Regulation of the intracellular calcium level in human blood platelets: Cyclic adenosine 3′,5′-monophosphate dependent phosphorylation of a 22 000 dalton component in isolated Ca-accumulating vesicles

Two protein kinase activities have been separated from the supernatants of homogenized human blood platelets by DEAE cellulose chromatography. One of them (peak I enzyme) is an efficient stimulator of the uptake of Ca²⁺ into isolated membrane vesicles in the presence of cyclic AMP and ATP. The second (peak II enzyme), although equally active towards histone, exerts only about one third of the activity of the peak I enzyme. The stimulation of Ca²⁺ uptake is accompanied by the phosphorylation of a membrane protein with an apparent molecular weight of 22 000, which appears to play an essential role in the regulation of the intracellular Ca²⁺ level and hence of platelet activity.     

Regulation of the intracellular calcium level in human blood platelets: Cyclic adenosine 3′,5′-monophosphate dependent phosphorylation of a 22 000 dalton component in isolated Ca2+-accumulating vesicles

Two protein kinase activities have been separated from the supernatants of homogenized human blood platelets by DEAE cellulose chromatography. One of them (peak I enzyme) is an efficient stimulator of the uptake of Ca²⁺ into isolated membrane vesicles in the presence of cyclic AMP and ATP. The second (peak II enzyme), although equally active towards histone, exerts only about one third of the activity of the peak I enzyme. The stimulation of Ca²⁺ uptake is accompanied by the phosphorylation of a membrane protein with an apparent molecular weight of 22 000, which appears to play an essential role in the regulation of the intracellular Ca²⁺ level and hence of platelet activity.     

Erythrocyte cytosolic free Ca2+ and plasma membrane Ca2+-ATPase activity in cystic fibrosis

The properties of the Ca2+, Mg2+-ATPase of erythrocyte membranes from patients with cystic fibrosis (CF) were extensively compared to that of healthy controls. Following removal of an endogenous membrane inhibitor of the ATPase, activation of the enzyme by Ca2+, calmodulin, limited tryptic digestion or oleic acid, as well as inhibition by trifluoperazine, were studied. The only properties found to be significantly different (CF cells vs controls) were calmodulin-stimulated peak activity (90 vs 101, P less than 0.02) and trypsin-activated peak activity (92 vs 102, P less than 0.02). No significant difference could be measured in the steady-state Ca2+-dependent phosphorylation of CF and control erythrocyte membranes indicating similar numbers of enzyme molecules per cell. The functional state of Ca2+ homeostasis in intact erythrocytes was investigated by measuring the resting cytosolic free Ca2+ levels using quin-2. Both CF and control erythrocytes maintained cytosolic free Ca2+ between 20 to 30 nM. Addition of 50 uM trifluoperazine resulted in an increase in erythrocyte cytosolic free Ca2+ to about 50 nM in both CF and control cells. Estimates of erythrocyte membrane permeability using the steady-state uptake of 45Ca into intact erythrocytes revealed no differences between CF and control cells. These results confirm that there is a small decrease in the calmodulin-stimulated activity of the erythrocyte Ca2+, Mg2+-ATPase in CF. However, this deficit is apparently not large enough to impair the ability of the CF erythrocyte to maintain normal resting levels of cytosolic free Ca2+.     

Functional involvement of carbonic anhydrase in calcium transport of the chick chorioallantoic membrane.

Carbonic anhydrase activity was demonstrated in the chick-embryonic chorioallantoic membrane and was correlated with the Ca2+-transport activity of the membrane. It is inhibited by sulphonamides and is expressed in the chorioallantoic membrane in an age-dependent fashion during embryonic development. Ca2+ uptake by the chorioallantoic membrane in vivo also increases in a similar age-dependent manner. The temporal increase in these activities is coincident with calcium deposition in the embryonic skeleton. Incubation of the chorioallantoic membrane in ovo with sulphonamides specifically inhibits both the carbonic anhydrase and the Ca2+ uptake activities of the membrane in vivo. Enzyme histochemistry revealed the carbonic anhydrase activity is localized in the Ca2+-transporting ectodermal cells of the chorioallantoic membrane. These results, taken together, indicate that carbonic anhydrase may be functionally important in the Ca2+-transport activity of the chorioallantoic membrane.     

