The effect of Mg2+ on hepatic microsomal Ca2+ and Sr2+ transport

Toxicity assessment of 3-O-[6-deoxy-3-O-methyl-β-D-allopyranosyl-(1 → 4)-β-D-oleandropyranosyl]-17β-marsdenin isolated from Gongronema latifolium leaf on selected brain and kidney function indices in mice

The safety of bioactive compounds, especially those isolated from medicinal plants, is a major concern for health authorities, pharmaceutical industries, and the public. Of recent, anti-tumor pregnane glycosides were isolated from Gongronema latifolium leaf, of which the toxicity of one, 3-O-[6-deoxy-3-O-methyl-β-D-allopyranosyl-(1 → 4)-β-D-oleandropyranosyl]-17β-marsdenin (3DMAOM), has not been evaluated. This study, therefore, evaluated the effects of 3DMAOM on selected brain and kidney function indices in mice. Female Swiss albino mice were randomly administered 5% dimethyl sulphoxide and different doses of 3DMAOM (0.5, 1, 2, and 4 mg/kg body weight) for fourteen (14) days, and their blood, brains, and kidneys were collected for biochemical analysis. There was no significant alteration in the activities of alkaline phosphatase (ALP), acetylcholinesterase, creatine kinase, Na+/K+-ATPase, Ca2+/Mg2+-ATPase, and Mg2+-ATPase in the brain of the treated groups compared to control. Also, no significant changes in the activities of ALP, gamma-glutamyltransferase, Na+/K+-ATPase, Ca2+/Mg2+-ATPase, and Mg2+-ATPase in the kidney of the treated groups compared to control. The plasma concentrations of Na+, K+, Cl-, PO43-, creatinine, and urea of mice were not significantly altered at all doses of the 3DMAOM compared to controls. However, the plasma concentration of Ca2+ was significantly reduced (p < 0.05) at all doses of the 3DMAOM, and the plasma concentration of uric acid was significantly reduced (p < 0.05) at 2 mg/kg body weight of the 3DMAOM compared to controls. These findings suggest that 3DMAOM isolated from Gongronema latifolium leaf may not adversely affect brain function but may affect calcium ion homeostasis in subjects.

Ca2+ release triggered by NAADP in hepatocyte microsomes

NAADP (nicotinic acid-adenine dinucleotide phosphate) is fast emerging as a new intracellular Ca2+-mobilizing messenger. NAADP induces Ca2+ release by a mechanism that is distinct from IP3 (inositol 1,4,5-trisphosphate)- and cADPR (cADP-ribose)-induced Ca2+ release. In the present study, we demonstrated that micromolar concentrations of NAADP trigger Ca2+ release from rat hepatocyte microsomes. Cross-desensitization to IP3 and cADPR by NAADP did not occur in liver microsomes. We report that non-activating concentrations of NAADP can fully inactivate the NAADP-sensitive Ca2+-release mechanism in hepatocyte microsomes. The ability of thapsigargin to block the NAADP-sensitive Ca2+ release is not observed in sea-urchin eggs or in intact mammalian cells. In contrast with the Ca2+ release induced by IP3 and cADPR, the Ca2+ release induced by NAADP was completely independent of the free extravesicular Ca2+ concentration and pH (in the range 6.4-7.8). The NAADP-elicited Ca2+ release cannot be blocked by the inhibitors of the IP3 receptors and the ryanodine receptor. On the other hand, verapamil and diltiazem do inhibit the NAADP- (but not IP3- or cADPR-) induced Ca2+ release.

The specific binding site of 9-[3H]methyl-7-bromoeudistomin D, a caffeine-like Ca2+ releaser, in liver microsomes in distinct from that in skeletal sarcoplasmic reticulum

