Glucagon stimulation of ruthenium red-insensitive calcium ion transport in developing rat liver

Evidence for multiple glucuronide transporters in rat liver microsomes

The transport of glucuronides across the endoplasmic reticulum membrane is an important step in the overall process of biotransformation, although the mechanism remains unclear and the participating transporters are unidentified. Using a rapid filtration assay in combination with liquid chromatography-mass spectrometry, we measured the transport of a variety of beta-D-glucuronides in rat liver microsomes and investigated the substrate specificity of the participating transporter(s) by inhibition studies. Time-dependent and bi-directional transport of phenolphthalein glucuronide was detected and the kinetic parameters for transport were determined. The K(m) and V(max) values of high affinity transport were 26microM and 3.9nmol/min/mg protein, respectively. Phenolphthalein glucuronide transport was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and N-ethylmaleimide. Transport inhibition studies revealed competition between three glucuronides: phenolphthalein glucuronide, estradiol 17-glucuronide and naphthol AS-BI glucuronide indicating that they share a common transporter in the endoplasmic reticulum membrane. Their transport was inhibited by phenolphthalein, but was not affected by p-nitrophenyl glucuronide, naphthyl glucuronide or d-glucuronate. Morphine 3-glucuronide transport was not inhibited by any of the latter four compounds or by phenolphthalein glucuronide. This novel experimental approach has produced data consistent with the presence of multiple (at least three) transporters catalyzing the transport of glucuronides through the endoplasmic reticulum membrane. These data also indicate that the size and/or shape of the aglycone rather than the glucuronic acid moiety per se is an important determinant of transporter specificity.

Modulation of the bilayer thickness of exocytic pathway membranes by membrane proteins rather than cholesterol

A biological membrane is conceptualized as a system in which membrane proteins are naturally matched to the equilibrium thickness of the lipid bilayer. Cholesterol, in addition to lipid composition, has been suggested to be a major regulator of bilayer thickness in vivo because measurements in vitro have shown that cholesterol can increase the thickness of simple phospholipid/cholesterol bilayers. Using solution x-ray scattering, we have directly measured the average bilayer thickness of exocytic pathway membranes, which contain increasing amounts of cholesterol. The bilayer thickness of membranes of the endoplasmic reticulum, the Golgi, and the basolateral and apical plasma membranes, purified from rat hepatocytes, were determined to be 37.5 +/- 0.4 A, 39.5 +/- 0.4 A, 35.6 +/- 0.6 A, and 42.5 +/- 0.3 A, respectively. After cholesterol depletion using cyclodextrins, Golgi and apical plasma membranes retained their respective bilayer thicknesses whereas the bilayer thickness of the endoplasmic reticulum and the basolateral plasma membrane decreased by 1.0 A. Because cholesterol was shown to have a marginal effect on the thickness of these membranes, we measured whether membrane proteins could modulate thickness. Protein-depleted membranes demonstrated changes in thickness of up to 5 A, suggesting that (i) membrane proteins rather than cholesterol modulate the average bilayer thickness of eukaryotic cell membranes, and (ii) proteins and lipids are not naturally hydrophobically matched in some biological membranes. A marked effect of membrane proteins on the thickness of Escherichia coli cytoplasmic membranes, which do not contain cholesterol, was also observed, emphasizing the generality of our findings.

Characterization of two Mg2+ transporters in sealed plasma membrane vesicles from rat liver

