Regulation of intracellular pH in LLC-PK1 cells by Na+/H+ exchange

Assessing intracellular pH regulation in H+-ATPase-rich ionocytes in zebrafish larvae using in vivo ratiometric imaging

The H+-ATPase-rich (HR) cells of zebrafish larvae are a sub-type of ion-transporting cell located on the yolk sac epithelium that are responsible for Na+ uptake and H+ extrusion. Current models of HR cell ion transport mechanisms in zebrafish larvae are well established, but little is known about the involvement of the various ion transport pathways in regulating intracellular acid-base status. Here, a ratiometric imaging technique was developed and validated to monitor intracellular pH (pHi) continuously in larval zebrafish HR cells in vivo Gene knockdown or CRISPR/Cas9 knockout approaches were used to evaluate the roles of the two principal apical membrane acid excretory pathways, the Na+/H+ exchanger (NHE3b; slc9a3.2) and the H+-ATPase (atpv1aa). Additionally, the role of HR cell cytosolic carbonic anhydrase (CAc) was investigated because of its presumed role in providing H+ for Na+/H+ exchange and H+-ATPase. The temporal pattern and extent of intracellular acidification during exposure of fish to 1% CO2 and the extent of post-CO2 alkalisation were altered markedly in fish experiencing knockdown/knockout of CAc, NHE3b or H+-ATPase. Although there were slight differences among the three knockdown/knockout experiments, the typical response was a greater degree of intracellular acidification during CO2 exposure and a reduced capacity to restore pHi to baseline levels post-hypercapnia. The metabolic alkalosis and subsequent acidification associated with 20 mmol l-1 NH4Cl exposure and its washout were largely unaffected by gene knockdown. Overall, the results suggest markedly different mechanisms of intracellular acid-base regulation in zebrafish HR cells depending on the nature of the acid-base disturbance.

Signaling pathways involved with the stimulatory effect of angiotensin II on vacuolar H+-ATPase in proximal tubule cells

It has been documented that angiotensin II (ANG II) (10(-9) M) stimulates proton extrusion via H(+)-adenosine triphosphatase (ATPase) in proximal tubule cells. In the present study, we investigated the signaling pathways involved in the effects of ANG II on H(+)-ATPase activity and on the cytosolic free calcium concentration in immortalized rat proximal tubule cells, a permanent cell line derived from rat proximal tubules. The effects of ANG on pH(i) and [Ca(+2)](i) were assessed by the fluorescent probes, 2',7-bis (2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxy-methyl ester and fluo-4-acetoxy-methyl ester, in the absence of Na(+) to block the Na(+)/H(+) exchanger. In the control situation, the pH recovery rate following intracellular acidification with NH(4)Cl was 0.073+/-0.011 pH units/min (n=12). This recovery was significantly increased with ANG II (10(-9 )M), to 0.12+/-0.015 pH units/min, n=10. This last effect was also followed by a significant increase of Ca(+2) (i), from 99.72+/-1.704 nM (n=21) to 401.23+/-33.91 nM (n=39). The stimulatory effect of ANG II was blocked in the presence of losartan, an angiotensin II subtype 1 (AT(1)) receptor antagonist. H89 [protein kinase A (PKA) inhibitor] plus ANG II had no effect on the pH recovery. Staurosporine [protein kinase C (PKC) inhibitor] impaired the effect of ANG II. Phorbol myristate acetate (PKC activator) mimicked in part the stimulatory effect of ANG II, but reduced Ca(+2) (i). 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (intracellular calcium chelator) alone reduced the pH(i) recovery rate below control levels and impaired the effect of ANG II, in a way similar to that of trimethoxy benzoate (a blocker of Ca(+2) (i) mobilization). We conclude that ANG II regulates rat proximal tubule vacuolar H(+)-ATPase by a PKA-independent mechanism and that PKC and intracellular calcium play a critical role in this regulation.

Increased NHE1 expression is associated with serum deprivation-induced differentiation in immortalized rat proximal tubule cells

We studied the proton secretion mechanisms involved with pHi regulation in immortalized rat proximal tubule cells (IRPTC), a SV40-immortalized cell line derived from rat proximal tubule, and characterized the effects of serum deprivation on them. Using pHi measurements with the fluorescent probe BCECF, we demonstrated that the IRPTC express both Na+/H+ exchanger and H+-ATPase, but only NHE1 is modulated by serum deprivation. In these cells, 24 h of serum starvation increased pHi from 7.08+/-0.008 (n=34) to 7.18+/-0.018 (n=33) as well as the pH recovery rate from intracellular acidification with NH4Cl from 0.29+/-0.022 pH U/min (n=14) to 0.50+/-0.024 pH U/min (n=14), without modifying their buffering capacity. These effects were followed by several modifications in morphological features, indicating an increase in differentiation status. The altered activity of NHE1 was consistent with an increase of both transcription and translation of the antiporter, as the utilization of actinomycin D and cycloheximide significantly inhibited the upregulation of NHE1 induced by serum withdrawal. Inhibition of tyrosine phosphorylation by genistein blocked the serum deprivation-dependent activation of NHE. Moreover, the pharmacological inhibition of MEK1/2, the upstream activator of ERK1/2 by UO-126, significantly inhibited the stimulatory effect of serum starvation on Na+/H+ exchanger activity, whereas the putative p38 MAPK inhibitor SB-203580 failed to cause any effect on pHi recovery rates. Our findings indicate that during IRPTC differentiation by serum deprivation, there was a net enhancement of NHE1 activity. This upregulation of NHE by serum removal was consistent with an increase of RNA and protein synthesis of the exchanger, which depends on tyrosine kinase phosphorylation and ERK pathway activation.