Differential regulation of calcium-dependent and calcium-independent cyclic nucleotide phosphodiesterases from heart by palmitoylcarnitine and palmitoyl coenzyme A

Regulation of Ca2+-dependent (peak I) and Ca2+-independent (peak II) phosphodiesterases from the heart by various fatty acyl esters and phospholipids were studied. DL-Palmitoylcarnitine stimulated the basal activity (in the absence of Ca2+) of peak I enzyme, while non-competitively inhibiting peak II enzyme with respect to cyclic AMP. It had no effect on other species of Ca2+-independent phosphodiesterases, including cyclic AMP- and cyclic GMP-specific enzymes from the lung, and cyclic CMP enzyme from the liver Palmitoyl-CoA and phosphatidylserine also stimulated the basal activity of peak I enzyme, but they were without effect on peak II enzyme. In comparison, DL-palmitoylcarnitine inhibited Ca2+-dependent activity of cardiac myosin light chain kinase, whereas phosphatidylserine was without effect. It is conceivable that differential regulation of phosphodiesterases by these lipids could profoundly alter the levels or effects, or both, of cyclic nucleotides and Ca2+ in the myocardium.     

Effect of insulin secretagogues and potential modulators of secretion on a plasma membrane (Ca2+ + Mg2+)-ATPase activity in islets of Langerhans

Studies were undertaken to determine whether factors which affect insulin secretion may exert their effects by altering the activity of an islet-cell plasma membrane Ca2+ extrusion pump. The insulin secretagogue, D-glucose, and a variety of phosphorylated hexoses, glucose 6-P, glucose 1,6-P, fructose 6-P, and fructose 2,6-P, were evaluated for their effect on an islet-cell plasma membrane (Ca2+ + Mg2+)-ATPase and were found to be ineffective in altering enzyme activity. D-Glucose also did not alter the rate of ATP-dependent Ca2+ uptake into plasma membrane vesicles. Similarly, cAMP, the catalytic subunit of cAMP-dependent protein kinase, arachidonic acid, or prostaglandin E2 did not affect either the plasma membrane (Ca2+ + Mg2+)-ATPase or the rate of ATP-dependent Ca2+ uptake into plasma membrane vesicles. Whereas previous studies have suggested that D-glucose and/or cAMP may inhibit ATPase activities in islets, these results indicate that the agents, i.e., D-glucose and cAMP, which stimulate and/or potentiate insulin secretion from the islet cell, do not modify Ca2+ fluxes by directly regulating the islet-cell plasma membrane (Ca2+ + Mg2+)-ATPase. In contrast, the acidic phospholipids, phosphatidic acid and phosphatidylserine, stimulated the enzyme activity in a concentration-dependent manner whereas phosphatidylcholine had only a minimal effect. The diacylglycerol, dilinolein, stimulated the (Ca2+ + Mg2+)-ATPase activity in the presence of phosphatidylserine, but not in the absence of phospholipids. These effects were independent of phospholipid-stimulated protein phosphorylation in the islet-cell plasma membrane under the conditions of the ATPase assay.     

Subcellular fractionation of pig stomach smooth muscle. A study of the distribution of the (Ca2+ + Mg2+)-ATPase activity in plasmalemma and endoplasmic reticulum

Isolated membrane vesicles from pig stomach smooth muscle (antral part) were subfractionated by a density gradient procedure modified in order to obtain an efficient extraction of extrinsic proteins. By using this method in combination with digitonin-treatment, an endoplasmic reticulum fraction contaminated with maximally 10 to 20% of plasma membranes was isolated, together with a plasma membrane fraction containing at most 30% endoplasmic reticulum. The endoplasmic reticulum and plasma membrane fractions differed in protein composition, reaction to digitonin, binding of wheat germ agglutinin, activities of marker enzymes and in the characteristics of the Ca2+ uptake. The Ca2+ uptake by the endoplasmic reticulum was much more stimulated by oxalate than the uptake by plasma membranes. Both fractions showed a (Ca2+ + Mg2+)-ATPase activity, but the largest amount of this enzyme was present in the plasma membranes. The study of the phosphorylated intermediates of the (Ca2+ + Mg2+)-ATPase by polyacrylamide gel electrophoresis revealed two phosphoproteins one of 130 kDa and one of 100 kDa (Wuytack, F., Raeymaekers, L., De Schutter, G. and Casteels, R. (1982) Biochim. Biophys. Acta 693, 45-52). The 130 kDa enzyme was predominant in the fraction enriched in plasma membrane whereas the distribution of the 100 kDa polypeptide correlated with the endoplasmic reticulum markers. The 130 kDa ATPase was the main 125I-calmodulin binding protein detected on nitrocellulose blots of proteins separated by gel electrophoresis. The (Ca2+ + Mg2+)-ATPase activity of the plasma membranes was higher than the (Na+ + K+)-ATPase activity, suggesting that the Ca2+ extrusion from these cells depends much more on the activity of the (Ca2+ + Mg2+)-ATPase than on Na+-Ca2+ exchange.     