The plasma membrane of mammalian cells possesses rapid Mg2+ transport mechanisms. The identity of Mg2+ transporters is unknown, and so are their properties. In this study, Mg2+ transporters were characterized using a biochemically and morphologically standardized preparation of sealed rat liver plasma membranes (LPM) whose intravesicular content could be set and controlled. The system has the advantages that it is not regulated by intracellular signaling machinery and that the intravesicular ion milieu can be designed. The results indicate that 1) LPM retain trapped intravesicular total Mg2+ with negligible leak; 2) the addition of Na+ or Ca2+ induces a concentration- and temperature-dependent efflux corresponding to 30-50% of the intravesicular Mg2+; 3) the rate of flux is very rapid (137.6 and 86.8 nmol total Mg2+ . micrometer -2 . min-1 after Na+ and Ca2+ addition, respectively); 4) coaddition of maximal concentrations of Na+ and Ca2+ induces an additive Mg2+ efflux; 5) both Na+- and Ca2+-stimulated Mg2+ effluxes are inhibited by amiloride, imipramine, or quinidine but not by vanadate or Ca2+ channel blockers; 6) extracellular Na+ or Ca2+ can stimulate Mg2+ efflux in the absence of Mg2+ gradients; and 7) Mg2+ uptake occurs in LPM loaded with Na+ but not with Ca2+, thus indicating that Na+/Mg2+ but not Ca2+/Mg2+ exchange is reversible. These data are consistent with the operation of two distinct Mg2+ transport mechanisms and provide new information on rates of Mg2+ transport, specificity of the cotransported ions, and reversibility of the transport.

Effect of the aminosteroid, U73122, on Ca2+ uptake and release properties of rat liver microsomes

The putative phospholipase C inhibitor, U73122, transiently increases the cytosolic free Ca2+ concentration in rabbit pancreatic acinar cells by stimulating the release of Ca2+ from intracellular stores [Willems, Van de Put, Engbersen, Bosch, Van Hoof & De Pont (1994) Pflügers Arch. 427, 233-243]. In order to elucidate the exact mechanism of action of U73122 we studied its effects on both Ca(2+)-stimulated Mg(2+)-dependent ATPase activity and Ca(2+)-stimulated ATP-dependent Ca2+ uptake in rat liver microsomes. In addition, we studied its effects on Ca2+ release from steady-state loaded microsomes. The effects of U73122 were compared with those of thimerosal, described in the literature as inhibiting Ca(2+)-ATPases and sensitizing inositol 1,4,5-trisphosphate-operated Ca2+ release channels, and thapsigargin, a specific inhibitor of sarcoplasmic and endoplasmic reticulum Ca(2+)-ATPases. Both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 11 microM) dose-dependently inhibited Ca(2+)-stimulated Mg(2+)-dependent ATPase activity, without significantly affecting Mg(2+)-stimulated ATPase activity. Similarly, both U73122 (IC50 = 9 microM) and thimerosal (IC50 = 14 microM) dose-dependently inhibited ATP-dependent Ca2+ uptake. At concentrations beyond 20 microM, U73122 stimulated Ca2+ release from steady-state loaded microsomes at a rate considerably higher than obtained with a maximally inhibitory concentration of thapsigargin (1 microM). This observation, which was not reached with equally inhibitory concentrations of thimerosal, demonstrates that higher U73122 concentrations cause an additional increase of passive Ca2+ leak. The data presented demonstrate that U73122 stimulates the release of actively stored Ca2+ primarily through inhibition of the internal Ca2+ pump.

Basal Mg(2+)-dependent ATPase activity of rat liver microsomes is not influenced by ambient free Ca2+

The potent inhibitor of Ca2+, Mg(2+)-ATPase activity, thapsigargin, has been used to investigate the effect of ambient free Ca2+ on basal Mg(2+)-dependent ATPase activity in rat liver microsomes. Thapsigargin non-competitively inhibited both Ca(2+)-stimulated ATP-dependent Ca2+ uptake and Ca(2+)-stimulated Mg(2+)-dependent ATPase activity. At a concentration of 1 microM, thapsigargin completely inhibited the Ca(2+)-pump activity, without affecting Mg(2+)-dependent ATPase activity measured in the absence of Ca2+. Increasing the ambient free Ca2+ concentration did not alter the basal Mg(2+)-dependent ATPase activity. The data presented indicate that ATPase activity measured in the absence of Ca2+ is a reliable measure for the basal Mg(2+)-dependent ATPase activity and that, consequently, the Ca(2+)-stimulated Mg(2+)-dependent ATPase activity can indeed be defined as the difference between the ATPase activity measured in the presence and the absence of Ca2+.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Interaction of ruthenium red with Ca2(+)-binding proteins