Stable, polarised, functional expression of Kir1.1b channel protein in Madin-Darby canine kidney cell line

1. The family of Kir1.1 (ROMK) channel proteins constitute a secretory pathway for potassium in principal cells of cortical collecting duct and thick ascending limb of Henle's loop. Mutations in Kir1.1 account for some types of Bartter's syndrome. 2. Here we report that stable transfection of Kir1.1b (ROMK2) in Madin-Darby canine kidney (MDCK) cell line results in expression of inwardly rectifying K+ currents and transmonolayer electrical and transport properties appropriate to Kir1.1 function. When grown on permeable supports, transfected monolayers secreted K+ into the apical solution. This secretion was inhibited by application of barium to the apical membrane, or by reduction in expression temperature from 37 to 26 C. However, whole-cell voltage clamp electrophysiology showed that K+ conductance was higher in cells expressing Kir1.1b at 26C. 3. To investigate this further, Kir1.1b was tagged with (EGFP), a modification that did not affect channel activity. Protein synthesis was inhibited with cycloheximide. Spectrofluorimetry was used to compare protein degradation at 37 and 26 C. The increased level of Kir1.1b at the plasma membrane at 26 C was due to an increase in protein stability. 4. Confocal microscopic investigation of EGFP-Kir1. 1b fluorescence in transfected cells showed that the channel protein was targeted to the apical domain of the cell. 5. These results demonstrate that Kir1.1b is capable of appropriate trafficking and function in MDCK cell lines at physiological temperatures. In addition, expression of Kir1.1b in MDCK cell lines provides a useful and convenient tool for the study of functional activity and targeting of secretory K+ channels.

Regulation of the high-affinity H+/peptide cotransporter in renal LLC-PK1 cells

Di- and tripeptides and peptide mimetics such as beta-lactam antibiotics are efficiently reabsorbed from the tubular lumen by a high-affinity peptide transporter. We have recently identified and characterized this H+-coupled high-affinity peptide transport system in the porcine proximal tubular cell line LLC-PK1. Here we describe for the first time the regulation of the renal high-affinity peptide cotransporter at the cellular level. Uptake of 5 microM 3H-D-Phe-L-Ala into LLC-PK1 cells was significantly increased by lowering [Ca2+]in and decreased by increasing [Ca2+] in. Moreover, it was shown that the [Ca2+]in effects on peptide transport activity were dependent on Ca2+ entry from the extracellular site (e.g., via a store-regulated capacitative Ca2+ influx). Protein kinase C (PKC) was found to transmit the effects of [Ca2+]in on peptide transport. Although we demonstrate by pHin measurements that the PKC inhibitor staurosporine did decrease the transmembrane H+ gradient and consequently should have reduced the driving force for peptide uptake, the only effect on transport kinetics of 3H-D-Phe-L-Ala observed was a significant decrease in Km from 22.7+/-2.5 microM to 10.2+/-1.9 microM with no change in maximal velocity.

Characterization of plasma membrane Na+/H+ exchange in eel (Anguilla anguilla) intestinal epithelial cells

The ability of eel intestinal epithelial cells to recover from an acute acid load was analysed using the fluorescent dye 2',7'-bis-carboxy-ethyl-5,6-carboxyfluorescein (BCECF) and cell suspensions. Under these experimental conditions (bicarbonate-free solutions) the resting pHi in cells prepared from sea-water (7.52 +/- 0.031) and fresh-water (7.50 +/- 0.094) adapted animals proved to be similar. The recovery rate (following an acid load) increases by increasing the Na ion concentration in the extracellular medium. This pHi recovery is competitively inhibited by the specific inhibitor dimethylamiloride (DMA) with a low Ki in sea- (1.2 microM) as well as in fresh-water (1.3 microM) adapted animals, indicating the presence of a specific Na/H exchange activity in these cells. Using basolateral membrane vesicles it could be demonstrated that this activity is located on the basolateral side of the enterocyte membrane. The kinetic parameters (Kapp and Jmax) of this exchanger are similar in fresh-water and sea-water adapted animals suggesting that no salinity adaptation occurs, thus excluding the involvement of the antiporter in the osmoregulatory processes. These results are in agreement with the presence in the plasma membrane of the eel enterocytes of a Na/H-1 (housekeeper) form of the antiporter.

