Fe2+ and other divalent metal ions uncouple Ca2+ transport from (Ca2+-Mg2+)-ATPase in rat liver plasma membranes

Is there a specific role for the plasma membrane Ca2+ -ATPase in the hepatocyte?

The plasma membrane Ca2+ -ATPase (PMCA) is responsible for the fine, long-term regulation of the cytoplasmic calcium concentration by extrusion of this cation from the cell. Although the general kinetic mechanisms for the action of both, well coordinated hydrolytic activity and calcium transport are reasonably understood in the majority of cell types, due to the complex physiologic and biochemical characteristics shown by the hepatocyte, the study of this enzyme in this cell type has become a real challenge. Here, we review the various molecular aspects known to date to be associated with liver PMCA activity, and outline the strategies to follow for establishing the role of this enzyme in the overall physiology of the hepatocyte. In this way, we first concentrate on the basic biochemical aspects of liver cell PMCA, and place an important emphasis on expression of its molecular forms to finally focus on the critical hormonal regulation of the enzyme. Although these complex aspects have been studied mainly under normal conditions, the significance of PMCA in the calcium homeostasis of an abnormal liver cell is also reviewed.

Manganese alters mitochodrial integrity in the hearts of swine marginally deficient in magnesium

It was previously reported that pigs marginally deficient in magnesium (Mg) and fed diets high in manganese (Mn) died suddenly with signs of sudden cardiac death. Manganese, which has properties similar to Mg, may exacerbate Mg-deficiency and be accumulated by mitochondria resulting in ultrastructural damage. The objective of this study was to determine whether deaths of the type previously observed were mediated by adverse interactions of Mn and Mg resulting in ultrastructural damage to the myocardium, alterations in electrocardiographic recordings and tissue retention of Mn, Mg and calcium (Ca). Forty-eight pigs were fed one of six diets in a 2 X 3 factorial arrangement of Mg (100 or 1000 mg Mg/kg) and Mn (5, 50 or 500 mg Mn/kg) for 8 weeks. Left ventricle muscle samples were collected for examination by transmission electron microscopy. No differences in heart muscle ultrastructure were observed between pigs fed low and adequate dietary Mg. However, marked myocardial necrosis and mitochondrial swelling were observed in pigs fed high dietary Mn when combined with low Mg. Feeding low dietary Mg elevated minimum (P < 0.01), maximum (P < 0.05) and average (P < 0.001) heart rates. Low dietary Mg resulted in a 55% probability of a ventricular beat being recorded (P = 0.05) and lower Mg (P < 0.02) and Ca (P < 0.04) contents in heart atria and ventricles. These results suggest that high Mn, when fed in combination with low Mg, disrupts mitochondrial ultrastructure and is associated with the sudden deaths previously reported.

Differential inhibitory effects of carbon tetrachloride on the hepatic plasma membrane, mitochondrial and endoplasmic reticular calcium transport systems: implications to hepatotoxicity

Mitochondrial, endoplasmic reticular and plasma membrane fractions were isolated by a new method from control male Fischer 344 rats and rats given CCl4 by gavage. After 1 h of CCl4 treatment, rats were in glucose and pancreatic hormone balance but plasma levels of T3 and T4 were decreased 29 and 22%, respectively. After 24 hours of CCl4 treatment, rats were: hypoglycaemic and insulin and glucagon levels were increased 33- and 35-fold, respectively; total T4 levels were decreased 62%; while total T3 levels were normalized. In liver fractions from CCl4-treated rats, 1 h after CCl4 administration: (i) calcium binding was decreased 65% in the mitochondrial fraction, 66% in the endoplasmic reticular fraction and 46% in the plasma membrane fraction; (ii) calcium uptake was decreased 59% in the mitochondrial fraction, 46% in the endoplasmic reticular fraction and 37% in the plasma membrane fraction. After 24 h of CCl4 administration: (i) calcium binding was decreased 57% in the mitochondrial fraction, 50% in the endoplasmic reticular fraction and 71% in the plasma membrane fraction; (ii). calcium uptake was decreased 55% in the mitochondrial fraction, 17% in the endoplasmic reticular fraction and 53% in the plasma membrane fraction. In vitro studies indicated the plasma membrane calcium transport system to be rapidly (within a minute) and strongly (>90%) inhibited by CCl4. We conclude that CCl4 produces a differential inhibitory effect on the hepatocyte calcium pumps that are implicated with hepatocellular damage.