The interaction of ruthenium red, [(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]Cl6.4H2O, with various Ca2(+)-binding proteins was studied. Ruthenium red inhibited Ca2+ binding to the sarcoplasmic reticulum protein, calsequestrin, immobilized on Sepharose 4B. Furthermore, ruthenium red bound to calsequestrin with high affinity (Kd = 0.7 microM; Bmax = 218 nmol/mg protein). The dye stained calsequestrin in sodium dodecyl sulfate-polyacrylamide gels or on nitrocellulose paper and was displaced by Ca2+ (Ki = 1.4 mM). The specificity of ruthenium red staining of several Ca2(+)-binding proteins was investigated by comparison with two other detection methods, 45Ca2+ autoradiography and the Stains-all reaction. Ruthenium red bound to the same proteins detected by the 45Ca2+ overlay technique. Ruthenium red stained both the erythrocyte Band 3 anion transporter and the Ca2(+)-ATPase of skeletal muscle sarcoplasmic reticulum. Ruthenium red also stained the EF hand conformation Ca2(+)-binding proteins, calmodulin, troponin C, and S-100. This inorganic dye provides a simple, rapid method for detecting various types of Ca2(+)-binding proteins following electrophoresis.

Characterization, localization and pharmacological profile of a high-affinity [3H]lidocaine binding site

Electrophysiological findings support the existence of voltage-dependent, sodium channel-associated receptors for class I antiarrhythmics. We have tried to identify such receptors with tritiated lidocaine. High-affinity binding sites were discovered in heart and brain membranes, but liver and kidney particulate fractions had the highest density of sites. The dissociation constants were 75 nM in bovine heart and 29 nM in guinea-pig liver membranes. Binding was reversible (t 1/2: 102 s at 2 degrees C), optimal at pH 9-10 and was only partly destroyed by heat treatment. Subcellular fractionation experiments excluded a plasmalemmal association of the lidocaine site in heart. The competition profile of 16 antiarrhythmics indicated chemical comparability of the sites in heart and liver. These data greatly challenge the applicability of labeled lidocaine as sodium channel probe. The pharmacological significance of the site described here remains to be clarified.

Passive calcium influx by plasma membrane vesicles isolated from rat liver

Passive Ca2+ influx independent of ATP addition to the incubation medium, took place in plasma membrane vesicles isolated from rat liver. The rate of Ca2+ influx was found to depend on the concentration of added Ca2+, and on the incubation temperature, and was inhibited by La3+, Hg2+ and by p-chloromercuribenzoate. Influx was not blocked by calcium channel blockers, or affected by a range of uncouplers. Addition of the Ca2+ ionophore A23187 to vesicles that had taken up the ion induced a rapid efflux of Ca2+ especially when EGTA also was added to the incubation medium. A number of divalent cations inhibited Ca2+ influx. The vesicles could be frozen and stored overnight with little loss in activity. The kinetics of Ca2+ influx could be related to that which occurs in the unstimulated perfused rat liver. The data suggest that the plasma membrane vesicle preparation may be useful for further studies on the basal liver cell Ca2+ influx system in vitro.

Ca2+ transport by rat liver plasma membranes: the transporter and the previously reported Ca2+-ATPase are different enzymes

A rat liver plasma membrane fraction showed an ATP-dependent uptake of Ca2+ which was released by the ionophore A23187. This activity represents a plasma membrane component and is not due to microsomal contamination. The Ca2+ transport displayed several properties which were different from those of the high-affinity Ca2+-ATPase previously observed in these membranes (Lotersztajn et al. (1981) J. Biol. Chem. 256, 11209-11215; Birch-Machin, M.A. and Dawson, A.P. (1986) Biochim. Biophys. Acta 855, 277-285). These observations have shown that Ca2+-ATPase does not require added Mg2+ whereas we have demonstrated that, in the same membrane preparation, Ca2+ uptake required millimolar concentrations of added Mg2+. The Ca2+-ATPase has a broad specificity for the nucleotides ATP, GTP, UTP and ITP while Ca2+ uptake remains specific for ATP. Ca2+ uptake also displayed different affinities for free Ca2+ and MgATP compared to Ca2+-ATPase activity, with apparent Km values of 0.25 microM Ca2+, 0.15 mM MgATP and 1.0 microM Ca2+, 4 microM MgATP respectively. The apparent maximum rate of Ca2+ uptake was about 150-fold less than Ca2+-ATPase activity. These features suggest that the high-affinity Ca2+-ATPase is not the enzymic expression of the ATP-dependent Ca2+ transport mechanism.

