Extra- and intracellular hydrogen ion-selective microelectrode based on neutral carriers with extended pH response range in acid media

Improvement of the Upper Detection Limit of Ionophore-Based H+-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H+-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions. This work shows that coextraction of H+ and a sample anion into the sensing membrane alone does not explain these super-Nernstian responses, even when membrane-internal diffusion potentials are taken into account. Instead, these super-Nernstian responses are explained by the formation of complexes between that anion and at least two protonated ionophore molecules. As demonstrated by experiments and explained with quantitative phase boundary models, the apparently super-Nernstian responses at low pH can be eliminated by restricting the molecular ratio of ionophore and ionic sites. Notably, this conclusion results in recommendations for the optimization of sensing membranes that, in some instances, will conflict with previously reported recommendations from the ionic site theory for the optimization of the lower detection limit. This mechanistic insight is key to maximizing the response range of these ionophore-based ISEs.

Covalently attached ionophores extend the working range of potentiometric pH sensors with poly(decyl methacrylate) sensing membranes

The pH working range of solid-contact ion-selective electrodes (ISEs) with plasticizer-free poly(decyl methacrylate) sensing membranes is shown to be expanded by covalent attachment of H+ ionophores to the polymeric membrane matrix. In situ photopolymerization not only incorporates the ionophores into the polymer backbone, but at the same time also attaches the sensing membranes covalently to the underlying inert polymer and nanographite solid contact, minimizing sensor drift and preventing failure by membrane delamination. A new pyridine-based H+ ionophore, 3-(pyridine-3-yl)propyl methacrylate, has lower basicity than trialkylamine ionophores and expands the upper detection limit. This reduces in particular the interference from hydrogen phthalate, which is a common component of commercial pH buffers. Moreover, the lower detection limit is improved by replacing the CH2CH2 spacer of previously reported dialkylaminoethyl methacrylates with a (CH2)10 spacer, which increases its basicity. Notably, for the more basic and highly cation-selective ionophore 10-(diisopropylamino)decyl methacrylate, the extent of counterion interference from hydrogen phthalate shifted the upper detection limit to lower pH by nearly one pH unit when the crosslinker concentration was decreased.

Recent Advances in Wearable Potentiometric pH Sensors

Wearable sensors reflect the real-time physiological information and health status of individuals by continuously monitoring biochemical markers in biological fluids, including sweat, tears and saliva, and are a key technology to realize portable personalized medicine. Flexible electrochemical pH sensors can play a significant role in health since the pH level affects most biochemical reactions in the human body. pH indicators can be used for the diagnosis and treatment of diseases as well as the monitoring of biological processes. The performances and applications of wearable pH sensors depend significantly on the properties of the pH-sensitive materials used. At present, existing pH-sensitive materials are mainly based on polyaniline (PANI), hydrogen ionophores (HIs) and metal oxides (MOx). In this review, we will discuss the recent progress in wearable pH sensors based on these sensitive materials. Finally, a viewpoint for state-of-the-art wearable pH sensors and a discussion of their existing challenges are presented.

Fluorous-Phase Ion-Selective pH Electrodes: Electrode Body and Ionophore Optimization for Measurements in the Physiological pH Range

Because of their low polarity and polarizability, fluorous sensing membranes are both hydrophobic and lipophobic and exhibit very high ion selectivities. Here, we report on a new fluorous-membrane ion-selective electrode (ISE) with a wide sensing range centered around physiologically relevant pH values. The fluorophilic tris[perfluoro(octyl)butyl]amine (N[(CH₂)₄R_f8]₃) was synthesized and tested as a new H⁺ ionophore using a redesigned electrode body that provides excellent mechanical sealing and much improved measurement reliability. In a challenging 1 M KCl background, these fluorous-phase ISEs exhibit a sensing range from pH 2.2 to 11.2, which is one of the widest working ranges reported to date for ionophore-based H⁺ ISEs. High selectivities against common interfering ions such as K⁺, Na⁺, and Ca²⁺ were determined (selectivity coefficients: logK _H, K ^pot = - 11.6; logK _H, Na ^pot = - 12.4; logK _H, Ca ^pot < - 10.2). The use of the N[(CH₂)₄R_f8]₃ ionophore with its -(CH₂)₄- spacers separating the amino group from the strongly electron-withdrawing perfluorooctyl groups improved the potentiometric selectivity as compared to the less basic tris[perfluoro(octyl)propyl]amine ionophore. The use of N[(CH₂)₄R_f8]₃ also made the ISE less prone to counter anion failure (i.e., Donnan failure) at low pH than the use of tris[perfluoro(octyl)pentyl]amine with its longer -(CH₂)₅- spacers, which more effectively shield the amino center from the perfluorooctyl groups. In addition, we exposed both conventional plasticized PVC-phase pH ISEs and fluorous-phase pH ISEs to 10% serum for 5 days. Results show that the PVC-phase ISEs lost selectivity while their fluorous-phase counterparts did not.

