Na(+)-H(+) exchange in salivary secretory cells is controlled by an intracellular Na(+) receptor

Intracellular Ca2+ oscillation frequency and amplitude modulation mediate epithelial apical and basolateral membranes crosstalk

Since the early seminal studies on epithelial solute transport, it has been understood that there must be crosstalk among different members of the transport machinery to coordinate their activity and, thus, generate localized electrochemical gradients that force solute flow in the required direction that would otherwise be thermodynamically unfavorable. However, mechanisms underlying intracellular crosstalk remain unclear. We present evidence that crosstalk between apical and basolateral membrane transporters is mediated by intracellular Ca2+ signaling in insect renal epithelia. Ion flux across the basolateral membrane is encoded in the intracellular Ca2+ oscillation frequency and amplitude modulation and that information is used by the apical membrane to adjust ion flux accordingly. Moreover, imposing experimentally generated intracellular Ca2+ oscillation modulation causes cells to predictably adjust their ion transport properties. Our results suggest that intracellular Ca2+ oscillation frequency and amplitude modulation encode information on transmembrane ion flux that is required for crosstalk.

Regulation of ion transport from within ion transit pathways

All cells must control the activities of their ion channels and transporters to maintain physiologically appropriate gradients of solutes and ions. The complexity of underlying regulatory mechanisms is staggering, as exemplified by insulin regulation of transporter trafficking. Simpler strategies occur in single-cell organisms, where subsets of transporters act as solute sensors to regulate expression of their active homologues. This Viewpoint highlights still simpler mechanisms by which Na transporters use their own transport sites as sensors for regulation. The underlying principle is inherent to Na/K pumps in which aspartate phosphorylation and dephosphorylation are controlled by occupation of transport sites for Na and K, respectively. By this same principle, Na binding to transport sites can control intrinsic inactivation reactions that are in turn modified by extrinsic signaling factors. Cardiac Na/Ca exchangers (NCX1s) and Na/K pumps are the best examples. Inactivation of NCX1 occurs when cytoplasmic Na sites are fully occupied and is regulated by lipid signaling. Inactivation of cardiac Na/K pumps occurs when cytoplasmic Na-binding sites are not fully occupied, and inactivation is in turn regulated by Ca signaling. Potentially, Na/H exchangers (NHEs) and epithelial Na channels (ENaCs) are regulated similarly. Extracellular protons and cytoplasmic Na ions oppose secondary activation of NHEs by cytoplasmic protons. ENaCs undergo inactivation as cytoplasmic Na rises, and small diffusible molecules of an unidentified nature are likely involved. Multiple other ion channels have recently been shown to be regulated by transiting ions, thereby underscoring that ion permeation and channel gating need not be independent processes.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Na/K pump inactivation, subsarcolemmal Na measurements, and cytoplasmic ion turnover kinetics contradict restricted Na spaces in murine cardiac myocytes

The Na/K pump exports cytoplasmic Na ions while importing K ions, and its activity is thought to be affected by restricted intracellular Na diffusion in cardiac myocytes. Lu and Hilgemann find instead that the pump can enter an inactivated state and that inactivation can be relieved by cytoplasmic Na.

Decades ago, it was proposed that Na transport in cardiac myocytes is modulated by large changes in cytoplasmic Na concentration within restricted subsarcolemmal spaces. Here, we probe this hypothesis for Na/K pumps by generating constitutive transsarcolemmal Na flux with the Na channel opener veratridine in whole-cell patch-clamp recordings. Using 25 mM Na in the patch pipette, pump currents decay strongly during continuous activation by extracellular K (τ, ∼2 s). In contradiction to depletion hypotheses, the decay becomes stronger when pump currents are decreased by hyperpolarization. Na channel currents are nearly unchanged by pump activity in these conditions, and conversely, continuous Na currents up to 0.5 nA in magnitude have negligible effects on pump currents. These outcomes are even more pronounced using 50 mM Li as a cytoplasmic Na congener. Thus, the Na/K pump current decay reflects mostly an inactivation mechanism that immobilizes Na/K pump charge movements, not cytoplasmic Na depletion. When channel currents are increased beyond 1 nA, models with unrestricted subsarcolemmal diffusion accurately predict current decay (τ ∼15 s) and reversal potential shifts observed for Na, Li, and K currents through Na channels opened by veratridine, as well as for Na, K, Cs, Li, and Cl currents recorded in nystatin-permeabilized myocytes. Ion concentrations in the pipette tip (i.e., access conductance) track without appreciable delay the current changes caused by sarcolemmal ion flux. Importantly, cytoplasmic mixing volumes, calculated from current decay kinetics, increase and decrease as expected with osmolarity changes (τ >30 s). Na/K pump current run-down over 20 min reflects a failure of pumps to recover from inactivation. Simulations reveal that pump inactivation coupled with Na-activated recovery enhances the rapidity and effectivity of Na homeostasis in cardiac myocytes. In conclusion, an autoregulatory mechanism enhances cardiac Na/K pump activity when cytoplasmic Na rises and suppresses pump activity when cytoplasmic Na declines.