PGF2 alpha activation of Na/H antiporter and ammoniagenesis in parent/variant LLC-PK1 cells

A novel variant of the LLC-PK1 cell line was used to examine directly the mechanism whereby PGF2 alpha and TPA inhibit renal ammoniagenesis. The variant cells, which exhibit a growth pattern and morphology similar to the parent cell line, were isolated by a self selection process utilizing long-term cultures of parent cells maintained under conditions of continuous gentle rocking of the media fluid. Incubation of both parent and variant LLC-PK1 cells for one hour in a glutamine supplemented Krebs-Hensleit media of low pH (pH 6.8) increased ammonia and alanine production in comparison to the basal rates at pH 7.4. The phorbol ester TPA and also PGF2 alpha inhibited the low pH-induced increases in ammonia and alanine formation in parent cells; however, neither TPA nor PGF2 alpha inhibited ammonia or alanine metabolism in variant cells. TPA and PGF2 alpha activated PKC similarly in the parent and variant cells as demonstrated by a significant increase in membrane bound enzyme activity. BCECF labeling of cells indicated that the parent and variant cells possess an amiloride sensitive Na+/H+ antiporter of comparable activity. Exposure of parent cells to PGF2 alpha or TPA resulted in the activation of Na+/H+ antiporter activity. By contrast, neither compound stimulated antiporter activity in variant cells. These studies strongly suggest that PKC mediated activation of the Na+/H+ antiporter accounts for the inhibition of ammonia production produced by both PGF2 alpha and TPA. In addition, this novel variant of LLC-PK1 cells should provide a valuable tool to investigate various normal and pathophysiological functions involving mediation by PKC and/or Na+/H+ antiporter activity.

pHi dependence of Na-H exchange and H delivery in IEC-6 cells

The intracellular pH (pHi) dependencies of the Na-H exchanger and also of unidentified acid-loading processes ("H delivery") in IEC-6, a rat intestinal crypt cell line, were determined using microspectrofluorimetry of the pH-sensitive dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein on groups of cells grown on glass cover slips mounted in a perfusion chamber in HCO3-free solutions (to inactivate Cl-HCO3 exchange and Na-HCO3 cotransport). Na-H exchange and H delivery rates were measured using two methods. 1) Cells were acid loaded with NH4Cl, and rates of pHi recovery and acidification were measured just before and then just after adding amiloride at various times during the recovery. Multiplying rates of pHi recovery times the intrinsic buffer capacity (beta i) yielded net H efflux rates, rates of pHi decrease (during amiloride treatment) times beta i yielded H delivery rates, and Na-H exchange equals net H efflux plus H delivery. 2) In control cells, time courses of amiloride-induced pHi decreases (from 7.36 to 6.65) and of pHi recovery following amiloride removal (from 6.65 to 7.49) were curve fit by computer; rates of pHi decrease and recovery were calculated; and H delivery, net H efflux, and Na-H exchange were then calculated at the different pHi as above. H delivery and Na-H exchange were both pHi dependent, but in the opposite directions. H delivery was 2-7 mM/min at the control pHi, decreasing to zero around pHi 6.6. Na-H exchange was also equal to 2-7 mM/min at the control pHi, and this increased as pHi decreased; at pHi 6.6, Na-H exchange was 20-35 mM/min.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarized expression of Na+/H+ exchange activities in clonal LLC-PK1 cells (Clone4 and PKE20) I. Basic characterization

We have analysed the mechanisms of Na(+)-dependent pHi recovery from an acid load in LLC-PK1/Clone4 and LLC-PK1/PKE20 cells by using the intracellular pH indicator 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl ester. By analysis using single-cell microspectrofluorometry, we obtained evidence for polarized expression of Na+/H+ exchange activities with different properties in apical and basolateral cell surfaces, respectively. In Clone4 cells, Na+/H+ exchange activity is only visible on the basolateral cell surface; in PKE20 cells, Na+/H+ exchange activities with equal capacities are present on both cell surfaces. In Clone4 cells, the apparent Km value for Na+ is around 10 mM; in PKE20 cells it is around 20 mM and indistinguishable for the two cell poles. Ethylisopropylamiloride (EIPA) inhibition for all three activities measured in monolayer configuration is reduced by increasing Na+ concentration. Measured in the same cells, EIPA inhibition of transport of PKE20 cells is weaker for apical Na+/H+ exchange as compared to basolateral activity. In Clone4 and PKE20 cells kept in suspension, Na+/H+ exchange activities with similar properties for the two cell lines are observed. However, Na+/H+ exchange activities in cells in suspension are different from either activity measured in monolayer configuration: affinity for Na+ is higher (PKE20 cells) and inhibition by amiloride is weak and not influenced by increasing Na+ concentrations (PKE20 and Clone4 cells). It is concluded that PKE20 cells contain different Na+/H+ exchange activities on the two cell surfaces; this cell line should be a useful model to study regulatory aspect of different Na+/H+ exchange functions ("epithelial'/"housekeeping').(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the kinetics of Na+/H+ exchange in OK cells: introduction of a new device for the analysis of polarized transport in cultured epithelia