Microfabricated reference electrodes and their biosensing applications

Over the past two decades, there has been an increasing trend towards miniaturization of both biological and chemical sensors and their integration with miniaturized sample pre-processing and analysis systems. These miniaturized lab-on-chip devices have several functional advantages including low cost, their ability to analyze smaller samples, faster analysis time, suitability for automation, and increased reliability and repeatability. Electrical based sensing methods that transduce biological or chemical signals into the electrical domain are a dominant part of the lab-on-chip devices. A vital part of any electrochemical sensing system is the reference electrode, which is a probe that is capable of measuring the potential on the solution side of an electrochemical interface. Research on miniaturization of this crucial component and analysis of the parameters that affect its performance, stability and lifetime, is sparse. In this paper, we present the basic electrochemistry and thermodynamics of these reference electrodes and illustrate the uses of reference electrodes in electrochemical and biological measurements. Different electrochemical systems that are used as reference electrodes will be presented, and an overview of some contemporary advances in electrode miniaturization and their performance will be provided.

Nano-pH Sensor for the Study of Reactive Materials

We report the development of a new iridium oxide nano-pH sensor designed to work in the range of pH 3-14. The fabrication process of reproducible iridium nanotips is described. The nanotips are covered by an insulating layer of parylene to ensure a chemical insulation. The use of a gallium focused ion beam enables the opening of the apex, leading to a sensing area of 100 nm diameter. A 12 h oxidation of the iridium tip in an oxygen atmosphere gives in a stable pH response. The calibration curve in buffer solutions exhibited a Nernstian behavior (slope 59.2 mV/pH). The distance control between the sample and the nanosensor is performed by atomic force microscopy (AFM), using either a shear force control or an inverted AFM configuration. The results of the hydration of two reactive samples, tricalcium silicate and tricalcium aluminate, having a size of 50 microm only, are presented.

Bradykinin-Induced Nuclear Factor of Activated T-Cells-Dependent Transcription in Rat Dorsal Root Ganglion Neurons

Bradykinin produced at sites of tissue injury and inflammation elicits acute pain and alters the sensitivity of nociceptive neurons to subsequent stimuli. We tested the hypothesis that bradykinin could elicit long-lasting changes in nociceptor function by activating members of the nuclear factor of activated T-cells (NFAT) family of transcription factors. Bradykinin activation of B2 receptors evoked concentration-dependent (EC50 = 6.0 +/- 0.3 nM) increases in intracellular Ca2+ concentration ([Ca2+]i) in a proportion of dorsal root ganglion neurons in primary culture. These [Ca2+] increases were sensitive to inhibition of phospholipase C (PLC) and depletion of Ca2+ stores. In neurons expressing a green fluorescent protein (GFP)-NFAT4 fusion protein, a 2-min exposure to bradykinin induced the translocation of GFP-NFAT4 from the cytoplasm to the nucleus. Translocation was partially inhibited by the removal of extracellular Ca2+ and was blocked by inhibition of calcineurin. Furthermore, bradykinin triggered a concentration-dependent increase in NFAT-mediated transcription of a luciferase gene reporter (EC50 = 24.2 +/- 0.1 nM). This depended on the B2 receptor, PLC activation, and inositol triphosphate-mediated Ca2+ release. Transcription was not inhibited by capsazepine. Finally, as indicated by quantitative reverse transcription-polymerase chain reaction, bradykinin elicited an increase in cyclooxygenase mRNA. This increase was sensitive to calcineurin and B2 receptor inhibition. These findings suggest a mechanism by which short-lived bradykinin-mediated stimuli can enact lasting changes in nociceptor function and sensitivity.

Neuron-glial trafficking of NH4+ and K+: separate routes of uptake into glial cells of bee retina

Ammonium (NH4+ and/or NH3) and K+ are released from active neurons and taken up by glial cells, and can modify glial cell behaviour. Study of these fluxes is most advanced in the retina of the honeybee drone, which consists essentially of identical neurons (photoreceptors) and identical glial cells (outer pigment cells). In isolated bee retinal glial cells, ammonium crosses the membrane as NH4+ on a Cl- cotransporter. We have now investigated, in the more physiological conditions of a retinal slice, whether the NH4+-Cl- cotransporter can transport K+ and whether the major K+ conductance can transport NH4+. We increased [NH4+] or [K+] in the superfusate and monitored uptake by recording from the glial cell syncytium or from interstitial space with microelectrodes selective for H+ or K+. In normal superfusate solution, ammonium acidified the glial cells but, after 6 min superfusion in low [Cl-] solution, ammonium alkalinized them. In the same low [Cl-] conditions, the rise in intraglial [K+] induced by an increase in superfusate [K+] was unchanged, i.e. no K+ flux on the Cl- cotransporter was detected. Ba2+ (5 mm) abolished the glial depolarization induced by K+ released from photoreceptors but did not reduce NH4+uptake. We estimate that when extracellular [NH4+] is increased, 62-100% is taken up by the NH4+-Cl- cotransporter and that when K+ is increased, 77-100% is taken up by routes selective for K+. This separation makes it possible that the glial uptake of NH4+ and of K+, and hence their signalling roles, might be regulated separately.