Cevimeline-induced monophasic salivation from the mouse submandibular gland: decreased Na+ content in saliva results from specific and early activation of Na+/H+ exchange

Cevimeline and pilocarpine are muscarinic agonists used clinically to treat dry mouth. In this study, we explored fluid secretion from mouse submandibular glands to determine the mechanism of cevimeline, pilocarpine, and an experimentally used agent carbachol. Cevimeline evoked almost the same amount of secretion at concentrations from 30 μM to 1 mM. Pilocarpine also induced secretion at a concentration as low as 1 μM and was the most powerful secretagogue at 10 μM. Secretion was induced by carbachol at 0.1 μM, with maximum secretion at 1.0 μM. Cevimeline induced monophasic secretion at all concentrations tested, whereas higher concentrations of pilocarpine and carbachol induced secretion with variable kinetics, i.e., an initial transient high flow rate, followed by decreased secretion after 2 to 3 min. In the presence of an epithelial Na(+) channel blocker, amiloride, neither carbachol nor pilocarpine affected the Na(+) level of secreted saliva; however, it significantly increased the Na(+) content of cevimeline-induced saliva. The intracellular Ca(2+) response of acinar cells was almost identical among all three agents, although recovery after drug removal was slower for cevimeline and pilocarpine. A profound decrease in intracellular pH was observed during pilocarpine and carbachol treatment, whereas intracellular acidification induced by cevimeline was only seen in the presence of a Na(+)/H(+) exchange inhibitor. When external HCO(3)(-) was removed, cevimeline-induced saliva significantly decreased. These findings suggest that cevimeline specifically activates Na(+)/H(+) exchange and may promote Na(+) reabsorption by stabilizing epithelial sodium channel activity.

Steady-state function of the ubiquitous mammalian Na/H exchanger (NHE1) in relation to dimer coupling models with 2Na/2H stoichiometry

We describe the steady-state function of the ubiquitous mammalian Na/H exchanger (NHE)1 isoform in voltage-clamped Chinese hamster ovary cells, as well as other cells, using oscillating pH-sensitive microelectrodes to quantify proton fluxes via extracellular pH gradients. Giant excised patches could not be used as gigaseal formation disrupts NHE activity within the patch. We first analyzed forward transport at an extracellular pH of 8.2 with no cytoplasmic Na (i.e., nearly zero-trans). The extracellular Na concentration dependence is sigmoidal at a cytoplasmic pH of 6.8 with a Hill coefficient of 1.8. In contrast, at a cytoplasmic pH of 6.0, the Hill coefficient is <1, and Na dependence often appears biphasic. Results are similar for mouse skin fibroblasts and for an opossum kidney cell line that expresses the NHE3 isoform, whereas NHE1^−/− skin fibroblasts generate no proton fluxes in equivalent experiments. As proton flux is decreased by increasing cytoplasmic pH, the half-maximal concentration (K_1/2) of extracellular Na decreases less than expected for simple consecutive ion exchange models. The K_1/2 for cytoplasmic protons decreases with increasing extracellular Na, opposite to predictions of consecutive exchange models. For reverse transport, which is robust at a cytoplasmic pH of 7.6, the K_1/2 for extracellular protons decreases only a factor of 0.4 when maximal activity is decreased fivefold by reducing cytoplasmic Na. With 140 mM of extracellular Na and no cytoplasmic Na, the K_1/2 for cytoplasmic protons is 50 nM (pH 7.3; Hill coefficient, 1.5), and activity decreases only 25% with extracellular acidification from 8.5 to 7.2. Most data can be reconstructed with two very different coupled dimer models. In one model, monomers operate independently at low cytoplasmic pH but couple to translocate two ions in “parallel” at alkaline pH. In the second “serial” model, each monomer transports two ions, and translocation by one monomer allosterically promotes translocation by the paired monomer in opposite direction. We conclude that a large fraction of mammalian Na/H activity may occur with a 2Na/2H stoichiometry.

