Role of chloride/bicarbonate antiport in the control of cytosolic pH. Cell-line differences in activity and regulation of antiport

Polyamines (PAs) but not small peptides with closely spaced positively charged groups interact with DNA and RNA, but they do not represent a relevant buffer system at physiological pH values

Polyamines (PAs) including putrescine (PUT), spermidine (SPD) and spermine (SPM) are small, versatile molecules with two or more positively charged amino groups. Despite their importance for almost all forms of life, their specific roles in molecular and cellular biology remain partly unknown. The molecular structures of PAs suggest two presumable biological functions: (i) as potential buffer systems and (ii) as interactants with poly-negatively charged molecules like nucleic acids. The present report focuses on the question, whether the molecular structures of PAs are essential for such functions, or whether other simple molecules like small peptides with closely spaced positively charged side chains might be suitable as well. Consequently, we created titration curves for PUT, SPD, and SPM, as well as for oligolysines like tri-, tetra-, and penta-lysine. None of the molecules provided substantial buffering capacity at physiological intracellular pH values. Apparently, the most important mechanism for intracellular pH homeostasis in neurons is not a buffer system but is provided by the actions of the sodium-hydrogen and the bicarbonate-chloride antiporters. In a similar approach we investigated the interaction with DNA by following the extinction at 260 nm when titrating DNA with the above molecules. Again, PUT and tri-lysine were not able to interact with herring sperm DNA, while SPD and SPM were. Obviously, the presence of several positively charged groups on its own is not sufficient for the interaction with nucleic acids. Instead, the precise spacing of these groups is necessary for biological activity.

Monocarboxylate transporters in the brain and in cancer

Monocarboxylate transporters (MCTs) constitute a family of 14 members among which MCT1-4 facilitate the passive transport of monocarboxylates such as lactate, pyruvate and ketone bodies together with protons across cell membranes. Their anchorage and activity at the plasma membrane requires interaction with chaperon protein such as basigin/CD147 and embigin/gp70. MCT1-4 are expressed in different tissues where they play important roles in physiological and pathological processes. This review focuses on the brain and on cancer. In the brain, MCTs control the delivery of lactate, produced by astrocytes, to neurons, where it is used as an oxidative fuel. Consequently, MCT dysfunctions are associated with pathologies of the central nervous system encompassing neurodegeneration and cognitive defects, epilepsy and metabolic disorders. In tumors, MCTs control the exchange of lactate and other monocarboxylates between glycolytic and oxidative cancer cells, between stromal and cancer cells and between glycolytic cells and endothelial cells. Lactate is not only a metabolic waste for glycolytic cells and a metabolic fuel for oxidative cells, but it also behaves as a signaling agent that promotes angiogenesis and as an immunosuppressive metabolite. Because MCTs gate the activities of lactate, drugs targeting these transporters have been developed that could constitute new anticancer treatments. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Cation-coupled bicarbonate transporters

Cation-coupled HCO3(-) transport was initially identified in the mid-1970s when pioneering studies showed that acid extrusion from cells is stimulated by CO2/HCO3(-) and associated with Na(+) and Cl(-) movement. The first Na(+)-coupled bicarbonate transporter (NCBT) was expression-cloned in the late 1990s. There are currently five mammalian NCBTs in the SLC4-family: the electrogenic Na,HCO3-cotransporters NBCe1 and NBCe2 (SLC4A4 and SLC4A5 gene products); the electroneutral Na,HCO3-cotransporter NBCn1 (SLC4A7 gene product); the Na(+)-driven Cl,HCO3-exchanger NDCBE (SLC4A8 gene product); and NBCn2/NCBE (SLC4A10 gene product), which has been characterized as an electroneutral Na,HCO3-cotransporter or a Na(+)-driven Cl,HCO3-exchanger. Despite the similarity in amino acid sequence and predicted structure among the NCBTs of the SLC4-family, they exhibit distinct differences in ion dependency, transport function, pharmacological properties, and interactions with other proteins. In epithelia, NCBTs are involved in transcellular movement of acid-base equivalents and intracellular pH control. In nonepithelial tissues, NCBTs contribute to intracellular pH regulation; and hence, they are crucial for diverse tissue functions including neuronal discharge, sensory neuron development, performance of the heart, and vascular tone regulation. The function and expression levels of the NCBTs are generally sensitive to intracellular and systemic pH. Animal models have revealed pathophysiological roles of the transporters in disease states including metabolic acidosis, hypertension, visual defects, and epileptic seizures. Studies are being conducted to understand the physiological consequences of genetic polymorphisms in the SLC4-members, which are associated with cancer, hypertension, and drug addiction. Here, we describe the current knowledge regarding the function, structure, and regulation of the mammalian cation-coupled HCO3(-) transporters of the SLC4-family.

Transmembrane domain histidines contribute to regulation of AE2-mediated anion exchange by pH

Activity of the AE2/SLC4A2 anion exchanger is modulated acutely by pH, influencing the transporter's role in regulation of intracellular pH (pHi) and epithelial solute transport. In Xenopus oocytes, heterologous AE2-mediated Cl−/Cl−and Cl−/HCO3−exchange are inhibited by acid pHior extracellular pH (pHo). We have investigated the importance to pH sensitivity of the eight histidine (His) residues within the AE2 COOH-terminal transmembrane domain (TMD). Wild-type mouse AE2-mediated Cl−/Cl−exchange, measured as DIDS-sensitive36Cl−efflux from Xenopus oocytes, was experimentally altered by varying pHiat constant pHoor varying pHo. Pretreatment of oocytes with the His modifier diethylpyrocarbonate (DEPC) reduced basal36Cl−efflux at pHo7.4 and acid shifted the pHovs. activity profile of wild-type AE2, suggesting that His residues might be involved in pH sensing. Single His mutants of AE2 were generated and expressed in oocytes. Although mutation of H1029 to Ala severely reduced transport and surface expression, other individual His mutants exhibited wild-type or near-wild-type levels of Cl−transport activity with retention of pHosensitivity. In contrast to the effects of DEPC on wild-type AE2, pHosensitivity was significantly alkaline shifted for mutants H1144Y and H1145A and the triple mutants H846/H849/H1145A and H846/H849/H1160A. Although all functional mutants retained sensitivity to pHi, pHisensitivity was enhanced for AE2 H1145A. The simultaneous mutation of five or more His residues, however, greatly decreased basal AE2 activity, consistent with the inhibitory effects of DEPC modification. The results show that multiple TMD His residues contribute to basal AE2 activity and its sensitivity to pHiand pHo.

Transporters involved in regulation of intracellular pH in primary cultured rat brain endothelial cells

Fluid secretion across the blood-brain barrier, critical for maintaining the correct fluid balance in the brain, entails net secretion of HCO(3)(-), which is brought about by the combined activities of ion transporters situated in brain microvessels. These same transporters will concomitantly influence intracellular pH (pH(i)). To analyse the transporters that may be involved in the maintenance of pH(i) and hence secretion of HCO(3)(-), we have loaded primary cultured endothelial cells derived from rat brain microvessels with the pH indicator BCECF and suspended them in standard NaCl solutions buffered with Hepes or Hepes plus 5% CO(2)/HCO(3)(-). pH(i) in the standard solutions showed a slow acidification over at least 30 min, the rate being less in the presence of HCO(3)(-) than in its absence. However, after accounting for the difference in buffering, the net rates of acid loading with and without HCO(3)(-) were similar. In the nominal absence of HCO(3)(-) the rate of acid loading was increased equally by removal of external Na(+) or by inhibition of Na(+)/H(+) exchange by ethylisopropylamiloride (EIPA). By contrast, in the presence of HCO(3)(-) the increase in the rate of acid loading when Na(+) was removed was much larger and the rate was then also significantly greater than the rate observed in the absence of both Na(+) and HCO(3)(-). Removal of Cl(-) in the presence of HCO(3)(-) produced an alkalinization followed by a resumption of the slow acid gain. Removal of Na(+) following removal of Cl(-) increased the rate of acid gain. In the presence of HCO(3)(-) and initial presence of Na(+) and Cl(-), DIDS inhibited the changes in pH(i) produced by removal of either Na(+) or Cl(-). These are the expected results if these cells possess an AE-like Cl(-)/HCO(3)(-) exchanger, a 'channel-like' permeability allowing slow influx of acid (or efflux of HCO(3)(-)), a NBC-like Cl(-)-independent Na(+)-HCO(3)(-) cotransporter, and a NHE-like Na(+)/H(+) exchanger. The in vitro rates of HCO(3)(-) loading via the Na(+)-HCO(3)(-) cotransporter could, if the transporter is located on the apical, blood-facing side of the cells, account for the net secretion of HCO(3)(-) into the brain.