A Hydrogen Ion-Selective Poly(aniline) Solid Contact Electrode Based on Dibenzylpyrenemethylamine Ionophore for Highly Acidic Solutions

Hydrogen-ion selective solid contact electrodes based on tribenzylamine, dibenzylnaphthalenemethylamine, and dibenzylpyrenemethylamine ionophores were prepared. With these electrodes, we showed that the response ranges were influenced by the number of phenyl rings in the ionophores. The lower limits for a linear pH response in acidic solutions were pH 2.50, 0.65, and 0.50, respectively. As the number of phenyl rings in the ionophores increased, the slopes of the EMF responses of these electrodes did not change significantly, but their response extended toward an acidic range (shifted to pH 0.50). Thus, their dynamic response range became wider. A solid contact electrode with dibenzylpyrenemethylamine ionophore, in particular, showed the best selectivity, from the interference of cations and anions, and the best reproducibility of the EMF. This electrode was stored in Tris buffer solutions, artificial serum, and hydrofluoric acid solutions for one month without any loss of performance. Their response time was 8 s. Satisfactory results were obtained when it was tested directly with artificial serum (in pH range 6.0 - 8.5) and hydrofluoric acid.

Specificity of anion exchange mediated by mouse Slc26a6

Recently, CFEX, the mouse orthologue of human SLC26A6, was localized to the brush border membrane of proximal tubule cells and was demonstrated to mediate Cl(-)-formate exchange when expressed in Xenopus oocytes. The purpose of the present study was to examine whether mouse Slc26a6 can mediate one or more of the additional anion exchange processes observed to take place across the apical membrane of proximal tubule cells. Influx of [(14)C]formate into Slc26a6-expressing oocytes was inhibited by sulfate, oxalate, and p-aminohippurate (PAH), indicating affinity for these anions. Measurements of uptake of [(14)C]oxalate, [(14)C]PAH, and [(35)S]sulfate indicated that Slc26a6 can mediate transport of oxalate and sulfate but not PAH. Studies of the effect of external anions on [(14)C]oxalate efflux demonstrated Slc26a6-mediated Cl(-)-oxalate, oxalate-formate, oxalate-oxalate, and oxalate-sulfate exchange. Two-electrode voltage clamp measurements indicated that Slc26a6-mediated Cl(-)-oxalate exchange is electrogenic. Intracellular pH recordings demonstrated that Slc26a6 can mediate Cl(-)-HCO(3)(-) exchange, but Cl(-)-OH(-) exchange was not detected. The presence of 100 microm oxalate inhibited the rate of Cl(-)-HCO(3)(-) exchange by 60%. We conclude that mouse Slc26a6 has affinity for oxalate, sulfate, and HCO(3)(-) in addition to Cl(-) and formate and can function in multiple exchange modes involving pairs of these anions. In the presence of high oxalate concentrations as found in renal tubular fluid and urine, Slc26a6 may largely function as an electrogenic Cl(-)-oxalate exchanger.

Mucus secretion from single submucosal glands of pig. Stimulation by carbachol and vasoactive intestinal peptide

Secretion rates of >700 individual glands in isolated tracheal mucosa from 56 adult pigs were monitored optically. "Basal" secretion of 0.7 +/- 0.1 nl x min(-1) gland(-1) was observed 1-9 h post-harvest but was near zero on day 2. Secretion to carbachol (10 microm) peaked at 2-3 min and then declined to a sustained phase. Peak secretion was 12.4 +/- 1.1 nl x min(-1) gland(-1); sustained secretion was approximately one-third of peak secretion. Thapsigargin (1 microm) increased secretion from 0.1 +/- 0.05 to 0.7 +/- 0.2 nl x min(-1) gland(-1); thapsigargin did not cause contraction of the trachealis muscles. Isoproterenol and phenylephrine (10 microm each) were ineffective, but vasoactive intestinal peptide (1 microm) and forskolin (10 microm) each produced sustained secretion of 1.0 +/- 0.5 and 1.7 +/- 0.2 nl x min(-1) gland(-1), respectively. The density of actively secreting glands was 1.3/mm(2). Secretion to either carbachol or forskolin was inhibited (approximately 50%) by either bumetanide or HCO(3)(-) removal and inhibited approximately 90% by the combined treatments. Mucus secreted in response to carbachol or forskolin was acidic by approximately 0.2 pH units relative to the bath and remained acidic by approximately 0.1 pH units after bumetanide. The strong secretory response to vasoactive intestinal peptide, the acidity of [cAMP](i)-stimulated mucus, and its inhibition by bumetanide were unexpected.