A novel 1,4-dihydropyridine-binding site on mitochondrial membranes from guinea-pig heart, liver and kidney

The 1,4-dihydropyridine (+/-)-[3H]nitrendipine reversibly binds to mitochondrial preparations from guinea-pig heart with a dissociation constant (Kd) of 593 +/- 77 nM and a maximum density of binding sites (Bmax.) of 1.75 +/- 0.27 nmol/mg of protein. This low-affinity high-capacity 1,4-dihydropyridine-binding site does not discriminate between the enantiomers of nitrendipine and is also found in mitochondrial membranes from guinea-pig liver (Kd 586 +/- 91 nM; Bmax. 0.36 +/- 0.04 nmol/mg of protein) and kidney (Kd 657 +/- 149 nM; Bmax. 0.56 +/- 0.12 nmol/mg of protein). Phenylalkylamines (e.g. verapamil) inhibit ( +/- )-[3H]nitrendipine binding with micromolar inhibition constants, but the benzothiazepine D-cis-diltiazem, a potent Ca2+-channel blocker, is without effect. The binding is heat-stable, shows a V-shaped pH-dependence with a minimum around pH 7.0, and is strongly dependent on ionic strength in the incubation medium. The cations La3+ greater than Cd2+ much greater than Co2+ greater than Ca2+ much greater than Ba2+ greater than Mg2+ greater than Li+ greater than Na+ and the anions NO3- greater than C1- greater than or equal to F- stimulate the binding, whereas PO4(3-) greater than SO4(2-) slightly inhibit it. The low-affinity ( +/- )-[3H]nitrendipine-binding site located on the mitochondrial inner membrane is biochemically and pharmacologically different from the 1,4-dihydropyridine-receptor domain of the L-type Ca2+ channel. Furthermore, it is not identical with any of the low-affinity 1,4-dihydropyridine-binding sites described so far.

The effect of GTP on inositol 1,4,5-trisphosphate-stimulated Ca2+ efflux from a rat liver microsomal fraction. Is a GTP-dependent protein phosphorylation involved?

GTP, when added to a rat liver microsomal fraction that had previously been allowed to accumulate Ca2+, causes a slow release of Ca2+, which is greatly enhanced by addition of inositol trisphosphate (IP3). The Ca2+ release caused by IP3 under these conditions is very much greater than that observed in the absence of GTP. The effect of GTP is dependent on the presence of polyethylene glycol in the incubation medium and is not due to inhibition of the Ca2+-accumulation system. The response to GTP is time-dependent, particularly at low (4 microM) GTP concentrations, and cannot be mimicked by ATP, ITP, CTP, UTP and GDP. Studies with [gamma-32P]GTP show that, during incubation with microsomal fractions, the terminal phosphate of GTP is transferred to two protein species, of Mr 38 000 and 17 000. These protein phosphorylations are still present when an excess of unlabelled ATP is included in the incubation mixture, but appear to be unaffected by the presence or absence of IP3 and polyethylene glycol. As a working hypothesis, it is suggested that a protein, phosphorylated by GTP, has to bind to the microsomal membranes before IP3 can stimulate Ca2+ release, and that, in vitro, the binding of this protein is favoured by the presence of polyethylene glycol.

Calcium sequestration activity in rat liver microsomes. Evidence for a cooperation of calcium transport with glucose-6-phosphatase