Bicarbonate exporting transporters in the ovine ruminal epithelium

In order to stabilize the intraruminal pH, bicarbonate secretion by the ruminal epithelium seems to be an important prerequisite. The present study therefore focussed on the characterization of bicarbonate exporting systems in ruminal epithelial cells. Intracellular pH (pH(i)) was measured spectrofluorometrically in primary cultured ruminal epithelial cells loaded with the pH-sensitive fluorescent dye, 2,7-bis(carboxyethyl)-5(6')-carboxyfluorescein acetomethyl ester. Switching from CO2/HCO3- -buffered to HEPES-buffered solution caused a rapid intracellular alkalinization followed by a counter-regulation towards initial pH(i). The recovery of pH(i) was dependent upon extracellular chloride, but independent of extracellular sodium. Adding 500 microM H2DIDS significantly reduced the increase of pH(i). For further characterization of the bicarbonate exporting systems, we tested the ability to reverse the direction from HCO3- export to import in the absence of sodium and chloride. Under sodium and chloride-free conditions, counter-regulation after CO2-induced pH(i) decrease did not differ from pH(i) recovery in the presence of sodium and chloride. Existence of bicarbonate exporting systems in cultured ruminal epithelial cells and intact ruminal epithelium was verified by reverse transcription polymerase chain reaction (RT-PCR). Using RT-PCR and subsequent sequencing, expression of mRNA encoding for AE2, DRA and PAT1 could be found. Bicarbonate exporting systems could therefore be detected both on the functional and structural level.

An Inhibitor of the F1 subunit of ATP synthase (IF1) modulates the activity of angiostatin on the endothelial cell surface

Angiostatin binds to endothelial cell (EC) surface F(1)-F(0) ATP synthase, leading to inhibition of EC migration and proliferation during tumor angiogenesis. This has led to a search for angiostatin mimetics specific for this enzyme. A naturally occurring protein that binds to the F1 subunit of ATP synthase and blocks ATP hydrolysis in mitochondria is inhibitor of F1 (IF1). The present study explores the effect of IF1 on cell surface ATP synthase. IF1 protein bound to purified F(1) ATP synthase and inhibited F(1)-dependent ATP hydrolysis consistent with its reported activity in studies of mitochondria. Although exogenous IF1 did not inhibit ATP production on the surface of EC, it did conserve ATP on the cell surface, particularly at low extracellular pH. IF1 inhibited ATP hydrolysis but not ATP synthesis, in contrast to angiostatin, which inhibited both. In cell-based assays used to model angiogenesis in vitro, IF1 did not inhibit EC differentiation to form tubes and only slightly inhibited cell proliferation compared with angiostatin. From these data, we conclude that inhibition of ATP synthesis is necessary for an anti-angiogenic outcome in cell-based assays. We propose that IF1 is not an angiostatin mimetic, but it can serve a protective role for EC in the tumor microenvironment. This protection may be overridden in a concentration-dependent manner by angiostatin. In support of this hypothesis, we demonstrate that angiostatin blocks IF1 binding to ATP synthase and abolishes its ability to conserve ATP. These data suggest that there is a relationship between the binding sites of IF1 and angiostatin on ATP synthase and that IF1 could be employed to modulate angiogenesis.

Calcium-pH crosstalks in rat mast cells: modulation by transduction signals show non-essential role for calcium in alkaline-induced exocytosis

Alkalinization of cytosolic pH with ammonium chloride (NH4Cl) was reported to be a stimulus for mast cell degranulation. This paper studied the modulatory role of drugs that target protein kinase C (PKC), adenosine 3',5'-cyclic monophosphate (cAMP), tyrosine kinase (TyrK) and phosphatidylinositol 3-kinase (PI3K) on this effect. We used Go6976 (100 nM) and low concentrations of GF109203X (Gf) (50 nM) to inhibit calcium-dependent PKC isozymes. For calcium-independent isozymes, we used 500 nM Gf, and 10 microM rottlerin to specifically inhibit PKC delta, and chelerythrine as non-specific PKC inhibitor. Genistein (10 microM) and lavendustin A (1 microM) were used as unspecific TyrK inhibitors, and 10 nM wortmannin as a PI3K inhibitor. Chelerythrine and 50 nM Gf inhibit histamine release in the presence of external calcium. The inhibition caused by wortmannin was strictly internal calcium-dependent. cAMP-active drugs did not modify the response to NH4Cl. The effect of NH4Cl on histamine release was triggered by a transient elevation on cytosolic pH, which was simultaneous to an elevation on cytosolic calcium and followed by a probable Ca2+-H+ exchange after addition of external calcium. EGTA inhibit the response to suboptimal concentrations of NH4Cl, and BAPTA increased the effect of NH4Cl. There is a clear relationship between NH4Cl-mediated calcium release and histamine release, since those drugs that inhibit this release also inhibit NH4Cl-mediated histamine release; nevertheless, NH4Cl-mediated histamine release was possible in the absence of any calcium release, as shown with BAPTA. This data, in combination with the results with PKC inhibitors, suggest that calcium is not only unnecessary to trigger cell activation, but also that it may be a negative modulator of NH4Cl-mediated exocytosis.

Acute pH-dependent Regulation of AE2-mediated Anion Exchange Involves Discrete Local Surfaces of the NH2-terminal Cytoplasmic Domain

We have previously defined in the NH2-terminal cytoplasmic domain of the mouse AE2/SLC4A2 anion exchanger a critical role for the highly conserved amino acids (aa) 336-347 in determining wild-type pH sensitivity of anion transport. We have now engineered hexa-Ala ((A)6) and individual amino acid substitutions to investigate the importance to pH-dependent regulation of AE2 activity of the larger surrounding region of aa 312-578. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during changes in pHi or pHo in HEPES-buffered and in 5% CO2/HCO3- -buffered conditions. Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited at low pHo, with a pHo(50) value = 6.75 +/- 0.05 and was stimulated up to 10-fold by intracellular alkalinization. Individual mutation of several amino acid residues at non-contiguous sites preceding or following the conserved sequence aa 336-347 attenuated pHi and/or pHo sensitivity of 36Cl- efflux. The largest attenuation of pH sensitivity occurred with the AE2 mutant (A)6357-362. This effect was phenocopied by AE2 H360E, suggesting a crucial role for His360. Homology modeling of the three-dimensional structure of the AE2 NH2-terminal cytoplasmic domain (based on the structure of the corresponding region of human AE1) predicts that those residues shown by mutagenesis to be functionally important define at least one localized surface region necessary for regulation of AE2 activity by pH.

pH regulation and swimbladder function in fish

Gas gland cells of the European eel (Anguilla anguilla) are specialized for the production and secretion of acidic metabolites. Although typically exposed to high oxygen partial pressures, they convert glucose mainly into lactate, but also produce CO2 in the pentose phosphate shunt. Only a very small fraction of glucose is oxidized via aerobic metabolism. Although the buffer capacity of gas gland cells appears to be high, even at low extracellular pH values intracellular pH is always kept about 0.2-0.3 pH-units more acidic. Thus, under all physiological conditions proton concentration within gas gland cells is higher than in the extracellular fluid, facilitating proton extrusion. Diffusion of CO2, Na+/H+-exchange, sodium-dependent anion exchange and a V-ATPase represent the pathways available for proton secretion. While under resting conditions the sodium-dependent pathways and diffusion of CO2 appear to be the dominating mechanisms for acid secretion, at low intracellular pH the contribution of Na+/H+-exchange and of V-ATPase appear to increase, while sodium-dependent anion exchange becomes less important. The mechanisms regulating the activity of these acid-secreting pathways and of the metabolism responsible for the production of protons are largely unknown.