Cloning, characterization, and chromosomal mapping of a human electroneutral Na(+)-driven Cl-HCO3 exchanger

The electroneutral Na(+)-driven Cl-HCO3 exchanger is a key mechanism for regulating intracellular pH (pH(i)) in neurons, glia, and other cells. Here we report the cloning, tissue distribution, chromosomal location, and functional characterization of the cDNA of such a transporter (NDCBE1) from human brain (GenBank accession number AF069512). NDCBE1, which encodes 1044 amino acids, is 34% identical to the mammalian anion exchanger (AE2); approximately 50% to the electrogenic Na/HCO3 cotransporter (NBCe1) from salamander, rat, and humans; approximately 73% to mammalian electroneutral Na/HCO3 cotransporters (NBCn1); 71% to mouse NCBE; and 47% to a Na(+)-driven anion exchanger (NDAE1) from Drosophila. Northern blot analysis of NDCBE1 shows a robust approximately 12-kilobase signal in all major regions of human brain and in testis, and weaker signals in kidney and ovary. This human gene (SLC4A8) maps to chromosome 12q13. When expressed in Xenopus oocytes and running in the forward direction, NDCBE1 is electroneutral and mediates increases in both pH(i) and [Na(+)](i) (monitored with microelectrodes) that require HCO3(-) and are blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The pH(i) increase also requires extracellular Na(+). The Na(+):HCO3(-) stoichiometry is 1:2. Forward-running NDCBE1 mediates a 36Cl efflux that requires extracellular Na(+) and HCO3(-) and is blocked by DIDS. Running in reverse, NDCBE1 requires extracellular Cl(-). Thus, NDCBE1 encodes a human, electroneutral Na(+)-driven Cl-HCO3 exchanger.

Delta-opioid-induced liberation of Gbetagamma mobilizes Ca2+ stores in NG108-15 cells

Activation of delta-opioid receptors in NG108-15 cells releases Ca2+ from an intracellular store through activation of a pertussis toxin-sensitive G protein. We tested the hypothesis that activation of delta-opioid receptors mobilizes inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca2+ stores via liberation of Gbetagamma. Fura-2-based digital imaging was used to study the mechanism of opioid-induced increases in [Ca2+](i) in NG108-15 cells. Exposure to D-Ala(2)-D-Leu(5) enkephalin (100 nM) for 90 s induced increases in [Ca2+](i) that were blocked by microinjection of the IP(3) receptor antagonist heparin (pipette concentration = 100 mg/ml) but not by sham injection. Microinjection of a peptide that binds Gbetagamma (QEHA, 1 mM) decreased the D-Ala(2)-D-Leu(5) enkephalin-evoked response. Microinjection of an inactive peptide (SKEE, 1 mM) that does not bind to Gbetagamma failed to inhibit the opioid-induced increase in [Ca2+](i). Microinjection of a peptide (QLKK, 15 mM) that binds to free Galpha(q) blocked the increase evoked by 3 nM bradykinin, but microinjection of an inactive peptide (ADRK, 15 mM) did not. Microinjection of QLKK did not significantly affect the opioid-induced increase in [Ca2+](i). Collectively, these data demonstrate that activation of delta-opioid receptors induces the release of Ca2+ from IP(3)-sensitive stores in NG108-15 cells through activation of the betagamma subunits of inhibitory G proteins.

Effects of photoreceptor metabolism on interstitial and glial cell pH in bee retina: evidence of a role for NH4+

1. Measurements were made with pH microelectrodes in superfused slices of the retina of the honey-bee drone. In the dark, the mean +/- S.E.M. pH values in the three compartments of the tissue were: neurones (photoreceptors), 6.99 +/- 0.04; glial cells (outer pigment cells), 7.31 +/- 0.03; extracellular space, 6.60 +/- 0.03. 2. Stimulation of the photoreceptors with light caused transient pH changes: a decrease in the photoreceptors (pHn) and in the glial cells (pHg), and an increase in the interstitial clefts (pHo). 3. The effects of inhibition and activation of aerobic metabolism showed that part, perhaps all, of the light-induced delta pHo resulted from the increased aerobic metabolism in the photoreceptors. 4. Addition of 2 mM NH4+ to the superfusate produced changes in pHo and pHg of the same sign as and similar amplitude to those caused by light stimulation. Manipulation of transmembrane pH gradients had similar effects on changes in pHo induced by light or by exogenous NH4+. 5. Measurements with NH(4+)-sensitive microelectrodes showed that stimulation of aerobic metabolism in the photoreceptors increased [NH4+]o and also that exogenous NH4+/NH3 was taken up by cells, presumably the glial cells. 6. We conclude that within seconds of an increase in the aerobic metabolism in the photoreceptors, they release an increased amount of NH4+/NH3 which affects pHo and enters glial cells. Other evidence suggests that in drone retina the glial cells supply the neurones with amino acids as substrates of energy metabolism; the present results suggest that fixed nitrogen is returned to the glial cells as NH4+/NH3.

