Kinetics of release of serotonin from isolated secretory granules. II. Ion exchange determines the diffusivity of serotonin.

We measured the efflux of 5-hydroxytryptamine (5-HT, serotonin) from an intact secretory granule extracted from the mast cell of the beige mouse. The efflux was measured with amperometry after rupture of the granule membrane was triggered by electroporation. We determined the diffusivity of 5-HT within the secretory granule to be 2.0 x 10(-8) cm2 s(-1) when the granule is in contact with a physiological saline and found that this diffusivity depends on the valence of the cation in the external electrolyte. There is a fivefold increase in the diffusion coefficient of 5-HT determined in CsCl (150 mM, pH 7.2) at 3.7 x 10(-8) cm2 s(-1) compared to that determined in histamine dihydrochloride (Hi, 100 mM at pH 4.5) at 0.7 x 10(-8) cm2 s(-1). We found that the rate of expansion of the granule matrix observed in physiological medium correlates with the efflux of 5-HT, and that the rate of swelling of the matrix and the efflux depend on the microviscosity within the granule matrix and not the bulk viscosity of the external solution. The low diffusivity of 5-HT (approximately 500-fold less than in the bulk), the observation that the valence of the counterion affects this diffusivity, and the relationship between the volume changes of the matrix and the efflux suggest that 5-HT is released from the granule by ion exchange. We discuss the implications of this result for exocytotic release in mast cells and propose that an ion exchange mechanism could control the rate of release in other secretory systems.     

Elastic behavior of zymogen granule membranes in response to changes in pH and pCa.

In the process of secretion, the membrane of secretory granules is expected to change its elastic behavior. Elastic modulus of the membrane of zymogen granules, prepared from the rat pancreas acinar cell, was measured by an osmotic swelling method. The elastic modulus of the granule membrane at pCa 8 reduced from the maximal value of 230 dyn/cm at pH 6.0 to almost zero at pH 7.5. In a cytosol of an acinar cell, calcium ions play an important role as a second messenger in secretion. The elastic modulus of the granule membrane reduced in a sigmoidal fashion at pCa between 7.0 and 6.0. This range of pCa corresponds to a physiological rise of free Ca2+ concentrations in the cell cytosol when stimulated by external secretagogues. Reduction of the elastic modulus indicates that the state of the granule membrane switches to a more flexible one in which the granule is easy to appose to the cell plasma membrane and then swell as a final step of exocytosis.     

Valinomycin-induced chloride permeability in isolated rat liver mitochondria

1. Ionophore-induced osmotic swelling was used to study Cl- transport in isolated rat liver mitochondria. 2. Energy-dependent, neutral ionophore-induced swelling in Cl- salts at pH 7.2 required K+ and was preceded by a brief lag phase that was absent in chlorotributyltin-induced swelling. 3. Treatments that stimulated or inhibited mitochondrial K+/H+ exchange had qualitatively similar effects on both valinomycin-induced swelling and the associated lag phase. 4. The results suggest that valinomycin-induced Cl- permeability results from an interaction between the K+/H+ antiporter and neutral ionophore K+ complexes.     

Relative lack of ATP-driven H+ translocase activity in isolated parotid secretory granules

The possible presence of ATP-driven H+ translocase activity in isolated rat parotid secretory granules has been examined by several approaches. First the transmembrane pH difference measured by either [14C] methylamine or [3H]acetate distribution is not substantially affected by ATP in the presence of membrane-permeating anions. Second, despite a low intrinsic H+ permeability of parotid granule membranes, only a small variably detectable inside-positive transmembrane potential is observed (by altered distribution of radioactive ions) when ATP is added in the absence of permeant anions. Third, ATP-induced lysis of parotid granules is minor and appears to be independent of ATP hydrolysis. Finally, ATP-hydrolase activity of the parotid granule fraction is not stimulated by an H+ ionophore, nor is it susceptible to inhibition by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole at a concentration which decreases the measured ATPase of purified chromaffin granule membranes by more than 80%. These findings suggest that this exocrine secretory granule type, which is characterized by storage of a heterogeneous mixture of secretory proteins, exhibits H+ pump activity which is at most a small fraction of that observed in biogenic amine storage granules of neural and endocrine tissues.     