Activatory effect of regucalcin on hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase is impaired by liver injury with carbon tetrachloride administration in rats

The alteration of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver of rats administered orally carbon tetrachloride (CCl4) solution was investigated. Rats received a single oral administration of CCl4 (10, 25 and 50%, 1.0 ml/100 g body weight), and 3 or 24 h later they were sacrificed. CCl4 administration caused a remarkable elevation of liver calcium content and a corresponding increase in liver plasma membrane (Ca(2+)-Mg2+)-ATPase activity, indicating that the increased Ca2+ pump activity is partly involved in calcium accumulation in liver cells. Moreover, the participation in regucalcin, which is an intracellular activating factor on the enzyme, was examined by using anti-regucalcin IgG. The plasma membrane (Ca(2+)-Mg2+)-ATPase activity increased by CCl4 administration was not entirely inhibited by the presence of anti-regucalcin IgG (1.0 and 2.5 ug/ml) in the enzyme reaction mixture. However, the effect of regucalcin (0.25-1.0 uM) to activate (Ca(2+)-Mg2+)-ATPase in the liver plasma membranes of normal rats was not revealed in the liver plasma membranes obtained from CCl4-administered rats. Also, the effect of regucalcin was not seen when the plasma membranes were washed with 1.0 mM EGTA, indicating that the disappearance of regucalcin effect is not dependent on calcium binding to the plasma membranes due to liver calcium accumulation. Now, the presence of dithiothreitol (5 mM) or heparin (20 ug/ml) caused a remarkable elevation of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver obtained from CCl4-administered rats. Thus, the regucalcin effect differed from that of dithiothreitol or heparin. The present study suggests that the impairment of regucalcin effect on Ca2+ pump activity in liver plasma membranes is partly contribute to hepatic calcium accumulation induced by liver injury with CCl4 administration.

Inorganic cation dependence of putrescine and spermidine transport in human breast cancer cells

The mechanism of polyamine uptake in mammalian cells is still poorly understood. The role of inorganic cations in polyamine transport was investigated in ZR-75-1 human breast cancer cells. Although strongly temperature dependent, neither putrescine nor spermidine uptake was mediated by a Na+ cotransport mechanism. In fact, Na+ and cholinium competitively inhibited putrescine uptake relative to that measured in a sucrose-based medium. On the other hand, ouabain, H+, Na+, and Ca2+ ionophores, as well as dissipation of the K+ diffusion potential, strongly inhibited polyamine uptake in keeping with a major role of membrane potential in that process. Polyamine transport was inversely dependent on ambient osmolality at near physiological values. Putrescine transport was inhibited by 70% by decreasing extracellular pH from 7.2 to 6.2, whereas spermidine uptake had a more acidic optimum. Deletion of extracellular Ca2+ inhibited putrescine uptake more strongly than chelation of intracellular Ca2+. In fact, bound divalent cations were absolutely required for polyamine transport, as shown after brief chelation of the cell monolayers with EDTA. Either Mn2+, Ca2+, or Mg2+ sustained putrescine uptake activity with high potency (Km = 50-300 microM). Mn2+ was a much stronger activator of spermidine than putrescine uptake, suggesting a specific role for this metal in polyamine transport. Other transition metals (Co2+, Ni2+, Cu2+, and Zn2+) were mixed activators/antagonists of carrier activity, while Sr2+ and Ba2+ were very weak agonists, while not interfering with Ca2+/Mg(2+)-dependent transport. Thus, polyamine uptake in human breast tumor cells is negatively affected by ionic strength and osmolality, and is driven, at least in part, by the membrane potential, but not by the Na+ electrochemical gradient. Moreover, the polyamine carrier, or a tightly coupled accessory component, appears to have a high-affinity binding site for divalent cations, which is essential for the uptake mechanism.