3H-labeled 9-methyl-7-bromoeudistomin D ([3H]MBED), a powerful caffeine-like Ca2+ releaser, binds to the caffeine binding site of terminal cisternae (TC) of skeletal muscle sarcoplasmic reticulum (SR) (Fang, Y-I., Adachi, M., Kobayashi, J., and Ohizumi, Y. (1993). J. Biol. Chem. 268, 18622-18625.) and activates Ca(2+)-induced Ca2+ release (CICR). [3H]MBED, however, bound to rabbit hepatic microsomes with a comparable affinity (Kd = 50 nM) and with a more than 30-fold greater receptor density (Bmax = 350 pmol/mg of protein), compared with those in SR. Caffeine (0.1-10 mM) caused a concentration dependent inhibition of [3H]MBED binding to hepatic microsomes with the IC50 value of 0.3 mM. The mode of inhibition by caffeine was allosteric, indicating that the binding site of the ligand is distinct from but related to that of caffeine. Procaine (1-10 mM), a representative inhibitor of CICR, which suppresses [3H]MBED binding to TC-SR, inhibited ligand binding to hepatic microsomes only slightly. Moreover, ligand binding to the hepatic binding site was not affected by adenosine 5'-(beta, gamma-methylene) triphosphate (AMP-PCP) (10-100 microM), which is an activator of CICR and potentiates [3H]MBED binding to TC-SR. Inhibitors of [3H]MBED binding to liver microsomes other than caffeine were nucleotides such as ADP, ATP, GTP, UTP (1 mM), while CTP, cAMP, AMP, adenosine (1 mM), ryanodine (0.1-100 mM) and inositol 1,4,5-trisphosphate (1 microM) were not effective. These features of the hepatic microsomal [3H]MBED binding site distinguish it from that of skeletal muscle SR. [3H]MBED, which binds to the different sites which are both sensitive to caffeine, is useful as a probe to investigate the actions of caffeine at the molecular level.

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 transport and calcium activated ATPase activity in microsomal vesicles of rat gastric mucosa

1. Microsomal and plasma membrane vesicles, isolated from rat gastric mucosa, were found to exhibit Ca(2+)-dependent ATPase activities of 14.1 +/- 1.4 and 7.8 +/- 1.1 mumol/mg/hr, respectively. The optimum conditions for the microsomal Ca(2+)-ATPase was pH 6-7, and required Mg2+, while divalent cation such as Cu2+, Zn2+, Fe2+, Ba2+ and Cd2+ had no significant effect. 2. As in the case of Ca2+, Mg(2+)-ATPase, the Ca2+ uptake activity of the microsomal membrane required Mg2+. Both processes were stimulated by submicro molar concentrations of Ca2+ and the apparent Km for Ca2+, Mg2+ ATPase and Ca2+ uptake activities were 0.06 microM and 0.02 microM, respectively. 3. Divalent cations Ba2+ and Fe2+, inhibited both microsomal activities, while Zn2+ and Cd2+ showed no effect on them. However, the monovalent cation K+ did not stimulate Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities. 4. The Ca2+ pumping ATPase of rat gastric mucosal microsome cross-reacted with a monoclonal antibody (mAb-5F10) against the human erythrocyte Ca2+ pump. The apparent molecular weight of mucosal Ca2+ pump was 98 kDa. 5. Close relationship between the kinetic parameters of Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities, and the cross reaction of 98 kDa protein of mucosal microsome with erythrocyte Ca2+ pump antibody, strongly suggest the expression of Ca2+ pump in rat gastric mucosa.

Characterization of high-affinity ryanodine-binding sites of rat liver endoplasmic reticulum. Differences between liver and skeletal muscle

In this study, the binding of [3H]ryanodine to liver microsomal subfractions was investigated. The specific binding of [3H]ryanodine, as determined both by vacuum filtration and by ultracentrifugation, is to a single class of high-affinity binding sites with a Kd of 10 +/- 2.5 nM and density of 500 +/- 100 and 1200 +/- 200 fmol/mg of protein by the filtration and centrifugation methods respectively. [3H]Ryanodine binding reached equilibrium in about 1 min and 2 min at 36 degrees C and 24 degrees C respectively, and the half-time of dissociation at 37 degrees C was approx. 15 s. The binding of [3H]ryanodine is Ca(2+)-independent: it is slightly stimulated by NaCl, Mg2+, ATP and InsP3 but strongly inhibited by caffeine, diltiazem and sodium dantrolene. Thus the binding of ryanodine to endoplasmic reticulum membranes shares some of the characteristics of its binding to the sarcoplasmic reticulum but also differs from it in several important properties, such as its Ca(2+)-independence, its rapid association and dissociation, and its inhibition by caffeine. The structural similarities between the skeletal muscle and liver binding sites were further explored by employing in vitro DNA amplification techniques, using the known sequence of the skeletal muscle receptor as reference point. The data obtained with this method indicate that the liver does not process mRNA for the skeletal muscle ryanodine receptor.