Intravesicular calcium release mediates the motion and exocytosis of secretory organelles: a study with adrenal chromaffin cells

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient toward the cytosol (pH 5.5 versus 7.2) promoted by the V-ATPase activity. This gradient of pH is also responsible for the accumulation of amines and Ca2+ because their transporters use H+ as the counter ion. We have recently shown that alkalinization of secretory vesicles slowed down exocytosis, whereas acidification caused the opposite effect. In this paper, we measure the alkalinization of vesicular pH, caused by the V-ATPase inhibitor bafilomycin A1, by total internal reflection fluorescence microscopy in cells overexpressing the enhanced green fluorescent protein-labeled synaptobrevin (VAMP2-EGFP) protein. The disruption of the vesicular gradient of pH caused the leak of Ca2+, measured with fura-2. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that bafilomycin directly released Ca2+ from freshly isolated vesicles. The Ca2+ released from vesicles to the cytosol dramatically increased the granule motion of chromaffin- or PC12-derived granules and triggered exocytosis (measured by amperometry). We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of cytosolic Ca2+ and in two of the major functions of secretory cells, vesicle motion and exocytosis.     

Energy-dependent contraction of swollen mitochondria: activation by nigericin.

The rate, extent, and efficiency of the energy-dependent contraction of heart mitochondria swollen in Na+ or K+ nitrate are all strongly activated by nigericin, an antibiotic which is known to support cation/H+ exchange in natural and model membranes. In the absence of nigericin, the cation selectivity sequence of energy-dependent contraction (Na+>Li+>K+>choline+) is identical to that of passive swelling in acetate salts, a reaction which is presumed to be dependent on an endogenous cation/H+ exchanger. These results strongly favor an osmotic mechanism for energy-dependent contraction which depends on electrogenic H+ ejection, H+/cation exchange, and electrophoretic anion efflux.     

Existence of an adenosine 5'-triphosphate dependent proton translocase in bovine neurosecretory granule membrane

The addition of ATP to bovine neurohypophysial secretory granules suspended in isotonic sucrose medium induces a positive polarization, delta psi, of their interior without affecting their internal pH. In KCl-containing media, ATP failed to generate large delta psi but induced a pH gradient (delta pH; interior acidic). These observations are consistent with the existence in the neurosecretory granule membrane of an ATP-dependent inward electrogenic H+ translocase (H+ pump), capable in KCl-containing media of acidifying the granule matrix by H+-Cl- cotransport. The delta psi and delta pH generated by the H+ pump, defined as the ATP-induced changes sensitive to the H+ ionophore carbonyl cyanide m-chlorophenylhydrazone (CCCP), were blocked by N,N'-dicyclohexylcarbodiimide, an inhibitor of all H+ pumps, and were insensitive to oligomycin, a mitochondrial ATPase inhibitor. In sucrose medium, measurements were complicated by a Donnan equilibrium reflecting the presence in the granule of peptide hormones and neurophysins which resulted in a CCCP-resistant resting delta pH. In KCl-containing media, the Donnan equilibrium was destroyed since the membrane is permeable to cations, but under these conditions a CCCP-resistant K+-diffusion potential was observed. The ATP-induced delta psi was also monitored by the extrinsic fluorescent probe bis(3-phenyl-5-oxoisoxazol-4-yl)pentamethine oxonol. The hypothesis of a granule H+ pump is further supported by the presence of an oligomycin-resistant ATPase in the preparation and the ultrastructural localization of such an activity on the granule membrane. The H+ pump has been found in both newly formed and aged neurosecretory granules. Its possible physiological function is discussed with reference to that of chromaffin granules, with which it has many similarities.     