Mechanisms regulating the energy-dependent calcium sequestering activity of liver microsomes were studied. The possibility for a physiologic mechanism capable of entrapping the transported Ca2+ was investigated. It was found that the addition of glucose 6-phosphate to the incubation system for MgATP-dependent microsomal calcium transport results in a marked stimulation of Ca2+ uptake. The uptake at 30 min is about 50% of that obtained with oxalate when the incubation is carried out at pH 6.8, which is the pH optimum for oxalate-stimulated calcium uptake. However, at physiological pH values (7.2-7.4), the glucose 6-phosphate-stimulated calcium uptake is maximal and equals that obtained with oxalate at pH 6.8. The Vmax of the glucose 6-phosphate-stimulated transport is 22.3 nmol of calcium/mg protein per min. The apparent Km for calcium calculated from total calcium concentrations is 31.9 microM. After the incubation of the system for MgATP-dependent microsomal calcium transport in the presence of glucose 6-phosphate, inorganic phosphorus and calcium are found in equal concentrations, on a molar base, in the recovered microsomal fraction. In the system for the glucose 6-phosphate-stimulated calcium uptake, glucose 6-phosphate is actively hydrolyzed by the glucose-6-phosphatase activity of liver microsomes. The latter activity is not influenced by concomitant calcium uptake. Calcium uptake is maximal when the concentration of glucose 6-phosphate in the system is 1-3 mM, which is much lower than that necessary to saturate glucose-6-phosphatase. These results are interpreted in the light of a possible cooperative activity between the energy-dependent calcium pump of liver microsomes and the glucose-6-phosphatase multicomponent system. The physiological implications of such a cooperation are discussed.

GTP enhances inositol trisphosphate-stimulated Ca2+ release from rat liver microsomes

Low concentrations of GTP (10-50 microM) greatly enhance the inositol 1,4,5-trisphosphate stimulated Ca2+ release from rat liver microsomal vesicles. The effect of GTP depends on the presence of low concentrations of polyethylene glycol in the incubation medium. Guanylyl imidodiphosphate is ineffective at mimicking the GTP effect and inhibits the action of GTP added subsequently.

A procedure for the rapid isolation from rat liver of plasma membrane vesicles exhibiting Ca2+-transport and Ca2+-ATPase activities

A technique is described for the isolation of a plasma membrane-enriched preparation from a rat liver post-mitochondrial fraction by using discontinuous Percoll density-gradient centrifugation. The procedure is simple, of high reproducibility and yield and requires a total isolation time of only 90 min. The preparation consists almost exclusively of membrane vesicles and is enriched approx. 26-fold in plasma membrane-localized enzymes with minor contamination (less than 10%) with membranes derived mainly from the endoplasmic reticulum and Golgi apparatus. Approx. 20% of the fraction comprises tightly-sealed vesicles in the inverted orientation which are capable of accumulating calcium ions and exhibiting vanadate-insensitive Ca2+-ATPase activity. The properties of these activities, including insensitivity to vanadate, oxalate, and to p-chloromercuribenzoate as well as a lack of requirement for added Mg2+, contrast markedly with the reported properties of Ca2+ transport by the endoplasmic reticulum isolated from rat liver. The technique may have wide application in the study of plasma membrane-associated activities in rat liver, particularly in relation to sinusoidal membrane surface-related events.

Effect of in vivo changes in phospholipid composition on liver microsomal calcium transport

We recently reported that the phospholipid composition of mouse liver microsomes could be altered in vivo by a combination of dietary choline deprivation and administration of the methylation inhibitors periodate-oxidized adenosine and cycloleucine (D.M. Boyle & W.L. Dean (1982) Biochim. Biophys. Acta 688, 667-670). We have now determined the effect of this in vivo change in phospholipid composition on 7 microsomal enzyme activities and 2 cytochromes. The specific contents of cytochromes b5 and P-450 were unaffected by the treatment. Similarly, NADH-cytochrome c reductase, cytochrome P-450 reductase, cyclohexane hydroxylase and Mg2+-ATPase were not significantly altered. In addition, the phospholipid/protein ratio was not changed. In contrast, Ca2+-ATPase and Ca2+ transport rates were reduced by more than 60%. This result suggests that the mouse liver microsomal Ca2+-ATPase is extremely sensitive to the phospholipid composition of the membrane in which it is embedded and that one mode of control of calcium metabolism in liver cells could be at the level of membrane phospholipid composition.

