Sodium fluxes in human fibroblasts: Effect of serum,Ca+2, and amiloride

Human fibroblasts that have been serum deprived for 4 hours have a digitoxin-insensitive Na influx of 9.5 ± 1.0 (n = 4) μmol/g prot/min which is not significantly different from the influx of 9.4 ± 0.6 (n = 3) μmol/g prot/min measured in cells arrested in the G₁/G₀ state by serum-deprivation for a period of four days. The Na influx in serum-deprived cells is rapidly stimulated (within one minute) simply by assaying the cells in medium containing 10% fetal bovine serum (FBS). The digitoxin-insensitive Na influx for cells in the presence of 10% FBS is 22.9 ± 1.1 (n = 6) μmol/g prot/min. the stimulation of Na influx in serumdeprived cells can also be achieved by the addition of the purified mitogen, epidermal growth factor (EGF). Addition of EGF to serum-deprived cells gives a maximal stimulation of Na influx of approximately 1.6-fold, with the concentration for half-maximal stimulation being 7.5 ng/ml. The stimulation of Na influx results from the activation of an amiloride-sensitive pathway, which appears to be minimally active in serum-deprived cells. Kinetic analysis of Na influx experiments in the presence of 10% FBS and varying concentrations of amiloride indicate that at infinite concentrations of amiloride the Na flux would be reduced to 8.9 μmol/g prot/min, which is comparable to the level of Na flux measured in serum-deprived cells in the presence of 5 mM amiloride. Thus, amiloride can totally inhibit the serum-stimulated component of Na influx while inhibiting less than 10% of the Na influx in serum-deprived cells. The Na influx in serum-deprived cells can also be stimulated 2.5-fold by preincubating cells in the presence of the Ca⁺ ionophore A23187 to elevate the intracellular Ca content. This stimulation of Na influx by intracellular Ca⁺² can be virtually eliminated by adding 1 mM amiloride.     

Sodium fluxes in human fibroblasts: kinetics of serum-dependent and serum-independent pathways

Sodium influx in serum-deprived human fibroblasts is by way of a pathway which shows saturation kinetics. A plot of initial Na influx versus [Na]0 ([Na]i approximately equal to 10 mM) gives a simple Michaelis-Menten type of curve with a K1/2 = 70.0 +/- 8.1 mM and a Vmax = 14.5 +/- 1.9 mumol/g prot/min. A similar plot of initial Na influx versus [Na]0 in the presence of 10% fetal bovine serum (FBS) gives a nonsaturating curvilinear response which appears to be biphasic. A plot of the serum-dependent Na influx versus [Na]0 (obtained by subtracting the curve in the absence of FBS from the curve in the presence of 10% FBS) shows that there is a linear relationship between serum-induced Na influx and external [Na]. At physiological Na concentrations, in the presence of FBS, the serum-induced Na influx is equal to the amiloride-sensitive Na flux, whereas in the absence of serum amiloride inhibits less than 10% of the Na influx. The effect of intracellular Na on Na flux was tested by preloading cells with Na in a digitoxin-containing medium prior to measurement of Na flux. A plot of steady-state Na exchange flux versus [Na]0 ([Na]i approximately equal to [Na]0) in the absence of serum gives a curve that appears to saturate at approximately 100 mM Na (flux = 100 mumol/g prot/min) and then declines with increasing [Na] (flux = 40 mumol/g prot/min at 150 mM). In contrast to Na influx in control serum-deprived cells, Na flux in Na-loaded cells in dramatically inhibited by the presence of amiloride. Since the peak Na exchange flux of 100 mumol/g prot/min is greatly in excess of the Vmax for Na influx in control serum-deprived cells and the enhanced Na flux is amiloride-sensitive, elevating intracellular Na must somehow activate the amiloride-sensitive Na transport system, which is normally only minimally active in the absence of serum.     

