawley rats were infused with furosemide (12 mg/kg per d) for 6 d, and the expression of cyclooxygenase-1 and -2 (Cox-1 and Cox-2) was analyzed in the different kidney zones. Furosemide increased Cox-2 mRNA expression approximately twofold in the cortex, but it left Cox-1 mRNA expression unaltered there. In the outer medulla, furosemide changed neither Cox-1 nor Cox-2 mRNA expression. In the inner medulla, however, furosemide decreased Cox-1 and Cox-2 mRNA levels to approximately 30% and 60% of their control levels, respectively. The downregulation of mRNA was paralleled by a decrease of Cox protein in the collecting ducts and interstitial cells. Moreover, tissue prostaglandin E(2) (PGE(2)) concentrations in the papilla were markedly decreased by furosemide to about 30% of the control level. Furosemide lowered urine osmolality from 1550 mosmol/kg to 480 mosmol/kg; therefore, further consideration was given to the influence of tonicity as a possible mediator of the effects of furosemide on the Cox expression. Water loading was therefore used to reduce the medullary tonicity by a second maneuver. Water loading led to a similar reduction in papillary Cox mRNA expression and PGE(2) content like furosemide. To investigate the influence of the osmolarity on the expression of Cox and the production of PGE(2) under defined in vitro conditions, inner medullary collecting duct cells were incubated with culture medium containing graded amounts of NaCl ranging from 200 mmol/L to 600 mmol/L, and Cox-1 and Cox-2 mRNA abundance were determined after 24 h an 48 h. Cox-1 and Cox-2 mRNA abundance changed in parallel with the osmolarity. The data suggest that loop diuretics decrease the expression of cyclooxygenases and consequently tissue PGE(2) concentrations in the kidney inner medulla. This effect could be related to the breakdown of the papillary osmotic gradient induced by loop diuretics.

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

Rat renal function four days after bile-duct ligation: effects of indomethacin and vasoactive agents

The purpose of this study was to assess the effects of the cyclooxygenase inhibitor, indomethacin, and some vasoactive agents on the renal functional parameters during the early liver injury induced by four days bile duct ligation (BDL). Wistar rats with four days-BDL and control-sham operated were used. Renal function was measured in anesthetized rat treated with a single dose of indomethacin (control, 0.3, 1.0, 3.0 mg/kg b.w.; i.v.) one hour before clearance studies. Sulindac effects were also evaluated (5 mg/kg b.w., i.p). Isolated rat kidney preparations from control and BDL donor rats were used to study renal vascular response to noradrenaline, carbachol or sodium nitroprusside. The bile duct ligation promoted a diminished renal cortical plasma flow (RCPF) on the fourth day post surgery accompanied with a diminution in the glomerular filtration rate (GFR), increased filtration fraction and increased fractional excretion of water and sodium. Indomethacin 0.3 mg/kg induced an increase in GFR and RCPF, maintaining the high filtration fraction in BDL rats. The other doses did not alter these parameters as compared with bile duct ligated rats without treatment, but indomethacin 3 mg/kg caused a significant increase in filtration fraction. Indomethacin induced dose-dependent diminution in natriuresis in sham and BDL groups. Sulindac did not modify hemodynamic parameters, but induced antinatriuresis and antidiuresis in both experimental groups. Maximal vascular responses to noradrenaline measured in isolated rat kidneys were statistically diminished in BDL-rats as compared with controls (C, n=7; 35.0+/-2.3 mmHg ml(-1) min; BDL-rats, n=5; 23.8+/-0.7 mmHg ml(-1) min; p<0.02), without changes in EC15. Maximal relaxation induced by sodium nitroprusside in the phenylephrine (PHE)-pre-constricted renal vasculature in control preparations did not differ from that observed in BDL group (C: n=6; 49.5+/-2.3%). Values of EC50 were 1.26+/-0.07 microM (n=6) in control preparations and 0.34+/-0.03 microM (n=4) in kidneys from BDL-rats (p<0.001). Carbachol induced a biphasic relaxation of PHE-pre-constricted renal vasculature. No differences in maximal responses were found. EC50 value of the second phase in BDL group was significantly decreased compared to control preparations (C: n=6, 0.47+/-0.05 microM; BDL: n=6, 0.22+/-0.03 microM p<0.001). The present results show that the altered renal function after a short time post bile duct ligation is determined, at least in part, by increased release of arachidonic derivatives in vascular bed and tubular cells. At this stage of liver injury, the alteration in the renal vascular response to different vasoactive agents is remarkable.