Inositol (1,4,5)trisphosphate-promoted Ca2+ release from microsomal fractions of rat liver

Crude mitochondrial fractions containing a substantial amount of microsomes accumulate Ca2+ in the presence of ATP, ruthenium red and oligomycin. A proportion of this accumulated Ca2+ is released by the addition of low concentrations (ca. 1 microM) of inositol (1,4,5) trisphosphate . Under some conditions the release is transient, and evidence is presented which suggests that this is due to inhomogeneity in the vesicle population. (1,4,5)inositol trisphosphate -induced Ca2+ release can also be demonstrated, under appropriate experimental conditions, in a more purified microsomal fraction essentially free of mitochondria.

45Ca2+ uptake and phospholipid methylation in isolated rat liver microsomes

The effects of glucagon, epinephrine and insulin on hepatic phospholipid methylation were studied. Glucagon, either injected into rats or added to perfused livers, stimulated methylation in subsequently isolated microsomes. Epinephrine also increased phospholipid methylation. Insulin by itself did not influence the rate of the reaction, but, when administered prior to glucagon, it blocked the effect of the latter. The possibility that the observed stimulation of phospholipid methylation might be causally linked to the reported stimulation by glucagon of 45Ca2+ uptake in subsequently isolated liver microsomes was examined. Both the substrate and the competitive inhibitor of the methylation reaction, S-adenosylmethionine and S-adenosylhomocysteine, had profound effect on the rate of phospholipid methylation, without having comparable effects on Ca2+ uptake. S-adenosylmethionine in increasing concentration stimulated methylation four-fold, while no significant changes in 45Ca2+ uptake were seen. S-adenosylhomocysteine did not inhibit 45Ca2+ uptake even at levels causing more than 95% decrease in methylation. In conclusion, while both phospholipid methylation and 45Ca2+ uptake seem to be hormonally controlled, the correlation between these two processes was not sufficient to support the notion that the changes in 45Ca2+ uptake are caused by the changes in phospholipid methylation.

Some properties of the Ca2+-stimulated ATPase of a rat liver microsomal fraction

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.

Calcium ion fluxes induced by the action of alpha-adrenergic agonists in perfused rat liver

Phenylephrine (2.0 microM) induces an alpha 1-receptor-mediated net efflux of Ca2+ from livers of fed rats perfused with medium containing physiological concentrations (1.3 mM) of Ca2+. The onset of efflux (7.1 +/- 0.5 s; n = 16) immediately precedes a stimulation of mitochondrial respiration and glycogenolysis. Maximal rates of efflux are observed between 35 s and 45 s after alpha-agonist administration; thereafter the rate decreases, to be no longer detectable after 3 min. Within seconds of terminating phenylephrine infusion, a net transient uptake of Ca2+ by the liver is observed. Similar effects were observed with vasopressin (1 m-unit/ml) and angiotensin (6 nM). Reducing the perfusate [Ca2+] from 1.3 mM to 10 microM had little effect on alpha-agonist-induced Ca2+ efflux, but abolished the subsequent Ca2+ re-uptake, and hence led to a net loss of 80-120 nmol of Ca2+/g of liver from the tissue. The administration at 5 min intervals of short pulses (90 s) of phenylephrine under these conditions resulted in diminishing amounts of Ca2+ efflux being detected, and these could be correlated with decreased rates of alpha-agonist-induced mitochondrial respiration and glucose output. An examination of the Ca2+ pool mobilized by alpha-adrenergic agonists revealed that a loss of Ca2+ from mitochondria and from a fraction enriched in microsomes accounts for all the Ca2+ efflux detected. It is proposed that the alpha-adrenergic agonists, vasopressin and angiotensin mobilize Ca2+ from the same readily depleted intracellular pool consisting predominantly of mitochondria and the endoplasmic reticulum, and that the hormone-induced enhanced rate of mitochondrial respiration and glycogenolysis is directly dependent on this mobilization.

