Structural and functional properties of a Ca2+-ATPase from human platelets

An antibody prepared against highly purified rabbit muscle Ca2+-ATPase from sarcoplasmic reticulum has been observed to cross-react with proteins in human platelet membrane vesicles. The antibody specifically precipitated Ca2+-ATPase activity from solubilized human platelet membranes and recognized two platelet polypeptides denatured in sodium dodecyl sulfate with Mr = 107,000 and 101,000. Ca2+-ATPase activity from Brij 78-solubilized platelet membranes was purified up to 10-fold. The purified preparation consisted mainly of two polypeptides with Mr approximately 100,000, and 40,000. The lower molecular weight protein appeared unrelated to Ca2+-ATPase activity. The Ca2+-ATPase in human platelet membrane vesicles exhibited "negative cooperativity" with respect to the kinetics of ATP hydrolysis. The apparent Km for Ca2+ activation of ATPase activity was 0.1 microM. Ca2+-dependent phosphorylation of platelet vesicles by [gamma-32P]ATP at 0 degrees C yielded a maximum of 0.2-0.4 nmol of PO4/mg of protein that was labile at pH 7.0 and 20 degrees C. This result suggests that only about 2-4% of the total protein in platelet membrane vesicles is the Ca2+-ATPase, which agrees with an estimate based on the specific activity of the Ca2+-ATPase in platelet membranes (20-50 nmol of ATP hydrolyzed/min/mg of protein at 30 degrees C). Calmodulin resulted in only a 1.6-fold stimulation of Ca2+-ATPase activity even after extensive washing of membranes with a calcium chelator or chlorpromazine. It is concluded that human platelets contain a Ca2+-ATPase immunochemically related to the Ca2+ pump from rabbit sarcoplasmic reticulum and that the enzymatic characteristics and molecular weight of the platelet ATPase are quite similar to those of the muscle ATPase.     

Ca2+-calmodulin-dependent phosphorylation and passive transport of Ca2+ in the myocardial sarcolemma

Highly purified vesicles of rabbit myocardium sarcolemma with predominant inside-out orientation possess the Ca2+-calmodulin-dependent protein kinase activity. At optimal concentrations of calmodulin (0.5 microM) and Ca2+ (0.1 mM), the activity of protein kinase is 0.21 nmol 32P X min X mg of protein. The Km(app) value for ATP is 3.0 X 10(-6) M, V = 0.27 nmol 32P X mg of protein X min. Endogenous Ca2+-calmodulin-dependent protein kinase phosphorylates four protein substrates in sarcolemmal vesicles (Mr = 145, 22, 11.5, and 6-8 KD). Studies with passive efflux of Ca2+ from the SL vesicles showed that the Ca2+-calmodulin-dependent phosphorylation of protein components of sarcolemma inhibits this reaction.     

Identification and partial characterization of caltrin-like proteins in the reproductive tract of the guinea pig

The presence of caltrin-like proteins in reproductive tract fluid (RTF) and seminal vesicle content from male guinea pigs has been determined. Two fractions with electrophoretic mobility corresponding to Mr = 6200 (main band) and 5100 were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. Isoelectric focusing in thin-layer agarose gels revealed three bands with acidic pIs of 5.3, 6.0, and 6.2, respectively. RTF prevented the enhancement of calcium permeability induced by incubating guinea pig epididymal spermatozoa in medium for capacitation. Spermatozoa incubated for 2 h in minimal culture medium plus pyruvate and lactate containing RTF accumulated less than 30% of the 45Ca2+ accumulated by cells maintained in absence of this fluid. Calcium uptake by preincubated spermatozoa was also inhibited by RTF. Inhibition of calcium transport activity by RTF and seminal vesicle proteins was not decreased by heating the dialyzed preparations at 60 degrees C for 5 min. After this treatment, the inhibitory activity and the protein pattern were stable for 3 wk when stored at 4 degrees C. Unheated extracts lost calcium transport inhibitory activity after 2 or 3 days at 4 degrees C. In spite of the differences in pIs among the proteins from the guinea pig reproductive tract and bovine caltrin, several features indicate they may play a similar role in both species by controlling Ca2+ movement across the plasma membrane. By this mechanism, these proteins could regulate physiologic events essential for the fertilization process.     