Effects of aging on the microclimate pH of the rat jejunum

The acidic microclimate layer in the vicinity of the cell surface of mammalian jejunum is important for absorption of some nutrients, such as small peptides and folate. The present study was undertaken to investigate the effect of aging on the cell surface pH (microclimate pH) of the jejunum of rats. The microclimate pH was measured in vitro in superfused preparations using single-barreled pH-sensitive microelectrodes filled with a liquid ion exchanger. The thickness of the microclimate layer was estimated by reading the distance of microelectrode advancements. The existence of a microclimate pH in the jejunum was confirmed in the senescent rats, but the value of the microclimate pH was significantly higher in the senescent (24 mo) rats (6.52 +/- 0.02) than in the young-adult (6 mo) rats (6.09 +/- 0.01) (P < 0.01). Na+ removal from the perfusate or the addition of amiloride elevated the pH in the senescent rats as well as in the young-adult rats. The microclimate layer was slightly thinner in the senescent rats than in the young-adult rats. The acidity of the microclimate layer of intestinal surface is lower in senescent animals than in the young-adult ones. One of reasons for this is the thinner mucus layer in senescent animals.

Disruptive effects of protein on performance of liquid membrane-based ion-selective microelectrodes

The effects of addition of protein [bovine serum albumin (BSA)] to the calibration solutions of various liquid membrane-based ion-selective microelectrodes have been assessed. Protein at typically cytosolic levels of 100 mg/ml had no effect on the response of Na(+)-, K(+)-, Mg(2+)-, Ca(2+)-, H(+)-, NO3(-)-, and Cl(-)- selective microelectrodes at ion concentrations well above the respective detection limits. By contrast, this level of protein resulted in a pronounced increase in the detection limits of Na(+)-, K(+)-, Mg(2+)-, Ca2+, and Cl(-)-selective microelectrodes; H+ and NO3- microelectrodes were largely unaffected by the protein. The effect of BSA on detection limit is independent of pH and ionic strength, nor is it due to hysteresis, to binding of ions to the protein, or to the generation of junction potentials. More extensive studies on Na(+)-selective microelectrodes revealed that other proteins and large molecules (e.g., polyethylene glycol) also increase detection limits, with the extent of this effect apparently correlated positively with the molecular weight. Because biological macromolecules at physiological concentrations can influence the properties of alkali- and alkali earth cation-selective microelectrodes, care must be taken if the use of these electrodes entails cytosolic measurements close to their detection limits.

Passive Ca buffering and SR Ca uptake in permeabilized rabbit ventricular myocytes

Passive Ca binding was measured with a Ca-selective minielectrode in suspensions of permeabilized rabbit ventricular myocytes equilibrated with 5 microM thapsigargin and 30 microM ruthenium red to prevent sarcoplasmic reticulum (SR) or mitochondrial Ca uptake. Passive Ca binding was obtained by titration of the myocytes with Ca and subtraction of Ca binding in a blank titration without myocytes. Passive Ca binding could be described by a Michaelis binding curve with two sites: K1 = 0.42 microM n1 = 1.27 nmol/mg cell protein and K2 = 79 microM, n2 = 4.13 nmol/mg cell protein. The passive Ca buffering over the physiological Ca concentration was approximately twice the value expected from the values compiled by Fabiato [A. Fabiato. Am. J. Physiol. 245 (Cell Physiol. 14): C1-C14, 1983]. The maximal SR Ca uptake in the presence of 30 microM ruthenium red was fit by an uptake curve with a maximal uptake of 5.16 nmol/mg cell protein and a K 1/2 of 1.0 microM. In the presence of 5 microM thapsigargin and no ruthenium red, a significant Ca uptake attributed to mitochondria was measured between 10 and 100 microM free Ca. Rapid changes in free Ca concentration ([Ca]) measured with a Ca electrode were slower than simultaneous measurements of free [Ca] with indo-1 in permeabilized myocytes. However, oxalate, which buffers Ca and maximizes SR Ca uptake, increased the uptake rate and eliminated the difference in free [Ca] measured with Ca electrode and indo-1. This suggests that spatial gradients of [Ca] exist in permeabilized myocytes without Ca buffering. The new estimates of the buffering of intracellular Ca in cardiac myocytes should be valuable in developing quantitative insights into cardiac Ca regulation.