Biochemical constitution of extracellular medium is critical for control of human breast cancer MDA-MB-231 cell motility

Although voltage-gated sodium channel (VGSC) activity, upregulated significantly in strongly metastatic human breast cancer cells, has been found to potentiate a variety of in vitro metastatic cell behaviors, the mechanism(s) regulating channel expression/activity is not clear. As a step toward identifying possible serum factors that might be responsible for this, we tested whether medium in which fetal bovine serum (FBS) was substituted with a commercial serum replacement agent (SR-2), comprising insulin and bovine serum albumin, would influence the VGSC-dependent in vitro metastatic cell behaviors. Human breast cancer MDA-MB-231 cells were used as a model. Measurements of lateral motility, transverse migration and adhesion showed consistently that the channel's involvement in metastatic cell behaviors depended on the extracellular biochemical conditions. In normal medium (5% FBS), tetrodotoxin (TTX), a highly specific blocker of VGSCs, suppressed these cellular behaviors, as reported before. In contrast, in SR-2 medium, TTX had opposite effects. However, blocking endogenous insulin/insulin-like growth factor receptor signaling with AG1024 eliminated or reversed the anomalous effects of TTX. Insulin added to serum-free medium increased migration, and TTX increased it further. In conclusion, (1) the biochemical constitution of the extracellular medium had a significant impact upon breast cancer cells' in vitro metastatic behaviors and (2) insulin, in particular, controlled the mode of the functional association between cells' VGSC activity and metastatic machinery.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

Molecular Control of Cardiac Sodium Homeostasis in Health and Disease

INTRODUCTION

Cardiac myocytes utilize three high-capacity Na transport processes whose precise function can determine myocyte fate and the triggering of arrhythmias in pathological settings. We present recent results on the regulation of all three transporters that may be important for an understanding of cardiac function during ischemia/reperfusion episodes.

METHODS AND RESULTS

Refined ion selective electrode (ISE) techniques and giant patch methods were used to analyze the function of cardiac Na/K pumps, Na/Ca exchange (NCX1), and Na/H exchange (NHE1) in excised cardiac patches and intact myocytes. To consider results cohesively, simulations were developed that account for electroneutrality of the cytoplasm, ion homeostasis, water homeostasis (i.e., cell volume), and cytoplasmic pH. The Na/K pump determines the average life-time of Na ions (3-10 minutes) as well as K ions (>30 minutes) in the cytoplasm. The long time course of K homeostasis can determine the time course of myocyte volume changes after ion homeostasis is perturbed. In excised patches, cardiac Na/K pumps turn on slowly (-30 seconds) with millimolar ATP dependence, when activated for the first time. In steady state, however, pumps are fully active with <0.2 mM ATP and are nearly unaffected by high ADP (2 mM) and Pi (10 mM) concentrations as may occur in ischemia. NCX1s appear to operate with slippage that contributes to background Na influx and inward current in heart. Thus, myocyte Na levels may be regulated by the inactivation reactions of the exchanger which are both Na- and proton-dependent. NHE1 also undergo strong Na-dependent inactivation, whereby a brief rise of cytoplasmic Na can cause inactivation that persists for many minutes after cytoplasmic Na is removed. This mechanism is blocked by pertussis toxin, suggesting involvement of a Na-dependent G-protein. Given that maximal NCX1- and NHE1-mediated ion fluxes are much greater than maximal Na/K pump-mediated Na extrusion in myocytes, the Na-dependent inactivation mechanisms of NCX1 and NHE1 may be important determinants of cardiac Na homeostasis.