Isolated secretion granules from parotid glands of chronically stimulated rats possess an alkaline internal pH and inward-directed H+ pump activity

Secretion granules have been isolated from the parotid glands of rats that have been chronically stimulated with the beta-adrenergic agonist, isoproterenol. These granules are of interest because they package a quantitatively different set of secretory proteins in comparison with granules from the normal gland. Polypeptides enriched in proline, glycine, and glutamine, which are known to have pI's greater than 10, replace alpha-amylase (pI's = 6.8) as the principal content species. The internal pH of granules from the treated rats ranges from 7.8 in a potassium sulfate medium to 6.9 in a choline chloride medium. The increased pH over that of normal parotid granules (approximately 6.8) appears to reflect the change in composition of the secretory content. Whereas normal mature parotid granules have practically negligible levels of H+ pumping ATPase activity (Arvan, P., G. Rudnick, and J. D. Castle, 1985, J. Biol. Chem., 260, 14945-14952) the isolated granules from isoproterenol-treated rats undergo a time-dependent internal acidification (approximately 0.2 pH unit) that requires the presence of ATP and is abolished by an H+ ionophore. Additionally, an inside-positive granule transmembrane potential develops after ATP addition that depends upon ATP hydrolysis. Two independent methods have been used that exclude the possibility that contaminating organelles are the source of the H+-ATPase activity. Together these data provide clear evidence for the presence of an H+ pump in the membranes of parotid granules from chronically stimulated rats. However, despite the presence of H+-pump activity, fluorescence microscopy with the weak base, acridine orange, reveals that the intragranular pH in live cells is greater than that of the cytoplasm.     

Alpha-granule secretion from alpha-toxin permeabilized, MgATP-exposed platelets is induced independently by H+ and Ca2+

In order to better understand granule release from platelets, we developed an alpha-toxin permeabilized platelet model to study alpha-granule secretion. Secretion of alpha-granules was analyzed by flow cytometry using P-selectin as a marker for alpha-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. Alpha-toxin-permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4-5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced alpha-granule secretion, while H+-induced alpha-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced alpha-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that alpha-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger alpha-granule release independent of Ca2+.     

Volume-regulating behavior of human platelets

Human platelets exposed to hypotonic media undergo an initial swelling followed by shrinking (regulatory volume decrease [RVD]). If the RVD is blocked, the degree of swelling is in accord with osmotic behavior. The cells could swell at least threefold without significant lysis. Two methods were used to follow the volume changes, electronic sizing and turbidimetry. Changes in shape produced only limited contribution to the measurements. The RVD was very rapid, essentially complete in 2 to 8 minutes, with a rate proportional to the degree of initial cell swelling. RVD involved a loss of KCl via volume-activated conductive permeability pathways for K+ and anions, presumably Cl-. In media containing greater than 50 mM KCl, the shrinking was inhibited and with higher concentrations was reversed (secondary swelling), suggesting that it is driven by the net gradient of K+ plus Cl-. The K+ pathway was specific for Rb+ and K+ compared to Li+ and Na+. The Cl- pathway accepted NO-3 and SCN- but not citrate or SO4(2-). In isotonic medium, the permeability of platelets to Cl- appeared to be low compared to that of K+. After hypotonic swelling both permeabilities were increased, but the Cl- permeability exceeded that of K+. The Cl- conductive pathway remained open as long as the cells were swollen. RVD was incomplete unless amiloride, an inhibitor of Na+/H+ exchange, was present or unless Na+ was replaced by an impermeant cation. In addition, acidification of the cytoplasm occurred upon cell swelling. This reduction in pHi appeared to activate Na+/H+ exchange, with a resultant uptake of Na+ and reduction in the rate and amount of shrinking. Like other cells, platelets responded to hypertonic shrinking with activation of Na+/H+ exchange, but regulatory volume increase was not detectable.     

Is swelling of the secretory granule matrix the force that dilates the exocytotic fusion pore?

The swelling of the secretory granule matrix which follows fusion has been proposed as the driving force for the rapid expansion of the fusion pore necessary for exocytosis. To test this hypothesis, we have combined simultaneous measurements of secretory granule swelling using videomicroscopy with patch clamp measurements of the time course of the exocytotic fusion pore in mast cells from the beige mouse. We show that isotonic acidic histamine solutions are able to inhibit swelling of the secretory granule matrix both in purified secretory granules lysed by electroporation and in intact cells stimulated to exocytose by guanine nucleotides. In contrast to the inhibitory effects on granule swelling, the rate of expansion of the exocytotic fusion pore is unaffected. Therefore, as the rate of granule swelling was more than 20 times slower under these conditions, swelling of the secretory granule matrix due to water entry through the fusion pore cannot be the force responsible for the characteristic rapid expansion of the exocytotic fusion pore. We suggest that tension in the secretory granule membrane, which has recently been demonstrated in fused secretory granules, might be the force that drives the irreversible expansion of the fusion pore.     