Mechanisms of cell volume regulation and possible nature of the cell volume sensor

In animal organisms, cell volume undergoes dynamic changes in many physiological and pathological processes. To protect themselves against lysis and apoptosis and to maintain an optimal concentration of intracellular enzymes and metabolites, most animal cells actively regulate their volume. In the present review, we shortly summarize the data on ion transport mechanisms involved in regulatory volume decrease (RVD) and regulatory volume increase (RVI) with an emphasis on unresolved aspects of this problem such as: (i) how cells sense their volume changes; (ii) what signals are generated upon cell volume alterations; and (iii) how these signals are transferred to the ion transport systems executing cell volume regulation.

Propofol protection of sodium-hydrogen exchange activity sustains glutamate uptake during oxidative stress

We investigated the role of intracellular pH in protection by propofol of glutamate uptake during oxidative stress. Exposure of primary astrocyte cultures to tert-butylhydroperoxide (t-BOOH, 300 microM) decreased the initial rate of Na-dependent glutamate uptake. Either propofol or alpha-tocopherol, administered 30 min after t-BOOH, attenuated this transport inhibition. These lipophilic antioxidants protected glutamate uptake whether the medium contained 25 mM bicarbonate or was nominally bicarbonate-free. t-BOOH also inhibited Na/H exchanger isoform 1 (NHE1) activation by intracellular protons and propofol prevented this inhibition. Blockade of NHE1 by the potent antagonist, 5-(N-ethyl-N-isopropyl) amiloride (1 microM), abolished the protective effects of small concentrations of propofol (1 microM) and alpha-tocopherol (40 microM) on glutamate uptake during oxidative stress in bicarbonate-free medium. 5-(N-ethyl-N-isopropyl) amiloride had no effect on antioxidant rescue of glutamate transport in medium containing 25 mM bicarbonate. These results indicate that regulation of intracellular pH may contribute to neuroprotection by propofol and other lipophilic antioxidants. Propofol concentrations that are associated with anesthesia and neuroprotection may prevent intracellular acidification during oxidative stress by preserving the NHE1 response to cytosolic protons. However, if intracellular acidification occurs nonetheless, then propofol protection of glutamate uptake activity becomes less effective and the extracellular glutamate concentration may increase to neurotoxic levels.

IMPLICATIONS

Anesthetic concentrations of propofol maintain the capacity of brain cells to extrude protons during oxidative stress. However, if intracellular acidification occurs nonetheless, then propofol's protection of glutamate clearance mechanisms from oxidative damage becomes attenuated, and extracellular glutamate concentration may increase to neurotoxic levels.

The cytoskeleton and cell volume regulation

Although the precise mechanisms have yet to be elucidated, early events in osmotic signal transduction may involve the clustering of cell surface receptors, initiating downstream signaling events such as assembly of focal adhesion complexes, and activation of, e.g. Rho family GTPases, phospholipases, lipid kinases, and tyrosine- and serine/threonine protein kinases. In the present paper, we briefly review recent evidence regarding the possible relation between such signaling events, the F-actin cytoskeleton, and volume-regulatory membrane transporters, focusing primarily on our own work in Ehrlich ascites tumer cells (EATC). In EATC, cell shrinkage is associated with an increase, and cell swelling with a decrease in F-actin content, respectively. The role of the F-actin cytoskeleton in cell volume regulation in various cell types has largely been investigated using cytochalasins to disrupt F-actin and highly varying effects have been reported. Findings in EATC show that the effect of cytochalasin treatment cannot always be assumed to be F-actin depolymerization, and that, moreover, there is no well-defined correlation between effects of cytochalasins on F-actin content and their effects on F-actin organization and cell morphology. At a concentration verified to depolymerize F-actin, cytochalasin B (CB), but not cytochalasin D (CD), inhibited the regulatory volume decrease (RVD) and regulatory volume increase (RVI) processes in EATC. This suggests that the effect of CB is related to an effect other than F-actin depolymerization, possibly its F-actin severing activity.

Macromolecular crowding and its role as intracellular signalling of cell volume regulation

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.