Angiostatin induces intracellular acidosis and anoikis in endothelial cells at a tumor-like low pH

Angiostatin inhibits angiogenesis by binding to endothelial cells (ECs) lining the vasculature of growing tumors. These cells are in a dynamic state during angiogenesis and are thus not firmly attached to the extracellular matrix. This makes them more vulnerable to anoikis, a process resulting in cell death initiated by or promoted by loss of attachment. Another potential source of EC vulnerability during tumor angiogenesis is that tumor extracellular pH is typically lower than in normal tissues. This presents an additional challenge to ECs in terms of maintaining ionic homeostasis. We report here that the lethality of angiostatin is significantly enhanced both by reduced matrix attachment during exposure and lowered extracellular pH (pH(e)). Another effect of angiostatin at reduced pH(e) is a decreased intracellular pH (pH(i)). These effects were observed in three model systems: aortic ring sprouts, ECs during tube formation, and ECs in a scratch/migration assay. In these three dynamic assays, angiostatin-induced cell death and intracellular acidification were clearly seen when pH(e) was reduced to 6.7. The intracellular acidification was far greater than that induced by pH(e) reduction alone. In contrast, the effect of angiostatin on pH(i) and on viability were not observed in a subconfluent monolayer in which the cells were allowed to attach to substrate for 48 h prior to exposure to angiostatin. These data suggest that low pH(e) and reduced adhesion to matrix play a role in the specificity of angiostatin for tumor neovasculature in contrast to wound healing and other normal angiogenic processes. The results also implicate roles for both pH(e) and pH(i) regulation in the mechanism of angiostatin action.

Regulation of AE2-mediated Cl- transport by intracellular or by extracellular pH requires highly conserved amino acid residues of the AE2 NH2-terminal cytoplasmic domain

We reported recently that regulation by intracellular pH (pH(i)) of the murine Cl-/HCO(3)(-) exchanger AE2 requires amino acid residues 310-347 of the polypeptide's NH(2)-terminal cytoplasmic domain. We have now identified individual amino acid residues within this region whose integrity is required for regulation of AE2 by pH. 36Cl- efflux from AE2-expressing Xenopus oocytes was monitored during variation of extracellular pH (pH(o)) with unclamped or clamped pH(i), or during variation of pH(i) at constant pH(o). Wild-type AE2-mediated 36Cl- efflux was profoundly inhibited by acid pH(o), with a value of pH(o50) = 6.87 +/- 0.05, and was stimulated up to 10-fold by the intracellular alkalinization produced by bath removal of the preequilibrated weak acid, butyrate. Systematic hexa-alanine [(A)6]bloc substitutions between aa 312-347 identified the greatest acid shift in pH(o(50)) value, approximately 0.8 pH units in the mutant (A)6 342-347, but only a modest acid-shift in the mutant (A)6 336-341. Two of the six (A)6 mutants retained normal pH(i) sensitivity of 36Cl- efflux, whereas the (A)6 mutants 318-323, 336-341, and 342-347 were not stimulated by intracellular alkalinization. We further evaluated the highly conserved region between aa 336-347 by alanine scan and other mutagenesis of single residues. Significant changes in AE2 sensitivity to pH(o) and to pH(i) were found independently and in concert. The E346A mutation acid-shifted the pH(o(0) value to the same extent whether pH(i) was unclamped or held constant during variation of pH(o). Alanine substitution of the corresponding glutamate residues in the cytoplasmic domains of related AE anion exchanger polypeptides confirmed the general importance of these residues in regulation of anion exchange by pH. Conserved, individual amino acid residues of the AE2 cytoplasmic domain contribute to independent regulation of anion exchange activity by pH(o) as well as pH(i).

Regulation of AE2 anion exchanger by intracellular pH: critical regions of the NH(2)-terminal cytoplasmic domain

The role of intracellular pH (pH(i)) in regulation of AE2 function in Xenopus oocytes remains unclear. We therefore compared AE2-mediated (36)Cl(-) efflux from Xenopus oocytes during imposed variation of extracellular pH (pH(o)) or variation of pH(i) at constant pH(o). Wild-type AE2-mediated (36)Cl(-) efflux displayed a steep pH(o) vs. activity curve, with pH(o(50)) = 6.91 +/- 0.04. Sequential NH(2)-terminal deletion of amino acid residues in two regions, between amino acids 328 and 347 or between amino acids 391 and 510, shifted pH(o(50)) to more acidic values by nearly 0.6 units. Permeant weak acids were then used to alter oocyte pH(i) at constant pH(o) and were shown to be neither substrates nor inhibitors of AE2-mediated Cl(-) transport. At constant pH(o), AE2 was inhibited by intracellular acidification and activated by intracellular alkalinization. Our data define structure-function relationships within the AE2 NH(2)-terminal cytoplasmic domain, which demonstrates distinct structural requirements for AE2 regulation by intracellular and extracellular protons.

Mechanism of intracellular pH increase during parthenogenetic activation of In vitro matured porcine oocytes

Parthenogenetic activation of porcine oocytes by using 7% ethanol, 50 or 100 microM A23187 results in an increase in intracellular pH as does prolonged exposure to thimerosal. We attempt to specify which transporters or mechanisms are involved in the observed increase in intracellular pH during oocyte activation. Experiments were performed in the absence of sodium; the presence of 2.5 mM amiloride, a potent inhibitor of the Na(+)/H(+) antiport; in the absence of bicarbonate; and in the presence of 4, 4'-diisothiocyanatodihydrostilbene-2,2'-di-sulfonic acid, disodium salt (H(2)DIDS) for all three activation methods. These treatments had no effect on the increase in intracellular pH induced by the calcium ionophore or thimerosal, but all reduced the increase in pH (P < 0.001) in the 7% ethanol group. This suggests that the Na(+)/H(+) antiport and the HCO(3)(-)/Cl(-) exchangers are not playing a role during treatment with calcium ionophore or thimerosal, and the pH increase observed during treatment with 7% ethanol may be dependent upon a sodium or bicarbonate flux (or both) into the oocyte. Bafilomycin A1 (500 nm), an inhibitor of vacuolar-type H(+) ATPases, had no effect on 7% ethanol or thimerosal treatments, but significantly reduced the increase in intracellular pH observed during calcium ionophore treatment. This may be the result of an initial local increase in intracellular free calcium levels.