Characteristics of an amiloride-sensitive sodium entry pathway in cultured rodent glial and neuroblastoma cells

We have studied the induction of an amiloride-sensitive sodium influx into C6 glioma, NIE, and NB2A neuroblastoma cell lines. In late log phase, cells grown continuously in the presence of 10% fetal calf serum showed Na⁺ influxes of approximately 25–30 nmol/mg protein min; < 5% of this flux was inhibited by amiloride. Removal of serum for 24 h caused a decrease in the total Na⁺ influx to 15–20 nmol/mg protein/min. Upon readdition of serum to the incubation medium, there was an increase in total Na⁺ influx, depending on the cell type, of 20–400% within 2 min. This increment in Na⁺ influx represented an increase in amiloride-sensitive Na⁺ transport with an apparent K′, of 0.4 mM. By adding serum back at various times after serum deprivation, it was determined that 4 h was required to observe a detectable increase in the amiloride-sensitive Na⁺ flux. Thus, serum removal results in the induction of the amiloride transport system which, however, remains latent until the reintroduction of serum to the medium. Addition of 5 μg/ml of cycloheximide blocked the increase in Na⁺ transport, indicating that de novo protein synthesis mediated this serum deprivation–induced increase in Na⁺ transport. Moreover, inhibition of de novo lipid synthesis by 0.1 mM fenfluramine also blocked the induction of this transport activity, suggesting that a coordinated synthesis of lipid and protein is required for the expression of this sodium transport site. We have also found that this serum stimulated Na⁺ influx did not saturate with Na⁺ concentration, up to 140 mM. Also, among commonly used inhibitors of passive Na⁺ entry into epithelial tissues, only amiloride was capable of inhibiting this transport system in these neural cell lines.     

Liver plasma membrane calcium transport. Evidence for a Na+-dependent Ca2+ flux

Plasma membrane vesicles isolated from rat liver exhibited an azide-insensitive Mg2+-ATP-dependent Ca2+ pump which accumulated Ca2+ at a rate of 5.1 +/- 0.5 nmol of calcium/mg of protein/min and reached a total accumulation of 33.2 +/- 2.6 nmol of calcium/mg of protein in 20 microM Ca2+ at 37 degrees C. Equiosmotic addition of 50 mM Na+ resulted in a loss of accumulated calcium. Measurement of Mg2+-ATP-dependent Ca2+ uptake in the presence of 50 mM Na+ revealed no effect of Na+ on the initial rate of Ca2+ uptake, but a decrease in the total accumulation. The half-maximal effect of Na+ on Ca2+ accumulation was achieved at 14 mM. The Ca2+ efflux rate constant in the absence of Na+ was 0.16 +/- 0.01 min-1, whereas the efflux rate constant in the presence of 50 mM Na+ was 0.25 +/- 0.02 min-1. Liver homogenate sedimentation fractions from 1,500 to 105,000 X g were assayed for azide-insensitive Mg2+-ATP-dependent Ca2+ accumulation. Na+-sensitive Ca2+ uptake activity was found to specifically co-sediment with the plasma membrane-associated enzymes, 5'-nucleotidase and Na+/K+-ATPase, whereas Na+-insensitive Ca2+ uptake was found to co-sediment with the endoplasmic reticulum-associated enzyme, glucose-6-phosphatase. The plasma membrane Ca2+ pump was also distinguished from the endoplasmic reticulum Ca2+ pump by its sensitivity to inhibition by vanadate. Half-maximal inhibition of plasma membrane Ca2+ uptake occurred at 0.8 microM VO4(3-), whereas half-maximal inhibition of microsomal Ca2+ uptake occurred at 40 microM.     

Calcium and EDTA fluxes in dialyzed squid axons

Ca efflux in dialyzed squid axons was measured with 45Ca as a function of internal ionized Ca in the range 0.005-10 muM. Internal Ca stores were depleted by treatment with CN and dialysis with media free of high energy compounds. The [Ca]iota was stabilized with millimolar concentrations of EDTA, EGTA, or DTPA. Nonspecific leak of chelated Ca was measured with [14C]-EDTA and found to be 0.02 pmol/cm2s/mM EDTA. Correction of the measured Ca efflux for this leak of chelated calcium was made when appropriate. Ca efflux was roughly linear with internal free Ca in the range 0.005-0.1 muM. Above 0.1 muM, efflux was less than proportional to concentration but did not saturate at the highest concentration studied. Ca efflux was reduced about 50% by replacement of external Na with Li at Caiota approximately 1 muM, but was insensitive to such replacement for Ca less than 0.1 muM. Ca efflux was insensitive to internal Mg in the range 0-4 mM, indicating that the Ca pump favors Ca over Mg by a factor of about 10(6). Ca efflux was reduced about 60% by increasing internal Na from 1 to 80 mM. This effect could represent weak interference of a Ca carrier by Na or a loss of driving force because of a reduction in ENa - Em occasioned by an increase in Naiota. A few measurements were made of Ca influx in intact and in dialyzed fibers. In both cases, Ca influx increased when external Na was replaced by Li.     