Characterization of Ca2+ uptake in plasma membrane vesicles isolated from guinea pig ileum smooth muscle

We have characterized ATP-dependent Ca2+ transport into highly purified plasma membrane fraction isolated from guinea pig ileum smooth muscle. The membrane fraction contained inside-out sealed vesicles and was enriched 30-40-fold in 5'-nucleotidase and phosphodiesterase I activity as compared to post nuclear supernatant. Plasma membrane vesicles showed high rate (76 nmol/mg/min) and high capacity for ATP dependent Ca2+ transport which was inhibited by addition of Ca2+ ionophore A23187. The inhibitors of mitochondrial Ca2+ transport, i.e., sodium azide, oligomycin and ruthenium red did not inhibit ATP-dependent Ca2+ uptake into plasma membrane vesicles. The energy dependent Ca2+ uptake into plasma membranes showed very high specificity for ATP as energy source and other nucleotide triphosphates were ineffective in supporting Ca2+ transport. Phosphate was significantly better as Ca2+ trapping anion to potentiate ATP-dependent Ca2+ uptake into plasma membrane fraction as compared to oxalate. Orthovanadate, an inhibitor of cell membrane (Ca2+-Mg2+)-ATPase activity, completely inhibited ATP-dependent Ca2+ transport and the Ki was approximately 0.6 microM. ATP-dependent Ca2+ transport and formation of alkali labile phosphorylated intermediate of (Ca2+-Mg2+)-ATPase increased with increasing concentrations of free Ca2+ in the incubation mixture and the Km value for Ca2+ was approximately 0.6-0.7 microM for both the reactions.     

The modulation of Ca2+-ATPase activity of sarcoplasmic reticulum by membrane cholesterol. The effect of enzyme coupling

The Ca2+-ATPase activity of sarcoplasmic reticulum is relatively low (less than 2 I.U.) in vesicles where enzyme activity is geared to calcium accumulation. Modulation of membrane fluidity by enriching the membrane with cholesterol has no significant effect on enzyme activity. Collapsing the Ca2+ gradient with the calcium ionophore, A23187, unmasks the inhibitory effect of membrane cholesterol on enzyme activity.     

Ca(2+) sequestering in the early-branching amitochondriate protozoan Tritrichomonas foetus: an important role of the Golgi complex and its Ca(2+)-ATPase

Total membrane vesicles isolated from Tritrichomonas foetus showed an ATP-dependent Ca(2+) uptake, which was not sensitive to 10 microM protonophore FCCP but was blocked by orthovanadate, the inhibitor of P-type ATPases (I(50)=130 microM), and by the Ca(2+)/H(+) exchanger, A-23187. The Ca(2+) uptake was prevented also by thapsigargin, an inhibitor of the SERCA Ca(2+)-ATPases. The sensitivity of the Ca(2+) uptake by the protozoan membrane vesicles to thapsigargin was similar to that of Ca(2+)-ATPase from rabbit muscle sarcoplasmic reticulum. Fractionation of the total membrane vesicles in sucrose density gradient revealed a considerable peak of Ca(2+) transport activity that co-migrated with the Golgi marker guanosine diphosphatase (GDPase). Electron microscopy confirmed that membrane fractions of the peak were enriched with the Golgi membranes. The Golgi Ca(2+)-ATPase contributed to the Ca(2+) uptake by all membrane vesicles 80-85%. We conclude that: (i) the Golgi and/or Golgi-like vesicles form the main Ca(2+) store compartment in T. foetus; (ii) Ca(2+) ATPase is responsible for the Ca(2+) sequestering in this protozoan, while Ca(2+)/H(+) antiporter is not involved in the process; (iii) the Golgi pump of this ancient eukaryotic microorganism appears to be similar to the enzymes of the SERCA family by its sensitivity to thapsigargin.     