Intracellular pH regulation in a nonmalignant and a derived malignant human breast cell line

Tumor cells in vivo often exist in an ischemic microenvironment that would compromise the growth of normal cells. To minimize intracellular acidification under these conditions, these cells are thought to upregulate H(+) transport mechanisms and/or slow the rate at which metabolic processes generate intracellular protons. Proton extrusion has been compared under identical conditions in two closely related human breast cell lines: nonmalignant but immortalized HMT-3522/S1 and malignant HMT-3522/T4-2 cells derived from them. Only the latter were capable of tumor formation in host animals or long-term growth in a low-pH medium designed to mimic conditions in many solid tumors. However, detailed study of the dynamics of proton extrusion in the two cell lines revealed no significant differences. Thus, even though the ability to upregulate proton extrusion in a low pH environment (pH(e)) may be important for cell survival in a tumor, this ability is not acquired along with the capacity to form solid tumors and is not unique to the transformed cell. This conclusion was based on fluorescence measurements of intracellular pH (pH(i)) on cells that were plated on extracellular matrix, allowing them to remain adherent to proteins to which they had become attached 24 to 48 h earlier. Proton translocation under conditions of low pH(e) was observed by monitoring pH(i) after exposing cells to an acute acidification of the surrounding medium. Proton translocation at normal pH(e) was measured by monitoring the recovery after introduction of an intracellular proton load by treatment with ammonium chloride. Even in the presence of inhibitors of the three major mechanisms of proton translocation (sodium-proton antiport, bicarbonate transport, and proton-lactate symport) together with acidification of their medium, cells showed only about 0.4 units of reduction in pH(i). This was attributed to a slowing of metabolic proton generation because the inhibitors were shown to be effective when the same cells were given an intracellular acidification.

Modulatory effect of HCO3- on rat mast cell exocytosis: cross-talks between bicarbonate and calcium

HCO-3 modulation of histamine release and its relationship with the Ca2+ signal were studied in serosal rat mast cells. Histamine release was induced by Ca2+ mobilizing stimuli, namely compound 48/80, thapsigargin, Ca2+ chelators, ionophore A23187, and PMA and ionophore A23187 in a HCO-3-buffered medium or a HCO-3-free medium. The presence of HCO-3 reduced histamine release by 48/80, Ca2+ chelators, A23187, and PMA/A23187, but increased histamine release induced by thapsigargin. Histamine release by PMA was significantly higher in a HCO-3-free medium than in a HCO-3-free medium, as it was the PMA potentiation of histamine release by A23187. [Ca2+]i changes induced by these drugs were measured in fura-2-loaded mast cells. In thapsigargin and EGTA or BAPTA preincubated mast cells [Ca2+]i increase was higher in a HCO-3-buffered medium than in a HCO-3-free medium in the presence of Ca2+. On the contrary, in compound 48/80 and PMA/A23187 activated mast cells the [Ca2+]i increase is the same both in the presence and in the absence of HCO-3. The effect of HCO-3 on histamine release in serosal rat mast cells depends on the stimulus, but it is not related to the presence of Cl-. In thapsigargin-stimulated mast cells the effect of HCO-3 on histamine release may be related to the Ca2+ signal, but in compound 48/80, EGTA, and PMA/A23187-activated mast cells there is no relationship between intracellular Ca2+ and the inhibitory effect of HCO-3 on histamine release. Additionally, the PKC pathway is implicated in the inhibitory effect of HCO-3 on histamine release, the higher the chelation of calcium rendering the higher the enhancement of the response after adding calcium in the absence of HCO-3.

Role of HCO3- ions in cytosolic pH regulation in rat mast cells: evidence for a new Na+-independent, HCO3--dependent alkalinizing mechanism

The role of external HCO3- in pHi regulation in rat mast cells was studied with BCECF. In a HCO3--free medium cells undergo an alkalinization after the addition of 40 mM HCO3Na. This alkalinization is unaffected by pH. In a Na+-free medium, the addition of 20 mM HCO3Na induced a higher alkalinization than 20 mM HCO3K. Amiloride (1 mM), a Na+/H+ exchanger inhibitor, inhibited by 45% the alkalinization induced by HCO3Na, but it did not change that induced by HCO3K. The anion exchanger inhibitor DIDS reduced 20% the alkalinization induced by both salts. An alkalinization of 0.085 units is observed after the addition of 20 mM HCO3K, even when these exchangers are inhibited (in the absence of Na+ and presence of DIDS). We conclude that the Na+/H+ exchanger and the Cl-/HCO3- exchangers are alkalinizing mechanisms that regulate pHi in these cells. Also, there is some HCO3--dependent, Na+-independent mechanism responsible for the intracellular alkalinization.

Human cytomegalovirus infection stimulates Cl-/HCO-3 exchanger activity in human fibroblasts

The effects of human cytomegalovirus (HCMV) infection on Cl-/HCO-3 exchanger activity in human lung fibroblasts (MRC-5 cells) were studied using fluorescent, ion-sensitive dyes. The intracellular pH (pHi) of mock- and HCMV-infected cells bathed in a solution containing 5% CO2-25 mM HCO-3 were nearly the same. However, replacement of external Cl- with gluconate caused an H2DIDS-inhibitable (100 microM) increase in the pHi of HCMV-infected cells but not in mock-infected cells. Continuous exposure to hyperosmotic external media containing CO2/HCO-3 caused the pHi of both cell types to increase. The pHi remained elevated in mock-infected cells. However, in HCMV-infected cells, the pHi peaked and then recovered toward control values. This pHi recovery phase was completely blocked by 100 microM H2DIDS. In the presence of CO2/HCO-3, there was an H2DIDS-sensitive component of net Cl- efflux (external Cl- was substituted with gluconate) that was less in mock- than in HCMV-infected cells. When nitrate was substituted for external Cl- (in the nominal absence of CO2/HCO-3), the H2DIDS-sensitive net Cl- efflux was much greater from HCMV- than from mock-infected cells. In mock-infected cells, H2DIDS-sensitive, net Cl- efflux decreased as pHi increased, whereas for HCMV-infected cells, efflux increased as pHi increased. All these results are consistent with an HCMV-induced enhancement of Cl-/HCO-3 exchanger activity.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

Effects of extracellular pH on intracellular pH-regulation and growth in a human colon carcinoma cell-line

Mechanisms of intracellular pH (pHi) regulation seem to be involved in cellular growth and cell division. Little is known about how extracellular acidosis, known to occur in central regions of solid tumors, or alkaline conditions affect pHi regulation in colonic tumors. pHi changes in the colonic adenocarcinoma cell-line SW-620 were recorded by spectrofluorimetric monitoring of the pH-sensitive, fluorescent dye BCECF, and proliferative activity was assessed by [3H]thymidine uptake. Resting pHi in Hepes-buffered solution was 7.53 +/- 0.01 (n = 36). Both 1 mM amiloride and Na(+)-free solution inhibited pHi recovery from acidification and decreased pHi in resting cells. In HCO3-/CO2-buffered media resting pH1 was 7.42 +/- 0.01 (n = 36). Recovery from acidification was Na(+)-dependent, CI(-)-independent, and only partially blocked by 1 mM amiloride. In the presence of amiloride and 200 microM H2DIDS pHi recovery was completely inhibited. In Na(+)-free solution pHi decreased from 7.44 +/- 0.04 to 7.29 +/- 0.03 (n = 6) and no alkalinization was observed in CI(-)-free medium. Addition of 5 microM tributyltin bromide (an anion/OH-exchange ionophore) caused pHi to decrease from 7.43 +/- 0.05 to 7.17 +/- 0.08 (n = 5). The effects of pH0 on steady-state pHi, pHi recovery from acidification and proliferative activity after 48 h were investigated by changing buffer [CO2] and [HCO3-]. In general, increases in pH0 between 6.7 and 7.4 increased pHi recovery, steady-state pHi and growth rates. In summary, SW-620 cells have a resting pHi > 7.4 at 25 degrees C, which is higher than other intestinal cells. Acid extrusion in physiological bicarbonate media is accomplished by a pHi-sensitive Na+/H+ exchanger and a pHi-insensitive Na(+)-HCO3-cotransporter, both of which are operational in control cells at the resting pHi. No evidence for activity of a CI-/HCO3- exchanger was found in these cells, which could account for the high pHi observed and may explain why the cells continue to grow in acidic tumor environments.