Rapid ionic events and the initiation of growth in serum-stimulated neuroblastoma cells

Rapid effects of serum stimulation on electrical and ionic membrane properties and their relationship to the initiation of DNA synthesis and cell division have been investigated in mouse N1E-115 neuroblastoma cells. Addition of 10% fetal calf serum to serum-deprived N1E-115 cells results in the initiation of DNA synthesis after a lag of approximately 10 hr. The earliest events following serum addition include: transient membrane potential and resistance changes, detectable within seconds and lasting 5--10 min; a persistent increase in the initial rate of 22Na+ influx, the major part of which is not of electrodiffusional origin, and which is potentiated by weak acid anions; and an external Na+-dependent increase in the rate of the Na+, K+ pump. In the absence of serum the stimulation of the Na+, K+ pump can be mimicked by increasing net Na+ influx with monensin or neurotoxins. Growth-depleted serum fails to induce any of the electrical and ionic events. The diuretic amiloride (0.4 mM) inhibits serum-induced Na+ influx, Na+, K+ pump stimulation and DNA synthesis, but does not affect the electrical response or the basal influx rates. The results suggest that serum growth factors act, at least in part, by stimulating an electroneutral, amiloride-sensitive Na+/H+ exchange mechanism. The enhanced Na+ influx then results in the observed stimulation of the Na+, K+ pump, while the simultaneous efflux of protons may raise the intracellular pH.     

Role of divalent metal ions in serum complement activity of the American alligator (Alligator mississippiensis)

Treatment of alligator serum with different concentrations of EDTA resulted in a concentration-dependent inhibition of serum-mediated sheep red blood cell (SRBC) hemolysis. This inhibition of serum-dependent hemolysis was observed for other chelators of divalent metal ions, such as phosphate and citrate. Treatment of alligator serum with 5 mM EDTA completely inhibited SRBC hemolysis, which could be totally restored by the addition of 5 mM Ca(2+) or Mg(2+), but not Cu(2+) or Ba(2+). These data indicate a specific need for Ca(2+) and/or Mg(2+) in the serum-mediated hemolysis of SRBCs. Kinetic analyses revealed that the addition of 30 mM EDTA 1 min after incubation of SRBCs with serum resulted in only 30% inhibition of hemolytic activity. However, addition of EDTA as early as 3 min post-incubation resulted in complete SRBC hemolysis. Pretreatment of serum with EDTA inhibited the hemolytic activity, but the activity could be restored in a time-dependent manner by the addition of Ca(2+)or Mg(2+). These data indicate that, as in human serum, the need for divalent metal ions occurs early in the alligator serum complement cascade.     

Sodium transport in erythrocytes of the mollusc Anadara broughtonii: Evidence for inhibition by quinine

Sodium transport through the molluscan erythrocyte membrane was examined using ²²Na as a tracer. Incubation of the red cells in standard saline resulted in a rapid ²²Na uptake reaching steady state concentration (about 21.5 mmol/l cells) in the first 60 min. A similar pattern in the time course of ²²Na uptake was seen in the erythrocytes incubated in mantle fluid. The average value of unidirectional Na⁺ influx, measured as a 5-min ²²Na uptake, was 7.76 ± 0.36 mmol/1 cells/5 min or 93 ± 4.3 mmol/1 cells/hr. The initial rate of Na⁺ influx increased in a saturable fashion as a function of external Na⁺ concentration with apparent AT., of 380±12mM and V_max of 14.3 ± 2.4 mmol/1 cells/5 min. Amiloride (1 mM), furosemide (1 mM), and DIDS (0.1 mM) had no effect on either initial Na⁺ influx (5 min ²²Na uptake) or equilibrium Na⁺ concentration (60 min and 120min ²²Na uptake) in the molluscan red cells exposed to standard saline. Quinine (1 mM) caused a significant fall in the initial Na⁺ influx (by 48%) and in 60-min ²²Na uptake (by 32%) as compared with control levels. In the presence of 0.1 mM ouabain, ²²Na uptake into the red cells was enhanced by an average 27% and 44% during 60 min and 120 min of cell incubation, respectively. The ouabain-sensitive Na⁺ accumulation in the red cells reflected a contribution of the Na, K-pump to Na⁺ transport and the mean value was 5.6 ± 1.0 mmol/1 cells/hr.     