Na+/Ca2+ exchange in coated microvesicles.

Coated microvesicles isolated from bovine neurohypophyses could be loaded with Ca2+ in two different ways, either by incubation in the presence of ATP or by imposition of an outwardly directed Na+ gradient. Na+, but not K+, was able to release Ca2+ accumulated by the coated microvesicles. These results suggest the existence of an ATP-dependent Ca2+-transport system as well as of a Na+/Ca2+ carrier in the membrane of coated microvesicles similar to that present in the membranes of secretory vesicles from the neurohypophysis. A kinetic analysis of transport indicates that the apparent Km for free Ca2+ of the ATP-dependent uptake was 0.8 microM. The average Vmax. was 2 nmol of Ca2+/5 min per mg of protein. The total capacity of microvesicles for Ca2+ uptake was 3.7 nmol/mg of protein. Both nifedipine (10 microM) and NH4Cl (50 mM) inhibited Ca2+ uptake. The ATPase activity in purified coated-microvesicles fractions from brain and neurohypophysis was characterized. Micromolar concentrations of Ca2+ in the presence of millimolar concentrations of Mg2+ did not change enzyme activity. Ionophores increasing the proton permeability across membranes activated the ATPase activity in preparations of coated microvesicles from brain as well as from the neurohypophysis. Thus the enzyme exhibits properties of a proton-transporting ATPase. This enzyme seems to be linked to the ion accumulation by coated microvesicles, although the precise coupling of the proton transport to Ca2+ and Na+ fluxes remains to be determined.     

Calcium ion uptake in isolated pancreas cells induced by secretagogues.

1. Secretagogues of pancreatic enzyme secretion: pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin, caerulein and the Ca2+ ionophore A 23187 stimulate 45Ca uptake into isolated rat pancreatic cells, whereas adrenaline, isoproterenol, secretin, dibutyrylic cyclic adenosine 3',5'-monophosphate and dibutyrylic cyclic guanosine 3',5'-monophosphate have no effect on 45Ca uptake. 2. A graphical analysis of the Ca2+ uptake curves reveals at least two phases: a fast phase, probably due to binding of Ca2+ to the membrane and a slow phase representing Ca2+ transport into cells. Both phases are stimulated by pancreozymin and carbamylcholine. 3. The 45Ca-exchangeable pool size is increased by both carbamylcholine and pancreozymin, whereas a significant increase of total content of cell calcium was too small to be detected. 4. Atropine blocks the stimulatory effect of carbamylcholine completely but not that of pancreozymin. The Ca2+ antagonist D600 blocks the stimulatory effects of both carbamylcholine and pancreozymin only partially. 5. The data suggest that secretagogues of pancreatic enzyme secretion act by increasing the rate of Ca2+ transfer into the cell most probably through an increase of the cell membrane permeability for Ca2+.     

Studies on the bivalent-cation-activated ATPase activities of highly purified human platelet surface and intracellular membranes.