Thermotolerance and intracellular pH in two Chinese hamster cell lines adapted to growth at low pH

As an in vitro model for the low extracellular pH (pHe) which has frequently been observed in tumors, cell lines have been grown in a low-pH medium in order to allow cell adaptation to that milieu. Two Chinese hamster cell lines [Chinese hamster ovary (CHO) and Chinese hamster ovarian carcinoma (OvCa)] were compared, both of which acquired thermotolerance during 42 degrees C heating in pHe = 7.3 buffer, but not in pHe = 6.7 medium unless grown at that pH long enough to become adapted. CHO cells, even when acutely acidified, showed higher intracellular pH (pHi) values in a suspension assay than OvCa cells, which confirmed the danger of comparing absolute values of pHi between cell lines. Despite this fundamental difference, relative changes in pHi were similar in that both lines showed a higher pHi in adapted than in unadapted cells, over the range of pHe values tested. The upregulation of pHi was statistically significant, but the two lines differed in the time frame over which adaptation occurred. OvCa cells acquired an enhanced ability to develop tolerance to 42 degrees heat at pHe = 6.7 in 4 days, but the CHO cells acquired this ability more progressively, achieving a maximum ability at approximately 100 days. In contrast, both lines were able to upregulate their pHi within 4 hours of being exposed to pH 6.7 medium. A further indication of different biochemical mechanisms at work was the opposite effects seen on pHi in the two cell lines upon the removal of extracellular CO2/HCO3-. The differential between adapted and unadapted OvCa cells was enhanced by removal of bicarbonate, whereas CHO cells seemed less stable and the data with greater scatter failed to show any difference between adapted and unadapted cells.

Modification of intracellular pH and thermotolerance development by amiloride

The intracellular pH (pHi) of cells heated at 45.0 degrees C in the presence or absence of amiloride and in choline chloride substituted sodium-free medium was measured with flow cytometry using the pH sensitive dye, carboxy-seminaphthorhodafluor (SNARF-1). Chinese hamster ovary (CHO) cells at pH 7.3 and low-pH-resistant (PHV2) cells at pH 6.6 were studied. Bimodal population distributions of pHi were obtained for both CHO and PHV2 cells following a treatment in which cells were heated 10 min at 45.0 degrees C, incubated 4 to 10 h at 37 degrees C, then reheated at 45 min at 45.0 degrees C. Amiloride or sodium-free medium modified the changes in pHi, but did not eliminate them entirely. Cells were sorted from the higher pHi and lower pHi subpopulations and plated for cell survival. The survival after both heat treatments was three to five-fold higher for cells sorted from the higher pHi subpopulation than cells sorted from the low pHi subpopulation. The development of thermotolerance was delayed in CHO cells but not in PHV2 cells when amiloride was present throughout the treatment regimen. Combining low pH with amiloride caused an even greater delay in thermotolerance development in CHO cells. However, the final fraction of thermotolerant cells after 14 h incubation was nearly identical, regardless of medium pH or the presence of amiloride.

Sulfate transport mediated by the mammalian anion exchangers in reconstituted proteoliposomes

The kinetic properties of sulfate transport mediated by the anion exchangers AE1 and AE2 have been examined. Microsomes isolated from HEK cells transiently overexpressing either protein were reconstituted in unilamellar, 200-600-nm diameter proteoliposomes. Transport mediated by the exchangers was monitored by loading the reconstituted proteoliposomes with the slowly transportable anion SO4(2-) using [35S]SO4(2-) as a tracer and performing [35S]SO4(2-)/SO4(2-) exchange. The following data suggest that AE1 and AE2 have been functionally reconstituted: (i) the rate of SO4(2-) transport in AE1 and AE2 containing proteoliposomes was 10-20 times higher than in proteoliposomes derived from control microsomes; (ii) the transport of SO4(2-) was strongly dependent on the presence of a trans anion; and (iii) the anion exchanger inhibitors, 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) and 4,4'-dinitrostilbene-2,2'-di-sulfonate (DIDS) totally abolished SO4(2-) transport. furthermore, DIDS inhibits SO4(2-) transport only when occluded inside the vesicles, indicating a uniform, asymmetrical, inside-out orientation of the reconstituted exchangers. The Ki values of the stilbene disulfonate compound DNDS were 2.5 and 4 microM for AE1 and AE2, respectively, suggesting that the two exchangers possess similar high affinity sites for stilbene compounds. Both AE1 and AE2 showed the same steep pH dependence of sulfate transport, which was maximal at pH 5.5 and reduced to less than 10% (of the value at pH 5.5) at pH 8.5, suggesting that an acidic residue shared by AE1 and AE2 participates in the pH regulation of sulfate transport.

Bicarbonate ions and pH regulation of Leishmania major promastigotes

Leishmania major promastigotes are parasites endowed with a plasma membrane electrogenic H+ pump and anionic channels. These systems have been thought to contribute to pH homeostasis of parasites and environmental adaptation by mediating extrusion of protons which are either generated metabolically or result from exogenous acid loads. In this work we show that HCO-3 transport plays a physiological role in supporting pH regulation of parasites. Intracellular pH (pHi) and the membrane potential (Vm) were assessed fluorometrically with pH sensitive and potentiometric dyes. We show that intracellular acidification, caused either by blocking the pump or the putative anion channel or by depleting Cl- from cells, could be largely overcome by addition of HCO-3. Likewise, addition of HCO-3 raises the steady state intracellular pH of untreated cells from 6.76 +/- 0.01 to 6.98 +/- 0.02 and induces membrane hyperpolarization in pump-inhibited cells. We provide evidence for the involvement of HCO-3 transport systems that subserve pH homeostasis in Leishmania promastigotes. A major anionic pathway which is sensitive to anion transport blockers is apparently conductive in nature and accomodates ions such as HCO-3 and Cl-. In physiological conditions, the primary role of H+ pumping is the generation of a relatively large membrane potential (Vm = -113 +/- 4 mV) which subserves electrochemical-driven uptake of nutrients. The involvement of H+ pumping in physiological pH regulation of promastigotes is apparently of a secondary nature.

Growth inhibition of rat glioma cells in vitro and in vivo by aspirin

The effect of non-steroidal anti-inflammatory drugs (NSAIDs), acetylsalicylic acid (commonly known as aspirin), salicylic acid, piroxicam and indomethacin on the growth of rat glioma cells (RG 2) in vitro and aspirin in vivo was studied. The in vitro studies reveal that aspirin and salicylic acid strongly inhibit growth of rat glioma (RG 2) cells in concentrations used in medicine for treatment of rheumatic diseases. On the other hand, indomethacin and piroxicam had no effect, indicating that the inhibitory effect on tumor growth is not due to the inhibition of prostaglandin synthesis. The synthesis of ATP was markedly reduced (34% of control) in the presence of drugs, whereas protein synthesis measured as 3H-leucine incorporation was slightly more inhibited (73% of control) than cell growth. Aspirin administered to Fischer 344 rats inhibited growth of RG 2 cells inoculated into the caudate nucleus in vivo, both when administered the day before inoculation of tumor cells and when tumors had formed, i.e. 5 days post inoculation.

Thermosensitization by increasing intracellular acidity with amiloride and its analogs

PURPOSE

The major mechanisms that regulate the intracellular acidity of pHi in mammalian cells are the Na+/H+ exchange and HCO3-/Cl- exchange through the plasma membrane. The purpose of this study was to investigate the feasibility of increasing the thermosensitivity of tumors by increasing intracellular acidity with the use of drugs that inhibit the pHi regulatory mechanisms.

