Growth factor action and intracellular pH regulation in fibroblasts. Evidence for a major role of the Na+/H+ antiport

Chinese hamster lung fibroblasts (CCl39) possess in their plasma membrane an amiloride-sensitive Na+/H+ antiport, activated by growth factors. Measurements of intracellular pH (pHi), using equilibrium distribution of benzoic acid, provide evidence for a major role of this antiport in 1) regulation of cytoplasmic pH, in response to an acute acid load or to varying external pH values, and 2) the increase in cytoplasmic pH (by 0.2-0.3 pH unit) upon addition of growth factors (alpha-thrombin and insulin) to G0/G1-arrested cells. Indeed, these two processes are Na+-dependent and amiloride-sensitive; furthermore, CCl39-derived mutant cells, lacking the Na+/H+ exchange activity, are greatly impaired in pHi regulation and present no cytoplasmic alkalinization upon growth factor addition. In wild type G0-arrested cells, the amplitude of the mitogen-induced alkalinization reflects directly the activity of the Na+/H+ antiport, and is tightly correlated with the magnitude of DNA synthesis stimulation. Therefore, we conclude that cytoplasmic pH, regulated by the Na+/H+ antiport, is of crucial importance in the mitogenic response.     

Control of cytoplasmic pH by Na+/H+ exchange in rat peritoneal macrophages activated with phorbol ester

The mechanisms underlying cytoplasmic pH (pHi) regulation in elicited rat peritoneal macrophages were investigated by electronic sizing and fluorescence determinations. Acid-loaded cells rapidly regained normal pHi by means of an amiloride-sensitive Na+/H+ exchange. When stimulated by 12-O-tetradecanoyl phorbol 13-acetate, macrophages displayed a biphasic pHi change: a marginal acidification followed by an alkalinization. The latter results from activation of Na+/H+ exchange, since it is Na+-dependent and prevented by amiloride. When the antiport is inhibited, the full magnitude of the initial acidification can be appreciated. This acidification is independent of the nature of the ionic composition of the medium and probably reflects accumulation of protons generated during the metabolic burst. Under physiological conditions, these protons are rapidly extruded by the Na+/H+ antiport.     

Calcium-dependent intracellular acidification dominates the pH response to mitogen in human T cells

The effects of a phorol ester and a mitogenic lectin on the intracellular pH (pHi) of human T lymphocytes was investigated. In contrast to the cytoplasmic alkalinization induced by 12-0-tetradecanoylphorbol-13-acetate, an acidification was recorded in cells treated with phytohemagglutinin. This decrease in pHi was magnified in Na+-free medium or in the presence of amiloride analogues, suggesting that activation of Na+/H+ exchange partially counteracts the phytohemagglutinin-induced acidification. The decrease in pHi was dependent on a sustained increase in cytosolic free Ca2+ and could be mimicked by addition of the divalent cation ionophore, ionomycin. The elevation of cytosolic free Ca2+ leads to metabolic H+ (equivalent) generation with consequent cytoplasmic acidification, which in human T cells predominates over the concurrent activation of the Na+/H+ antiport. These findings argue against the notion that activation of Na+/H+ exchange is a signal for the initiation of proliferation.     

Okadaic acid, a phosphatase inhibitor, induces activation and phosphorylation of the Na+/H+ antiport.

We determined the effect of okadaic acid (OA), a potent phosphoprotein phosphatase inhibitor, on the intracellular pH (pHi) of rat thymic lymphocytes and human bladder carcinoma cells. OA induced a rapid and sustained cytosolic alkalinization. This pHi increase was Na(+)-dependent and was inhibited by 5,N-disubstituted analogs of amiloride, indicating mediation by the Na+/H+ antiport. As described for other stimulants, such as mitogens and hypertonic challenge, activation of the antiport by OA is attributable to an upward shift in its pHi dependence. Accordingly, the alkalinization produced by the phosphatase inhibitor was not additive with that induced osmotically. Activation of the antiport by OA was accompanied by a marked increase in phosphoprotein accumulation, revealing the presence of active protein kinases in otherwise unstimulated cells. We considered the possibility that phosphorylation of the antiport itself or of an ancillary protein is responsible for activation of Na+/H+ exchange. Consistent with this notion, the alkalinization induced by OA was absent in ATP depleted cells. More importantly, immunoprecipitation experiments demonstrated increased phosphorylation of the antiport following treatment with OA. We conclude that, upon inhibition of phosphoprotein phosphatase activity, constitutively active kinases induce the activation of Na+/H+ exchange, possibly by direct phosphorylation of the antiport.     