Isolation and characterization of tryptic fragments of the adenosine triphosphatase of sarcoplasmic reticulum.

Exposure of sarcoplasmic reticulum to trypsin in the presence of 1 M sucrose results in degradation of the Mr = 102,000 ATPase enzyme to two fragments of Mr = 55,000 and 45,000 with subsequent appearance of fragments of Mr = 30,000 and 20,000. These fragments were purified by column chromatography in sodium dodecyl sulfate. Antibodies were raised against the ATPase and the Mr = 55,000, 45,000, and 20,000 fragments. There was no antigenic cross-reactivity between the Mr = 55,000 and 45,000 fragments, indicating that they were derived from a single linear cleavage of the larger enzyme. There was antigenic cross-reactivity between the Mr = 20,000 and 55,000 fragments, indicating an origin of the Mr = 20,000 fragment in the Mr = 55,000 fragment. None of the antibodies inhibited (Ca2+ + Mg2+)-dependent ATPase or Ca2+ transport. The Mr = 20,000 fragment and the Mr = 55,000 fragment were active in Ca2+ ionophore assays. The active site of ATP hydrolysis was labeled with [gamma-32P]ATP and the site of ATP binding was labeled with tritiated N-ethylmaleimide. In both cases radioactivity was found in the intact ATPase and in the Mr = 55,000 and 30,000 fragments, indicating that the Mr = 30,000 fragment was also derived from the Mr = 55,000 fragment. Amino acid composition data showed that the Mr = 45,000 fragment contained about 60% nonpolar and 40% polar amino acids, while the Mr = 55,000 fragment and the Mr = 20,0000 fragment contained about equal amounts of polar and nonpolar amino acids. Studies of the reaction of various antibodies at the external surface of sarcoplasmic reticulum vesicles showed that the ATPase was exposed, whereas calsequestrin and the high affinity Ca2+-binding protein were not. The use of antibodies against the various fragments indicated that the Mr = 55,000 fragment was in large part exposed, whereas the Mr = 20,000 and the 45,000 fragments were only poorly exposed. It is probable that the site of ATP hydrolysis in the Mr = 55,000 fragment is external, whereas the ionophore site is only partially exposed and the Mr = 45,000 fragment is largely buried within the membrane.     

ATP-dependent transport of Ca2+ in myocardium sarcolemma vesicles and its activation by phorbol esters

Highly purified pig myocardium sarcolemma vesicles possess the Ca2+,Mg2+-ATPase activity (4.1 mumol Pi/mg protein/hour) and induce the ATP-dependent accumulation of 45Ca2+ (6.0 nmol/mg protein/min). This reaction is not stimulated by oxalate; Ca2+ are released from the vesicles by saponin and Na+ treatment, which suggests that Ca2+ transport against the concentration gradient is induced by myocardium sarcolemma vesicles and not by sarcoplasmic reticulum fragments. The phorbol ester possessing a biological activity of a growth-promoting factor and activating membrane-bound protein kinase C stimulates the Ca2+,Mg2+-ATPase activity and the ATP-dependent accumulation of Ca2+, whereas its counterpart devoid of biological activity does not influence Ca2+ transport. Polymixin B, a specific inhibitor of protein kinase C, prevents the activating effect of phorbol esters on Ca2+ accumulation inside the vesicles. It is suggested that the ATP-dependent transport of Ca2+ in myocardium sarcolemma is controlled by Ca2+-phospholipid-dependent phosphorylation catalyzed by protein kinase C.     