Distinct ryanodine- and inositol 1,4,5-trisphosphate-binding sites in hepatic microsomes

A light hepatic microsomal preparation was fractionated by sucrose-density centrifugation into one rough, one intermediate and two smooth fractions. The four fractions were characterized with respect to parameters relevant to Ca2+ sequestration. Ca2(+)-ATPase activity was similar in the rough, intermediate and smooth I fractions, but lower in the smooth II fraction. Ca2+ accumulation was the highest in the smooth I and intermediate fractions. On the other hand, Ca2+ efflux from the rough fraction was several-fold faster than from the smooth I fraction. All four subfractions exhibited specific binding sites for inositol 1,4,5-trisphosphate (IP3) and ryanodine; however, the receptors were especially enriched in the smooth I fraction. The total binding sites for ryanodine in that fraction exceeded the number of binding sites for IP3 by about 10-fold. The two receptors responded differently to pharmacological agents; caffeine and dantrolene strongly inhibited ryanodine binding but not IP3 binding, whereas heparin inhibited IP3 binding only. Thus the two receptors are distinct entities. The four fractions also showed distinct gel electrophoretic patterns. The use of two different SDS/polyacrylamide-gel gradients and two protein-staining methods revealed major differences in the distribution of the bands corresponding to Mr values of (x 10(-3) 380, 320, 260, 170, 90, 29 and 21. These proteins were enriched in the smooth fraction. The results indicate that the smooth I fraction might have special importance in stimulus-evoked Ca2(+)-release processes.

The effects of Mg2+ on rat liver microsomal Ca2+ sequestration

The effects of Mg2+ on the hepatic microsomal Ca2(+)-sequestering system was tested. Ca2(+)-ATPase activity and Ca2+ uptake were both dependent on the concentration of free Mg2+, reaching maximum levels at 2 mM. The effects of Mg-ATP were also influenced by the concentration of free Mg2+, being maximally effective at a ratio of 1:1. The results suggest that Mg2+ influences Ca2+ sequestration at various steps, namely in addition to forming the substrate of the Ca2(+)-ATPase reaction, Mg-ATP, Mg2+ stimulates the reaction at an additional step, as indicated by its stimulatory effect on the Ca2(+)-ATPase reaction and on Ca2+ uptake, even at optimal Mg-ATP levels. The stimulatory effect of Mg2+ was evident at various pH levels tested, and it was nucleotide specific. The stimulatory effect of Mg2+ might be exerted at the dephosphorylation step of the enzymatic reaction or at an other, yet undefined, site. The results demonstrate a plural effect of Mg2+ on the hepatic microsomal sequestration system. This indicates that, depending on its magnitude, changes in Mg2+ distribution might influence cytosolic Ca2+ levels.

High affinity ryanodine binding sites in rat liver endoplasmic reticulum

The binding of [3H]ryanodine to liver microsomal subfractions was investigated. The smooth microsomal membranes were enriched with ryanodine binding sites and also with a polypeptide of 360 kDa. Caffeine completely inhibited [3H]ryanodine binding. Ryanodine also affected the membrane Ca2+ permeability. At low concentrations (less than 10 microM) ryanodine stimulated Ca2+ efflux and at higher concentrations (greater than 50 microM) it blocked Ca2+ efflux. These results suggest that hepatic microsomes contain ryanodine binding sites which can modify the membrane permeability for Ca2+.

MgATP-dependent, glucose 6-phosphate-stimulated liver microsomal Ca2+ accumulation: difference between rough and smooth microsomes

Some features of the MgATP-dependent Ca2+-accumulating capacity of rough as compared to smooth liver microsomal fraction were studied. Smooth microsomes accumulate somewhat higher amounts of Ca2+ than rough ones in the presence of MgATP. In the presence of glucose 6-phosphate, which markedly stimulates MgATP-dependent Ca2+ accumulation in both fractions, smooth microsomes exhibit a much higher Ca2+-accumulating capacity than rough ones. Possible reasons of the differences observed between the two fractions were investigated. Smooth microsomes exhibit a higher Ca2+-dependent ATPase activity, suggesting a higher Ca2+ inward transport into smooth vesicles. Also, following the inhibition of active Ca2+ transport by means of vanadate, smooth microsomes appear to release the Ca2+ previously accumulated--both in the absence (i.e., with MgATP only) and in the presence of glucose 6-phosphate--at a lower rate than rough ones. This indicates a lower passive backflux of Ca2+ accumulated in smooth vesicles. On the basis of these data, differences can be envisaged with respect to cellular Ca2+ handling by different domains of endoplasmic reticulum in the liver.