Bradykinin B2 receptors activate Na+/H+ exchange in mIMCD-3 cells via Janus kinase 2 and Ca2+/calmodulin

We used a cultured murine cell model of the inner medullary collecting duct (mIMCD-3 cells) to examine the regulation of the ubiquitous sodium-proton exchanger, Na+/H+ exchanger isoform 1 (NHE-1), by a prototypical G protein-coupled receptor, the bradykinin B2 receptor. Bradykinin rapidly activates NHE-1 in a concentration-dependent manner as assessed by proton microphysiometry of quiescent cells and by 2'-7'-bis[2-carboxymethyl]-5(6)-carboxyfluorescein fluorescence measuring the accelerated rate of pH(i) recovery from an imposed acid load. The activation of NHE-1 is blocked by inhibitors of the bradykinin B2 receptor, phospholipase C, Ca2+/calmodulin (CaM), and Janus kinase 2 (Jak2), but not by pertussis toxin or by inhibitors of protein kinase C and phosphatidylinositol 3'-kinase. Immunoprecipitation studies showed that bradykinin stimulates the assembly of a signal transduction complex that includes CaM, Jak2, and NHE-1. CaM appears to be a direct substrate for phosphorylation by Jak2 as measured by an in vitro kinase assay. We propose that Jak2 is a new indirect regulator of NHE-1 activity, which modulates the activity of NHE-1 by increasing the tyrosine phosphorylation of CaM and most likely by increasing the binding of CaM to NHE-1.

Genistein inhibits osmotic activation of Na(+) /H( +) exchange in human platelets

The osmotic shrinkage is an important activator of the Na(+)/H( *) exchanger. The intracellular signaling mechanisms by which shrinkage changes intracellular pH have not been fully elucidated. In human platelets, the removal of calcium did not prevent the osmotic activation of the exchanger. The increase of pH(i) after an hyperosmotic stress was reduced by W-7 (63 micromol l(-1)), and by ML-7 (25 micromol l(-1)), inhibitors of responses mediated by calmodulin or by myosin light chain kinase, but the high concentrations needed suggested that non-specific effects could be involved. Although the exchanger was quiescent during preincubation in hypertonic sodium free solutions, some steps of the signal transduction chain that links the shrinkage to the exchanger activation suffers a modification. Therefore, upon exposure to isotonic sodium-containing media, the rate of recovery from acid loads was increased. The presence of genistein (100 micromol l( -1)) during the preincubation inhibited this activation of Na(+)/H( +) exchanger. We propose that shrinkage induce activation of tyrosine kinases, which in turn leads to the activation of Na(+)/H(+) exchanger and contributes to the restoration of cell volume in human platelets.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Modulation of Na(+) x H(+) exchange by osmotic shock in isolated bovinearticular chondrocytes

The effects of hyperosmotic shock on intracellular pH (pHi) have been characterized in bovine articular chondrocytes. Osmotic shock is one of a variety of physicochemical stimuli experienced by chondrocytes upon cartilage loading. Cells were isolated from their extracellular matrix, and loaded with the pH-sensitive fluorophore 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Hyperosmotic shocks were imposed by addition of KCl or sucrose to the extracellular medium. For cells at steady-state pHi, resuspension in hyperosmotic solutions elicited an alkalinization, which was significantly inhibited by removal of extracellular Na+ ions, or treatment with amiloride (1 mM) or HOE-694 (10 microM), both inhibitors of Na+ x H+ exchange. For cells acidified by ammonium rebound, recovery of pHi towards resting levels was significantly stimulated by exposure to hyperosmotic solutions, and the effect was again attenuated by inhibition of Na+ x H+ exchange. Determination of the rate of acid extrusion at different levels of acidification indicated that the affinity of acid extrusion systems for H+ ions was increased by hypertonic shock. The response to hyperosmotic media could be abolished by treatment of chondrocytes with the non-specific kinase inhibitor staurosporine (10 nM), while the phosphatase inhibitor okadaic acid (1 mM) was able to augment recovery rates to values similar to those measured under hyperosmotic conditions. The osmotic sensitivity of recovery was unaffected by exposure to the protein kinase C inhibitor calphostin C, but was abolished in cells treated with ML-7, a specific inhibitor of myosin light chain kinase. These results confirm that - as for other components of mechanical load - increased osmolarity can modulate the activity of Na+ x H+ exchange, in this case by altered patterns of phosphorylation of transporter-associated myosin. The changes of pHi which will result dictate in part the rate of cartilage macromolecule synthesis.