Mechanisms of Ca2+ transport in plasma membrane vesicles prepared from cultured pituitary cells. II. (Ca2+ + Mg2+)-ATPase-dependent Ca2+ transport activity

Purified plasma membrane vesicles from GH3 rat anterior pituitary cells exhibit a Mg2+-ATP-dependent Ca2+ transport activity. Concentrative uptake of Ca2+ is abolished by exclusion of either Mg2+ or ATP or by inclusion of the Ca2+ ionophore A23187. Furthermore, addition of A23187 to vesicles which have reached a steady state of ATP-supported Ca2+ accumulation rapidly and completely discharges accumulated cation. Ca2+ uptake is unaffected by treatment of vesicles with oligomycin, the uncoupler CCCP, or valinomycin and is greatly reduced in non-plasma membrane fractions. Likewise, Ca2+ accumulation is not stimulated by oxalate, consistent with the plasma membrane origin of this transport system. (Na+, K+)-ATPase participation in the Ca2+ transport process (i.e. via coupled Na+/Ca2+ exchange) was eliminated by omitting Na+ and including ouabain in the reaction medium. Ca2+ transport activity in GH3 vesicles has a similar pH dependence as that seen in a number of other plasma membrane systems and is inhibited by orthovanadate in the micromolar range. Inhibition is enhanced if the membranes are preincubated with vanadate for a short time. A kinetic analysis of transport indicates that the apparent Km for free Ca2+ and ATP are 0.7 and 125 microM, respectively. The average Vmax is 3.6 nmol of Ca2+/min/mg of protein at 37 degrees C. Addition of exogenous calmodulin or calmodulin antagonists had no significant effect on these kinetic properties. GH3 plasma membranes also contain a Na+/Ca2+ exchange system. The apparent Km for Ca2+ is almost 10-fold higher in this system than that for ATP-driven Ca2+ uptake. When both processes are compared under similar conditions, the Vmax of the exchanger is approximately 2-3 times that of ATP-dependent Ca2+ accumulation. Similar results are obtained when purified plasma membranes from bovine anterior pituitary glands were investigated. It is suggested that both Na+/Ca2+ exchange and the (Ca2+ + Mg2+)-ATPase are important in controlling intracellular levels of Ca2+ in anterior pituitary cells.     

The interaction of Ca2+ with the calmodulin-sensitive adenylate cyclase from Bordetella pertussis

Bordetella pertussis, the etiologic agent of whooping cough, produces a calmodulin-sensitive adenylate cyclase which elevates intracellular cAMP in a variety of eucaryotic cells. Exogenous calmodulin added to the partially purified adenylate cyclase has been shown to inhibit invasion of animal cells by this enzyme (Shattuck, R. L., and Storm, D. R. (1985) Biochemistry 24, 6323-6328). In this study, several properties of the calmodulin-sensitive adenylate cyclase are shown to be influenced by Ca2+ in the absence of calmodulin. The presence or absence of Ca2+ during QAE-Sephadex ion exchange chromatography produced two distinct chromatographic patterns of adenylate cyclase activity. Two different forms of the enzyme (Pk1 and Pk2EGTA) were isolated by this procedure. Pk1 adenylate cyclase readily elevated intracellular cAMP levels in mouse neuroblastoma cells (N1E-115) while Pk2EGTA adenylate cyclase had no effect on cAMP levels in these cells. Gel exclusion chromatography of Pk1 adenylate cyclase gave apparent Stokes radii (RS) of 43.5 A (+/- 1.3) in the presence of 2 mM CaCl2 and 33.8 A (+/- 0.94) in the presence of 2 mM EGTA [( ethylenebis (oxyethylenenitrilo)]tetraacetic acid). These Stokes radii are consistent with molecular weights of 104,000 (+/- 6,400) and 61,000 (+/- 3,600), respectively. Pk2EGTA adenylate cyclase had an apparent RS of 33.0 (+/- 1.2) (Mr = 60,600 (+/- 2,800] in the presence of Ca2+ or excess EGTA. At 60 degrees C, Pk1 adenylate cyclase exhibited a Ca2+-dependent heat stability with a half-life for loss of enzyme activity of 10.3 min in 5 mM CaCl2 and a half-life of 2.8 min in the presence of 0.1 microM CaCl2. The stability of Pk2EGTA adenylate cyclase was not affected by changes in free Ca2+. The adenylate cyclase preparations described above were submitted to sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and enzyme activity was recovered from gel slices by extraction with detergent containing buffers. The catalytic subunit isolated from SDS-polyacrylamide gels was activated 7-fold in the presence of Ca2+ with maximum activity observed at 1 microM free Ca2+. With both preparations, the apparent molecular weight of the catalytic subunit on SDS gels was 51,000 in the presence of 2 mM CaCl2 and 45,000 in the presence of 2 mM EGTA. The catalytic subunit of the enzyme was purified to apparent homogeneity by preparative SDS-polyacrylamide gel electrophoresis and resubmitted to SDS gel electrophoresis in the presence or absence of free Ca2+. The purified catalytic subunit also exhibited a Ca2+-dependent shift in its mobility on SDS gels.(ABSTRACT TRUNCATED AT 400 WORDS)     