CONCLUSIONS

Na/K pumps appear to be optimized to continue operation when energy reserves are compromised. Both NCX1 and NHE1 activities are regulated by accumulation of cytoplasmic Na. These principles may importantly control cardiac cytoplasmic Na and promote myocyte survival during ischemia/reperfusion episodes by preventing Ca overload.

Cellular mechanisms of direct-current electric field effects: galvanotaxis and metastatic disease

Endogenous direct-current electric fields (dcEFs) occur in vivo in the form of epithelial transcellular potentials or neuronal field potentials, and a variety of cells respond to dcEFs in vitro by directional movement. This is termed galvanotaxis. The passive influx of Ca2+ on the anodal side should increase the local intracellular Ca2+ concentration, whereas passive efflux and/or intracellular redistribution decrease the local intracellular Ca2+ concentration on the cathodal side. These changes could give rise to `push-pull' effects, causing net movement of cells towards the cathode. However, such effects would be complicated in cells that possess voltage-gated Ca2+ channels and/or intracellular Ca2+ stores. Moreover, voltage-gated Na+ channels, protein kinases, growth factors, surface charge and electrophoresis of proteins have been found to be involved in galvanotaxis. Galvanotactic mechanisms might operate in both the short term (seconds to minutes) and the long term (minutes to hours), and recent work has shown that they might be involved in metastatic disease. The galvanotactic responses of strongly metastatic prostate and breast cancer cells are much more prominent, and the cells move in the opposite direction compared with corresponding weakly metastatic cells. This could have important implications for the metastatic process and has clinical implications. Galvanotaxis could thus play a significant role in both cellular physiology and pathophysiology.

Intracellular Na+ inhibits voltage-dependent N-type Ca2+ channels by a G protein betagamma subunit-dependent mechanism

N-type voltage-dependent Ca(2+) channels (N-VDCCs) play important roles in neurotransmitter release and certain postsynaptic phenomena. These channels are modulated by a number of intracellular factors, notably by Gbetagamma subunits of G proteins, which inhibit N-VDCCs in a voltage-dependent (VD) manner. Here we show that an increase in intracellular Na(+) concentration inhibits N-VDCCs in hippocampal pyramidal neurones and in Xenopus oocytes. In acutely dissociated hippocampal neurones, Ba(2+) current via N-VDCCs was inhibited by Na(+) influx caused by the activation of NMDA receptor channels. In Xenopus oocytes expressing N-VDCCs, Ba(2+) currents were inhibited by Na(+) influx and enhanced by depletion of Na(+), after incubation in a Na(+)-free extracellular solution. The Na(+)-induced inhibition was accompanied by the development of VD facilitation, a hallmark of a Gbetagamma-dependent process. Na(+)-induced regulation of N-VDCCs is Gbetagamma dependent, as suggested by the blocking of Na(+) effects by Gbetagamma scavengers and by excess Gbetagamma, and may be mediated by the Na(+)-induced dissociation of Galphabetagamma heterotrimers. N-VDCCs may be novel effectors of Na(+)ion, regulated by the Na(+) concentration via Gbetagamma.

Stimulation of the plasma membrane Na+/H+ exchanger NHE1 by sustained intracellular acidosis. Evidence for a novel mechanism mediated by the ERK pathway

