Ca2+-Mg2+-activated adenosine triphosphatase in plasma and granule membranes in non-secreting and secreting mast cells

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Polyphosphate is a novel pro-inflammatory regulator of mast cells and is located in acidocalcisomes

Polyphosphate (polyP) is a pro-inflammatory agent and a potent modulator of the human blood-clotting system. The presence of polyP of 60 phosphate units was identified in rat basophilic leukemia (RBL-2H3) mast cells using specific enzymatic assays, urea-polyacrylamide gel electrophoresis of cell extracts, and staining of cells with 4,6-diamidino-2-phenylindole (DAPI), and the polyP-binding domain of Escherichia coli exopolyphosphatase. PolyP co-localizes with serotonin- but not with histamine-containing granules. PolyP levels greatly decreased in mast cells stimulated to degranulate by IgE. Mast cell granules were isolated and found to be acidic and decrease their polyP content upon alkalinization. In agreement with these results, when RBL-2H3 mast cells were loaded with the fluorescent calcium indicator fura-2 acetoxymethyl ester to measure their intracellular Ca(2+) concentration ([Ca(2+)](i)), they were shown to possess a significant amount of Ca(2+) stored in an acidic compartment different from lysosomes. PolyP derived from RBL-2H3 mast cells stimulated bradykinin formation, and it was also detected in human basophils. All of these characteristics of mast cell granules, together with their known elemental composition, and high density, are similar to those of acidocalcisomes. The results suggest that mast cells polyP could be an important mediator of their pro-inflammatory and pro-coagulant activities.

Role of a Ca2+-Mg2+ ATPase on the mast cell surface in calcium transport and histamine secretion

We have previously reported the presence of an ATPase, stimulated by calcium and magnesium, on the outer surface of the rat peritoneal mast cell. Experiments in which the enzyme activity was enhanced or inhibited showed a relationship to histamine secretion. Enhanced enzyme activity with increasing concentrations of the substrate (ATP) was associated with a potentiation of histamine release, and a pronounced inhibition of the enzyme caused an inhibition of the release. In the present work we have studied the influx and efflux of calcium in mast cells in relation to the activity of the Ca2+-Mg2+ ATPase on the mast cell membrane. The enzyme activity is shown to be related to calcium influx and has no effect on calcium efflux. Stimulation of the enzyme with ATP is associated with increased calcium influx into the mast cell, and inhibition of the enzyme with AMP causes inhibition of the calcium uptake. In both cases calcium efflux is unaffected. The function of the enzyme is thus different from the calcium efflux enzyme on the cytoplasmic surface, described in other cells. In addition, the Ca2+-Mg2+ ATPase on the mast cell surface is neither stimulated by calmodulin nor inhibited by the calmodulin antagonists, trifluoperazine and W-7. In mast cells the low cytosolic calcium concentration seems to be maintained by Na+-Ca2+ countertransport. Phosphorylation of the Ca2+-Mg2+ ATPase on the mast cell is likely to be associated with Ca2+ release at the cytoplasmic surface of the plasma membrane. It is thus possible that ATP hydrolysis in the membrane stimulates the contraction of microfilaments in the membrane and the cytoskeleton, and promotes the migration of the granules to the plasma membrane.

Amine uptake into intact mast cell granules in vitro

Histamine, the principal amine of rat peritoneal mast cells, is taken up into isolated granules with intact membranes. Uptake is pH- and concentration-dependent and is not stimulated by the addition of Mg2+-ATP. The saturable uptake has a Km of 91.1 microM and a Vmax of 95.4 pmol (mg of protein)-1 min-1. Uptake is abolished by 5 mM ammonium ion. 5-HT, the other endogenous amine of the granules, and dopamine and tyramine, which do not occur naturally in rat mast cells, each competitively inhibits [3H]-histamine uptake with Ki's close to 1 microM. Reserpine, a putative amine carrier blocker, inhibits uptake at nanomolar concentrations. At high concentrations, uptake of [3H]-5-HT is nonsaturable; at low concentrations, a saturable component is observed with a Km of 1.6 microM. Uptake of [3H]-5-HT is not enhanced by Mg2+-ATP. It is pH-dependent but with a lower apparent pKa than that of histamine. [3H]-5-HT uptake can be completely inhibited by ammonium ions. Amine inhibition of [3H]-5-HT gives nonlinear Dixon plots, and high concentrations of the competing amines or reserpine cannot completely block uptake. We propose a model consistent with these results in which amine uptake occurs by several distinct saturable transport systems. According to the model, histamine is transported by a single system, which also transports 5-HT and dopamine. 5-HT and dopamine are transported by one or more other systems.