Activation of latent K+ uniport in mitochondria treated with the ionophore A23187

Addition of A23187 plus EDTA to energized mitochondria in KCl medium determines a rapid osmotic swelling due to K+ uptake. The swelling is fully reversed by uncoupler, is stimulated by quinine, and is accompanied by membrane depolarization and increased rate of respiration. A23187-treated mitochondria passively swell in K+ thiocyanate at neutral pH, under conditions where the H+-K+ antiporter appears to be silent. These data indicate that A23187 activates electrophoretic K+ flux, supporting the notion that Mg2+ depletion unmasks several ionic conductance pathways whose concerted interplay could provide a sensitive regulation of mitochondrial volume homeostasis.     

Activation of Na+/H+ exchange in lymphocytes by osmotically induced volume changes and by cytoplasmic acidification

After swelling in hypotonic solutions, peripheral blood mononuclear cells (PBM) shrink toward their original volumes. Upon restoration of isotonicity, the cells initially shrink but then regain near-normal size again. This regulatory volume increase (RVI) is abolished by removal of Na+o or Cl-o or by addition of amiloride. RVI is unaffected by removal of K+o or by ouabain and is only partially inhibited by 1 mM furosemide. As a result of increased influx, the cells gain both Na+ and K+ during reswelling. In contrast, only Na+ content increases in the presence of ouabain. Amiloride largely eliminates the changes in the content of both cations. Using diS-C3-(5), no significant membrane potential changes were detected during RVI, which suggests that the fluxes are electroneutral. The cytoplasmic pH of volume-static cells was measured with 5,6-dicarboxyfluorescein. After acid loading, the addition of extracellular Na+ induced an amiloride-inhibitable alkalinization, which is consistent with Na+/H+ exchange. Cytoplasmic pH was not affected by cell shrinkage itself, but an internal alkalinization, which was also amiloride sensitive and Na+ dependent, developed during reswelling. In isotonic lightly buffered solutions without HCO-3, an amiloride-sensitive acidification of the medium was measurable when Na+ was added to shrunken PBM. K+ was unable to mimic this effect. The observations are compatible with the model proposed by Cala (J. Gen. Physiol. 1980. 76:683-708), whereby an electroneutral Na+o/H+i exchange is activated by osmotic shrinking. Cellular volume gain occurs as Cl-o simultaneously exchanges for either HCO-3i or OH-i. Na+i is secondarily replaced by K+ through the pump, but this step is not essential for RVI.     

Regulatory volume decrease in nematocytes isolated from acontia of Aiptasia diaphana.

Regulatory volume decrease (RVD) and the mechanisms of its regulation were investigated in microbasic mastigophore nematocytes isolated from the acontia of Aiptasia diaphana (Coelenterates, Cnidaria), a marine species that can be exposed to considerable changes in osmotic pressure. Exposure of isolated cells to a 35% hypoosmotic shock lead to the expected osmotic swelling followed by a rapid RVD. RVD was blocked if Ca2+ influx was prevented either by applying a Ca2+-free medium or by treating the cells with Gd3+. Furthermore, the calmodulin action inhibitor trifluoperazine (TFP), prevented RVD and also caused a larger swelling than that induced by preventing Ca2+ influx. Treatment of nematocytes with quinine completely blocked the RVD. Such an effect was prevented by gramicidine. A partial inhibition of RVD was caused by treatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). It is concluded that: i) the nematocytes regulate volume under hypoosmotic shock; ii) the regulatory mechanisms consist mainly in increased conductance to K+, and consequently, of Cl-, and, to a lesser extent, in H+/K+-Cl-/HCO3- exchange, and iii) the ionic fluxes are triggered by increased [Ca2+]i with the possible involvement of calmodulin.     