Activity of the Na+/H+ exchanger (NHE) isoform 1 (NHE1) is increased by intracellular acidosis through the interaction of intracellular H+ with an allosteric modifier site in the transport domain. Additional regulation is achieved via kinase-mediated modulation of the NHE1 regulatory domain. To determine if intracellular acidosis stimulates NHE1 activity solely by the allosteric mechanism, we subjected cultured neonatal rat ventricular myocytes (NRVM) with native NHE1 expression to intracellular acidosis (pHi approximately 6.6) for up to 6 min by transient exposure to NH4Cl and its washout in the presence of NHE inhibition (by zero [Na+]o or the NHE1 inhibitor cariporide) in HCO3- -free medium. After the desired duration of acidosis, NHE was reactivated (by reintroduction of [Na+]o or removal of cariporide), and the rate of recovery of pHi (dpHi/dt) was measured as the index of NHE activity. Regardless of the method used when intracellular acidosis was sustained for > or =3 min, subsequent NHE activity was significantly increased (>4-fold). Similar NHE stimulatory effects of sustained acidosis were observed in adult rat ventricular myocytes and COS-7 cells. Sustained (3 min) intracellular acidosis activated several NHE1 kinases in NRVM, in an in-gel kinase assay using as substrate a glutathione S-transferase fusion protein of the NHE1 regulatory domain. Detailed investigation of ERK and its downstream effector p90RSK, two putative NHE1 kinases, revealed time-dependent activation of both by intracellular acidosis in NRVM. Furthermore, inhibition of MEK1/2 by pretreatment of NRVM with two structurally distinct inhibitors, PD98059 (30 microM) or UO126 (3 microM), inhibited the activation of ERK and p90RSK and abolished the stimulation of NHE activity by sustained (3 min) intracellular acidosis. Our data show that not only the extent but also the duration of intracellular acidosis regulates NHE1 activity and suggest that the stimulatory effect of sustained intracellular acidosis occurs through a novel mechanism mediated by activation of the ERK pathway.

Contribution of functional voltage-gated Na+ channel expression to cell behaviors involved in the metastatic cascade in rat prostate cancer: II. Secretory membrane activity

The secretory membrane activities of two rat prostate cancer cell lines of markedly different metastatic potential, and corresponding electrophysiological characteristics, were studied in a comparative approach. In particular, voltage-gated Na(+) channels (VGSCs) were expressed in the strongly metastatic MAT-LyLu but not in the closely related, but weakly metastatic, AT-2 cells. Uptake and release of the non-cytotoxic marker horseradish peroxidase (HRP) were used as indices of general endocytotic and exocytotic membrane activity, respectively. The amount of tracer present in a given experimental condition was quantified by light microscopic digital imaging. The uptake of HRP was an active process, abolished completely by incubating the cells at low temperature (5 degrees C) and suppressed by disrupting the cytoskeleton. Interestingly, the extent of HRP uptake into the strongly metastatic MAT-LyLu cells was almost twice that into the weakly metastatic AT-2 cells. Vesicular uptake of HRP occurred in a fast followed by a slow phase; these appeared to correspond to cytoplasmic and perinuclear pools, respectively. Importantly, the overall quantitative difference in the uptake disappeared in the presence of 1 microM tetrodotoxin which significantly reduced the uptake of HRP into the MAT-LyLu cells. There was no effect on the AT-2 cells, consistent with functional VGSC expression occurring selectively in the former. A similar effect was observed in Na(+)-free medium. The uptake was partially dependent upon extracellular Ca(2+) but was not affected by raising the extracellular K(+) concentration. We suggest that functional VGSC expression could potentiate prostate cancer cells' metastatic ability by enhancing their secretory membrane activity.

Sensitivity of oocyte-expressed epithelial Na+ channel to glibenclamide

The effect of glibenclamide on heterologously expressed amiloride-sensitive sodium channels (ENaCs) was investigated in Xenopus oocytes. The ENaC is a heteromer and consists of alpha-, beta- and gamma-subunits and the alpha- and beta-subunits have previously been shown to confer sensitivity to glibenclamide. We coexpressed either colonic rat alpha- (ralpha) or guinea-pig alpha-subunit (gpalpha) with Xenopus betagamma-subunits. The gpalphaxbetagamma was significantly stimulated by glibenclamide (100 microM) (184+/-15%), whereas the ralpha-combination was slightly down-regulated by the sulfonylurea (79+/-4%). The stimulating effect did not interfere with Na(+)-self-inhibition resulting from intracellular accumulation of Na(+)-ions. We exchanged cytosolic termini between both orthologs but the gpalpha-chimera with the termini from rat retained sensitivity to glibenclamide. The effect of glibenclamide on Xenopus ENaC (xENaC) was inhibited by ADP-beta-S but not by ATP-gamma-S, when applied intracellularly. Intracellular loading with Na(+)-ions after inhibition of Na(+)/K(+)-ATPases with ouabain prevented an up-regulation of ENaC activity by glibenclamide. Pretreatment of oocytes expressing xENaC with edelfosine (ET-18-OCH(3)) slightly reduced stimulation of I(ami) (118+/-12%; control: 132+/-9%) while phosphatidylinositol-4,5-biphosphate (PIP(2)) significantly reduced the effect of glibenclamide to 101+/-3%.