Different sensitivity to trypsin of the human platelet plasma and intracellular membrane Ca2+ pumps

The Ca2+ pumps associated with human platelet plasma and intracellular membranes have been further characterized by their sensitivity to trypsin. (a) Tryptic degradation of the Ca2+-ATPases has been followed by immunoblotting. It resulted in fragmentation into peptides of 80, 55, 35, and 24 kDa for both enzymes. Subcomplete hydrolysis obtained with a ratio of trypsin/membrane protein of 0.05-0.1 for the two Ca2+ pumps resulted in the total disappearance of the 100-, 80-, and 35-kDa fragments. However, maximum degradation was reached within 1 min for the intracellular enzyme but needed 5 min of incubation for the plasma membrane enzyme. (b) This effect of trypsin has been correlated with its effect on both the Ca2+-ATPase activities. The plasma membrane enzyme showed a maximum inhibition of 50-60% which was obtained using a trypsin/protein ratio of 0.1 and 5 min of incubation. A much higher trypsin sensitivity was observed for the intracellular enzyme because the maximum inhibition reached 80% after only 1 min of incubation. (c) Finally, the two Ca2+ transport systems studied showed different trypsin reactivities; the Ca2+ uptake by the plasma membrane vesicles was inhibited by 20-25%, and this maximum inhibition was observed after 5 min of incubation with trypsin. In contrast, the Ca2+ transport associated with the intracellular membrane vesicles was difficult to detect after trypsin treatment. Taken together, the results show that the two Ca2+ pumps can be distinguished by their trypsin sensitivity.     

Restoration of Ca2+-transport in sarcoplasmic reticulum ATPase solubilized with octaethyleneglycol mono n-dodecyl ether

One mg protein/ml of sarcoplasmic reticulum (SR) membranes isolated from rabbit skeletal muscle were solubilized with 50 mg/ml of octaethyleneglycol mono n-dodecyl ether (C12E8) in a solution containing 5 mM CaCl2, 0.1 M KCl, and 20% glycerol at pH 7.5. When 30 mg/ml of soybean lecithin was added to this mixture and then incubated with Bio-beads SM-2 at 20 degrees C for 1.5 h to remove the detergent from the mixture, proteoliposomes were formed. This process restored Ca2+-uptake activity to approximately 50% of that of control sR. However, Ca2+-transport was not observed when SR membranes were formed without the addition of soybean lecithin. The reconstituted vesicles also catalyze Ca2+-release, which is coupled to the backward reaction which forms ATP from ADP and P1 in the presence of a Ca2+-gradient across the membrane. When the reconstituted vesicles were subjected to equilibrium centrifugation in a 5 to 25% glycerol density gradient, all of the Ca2+-transport activity was closely associated with the fraction containing soybean liposome.     

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.     