Phosphatidic acid osmotically destabilizes lysosomes through increased permeability to K+ and H+

Lysosomal destabilization is a critical event not only for the organelle but also for living cells. However, what factors can affect lysosomal stability is not fully studied. In this work, the effects of phosphatidic acid (PA) on the lysosomal integrity were investigated. Through the measurements of lysosomal beta-hexosaminidase free activity, intralysosomal pH, leakage of lysosomal protons and lysosomal latency loss in hypotonic sucrose medium, we established that PA could increase the lysosomal permeability to K+ and H+, and enhance the lysosomal osmotic sensitivity. Treatment of lysosomes with PA promoted entry of K+ into the organelle via K+/H+ exchange, which could produce osmotic stresses and osmotically destabilize the lysosomes. In addition, PA-induced increase in the lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shocks. The results suggest that PA may play a role in the lysosomal destabilization.     

cis-Unsaturated fatty acids induce the fusion of chromaffin granules aggregated by synexin

When isolated chromaffin granules were aggregated by synexin (a Ca2+-binding protein present in chromaffin and other secretory tissues) and then exposed to cis-unsaturated fatty acids at 37 degrees C, they fused together to form large vesicles. The fusion was monitored by phase and electron microscopy and by turbidity measurements on the granule suspension. Arachidonic acid was the most effective fusogen, whereas trans-unsaturated fatty acids, saturated fatty acids, detergents or lysolecithin were inactive. During fusion some of the epinephrine of the granules was released but the soluble core proteins remained trapped in the resulting vesicles. These vesicles swelled to enclose the maximum volume. Although this swelling could be inhibited by increasing the osmotic strength of the medium, it did not appear to depend on the chemiosmotic properties of the granule membranes as it was not influenced by ATP, a proton ionophore, or an anion transport inhibitor. The regulators of this in vitro fusion--Ca2+, synexin, and free, cis-unsaturated fatty acids--may be present in the cytoplasm of the chromaffin cell when it is stimulated to release epinephrine and granule proteins by exocytosis. Therefore, this fusion event may be the same that occurs between chromaffin granules undergoing compound exocytosis.     

K+/H+ antiport in heart mitochondria

Heart mitochondria depleted of endogenous divalent cations by treatment with A23187 and EDTA swell in (a) K+ acetate or (b) K+ nitrate when an uncoupler is present. These mitochondria also exchange matrix 42K+ with external K+, Na+, or Li+ in a reaction that does not require respiration and is insensitive to uncouplers. Untreated control mitochondria do not swell in either medium nor do they show the passive cation exchange. Both the swelling and the exchange reactions are inhibited by Mg2+ and by quinine and other lipophilic amines. Swelling and exchange are both strongly activated at alkaline pH, and the exchange reaction is also increased markedly by hypotonic conditions. All of these properties correspond to those reported for a respiration-dependent extrusion of K+ from Mg2+-depleted mitochondria, a reaction attributed to a latent Mg2+- and H+-sensitive K+/H+ antiport. The swelling reactions are strongly inhibited by dicyclohexylcarbodiimide reacted under hypotonic conditions, but the exchange reaction is not sensitive to this reagent. Heart mitochondria depleted of Mg2+ show marked increases in their permeability to H+, to anions, and possibly to cations, and the permeability to each of these components is further increased at alkaline pH. This generalized increase in membrane permeability makes it likely that K+/H+ antiport is not the only pathway available for K+ movement in these mitochondria. It is concluded that the swelling, 42K+ exchange, and K+ extrusion data are all consistent with the presence of the putative K+/H+ antiport but that definitive evidence for the participation of such a component in these reactions is still lacking.     