The present study describes a new perfusion technique--based on the use of a routine spectrofluorometer--which enables fluorometric evaluation of polarity, regulation and kinetics of Na+/H+ exchange at the level of an intact monolayer. Na+/H+ exchange was evaluated in bicarbonate-free solutions in OK (opossum kidney) cells, a renal epithelial cell line. Na+/H+ exchange activity was measured by monitoring changes in intracellular pH (pHi) after an acid load, using the pH-sensitive dye 2'7'-bis (carboxyethyl) 5-6-carboxy-fluorescein (BCECF). Initial experiments indicated that OK cells grown on a permeable support had access to apical and basolateral perfusion media. They also demonstrate that OK cells express an apical pHi recovery mechanism, which is Na+ dependent, ethylisopropylamiloride (EIPA) sensitive and regulated by PTH. Compared to resting conditions (pHi = 7.68; pHo = 7.4) where Na+/H+ exchange is not detectable, transport rate increased as pHi decreased. A positive cooperativity characterized the interaction of internal H+ with the exchanger, and suggests multiple H+ binding sites. In contrast, extracellular [Na+] increased transport with simple Michaelis-Menten kinetics. The apparent affinity of the exchanger for Na+ was 19 mM at an intracellular pH of 7.1 and 60 mM at an intracellular pH of 6.6. Inhibition of Na+/H+ exchange activity by EIPA was competitive with respect to extracellular [Na+] and the Ki was 3.4 microM. In conclusion, the technique used in the present study is well suited for determination of mechanisms involved in control of epithelial cell pHi and processes associated with their polarized expression and regulation.

Na(+)-H+ exchange activity and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Na+/H+ exchange activity have been examined in isolated chicken enterocytes using pH sensitive fluorescence dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and in a nominally bicarbonate-free buffer. Under resting conditions the pHi (7.18) was higher than that observed in the presence of the proton ionophore FCCP (6.98), indicating that [H+] is below the value predicted for electrochemical equilibrium across the plasma membrane, i.e., pHi is regulated. Removal of extracellular Na+ lowered pHi by 0.28 units and subsequent addition of 80 mM Na+ rapidly increased pHi towards the control value. The acidification induced by Na(+)-removal was prevented by 1 mM amiloride. After an intracellular acidification by exposure to 30 mM NH4Cl during 5 min, the pHi decreased from approx. 7.18 to approx. 6.86. Subsequent alkalinization of cells back to control pHi was observed after addition of Na+ or Li+ but not TEA+. Na(+)-dependent recovery of pHi after an acid-load was unaffected by valinomycin, and was 82% reduced by 1 mM amiloride. The inhibitory action of amiloride was abolished by 10 microM monensin. The initial rate of pHi recovery from an acid-load following exposure to Na+ exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Na+ and maximum velocity of alkalinization of approx 0.2 pH units.min-1. The rate of pHi recovery was inversely proportional to pHi. The 'set point' for the exchanger is approx. 7.35. It is concluded that in chicken enterocytes the Na+/H+ exchange system is not quiescent at resting pHi and, thus, contributes to the maintenance of a steady-state pHi at neutral or slightly alkaline levels.

Heat induces intracellular acidification in human A-431 cells: role of Na(+)-H+ exchange and metabolism

The resting intracellular pH (pHi) of A-431 cells at 37 degrees C in Na+ Hanks' solution is 7.23 +/- 0.02. In the presence of amiloride (100 microM) pHi decreases to 7.08 +/- 0.03. Hyperthermia induces a temperature- and time-dependent intracellular acidification of 0.2 pH units in either bicarbonate-free or bicarbonate-buffered solutions. After heat treatment (45 degrees C, 10 min) pHi returns to normal 1 h after incubation at 37 degrees C. The activity of the Na(+)-H+ exchanger was examined in heated and unheated cells in the absence of bicarbonate. Unheated cells recover from an acid load in a [Na+]o-dependent and amiloride-sensitive manner. The apparent Michaelis constant for extracellular Na+ is 38 +/- 9 mM, and the apparent mean affinity constant for amiloride is 11 +/- 3 microM. In heated cells the apparent affinity of the Na(+)-H+ exchanger for extracellular Na+ is not changed, but the maximal recovery rate is approximately 40% slower than that of unheated cells. The rate of recovery from acid loading returns to normal 2 h after heat treatment. [Na+]i and intrinsic buffering power in heated cells are the same as those in unheated cells. Decreases in both intracellular ATP and lactic acid are observed in heated cells. 2-Deoxy-D-glucose and sodium azide induce an intracellular acidification but prevent most of the acidification induced by heat. Heat treatment causes no further acidification in cells that are acidified by both amiloride and 2-deoxy-D-glucose together. These data are the first to suggest that thermally induced intracellular acidification is due to both an inhibition of Na(+)-H+ exchange and an inhibition of metabolic pathways.