Phosphorylation of Brain Synaptic and Coated Vesicle Proteins by Endogenous Ca2+/Calmodulin- and cAMP-Dependent Protein Kinases

Phosphorylation of brain synaptic and coated vesicle proteins was stimulated by Ca2+ and calmodulin. As determined by 5-15% sodium dodecylsulfate (SDS) polyacrylamide gel electrophoresis (PAGE), molecular weights (Mr) of the major phosphorylated proteins were 55,000 and 53,000 in synaptic vesicles and 175,000 and 55,000 in coated vesicles. In synaptic vesicles, phosphorylation was inhibited by affinity-purified antibodies raised against a 30,000 Mr protein doublet endogenous to synaptic and coated vesicles. When this doublet, along with clathrin, was extracted from coated vesicles, phosphorylation did not take place, implying that the protein doublet may be closely associated with Ca2+/calmodulin-dependent protein kinase. Affinity-purified antibodies, raised against clathrin used as a control antibody, failed to inhibit Ca2+/calmodulin-dependent phosphorylation in either synaptic or coated vesicles. Immunoelectron cytochemistry revealed that this protein doublet was present in axon terminal synaptic and coated vesicles. Synaptic vesicles also displayed cAMP-dependent kinase activity; coated vesicles did not. The molecular weights of phosphorylated synaptic vesicle proteins in the presence of Mg2+ and cAMP were: 175,000, 100,000, 80,000, 57,000, 55,000, 53,000, 40,000, and 30,000. Based on the different phosphorylation patterns observed in synaptic and coated vesicles, we propose that brain vesicle protein kinase activities may be involved in the regulation of exocytosis and in retrieval of synaptic membrane in presynaptic axon terminals.     

Contribution of the Mg2+-, ATP- AND Na+-dependent Ca2+-transport systems to the regulation of Ca2+ concentration in myometrial cells

Studies with sarcolemma from cattle myometrium containing inside-out cytoplasmic vesicles, using Ca2+-EGTA buffer, showed that the affinity of ionized Ca2+ for the Mg2+- or ATP-dependent transport is higher than that for the Na+-Ca2+ exchange system (Kd = 3,2 X 10(-6) and (4.3-5.3) X 10(-5) M), respectively. The Km values for MgATP are 2.15 mM. Oxytocin added to the homogenization medium containing rabbit and cattle myometrium cells, i.e. during the formation of closed sarcolemmal fragments, resulted in inhibition of Mg2+, ATP-dependent accumulation of 45Ca2+ by plasma membranes. However, an addition of oxytocin to the incubation medium did not affect the kinetics of active accumulation of Ca2+. It was assumed that the system of non-electrogenic Na+-Ca2+ exchange in the myometrium possessing a low affinity for Ca2+ provides for the maintenance of ionized Ca2+ concentration in the myocytes at 10(-5) M. Therefore, this system cannot induce relaxation of mechanical tension of the uterus. Further decrease of Ca2+ in the myoplasm from 10(-5) to 10(-7) M and, correspondingly, the relaxation of myometrium is provided for by the Mg2+, ATP-dependent efflux of Ca2+ from the myocytes having a high affinity for this cation. The decrease of the activity of ATP-dependent Ca2+-pump by oxytocin is the cause of Ca2+ elevation in the myoplasm and, consequently, of myometrium contraction.     

ATP-dependent calcium pump and Na+-Ca2+ exchange in plasma membrane vesicles from squid optic nerve

Purified plasma membrane vesicles from the optic nerve of the squid Sepiotheutis sepioidea accumulate calcium in the presence of Mg2+ and ATP. Addition of the Ca2+ ionophore A23187 to vesicles which have reached a steady state of calcium-active uptake induces complete discharge of the accumulated cation. Kinetic analysis of the data indicates that the apparent Km for free Ca2+ and ATP are 0.2 muM and 21 muM, respectively. The average Vmax is 1 nmol Ca2+/min per mg protein at 25 degrees C. This active transport is inhibited by orthovanadate in the micromolar range. An Na+-Ca2+ exchange mechanism is also present in the squid optic nerve membrane. When an outwardly directed Na+ gradient is imposed on the vesicles, they accumulate calcium in the absence of Mg2+ and/or ATP. This ability to accumulate Ca2+ is absolutely dependent on the Na+ gradient: replacement of Na+ by K+, or passive dissipation of the Na+ gradient, abolishes transport activity. The apparent Km for Ca2+ of the Na+-Ca2+ exchange is more than 10-fold higher than that of the ATP-driven pump (app. Km=7.5 muM). While the apparent Km for Na+ is 74 mM, the Vmax of the exchanger is 27 nmol Ca2+/min per mg protein at 25 degrees C. These characteristics are comparable to those displayed by the uncoupled Ca pump and Na+-Ca2+ exchange previously described in dialyzed squid axons.     

