Buffer capacity of rat cortical tissue as well as of cultured neurons and astrocytes

High effective cytosolic H+ buffering in mouse cortical astrocytes attributable to fast bicarbonate transport

Cytosolic H(+) buffering plays a major role for shaping intracellular H(+) shifts and hence for the availability of H(+) for biochemical reactions and acid/base-coupled transport processes. H(+) buffering is one of the prime means to protect the cell from large acid/base shifts. We have used the H(+) indicator dye BCECF and confocal microscopy to monitor the cytosolic H(+) concentration, [H(+)]i, in cultured cortical astrocytes of wild-type mice and of mice deficient in sodium/bicarbonate cotransporter NBCe1 (NBCe1-KO) or in carbonic anhydrase isoform II (CAII-KO). The steady-state buffer strength was calculated from the amplitude of [H(+)]i transients as evoked by CO2/HCO3(-) and by butyric acid in the presence and absence of CO2/HCO3(-). We tested the hypotheses if, in addition to instantaneous physicochemical H(+) buffering, rapid acid/base transport across the cell membrane contributes to the total, "effective" cytosolic H(+) buffering. In the presence of 5% CO2/26 mM HCO3(-), H(+) buffer strength in astrocytes was increased 4-6 fold, as compared with that in non-bicarbonate, HEPES-buffered solution, which was largely attributable to fast HCO3 (-) transport into the cells via NBCe1, supported by CAII activity. Our results show that within the time frame of determining physiological H(+) buffering in cells, fast transport and equilibration of CO2/H(+)/HCO3(-) can make a major contribution to the total "effective" H(+) buffer strength. Thus, "effective" cellular H(+) buffering is, to a large extent, attributable to membrane transport of base equivalents rather than a purely passive physicochemical process, and can be much larger than reported so far. Not only physicochemical H(+) buffering, but also rapid import of HCO3(-) via the electrogenic sodium-bicarbonate cotransporter NBCe1, supported by carbonic anhydrase II (CA II), was identified to enhance cytosolic H(+) buffer strength substantially.

HCO3(-)-independent pH regulation in astrocytes in situ is dominated by V-ATPase

The mechanisms of HCO₃⁻-independent intracellular pH (pH_i) regulation were examined in fibrous astrocytes within isolated neonatal rat optic nerve (RON) and in cultured cortical astrocytes. In agreement with previous studies, resting pH_i in cultured astrocytes was 6.82 ± 0.06 and inhibition of the V-ATPase H⁺ pump by Cl⁻ removal or via the selective inhibitor bafilomycin had only a small effect upon resting pH_i and recovery following an acid load. In contrast, resting pH_i in RON astrocytes was 7.10 ± 0.04, significantly less acidic than that in cultured cells (p < 0.001), and responded to inhibition of V-ATPase with profound acidification to the 6.3–6.5 range. Fluorescent immuno-staining and immuno-gold labeling confirmed the presence V-ATPase in the cell membrane of RON astrocyte processes and somata. Using ammonia pulse recovery, pH_i recovery in RON astrocyte was achieved largely via V-ATPase with sodium-proton exchange (NHE) playing a minor role. The findings indicate that astrocytes in a whole-mount preparation such as the optic nerve rely to a greater degree upon V-ATPase for HCO₃⁻-independent pH_i regulation than do cultured astrocytes, with important functional consequences for the regulation of pH in the CNS.

Efficacy of synaptic inhibition depends on multiple, dynamically interacting mechanisms implicated in chloride homeostasis