22Na+ fluxes in thymic lymphocytes. II. Amiloride-sensitive Na+/H+ exchange pathway; reversibility of transport and asymmetry of the modifier site

22Na+ flux and cytoplasmic pH (pHi) determinations were used to study the reversibility, symmetry, and mechanism of activation of the Na+/H+ exchange system in rat thymic lymphocytes. In acid-loaded cells, the antiport can be detected as an Na+-induced, amiloride-sensitive alkalinization. At pHi greater than or equal to 7.0, amiloride- sensitive net H+ fluxes are not detectable. To investigate whether at this pHi the transporter is operative in a different mode, e.g., Na+/Na+ exchange, 22Na+ uptake was measured as a function of pHi. The results indicate that the antiport is relatively inactive at pHi greater than or equal to 7.0. Comparison of the rates of H+ efflux (or equivalent OH- uptake) and Na+ uptake indicate that Na+/Na+ countertransport through this system is negligible at all values of pHi and that the Na+:H+ stoichiometry is 1:1. Measurements of pHi in Na+- loaded cells suspended in Na+-free medium revealed an amiloride- sensitive cytoplasmic acidification, which is indicative of exchange of internal Na+ for external H+. The symmetry of the system was analyzed by measuring the effect of extracellular pH (pHo) on Na+ efflux. Unlike cytoplasmic acidification, lowering pHo failed to activate the antiport. The results indicate that the amiloride-sensitive Na+/H+ exchanger is reversible but asymmetric. The system is virtually inactive at pHi greater than or equal to 7.0 but can be activated by protonation of a modifier site on the cytoplasmic surface. Activation can also occur by depletion of cellular Na+. It is proposed that Na+ may also interact with the modifier site, stabilizing the unprotonated (inactive) form.     

Thrombin induces a calcium transient that mediates an activation of the Na+/H+ exchanger in human fibroblasts

The calcium dependence of growth factor-induced cytoplasmic alkalinization was determined in serum-deprived human fibroblasts (WS-1 cells). Intracellular pH (pHi) and intracellular calcium (Ca2+i) were measured using the fluorescent dyes 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein and fura2, respectively. Thrombin (10 nM) induced an alkalinization (0.18 +/- 0.01 pH units, n = 23) that was Na+-dependent and amiloride-sensitive, suggesting that the alkalinization was mediated by the Na+/H+ exchanger. Thrombin treatment caused a transient increase in Ca2+i (325 +/- 39 nM, n = 12) that preceded the observed increase in pHi. The increases in Ca2+i and pHi were dependent on the concentration of thrombin. The thrombin-induced increase in Ca2+i occurred in the absence of external calcium indicating that thrombin released calcium from internal stores. Inhibition of the thrombin-induced increase in Ca2+i with 8-diethylaminooctyl 3,4,5-trimethoxybenzoate hydrochloride or bis-(o-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid also inhibited the thrombin-stimulated increase in pHi. The calcium ionophore ionomycin was used to increase Ca2+i independent of growth factor stimulation. When Ca2+i was elevated with ionomycin, a concomitant increase in pHi was observed. The increase in pHi due to ionomycin was dependent on Na+ and sensitive to amiloride. The removal of external Ca2+i inhibited the ionomycin-induced elevation of both Ca2+i and pHi. The ionomycin-induced increases in Ca2+i and pHi were not inhibited by 8-diethylaminooctyl 3,4,5-trimethoxy-benzoate hydrochloride. The results suggest that thrombin treatment can activate the Na+/H+ exchanger, and this activation is mediated by an increase in Ca2+i.     