METHODS AND MATERIALS

The pHi of SCK tumor cells in vitro was determined with the fluorescence spectroscopy method. The thermosensitizing effects of the drugs on the cells in neutral (pH 7.2-7.5) and acidic (pH 6.6) media were determined by clonogenic assay. The thermosensitization of SCK tumors in vivo by the drugs was determined with the tumor growth delay and the in vivo-in vitro assay for clonogenic cells.

RESULTS

The pHi of SCK tumor cells in pH 7.2-7.5 media was similar to the media pH, while the pHi of the cells in pH 6.6 media was about 7.0. The pHi declined and the thermosensitivity of the tumor cells increased when the Na+/H+ exchange was inhibited with amiloride (3,5 diamino-6-chloro-N-(diaminomethylene) pyrazinecarboxamide) and its analogs, HMA (3-amino-6-chloro-5-(1-homopiperidyl)-N-(diaminomethylene) pyrazinecarboxamide) or EIPA (3-amino-6-chloro-5-(N-ethyl-N-isopropylamino)-N-diaminomethylene) pyrazinecarboxamide), especially in acidic medium. The potencies of HMA and EIPA to decrease the pHi and increase the thermosensitivity in vitro were more than 50 times greater than that of amiloride. DIDS (4,4-diiosothiocyanatostilbene-2,2'-disulfonic acid), an inhibitor of the Na(+)-dependent HCO3-/Cl- exchange, exerted little effect on the pHi and thermosensitivity of SCK cells in vitro, but it enhanced the effects of amiloride and its analogs. Amiloride and HMA also significantly enhanced the thermal effect on tumors in vivo, as judged by the tumor growth delay and also by the in vitro-in vivo assay for clonogenic cells. Combinations of DIDS with amiloride or HMA were more effective than either of them alone in increasing the thermal damage in vivo. As in vitro, HMA was far more potent than amiloride in increasing the thermosensitivity of tumor cells in vivo. However, EIPA was not effective in vivo, probably due to a rapid metabolic breakdown of the drug.

CONCLUSION

The drugs that interfere with the pHi regulatory mechanism significantly thermosensitized the tumor cells in vitro, particularly those in acidic media. The drugs were also effective in increasing the thermosensitivity of tumors. Because the interstitial environment in tumors is acidic relative to that in normal tissues, the thermosensitization by the drugs may be greater in tumors than that in normal tissues.

pHi and serum regulate AE2-mediated Cl-/HCO3- exchange in CHOP cells of defined transient transfection status

Anion exchanger (AE) protein-mediated anion exchange contributes to regulation of intracellular pH (pHi), Cl- concentration, and volume in vertebrate cells. We have extended the functional characterization of recombinant AE2-mediated Cl-/HCO3- exchange in single Chinese hamster ovary cells stably transfected with the polyoma large T antigen (CHOP cells) of defined transient transfection status using a novel surface marker coexpression vector. Marker expression and detection had minimal effect on the low endogenous Cl-/HCO3- exchange activity of CHOP cells, whereas coexpression of marker with AE2 elevated CHOP cell Cl-/HCO3- exchange activity 16-fold. Between pHi of 7.3 and 7.8, AE2-mediated flux of proton equivalents was activated > 11-fold by increasingly alkaline pHi without reaching saturation. This activation may be secondary to allosteric effects of pHi on AE2, in parallel with the obligatory increase in substrate intracellular HCO3- concentration. Nominal removal of CO2/HCO3- reduced AE2 activity by 90%. Addition of 10% calf serum slowly activated AE2 activity severalfold. This activation was slowly reversed after serum removal. Surface marker coexpression vectors improve both the efficiency and reliability of studies of recombinant protein function for a wide range of single cell assays in many cell types.

Effects of HMA, an analog of amiloride, on the thermosensitivity of tumors in vivo

PURPOSE

The effects of HMA (3-amino-6-chloro-5-(1-homopiperidyl)-N- (diaminomethylene)pyrazinecarboxamide), an analog of amiloride, on the intracellular pH (pHi) of SCK tumor cells in vitro and on the thermosensitivity of tumors in vivo were investigated.

METHODS AND MATERIALS

The pHi of SCK tumor cells in vitro was measured with the BCEC fluorescence spectroscopy method. The effect of HMA on the thermosensitivity of SCK tumors grown SC in the legs of A/J mice was assessed by the tumor growth delay method and the in vivo-in vitro excision assay method.

RESULTS

The pHi of SCK tumor cells in pH 7.5 and 6.6 medium was about 7.50 and 7.15, respectively. The presence of 10-50 microM of HMA lowered the pHi by 0.1-0.2 pH units both in pH 7.5 and 6.6 medium. Heating at 43 degrees C 120 min lowered the pHi by 0.2 and 0.3 pH units in pH 7.5 and 6.6 medium, respectively. When the cells were heated in the presence of 10-50 microM HMA, a marked decline in pHi occurred and and the decline in pHi resulting from the combination of heat and HMA was more pronounced in pH 6.6 medium than in pH 7.5 medium. Heating the SCK tumors grown SC in the legs of A/J mice at 43.5 degrees C for 1 h resulted in a growth delay of 3.7 days. When the host mice were i.v. injected with 0.1 mg/kg of HMA and the tumors were heated heated 20 min later, the tumor growth was delayed by 8.2 days, which was 4.5 days longer than that by heating alone. Heating the SCK tumor at 42.5 degrees C for 1 h caused a tumor growth delay of 0.9 days. An i.v. injection of 1 mg/kg or 10 mg/kg of HMA prior to heating at 42.5 degrees C for 1 h caused a tumor growth delay 2.1 and 3.1 days longer, respectively, than that by heating alone. Such an enhancement of heat-induced tumor growth delay by HMA was due to increased cell killing, as determined with the in vivo-in vitro excision assay of clonogenic cells in the tumors.

CONCLUSION

HMA is a potent thermosensitizer, particularly in an acidic environment. Thermosensitization by HMA may occur preferentially in tumors relative to normal tissues since the intratumor environment is acidic.

Kinetics of the chloride-anion exchanger of brush-border membrane vesicles isolated from chicken jejunum

The kinetic parameters of the apical Cl(-)-anion exchanger of chicken jejunum have been evaluated using brush-border membrane vesicles. SITS inhibited pH gradient-driven Cl- uptake in a dose-dependent manner with a IC50 of 700 microM. In the presence of a pH gradient (pH 7.7 inside, 5.5 outside), Cl- uptake tends to saturate with increasing external Cl- concentration. With 5 mM SITS the relationship between 36Cl- uptake and external Cl- concentration was linear. SITS-insensitive Cl- uptake may represent a diffusion component and has an apparent diffusion constant, Kd, of 0.068 +/- 0.002 nmol/mg protein per 15 s per mM. Eadie-Hofstee analysis of the data obtained by subtracting the SITS-insensitive component from the total uptake indicates a single transport system with a Km of 5 +/- 0.9 mM. A 50 mM outwardly directed bicarbonate gradient increased the Vmax from 3.63 +/- 0.33 to 6.40 +/- 0.54 nmol/mg protein per 15 s. The initial rate of H2-DIDS-sensitive Cl- uptake increased as the intravesicular pH increased, with a Hill coefficient greater than one. An intra- to extravesicular gradient of Cl-, I- and HCO3- stimulated Cl- uptake in the absence of a pH gradient.