Chloride homeostasis is a critical determinant of the strength and robustness of inhibition mediated by GABA(A) receptors (GABA(A)Rs). The impact of changes in steady state Cl(-) gradient is relatively straightforward to understand, but how dynamic interplay between Cl(-) influx, diffusion, extrusion and interaction with other ion species affects synaptic signaling remains uncertain. Here we used electrodiffusion modeling to investigate the nonlinear interactions between these processes. Results demonstrate that diffusion is crucial for redistributing intracellular Cl(-) load on a fast time scale, whereas Cl(-)extrusion controls steady state levels. Interaction between diffusion and extrusion can result in a somato-dendritic Cl(-) gradient even when KCC2 is distributed uniformly across the cell. Reducing KCC2 activity led to decreased efficacy of GABA(A)R-mediated inhibition, but increasing GABA(A)R input failed to fully compensate for this form of disinhibition because of activity-dependent accumulation of Cl(-). Furthermore, if spiking persisted despite the presence of GABA(A)R input, Cl(-) accumulation became accelerated because of the large Cl(-) driving force that occurs during spikes. The resulting positive feedback loop caused catastrophic failure of inhibition. Simulations also revealed other feedback loops, such as competition between Cl(-) and pH regulation. Several model predictions were tested and confirmed by [Cl(-)](i) imaging experiments. Our study has thus uncovered how Cl(-) regulation depends on a multiplicity of dynamically interacting mechanisms. Furthermore, the model revealed that enhancing KCC2 activity beyond normal levels did not negatively impact firing frequency or cause overt extracellular K(-) accumulation, demonstrating that enhancing KCC2 activity is a valid strategy for therapeutic intervention.

Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca²(+)), in salamander photoreceptor inner segments loaded with Ca²(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ∼0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH₄Cl) depolarized photoreceptors and stimulated Ca²(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca²(+)](i). Alkalinization-evoked [Ca²(+)](i) decreases were independent of voltage-operated and store-operated Ca²(+) entry, plasma membrane Ca²(+) extrusion, and Ca²(+) sequestration into internal stores. The [Ca²(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca²(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca²(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca²(+) stores, and intracellular Ca²(+) buffering in rod and cone inner segments.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

Effects of the nitric oxide donor, DEA/NO on cortical spreading depression

Cortical spreading depression (CSD) is a transient disruption of local ionic homeostasis that may promote migraine attacks and the progression of stroke lesions. We reported previously that the local inhibition of nitric oxide (NO) synthesis with Nomega-nitro-L-arginine methyl ester (L-NAME) delayed markedly the initiation of the recovery of ionic homeostasis from CSD. Here we describe a novel method for selective, controlled generation of exogenous NO in a functioning brain region. It is based on microdialysis perfusion of the NO donor, 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO). As DEA/NO does not generate NO at alkaline pH, and as the brain has a strong acid-base buffering capacity, DEA/NO was perfused in a medium adjusted at alkaline (but unbuffered) pH. Without DEA/NO, such a microdialysis perfusion medium did not alter CSD. DEA/NO (1, 10 and 100 microM) had little effect on CSD by itself, but it reversed in a concentration-dependent manner the effects of NOS inhibition by 1 mM L-NAME. These data demonstrate that increased formation of endogenous NO associated with CSD is critical for subsequent, rapid recovery of cellular ionic homeostasis. In this case, the molecular targets for NO may be located either on brain cells to suppress mechanisms directly involved in CSD genesis, or on local blood vessels to couple flow to the increased energy demand associated with CSD.

Developmental changes in intracellular pH regulation in medullary neurons of the rat

We examined intracellular pH (pH(i)) regulation in the retrotrapezoid nucleus (RTN), a CO(2)-sensitive site, and the hypoglossal nucleus, a nonchemosensitive site, during development (postnatal days 2-18) in rats. Respiratory acidosis [10% CO(2), extracellular pH (pH(o)) 7.18] caused acidification without pH(i) recovery in the RTN at all ages. In the hypoglossal nucleus, pH(i) recovered in young animals, but as animal age increased, the slope of pH(i) recovery diminished. In animals older than postnatal day 11, the pH(i) responses to hypercapnia were identical in the hypoglossal nucleus and the RTN, but hypoglossal nucleus and RTN neurons could regulate pH(i) during intracellular acidification at constant pH(o) at all ages. Recovery of pH(i) from acidification in the RTN depended on extracellular Na+ and was inhibited by amiloride but was unaffected by DIDS, suggesting a role for Na+/H+ exchange. Hence, pH(i) regulation during acidosis is more effective in the hypoglossal nucleus in younger animals, possibly as a requirement of development, but in older juvenile animals (older than postnatal day 11), pH(i) regulation is relatively poor in chemosensitive (RTN) and nonchemosensitive nuclei (hypoglossal nucleus).