Ca2+-activated Na+ fluxes in human red cells. Amiloride sensitivity

The effect of Ca2+ on the ouabain- and bumetanide-resistant Na+ fluxes in intact red cells was studied at relatively constant internal Ca2+, membrane potential, and cell volume. The red cell calcium concentration was modified using the ionophore A23187. In fresh red cells, the Na+ influx and efflux (1.2 +/- 0.13 and 0.26 +/- 0.07 mmol/liter cells x h, respectively) were not affected by amiloride (1 mM). When external Ca2+ was raised from 0 to 150 microM, in the presence of A23187, both the Na+ influx and efflux were stimulated (about 3.5-fold). The Ca2+-activated Na+ efflux and influx had an apparent Km for activation by Ca2+o of about 25 microM. The Ca2+-dependent Na+ transport was inhibited 30-60% by amiloride (ID50 = 17.3 +/- 8 microM). Amiloride, however, had no effect on the Ca2+-dependent K+ influx. The amiloride-sensitive (AS) transport pathway was a linear function of the Na+o concentration in the range from 0 to 75 mM. The Ca2+i activation seems to depend on the metabolic integrity of red cells. 1) It does not take place in ATP-depleted red cells; 2) ATP-repletion of ATP-depleted red cells fully restored AS Na influx; and 3) ATP-enrichment (ATP-red cells) enhanced the AS Na influx by about 100%. The Ca2+-activated AS Na+ influx was not affected by either DIDS or trifluoperazine. The present results indicate that in human erythrocytes an increase in internal Ca2+ activates on otherwise silent AS Na+-transport system, which is dependent on the metabolic integrity of the red cells.     

Membrane potential and sodium flux in neuroblastoma X glioma hybrid cells: Effects of amiloride and serum

The Na⁺ uptake into neuroblastoma x glioma hybrid cells was measured in Hepes-buffered EMEM containing 10% calf serum and 5 mM ouabain in the presence and absence of amiloride (1.0 mM). Amiloride was found to markedly inhibit net Na⁺ influx (by approximately 50%). Examination of the effect of amiloride on net Na⁺ influx in the absence of calf serum revealed that a significant amiloride-sensitive Na⁺ influx remains even under serum-deprived conditions, although the degree of amiloride inhibition (35%) is substantially lower than that found in the presence of serum. The amiloride-insensitive portion of Na⁺ influx was found to be independent of serum effects. Estimation of resting membrane potential was made by measurement of the steady state distribution of the lipophilic cation, TPP⁺, in the presence and absence of amiloride. A large, immediate increase in TPP⁺ uptake, indicative of a membrane hyperpolarization, was seen upon addition of amiloride. Determination of the effect of amiloride on resting membrane potential of serum-deprived cells showed that cells are hyperpolarized to a greater extent in the presence than in the absense of amiloride, and that serum exerts a depolarizing effect on the cells. Thus, serum-stimulation of Na⁺ influx results in a depolarization of resting membrane potential, while amiloride inhibition of Na⁺ influx causes a hyperpolarization. These data strongly suggest that NG108-15 cells possess an electrogenic Na⁺ influx pathway that is sensitive to amiloride inhibition and enhanced by serum.     

Effect of ATP,EDTA and EGTA on the simultaneous binding of Na,K, Mg and Ca by rat liver microsomes

In the presence of Na, K, Mg and Ca at physiological pH, complexing agents can affect cation binding by rat liver microsomes in a manner not always readily predictable simply from a knowledge of individual formation constants. Increasing concentrations (0 to 20 mM) of the strong nonbiological complexing agent, ethylenediaminetetraacetate (EDTA), produced a sharp decrease almost to zero in bound Ca, an increase to a high plateau in bound Na and K and an initial increase followed by a sharp decrease in bound Mg. Increasing concentrations of the Ca-preferring analogue of EDTA, ethylene bisglycol (β-aminoethylether) tetraacetate (EGTA), produced similar changes except that bound Mg increased and remained elevated, indicating that this agent complexes Mg very weakly at physiological pH. The biological complexing agent, adenosine triphosphate (ATP), caused a gradual rectilinear and parallel decrease in bound Mg and Ca and a concomitant and parellel increase in bound Na and K at about 4°C and pH 6.4. Results with EDTA and EGTA suggest, however, that under different conditions, enhancement by ATP of divalent cation binding may be possible. Reactions of this nature may be of significance in ATP stimulated divalent cation uptake by subcellular particles.     