Dependence of Na+/H+ antiport activation in cultured rat aortic smooth muscle on calmodulin, calcium, and ATP. Evidence for the involvement of calmodulin-dependent kinases

The role of Ca2+/calmodulin-dependent processes in the activation of the Na+/H+ antiport of primary cultures of rat aortic smooth muscle was studied using 22Na+ uptake and measurement of intracellular pH (pHi) with the fluorescent pH dye 2',7'-bis-(2-carboxyethyl)-5(and 6)-carboxyfluorescein. Antiport activation following exposure to serum and by the induction of an intracellular acidosis could be markedly attenuated by calmodulin antagonists. Ionomycin also transiently elevated pHi and 5-(N-ethyl-N-isopropyl) amiloride-sensitive 22Na+ influx, effects consistent with activation of the antiport; these effects were abolished in cells exposed to calmodulin antagonists or [ethylenebis(oxyethylenenitrilo)]tetraacetic acid. Activation of the antiport following intracellular acidosis was markedly affected by cellular ATP depletion. A comparison of the abilities of control and 2-deoxy-D-glucose-treated cells to increase 5-(N-ethyl-N-isopropyl)amiloride-sensitive 22Na+ influx in response to graded acidifications indicated that attenuation of Na+/H+ antiport activity was due to both a shift of its pHi dependence and to a reduction in maximal activity. The results suggest that the Na+/H+ antiport of rat aortic smooth muscle is dependent on Ca2+/calmodulin-dependent processes, presumably phosphorylation, which influences its activity by modulating (i) an intracellular proton dependent regulatory mechanism (allosteric site) and (ii) the maximum activity of the antiport.     

Activation of the Na+/H+ antiport is not required for lectin-induced proliferation of human T lymphocytes

Interaction of some mitogenic lectins and growth factors with the cell surface leads to activation of the Na+/H+ antiport and a resultant cytoplasmic alkalinization. Because amiloride inhibits both Na+/H+ exchange and cell proliferation, it has been hypothesized that activation of the antiport is an obligatory requirement and may, perhaps, be the "trigger" for proliferation. However, concentrations of amiloride which inhibit the antiport also inhibit several other intracellular processes, including protein synthesis and phosphorylation. To determine whether activation of the Na+/H+ antiport is necessary for lectin-induced proliferation, we examined the inhibitory activity of a series of potent amiloride analogs by measuring [3H]thymidine incorporation, cell cycle progression, and induction of the interleukin 2 (IL 2) receptor on human lymphocytes. In medium containing bicarbonate, and at concentrations at least 10 times higher than required to inhibit the antiport, these drugs did not inhibit the proliferative response of human peripheral blood T cells to the mitogen phytohemagglutinin. The amiloride analogs also failed to inhibit induction of the IL 2 receptor. Similarly, with human thymocytes, the amiloride analogs did not inhibit the co-mitogenic effects of the lectins phytohemagglutinin and concanavalin A together with IL 2 or the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. This finding suggests that Na+/H+ exchange through the antiport is not an obligatory requirement for activation or proliferation of human lymphocytes or thymocytes.     

Mechanism of activation of lymphocyte Na+/H+ exchange by concanavalin A. A calcium- and protein kinase C-independent pathway

Treatment of thymic lymphocytes with the mitogenic lectin concanavalin A (ConA) increases the intracellular free Ca2+ concentration and stimulates phosphoinositide turnover. ConA also induced a rapid, amiloride-sensitive, Na+-dependent increase in cytosolic pH of 0.13 +/- 0.01, indicative of stimulation of the Na+/H+ antiport. To investigate the mechanism underlying activation of Na+/H+ exchange by ConA, the intracellular free Ca2+ concentration changes induced by this lectin were precluded by loading the cells with Ca2+-buffering agents and suspension in Ca2+-free media. Under these conditions, the ConA-induced cytoplasmic alkalinization proceeded normally. Two approaches were used to assess the role of protein kinase C. First, this enzyme was inhibited by the addition of 1-(5-isoquinolinysulfonyl)-2-methylpiperazine. In the presence of this potent antagonist, stimulation of the antiport by 12-O-tetradecanoylphorbol-13-acetate was greatly inhibited. In contrast, stimulation by ConA was unaffected. Second, protein kinase C was depleted by overnight incubation with phorbol esters. Following this treatment, Na+/H+ exchange was no longer activated by 12-O-tetradecanoyl-13-acetate, but was still stimulated by ConA. These data suggest that a Ca2+- and protein kinase C-independent mechanisms mediates the activation of Na+/H+ exchange by ConA. The possible role of GTP-binding proteins in the activation was also studied. The antiport was not stimulated by either fluoroaluminate or vanadate. Moreover, pretreatment with pertussis toxin failed to inhibit the ConA-induced cytoplasmic alkalinization. In contrast, preincubation with cholera toxin partially inhibited activation. Under these conditions, cholera toxin significantly elevated intracellular cAMP levels. Inhibition was also observed in cells treated with forskolin at concentrations that increased [cAMP]. The data suggest that a novel cAMP-sensitive signaling mechanism not involving Ca2+ and protein kinase C is involved in the stimulation of Na+/H+ exchange by mitogens in T lymphocytes.     