Response of intracellular pH to acute anoxia in individual neurons from chemosensitive and nonchemosensitive regions of the medulla

The effect of acute (10 minutes) exposure to anoxia on intracellular pH (pHi) in individual brainstem neurons, in slices from neonatal (P7 to P11) rats, was studied using a fluorescence microscopy imaging technique. Neurons from 4 regions of the medulla were studied, two of which contained chemosensitive neurons (nucleus tractus solitarius, NTS, and ventrolateral medulla, VLM) and two regions which did not contain chemosensitive neurons (hypoglossal, Hyp, and inferior olivary, IO). Acute anoxia caused a rapid and maintained acidification of 0.1-0.3 pH unit that was not different in neurons from chemosensitive vs. nonchemosensitive regions. Blocking the contribution of Na+/H+ exchange (NHE) to pHi regulation by exposing neurons to acute anoxia in the presence of the exchange inhibitor amiloride (1 mM) did not affect the degree of acidification seen in neurons from the NTS and VLM region, but significantly increased acidification (to about 0.35 pH unit) in Hyp and IO neurons. In summary, anoxia-induced intracellular acidification is not different between neurons from chemosensitive and nonchemosensitive regions, but NHE activity blunts acidification in neurons from the latter regions. These data suggest that neurons from chemosensitive areas might have a smaller acid load in response to anoxia than neurons from nonchemosensitive regions of the brainstem.

Ammonium prepulse: effects on intracellular pH and bioelectric activity of CA3-neurones in guinea pig hippocampal slices

The ammonium prepulse technique was used to study influences of intracellular pH (pH(i)) on bioelectric activity of CA3-neurones in hippocampal slices. 60, 180 or 600 s lasting NH(4)Cl (10 mM) pulses led to a transient intracellular alkalosis (DeltapH(i): up to 0.2 pH-units) in about one-half of the neurones loaded with 2', 7-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethylester (BCECF-AM). No alkalosis was seen in the remainder cells. The amount of alkalosis depended on the actual pH(i) of each neurone and increased when the pH(i) decreased. Washout of NH(4)Cl induced a fall in pH(i) (DeltapH(i): 0.12-0.54 pH-units) which recovered within <20 min. Frequency of spontaneous action potentials remained unchanged during washin of ammonium (60 or 180 s). However, pre-treatment with low concentrations of bicuculline-methiodide (0. 01 microM) or caffeine (0.1 mM), both of which did not change bioelectric activity per se, permitted a burst-activity to occur during ammonium-washin in about one-half of the neurones. In all neurones, washout of ammonium inhibited spontaneous and epileptiform activity (elicited by 1 mM caffeine, 20-50 microM bicuculline-methiodide, or 50-75 microM 4-aminopyridine) for </=20 min. This inhibition was accompanied by an increased membrane conductance (up to 20%) and a hyperpolarisation of up to 10 mV. We conclude that intracellular alkalosis augments, whereas intracellular acidosis depresses bioelectric activity of CA3-neurones.

Increases in [Ca2+]i and changes in intracellular pH during chemical anoxia in mouse neocortical neurons in primary culture

The effect of chemical anoxia (azide) in the presence of glucose on the free intracellular Ca2+ concentration ([Ca2+]i) and intracellular pH (pHi) in mouse neocortical neurons was investigated using Fura-2 and BCECF. Anoxia induced a reversible increase in [Ca2+]i which was significantly inhibited in nominally Ca2+-free medium. A change in pHo (8.2 or 6.6), or addition of NMDA and non-NMDA receptor antagonists (D-AP5 and CNQX) in combination, significantly reduced the increase in [Ca2+]i, pointing to a protective effect of extracellular alkalosis or acidosis, and involvement of excitatory amino acids. An initial anoxia-induced acidification was observed under all experimental conditions. In the control situation, this acidification was followed by a recovery/alkalinization of pHi in about 50% of the cells, a few cells showed no recovery, and some showed further acidification. EIPA, an inhibitor of Na+/H+ exchangers, prevented alkalinization, pointing towards anoxia-induced activation of a Na+/H+ exchanger. In a nominally Ca2+-free medium, the initial acidification was followed by a significant alkalinization. At pHo 8.2, the alkalinization was significantly increased, while at pHo 6.2, the initial acidification was followed by further acidification in about 50% of the cells, and by no further change in the remaining cells.