Essential activation of Na(+)-H+ exchange by [H+]i in HL-60 cells

The intracellular pH (pHi) dependence of the rate of Na(+)-H+ exchange was determined in undifferentiated promyelocytic HL-60 cells by measuring alkalinization rates using the fluorescent pHi indicator 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). BCECF was calibrated in the pH range from 5 to 7 using the nigericin technique of Thomas and co-workers (J. A. Thomas, R. N. Buchsbaum, A. Zimniak, and E. Racker. Biochemistry 18: 2210-2218, 1979). Exchange rate increases as pHi is lowered below pH 7.00. At low pH (pH below 6.3), the dependence of Na(+)-H+ exchange rate on intracellular proton activity is well fitted by the Michaelis-Menten equation with a maximum exchange velocity of 33.7 +/- 2.4 mmol H(+).1 cell water-1.min-1 and a half-saturation constant of 1.35 +/- 0.28 microM (corresponding to a minus log of the Michaelis constant of 5.89). However, a Hill plot reveals that the Hill coefficient changes gradually from one to two when pH is changed from 5 to 7, ruling out Michaelian kinetics. The dependence of exchange flux on internal protons is well fit in the full pH range from 5 to 7 by a simple kinetic model (essential activation) with modifier and transport sites for internal proton binding. At low pH, failure to correct BCECF measurement of pHi for contribution to fluorescence signal from extracellular dye and for quenching of intracellular BCECF leads to an artifactual increase in the measured Hill coefficient. These two findings (increase in Hill coefficient as pHi is increased and artifactual increase in Hill coefficient because of methodological reasons) provide a good explanation for the wide range of Hill coefficients reported in the literature.

Evidence for probenecid-sensitive organic anion transporters on polarized thyroid cells in culture

Epithelial thyroid cells in primary cultures loaded with BCECF/AM rapidly released the impermeant fluorescent dye BCECF (bis(carboxyethyl)carboxyfluorescein) in the incubation medium. Cells organized into follicles rapidly cleared BCECF (80% within 10 min) whereas fluorescence microscopy did not show any fluorescence in the follicular cavity. Cells organized into monolayers on plastic exported BCECF into the medium (70% within 40 min) whereas fluorescence microscopy showed intense fluorescence under the domes. BCECF efflux was blocked by probenecid, one of the known inhibitors of organic anion transporters, with similar efficiency in both structures. Maximal and half-maximal effects were respectively observed for 5 mM and 0.4 mM probenecid. The polarity of BCECF efflux was studied by using monolayers on collagen-coated Nuclepore filters: 85% of BCECF released was found in the basal compartment and 15% in the apical compartment. These findings suggested that thyroid cells in culture expressed a transport mechanism for the anionic form of BCECF. Furthermore, the observed activation of the Na+/H+ exchanger by probenecid suggested that the presence of this blocker did not overcome problems arising in the use of BCECF as intracellular pH indicator for thyroid cells.

Regulation of intracellular pH by cultured opossum kidney cells

Opossum kidney (OK) cells (an epithelial cell line) were examined by flame photometry of cellular Na+ and K+ and by microfluorometric measurements of the intracellular pH (pHi) of single cells loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). The work concentrates on defining resting pHi values under different experimental conditions and examines factors that contribute to the maintenance of resting pHi. To use nigericin to calibrate the intracellular response of BCECF, cellular K+ levels were measured by a null point analysis, and the stability and magnitude of cellular Na+ and K+ levels were determined vs. time. Resting pHi in medium without added CO2 was high when measured by null point analysis of the population (pHi 7.6) and from measurements of single cells that have recovered from an acid load caused by NH4 prepulse (pHi 7.76 +/- 0.03, n = 20 cells). In single-cell measurements, addition of CO2-HCO3- to the medium results in cellular acidification of the steady-state pHi by 0.35 +/- 0.04 pH units. In medium equilibrated with room air, the resting pHi is shown to be a dynamic steady state composed of net flux due to apical Na(+)-dependent transport (Na(+)H+ exchange) plus acidifying processes. It is concluded that although 5-[N-ethyl-N-isopropyl]amiloride (EIPA) inhibits the forward reaction of Na(+)-H+ exchange, EIPA is either ineffective as an inhibitor of the reverse reaction of Na(+)-H+ exchange or Na(+)-H+ exchange does not reverse measurably in the OK cells.

Regulation of Na+/H+ exchange in opossum kidney cells by parathyroid hormone, cyclic AMP and phorbol esters