Refined estimation of kinetic parameters of the Na+/H+ antiport in human fibroblasts and platelets

A technique is presented to estimate the initial rates of Na(+)-dependent alkalinization of acidified human fibroblasts and platelets and assess the kinetics of the Na+/H+ antiport in these cells. Cytosolic pH (pHi) exhibits an exponential recovery following cellular acidification. Thus, the length of the time interval selected to monitor changes in pHi (delta pHi) is critical to estimating the kinetics of the Na+/H+ antiport. We compared kinetic parameters of the Na+/H+ antiport, using computed and observed changes in delta pHi, for arbitrarily selected time intervals following Na(+)-dependent activation. In both cells, significant increases in both the [Na+] for half-maximal activation (K0.5) and maximal velocities (Vmax) were observed as delta pHi was decreased. We conclude that kinetic parameters derived from initial rate determinations enable a more accurate characterization of the Na+/H+ antiport.     

Amiloride-sensitive Na+/H+ exchange in human neutrophils: mechanism of activation by chemotactic factors

The cytoplasmic pH (pHi) of human blood neutrophils was measured using trapped carboxyfluorescein derivatives. Cells were acid-loaded using propionate or by pretreatment with NH4+. Acid-loaded cells were found to regain near-normal pHi by means of a Na+-dependent process. A concomitant Na+ uptake was recorded as a change in cell volume. Both events were amiloride-sensitive, indicating involvement of a Na+/H+ antiport. Activation of Na+/H+ exchange was also observed with chemotactic factors. Studies of the pHi-dependence of the H+ extrusion rate indicate that chemotactic factors increase the [H+i] sensitivity of the antiport.     

Interleukin-3-stimulated haemopoietic stem cell proliferation. Evidence for activation of protein kinase C and Na+/H+ exchange without inositol lipid hydrolysis.

Interleukin 3 (IL-3) is an important regulator of haemopoietic stem cell proliferation both in vivo and in vitro. Little is known about the possible mechanisms whereby this growth factor acts on stem cells to stimulate cell survival and proliferation. Here we have investigated the role of intracellular pH and the Na+/H+ antiport in stem cell proliferation using the multipotential IL-3-dependent stem cell line, FDCP-Mix 1. Evidence is presented that IL-3 can stimulate the activation of an amiloride-sensitive Na+/H+ exchange via protein kinase C activation. IL-3-mediated activation of the Na+/H+ exchange is not observed in FDCP-Mix 1 cells where protein kinase C levels have been down-modulated by treatment with phorbol esters. Also the protein kinase C inhibitor H7 can inhibit IL-3-mediated increases in intracellular pH. This activation of Na+/H+ exchange via protein kinase C has been shown to occur with no measurable effects of IL-3 on inositol lipid hydrolysis or on cytosolic Ca2+ levels. Evidence is also presented that this IL-3-stimulated alkalinization acts as a signal for cellular proliferation in stem cells.     