Intracellular pH regulation in neurons from chemosensitive and nonchemosensitive areas of the medulla

Intracellular pH (pHi) regulation was studied in neurons from two chemosensitive [nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM)] and two nonchemosensitive [hypoglossal (Hyp) and inferior olive (IO)] areas of the medulla oblongata. Intrinsic buffering power (betaint) was the same in neurons from all regions (46 mM/pH U). Na+/H+ exchange mediated recovery from acidification in all neurons [Ritucci, N. A., J. B. Dean, and R. W. Putnam. Am. J. Physiol. 273 (Regulatory Integrative Comp. Physiol. 42): R433-R441, 1997]. Cl-/HCO-3 exchange mediated recovery from alkalinization in VLM, Hyp, and IO neurons but was absent from most NTS neurons. The Na+/H+ exchanger from NTS and VLM neurons was fully inhibited when extracellular pH (pHo) <7.0, whereas the exchanger from Hyp and IO neurons was fully inhibited only when pHo <6.7. The Cl-/HCO-3 exchanger from VLM, but not Hyp and IO neurons, was inhibited by pHo of 7.9. These pH regulatory properties resulted in steeper pHi-pHo relationships in neurons from chemosensitive regions compared with those from nonchemosensitive regions. These differences are consistent with a role for changes of pHi as the proximate signal in central chemoreception and changes of pHo in modulating pHi changes.

The acidophilic nature of neuronal Golgi impregnation

The mechanisms of Golgi impregnation of neurons has remained enigmatic for decades. Recently, it was suggested that divalent (di)chromate anions play a role in the Golgi impregnation process. Therefore, we incubated slices of (para)formaldehyde-fixed rat brain tissue in solutions of potassium (di)chromate, phosphate, chloride or nitrate at pH 6 or 7. Slices were then immersed in solutions of silver nitrate and processed for light microscopical analysis. At pH 6, dichromate probes resulted in dense and homogeneous impregnation of neuronal cytoplasm (typical impregnation). At pH 7, chromate probes showed solely partial cytoplasmic and heavy nuclear-region neuron impregnation (atypical impregnation). Phosphate probes at pH 6 resulted in typical impregnation, whereas at pH 7 phosphate probes gave atypical impregnation. Both at pH 6 and 7, chloride and nitrate probes did not yield any Golgi impregnation. These findings confirmed the pH-dependence of silver-chromate Golgi impregnation as well as the correctness of corresponding acidic silver-phosphate impregnation. Our study revealed a previously unknown, strong anion-dependence of Golgi impregnation, suggesting that hydrogenated monovalent anions are carriers of the neuron impregnation.

The pH buffering capacity of hippocampal slices from young adult and aged rats

We examined the hypothesis that aging alters the capacity of brain tissue to buffer intracellular pH changes. Intracellular buffering power was determined in hippocampal slices from young adult and aged rats by raising the partial pressure of CO2. Changes in intracellular and extracellular pH in response to increases and decreases in CO2 were measured simultaneously with spectrophotometry and pH-sensitive microelectrodes. The intrinsic buffering power did not differ between young adult (25.6 +/- 11.1 mM) and aged (28.2 +/- 8.6 mM) slices. However, the bicarbonate buffering power was higher in young adult slices (72.52 +/- 5.07 mM) compared with aged slices (58.67 +/- 5.78 mM) since the resting intracellular pH was about 0.1 unit lower in aged slices. This decreased bicarbonate buffering capacity may contribute to the increased vulnerability of the aged brain to metabolic stress.