Parathyroid hormone (PTH) controls two proximal tubular brush border membrane transport systems, Na+/phosphate co-transport and Na+/H+ exchange. In OK cells, a cell line with proximal tubular transport characteristics, PTH acts via kinase C and kinase A activation to inhibit Na+/phosphate co-transport [6, 8, 9, 19, 22]. In the present study, we show that PTH inhibits Na+/H+ exchange and that this effect can be mimicked by pharmacological activation of kinase A and kinase C. Ionomycin-dependent increases in cytoplasmic Ca2+ concentration do not induce inhibition of Na+/H+ exchange; PTH-dependent inhibition of Na+/H+ exchange is not prevented by ionomycin or by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (Ca2+ clamping). Detailed dose-response curves for the different agonists, given either alone or in combination, suggest that the two regulatory cascades (kinase A and kinase C) are operating independent of each other and reach a common final target, resulting in 40-50% inhibition of Na+/H+ exchange. An analysis of intracellular pH sensitivity of Na+/H+ exchange suggests that inhibition is not related to a shift in set point, but is rather explained by a reduced Vmax of Na+/H+ exchange and/or reduced affinity for protons at the internal membrane surface. It is suggested that kinase A as well as kinase C can mediate PTH inhibition of renal proximal tubular Na+/H+ exchange and that the relative importance of a particular regulatory cascade is determined by the PTH-concentration-dependent rates in the liberation of diacylglycerol (phospholipase C/kinase C) and cAMP (adenylate cyclase/kinase A).

Regulatory volume decrease by cultured renal cells

Volume regulatory responses of OK cells (a continuous epithelioid cell line from opossum kidney) are examined by electronic cell sizing and measurements of intracellular pH in cell suspensions. In response to a 40% reduction in osmolality, the cells swell and then subsequently shrink toward their starting volume. This regulatory volume decrease (RVD) is reduced by replacement of Cl- in the medium with acetate. Replacement of Cl- with NO3- accelerates the RVD. The RVD response is inhibited by 1 mM quinine or 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in the medium. The inhibitory effect of 100 microM DIDS (but not 1 mM quinine) is altered by replacement of Cl- by NO3- in the medium. Hypotonic challenge does not induce a DIDS-sensitive net flux of acid-base equivalents. Addition of (9 microM) valinomycin also inhibits the RVD response. It is suggested that the RVD response of OK cells involves activation of separate K+ and Cl- channels.

Na+/H+ exchange regulates intracellular pH in a cell clone derived from bovine pigmented ciliary epithelium

The regulation of intracellular pH (pHi) was monitored in a virus-transformed cell clone derived from bovine ciliary body exhibiting characteristics of pigmented ciliary epithelium. Data were obtained from confluent monolayers grown on plastic coverslips in nominally bicarbonate-free media using the pH-sensitive absorbance of 5- (and 6-) carboxy-4',5'-dimethylfluorescein. Under resting conditions, pHi averaged 6.98 +/- 0.01 (SEM; n = 57). When cells were acid loaded by briefly exposing them to Ringer containing NH4+ and then withdrawing the NH4+, pHi spontaneously regained its initial value. In the presence of 1 mM amiloride or in the absence of Na+, this process was blocked, indicating the involvement of an Na+/H+ exchanger in the regulation of pHi after an acid load. Removing Na+ during resting conditions decreased cytoplasmatic pH. This acidification could be slowed by amiloride, which is evidence for reversal of the Na+/H+ countertransport exchanging intracellular Na+ for extracellular protons. Application of 1 mM amiloride during steady state led to a slow acidification. Thus the Na+/H+ exchanger is operative during resting conditions extruding protons, derived from cellular metabolism, or from downhill leakage into the cell. Addition of Na+ to Na+ -depleted cells led to an alkalinization, which was sensitive to amiloride, with an IC50 of about 20 microM. This alkalinization was attributed to the Na+/H+ exchanger and exhibited saturation kinetics with increasing Na+ concentrations, with an apparent KM of 29.6 mM Na+. It is concluded that Na+/H+ exchange regulates pHi during steady state and after an acid load.

Sodium influxes in renal epithelial LLC-PK1/Cl4 cells monitored by 23Na NMR

23Na NMR experiments with a suspension of the renal epithelial cell line, LLC-PK1/Cl4, in the presence of the shift reagent dysprosium polyphosphate showed that the intracellular sodium was only NMR visible for 64 +/- 4%. Intracellular sodium content was found to be 30.6 +/- 1.2 mM (25 degrees C). Examination of the sodium influx during recovery from intracellular acidification showed that sodium is transported not only by Na+/H+ exchange but also by sodium-D-glucose cotransport with a stoichiometry of 1:1.

Separate control of regulatory volume increase and Na+-H+ exchange by cultured renal cells

Suspensions of OK cells (a continuous epithelioid cell line from opossum kidney) are examined by electronic cell sizing, measurements of intracellular pH, and measurements of cellular Na+ and K+. The response of the cells to hypertonic solutions is evaluated in most detail. When shrunken by exposure to hyperosmotic medium (430 mosmol/kg), the cells do not demonstrate a regulatory volume increase (RVI) independent of the solute that is used to increase osmolality [NaCl, N-methyl-D-glucamine-HCl (NMGCl), or sucrose]. In contrast, when cells are preexposed to 190 mosmol/kg medium and then shrunken by exposure to 310 mosmol/kg medium, a volume increase is observed after the addition of 120 mosmol/kg NaCl or NMGCl, but not sucrose. This RVI is sensitive to 1 mM furosemide and removal of Na+ or K+ from the medium, but it is not inhibited by 1 mM amiloride. In the presence of a propionate-induced cellular acidification, a Na+-H+ exchanger in the cells is shown to have a large capacity for net solute uptake and to be inhibited by 1 mM amiloride. Net solute uptake by the Na+-H+ exchanger is sensitive to addition of parathyroid hormone or 8-bromoadenosine 3',5'-cyclic monophosphate but is not stimulated in response to cell shrinkage.