Gd3+-sensitive Na+ transport across the integument of Hirudo medicinalis

Leeches Hirudo medicinalis were exposed to either artificial pond water (APW; 1 mM NaCl) or to high-salinity conditions (HS; 200 mM NaCl) for several days. The aim of the study was to assess whether transepithelial ion conductances in their dorsal integuments were affected by this long-term acclimation. In voltage-clamp experiments using Ussing-type chambers, the transepithelial potential V(T) was clamped to 0 mV, and amiloride-sensitive currents (I(ami)) and total Na(+) transport (I(Na)) were determined. Apical Ca(2+)-free conditions strongly increased I(ami) to a similar magnitude in both differently acclimated integuments. Apical application of the lanthanide gadolinium <0.1 mM decreased the short-circuit current (I(sc)). In contrast, higher concentrations up to 10 mM Gd(3+) upregulated I(sc) by an additional 90% in APW integuments and by an additional 300% in HS integuments. This Gd(3+) effect was due to a doubling of I(Na) in APW and a more than sixfold increase of I(Na) in HS integuments. In summary, the macroscopic electrophysiological variables, including I(Na), were generally not affected by long-term exposure to high salinity. However, the presence of Gd(3+)-sensitive Na(+) conductances or regulating structures were greatly upregulated during HS acclimation.

Annelid epithelia as models for electrogenic Na+ transport

The electrogenic Na(+) absorption across tight epithelia from invertebrates follows the principles analog to the mechanisms found in vertebrates. Extracellular Na(+)-ions pass the apical cell membranes through highly selective Na(+) channels and follow an electrochemical gradient which is sustained by the basolateral Na(+)/K(+)-ATPases. These apical Na(+) channels are selectively blocked by amiloride and represent the rate-limiting target for the control of transcellular Na(+) uptake. Although annelids express ADH-like peptide hormones, they lack the osmoregulatory mineralocorticoid system with the vertebrate-specific key hormone aldosterone. Thus, their epithelia may represent interesting models for investigation of ion transport regulation. While the formation of urine in the nephridia of, for example, leeches had been subject to intensive studies, the investigation of ion transport across their body wall was largely neglected. We use dissected segments of integuments from the limnic leech Hirudo medicinalis and, recently, from the earthworm Lumbricus terrestris for Ussing chamber experiments. We investigate transintegumental ion transport with focus on control of electrogenic Na(+) uptake and the amiloride-sensitive part of it and identified several extracellular factors as peptide hormones, tri- and divalent cations or purinergic molecules with regulatory effects on it. Meanwhile, there exists a macroscopic view on Na(+) absorption; however, other ion transport mechanisms across annelid integuments still await scientific effort. Here we present a concise synopsis about the electrophysiology of annelid integuments to illustrate the state of science and to evaluate whether further studies in this particular field may be of interest.

Effects of extracellular purines on ion transport across the integument of Hirudo medicinalis

Little is known about the long-term regulation of epithelial ion transport in invertebrates and the specific mediators involved. For some years, we have been investigating the short-term regulation of transepithelial ion transport across the dorsal integument of the leech Hirudo medicinalis, and we have established a model of Na+ uptake. In the present study, we investigated the effect of long-term acclimation on transintegumental ion transport by adapting leeches to high-salinity conditions. We dissected segments of dorsal integument and measured ion currents in Ussing chamber experiments. Electrophysiological variables, such as transepithelial potential(VT) and short-circuit-current (Isc),were profoundly affected by adaptation to high-salinity conditions. The total transepithelial Na+ current (INa) decreased from 7.66±0.82 to 4.6±0.54 μA cm-2 in preparations adapted to high salinity. The involvement of epithelial Na+channels was determined as current inhibition (Iami) by apical application of amiloride; Na+ channels were equally active in control epithelia and epithelia from leeches adapted to high salinity. Removal of Ca2+ from the apical solutions, which is believed to reduce intracellular Ca2+ concentrations, equalized transepithelial variables between high-salt-adapted integuments and control integuments.