Desensitization of the serum effect on Na+ influx in cultured human fibroblasts

Stimulation of an amiloride-sensitive Na+ influx pathway, which mediates Na+/H+ exchange, has been postulated to be an important step in the initiation of DNA synthesis in quiescent human fibroblasts. If the elevation of intracellular Na+ or the alkalinization of intracellular pH resulting from the activation of this system is a trigger for subsequent mitogenic events, then its inactivation may also be important to cellular functions. We investigated the duration of the activation of Na+ influx by serum in human foreskin fibroblasts (HSWP). It was found that activation of Na+ influx by 10% serum was transient, declining with a t 1/2 = 15 min. Similarly, the Na+ content of the cells rose rapidly following serum addition and decreased with a t 1/2 = 15 min. In addition, both the lys-bradykinin- and the vasopressin-stimulated Na+ influx and Na+ content declined with a t 1/2 of approximately 15 min. Similar results were obtained using both Tris-buffered and Hepes-buffered, amino-acid-free EMEM. Finally, the above experiments were repeated under conditions normally used to assess the mitogenic response of cells. It was found that in cells arrested in G0 by serum deprivation in CO2-buffered EMEM, the serum activated Na+ flux was also transient with a t 1/2 of approximately 20 min. The desensitization of cells to serum could be readily (t 1/2 = 20') reversed by a subsequent incubation of cells in serum-free medium. Stimulation of Na+ influx by both the divalent cation ionophore A23187 and the phospholipase activator melittin in also desensitized rapidly, suggesting the process is independent of receptor downregulation. The desensitization during serum preincubation occurred in both low Na+ and low pH medium suggesting that the process is not due to negative feedback on the transport system via a rise in cellular Na+ concentration or a rise in intracellular pH. Although the mechanism of desensitization is at present not known, it is likely to be a physiologically important event.     

The Mg,Ca, EDTA and ATP dependence of Na binding by rat liver microsomes

The role of ionic interactions in the adenosinetriphosphate (ATP) dependent Na binding by rat liver microsomes was investigated. In the concentration range of 0 to 20 mM, Mg and Ca are demonstrated to compete strongly against Na for microsome binding sites. In the presence of Ca, the nonbiological complexing agent ethylenediaminetetraacetate (EDTA) produced a marked increase in Na binding accompanied by a concomitant decrease in Ca binding. Under similar conditions ATP, which is a weaker complexing agent than EDTA, produced quantitatively smaller but qualitatively similar changes in binding. The data show that the effect of ATP on Na binding is not dependent upon the formation of a hypothetical Na binding intermediate in the hydrolysis of ATP as other investigators have postulated. Rather, the effect of ATP is demonstrated to depend upon the presence of unhydrolyzed ATP and its ability to complex divalent cations, and thereby to reduce divalent cation competition against monovalent cations for membrane binding sites.     

Membrane transport of sodium ions in erythrocytes of the American black bear, Ursus americanus

1. Membrane transport of Na ions was investigated in red blood cells of bears by methods of measurement of unidirectional isotopic fluxes. 2. Like red blood cells of dogs, bear red cells contain a high Na concentration and low concentrations of K and ATP. 3. As in dog red cells, Na efflux from bear cells was not inhibited by ouabain but was activated by the presence of Ca in the medium, possibly indicating the presence of a Na-Ca exchange mechanism. 4. ATP depletion of cells was accelerated by Ca in the medium, consistent with the presence of a strong ATP-dependent Ca pump. 5. As in other carnivore red cells, Na influx into bear cells was strongly activated by shrinkage and inhibited by swelling. Shrinkage-activated influx was blocked by amiloride. 6. Amiloride-sensitive influx was activated by cytoplasmic Ca and also correlated with the presence of a Na-dependent, amiloride-sensitive H loss. 7. Amiloride-sensitive Na influx exhibited a strong seasonal cycle with a minimum in the middle of the hibernation period, suggesting a possible avenue of cellular energy conservation.     

Mg2+--Ca2+--Na+ interactions in uterine smooth muscle: studies with cAMP.