Bile acids increase cellular free calcium in cultured kidney cells (LLC-PK)

Suspensions of LLC-PK1 cells were used to determine the effect of bile acids on the cellular homeostasis of inorganic ions. It is determined that bile acids alter cellular free calcium (Cai) levels in LLC-PK1 cells. A series of bile acids were compared and found to produce increases in Cai in the order: lithocholate sulfate (LCS) greater than deoxycholate greater than chenodeoxycholate greater than lithocholate glucuronide greater than cholate. LCS (300 microM) produces changes in Cai (measured using Fura-2) qualitatively similar to those produced by 1 microM ionomycin, except that only ionomycin is able to release calcium from intracellular stores. The effect on Cai is roughly proportional to LCS concentration between 50 and 300 microM. The presence of 40 mM Na in the extracellular medium reduces the LCS-induced rise in Cai to 20% of that observed in the absence of Na. This effect is specific for Na versus 150 mM extracellular K, Li, or TMA. The effect is not dependent on the Na gradient across the membrane. At concentrations of LCS which induce changes in Cai, no significant effect of LCS is observed on either cellular Na or K levels, or intracellular pH.

Regulation of Na+-H+ exchange in cultured opossum kidney cells by parathyroid hormone, atrial natriuretic peptide and cyclic nucleotides

The activity of Na+-H+, exchange was studied in a cultured cell line derived from opossum kidney (OK cells). The activity of the exchanger was measured either as the amiloride (2 mM) inhibitable 22Na flux in acid-loaded cells, or as the Na+-dependent and amiloride-sensitive recovery of intracellular pH (pHi) from an acid load. Initial rates of tracer flux were analyzed in confluent monolayers while changes in pHi were evaluated in suspensions of trypsinized cells which had been loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein. Both 8-bromo-cAMP and 8-bromo-cGMP inhibit the activity of the exchanger in a dose-dependent manner. Maximal inhibition due to 8-bromo-cAMP was about 50% and was attained with 0.75 mM of the cyclic nucleotide. Parathyroid hormone (10(-9)-10(-7) M) and atrial natriuretic peptide (10(-7) M) also inhibit the activity of the exchanger. By measuring the rate of Na+-dependent pHi recovery from different starting pHi values, evidence was obtained for a cyclic nucleotide-dependent decrease in the response of Na+-H+ exchange to intracellular acidification. We conclude that cAMP and cGMP are intracellular messengers in the hormone-dependent regulation of Na+-H+ exchange activity in renal epithelial cells.

Na+/H+ exchange in Ehrlich ascites tumor cells: activation by cytoplasmic acidification and by treatment with cupric sulphate

Exposure of Ehrlich cells to isotonic Na+-propionate medium induces a rapid cell swelling. This treatment is likely to impose an acid load on the cells. Cell swelling is absent in K+-propionate medium but may be induced by the ionophore nigericin, which mediates K+/H+ exchange. Cell swelling in Na+-propionate medium is blocked by amiloride, but an alternative pathway is introduced by addition of the ionophore monensin, which mediates Na+/H+ exchange. Consequently, swelling of Ehrlich cells in Na+-propionate medium is due to the operation of an amiloride-sensitive, Na+-specific mechanism. It is concluded that this mechanism is a Na+/H+ exchange system, activated by cytoplasmic acidification. We have previously demonstrated that the heavy metal salt CuSO4 in micromolar concentrations inhibits regulatory volume decrease (RVD) of Ehrlich cells following hypotonic swelling. The present work shows that CuSO4 inhibits RVD as a result of a net uptake of sodium, of which the major part is sensitive to amiloride. Measurements of intracellular pH show that CuSO4 causes significant cytoplasmic alkalinization, which is abolished by amiloride. Concomitantly, CuSO4 causes an amiloride-sensitive net proton efflux from the cells. The combined results confirm that a Na+/H+ exchange system exists in Ehrlich cells and demonstrate that the heavy metal salt CuSO4 activates this Na+/H+ exchange system.

Role of monocarboxylic acid transport in intracellular pH regulation of isolated proximal cells

Isolated proximal cells were prepared from rabbit kidney cortex by mechanical dissociation. The intracytoplasmic pH (pHi) was measured in HCO3(-)-free media (external pH (pHe), 7.3) using the fluorescent dye 2,7-biscarboxyethyl-5,6-carboxyfluorescein (BCECF). Cells were acid-loaded by the nigericin technique. Addition of 70 mM Na+ to the cells caused a rapid pHi recovery, which was blocked by 0.5 mM amiloride. When the cells were exposed to 5 mM sodium butyrate in the presence of 1 mM amiloride, the H+ efflux was significantly increased and followed Michaelis-Menten kinetics. Increasing pHe from 6.4 to 7.6 at a constant pHi of 6.4 enhanced the butyrate activation of the H+ efflux. Increasing pHi from 6.5 to 7.2 at a constant pHe of 7.2 reduced the butyrate effect. 22Na uptake experiments in the presence of 1 mM amiloride showed that 1.5 mM butyrate increased the Na+ flux in the proximal cells (pHi 7.10). The efficiency of monocarboxylic anions in promoting a pHi recovery increased with the length of their straight chain (acetate less than propionate less than butyrate less than valerate). The data show that when the Na+/H+ antiporter is blocked, the proximal cells can regulate their pHi by a Na+-coupled absorption of butyrate followed by non-ionic diffusion of butyric acid out of the cell and probably also by OH- influx by means of the OH-/anion exchanger.