Membrane-bound Ca2+-ATPases are responsible for the energy-dependent transport of Ca2+ across membrane barriers against concentration gradients. Such enzymes have been identified in sarcoplasmic reticulum of muscle tissues and in non-muscle cells in both surface membranes and endoplasmic-reticulum-like intracellular membrane complexes. In a previous study using membrane fractionation by density-gradient and free-flow electrophoresis, we reported that the intracellular membranes of human blood platelets were a major storage site for Ca2+ and involved in maintaining low cytosol [Ca2+] in the unactivated cell. In the present report we demonstrated that the intracellular membranes also exhibit a high-affinity Ca2+-ATPase which appears to be kinetically associated with the Ca2+-sequestering process. We found that both the surface membrane and the intracellular membrane exhibited a basal Mg2+-ATPase activity, but Ca2+ activation of this enzyme was confined only to the intracellular membrane. Use of Ca2+-EGTA buffers to control the extravesicle [Ca2+] allowed a direct comparison of the Ca2+-ATPase and the Ca2+-uptake process over a Ca2+ range of 0.01 microM to 1.0 mM, and it was found that both properties were maximally expressed in the range of external [Ca2+] 1-50 microM, with concentrations greater than 100 microM showing substantial inhibition. Double-reciprocal plots for the Ca2+-ATPase activity and Ca2+ uptake gave apparent Km values for Ca2+ of 0.15 and 0.13 microM respectively. However, similar plots for ATP with the enzyme revealed a discontinuity (two affinity sites, with Km 20 and 145 microM), whereas plots for the Ca2+ uptake gave a single Km value for Ca2+, 1.1 microM. Phosphorylation studies during Ca2+ uptake using [gamma-32P]ATP revealed two components of 90 and 95 kDa phosphorylated at extravesicle [Ca2+] of 3 microM. The Ca2+-ATPase activity, Ca2+ uptake and phosphorylation were all almost completely inhibited in the presence of 500 microM-Ca2+. Similar studies using mixed membranes revealed four other phosphoproteins (50, 40, 20 and 18 kDa) formed in addition to the 90 and 95 kDa components. The findings are discussed in the context of platelet Ca2+ mobilization for function and the mechanisms whereby Ca2+ homoeostasis is controlled in the unactivated cell.     

Role of calcium in exocrine pancreatic secretion. I. Calcium movements in the rabbit pancreas.

1. Calcium movements in the isolated rabbit pancreas and in rabbit pancreas fragments have been studied with the aid of 4 5 Ca2+. 2. Addition of 4 5 Ca2+ to the incubation medium of the isolated rabbit pancreas results in an immediate appearance of isotope in the secreted fluid reaching a constant specific activity in 30 min. The absolute activity in the secreted fluid is 30-40% of that in the incubation medium. 3. Addition of 10(-5) M carbachol after 2 h preincubation with 4 5 Ca2+ results in enzyme secretion accompanied by calcium release. There is also an increase in 4 5 Ca2+ secretion, but this is maximal 10 min after the protein and total calcium peaks. 4. Partial removal of 4 5 Ca2+ from the bathing medium, before stimulation, reduces the increase in 4 5 Ca2+ secretion nearly proportionally. 5. [3H]Mannitol, added to the bathing medium, appears in the secreted fluid and behaves upon carbachol stimulation similarly to 4 5 Ca2+. 6. Upon repeated stimulation with 10(-5) M acetylcholine, a 4 5 Ca2+ peak appears, even in virtual absence of enzyme secretion. In this case the peak coincides with a small total calcium peak. 7. Efflux studies of rabbit pancreas fragments, preloaded with 4 5 Ca2+, show a carbachol-stimulated 4 5 Ca2+ efflux in addition to a release of amylase. 8. These studies indicate that there are three calcium movements in rabbit pancreas which can all be influenced by cholinergic agents: (a) an extracellular route for calcium and other small molecules and ions; (b) a calcium release across the apical membrane along with the enzymes, originating from a pool which does not freely exchange with 4 5 Ca2+ in the bath; (c) a calcium flux across the serosal membrane, which involves calcium exchanging freely with 4 5 Ca2+ from the bath. The third flux is thought to result from an increase in cytoplasmic calcium, which may be involved in the stimulus-secretion coupling of pancreatic enzyme secretion.     

A calcium pump in plasma membrane vesicles from Leishmania braziliensis

A subcellular fraction highly enriched in plasma membrane vesicles was prepared from Leishmania promastigotes. This fraction showed (Ca2+ + Mg2+)-ATPase activity. This, however, represented a small fraction (about 25%) of the overall ATPase activity. The Ca2(+)-ATPase showed general characteristics common to plasma membrane ATPases involved in Ca2+ transport. Thus, the Ca2(+)-ATPase was activated by Ca2+ with a high affinity (Km about 0.7 microM), saturating at about 5 microM Ca2+. Furthermore, it was stimulated by calmodulin (about 70-80% with 5 micrograms/ml) and almost fully inhibited by trifluoperazine (100 microM). The above vesicles accumulated Ca2+ against a concentration gradient and released it after the addition of A23187, as shown independently by 45Ca2+ and Arsenazo III studies. The transport mechanism showed the same kinetics parameters as described for the enzyme, indicating a single molecular entity. In addition, Ca2(+)-ATPase activity and Ca2+ uptake were completely inhibited by vanadate (20 microM), indicating that an E1-E2 type mechanism is involved. The results clearly demonstrate the presence of a Ca2+ pump in the plasma membrane of Leishmania which is capable of maintaining a low cytoplasmic Ca2+ concentration.     