Effect of catecholamines on intracellular pH in sheep cardiac Purkinje fibres

1. It has been reported that catecholamines affect intracellular pH (pHi) in a number of tissues, generally by altering the kinetics of the Na(+)-H+ exchanger. We postulated that catecholamines might affect pHi in cardiac tissue. We tested this in resting sheep cardiac Purkinje fibres by measuring transmembrane potential and pHi with standard and H(+)-sensitive microelectrodes. 2. Adrenaline and the beta-adrenergic agonist isoprenaline, both 5.0 x 10(-6) M, resulted in depolarization and intracellular acidification (adrenaline, 0.03 +/- 0.01 pH units, n = 8, P = 0.005; isoprenaline, 0.08 +/- 0.01 pH units, n = 17, P = 0.0001). The alpha-adrenergic agonist phenylephrine, at concentrations up to 200 microM, had no significant effect on membrane potential or pHi. 3. Isoprenaline significantly attenuated the half-time (t0.5) for pHi recovery from intracellular acidification induced via the NH4Cl pulse technique. Isoprenaline also attenuated the hyperpolarization that is normally seen at the onset of pHi recovery. Phenylephrine slightly reduced the t0.5 for recovery, although the reduction did not reach statistical significance. 4. Forskolin, 7.5-10 x 10(-5) M, an agent that raises intracellular cyclic adenosine 3',5'-monophosphate (cyclic AMP), also induced depolarization and acidification, similar to that induced by adrenaline and isoprenaline. 5. In the presence of the Na(+)-H+ exchange blocker 5-dimethyl amiloride, 2-6 x 10(-5) M, isoprenaline-induced acidification was blunted but not abolished. When administered in Na(+)-free Tyrode solution, isoprenaline-induced acidification was also not abolished. Buffering power, tested using the NH4Cl method, was not decreased by isoprenaline, but rather, was slightly increased. Reversal of H+ driving force across the cell membrane from the normally inward direction to outward (achieved by increasing pHo to 8.3-8.5 and depolarizing the membrane with 10 mM K+ solutions) did not prevent intracellular acidification from occurring in the presence of isoprenaline. When glycolysis was inhibited by a 60 min exposure to glucose-free solution containing 5.5 mM 2-deoxyglucose, acidification by isoprenaline was nearly abolished. 6. We conclude that, in resting sheep Purkinje fibres, beta- but not alpha-adrenergic stimulation results in intracellular acidification and depolarization, probably mediated via an increase in cyclic AMP. beta- but not alpha-adrenergic stimulation slows the rate of recovery from intracellular acidification and blunts the hyperpolarization associated with this recovery. 7. The intracellular acidification appears to be due both to partial inhibition of Na(+)-H+ exchange and to stimulation of glycolysis by beta-adrenergic agents.

Intracellular pH and its relationship to regulation of ion transport in normal and cystic fibrosis human nasal epithelia

1. Intracellular pH (pHi) of cultured human airway epithelial cells from normal and cystic fibrosis (CF) subjects were measured with double-barrelled pH-sensitive liquid exchanger microelectrodes. The cells, which were grown to confluence on a permeable collagen matrix support, were mounted in a modified miniature Ussing chamber. All studies were conducted under open circuit conditions. Values are given as means +/- S.E.M. and n refers to the number of preparations. 2. Normal preparations (n = 15) were characterized by a transepithelial potential difference (Vt) of -18 +/- 2 mV, an apical membrane potential (Va) of -19 +/- 2 mV, a basolateral membrane potential (Vb) of -37 +/- 2 mV, a transepithelial resistance (Rt) of 253 +/- 15 omega cm2, a fractional apical membrane resistance (fRa) of 0.40 +/- 0.04 and an equivalent short circuit current (Ieq) of -73 +/- 7 microA cm-2. 3. CF preparations (n = 13) were characterized by a Vt of -46 +/- 7 mV, a Va of 3 +/- 5 mV, a Vb of -43 +/- 3 mV, Rt of 373 +/- 47 omega cm2, fRa of 0.44 +/- 0.04 and an Ieq of -130 +/- 16 microA cm-2. All parameters except Vb and fRa were significantly different (P < 0.025) from those of normal preparations. 4. Despite large differences in electrochemical driving force for proton flow across the apical cell membranes between normal and CF preparations (-4 +/- 3 mV and 20 +/- 7 mV, respectively), pHi was similar (7.15 +/- 0.02 and 7.11 +/- 0.05, respectively). The driving force across the basolateral membrane was similar in normal and CF preparations (22 +/- 3 and 26 +/- 3 mV, respectively). 5. Intracellular alkalinization achieved by removal of CO2 from the luminal Ringer solution or by luminal ammonium prepulse led to stimulation of Ieq in both normal (from -58 to -70 microA cm-2, n = 4; P < 0.05) and CF (from -144 to -163 microA cm-2, n = 4; P < 0.005) preparations. The increase in Ieq was associated with a reduction of Rt, increase in fRa, and hyperpolarization of Vb. All changes in bioelectric properties in response to intracellular alkalinization were fully reversible. 6. Intracellular acidification achieved by serosal ammonium prepulse led to marked reductions of Ieq in both normal (from -95 to -31 microA cm-2, n = 6; P < 0.05) and CF (from -111 to -67 microA cm-2, n = 7; P < 0.005) preparations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of intra- and extracellular H+ and Na+ concentrations on Na(+)-H+ antiport activity in the lacrimal gland acinar cells