H+ transporters in the main excretory duct of the mouse mandibular salivary gland

1. We used microspectrofluorimetry with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF) to study the regulation of cytosolic pH (pHi) in the isolated, perfused main excretory duct of the mouse mandibular gland. 2. In nominally HCO3(-)-free solutions, removal of Na+ from the lumen alone caused pHi to decline whereas removing it from the bath alone did not. 3. Readmission of Na+ to the lumen of ducts studied under zero-Na+ conditions caused pHi to recover fully. This recovery was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) with a half-maximum concentration of 0.5 mumol l-1, indicating the presence of an apical Na(+)-H+ exchanger. 4. Readmission of Na+ to the bath of ducts studied under zero-Na+ conditions also caused pHi to recover. This recovery was blocked by 100 mumol l-1 EIPA, indicating the presence of a basolateral Na(+)-H+ exchanger. 5. Measurements of H+ fluxes indicated that the apical Na(+)-H+ exchanger was approximately four times more active than the basolateral Na(+)-H+ exchanger. 6. In three sets of experiments (in the absence of Na+, in the presence of Na+, and in the presence of Na+ plus 100 mumol l-1 EIPA), the effects of changing luminal K+ concentration on pHi were examined. We found no evidence for the presence of K(+)-H+ exchange or Na(+)-coupled K(+)-H+ exchange in the apical membranes of duct cells. 7. pHi recovery under nominally HCO3(-)-free conditions following acidification with an NH4Cl pulse was abolished by removal of Na+ from the bath and luminal solutions, indicating that no Na(+)-independent systems such as H(+)-ATPases were present. 8. A repeat of the above experiments in the presence of 25 mmol l-1 HCO3- plus 5% CO2 did not reveal any additional H+ transport systems. The removal of luminal Cl-, however, caused a small rise in pHi. This latter effect was blocked by 500 mumol l-1 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulphonic acid (H2-DIDS), suggesting that a Cl(-)-HCO3- exchanger in the apical membrane might contribute in a minor way to pHi regulation. 9. We conclude that the predominant H+ transport systems in the mouse mandibular main excretory duct are Na(+)-H+ exchangers in the apical and the basolateral membranes. The model we postulate to account for electrolyte transport across the main duct in the mouse mandibular gland is quite different from that previously developed for the rat duct but is similar to that developed for the rabbit duct. The difference is in concordance with the known ability of the mandibular gland of the rat, but not the rabbit or the mouse, to secrete a HCO3(-)-rich final saliva.

Cell-cell coupling between CO2-excited neurons in the dorsal medulla oblongata

Anatomically coupled neurons (17 of 137) and non-coupled neurons (120 of 137), in and near the nucleus tractus solitarius and dorsal motor nucleus (i.e. solitary complex), were studied by rapid perforated patch recording in slices (rat, 150-350 microm thick, postnatal day 0-21) before, during and after exposure to hypercapnic acidosis. Anatomical coupling refers to the intercellular transfer of Lucifer Yellow and Biocytin into adjoining neurons, presumably via gap junctions [see Dean et al. (1997) Neuroscience 80, 21-40]. Eighty-six per cent of the anatomically coupled neurons (12 of 14) were depolarized by hypercapnic acidosis, a response referred to as CO2 excitation or CO2 chemosensitivity. In all, 28% (12 of 43) of the CO2-excited neurons were anatomically coupled to at least one other neuron. None (0 of 39) of the CO2-inhibited neurons were anatomically coupled, and only 4% (two of 46) of the CO2-insensitive neurons were anatomically coupled. Increasing the fractional concentration of CO2 from five to 10 and 15% in constant bicarbonate (26 mM) decreased intracellular pH (control 7.3 7.4, 22-25 degrees C) by approximately 1.0 and 1.5 pH units, respectively, as measured using the pH-sensitive fluorescent dye, 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein. Nine of the anatomically coupled neurons (six CO2-excited, one CO2-insensitive and two unidentified) exhibited spontaneous electrotonic postsynaptic potential-like activity, suggesting that they were also electrotonically coupled. During hypercapnic acidosis, the amplitudes of electrotonic postsynaptic potentials were unchanged, concomitant with small changes in input resistance. The frequency of electrotonic postsynaptic potentials increased during hypercapnic acidosis in many anatomically coupled neurons (eight of nine), indicating that both neurons of the coupled pair were stimulated. Cell-cell coupling occurred preferentially in and between CO2-excited neurons of the solitary complex. Further, CO2-excited neurons were not electrotonically uncoupled during intracellular acidosis, in contrast to the effect that decreased intracellular pH has on many other types of coupled cells. It was not determined whether anatomical coupling was affected by hypercapnic acidosis since dye mixture was always administered under normocapnic conditions. The high correlation between anatomical coupling, electrotonic coupling activity and CO2-induced depolarization suggests that cell-cell coupling is an important electroanatomical feature in CO2-excited neurons of the solitary complex. CO2-excited neurons have been hypothesized to function in central chemoreception for the cardiorespiratory control systems, suggesting that cell cell coupling may contribute in part to central chemoreception of CO2 and H+.