The influence of variation in the extracellular concentrations of Na+, Mg2+, and Ca2+ in the depolarizing medium on isoproterenol-induced increases in cAMP levels and relaxation was studied in rat uterus. Isoproterenol (10(-8) M) failed to increase cAMP levels in the high-K+ medium containing no Na+. When 80 mM Na+ was present in the medium, isoproterenol caused increases in cAMP levels similar to those observed in nondepolarized uterus. A similar effect of 2.5 mM Mg2+ was observed on the cAMP response. These effects of Na+ and Mg2+ were antagonized by increasing the extracellular concentration of Ca2+. The simultaneous presence of 80 mM Na+ and 2.5 mM Mg2+ did not produce an additive effect on the cAMP responses.     

Characterization of calcium transport by basal plasma membranes from human placental syncytiotrophoblast.

We have studied the mechanisms involved in calcium (Ca2+) transport through the basal plasma membranes (BPM) of the syncytiotrophoblast cells from full-term human placenta. These purified membranes were enriched 25-fold in Na+/K(+)-adenosine triphosphate (ATPase), 37-fold in [3H] dihydroalprenolol binding sites, and fivefold in alkaline phosphatase activity compared with the placenta homogenates. In the absence of ATP and Mg2+, a basal Ca2+ uptake was observed, which followed Michaelis-Menten kinetics, with a Km Ca2+ of 0.18 +/- 0.05 microM and Vmax of 0.93 +/- 0.11 nmol/mg/min. The addition of Mg2+ to the incubation medium significantly decreased this uptake in a concentration-dependent manner, with a maximal inhibition at 3 mM Mg2+ and above. The Lineweaver-Burk plots of Ca2+ uptake in the absence and in the presence of 1 mM Mg2+ suggest a noncompetitive type of inhibition. Preloading the BPM vesicles with 5 mM Mg2+ had no significant effect on Ca2+ uptake, eliminating the hypothesis of a Ca2+/Mg2+ exchange mechanism. This ATP-independent Ca2+ uptake was not sensitive to 10(-6) M nitrendipine nor to 10(-4) M verapamil. An ATP-dependent Ca2+ transport was also detected in these BPM, whose Km Ca2+ was 0.09 +/- 0.02 microM and Vmax 3.4 +/- 0.2 nmoles/mg/3 min. This Ca2+ transport requires Mg2+, the optimal concentration of Mg2+ being approximately 1 mM. Preincubation of the membrane with 10(-6) M calmodulin strongly enhanced the initial ATP-dependent Ca2+ uptake. Finally, no Na+/Ca2+ exchange process could be demonstrated.     

Chemotactic factor-induced activation of Na+/H+ exchange in human neutrophils. I. Sodium fluxes

The nature of Na+ fluxes in resting and in chemotactic factor-activated human neutrophils was investigated. In resting cells, ouabain-insensitive unidirectional 22Na+ in- and effluxes represented passive electrodiffusional fluxes through ion channels: they were nonsaturable and voltage-dependent (PNa = 4.3 X 10(-9) cm/s). Amiloride (1 mM) had little effect on resting 22Na+ influx (approximately 0.8 meq/liter X min), thereby suggesting a minor contribution of Na+/H+ exchange and a lack of amiloride-sensitive Na+ channels. When neutrophils were exposed to the chemotactic tripeptide N-formyl-methionyl-leucyl-phenylalanine (FMLP, 0.1 microM), 22Na+ influx was stimulated approximately 30-fold (initial rate approximately 22 meq/liter X min). The FMLP-induced 22Na+ influx was saturable with respect to external Na+ (Km 26-35 mM, Vmax approximately 28 meq/liter X min), was electroneutral, and could be competitively inhibited by amiloride (Ki 10.6 microM). From a resting value of approximately 30 meq/liter of cell water, internal Na+ in FMLP-stimulated cells rose exponentially to reach a concentration of approximately 60 meq/liter by 10-15 min. This uptake was blocked by amiloride. FMLP also stimulated the efflux of 22Na+ which followed a single exponential time course (rate coefficient approximately 0.16 min-1). The FMLP-induced 22Na+ fluxes were similar to those observed with 10 microM monensin, a known Na+/H+ exchanging ionophore. The data indicate that FMLP activates an otherwise quiescent, amiloride-sensitive Na+/H+ exchange. Furthermore, all of the FMLP-induced 22Na+ fluxes can be satisfactorily accounted for by transport through the exchanger, leaving little room for an appreciable increase in Na+ conductance.     