Kinetic properties of the Na(+)-H+ antiport in the acinar cells of the isolated, superfused mouse lacrimal gland were studied by measuring intracellular pH (pHi) and Na+ activity (aNai) with the aid of double-barreled H(+)- and Na(+)-selective microelectrodes, respectively. Bicarbonate-free solutions were used throughout. Under untreated control conditions, pHi was 7.12 +/- 0.01 and aNai was 6.7 +/- 0.6 mmol/l. The cells were acid-loaded by exposure to an NH4+ solution followed by an Na(+)-free N-methyl-D-glucamine (NMDG+) solution. Intracellular Na+ and H+ concentrations were manipulated by changing the duration of exposure to the above solutions. Subsequent addition of the standard Na+ solution rapidly increased pHi. This Na(+)-induced increase in pHi was almost completely inhibited by 0.5 mmol/l amiloride and was associated with a rapid, amiloride-sensitive increase in aNai. The rate of pHi recovery induced by the standard Na+ solution increased in a saturable manner as pHi decreased, and was negligible at pHi 7.2-7.3, indicating an inactivation of the Na(+)-H+ antiport. The apparent Km for intracellular H+ concentration was 105 nmol/l (pH 6.98). The rate of acid extrusion from the acid-loaded cells increased proportionally to the increase in extracellular pH. Depletion of aNai to less than 1 mmol/l by prolonged exposure to NMDG+ solution significantly increased the rate of Na(+)-dependent acid extrusion. The rate of acid extrusion increased as the extracellular Na+ concentration increased following Michaelis-Menten kinetics (Vmax was 0.55 pH/min and the apparent Km was 75 mmol/l at pHi 6.88).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of mepacrine, indomethacin, and nordihydroguaiaretic acid on the electrical properties of frog renal proximal tubules

In proximal renal tubules of the frog kidney, stimulation of sodium-coupled transport leads to a depolarization of the peritubular cell membrane, followed by partial repolarization. These alterations of the potential difference across the peritubular cell membrane (PDpt) are in part the result of altered peritubular potassium conductance. The repolarization has been blunted by the phospholipase A2 inhibitor mepacrine, but not by the cyclooxygenase inhibitor indomethacin. In the present study the effect of mepacrine, indomethacin and the lipoxygenase inhibitor nordihydroguaiaretic acid on the electrical properties of proximal renal tubules has been tested in the presence and absence of stimulated sodium-coupled transport. In the absence of inhibitors, addition of 10 mmol/l phenylalanine to the luminal perfusate leads to a rapid depolarization and partial repolarization of the peritubular cell membrane, a decrease of the luminal cell membrane resistance (Ra) and a small increase of the cellular core resistance (Rc). Removal of phenylalanine leads to rapid hyperpolarization, increase of Ra and decline Rc. Mepacrine (100 mumols/l) depolarizes the cell membrane and increases the peritubular cell membrane resistance (Rb), Rc and the intracellular pH. In the presence of mepacrine, phenylalanine leads to a sustained depolarization and a transient decrease of Ra. Indomethacin (10 mumol/l) does not significantly modify PDpt, the lumped resistance of both cell membranes (Rm) or Rc in the presence or absence of phenylalanine. Nordihydroguaiaretic acid (50 mumols/l) does not alter significantly PDpt, Ra, Rb or Rc prior to phenylalanine.(ABSTRACT TRUNCATED AT 250 WORDS)

Madin-Darby canine kidney cells. II. Aldosterone stimulates Na+/H+ and Cl-/HCO3- exchange