Calcium and osmotic regulation of the Na+/H+ exchanger in neonatal ventricular myocytes

Intracellular pH regulation in primary cultures of neonatal cardiac myocytes has been characterized. Myocytes were exposed to hyperosmolar solutions to examine the effects on pH regulation by the Na+/H+ exchanger. Exposure to 100 mM NaCl, sorbitol, N-methyl-D-glucamine, or choline chloride all caused significant increases in steady state pHi in myocytes. Omission of extracellular calcium or administration of calmodulin antagonists reduced the osmotic activation of the exchanger. The myosin light-chain inhibitor ML-7 completely blocked osmotic activation of the exchanger suggesting that myosin light-chain kinase is involved in osmotic activation of the exchanger in the myocardium. The calmodulin-dependent protein kinase II inhibitor KN-93 inhibited the rate of recovery from an acute acid load as did trifluoperazine (TFP) and the calmodulin blocker W7, [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide]. Addition of the calcium ionophore ionomycin caused a large increase in resting pHi in isolated myocytes. However, this effect was largely resistant to HMA (5-(N,N-hexamethylene)-amiloride) indicating that an alternative mechanism of pHi regulation is responsible. The results demonstrate that the Na+/H+ exchanger of the neonatal myocardium is responsive to calcium and osmotically responsive pathways and that myosin light-chain kinase is a key protein involved in mediating the osmotic response.

Extracellular Cl(-) modulates shrinkage-induced activation of Na(+)/H(+) exchanger in rat mesangial cells

To examine the effect of hyperosmolality on Na(+)/H(+) exchanger (NHE) activity in mesangial cells (MCs), we used a pH-sensitive dye, 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-AM, to measure intracellular pH (pH(i)) in a single MC from rat glomeruli. All the experiments were performed in CO(2)/HCO(-)(3)-free HEPES solutions. Exposure of MCs to hyperosmotic HEPES solutions (500 mosmol/kgH(2)O) treated with mannitol caused cell alkalinization. The hyperosmolality-induced cell alkalinization was inhibited by 100 microM ethylisopropylamiloride, a specific NHE inhibitor, and was dependent on extracellular Na(+). The hyperosmolality shifted the Na(+)-dependent acid extrusion rate vs. pH(i) by 0.15-0.3 pH units in the alkaline direction. Removal of extracellular Cl(-) by replacement with gluconate completely abolished the rate of cell alkalinization induced by hyperosmolality and inhibited the Na(+)-dependent acid extrusion rate, whereas, under isosmotic conditions, it caused no effect on Na(+)-dependent pH(i) recovery rate or Na(+)-dependent acid extrusion rate. The Cl(-)-dependent cell alkalinization rate under hyperosmotic conditions was partially inhibited by pretreatment with 5-nitro-2-(3-phenylpropylamino)benzoic acid, DIDS, and colchicine. We conclude: 1) in MCs, hyperosmolality activates NHE to cause cell alkalinization, 2) the acid extrusion rate via NHE is greater under hyperosmotic conditions than under isosmotic conditions at a wide range of pH(i), 3) the NHE activation under hyperosmotic conditions, but not under isosmotic conditions, requires extracellular Cl(-), and 4) the Cl(-)-dependent NHE activation under hyperosmotic conditions partly occurs via Cl(-) channel and microtubule-dependent processes.

ATP-mediated glia signaling

Glia calcium signaling has recently been identified as a potent modulator of synaptic transmission. We show here that the spatial expansion of calcium waves is mediated by ATP and subsequent activation of purinergic receptors. Ectopic expression of gap junction proteins, connexins (Cxs), leads to an increase in both ATP release and the radius of calcium wave propagation. Cx expression was also associated with a phenotypic transformation, and cortical neurons extended longer neurites when co-cultured with Cx-expressing than with Cx-deficient cells. Purinergic receptor activation mediated both these effects, because treatment with receptor antagonists restored the glia phenotype and slowed neurite outgrowth. These results identify a key role of ATP in both short-term calcium signaling events and in long-term differentiation regulated by glia.

Myosin light chain phosphorylation-dependent modulation of volume-regulated anion channels in macrovascular endothelium

The Rho/Rho-associated kinase (ROK) pathway has been shown to modulate volume-regulated anion channels (VRAC) in cultured calf pulmonary artery endothelial (CPAE) cells. Since Rho/ROK can increase myosin light chain phosphorylation, we have now studied the effects of inhibitors of myosin light chain kinase (MLCK) or myosin light chain phosphatase (MLCP) on VRAC in CPAE. Application of ML-9, an MLCK inhibitor, inhibited VRAC, both when applied extracellularly or when dialyzed into the cell. A similar inhibitory effect was obtained by dialyzing the cells with AV25, a specific MLCK inhibitory peptide. Conversely, NIPP1(191-210), an MLCP inhibitory peptide, potentiated the activation of VRAC by a 25% hypotonic stimulus. These data indicate that activation of VRAC is modulated by MLC phosphorylation.