Activating effect of regucalcin on (Ca(2+)-Mg2+)-ATPase in rat liver plasma membranes: relation to sulfhydryl group

The activating mechanism of regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca(2+)-Mg2+)-ATPase in the plasma membranes of rat liver was investigated. (Ca(2+)-Mg2+)-ATPase activity was markedly increased by a sulfhydryl (SH) group protecting reagent dithiothreitol (DTT; 2.5 and 5 mM as a final concentration), while the enzyme activity was significantly decreased by a SH group modifying reagent N-ethylmaleimide (NEM; 0.5-5 mM). The effect of DTT (5 mM) to increase the enzyme activity was clearly blocked by NEM (5 mM). Regucalcin (0.25-1.0 microM) significantly increased (Ca(2+)-Mg2+)-ATPase activity. This increase was completely blocked by NEM (5 mM). Meanwhile, digitonin (0.04%), which can solubilize the membranous lipids, significantly decreased (Ca(2+)-Mg2+)-ATPase activity. Digitonin did not have an effect on the DTT (5 mM)-increased enzyme activity. However, the effect of regucalcin (0.25 microM) increasing (Ca(2+)-Mg2+)-ATPase activity was entirely blocked by the presence of digitonin. The present results suggest that regucalcin activates (Ca(2+)-Mg2+)-ATPase by the binding to liver plasma membrane lipids, and that the activation is involved in the SH groups which are an active site of the enzyme.

Regucalcin modulates hormonal effect on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes

The interaction of various hormones and regucalcin on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes was investigated. The presence of epinephrine (10(-6)-10(-4) M), phenylephrine (10(-6)-10(-4) M), and insulin (10(-8)-10(-7) M) in the reaction mixture produced a significant increase in (Ca(2+)-Mg7+)-ATPase activity, while the enzyme activity was decreased significantly by calcitonin (3 x 10(-8)-3 x 10(-6) M). These hormonal effects, except for calcitonin, were clearly inhibited by the presence of vanadate (10(-4) M) which can inhibit the Ca(2+)-dependent phosphorylation of enzyme. Meanwhile, regucalcin (0.25 and 0.50 microM), isolated from rat liver cytosol, elevated significantly (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes, although this elevation was not inhibited by vanadate (10(-4) M). The epinephrine (10(-5) M) or phenylephrine (10(-4) M)-induced increase in (Ca(2+)-Mg2+)-ATPase activity was disappeared in the presence of regucalcin; in this case the effect of regucalcin was also weakened. However, the inhibitory effect of calcitonin (3 x 10(-6) M) was not weakened by the presence of regucalcin (0.5 microM). Moreover, GTP (10(-5) and 10(-4) M)-induced increase in (Ca(2+)-Mg2+)-ATPase activity was not seen in the presence of regucalcin (0.25 microM). The present finding suggests that the activating mechanism of regucalcin on (Ca(2+)-Mg2+)-ATPase is not involved on GTP-binding protein which modulates the receptor-mediated hormonal effect in rat liver plasma membranes.

Regulatory effect of regucalcin on (Ca(2+)-Mg2+)-ATPase in rat liver plasma membranes: comparison with the activation by Mn2+ and Co2+

The effect of various metals and regucalcin, a calcium-binding protein isolated from rat liver cytosol, on (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes of rat liver was investigated. Of various metals (Zn2+, Cu2+, Ni2+, Mn2+, Co2+ and Al3+; 100 microM as a final concentration), Mn2+ and Co2+ increased markedly (Ca(2+)-Mg2+)-ATPase activity, while other metals had no effect. When Ca2+ was not added into enzyme reaction mixture, Mn2+ and Co2+ (25-100 microM) did not significantly increase the enzyme activity, indicating that heavy metals act on Ca(2+)-stimulated phosphorylation of the enzyme. Meanwhile, regucalcin (0.25-1.0 microM) caused a remarkable elevation of (Ca(2+)-Mg2+)-ATPase activity. This increase was not inhibited by the presence of 100 microM vanadate, although the effects of Mn2+ and Co2+ (100 microM) were inhibited by vanadate. Also, the inhibition of the Mn2+ and Co2+ effects by vanadate was not seen in the presence of regucalcin. Moreover, regucalcin (0.5 microM) increased significantly the enzyme activity in the absence of Ca2+. This effect of regulcalcin was not altered by increasing concentrations of Ca2+ added, indicating that the regucalcin effect does not depend on Ca2+. The present results suggest that regucalcin activates directly (Ca(2+)-Mg2+)-ATPase in liver plasma membranes, and that the activation is not involved in the Ca(2+)-dependent phosphorylation of the enzyme.