Experiments in dome epithelium of Madin-Darby canine kidney (MDCK) cells were performed to elucidate aldosterone action on acid-base transport. By means of pH-sensitive microelectrodes the pH of the dome fluid was measured while the apical plasma membrane was superfused. In the absence of HCO3- the dome fluid (facing the basolateral cell membrane) alkalinized in response to 10(-7) mol/l aldosterone. Amiloride (10(-3) mol/l) inhibited dome formation and pH recovery of the dome fluid from an extracellular acid load. In the presence of HCO3- dome fluid acidified in response to aldosterone. The stilbene derivative diisothiocyanate-stilbene-2,2'-disulphonic acid (DIDS) or removal of Cl- from the apical perfusate inhibited this dome acidification. In aldosterone-depleted MDCK monolayers HCO3- was actively accumulated in the dome fluid in contrast to aldosterone-supplemented cells. The results indicate that aldosterone stimulates both amiloride-sensitive Na+/H+ exchange and DIDS-sensitive Cl-/HCO3- exchange in the apical cell membrane of MDCK cells. In the absence of aldosterone the HCO3- extrusion process is localized in the basolateral membrane in series with apical Na+/H+ exchange, while in the presence of aldosterone Cl-/HCO3- is mainly localized in the apical membrane in parallel with Na+/H+ exchange. Cl- exits the cell through apical Cl- channels and is absorbed via the paracellular route.

Madin-Darby canine kidney cells. III. Aldosterone stimulates an apical H+/K+ pump

Functionally and morphologically, Madin-Darby canine kidney (MDCK) cells resemble intercalated cells of urinary epithelia. Experiments were performed on domes of confluent MDCK monolayers to test for apical H+ secretion. Apical application of 10(-3) mol/l amiloride or of Na(+)-free solution significantly reduced the limiting pH gradient across the dome epithelium (delta pHd) consistent with inhibition of apical Na+/H+ exchange. Short-circuit current (SCC) measurements disclosed an acetazolamide-sensitive, (basolateral to apical) positive transepithelial current stimulated by 10(-7) mol/l aldosterone and inhibited by acidification of apical medium to pH = 4.5. Histochemical evaluation of carbonic anhydrase (CA) activity revealed cytoplasmic and apical-membrane-bound CA particularly in dome-forming cells. Apical substitution of Na+ by K+ increased delta pHd, whereas a reduction of K+ concentration to 0.5 mmol/l or addition of barium or omeprazole (10(-5) mol/l) to the apical superfusate reduced delta pHd by at least 75%. Aldosterone-stimulated SCC was completely abolished by the apical application of barium. We conclude that besides Na+/H+ exchange MDCK cells can express an apically located H(+)-K+ pump stimulated by aldosterone and inhibited directly by the anti-ulcer agent omeprazole or indirectly, either by blocking apical K+ recycling or by interfering with the CA-dependent intracellular formation of H+ ions.

A new double-barrelled, ionophore-based microelectrode for chloride ions

A new Cl- selective microelectrode based on the ionophore 5,10,15,20-tetraphenyl-21H,23H-porphin manganese(III) chloride is presented which discriminates better against HCO3- and several organic anions than electrodes containing the Corning 477913 ion-exchanger. Using a redesigned construction procedure, fine-tip double-barrelled microelectrodes were produced which had slopes of -52.4 +/- 0.6 mV (SE, n = 24), resistances of about 7.10(11) omega and a selectivity coefficient log KpotCLHCO3 of -1.40 +/- 0.03. Some electrodes showed a small unexplained sensitivity to pH greater than 7.6. When used to puncture cells of isolated S3 segments of rabbit renal proximal tubule during perfusion with HCO3- Ringer solution, the electrodes gave a membrane potential of -69.8 +/- 1.5 mV and an intracellular Cl- activity, [Cl-]i, of 35.3 +/- 2.6 mmol/l. Upon switching bath and lumen perfusions to Cl- -free solutions the "residual" [Cl-]i dropped to 1.20 +/- 0.03 mmol/l, while in similar measurements with ion-exchanger electrodes the "residual" [Cl-]i dropped only to 10.9 +/- 0.5 mmol/l. These observations demonstrate the superiority of the new electrode and prove that previously determined high [Cl-]i values in Cl- -free ambient solutions reflect interference problems rather than non-exchangeable intracellular chloride.

NH4+ ion-selective microelectrode based on the antibiotics nonactin/monactin

A liquid-membrane microelectrode (less than or equal to 1 micron tip diameter) using macrotetrolide antibiotics as ion-selective components is described. The electrode shows selectivities of NH4+ over K+, Na+ and H+ of 3.8, 100 and 150, respectively. The stability and reproducibility of the sensor signal and the response time are determined in solutions with a typical intracellular ion background. The microelectrode does not suffer from significant interference by inorganic and organic inhibitors and lipophilic cations, but high concentrations of lipophilic anions may interfere considerably.