JNK is a volume-sensitive kinase that phosphorylates the Na-K-2Cl cotransporter in vitro

Cell shrinkage phosphorylates and activates the Na-K-2Cl cotransporter (NKCC1), indicating the presence of a volume-sensitive protein kinase. To identify this kinase, extracts of normal and shrunken aortic endothelial cells were screened for phosphorylation of NKCC1 fusion proteins in an in-the-gel kinase assay. Hypertonic shrinkage activated a 46-kDa kinase that phosphorylated an NH2-terminal fusion protein, with weaker phosphorylation of a COOH-terminal fusion protein. This cytosolic kinase was activated by both hypertonic and isosmotic shrinkage, indicating regulation by cell volume rather than osmolarity. Subsequent studies identified this kinase as c-Jun NH2-terminal kinase (JNK). Immunoblotting revealed increased JNK activity in shrunken cells; there was volume-sensitive phosphorylation of NH2-terminal c-Jun fusion protein; immunoprecipitation of JNK from shrunken cells but not normal cells phosphorylated NKCC1 in gel kinase assays; and treatment of cells with tumor necrosis factor, a known activator of JNK, mimicked the effect of hypertonicity. We conclude that JNK is a volume-sensitive kinase in endothelial cells that phosphorylates NKCC1 in vitro. This is the first demonstration of a volume-sensitive protein kinase capable of phosphorylating a volume-regulatory transporter.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Cell shrinkage regulates Src kinases and induces tyrosine phosphorylation of cortactin, independent of the osmotic regulation of Na+/H+ exchangers

The signaling pathways by which cell volume regulates ion transporters, e.g. Na+/H+ exchangers (NHEs), and affects cytoskeletal organization are poorly understood. We have previously shown that shrinkage induces tyrosine phosphorylation in CHO cells, predominantly in an 85-kDa band. To identify volume-sensitive kinases and their substrates, we investigated the effect of hypertonicity on members of the Src kinase family. Hyperosmolarity stimulated Fyn and inhibited Src. Fyn activation was also observed in nystatin-permeabilized cells, where shrinkage cannot induce intracellular alkalinization. In contrast, osmotic inhibition of Src was prevented by permeabilization or by inhibiting NHE-1. PP1, a selective Src family inhibitor, strongly reduced the hypertonicity-induced tyrosine phosphorylation. We identified one of the major targets of the osmotic stress-elicited phosphorylation as cortactin, an 85-kDa actin-binding protein and well known Src family substrate. Cortactin phosphorylation was triggered by shrinkage and not by changes in osmolarity or pHi and was abrogated by PP1. Hyperosmotic cortactin phosphorylation was reduced in Fyn-deficient fibroblasts but remained intact in Src-deficient fibroblasts. To address the potential role of the Src family in the osmotic regulation of NHEs, we used PP1. The drug affected neither the hyperosmotic stimulation of NHE-1 nor the inhibition of NHE-3. Thus, members of the Src family are volume-sensitive enzymes that may participate in the shrinkage-related reorganization of the cytoskeleton but are probably not responsible for the osmotic regulation of NHE.

Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling

We have studied the role of actin fiber assembly on calcium signaling in astrocytes. We found that (1) after astrocytes have been placed in culture, it takes several hours for organization of the definitive actin cytoskeleton. Actin organization and the number of cells engaged in calcium signaling increased in parallel. (2) Disruption of the actin cytoskeleton attenuated the calcium wave propagation; cytochalasin D treatment reduced the number of astrocytes engaged in calcium signaling. (3) Propagation of calcium waves depends on cytoskeletal function; inhibition of myosin light chain kinase suppressed wave activity. (4) Astrocytic calcium signaling is mediated by release of ATP and purinergic receptor stimulation, because agents that interfere with this cascade attenuated or reduced calcium signaling. Because purinergic receptors are fully functional shortly after plating and not affected by cytochalasin D, these observations indicate that cytoskeleton organization is a prerequisite for interastrocytic calcium signaling mediated by release of ATP.