Alpha 2A-adrenergic receptors activate protein kinase C in human platelets via a pertussis toxin-sensitive G-protein

4,4'-Diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) stimulates human platelets via alpha 2A-adrenergic receptor-mediated activation of protein kinase C (PKC) independent of the phospholipase C pathway. Here we show, that in permeabilized platelets activation of PKC by DIDS (20 microM), measured as 32P incorporation in pleckstrin, is completely inhibited by guanosine 5'-(2-O-thio)diphosphate (200 microM), an inhibitor of heterotrimeric G-proteins. Also pertussin toxin (4 micrograms/ml), which ADP-ribosylates the alpha-subunits of Gi's and Go, prevents pleckstrin phosphorylation by DIDS. N-Ethylmaleimide (50 microM), which uncouples Gi from alpha 2A-adrenoceptors, inhibits pleckstrin phosphorylation by DIDS in intact platelets. Activation of PKC by 55 nM phorbol 12-myristate 13-acetate and 500 nM platelet-activating factor are not disturbed by NEM. DIDS inhibits by 40 +/- 5% (n = 4) the pertussis toxin-catalyzed [32P]ADP-ribosylation of a 41 kDa protein fraction previously shown to contain the alpha-subunits of Gi alpha-1, Gi alpha-2 and Gi alpha-3. Thus, the alpha 2A-adrenergic receptor activates PKC via a G-protein of the Gi-family.

Presence of chloride-formate exchange in vascular smooth muscle and cardiac cells

The presence of chloride-formate anion exchange in vascular smooth muscle cells (VSMCs) and cardiac myocytes was investigated. Imposing an outward chloride gradient in sarcolemmal microsomes isolated from canine aorta stimulated [14C]formate uptake compared with the absence of a chloride gradient (24.3 +/- 2.33 versus 9.8 +/- 1.41 pmol/mg protein for 30 seconds, P < .03) and induced transient uphill [14C]formate uptake. The chloride-formate exchange was significantly inhibited in the presence of 1 mmol/L 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or furosemide (57% and 61%, respectively). Incubation of rat cultured VSMCs in a medium containing [14C]formate resulted in uptake of formate that was significantly DIDS and furosemide sensitive (79.34 +/- 2.47, 43.03 +/- 2.37, and 44.65 +/- 1.68 pmol/mg protein for 4 minutes in control, DIDS, and furosemide groups, respectively). Preincubation of the VSMCs in chloride-free medium significantly reduced the DIDS-sensitive (36.31 versus 16.85 pmol/mg protein for 4 minutes, P < .001) and furosemide-sensitive (34.72 versus 8.78 pmol/mg protein for 4 minutes, P < .001) [14C]formate uptake. These results are compatible with the presence of chloride-formate exchange in VSMCs. Influx of [14C]formate into sarcolemmal vesicles isolated from canine heart was significantly higher in the presence of an outward chloride gradient than in its absence (18.1 +/- 2.3 versus 9.6 +/- 1.7 pmol/mg protein for 30 seconds, P < .03). The chloride-formate exchange was significantly inhibited in the presence of 1 mmol/L DIDS or furosemide (41% and 52%, respectively). We conclude that the distribution of chloride-formate exchange may be more universal than previously suggested.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Intracellular pH regulation in the early embryo

Intracellular pH (pHi) regulation is a homeostatic function of all cells. Additionally, the plasma membrane-based transporters controlling pHi are involved in growth factor activation, cell proliferation and salt transport--all processes active in early embryos. pHi regulation in the early embryos of many species exhibits unique features: in mouse preimplantation embryos, mechanisms for correcting excess acid apparently are inactive, while excess base is removed by the mechanism common in differentiated cells. Additionally, unlike differentiated cells, mouse preimplantation embryos are highly permeable to H+ until the blastocyst stage, where the epithelial cells surrounding the embryo are impermeable. In several non-mammalian species, of which the best-studied is sea urchin, cytoplasmic alkalinization at fertilization is necessary for development of the embryo, and elevated pHi must be maintained during early development. Thus, pHi regulatory mechanisms appear to be important for early embryo development in many species.

Thermosensitization by lowering intracellular pH with 5-(N-ethyl-N-isopropyl) amiloride

It has previously been reported that amiloride, a diuretic drug, sensitizes cells to hyperthermia by inhibiting the Na+/H+ exchange through the plasma membrane and thus decreasing the intracellular pH (pHi), particularly in a low extracellular pH (pHe) environment. In the present study, the efficacy of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), an analog of amiloride, to lower the pHi and sensitize tumor cells to hyperthermia was investigated. It was observed that 10 microM EIPA was as effective as 500 microM amiloride to lower the pHi and to increase the thermal sensitivity of SCK tumor cells in vitro. The fact that lowering the pHi and increasing thermal sensitivity of tumor cells by EIPA are more pronounced in acidic medium suggests that the acidic intratumor environment may be exploited to selectively increase the thermal damage in tumors relative to normal tissues by EIPA or its analogs.

Induction of protein phosphorylation, protein synthesis, immediate-early-gene expression and cellular proliferation by intracellular pH modulation. Implications for the role of hydrogen ions in signal transduction

In Syrian hamster embryo cells, intracellular acidification (but not alkalization) results in proliferation, immediate-early-gene expression and tyrosine phosphorylation. In addition, both intracellular acidification and alkalization result in serine/threonine phosphorylation and de novo protein synthesis of specific proteins. Calcium is not mobilized in response to either intracellular alkalization or acidification. Neither intracellular acidification nor alkalization altered the serum proliferative signal while intracellular alkalization (but not acidification) reduced the epidermal-growth-factor-induced proliferative signal, tyrosine phosphorylation and immediate-early-gene expression. Finally, intracellular acidification (but not alkalization) could induce immediate-early-gene expression in cells growing in the presence of serum, indicating that the pH signalling pathway is not down modulated by the serum signalling pathway. These results, while indirect, indicate that hydrogen ions may play an important role in mitogen-signal transduction in Syrian hamster embryo cells.

Tumour cell proliferation is abolished by inhibitors of Na+/H+ and HCO3-/Cl- exchange

Cell membrane-associated ion transporters, Na+/H+ exchanger and Na(+)-dependent HCO3-/Cl- antiport, were shown to be important in the regulation of acidic intracellular pH in different cell types. This study investigated the role of the ion exchangers and their inhibitors in the serum-induced proliferation of two murine tumour cell lines, P815 and L929. The presence of Na+/H+ exchanger [inhibited by amiloride and 5-(N-ethyl-N-isopropyl)amiloride (EIPA)] and Na(+)-dependent HCO3-/Cl- antiport [inhibited by 4,4'-diisothiocyanostilbene-2,2-disulphonic acid (DIDS)] was shown on the tumour cell line tested. EIPA suppressed tumour cell proliferation more strongly than amiloride, and its effect was further increased after intracellular acidification by nigericin. DIDS slightly inhibited proliferation of L929 cell line and did not influence proliferation of P815 cells. However, in nigericin acidified cells DIDS had a dose dependent antiproliferative effect. Furthermore, DIDS significantly increased antiproliferative effects of amiloride and EIPA, suggesting the activity of Na(+)-dependent HCO3-/Cl- antiport in tumour cell proliferation. These results demonstrate the importance of Na(+)-dependent HCO3-/Cl- exchange in addition to Na+/H+ antiport, in tumour cell proliferation and indicate the possibility that ion exchange inhibitors could act as antitumour reagents.