Evidence for a Na+/H+ antiport in stomach smooth muscle cells

Single smooth muscle cells were isolated from circular muscle of the canine gastric corpus by collagenase incubation. Cytoplasmic pH (pHi) of these cells was measured fluorometrically using the trapped dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Cells were examined for their Na+/H+ exchange activity after intracellular acidification. Cells acid-loaded by propionate exposure, the NH4+ prepulse technique or suspension in a Na+-depleted medium regained almost normal pHi upon exposure to a Na+ medium. The Na+-dependent alkalinization was amiloride sensitive. As well, addition of amiloride to cells suspended in a Na+ medium caused a concurrent decrease in pHi. The study indicates that a Na+/H+ antiport is present in these smooth muscle cells.     

Cytoplasmic [Ca2+] and intracellular pH in lymphocytes. Role of membrane potential and volume-activated Na+/H+ exchange

The effect of elevating cytoplasmic Ca2+ [( Ca2+]i) on the intracellular pH (pHi) of thymic lymphocytes was investigated. In Na+-containing media, treatment of the cells with ionomycin, a divalent cation ionophore, induced a moderate cytoplasmic alkalinization. In the presence of amiloride or in Na+-free media, an acidification was observed. This acidification is at least partly due to H+ (equivalent) uptake in response to membrane hyperpolarization since: it was enhanced by pretreatment with conductive protonophores, it could be mimicked by valinomycin, and it was decreased by depolarization with K+ or gramicidin. In addition, activation of metabolic H+ production also contributes to the acidification. The alkalinization is due to Na+/H+ exchange inasmuch as it is Na+ dependent, amiloride sensitive, and accompanied by H+ efflux and net Na+ gain. A shift in the pHi dependence underlies the activation of the antiport. The effect of [Ca2+]i on Na+/H+ exchange was not associated with redistribution of protein kinase C and was also observed in cells previously depleted of this enzyme. Treatment with ionomycin induced significant cell shrinking. Prevention of shrinking largely eliminated the activation of the antiport. Moreover, a comparable shrinking produced by hypertonic media also activated the antiport. It is concluded that stimulation of Na+/H+ exchange by elevation of [Ca2+]i is due, at least in part, to cell shrinking and does not require stimulation of protein kinase C.     

The regulation of intracellular pH in monkey kidney epithelial cells (BSC-1). Roles of Na+/H+ antiport, Na+-HCO3(-)-(NaCO3-) symport, and Cl-/HCO3- exchange

Using the pH-sensitive absorbance of 5 (and 6)-carboxy-4',5'- dimethylfluorescein, we investigated the regulation of cytoplasmic pH (pHi) in monkey kidney epithelial cells (BSC-1). In the absence of HCO3-, pHi is 7.15 +/- 0.1, which is not significantly different from pHi in 28 mM HCO3-, 5% CO2 (7.21 +/- 0.07). After an acid load, the cells regulate pHi in the absence of HCO3- by a Na+ (or Li+)-dependent, amiloride-inhibitable mechanism (indicative of Na+/H+ antiport). In 28 mM HCO3-, while still dependent on Na+, this regulation is only blocked in part by 1 mM amiloride. A partial block is also observed with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (1 mM). With cells pretreated with DIDS, 1 mM amiloride nearly totally inhibits this regulation. Cl- had no effect on pHi regulation in the acidic range. In HCO3(-)-free saline, Na+ removal leads to an amiloride-insensitive acidification, which is dependent on Ca2+. In 28 mM HCO3-, Na+ (and Ca2+) removal led to a pronounced reversible and DIDS-sensitive acidification. When HCO3- was lowered from 46 to 10 mM at constant pCO2 (5%), pHi dropped by a DIDS-sensitive mechanism. Identical changes in pHo (7.6 to 6.9) in the nominal absence of HCO3- led to smaller changes of pHi. In the presence but not in the absence of HCO3-, removal of Cl- led to a DIDS-sensitive alkalinization. This was also observed in the nominal absence of Na+, which leads to a sustained acidification. It is concluded that in nominally bicarbonate-free saline, the amiloride-sensitive Na+/H+ antiport is the predominant mechanism of pHi regulation at acidic pHi, while being relatively inactive at physiological values of pHi. In bicarbonate saline, two other mechanisms effect pHi regulation: a DIDS-sensitive Na+-HCO3- symport, which contributes to cytoplasmic alkalinization, and a DIDS-sensitive Cl-/HCO3- exchange, which is apparently independent of Na+.     

