Parotid microsomal Ca2+ transport. Subcellular localization and characterization.
ABSTRACT: Rat parotid gland homogenates were fractionated into mitochondrial, heavy microsomal and light microsomal fractions by differential centrifugation. ATP-dependent 45Ca2+ uptake by the subcellular fractions paralleled the distribution of NADPH-cytochrome c reductase, an enzyme associated with the endoplasmic reticulum. The highest rate of Ca2+ uptake was found in the heavy microsomal fraction. Ca2+ uptake by this fraction was dependent on the presence of ATP and was sustained at a linear rate by 5 mM-oxalate. Inhibitors of mitochondrial Ca2+ transport had no effect on the rate of Ca2+ uptake. Na+ and K+ stimulated Ca2+ uptake. At optimal concentrations. Na+ stimulated Ca2+ uptake by 120% and K+ stimulated Ca2+ uptake by 260%. Decreasing the pH from 7.4 to 6.8 had little effect on Ca2+ uptake. The Km for Ca2+ uptake was 3.7 microM free Ca2+ and 0.19 mM-ATP. Vanadate inhibited Ca2+ uptake; 60 microM-vanadate inhibited the rate of Ca2+ accumulation by 50%. It is concluded that the ATP-dependent Ca2+ transport system is located on the endoplasmic reticulum and may play a role in maintaining intracellular levels of free Ca2+ within a narrow range of concentration.
Project description:The existence of Na+ -dependent Ca2+ transport was investigated in microsomal fractions from the longitudinal smooth muscle of the guinea-pig ileum and from the rat aorta, and its activity was compared with that of the plasmalemmal ATP-dependent Ca2+ pump previously identified in these preparations. The rate of Ca2+ release from plasmalemmal vesicles previously loaded with Ca2+ through the ATP-dependent Ca2+ pump was transiently faster in the presence of 150 mM-NaCl in the medium than in the presence of 150 mM-KCl or -LiCl or 300 mM-sucrose. Na+-loaded vesicles took up Ca2+ when an outwardly directed Na+ gradient was formed across the membrane. The Ca ionophore A23187 induced a rapid release of 85% of the sequestered Ca2+, whereas only 15% was displaced by La3+. Ca2+ accumulated by the Na+-induced Ca2+ transport was released by the addition of NaCl, but not KCl, to the medium. Ca2+ uptake in Na+-loaded vesicles was inhibited in the presence of increasing NaCl concentration in the medium. Half-maximum inhibition was observed with 28 mM-NaCl. Data fitted the Hill equation, with a Hill coefficient (h) of 1.9. Na+-induced Ca2+ uptake was a saturable function of Ca2+ concentration in the medium. Half-maximum activity was obtained with 18 microM-Ca2+ in intestinal-smooth-muscle microsomal fraction and with 50 microM-Ca2+ in aortic microsomal fraction. The results suggest that in these membrane preparations a transmembrane movement of Ca2+ can be driven by a Na+ gradient. However, the Na+-induced Ca2+ transport had a lower capacity, a lower affinity and a slower rate than the ATP-dependent Ca2+ pump.
Project description:We have cloned the squid neuronal Na+-Ca2+ exchanger, NCX-SQ1, expressed it in Xenopus oocytes, and characterized its regulatory and ion transport properties in giant excised membrane patches. The squid exchanger shows 58% identity with the canine Na+-Ca2+ exchanger (NCX1.1). Regions determined to be of functional importance in NCX1 are well conserved. Unique among exchanger sequences to date, NCX-SQ1 has a potential protein kinase C phosphorylation site (threonine 184) between transmembrane segments 3 and 4 and a tyrosine kinase site in the Ca2+ binding region (tyrosine 462). There is a deletion of 47 amino acids in the large intracellular loop of NCX-SQ1 in comparison with NCX1. Similar to NCX1, expression of NCX-SQ1 in Xenopus oocytes induced cytoplasmic Na+-dependent 45Ca2+ uptake; the uptake was inhibited by injection of Ca2+ chelators. In giant excised membrane patches, the NCX-SQ1 outward exchange current showed Na+-dependent inactivation, secondary activation by cytoplasmic Ca2+, and activation by chymotrypsin. The NCX-SQ1 exchange current was strongly stimulated by both ATP and the ATP-thioester, ATP gamma S, in the presence of F- (0.2 mM) and vanadate (50 microM), and both effects reversed on application of a phosphatidylinositol-4',5'-bisphosphate antibody. NCX1 current was stimulated by ATP, but not by ATP gamma S. Like NCX1 current, NCX-SQ1 current was strongly stimulated by phosphatidylinositol-4',5'-bisphosphate liposomes. In contrast to results in squid axon, NCX-SQ1 was not stimulated by phosphoarginine (5-10 mM). After chymotrypsin treatment, both the outward and inward NCX-SQ1 exchange currents were more strongly voltage dependent than NCX1 currents. Ion concentration jump experiments were performed to estimate the relative electrogenicity of Na+ and Ca2+ transport reactions. Outward current transients associated with Na+ extrusion were much smaller for NCX-SQ1 than NCX1, and inward current transients associated with Ca2+ extrusion were much larger. For NCX-SQ1, charge movements of Ca2+ transport could be defined in voltage jump experiments with a low cytoplasmic Ca2+ (2 microM) in the presence of high extracellular Ca2+ (4 mM). The rates of charge movements showed "U"-shaped dependence on voltage, and the slopes of both charge-voltage and rate-voltage relations (1,600 s-1 at 0 mV) indicated an apparent valency of -0.6 charges for the underlying reaction. Evidently, more negative charge moves into the membrane field in NCX-SQ1 than in NCX1 when ions are occluded into binding sites.