Extracellular purines regulate transepithelial Cl- secretion and Na+ absorption. In a variety of tissues we tested ATP and adenosine for their effects on epithelial transport. Examination of integuments from pondwater- and high-salinity-adapted leeches revealed different sensitivities for these purines. Apical and basolateral application of ATP both stimulated transepithelial Na+ uptake and Iami. Adenosine upregulated non-Na+ currents and acted from the basolateral side only. Apical Ca2+-free conditions attenuated these effects of purines on transepithelial currents. Extracellular UTP had no effect on ion transport.

A slow pH-dependent conformational transition underlies a novel mode of activation of the epithelial Na+/H+ exchanger-3 isoform

Allosteric control of Na(+)/H(+) exchange by intracellular protons ensures rapid and accurate regulation of the intracellular pH. Although this allosteric effect was heretofore thought to occur almost instantaneously, we report here the occurrence of a slower secondary activation of the epithelial Na(+)/H(+) exchanger (NHE)-3 isoform. This slow activation mode developed over the course of minutes and was unique to NHE3 and the closely related isoform NHE5, but was not observed in NHE1 or NHE2. Activation of NHE3 was not due to increased density of exchangers at the cell surface, nor was it accompanied by detectable changes in phosphorylation. The association of NHE3 with the cytoskeleton, assessed by its retention in the detergent-insoluble fraction, was similarly unaffected by acidification. In contrast to the slow progressive activation elicited by acidification, deactivation occurred very rapidly upon restoration of the physiological pH. We propose that NHE3 undergoes a slow pH-dependent transition from a less active to a more active state, likely by changing its conformation or state of association.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Modulation of aldosterone-induced stimulation of ENaC synthesis by changing the rate of apical Na+ entry

Primary cultures of immunodissected rabbit connecting tubule and cortical collecting duct cells were used to investigate the effect of apical Na+ entry rate on aldosterone-induced transepithelial Na+ transport, which was measured as benzamil-sensitive short-circuit current (I(sc)). Stimulation of the apical Na+ entry, by long-term short-circuiting of the monolayers, suppressed the aldosterone-stimulated benzamil-sensitive I(sc) from 320 +/- 49 to 117 +/- 14%, whereas in the presence of benzamil this inhibitory effect was not observed (335 +/- 74%). Immunoprecipitation of [(35)S]methionine-labeled beta-rabbit epithelial Na+ channel (rbENaC) revealed that the effects of modulation of apical Na+ entry on transepithelial Na+ transport are exactly mirrored by beta-rbENaC protein levels, because short-circuiting the monolayers decreased aldosterone-induced beta-rbENaC protein synthesis from 310 +/- 51 to 56 +/- 17%. Exposure to benzamil doubled the beta-rbENaC protein level to 281 +/- 68% in control cells but had no significant effect on aldosterone-stimulated beta-rbENaC levels (282 +/- 68%). In conclusion, stimulation of apical Na+ entry suppresses the aldosterone-induced increase in transepithelial Na+ transport. This negative-feedback inhibition is reflected in a decrease in beta-rbENaC synthesis or in an increase in beta-rbENaC degradation.

Differences in intracellular pH regulation by Na(+)/H(+) antiporter among two-cell mouse embryos derived from females of different strains

Regulation of intracellular pH (pH(i)) by two-cell-stage embryos derived from female mice of three different strains (CF-1, Balb/c, and BDF) was investigated. Embryos recovered at a slow rate from intracellular acidosis produced by a pulse of NH(4)Cl; the rate did not differ significantly among strains. Recovery was reversibly inhibited by amiloride or the absence of Na(+), implicating Na(+)/H(+) antiporter activity. The threshold pH(i) (setpoint) below which Na(+)/H(+) antiporter activity was elicited was approximately 7.15 for each strain. No recovery from induced acidosis occurred in the absence of external Na(+) in any strain, and thus embryos could be maintained in acidosis for an extended period. Upon reintroduction of Na(+), embryos derived from either CF-1 or BDF females recovered at a slow rate comparable to that measured in embryos not maintained for a period in Na(+)-free medium, but embryos derived from Balb/c females consistently recovered at a highly accelerated rate. This accelerated recovery appeared to be due, in part, to an activation of the Na(+)/H(+) antiporter in Balb/c-derived embryos, which did not occur in CF-1- or BDF-derived embryos. Thus, embryos derived from different strains of female mice differ in their control of mechanisms for pH(i) regulation.