Ca2+-dependent modulation of intracellular Mg2+ concentration with amiloride and KB-R7943 in pig carotid artery

It has long been recognized that magnesium is associated with several important diseases, including diabetes, hypertension, cardiovascular, and cerebrovascular diseases. In the present study, we measured the intracellular free Mg2+ concentration ([Mg2+]i) using 31P nuclear magnetic resonance (NMR) in pig carotid artery smooth muscle. In normal solution, application of amiloride (1 mm) decreased [Mg2+]i by approximately 12% after 100 min. Subsequent washout tended to further decrease [Mg2+]i. In contrast, application of amiloride significantly increased [Mg2+]i (by approximately 13% after 100 min) under Ca2+-free conditions, where passive Mg2+ influx is facilitated. The treatments had little effect on intracellular ATP and pH (pHi). Essentially the same Ca2+-dependent changes in [Mg2+]i were produced with KB-R7943, a selective blocker of reverse mode Na+-Ca2+ exchange. Application of dimethyl amiloride (0.1 mM) in the presence of Ca2+ did not significantly change [Mg2+]i, although it inhibited Na+-H+ exchange at the same concentration. Removal of extracellular Na+ caused a marginal increase in [Mg2+]i after 100-200 min, as seen in intestinal smooth muscle in which Na+-Mg2+ exchange is known to be the primary mechanism of maintaining a low [Mg2+]i against electrochemical equilibrium. In Na+-free solution (containing Ca2+), neither amiloride nor KB-R7943 decreased [Mg2+]i, but they rather increased it. The results suggest that these inhibitory drugs for Na+-Ca2+ exchange directly modulate Na+-Mg2+ exchange in a Ca2+-dependent manner, and consequently produce the paradoxical decrease in [Mg2+]i in the presence of Ca2+.     

Amiloride-sensitive sodium transport in lamprey red blood cells: evidence for two distinct transport pathways

To determine Na+/H+ exchange in lamprey erythrocyte membranes, the cells were acidified to pH(i) 6.0 using the K+/H+ ionophore nigericin. Incubation of acidified erythrocytes in a NaCl medium at pH 8.0 caused a considerable rise in 22Na+ influx and H+ efflux during the first 1 min of exposure. In addition, exposure of acidified red cells to NaCl medium was associated with rapid elevation of intracellular Na+ content. The acid-induced changes in Na+ influx and H+ efflux were almost completely inhibited by amiloride and dimethylamiloride. In native lamprey erythrocytes, amiloride-sensitive Na+ influx progressively increased as the osmolality of incubation medium was increased by addition of 100, 200, or 300 mmol/l sucrose. Unexpectedly, the hypertonic stress induced a small, yet statistically significant decrease in intracellular Na+ content in these cells. The reduction in the cellular Na+ content increased with hypertonicity of the medium. The acid- and shrinkage-induced Na+ influxes were inhibited by both amiloride and 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) in a dose-dependent manner. For both blockers, the half-maximal inhibitory values (IC50) were much greater for the shrinkage-induced (44 and 15 micromol/l for amiloride and EIPA, respectively) than for the acid-induced Na+ influx (5.1 and 3.3 micromol/l, respectively). The data obtained are the first demonstration of the presence of a Na+/H+ exchanger with high activity in acidified (pH(i) 6.0) lamprey red blood cells (on average, 512 +/- 56 mmol/l cells/h, n = 13). The amiloride-sensitive Na+ influxes produced by hypertonic cell shrinkage and acid load are likely to be mediated by distinct ion transporters in these cells.     

The O2- generating oxidoreductase of human neutrophils: evidence of an obligatory requirement for calcium and magnesium for expression of catalytic activity

NADPH-dependent O2- generating oxidoreductase activity recovered from cell lysates of phorbol myristate acetate-stimulated human neutrophils exhibits dependence on Ca+2 and Mg+2 for full expression of its catalytic activity. O2- generating activity was completely abolished by exposure of the oxidoreductase to EDTA, then reconstituted by exposure of the enzyme to Ca+2 and Mg+2 in excess of the EDTA concentration used to block catalytic activity. The oxidoreductase responded maximally to either 0.25 mM Ca+2 or 0.80 mM Mg+2. The pH optimum of the oxidoreductase exposed to Ca+2 and Mg+2 is between pH 7.0 and 7.6. The molar ratio of NADPH oxidation to O2- production determined at pH 7.6 in the presence of Ca+2 and Mg+2 is 0.49, indicating 1 mole of NADPH oxidized per 2 moles of O2- formed. Particulate fractions recovered from cell lysates of resting neutrophils exhibited no oxidoreductase activity under the same conditions.