Project description: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.
Project description:1. The heavy microsomal fraction from rat liver apparently has very little Ca2+-stimulated ATPase activity, although it has an active, ATP-driven Ca2+ accumulation system. 2. The addition of ionophore A23187 to the ATPase assay, to allow continuous Ca2+ recycling during the assay time, reveals the presence of a substantial Ca2+-stimulated ATPase with Vmax. 160 nmol of Pi/10 min per mg of protein and Km for Ca2+ 0.19 microM. 3. The Ca2+-stimulated ATPase, but not the basal Mg2+-stimulated ATPase, is potently inhibited by orthovanadate. Both the Ca2+-stimulated ATPase and the vanadate inhibition are enhanced by the presence of Mg2+. 4. Ca2+-stimulated ATPase activity is not responsive to calmodulin or the calmodulin antagonist trifluoperazine.
Project description:Two microsomal subfractions from isolated rat pancreatic acini were produced by centrifugation through a discontinuous sucrose density gradient and characterized by biochemical markers. The denser fraction ( SF2 ) was a highly purified preparation of rough endoplasmic reticulum; the less-dense fraction ( SF1 ) was heterogeneous and contained Golgi, endoplasmic reticulum and plasma membranes. 45Ca2+ accumulation in the presence of ATP and its rapid release after treatment with the bivalent-cation ionophore A23187 were demonstrated in both fractions. The pH optimum for active 45Ca2+ uptake was approx. 6.8 for the rough endoplasmic reticulum ( SF2 ) and approx. 7.5 for SF1 . Initial rate measurements were used to determine the affinity of the rough-endoplasmic-reticulum uptake system for free Ca2+. An apparent Km of 0.16 +/- 0.06 microM and Vmax. of 21.5 +/- 5.6 nmol of Ca2+/min per mg of protein were obtained. 45Ca2+ uptake by SF1 was less sensitive to Ca2+, half-maximal uptake occurring at 1-2 microM-free Ca2+. When fractions were prepared from isolated acini stimulated with 3 microM-carbamylcholine, 45Ca2+ uptake was increased in the rough endoplasmic reticulum. The increased uptake was due to a higher Vmax. with no significant change in Km. No effect was observed on 45Ca2+ uptake by SF1 . In conclusion, two distinct non-mitochondrial, ATP-dependent calcium-uptake systems have been demonstrated in rat pancreatic acini. One of these is located in the rough endoplasmic reticulum, but the precise location of the other has not been determined. We have shown that the Ca2+-transporting activity in the rough endoplasmic reticulum may have an important role in maintaining the cytosolic free Ca2+ concentration in resting acinar cells and is involved in Ca2+ movements which occur during stimulation of enzyme secretion.
Project description:We have studied the effect of ATP on Ca2+ uptake in intestinal brush border membrane vesicles (BBMVs) of the teleost tilapia (Oreochromis mossambicus). ATP stimulated Ca2+ uptake 12-fold over the control, with a linear time course. Ionomycin and detergent treatment did not reduce BBMVs' Ca2+ content, indicating the binding of Ca2+ to a membrane component. A rank order of ATP > ADP > AMP was established for the stimulation of Ca2+ uptake. Adenosine, vanadate, adenosine 5'-[alpha, beta-methylene]triphosphate (a P2x purinoceptor agonist) and adenosine 5'-[gamma-thio]triphosphate (a P-type ATPase inhibitor) were without effect. 2-Methylthioadenosine 5'-triphosphate, a P2y purinoceptor agonist, mimicked the stimulation by ATP. As judged from a kinetic comparison, ATP hydrolysis and the stimulation by ATP of Ca2+ uptake were not compatible. The P2 purinoceptor antagonist suramin and the P2y purinoceptor antagonist Reactive Blue-2 inhibited the Ca2+ uptake stimulated by 1 mM ATP (IC50 0.17 mM and 58 microM respectively). We conclude that ATP-stimulated Ca2+ uptake in tilapia intestine is dissociated from ATPase activity and is mediated through a P2 purinoceptor.