Distances between functional sites of the Ca2+ + Mg2(+)-ATPase from sarcoplasmic reticulum using Co2+ as a spectroscopic ruler

Cobalt ion inhibits the Ca2+ + Mg2(+)-ATPase activity of sealed sarcoplasmic reticulum vesicles, of solubilized membranes and of the purified enzyme. To use Co2+ appropriately as a spectroscopic ruler to map functional sites of the Ca2+ + Mg2(+)-ATPase, we have carried out studies to obtain the kinetic parameters needed to define the experimental conditions to conduct the fluorimetric studies. 1. The apparent K0.5 values of inhibition of this ATPase are 1.4 mM, 4.8 mM and 9.5 mM total Co2+ at pH 8.0, 7.0 and 6.0, respectively. The inhibition by Co2+ is likely to be due to free Co2+ binding to the enzyme. Millimolar Ca2+ can fully reverse this inhibition, and also reverses the quenching of the fluorescence of fluorescein-labeled sarcoplasmic reticulum membranes due to Co2+ binding to the Ca2+ + Mg2(+)-ATPase. Therefore, we conclude that Co2+ interacts with Ca2+ binding sites. 2. Co2+.ATP can be used as a substrate by this enzyme with Vmax of 2.4 +/- 0.2 mumol ATP hydrolyzed min-1 (mg protein)-1 at 20-22 degrees C and pH 8.0, and with a K0.5 of 0.4-0.5 mM. 3. Co2+ partially quenches, about 10 +/- 2%, the fluorescence of fluorescein-labeled sarcoplasmic reticulum Ca2+ + Mg2(+)-ATPase upon binding to this enzyme at pH 8.0. From the fluorescence data we have estimated an average distance between Co2+ and fluorescein in the ATPase of 1.1-1.8 nm or 1.3-2.1 nm for one or two equidistant Co2+ binding sites, respectively. 4. Co2+.ATP quenches about 20-25% of the fluorescence of fluorescein-labeled Ca2+ + Mg2(+)-ATPase, from which we obtain a distance of 1.1-1.9 nm between Co2+ and fluorescein located at neighbouring catalytic sites.

CCl4-induced lipoperoxidation triggers a lethal defect in the liver plasma membranes

Loss of calcium regulation across the plasma membrane of hepatocytes is responsible for irreversible cell damage by CCl4. The mode of action of colchicine in CCl4 acute liver damage is not completely understood. We followed the time courses of the changes in lipoperoxidation, the activities of liver plasma membrane Ca2(+)-ATPase, gamma-glutamyl transpeptidase and alkaline phosphatase, as well as the time courses of serum markers of liver damage in rats acutely intoxicated with CCl4. We assessed the effects of colchicine in this model and evaluated the effect of this drug on liver cytochrome P-450. Increased lipoperoxidation is the earliest and shortest lasting effect of CCl4 in the liver and is followed by a decrease in the activities of plasma membrane-bound enzymes. The alterations in serum enzymes showed a slower onset and were more protracted. Colchicine pretreatment produced a small decrease in cytochrome P-450 in the liver but completely prevented most of the changes produced by CCl4 in lipoperoxidation, liver plasma membrane enzyme activities and serum enzyme activities. We conclude that CCl4 metabolites trigger lipoperoxidation and then produce a longer lasting change in the plasma membrane, which thus allows calcium accumulation. Colchicine prevents the early mechanisms of CCl4 damage, and its effect on cytochrome P-450 perhaps plays only a contributory role.