Topological analysis of NHE1, the ubiquitous Na+/H+ exchanger using chymotryptic cleavage

Proteases, glycosidases, and impermeant biotin derivatives were used in combination with antibodies to analyze the subcellular distribution and transmembrane disposition of the Na+/H+ exchanger NHE1. Both native human NHE1 in platelets and epitope-tagged rat NHE1 transfected into antiport-deficient cells were used for these studies. The results indicated that 1) the entire population of exchangers is present on the surface membrane of unstimulated platelets, ruling out regulation by recruitment of internal stores of NHE1; 2) the putative extracellular loops near the NH2 terminus are exposed to the medium and contain all the N- and O-linked carbohydrates; 3) by contrast, the putative extracellular loops between transmembrane domains 9-10 and 11-12 are not readily accessible from the outside and may be folded within the protein, perhaps contributing to an aqueous ion transport pathway; 4) the extreme COOH terminus of the protein was found to be inaccessible to extracellular proteases, antibodies, and other impermeant reagents, consistent with a cytosolic localization; and 5) detachment of approximately 150 amino acids from the NH2-terminal end of the protein had little effect on the transport activity of NHE1.

Na-H exchange acts downstream of RhoA to regulate integrin-induced cell adhesion and spreading

The ubiquitously expressed Na-H exchanger NHE1 functions in regulating intracellular pH and cell volume. NHE1 activity is stimulated by hormones, growth factors, and activation of integrin receptors. We recently determined that NHE1 activity is also stimulated by activation of the low molecular weight GTPase RhoA and that increases in NHE1 activity are necessary for RhoA-induced formation of actin stress fibers. We now show that NHE1 acts downstream of RhoA to modulate initial steps in integrin signaling for the assembly of focal adhesions. Adhesion of CCL39 fibroblasts on fibronectin was markedly delayed in the presence of the NHE inhibitor ethylisopropylamiloride. In mutant PS120 cells, derived from CCL39 fibroblasts but lacking NHE1, adhesion was also delayed but was rescued in PS120 cells stably expressing NHE1. In the absence of NHE1 activity, cell spreading was inhibited, and the accumulation of integrins, paxillin, and vinculin at focal contacts was impaired. Additionally, tyrosine phosphorylation of p125(FAK) induced by integrin clustering was also impaired. Inactivation of RhoA with C3 transferase and inhibition of the Rho-kinase p160ROCK with the pyridine derivative Y-27632 completely abolished activation of NHE1 by integrins but not by platelet-derived growth factor. These findings indicate that NHE1 acts downstream of RhoA to contribute a previously unrecognized critical signal to proximal events in integrin-induced cytoskeletal reorganization.

Calcium-calmodulin modulation of the activity of Na+/H+ exchanger in human platelets

This study aimed at establishing the role of calmodulin in regulating pHi of human platelets under acid loads and in stimulated states. The response of human platelets to thrombin was an initial drop of pHi followed by a recovery with a significant increase above the pre-stimulation level in control experiments and a recovery to initial values in platelets maintained in the presence of 19 mmol/l TFP (trifluoperazine = 2 trifluoromethyl-10 [3'-(1 methyl-4-piperazinyl) propyl] phenothiazine). The change in pHi after 8 min was 0.130 +/- 0.030 in the control and 0.001 +/- 0.011 pH units in TFP (P < 0.05). The initial velocity of recovery from an acid load was reduced to 56.7 +/- 6% of the control (n = 6, P < 0.05) with 50 mumol/l W7 (N-(6 aminohexyl)-5-chloro-l-naphthalene sulphonamide), and to 29.7 +/- 4.3% of the control (n = 8, P < 0.05) with 19 mumol/l TFP. The initial velocity of recovery was significantly greater in recalcified platelets than in the preparations kept in the nominal absence of extracellular calcium (1.08 +/- 0.12 vs 0.66 +/- 0.12 pH units per min, P < 0.05). Lower concentration of TFP had an inhibitory effect only in the presence of calcium. The velocities of recovery reached similar values at higher TFP concentration. The significant interaction between Ca2+ and TFP concentrations indicates that the Ca-calmodulin complex, rather than an unspecified direct action of TFP, is responsible for the modulation of the Na+/H+ exchanger. These findings indicate that calcium-calmodulin participates in both the recovery of pH after an acid load and the increase of pHi in stimulated states of human platelets.