Free radical production from the aerobic oxidation of reduced pyridine nucleotides catalysed by phenazine derivatives

Calcium-accumulating vesicles were isolated by differential centrifugation of sonicated platelets. Such vesicles exhibit a (Ca2+ + Mg2+)-ATPase activity of about 10 nmol (min . mg)-1 and an ATP-dependent Ca2+ uptake of about 10 nmol (min . mg)-1. When incubated in the presence of Mg[gamma-32P]ATP, the pump is phosphorylated and the acyl phosphate bond is sensitive to hydroxylamine. The [32P]phosphate-labeled Ca2+ pump exhibits a subunit molecular weight of 120 000 when analyzed by lithium dodecyl sulfate-polyacrylamide gel electrophoresis. Platelet calcium-accumulating vesicles contain a 23 kDa membrane protein that is phosphorylatable by the catalytic subunit of cAMP-dependent protein kinase but not by protein kinase C. This phosphate acceptor is not phosphorylated when the vesicles are incubated in the presence of either Ca2+ or Ca2+ plus calmodulin. The latter protein is bound to the vesicles and represents 0.5% of the proteins present in the membrane fraction. Binding of 125I-labeled calmodulin to this membrane fraction was of high affinity (16 nM), and the use of an overlay technique revealed four major calmodulin-binding proteins in the platelet cytosol (Mr = 94 000, 87 000, 60 000 and 43 000). Some minor calmodulin-binding proteins were enriched in the membrane fractions (Mr = 69 000, 57 000, 39 000 and 37 000). When the vesicles are phosphorylated in the presence of MgATP and of the catalytic subunit of cAMP-dependent protein kinase, the rate of Ca2+ uptake is essentially unaltered, while the Ca2+ capacity is diminished as a consequence of a doubling in the rate of Ca2+ efflux. Therefore, the inhibitory effect of cAMP on platelet function cannot be explained in such simple terms as an increased rate of Ca2+ removal from the cytosol. Calmodulin, on the other hand, was observed to have no effect on the initial rate of calcium efflux when added either in the absence or in the presence of the catalytic subunit of the cyclic AMP-dependent protein kinase, nor did the addition of 0.5 microM calmodulin result in increased levels of vesicle phosphorylation.     

Ca2+-induced down-regulation of Na+ channels in toad bladder epithelium

Regulation of epithelial Na+ channels was investigated by measuring the amiloride-blockable 22Na+ fluxes in apical membrane vesicles, derived from cells exposed to various treatments. Maximal amiloride-blockable 22Na+ uptake into vesicles was obtained if the cells were preincubated at 25 degrees C in a Ca2+-free [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) solution. Including 10(-5) M Ca2+ in the cell incubating medium blocked nearly all of the amiloride-sensitive flux in vesicles, even though the Ca2+ was removed before homogenization of the cells. This Ca2+-dependent inhibition of Na+ channels could be induced in whole cells only; incubating cell homogenates with Ca2+ had no effect on the transport in vesicles. The dose-response relationships of this effect were measured by equilibrating cell aliquots with various Ca2+-EGTA buffers, preparing membrane vesicles (in the absence of Ca2+ ions), and assaying them for amiloride-sensitive Na+ permeability. It was found that the Ca2+ blockage is highly cooperative (Hill coefficient of nearly 4) and is characterized by an inhibition constant which varies between 6.4 X 10(-8) to 8.15 X 10(-6)M Ca2+. Thus, it is likely that the above process is involved in the physiological control of Na+ transport. The Ca2+-dependent transport changes were not affected by the calmodulin inhibitor trifluoperasine, vanadate (VO3-), phorbol ester, colchicine, cytochalasin B, 3-deazaadenosine, and 8-bromo-cAMP. Vanadyl (VO2+) ions, on the other hand, produced a "Ca2+-like" inhibition of transport.     