Calcium transport activities of plasma membranes isolated from the livers of various animal species

1. Plasma membranes of comparable yield and purity were isolated from the livers of various animal species belonging to phylogenetic groups from Amphibia to Mammalia. 2. Calcium transport activity was observed in all liver plasma membranes examined. 3. No phylogenetic pattern of expression of the liver plasma membrane calcium transport system was observed, with the order of activity being: guinea pig greater than rabbit greater than frog greater than chicken = hamster greater than rat = budgerigar = turtle greater than beef cattle greater than mouse = duck. 4. Calcium transport activity was only 9.7 and 8.7% of adult frog levels in plasma membranes isolated from the livers of tadpoles without and with limbs, respectively. 5. Liver plasma membrane calcium transport activity was 25% higher in adult chickens than in day-old chicks. 6. A possible role for thyroid hormone in the development of the liver plasma membrane calcium transport system is discussed.

Effect of cobalt on Ca2+-Mg2+ ATPase in rat incisor maturation ameloblasts

Rat incisor maturation ameloblasts were studied to determine the effect of injected cobalt on the distribution and intensity of Ca2+-Mg2+ ATPase. The dosage of cobalt utilized temporarily inhibits enamel mineralization and alters ameloblast-associated calcium. A modified Wachstein-Meisel medium containing cerium as the capturing ion was used to localize Ca2+-Mg2+ ATPase cytochemically. The distribution and intensity of the reaction product in normal maturation ameloblasts was, as previously reported, primarily in association with the plasma membranes. The lateral cell membranes of both smooth-ended and ruffle-ended ameloblasts were reactive. The ruffled border region contained the heaviest concentration of reaction product. Although cobalt did not alter the general pattern of distribution of the reaction product in either cell type, in all regions of activity the intensity was noticeably increased. Cells modulating from smooth-ended to ruffle-ended ameloblasts and under the influence of cobalt exhibited an irregular dense layer along the enamel surface, and large focal accumulations of electron-dense material in the various extracellular compartments. This may indicate interference with a putative resorptive activity of these cells.

Hypoxic and hypercapnic responses of [Ca2+]o and [K+]o in the cat carotid body in vitro

Measurements of extracellular Ca2+ and K+ activities [( Ca2+]o, [K+]o) in the superfused cat carotid body in vitro with triple-barrelled ion-selective electrodes have shown that hypoxia induced a decrease in [Ca2+]o of 0.035 +/- 0.17 mM (mean +/- S.D.; n = 17) and a biphasic change in [K+]o which consisted of an increase of 2.3 +/- 1.8 mM followed by an undershoot of -0.52 +/- 0.34 mM (mean +/- S.D.; n = 17). Hypercapnia induced a monophasic upward deflection increase of both [Ca2+]o and [K+]o of about 0.037 +/- 0.013 mM and 0.33 +/- 0.15 mM, respectively (n = 17). During hypoxia, lowering [Ca2+] in the medium to 0.1 mM resulted in a reversed [Ca2+]o response, attenuated [K+]o increase and absence of chemosensory nerve discharges. TTX generally did not affect the hypoxic and hypercapnic induced ionic changes, although the [K+]o undershoot was reduced by 30%. Co2+ competitively blocked the changes in [Ca2+]o and the increase in the sensory nerve discharge elicited by hypoxia and, not competitively, the changes of [K+]o. The ionic changes to hypercapnia were less affected by Co2+. Ouabain inhibited the [K+]o undershoot induced by hypoxia, as did the removal of Na+ from medium. It is concluded that changes in extracellular free Ca2+ and K+ ions concentration induced by hypoxia and hypercapnia represent ionic fluxes related to the transduction process of carotid body cells (glomus and/or sustentacular).

The liver plasma membrane Ca2+ pump: hormonal sensitivity

The liver plasma membrane Ca2+ pump is supposed to extrude cytosolic calcium out of the cell. This system has now been well defined on the basis of its plasma membrane origin, its high affinity Ca2+ -stimulated ATPase activity, its Ca2+ transport activity, its phosphorylated intermediate. The liver calcium pump appears to be a target of hormonal action since it has been shown that glucagon and calcium mobilizing hormones namely alpha 1-adrenergic agonists, vasopressin, angiotensin II inhibit this system. The present review details the mechanism of calcium pump inhibition by glucagon and points out its difference from the inhibition process induced by calcium mobilizing hormones. We conclude that the inhibitory action of the Ca2+ mobilizing hormones and glucagon on the liver plasma membrane Ca2+ pump might play a key role in the actions of these hormones by prolonging the elevation in cytosolic free Ca2+.