Regulation of intracellular pH in cultured bovine retinal pigment epithelial cells

Regulation of intracellular pH (pHi) in bovine retinal pigment epithelium (RPE) was investigated in cell culture. pHi was measured using the pH-sensitive absorbance of intracellularly trapped 5 (and 6)-carboxy-dimethyl-fluorescein (CDMF). (1) Regulation of pHi after induction of an acid load by removal of NH4Cl could be blocked either totally by removal of extracellular sodium, or subtotally (about 90%) by application of amiloride (1 mmol/l). Additional flux measurements revealed a dose-dependent, amiloride-sensitive 22Na+-uptake into Na+-loaded cells. Both results suggest the presence of a Na+/H+ antiport. (2) When alkalinization of the cells was induced by preincubation with 50 mmol/l acetate in HCO3(-)-Ringer's and subsequent removal of the weak acid, the following regulation was dependent on the presence of extracellular chloride. This process could be blocked with DIDS (1 mmol/l), suggesting the presence of a Cl-/HCO3- exchange mechanism. (3) We found no evidence for a Na+/HCO3(-)-cotransport, which had been postulated to be present in RPE by others. We conclude that two processes are involved in regulation of pHi in RPE: A Na+/H+ antiport responsible for recovery of pHi from acid load, and a DIDS-sensitive Cl-/HCO3- exchange mechanism responsible for recovery of pHi after alkalinization.

Intracellular pH in human and experimental hypertension

31P NMR spectroscopy was utilized to evaluate intracellular pH in erythrocytes from normotensive (n = 15) and from untreated (n = 16) and treated (n = 24) human essential hypertensive individuals. Intracellular erythrocyte pH was also measured in normotensive rats on different dietary calcium intakes as well as in volume-dependent deoxycorticosterone/saline and renin-dependent, 2 kidney, 1 clip (2K-1C) Goldblatt hypertensive rat models. Untreated essential hypertensives had significantly lower intracellular pH values compared with normotensive subjects [7.17 +/- 0.02 vs. 7.28 +/- 0.02 (mean +/- SEM), significance level = 0.01]. Treated hypertensives had intracellular pH values indistinguishable from normotensives [7.27 +/- 0.02 (mean +/- SEM)]. Similarly, pH values for each rat model varied inversely with blood pressure, regardless of whether increased dietary calcium intake lowered pressure (normotensive and deoxycorticosterone/saline hypertensive rats) or elevated it (2K-1C Goldblatt hypertensive rats). These results demonstrate that lower intracellular pH values are commonly observed in various hypertensive states and suggest that they may contribute to the pathophysiology of the hypertensive process. Alterations in intracellular pH may also underlie the clinically observed linkage of hypertension with other disease syndromes, such as diabetes mellitus and obesity.

Measurements of intracellular pH in single LLC-PK1 cells: recovery from an acid load via basolateral Na+/H+ exchange

LLC-PK1 cells (a continuous epithelioid cell line with renal characteristics) are examined by microspectrofluorometry as single cells, in order to determine the mechanism of intracellular pH (pHi) recovery from an acid load imposed by ammonium preincubation and removal (NH4 prepulse). Initial experiments evaluate the intracellular K+ levels through a null point analysis of total cellular K+ with flame photometry. The response of BCECF (a pH-sensitive fluorescent dye) is then calibrated, using saturating concentrations of nigericin to cause defined changes in pH. For experiments with the microspectrofluorometer, LLC-PK1 cells were grown on either glass coverslips or filters (the latter attached to plastic coverslips with a hole under the filter). The cells on glass coverslips demonstrate a Na+-dependent recovery from an (NH4 prepulse) acid load which is sensitive to 1 microM ethylisopropylamiloride. They also demonstrate a 'set point' of activation of Na+/H+ exchange. When examined for changes in pH, due to changes in membrane potential, plasma membrane proton conductance could not be detected at resting pHi. Cells grown on filters also demonstrate a pHi recovery from an acid load which is Na+ dependent and ethylisopropylamiloride sensitive, but in this configuration, the majority of cells (22/23 preparations) require Na+ at the basolateral membrane for rapid pHi recovery. The morphology and polarity of the cells grown on permeable supports appears normal at the electron-microscopic level. The results are not affected by changes in cell seeding density or collagen treatment of the filters.