Regulation of intracellular pH in salamander retinal rods

1. We measured intracellular pH (pHi) in rods isolated from the retina of the axolotl salamander, Ambystoma mexicanum, using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and -6)-carboxyfluorescein (BCECF). 2. The light exposures associated with data acquisition had no marked effect on pHi. There was no sharp change between the value obtained from the first exposure of dark-adapted rods and subsequent readings. Increasing the acquisition frequency from 1 to 10 min-1 either had no effect, or brought about a slow acidification, which was stopped or reversed when the low frequency was restored. 3. In nominally HCO3(-)-free solution at pH 7.5, the rods had a steady-state pHi of 7.09 +/- 0.02 (n = 46) and a buffering power (beta i) of 24 +/- 1 mM (pH unit)-1 (n = 48). The buffering power was virtually constant in the pH range 6.6-8.0. In the same range, pHi dependent linearly on perfusion pH (pHo) with regression coefficients of 0.4-0.5. 4. There were no significant differences between the inner and outer segment of intact rods as regards steady-state pHi or responses to experimental treatments. 5. Recovery from an intracellular acid load imposed by sodium propionate or an NH4Cl prepulse in nominally bicarbonate-free perfusate was completely blocked by decreasing the extracellular Na+ concentration to 7 mM, and slowed by 86% by applying 1 mM amiloride. 6. Introduction of 2% CO2-13 mM HCO3- caused an alkalinization that was often preceded by a transient acidification. Steady-state pHi was on average 0.1 pH units higher than in nominally bicarbonate-free solution. The mean acid extrusion rate, calculated on the assumption that CO2-HCO3- behaves as an open system, was 19% higher (31 +/- 2 mM h-1) than in a solution buffered only by Hepes (26 +/- 2 mM h-1). 7. In the presence of CO2-HCO3-, 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) decreased the acid extrusion rate by 20% on average. Lowering the extracellular Cl-concentration to 7 mM raised pHi, but did not significantly affect the acid extrusion rate. 8. We conclude that retinal rods regulate pHi by both Na(+)-H+ exchange and mechanism(s) involving HCO3(-)-Cl- exchange. In the present conditions, the Na(+)-H+ exchanger appears as the dominant mechanism for acid extrusion.

A fluorescence technique to measure intracellular pH of single neurons in brainstem slices

We have developed a technique to measure the pH, of single neurons in brainstem slices using a fluorescence imaging system. Slices were loaded with the pH-sensitive fluorescent dye BCECF and fluorescence was visualized by exciting the slices alternately at 500 and 440 nm. The emitted fluorescence at 530 nm was directed through an MTI GenIISys image intensifier and MT1 CCD72 camera. The images were processed by image-1/FL software. The ratio of emitted fluorescence at excitation wavelengths of 500 and 440 nm was measured and converted to pH by constructing a calibration curve using high K+/nigericin solutions at pH values ranging from 5.8 to 8.6. BCECF-loaded slices showed distinct spheres of intense fluorescence and diffuse background fluorescence. Slices labeled with a neuron-specific antibody, neuron-specific enolase, showed staining that correlated with the spheres of intense fluorescence of BCECF-loaded cells. Slices labeled with a glial-specific antibody, glial fibrillary acidic protein, showed a diffuse, background staining. Neurons that were retrograde-labeled with rhodamine beads fluoresced as large spheres that exactly correlated with the fluorescence from BCECF-loaded cells. Further, large fluorescent spheres had membrane potentials of about -60 mV and generated action potentials. These findings indicate that the large fluorescent spheres are neurons. pHi was measured in these large spheres (neurons) in the dorsal and ventral medullary chemosensitive regions, and was 7.32 +/- 0.02 (n = 110) and 7.38 +/- 0.02 (n = 85), respectively.