ATPase activity and Ca2+ transport by reconstituted tryptic fragments of the Ca2+ pump of the erythrocyte plasma membrane

The purified Ca2+ ATPase of the erythrocyte plasma membrane has been submitted to controlled trypsin proteolysis under conditions that favor either its (putative) E1 or E2 configurations. The former configuration has been forced by treating the enzyme with Ca2+-saturated calmodulin, the latter with vanadate and Mg2+. The E1 conformation leads to the accumulation of a polypeptide of Mr 85 KDa which still binds calmodulin, the E2 conformation to the accumulation of one of Mr 81 KDa which does not. Both fragments arise from the hydrolysis of a transient 90 KDa product which has Ca2+-calmodulin dependent ATPase activity, and which retains the ability to pump Ca2+ in reconstituted liposomes. Highly enriched preparations of the 85 and 81 KDa fragments have been obtained and reconstituted into liposomes. The former has limited ATPase and Ca2+ transport ability and is not stimulated by calmodulin. The latter has much higher ATPase and Ca2+ transport activity. It is proposed that the Ca2+ pumping ATPase of erythrocytes plasma membrane contains a 9 KDa domain which is essential for the interaction of the enzyme with calmodulin and for the full expression of the hydrolytic and transport activity. This putative 9 KDa sequence contains a 4 KDa "inhibitory" domain which limits the activity of the ATPase. In the presence of this 4 KDa sequence, i.e., when the enzyme is degraded to the 85 KDa product, calmodulin can still be bound, but no longer stimulates ATPase and Ca2+ transport.     

Stimulation of Na+-Ca2+ exchange in cardiac sarcolemmal vesicles by phospholipase D

Treatment of canine cardiac sarcolemmal vesicles with phospholipase D resulted in a large stimulation (up to 400%) of Na+-Ca2+ exchange activity. The phospholipase D treatment decreased the apparent Km (Ca2+) for the initial rate of Nai+-dependent Ca2+ uptake from 18.2 +/- 2.6 to 6.3 +/- 0.3 microM. The Vmax increased from 18.0 +/- 3.6 to 31.5 +/- 3.6 nmol of Ca2+/mg of protein/s. The effect was specific for Na+-Ca2+ exchange; other sarcolemmal transport enzymes ((Na+, K+)-ATPase; ATP-dependent Ca2+ transport) are inhibited by incubation with phospholipase D. Phospholipase D had little effect on the passive Ca2+ permeability of the sarcolemmal vesicles. After treatment with 0.4 unit/ml of phospholipase D (20 min, 37 degrees C), the sarcolemmal content of phosphatidic acid rose from 0.9 +/- 0.2 to 8.9 +/- 0.4%; simultaneously, Na+-Ca2+ exchange activity increased 327 +/- 87%. It is probable that the elevated phosphatidic acid level is responsible for the enhanced Na+-Ca2+ exchange activity. In a previous study (Philipson, K. D., Frank, J. S., and Nishimoto, A. Y. (1983) J. Biol. Chem. 258, 5905-5910), we hypothesized that negatively charged phospholipids were important in Na+-Ca2+ exchange, and the present results are consistent with this hypothesis. Stimulation of Na+-Ca2+ exchange by phosphatidic acid may be important in explaining the Ca2+ influx which accompanies the phosphatidylinositol turnover response which occurs in a wide variety of tissues.     