Regulation of cytoplasmic pH in resident and activated peritoneal macrophages

Cytoplasmic pH (pHi) has been shown to be an important determinant of the activity of the NADPH oxidase in phagocytic cells. We hypothesized that a difference in pHi and/or its regulation existed between activated and resident macrophages (RES MOs) which might explain the increased NADPH oxidase activity observed in the former. The pHi of RES and lipopolysaccharide (LPS)-elicited MOs was examined using the fluorescent dye BCECF. Resting pHi did not differ between resident (RES) and elicited (ELI) MOs (7.16 +/- 0.05 and 7.20 +/- 0.05, respectively). pHi recovery after intracellular acid loading was partially dependent on the presence of Na+ in the extracellular medium, and was partially inhibited by the Na+/H+ antiport inhibitor, amiloride. At comparable pHi, the rate of acid extrusion during recovery was not different in RES and ELI MOs (1.48 +/- 0.12 and 1.53 +/- 0.06 mM/min, respectively). In both RES and ELI MOs, approx. 40% of total pHi recovery was insensitive to amiloride and independent of extracellular Na+. In both RES and ELI MOs, stimulation with TPA resulted in a biphasic pHi response: an initial acidification followed by a sustained alkalinization to a new steady-state pHi. This alkalinization was Na(+)-dependent and amiloride-sensitive, consistent with a TPA-induced increase in Na+/H+ antiport activity. The new steady-state pHi attained after TPA stimulation was equivalent in RES and ELI MOs (7.28 +/- 0.04 and 7.31 +/- 0.06, respectively), indicating comparable stimulated Na+/H+ antiport activity. However, the initial acidification induced by TPA was greater in ELI than in RES MOs (0.18 +/- 0.02 vs. 0.06 +/- 0.02 pH unit, respectively, P less than 0.05). The specific NADPH oxidase inhibitor diphenylene iodonium (DPI) completely inhibited the respiratory burst but reduced the magnitude of this pHi reduction by only about 50%. This suggested that the TPA-induced pHi reduction was due in part to acid produced via the respiratory burst, and in part to other acid-generating pathways stimulated by TPA.     

Mitogen-independent activation of Na+/H+ exchange in human epidermoid carcinoma A431 cells: regulation by medium osmolarity

Previous studies have documented the activation of Na+/H+ exchange in A431 cells by the addition of epidermal growth factor or serum (Rothenberg et al., 1983b). Here we show that exposure of A4 31 cells to medium of increased osmolarity also leads to activation of Na+/H+ exchange and to an increase in intracellular pH (pHi), which under a variety of conditions displays similar kinetics to that observed upon addition of mitogens to the cells. Measurements of cell volume using the 3-0-methylglucose equilibration technique clearly show that mitogens do not activate Na+/H+ exchange by an osmotic mechanism (i.e., a decrease in cell volume). In fact, mitogens can induce further intracellular alkalinization if added to cells which have been shrunken in hypertonic medium. Activation of the Na+/H+ antiport does not lead to an obligatory change in pHi. Addition of epidermal growth factor of hypertonic solution to A431 cells in bicarbonate buffer activates Na+/H+ exchange without a concomitant increase in pHi. Under these conditions the increased proton efflux via Na+/H+ exchange must therefore be compensated by other mechanisms that control cytoplasmic pH.     

Cytoplasmic pH regulation in normal and abnormal neutrophils. Role of superoxide generation and Na+/H+ exchange

The cytoplasmic pH of human neutrophils was determined fluorometrically using carboxylated fluorescein derivatives. When normal neutrophils were activated by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) in Na+-containing medium, the cytoplasmic pH initially decreased but then returned to near normal values. In Na+-free media or in Na+ medium containing amiloride, TPA induced a marked monophasic intracellular acidification. The cytoplasmic acidification is associated with net H+ equivalent efflux, suggesting metabolic acid generation. The metabolic pathways responsible for the acidification were investigated by comparing normal to chronic granulomatous disease neutrophils. These cells are unable to oxidize NADPH and generate superoxide. When treated with TPA in Na+-free or amiloride-containing media, chronic granulomatous disease cells did not display a cytoplasmic acidification. This suggests that in normal cells NADPH oxidation and/or the accompanying activation of the hexose monophosphate shunt are linked to the acidification. Unlike normal neutrophils, chronic granulomatous disease cells treated with TPA in Na+-containing medium displayed a significant cytoplasmic alkalinization. The alkalinization was Na+-dependent and amiloride-sensitive, indicating activation of Na+/H+ exchange. Thus, the Na+/H+ antiport, which can be indirectly stimulated by the metabolic cytoplasmic acidification, is also directly activated by the phorbol ester.     