Salutary and deleterious effects of acidity on an indirect measure of metabolic rate and ATP concentrations in CNS cultures

Acidosis has traditionally been considered to mediate certain types of hypoxic-ischemic injury to the brain. However, the recent demonstration that moderate acidosis will reduce NMDA-mediated currents suggested that acidity could actually protect against types of ischemia and excitotoxicity, and in vitro studies now support this idea. Prompted by this, we have utilized the silicon microphysiometer, a recently-developed instrument that allows for indirect real-time measurement of metabolic rate by detecting proton efflux from small numbers of cultured cells, to determine whether acidity has protective effects upon cellular metabolism. Reducing extracellular pH from 7.4 to as low as 6.0 caused prompt, step-wise, and reversible inhibition of proton efflux rate in cortical and hippocampal cultures both normally and restricted to either glycolysis or oxidative metabolism. Approximately half of the inhibition was due to acidotic effects of NMDA-mediated currents, as demonstrated with NMDA receptor antagonists. Such an inhibition of this indirect metabolic measure could be associated with constant or increased ATP concentrations and represent a beneficial decrease in energy demands upon a neuron. Alternatively, an inhibition of proton efflux rate could be associated with ATP depletion and reflect impaired energy production. We observed a complex interplay between these opposing patterns. Reducing pH to 6.7 for 20 min caused significantly increased ATP concentrations, and prevented excitotoxin-induced ATP depletion. These effects of acidosis involved both NMDA-dependent and- independent actions. More severe (less than pH 6.7) acidosis did not cause ATP concentrations to rise, and if sustained for more than an hour caused a significant decline in ATP concentrations. Thus, despite the recent emphasis on the surprising neuroprotective potential of acidosis, a drop in pH is still likely to have complex and mixed consequences for brain tissue.

pH-associated Brain Injury in Cerebral Ischemia and Circulatory Arrest

Neuronal injury remains a major limitation in therapies directed toward cardiopulmonary resuscitation and cerebral ischemia. We summarize clinical and experimental information regarding pH-modulated mechanisms of cerebral ischemic injury and the status of antiacidosis therapies relative to the brain. A large body of evidence in animals and humans indicates that cerebral pH can modulate, and perhaps mediate, ischemic brain pathology and influence functional outcome. The importance of low pH and brain bicarbonate levels during reperfusion as a secondary injury remains an open question of therapeutic importance. Under specific conditions, acidosis may be neuroprotective, but this is an area of current controversy. Effective antiacidosis therapy must address the possibility of synergism and competition among multiple injury mechanisms.

The influence of calcium transients on intracellular pH in cortical neurons in primary culture

The objective of this study was to assess the influence of Ca2+ influx on intracellular pH (pHi) of neocortical neurons in primary culture. Neurons were exposed to glutamate (100-500 microM) or KCl (50 mM), and pHi was recorded with microspectrofluorometric techniques. Additional experiments were carried out in which calcium influx was triggered by ionomycin (2 microM) or the calcium ionophore 4-Br-A23187 (2 microM). Glutamate exposure either caused no, or only a small decrease in pHi (delta pH approximately 0.06 units). When a decrease was observed, a rebound rise in pHi above control was observed upon termination of glutamate exposure. In about 20% of the cells, the acidification was more pronounced (delta pH approximately 0.20 units), but all these cells had high control pHi values, and showed gradual acidification. Exposure of cells to 50 mM KCl consistently increased pHi. Since this increase was similar in the presence and nominal absence of HCO3-, it probably did not reflect influx of HCO3- via a Na(+)-HCO3- symporter. Furthermore, since it occurred in the absence of external Ca2+ (or a measurable rise in Cai2+) it seemed independent of Ca2+ influx. It is tentatively concluded that the rise in pHi was due to reduced passive influx of H+ along the electrochemical gradient, which is reduced by depolarization. In Ca(2+)-containing solutions, depolarization led to a rebound increase in pHi above control. This, and the rebound found after glutamate transients, may reflect Ca(2+)-triggered phosphorylation and upregulation of the Na+/H+ antiporter which extrudes H+ from the cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of intra- and extracellular pH in the rat brain in acute hypercapnia: a re-appraisal