Effect of caffeine on active Ca2+ ion transport in a homogenate of skeletal muscles and myocardium

A Ca2-selective electrode was used to study active transport of Ca2+ by sarcoplasmic reticulum fragments of rabbit skeletal muscle and myocardium homogenates. The specific Ca2+ transport activities (mumol Ca2+/min/mg tissue) are 40 = 60 and 3 = 5 units for fast and slow muscles and the myocardium, respectively. Caffeine (5 mM) exerts a powerful inhibitory influence on Ca2+ transport in skeletal muscle homogenates. For fast muscles, the degree of inhibition exceeds 50%. The rate of Ca2+ transport in the myocardium homogenate increases in the presence of creatine phosphate. The latter produces no effect on Ca2+ transport in skeletal muscle homogenates. The high sensitivity of Ca2 transport to caffeine, a specific blocker of Ca2+ transport to the terminal cisterns of the sarcoplasmic reticulum, suggests that the terminal cisterns, apart from being a reservoir for Ca2+ needed for contraction trigger, may play an essential role in muscle relaxation.     

Induction of Ca2+ transport in liposomes by insulin.

The requirement of extracellular Ca2+ for insulin action has been indicated by past studies. With a view to understand the interaction of insulin with Ca2+ in the vicinity of the cell membrane, we have examined the ability of insulin and its constituent polypeptide chains A and B to translocate Ca2+ and Mg2+ across the lipid bilayer in two sets of synthetic liposomes. The first were unilamellar vesicles made of dimyristoylphosphatidylcholine and contained the Ca2+ sensor dye arsenazo III. Peptide-mediated Ca2+ and Mg2+ transport in these vesicles was monitored at 37 degrees C in a neutral buffer containing CaCl2 or MgCl2 using a difference absorbance method. In the second set, multilamellar vesicles of egg lecithin containing trapped fura-2 were employed and the cation transport was followed at 20 degrees C by fluorescence changes in the dye. Control experiments indicated that the hormonal peptides caused no appreciable perturbation of the vesicles leading to leakage of contents or membrane fusion. In both liposome systems, substantial Ca2+ and Mg2+ transport was observed with insulin and the B chain; the A chain was less effective as an ionophore. Quantitative analysis of the transport kinetic data on the B chain showed a 1:1 peptide-Ca2+ complex formed inside the membrane. In light of the available structural data on Ca2+ binding by insulin and insulin receptor, our results suggest the possibility of the hormone interacting with the receptor with the bound Ca2+.     

Modulation of ATP-dependent Ca2+ transport in rat parotid basolateral membrane vesicles by K+ + Cl- flux

In basolateral membrane vesicles (BLMV) isolated from rat parotid glands, the initial rate of ATP-dependent Ca2+ transport, in the presence of KCl, was approx. 2-fold higher than that obtained with mannitol, sucrose or N-methyl-D-glucamine (NMDG)-gluconate. Only NH4+, Rb+, or Br- could effectively substitute for K+ or Cl-, respectively. This KCl activation was concentration dependent, with maximal response by 50 mM KCl. An inwardly directed KCl gradient up to 50 mM KCl had no effect on Ca2+ transport, while equilibration of the vesicles with KCl (greater than 100 mM) increased transport 15-20%. In presence of Cl-, 86Rb+ uptake was 2.5-fold greater than in the presence of gluconate. 0.5 mM furosemide inhibited 86Rb+ flux by approx. 60% in a Cl- medium and by approx. 20% in a gluconate medium. Furosemide also inhibited KCl activation of Ca2+ transport with half maximal inhibition either at 0.4 mM or 0.05 mM, depending on whether 45Ca2+ transport was measured with KCl (150 mM) equilibrium or KCl (150 mM) gradient. In a mannitol containing assay medium, potassium gluconate loaded vesicles had a higher (approx. 25%) rate of Ca2+ transport than mannitol loaded vesicles. Addition of valinomycin (5 microM) to potassium gluconate loaded vesicles further stimulated (approx. 30%) the Ca2+ transport rate. These results suggest that during ATP dependent Ca2+ transport in parotid BLMV, K+ can be recycled by the concerted activities of a K+ and Cl- coupled flux and a K+ conductance.