Mechanism of osmotic activation of Na+/H+ exchange in rat thymic lymphocytes

The activity of the Na+/H+ exchange system of rat thymic lymphocytes was determined by means of intracellular (pHi) and extracellular pH (pH0) measurements. In isotonic media, the antiport is virtually quiescent at physiological pHi (7.0-7.1), but is greatly activated by cytoplasmic acidification. At normal pHi, the antiport can also be activated by osmotic shrinking. Osmotic activation occurs after a delay of 20-30 s and is reversed several minutes after iso-osmolarity is restored. The mechanism of activation was analyzed by comparing the kinetic parameters of transport in resting (isotonic) and hyperosmotically stressed cells. The affinities of the external substrate site for Na+ and H+ are not altered in shrunken cells. In contrast, the Hi+ sensitivity of the antiport (which is largely dictated by an allosteric modifier site) was increased, which accounted for the activation. The concentration of free cytoplasmic Ca2+ [( Ca2+]i) increased after osmotic shrinking. This increase was dependent on the presence of extracellular Ca2+ and Na+ and was blocked by inhibitors of Na+/H+ exchange, which suggests that it is a consequence, rather than the cause, of the activation of the antiport. It is concluded that the shift in the pHi dependence of the modifier site of the Na+/H+ antiport is the primary event underlying the regulatory volume increase that follows osmotic shrinkage.     

IL-1 activates the Na+/H+ antiport in a murine T cell

One of the early events following growth factor exposure is elevation of intracellular pH, a process mediated by the Na+/H+ antiport. We studied the effects of human rIL-1 alpha (HrIL-1 alpha) on intracellular pH (pHi) and calcium ([Ca2+]i) in a murine T cell line (MD10 cells), which proliferates in response to IL-1 alone. By using the intracellularly trapped fluorescent dyes (2(1),7(1)-bis-2-carboxyethyl)-5(and -6) carboxyfluorescein) and indo-1, we monitored immediate to early changes of pHi and [Ca2+]i in response to HrIL-1 alpha. Exposure to HrIL-1 alpha (120 pM) leads to an early, sustained intracellular alkalinization (delta pH = + 0.09 +/- 0.03) that plateaus within 20 min. Lower concentrations of the monokine (12 pM, 1.2 pM) have a positive but not statistically significant effect on pHi. These effects parallel the degree of MD10 IL-1R saturation predicted by the KD (49 pM) as assessed by 125I-HrIL-1 alpha binding by MD10 cells (Bmax = approximately 1300). Both the MD10 IL-1 receptor KD and the HrIL-1 alpha concentration required to induce early measurable alkaline pH shifts, however, exceed by three orders of magnitude the HrIL-1 alpha ED50 (50 fM) required for MD10 proliferation. The IL-1-induced rise in pHi is both sodium dependent and amiloride sensitive, indicative of activation of the Na+/H+ antiport. Additionally, PMA (100 nM) and IL-2 (2 nM) alkalinize MD10 cells, with the rise in pHi as a result of PMA exceeding the maximal IL-1 effect (delta pH = + 0.13 +/- 0.04). Furthermore, although PMA alkalinizes cells previously exposed to HrIL-1 alpha, the monokine does not alter the pHi of PMA-treated MD10 cells. Importantly, intracellular alkalinization induced by either HrIL-1 alpha or PMA is inhibited by staurosporine (1 mu iM). Finally, HrIL-1 alpha does not change MD10 [Ca2+]i, in either an acute or sustained fashion. These results indicate that IL-1 activates the Na+/H+ antiport in T cells by a mechanism that is unrelated to changes in [Ca2+]i but may involve protein kinase C activation.