Project description:The effects of myo-inositol 1,4,5-trisphosphate (IP3) on Ca2+ uptake and release from isolated adipocyte endoplasmic reticulum and plasma membrane vesicles were investigated. Effects of IP3 were initially characterized using an endoplasmic reticulum preparation with cytosol present (S1-ER). Maximal and half-maximal effects of IP3 on Ca2+ release from S1-ER vesicles occurred at 20 microM- and 7 microM-IP3, respectively, in the presence of vanadate which prevents the re-uptake of released Ca2+ via the endoplasmic reticulum Ca2+ pump. At saturating IP3 concentrations, Ca2+ release in the presence of vanadate was 20% of the exchangeable Ca2+ pool. IP3-induced release of Ca2+ from S1-ER was dependent on extravesicular free Ca2+ concentration with maximal release occurring at 0.13 microM free Ca2+. At 20 microM-IP3 there was no effect on the initial rate of Ca2+ uptake by S1-ER. IP3 promoted Ca2+ release from isolated endoplasmic reticulum vesicles (cytosol not present) to a similar level as compared with S1-ER. Addition of cytosol to isolated endoplasmic reticulum vesicles did not affect IP3-induced Ca2+ release. The endoplasmic reticulum preparation was further fractionated into heavy and light vesicles by differential centrifugation. Interestingly, the heavy fraction, but not the light fraction, released Ca2+ when challenged with IP3. IP3 (20 microM) did not promote Ca2+ release from plasma membrane vesicles and had no effect on the (Ca2+ + Mg2+)-ATPase activity or on the initial rate of ATP-dependent Ca2+ uptake by these vesicles. These results support the concept that IP3 acts exclusively at the endoplasmic reticulum to promote Ca2+ release.
Project description:Ca2+ + Mg2+-dependent ATPase from sarcoplasmic reticulum was inhibited by preincubation with vanadate. When the inhibited enzyme was preincubated in the presence of vanadate and assayed in its absence, a slow reactivation process was observed. This slow, hysteretic, process was exploited to study the influence of Ca2+ and ATP on the dissociation of vanadate. Ca2+ alone slowly displaced vanadate from the inhibited enzyme, and a rate constant of 0.1 min-1, at 25 degrees C, was calculated for this re-activation process. However, ATP re-activated with an apparent constant that hyperbolically depended on ATP concentration, and from it a rate constant for vanadate dissociation induced by ATP of 0.5 min-1 was calculated. It is deduced from the kinetic studies that ATP binds to the enzyme-vanadate complex, forming a ternary complex, with a dissociation constant of 4 microM, and that this binding accelerates vanadate dissociation. Binding experiments with [14C]ATP showed that ATP binds to the enzyme-vanadate complex with a dissociation constant of 12 microM, i.e. the affinities calculated with the isotope technique and the kinetic procedure are of the same order of magnitude.
Project description:Bovine chromaffin-granule ghosts accumulate 45Ca2+ in a temperature- and osmotic-shock-sensitive process; the uptake is saturable, with Km 38 microM and Vmax. 28 nmol/min per mg at 37 degrees C. Entry occurs by exchange with Ca2+ bound to the inner surface of the membrane. It is inhibited non-competitively by Na+, La3+ and Ruthenium Red (Ki 10.7 mM, 7 microM and 2 microM respectively), and competitively by Mg2+ (ki 0.9 mM). Uptake was not stimulated by ATP. Na+ induces Ca2+ efflux; Ca2+ can re-enter the ghosts by a process of Ca2+/Na+ exchange. La3+ inhibits Ca2+ efflux during Ca2+-exchange, and Ca2+ efflux induced by Na+, suggesting that Ca2+ uptake and efflux, and Ca2+/Na+ exchange, are catalysed by the same protein. Na+ enters ghosts during CA2+ efflux, but the kinetics of its entry are not exactly similar to the kinetics of Ca2+ efflux. Initially 1-2 Na+ enter per Ca2+ lost, but at equilibrium 3-4 Na+ have replaced each Ca2+. There is no evidence that either Ca2+ uptake or efflux by Ca2+/Na+ exchange is electrogenic, suggesting that the stoichiometry of exchange is Ca2+/2Na+. This exchange reaction may have a role in depleting cytoplasmic Ca2+ after depolarization-induced Ca2+ entry through the adrenal medulla plasma membrane; there is some evidence that there may be an additional entry mechanism for Na+ across the granule membrane.
Project description: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.