Presence of a high-affinity Ca2+- and Mg2+-dependent ATPase in rat peritoneal mast-cell membranes

Purified perigranular and plasma membranes isolated from rat peritoneal mast cells were examined for Ca2+- and Mg2+-dependent ATPase activity. Isolated perigranular membranes contained only a low-affinity Ca2+- or Mg2+-dependent ATPase (Km greater than 0.5 mM). The plasma membranes contained both a low-affinity Ca2+- or Mg2+-dependent ATPase (Km = 0.4 mM, Vmax. = 20 nmol of Pi/min per mg), as well as a high-affinity Ca2+- and Mg2+-dependent ATPase (Km = 0.2 microM, Vmax. = 6 nmol of Pi/min per mg).

Histamine release from mast cell granules

Rat peritoneal mast cells were separated from other cells by differential centrifugation in concentrated serum albumin. Granules were isolated form these cells by ultrasonic disintegration and subsequent centrifugation. 60-80% of the granules had intact membranes and retained their histamine store in a physiological salt solution. Histamine was released from the granules with intact membranes by Ca2+, 10 mM, in the presence of phosphatidyl serine, 25--50 microgram/ml. The release was initiated in 15 sec and completed in 16 min. Other divalent cations in 10 mM concentration, viz. Mg2+, Ba2+, Sr2+, Ni2+ and Mn2+, also released histamine from the granules in the presence of phosphatidyl serine. When histamine release was induced by calcium and phosphatidyl serine the calcium uptake in the granules was remarkably increased. Ca2+ could thus displace histamine from the granule matrix. The possibility that calcium with phosphatidyl serine may change the granule membrane permeability is discussed.

Stimulus-secretion coupling in rat mast cells: inactivation of extracellular calcium dependent secretion

1. Stimulation by compound 48/80 of mast cells deprived of Ca failed to release histamine. Secretion of histamine was elicited from such cells by the subsequent introduction of Ca. 2. Histamine secretion declined as the interval between stimulation by compound 48/80 and the introduction of Ca increased. This decline is called inactivation. 3. The addition of the ionophore, A23187, with Ca restored maximum histamine secretion overcoming inactivation. 4. Increasing the concentration of Ca introduced after stimulation, from 2 to 8 mM, or to 20 mM reduced the amount of histamine released. This reduction was proportional to the interval between stimulation and the introduction of Ca. The addition of A23187 with the higher concentrations of Ca fully restored histamine secretion. 5. Stimulation of mast cells in Ca-free media by the secretagogues polymyxin B or bradykinin, and the subsequent introduction of Ca, resulted in a similar inactivation or decline in histamine release. 6. Mast cells inactivated by compound 48/80 stimulation in Ca-free media showed no increase in histamine release when the secretagogues polymyxin B plus Ca or bradykinin plus Ca were added. However, when A23187 plus Ca was added, a full secretory response was obtained. 7. It is suggested that the process of inactivation involves time-dependent change in the Ca permeability of the mast cell membrane. The concentration of introduced Ca is suggested to influence the regulation of this permeability.

The role of plasma membrane Ca++-Mg++ activated adenosine triphosphatase of rat mast cells on histamine release

The role of a Ca++-MG++ activated ATPase, demonstrated on the outer surface of rat peritoneal mast cells, on histamine release induced by antigen (anaphylactic reaction), compound 48/80 and ionophore A23187 has been studied. A high level of the enzyme activity is retained at the optimal pH for histamine release induced by the three releasing agents. The effect of fourteen inhibitors of ATPase has been studied, viz. quinidine, fluoride, platinum salt, suramin, ethacrynic acid, ethyl alcohol, N-ethylmaleimide, Mn++, Ni++, ADP, AMP and the flavones: kaempferol, quercetin, morin. All the inhibitors, which caused varying degrees of inhibition of ATPase, also inhibited histamine release. The inhibition of the enzyme was competitive with ADP, AMP, ethacrynic acid, suramin and morin and non-competitive with the others. The degree of inhibition of ATPase and of histamine release tended to be similar with six inhibitors. With the others the extent of the inhibition of the release and of the enzyme varied. But a marked inhibition of the enzyme was always associated with a pronounced inhibition of histamine release. ATP in lower concentrations (10-20 microM) has been shown to potentiate histamine release induced by all the three releasers, possibly through its utilization by plasma membrane ATPase. The observations agree with the hypothesis that plasma membrane ATPase participates in the histamine release process.