Modulation of Na+/H+ exchange activity by Cl-

Na+/H+ exchanger (NHE) activity is exquisitely dependent on the intra- and extracellular concentrations of Na+ and H+. In addition, Cl- ions have been suggested to modulate NHE activity, but little is known about the underlying mechanism, and the Cl- sensitivity of the individual isoforms has not been established. To explore their Cl- sensitivity, types 1, 2, and 3 Na+/H+ exchangers (NHE1, NHE2, and NHE3) were heterologously expressed in antiport-deficient cells. Bilateral replacement of Cl- with nitrate or thiocyanate inhibited the activity of all isoforms. Cl- depletion did not affect cell volume or the cellular ATP content, which could have indirectly altered NHE activity. The number of plasmalemmal exchangers was unaffected by Cl- removal, implying that inhibition was due to a decrease in the intrinsic activity of individual exchangers. Analysis of truncated mutants of NHE1 revealed that the anion sensitivity resides, at least in part, in the COOH-terminal domain of the exchanger. Moreover, readdition of Cl- into the extracellular medium failed to restore normal transport, suggesting that intracellular Cl- is critical for activity. Thus interaction of intracellular Cl- with the COOH terminus of NHE1 or with an associated protein is essential for optimal activity.

VHL tumor suppressor regulates Cl-/HCO3- exchange and Na+/H+ exchange activities in renal carcinoma cells

Mutations in the von Hippel-Lindau (VHL) tumor suppressor gene are thought to play a critical role in the pathogenesis of both sporadic and VHL disease-associated clear-cell renal carcinomas (RCC). Differential display-PCR identified the AE2 anion exchanger as a candidate VHL target gene. AE2 mRNA and polypeptide levels were approximately threefold higher in 786-O VHL cells than in 786-O Neo cells. In contrast, Cl(-)/HCO(3)(-) exchange activity in 786-O VHL cells was 50% lower than in 786-O Neo cells. Since resting intracellular pH (pH(i)) values were indistinguishable, we postulated that Na(+)/H(+) exchange activity (NHE) might be similarly reduced in 786-O VHL cells. NHE-mediated pH(i) recovery from acid load was less than 50% that in 786-O Neo cells, whereas hypertonicity-stimulated, amiloride-sensitive NHE was indistinguishable in the two cell lines. The NHE3 mRNA level was higher in 786-O VHL than 786-O Neo cells, but NHE1 mRNA levels did not differ. AE2 and NHE3 are the first transcripts reported to be upregulated by pVHL. Elucidation of mechanisms responsible for downregulation of both ion exchange activities will require further investigation.

Mechanisms of endothelial cell swelling from lactacidosis studied in vitro

One of the early sequelae of ischemia is an increase of circulating lactic acid that occurs in response to anaerobic metabolism. The purpose of the present study was to investigate whether lactic acidosis can induce endothelial swelling in vitro under closely controlled extracellular conditions. Cell volume of suspended cultured bovine aortic endothelial cells was measured by use of an advanced Coulter technique employing the "pulse area analysis" signal-processing technique (CASY1). The isosmotic reduction of pH from 7.4 to 6.8 had no effect on cell volume. Lowering of pH to 6.6, 6.4, or 6.0, however, led to significant, pH-dependent increases of cell volume. Swelling was more pronounced in bicarbonate-buffered media than in HEPES buffer. Specific inhibition of Na(+)/H(+) exchange by ethylisopropylamiloride completely prevented swelling in HEPES-buffered media. Pretreatment with ouabain to partially depolarize the cells did not affect the degree of acidosis-induced swelling. In bicarbonate-buffered media, the inhibition of transmembrane HCO(3)(-) transport by DIDS reduced swelling to a level comparable with that seen in the absence of bicarbonate ions. Lactacidosis-induced endothelial swelling, therefore, is a result of intracellular pH regulatory mechanisms, namely, Na(+)/H(+) exchange and bicarbonate-transporting carriers.