Involvement of AQP6 in the Mercury-Sensitive Osmotic Lysis of Rat Parotid Secretory Granules

In secretory granules and vesicles, membrane transporters have been predicted to permeate water molecules, ions and/or small solutes to swell the granules and promote membrane fusion. We have previously demonstrated that aquaporin-6 (AQP6), a water channel protein, which permeates anions, is localized in rat parotid secretory granules (Matsuki-Fukushima et al., Cell Tissue Res 332:73–80, 2008). Because the localization of AQP6 in other organs is restricted to cytosolic vesicles, the native function or functions of AQP6 in vivo has not been well determined. To characterize the channel property in granule membranes, the solute permeation-induced lysis of purified secretory granules is a useful marker. To analyze the role of AQP6 in secretory granule membranes, we used Hg²⁺, which is known to activate AQP6, and investigated the characteristics of solute permeability in rat parotid secretory granule lysis induced by Hg²⁺ (Hg lysis). The kinetics of osmotic secretory granule lysis in an iso-osmotic KCl solution was monitored by the decay of optical density at 540 nm using a spectrophotometer. Osmotic secretory granule lysis was markedly facilitated in the presence of 0.5–2.0 μM Hg²⁺, concentrations that activate AQP6. The Hg lysis was completely blocked by β-mercaptoethanol which disrupts Hg²⁺-binding, or by removal of chloride ions from the reaction medium. An anion channel blocker, DIDS, which does not affect AQP6, discriminated between DIDS-insensitive and sensitive components in Hg lysis. These results suggest that Hg lysis is required for anion permeability through the protein transporter. Hg lysis depended on anion conductance with a sequence of NO₃ ^? > Br^? > I^? > Cl^? and was facilitated by acidic pH. The anion selectivity for NO₃ ^? and the acidic pH sensitivity were similar to the channel properties of AQP6. Taken together, it is likely that AQP6 permeates halide group anions as a Hg²⁺-sensitive anion channel in rat parotid secretory granules.     

The voltage-sensitive Na+/H+ exchange in sea urchin spermatozoa flagellar membrane vesicles studied with an entrapped pH probe

Flagellar plasma membrane vesicles were isolated from sea urchin sperm using osmotic lysis. A membrane impermeant fluorescence pH indicator, pyranine, was incorporated into the vesicles as they resealed after lysis and was used to measure the intravesicular pH (pHi). Addition of Na+ rapidly alkalinized the pHi of vesicles prepared with an internal acidic pH gradient. The pHi increase showed ionic selectivity in the order of Na+ greater than Li+ much greater than K+ approximately equal to Cs+ approximately equal to O. Complete removal of monovalent anions such as Cl- and HCO3- did not affect the exchange, thus ruling out the participation of an anion carrier in the process. The optimal operation of the exchanger, however, required the presence of a transmembrane potential, which could be generated by the diffusion potential of either K+, a naturally permeant ion, or Cs+ which was artificially made permeant by the ionophore valinomycin. Depolarization inhibited the exchange in both the forward and the reverse directions, which is consistent with the voltage-gated electroneutral exchange mechanism proposed previously for this exchanger (Lee, H. C. J. Biol. Chem. (1984) 259, 15315-15319). The voltage sensitivity of the Na+/H+ exchanger was found to be modulated by the presence of Mg2+. A model involving the screening of the internal surface potential was proposed to account for the Mg2+ effect. The vesicle preparation used in this study allows complete control of the internal contents and represents a major simplification of the system as compared with the intact sperm and the isolated flagella used previously.     

Molecular cloning and characterization of a novel (Na+,K+)/H+ exchanger localized to the trans-Golgi network

The luminal pH of organelles along the secretory and endocytic pathways of mammalian cells is acidic and tightly regulated, with the [H+] varying up to 100-fold between compartments. Steady-state organellar pH is thought to reflect a balance between the rates of H+ pumping by the vacuolar-type H+-ATPase and H+ efflux through ill-defined pathways. Here, we describe the cloning of a novel gene (NHE7) in humans that is homologous to Na+/H+ exchangers, is ubiquitously expressed, and localizes predominantly to the trans-Golgi network. Significantly, NHE7 mediates the influx of Na+ or K+ in exchange for H+. The activity of NHE7 was also found to be relatively insensitive to inhibition by amiloride but could be antagonized by the analogue benzamil and the unrelated compound quinine. Thus, NHE7 displays unique functional and pharmacological properties and may play an important role in maintaining cation homeostasis of this important organelle.