Effects of cAMP- and cGMP-dependent protein kinases, and calmodulin on Ca2+ uptake by highly purified sarcolemmal vesicles of vascular smooth muscle

Sarcolemmal fractions of vascular smooth muscles were prepared from porcine thoracic aortae by differential and sucrose density gradient centrifugation. In these fractions, there was a high activity of 5'-nucleotidase, a putative marker enzyme of plasma membrane, and a low activity of rotenone insensitive NADH-cytochrome c reductase a marker of sarcoplasmic reticulum. In these fractions, the Ca2+ uptake was ATP-dependent. A low concentration of saponin which inhibited Ca2+ uptake by the plasma membrane but not by the sarcoplasmic reticulum, inhibited 65% of the Ca2+ uptake of this fraction. The Ca2+ uptake of this fraction was enhanced by cAMP- and cGMP-dependent protein kinases, and by calmodulin. The cAMP-dependent protein kinase enhanced the phosphorylation of 28 and 22 kDa proteins, while the cGMP-dependent protein kinase phosphorylated the 35 kDa protein. The phosphorylation of 100, 75, 65, 41 and 22 kDa proteins was enhanced by Ca2+ and calmodulin. These results indicate that cAMP- and cGMP-dependent protein kinases as well as calmodulin play important roles in Ca2+ transport in the sarcolemma, and that the phosphorylated proteins may be associated with an enhancement of Ca2+ transport in the sarcolemma.     

Comparison between calcium transport and adenosine triphosphatase activity in membrane vesicles derived from rabbit kidney proximal tubules

Characteristics of Ca2+ uptake were studied in a vesicular preparation of proximal tubule plasma membranes from rabbit kidney and compared with the properties of both membrane-bound and solubilized Ca2+-ATPase activities. Calcium uptake required both ATP and MgCl2 and revealed two kinetic components with respect to Ca2+ concentration requirements, one with a high affinity for Ca2+ (1.8 microM), operative in the range of cytosolic Ca2+ activity, and one with a low affinity for Ca2+ (250 microM) which may become active only at abnormally high cytosolic Ca2+ concentrations. The high- and low-affinity components were stimulated to similar extents by phosphate, and required similar concentrations of ATP (0.6 mM) for half-maximal activity. The amount of membrane-bound phosphoenzyme formed from ATP in the presence of Ca2+ was the same regardless of whether only one or both sites were saturated, suggesting that occupancy of the second Ca2+ binding site accelerates the enzyme turnover. Inhibition of Ca2+ transport by Na+ was reversed by the addition of ouabain or an ATP-regenerating system, indicating that this inhibitory effect of Na+ on Ca2+ uptake may be due to the accumulation of ADP in the medium as a result of Na+ pump activity. Low concentrations of carbonyl cyanide p-trifluoromethoxyphenylhydrazone and valinomycin (2.5 and 1 microM, respectively) were without effect on Ca2+ uptake in the presence of phosphate, whereas higher concentrations of the ionophores (200 and 100 microM, respectively) reduced uptake by 60% or more. The calmodulin antagonist 48/80 also reduced Ca2+ uptake with half-maximal effectiveness at 100 micrograms/ml. None of these drugs affected either ATPase activity or the EGTA-induced Ca2+ efflux from preloaded vesicles. The Ca2+ dependence of ATP hydrolysis by the membrane-bound enzyme preparation was similar to that observed for Ca2+ uptake by the vesicles. However, with solubilized enzyme, concentrations of Ca2+ similar to that found in the plasma reduced Ca2+-stimulated ATP hydrolysis to one-half of its maximal rate. This indicates that peritubular Ca2+ may play a role in the regulation of Ca2+ transport across the tubular epithelium. ATP could not be replaced by ITP as a substrate for Ca2+ uptake, and the (Ca2+ + Mg2+)ITPase activity of soluble enzyme was 25-fold lower than in the presence of ATP. This is an indication that the active Ca2+ pumping mechanism in proximal tubules is critically dependent on the nucleoside moiety of the substrate.     