Characterization of the Mg2+-activated ATPase activity in smooth-muscle membranes. NADH oxidase and adenylate kinase interfere with the NADH-coupled enzyme assay

The apparent Mg2+-activated ATPase activity measured by the continuous NADH-coupled enzyme assay was studied in a number of microsomal preparations obtained from smooth muscle of the myometrium from pregnant or 17 beta-oestradiol-pretreated rats, the bovine aorta, the guinea-pig taenia coli, the rabbit ear artery and pig antrum. It was shown that this ATPase assay is prone to the effects of a number of artefacts that are tissue-dependent. The apparent Mg2+-ATPase activity in microsomes (microsomal fractions) from myometrium, aorta and taenia coli declines non-linearly during the assay. Its initial high rate gradually diminishes over 15-60 min, depending on the type of smooth muscle, to a constant value. This decline depends on the presence of ATP and can be partially prevented by concanavalin A. The non-linearity is limited in microsomes from rabbit ear artery. In microsomes from antrum the apparent Mg2+-ATPase activity actually increases with time, albeit gradually. Storage on ice of the microsomes of the aorta, and especially of myometrium of pregnant rats and of taenia coli, is accompanied over a few hours after their preparation by a gradual suppression of the component of the Mg2+-ATPase activity that is inhibited by ATP. The Mg2+-ATPase activity in microsomes from antrum remains constant. NADH oxidase activity accounts for 10% of the Mg2+-ATPase activity in microsomes from stomach smooth muscle. The apparent initial non-linearity of the Mg2+-ATPase activity in that tissue is due to a time-dependent decrease of a rotenone-sensitive NADH oxidase activity. The adenylate kinase activity, as deduced from the effect of the adenylate kinase inhibitor P1,P5-di(adenosine-5') pentaphosphate, could account for 45.0, 35.0 and 31.0% respectively of the Mg2+-ATPase activity in microsomes from stomach, myometrium and aorta. No adenylate kinase activity could be detected in microsomes from ear artery and taenia coli. When microsomes from stomach smooth muscle were separated on a sucrose gradient, the contribution of adenylate kinase and NADH oxidase to the Mg2+-ATPase activity was most pronounced in the higher-density fractions. Part of the NADH oxidase activity and of the Mg2+-ATPase activity, and most of the adenylate kinase activity, are not sedimented at 224000 gmax. for 30 min and may therefore be present as soluble enzymes.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitor of anion transport, DIDS, releases Ca2+ from hepatic microsomes

Addition of 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) to Ca2+ loaded hepatic microsomal vesicles evoked a dose-dependent release of the accumulated Ca2+. Ca2+ uptake was also inhibited. The effects of DIDS do not seem to be due to the inhibitions of either Cl- or proton fluxes. The results indicate that DIDS inhibits Ca2+ uptake and releases Ca2+ by inhibiting the Ca2+-ATPase and the formation of the phosphorylated intermediate of the enzyme, and that it might interact with a specific site on the vesicle which is involved in the translocation of Ca2+ across the microsomal and mitochondrial membranes.

Inhibition of rat liver microsomal Ca2+-ATPase by fluorescein-5'-isothiocyanate

Rat liver microsomal fraction was incubated at pH 8.8 with fluorescein-5'-isothiocyanate in a Tris-buffered sucrose medium. This treatment completely inhibited ATP-dependent Ca2+ transport, Ca2+-ATPase activity, and Ca2+-ATPase phosphoenzyme intermediate formation. Inhibition of Ca2+ transport and phosphoenzyme intermediate formation by fluorescein-5'-isothiocyanate was partially prevented by including ATP in the treatment medium. These data taken together are consistent with the proposal that fluorescein-5'-isothiocyanate binds the Ca2+-ATPase ATP-binding site, suggesting the presence of a lysine residue in this domain. Fluorescein-5'-isothiocyanate labeling of microsomal proteins had no measurable effect on the basal, Mg2+-ATPase activity. Using fluorescein-5'-isothiocyanate-labeled microsomal fraction, we demonstrated that the Mg2+-ATPase activity was inhibited by Ca2+.