Regulation of cytoplasmic pH in rat sublingual mucous acini at rest and during muscarinic stimulation

The regulation of intracellular pH (pHi) in rat sublingual mucous acini was monitored using dual-wavelength microfluorometry of the pH-sensitive dye BCECF (2',7'-biscarboxyethyl-5(6)-carboxyfluorescein). Acini attached to coverslips and continuously superfused with HCO3(-)-containing medium (25 mM NaHCO3/5% CO2; pH 7.4) have a steady-state pHi of 7.25 +/- 0.02. Acid loading of acinar cells using the NH4+/NH3 prepulse technique resulted in a Na(+)-dependent, MIBA-inhibitable (5-(N-methyl-N-isobutyl) amiloride, Ki approximately 0.42 microM) pHi recovery, the kinetics of which were not influenced by the absence of extracellular Cl-. The rate and magnitude of the pHi recovery were dependent on the extracellular Na+ concentration, indicating that Na+/H+ exchange plays a critical role in maintaining pHi above the pH predicted for electrochemical equilibrium. When the NH4+/NH3 concentration was varied, the rate of pHi recovery was enhanced as the extent of the intracellular acidification increased, demonstrating that the activity of the Na+/H+ exchanger is regulated by the concentration of intracellular protons. Switching BCECF-loaded acini to a Cl(-)-free medium did not significantly alter resting pHi, suggesting the absence of Cl-/HCO3- exchange activity. Muscarinic stimulation resulted in a rapid and sustained cytosolic acidification (t 1/2 < 30 sec; 0.16 +/- 0.02 pH unit), the magnitude of which was amplified greater than two-fold in the presence of MIBA (0.37 +/- 0.05 pH unit) or in the absence of extracellular Na+ (0.34 +/- 0.03 pH unit). The agonist-induced intracellular acidification was blunted in HCO3(-)-free media and was inhibited by DPC (diphenylamine-2-carboxylate), an anion channel blocker. In contrast, the acidification was not influenced by removal of extracellular Cl-. The Ca2+ ionophore, ionomycin, mimicked the effects of stimulation, whereas preloading acini with BAPTA (bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetra-acetic acid) to chelate intracellular Ca2+ blocked the agonist-induced cytoplasmic acidification. The above results indicate that during muscarinic stimulation an intracellular acidification occurs which: (i) is partially buffered by increased Na+/H+ exchange activity; (ii) is most likely mediated by HCO3- efflux via an anion channel; and (iii) requires an increase in cytosolic free [Ca2+].

Chemical probes for anion transporters of mammalian cell membranes

Mammalian cell membranes harbor several types of chloride channels, chloride-cation symporters/cotransporters, and several classes of anion exchangers/antiporters. These transport systems subserve different cellular or organismic functions, depending on the nature of the cell, the spatial organization of transporters, and their functional interplay. Chemical probing has played a central role in the structural and functional delineation of the various anion transport systems. The design of specific probes or their selection from existing sources coupled with their judicious application to the most appropriate biological system had led to the identification of specific anion transporters and to the elucidation of the underlying molecular transport mechanism. In many instances, chemical probing has remained the major or exclusive analytical tool for the functional definition or identification of a given transport system, particularly for discerning among the various anion transporters which operate in highly heterogeneous cell membrane systems. This work critically reviews the present state of the chemical armamentarium available for the most common anion transporters found in mammalian cell membranes. It encompasses the description of the most useful or commonly used probes in terms of their chemical, biochemical, physiological, and pharmacological properties. The review deals primarily with what chemical probes tell about anion transporters and, most importantly, with the limitations inherent in the use of probes in transport studies.

Importance of bicarbonate ion for intracellular pH regulation in antigen- and ionomycin-stimulated RBL-2H3 mast cells

In RBL-2H3 rat basophilic leukemia cells, Ca2+ influx and secretion are activated by antigens that crosslink IgE-receptor complexes and by the Ca2+ ionophore, ionomycin. Here we report that antigen-stimulated Ca2+ influx and secretion are impaired and ionomycin-induced responses are strongly inhibited following the removal of HCO3- from the medium. These results raised the possibility that HCO3(-)-dependent pH regulation mechanisms play a role in the cascade of events leading to mast cell activation. To test this hypothesis, intracellular pH (pHi) was measured by ratio imaging microscopy in individual RBL-2H3 cells labeled with 2',7'-bis-(2-carboxyethyl)-5-(6) carboxyfluorescein (BCECF). In unstimulated cells, it was found that basal pHi in the presence of HCO3- is 7.26, significantly greater than pHi in its absence, 7.09 (P less than 10(-6]. These results, as well as evidence that pHi increases rapidly when HCO3- is added to cells initially incubated in HCO3(-)-free medium, indicate that unstimulated cells use a HCO3(-)-dependent mechanism to maintain cytoplasmic pH. Further analyses comparing unstimulated with stimulated cells showed that antigen causes a small transient acidification in medium containing HCO3- and a larger sustained acidification in HCO3(-)-depleted medium. Ionomycin is a more potent acidifying agent, stimulating a sustained acidification in complete medium and causing further acidification in HCO3(-)-free medium. These results support the hypothesis that the inhibition of antigen- and ionomycin-induced 45Ca2+ influx and secretion in cells incubated in HCO3(-)-free medium is at least partially due to the inactivation of HCO3(-)-dependent mechanisms required to maintain pH in unstimulated cells and to permit pH recovery from stimulus-induced acidification.

Intracellular pH in the OK cell. I. Identification of H+ conductance and observations on buffering capacity

The regulation of intracellular pH (pHi) in the opossum kidney (OK) cell line was studied in vitro using the pH-sensitive excitation ratio of 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Recovery from an NH4Cl acid load disclosed a Na-dependent component blocked by amiloride and a smaller Na-independent component. The Na-independent recovery rate was proportional to the H+ gradient from cell to buffer and was zero in the absence of an electrochemical gradient. The Na-independent recovery was not affected by N-ethylmaleimide, dicyclohexylcarbodiimide, HCO3, phloretin, or ZnCl2 but was accelerated in depolarized cells and by membrane-fluidizing drugs and was inhibited by glutaraldehyde. The apparent cellular buffering capacity changed in proportion to this H+ conductance. Consistent with an electrogenic H+ leak, steady-state cell pH alkalinized with depolarization and acidified with hyperpolarization. Removal of buffer Na+ produced a profound acidification, as did amiloride. In 0-Na+ buffers, extremely large cell-to-buffer H+ gradients were present and proportional to buffer pH. 4-Acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid had no effect on steady-state pHi. Measurements of intracellular buffering capacity were derived from the change of cell pH induced by withdrawing NH4Cl. This buffering capacity was increased threefold in Na-free buffers, whereas the value measured by direct titration of cell lysate was the same or less than that of control cells. The NH4Cl-derived buffering capacity varied in direct proportion to the magnitude of the H+ leak. Drugs that changed H+ permeability produced the apparent changes of the measured buffering capacity within a few minutes. We conclude that, in HCO3-free buffer, the OK cell uses two membrane acid-base transport pathways: a Na-H antiporter active at physiological pH and a substantial passive H+ conductance. The results also reveal that the NH4Cl-derived buffering capacity is subject to artifacts, possibly due to a finite leak of ionic NH4+.

Effect of ionophore A23187 on the membrane permeability in mouse fibroblasts

Addition of the divalent cation ionophore A23187 to transformed mouse fibroblasts (3T6) resulted in an increase in the cell membrane permeability to normally impermeant solutes (e.g., nucleotides). The membrane permeability was assessed by following the efflux of prelabeled adenine nucleotides, the influx of p-nitrophenyl phosphate in cells attached to plastic dishes and reconstitution of intracellular protein synthesis in the presence of exogenously added normally impermeant factors required for macromolecular synthesis. The permeability change of 3T6 cells was found to be dependent on the specific presence of external calcium ion. The permeabilization was found to occur preferably in alkaline pH and specific to certain transformed cells. It is preceded by rapid efflux of K+, influx of Na+ and partial hydrolysis of cellular nucleotides in 3T6 cells. Similar ion fluxes were previously found to precede cell permeabilization by electrogenic ionophores for monovalent ions and by exogenous ATP. Our data suggest that a calcium dependent process caused the K+ release and excess Na+ entry, causing dissipation of the membrane potential and subsequent formation of aqueous channels.