Characteristics of the Ca2+ pump and Ca2+-ATPase in the plasma membrane of rat myometrium.

A plasma membrane-enriched fraction from rat myometrium shows ATP-Mg2+-dependent active calcium uptake which is independent of the presence of oxalate and is abolished by the Ca2+ ionophore A23187. Ca2+ loaded into vesicles via the ATP-dependent Ca2+ uptake was released by extravesicular Na+. This showed that the Na+/Ca2+ exchange and the Ca2+ uptake were both occurring in plasma membrane vesicles. In a medium containing KCl, vanadate readily inhibited the Ca2+ uptake (K1/2 5 microM); when sucrose replaced KCl, 400 microM-vanadate was required for half inhibition. Only a slight stimulation of the calcium pump by calmodulin was observed in untreated membrane vesicles. Extraction of endogenous calmodulin from the membranes by EGTA decreased the activity and Ca2+ affinity of the calcium pump; both activity and affinity were fully restored by adding back calmodulin or by limited proteolysis. A monoclonal antibody (JA3) directed against the human erythrocyte Ca2+ pump reacted with the 140 kDa Ca2+-pump protein of the myometrial plasma membrane. The Ca2+-ATPase activity of these membranes is not specific for ATP, and is not inhibited by mercurial agents, whereas Ca2+ uptake has the opposite properties. Ca2+-ATPase activity is also over 100 times that of calcium transport; it appears that the ATPase responsible for transport is largely masked by the presence of another Ca2+-ATPase of unknown function. Measurements of total Ca2+-ATPase activity are, therefore, probably not directly relevant to the question of intracellular Ca2+ control.     

The effect of thyroid hormone on the high affinity Ca2+-ATPase in rat liver plasma membrane

The effect of thyroid hormone on the high affinity Ca2+-ATPase activity in rat liver plasma membrane was studied. The high affinity Ca2+-ATPase activity in plasma membrane was activated by 10(-7)-10(-5) M of Ca2+ and was inhibited by 70 microM trifluoperazine. Thyroidectomy of rats was associated with an increase in the activity of high affinity Ca2+-ATPase. The increased enzyme activity was normalized by T4 administration to the animals. On the other hand, Na+-K+-ATPase activity in the membrane was decreased by thyroidectomy and the decreased enzyme activity was normalized by T4 administration. The results suggest that thyroid hormone inhibits the Ca2+ extrusion system by inhibiting calmodulin-independent high affinity Ca2+-ATPase in liver plasma membrane.     

Interaction of chemical probes with sarcoplasmic reticulum membranes

The labelling of the sarcoplasmic reticulum membranes by the chemical probes, trinitrobenzenesulfonate (TNBS) and fluorodinitrobenzene (FDNB) has been investigated. The incorporation of TNBS, but not of FDNB, depends on the binding of Ca2+ or Mg2+ to the membranes. The labelling of lipids and of the various reticulum proteins by TNBS is increased by those agents, but the effect is not uniform for all membrane proteins. The Ca2+ -ATPase contributes only 2.2% for the total labelling of the sarcoplasmic reticulum proteins, whereas the proteins of molecular weight 90 000 and 30 000 contribute about 34 and 56%, respectively. However, the Ca2+-ATPase isolated from the membrane reacts with an amount of TNBS 5-fold higher than that which reacts with the enzyme in situ. Both probes, TNBS and FDNB, inhibit the Ca2+-ATPase activity and the Ca2+ uptake by sarcoplasmic reticulum, whereas the Mg2+-ATPase remains unaffected. The results indicate that FDNB is maximally incorporated into the sarcoplasmic reticulum membrane, whereas only some of the membrane amino groups are accessible to TNBS in the absence of Ca2+, Mg2+ or ATP which, when present, make additional amino groups available to TNBS. The highest degree of TNBS incorporation takes place into proteins, other than the ATPase, but sufficient reaction occurs with the enzyme to inhibit its activity.