Recent results have demonstrated that intracellular pH (pHi) in nerve and glial cells is not regulated back to normal during CO2 exposure if extracellular pH (pHe) is reduced. This raises the question about regulation of pHi and pHe in vivo. In order to successively reduce pHe we exposed animals to incremental increases in CO2 tension (11, 27.5, 42.5%) and studied regulation of pHi during the first 90 min of hypercapnia. Extracellular pH, as well as Na+, K+, and Cl- concentrations, were also measured, as were whole tissue contents of Na+, K+, and Cl-. At all CO2 tensions studied, pHe slowly increased during CO2 exposure. In animals breathing 11% CO2 (delta pHe approximately 0.2 units), pHi increased slowly. However, in animals exposed to 27.5% CO2 or 42.5% CO2 (delta pHe > 0.4 units), no regulation of pHi was observed. Extracellular HCO3- concentrations increased substantially already during the first 15 min of hypercapnia (not significant in animals breathing 42.5% CO2), and then gradually rose. These increases were accompanied by a decrease in Cl- and an increase in Na+ concentration, K+ concentration remaining constant. The total tissue content of these ions remained constant, suggesting that extracellular HCO3- concentration increases by Cl-/HCO3- antiport and/or by Na+.2HCO3- symport, the HCO3- emanating from intracellular sources. The results challenge the dogma of the supremacy of mechanisms regulating pHi, and suggest that brain cells, possibly astrocytes, regulate pHe at the expense of their own pH homeostasis. By inference, we further conclude that regulation of pHi normally occurs only if pHe is first regulated back close to normal value.

The regulation of intracellular pH is strongly dependent on extracellular pH in cultured rat astrocytes and neurons

We studied the mechanisms regulating intracellular pH (pHi) in cultured rat astrocytes and neurons, with particular reference to the influence of extracellular pH (pHe) on these mechanisms, using microspectrofluorometric monitoring from single cells, loaded with the pH-sensitive fluorophore BCECF. The pH regulatory mechanisms differ between neurons and astrocytes. The experimental data suggest the presence of a Na+/H+ and a Na(+)-independent HCO3-/Cl- exchanger in both types of cells, while astrocytes, in addition, utilise a Na(+)-dependent HCO3-/Cl- exchanger for regulating acid transients. In both cell types the pH regulatory mechanisms are strongly dependent on pHe. Thus, at pHe 6.85 or below, there was no recovery of pHi. Steady state pHi was also strongly dependent on pHe, in both astrocytes and neurons. The pHi recovery following normalisation of pHe was very rapid, (indicating that a prolonged exposure to a low pH stimulates pH regulating mechanisms), and was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and amiloride, or in the absence of Na+. The results challenge the concept of a H(+)-regulatory site solely at the internal side of the exchanger regulating pHi to a constant value.

Regulation of intracellular pH in single rat cortical neurons in vitro: a microspectrofluorometric study

Intracellular pH (pHi) and the mechanisms of pHi regulation in cultured rat cortical neurons were studied with microspectrofluorometry and the pH-sensitive fluorophore 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Steady-state pHi was 7.00 +/- 0.17 (mean +/- SD) and 7.09 +/- 0.14 in nominally HCO3(-)-free and HCO3(-)-containing solutions, respectively, and was dependent on extracellular Na+ and Cl-. Following an acid transient, induced by an NH1 prepulse or an increase in CO2 tension, pHi decreased and then rapidly returned to baseline, with an average net acid extrusion rate of 2.6 and 2.8 mmol/L/min, in nominally HCO3(-)-free and HCO3(-)-containing solutions, respectively. The recovery was completely blocked by removal of extracellular Na+ and was partially inhibited by amiloride or 5-N-methyl-N-isobutylamiloride. In most cells pHi recovery was completely blocked in the presence of harmaline. The recovery of pHi was not influenced by addition of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or removal of Cl-. The rapid regulation of pHi seen following a transient alkalinization was not inhibited by amiloride or by removal of extracellular Na+, but was partially inhibited by DIDS and by removal of extracellular Cl-. The results are compatible with the presence of at least two different pHi-regulating mechanisms: an acid-extruding Na+/H+ antiporter, possibly consisting of different subtypes, and a passive Cl-/HCO3- exchanger, mediating loss of HCO3- from the cell.