Kinetic properties of the Ca2+-accumulation system of a rat liver microsomal fraction

1. By using Ca-EGTA buffers, the Km for Ca2+ uptake into rat liver heavy microsomes (microsomal fraction) was found to be 0.2 microM free Ca2+. 2. In the absence of oxalate, these vesicles accumulate about 20 nmol of Ca2+/mg of protein. Efflux of Ca2+ from the vesicles is much faster at pH 7.6 than at pH 6.8, but does not apparently show saturation kinetics or any stringent requirement for external ions. 3. The steady-state distribution of Ca2+ between the microsomes and the medium in the presence of ATP and the absence of oxalate is dependent on Ca2+ load. When the vesicles are loaded to 50% capacity, the external free Ca2+ concentration is 70 nM. 4. The affinity of heavy microsomes for Ca2+ is such that is seems likely that they has a dominant role in the determination of cytoplasmic free Ca2+ concentrations.

Ruthenium red-insensitive calcium transport in ascites-sarcoma 180/TG cells

1. Ruthenium Red-insensitive Ca2+ transport in the mouse ascites sarcoma 180/TG is enriched in a 'heavy' microsomal fraction (microsomes) sedimented at 35 000 g for 20 min. The subcellular distribution of this Ca2+ transport differed from that of Ruthenium Red-sensitive Ca2+ transport and (Na+ + K+)-dependent ATPase activity, but was similar to that of glucose 6-phosphatase. 2. The affinity of this transport system for 'free' Ca2+ is high (Km approx. 6 microM) and that for MgATP somewhat lower (Km approx. 100 microM). Ca2+ transport by the tumour microsomes, by contrast with that by liver microsomes, was greatly stimulated by low concentrations of P1. 3. Although incubation of intact ascites cells with glucagon led to an increase in intracellular cyclic AMP, no stable increase in the initial rate of Ca2+ transport in the subsequently isolated 'heavy' microsomes could be detected as in similar experiments carried out previously with rat liver cells. Reconstitution experiments suggest that a deficiency exists in the tumour microsomal membrane such that an action of glucagon that is normally present in rat liver microsomes is not evoked.

Maturation in liver mitochondria of Ruthenium Red-sensitive calcium-ion-transport activity and the influence of glucagon administration in vivo and in utero

The maturation of Ca(2+) transport in mitochondria isolated from rat liver was examined, from 5 days before birth. The mitochondria used were isolated from liver homogenates by centrifugation at 22000g-min. Ca(2+) transport by mitochondria isolated from foetal liver is energy-dependent and Ruthenium Red-sensitive. The transmembrane pH gradient in these mitochondria is higher by about 7mV and the membrane potential lower by about 20mV than in adult mitochondria. The inclusion of 2mm-P(i) in the incubation medium enhances the protonmotive force by approx. 30mV. The rate of Ca(2+) influx in foetal mitochondria measured in buffered KCl plus succinate is low until about 2-3h after birth, when it increases to about 60% of adult values; approx. 24h later it has reached near-adult values. Higher rates of Ca(2+) influx are observed in the presence of 2mm-P(i); 3-5 days before birth the rates are about one-third of adult values and decline slightly as birth approaches. By 2-3h post partum they have reached adult values. The inclusion of 12.5mum-MgATP with the P(i) stimulates further the initial rate of Ca(2+) influx in foetal mitochondria. The rates observed are constant over the prenatal period examined and are 50-60% of those observed in adult mitochondria. Mitochondria isolated from foetal livers 4-5 days before birth retain the accumulated Ca(2+) for about 50min in the presence of 2mm-P(i). In the period 2 days before birth to birth, this ability is largely lost, but by 2-3h after birth Ca(2+) retention is similar to that of adult mitochondria. The presence of 12.5mum-MgATP progressively enhances the Ca(2+) retention time as development proceeds until 2-3h after birth, when it becomes less sensitive to added MgATP. Glucagon administration to older foetuses in utero enhances both the rate of mitochondrial Ca(2+) influx assayed in the presence of 2mm-P(i) and the time for which mitochondria retain accumulated Ca(2+) in the presence of 12.5mum-MgATP and 2mm-P(i). Its administration to neonatal animals leads to an increase in mitochondrial Ca(2+) retention similar to that seen in adult mitochondria. The data provide evidence that the Ruthenium Red-sensitive Ca(2+) transporter is potentially as active in foetal mitochondria 5 days before birth as it is in adult mitochondria. They also show that foetal mitochondria have an ability to retain accumulated Ca(2+) reminiscent of mitochondria from tumour cells and from hormone-challenged rat liver.