Calcium-binding properties and ATPase activities of rat liver plasma membranes

Na(+)-Ca2+ antiporter activity of rat hepatocytes. Effect of adrenalectomy on Ca2+ uptake and release from plasma membrane vesicles

The presence and mode of Na(+)-Ca2+ antiporter activity were studied in hepatocytes isolated from sham-operated or adrenalectomized rats and in inside-out plasma membrane vesicles isolated from rat liver. Decreasing extracellular Na+ (Na+o) immediately increased cytosolic free calcium (Ca2+i). The rise in Ca2+i was proportional to the reduction in Na+o and was caused by an increased calcium influx, presumably on the Na(+)-Ca2+ antiporter operating in the reverse mode. Perfusing the cells with Ca(2+)-free media stimulated Ca2+ efflux and decreased Ca2+i, an effect dependent on Na+o. This suggests an activation of the forward mode of Na(+)-Ca2+ exchange. There was little difference in these parameters between sham and adx groups. In contrast, steady-state calcium uptake by inside-out plasma membrane vesicles was inhibited 40% after adrenalectomy. The decreased calcium uptake was not caused by a deficiency in the ATP-dependent Ca2+ pump, whose Km and Vmax were unaffected by adrenalectomy, but by an Na(+)-dependent leak from the vesicles. Ca2+ efflux was proportional to the extravesicular Na+ concentration, suggesting that the calcium leak may take place on a Na(+)-Ca2+ antiporter. This Na(+)-dependent calcium efflux was significantly increased in vesicles prepared from adx rat livers. These results suggest that hepatocytes have functional Na(+)-Ca2+ antiporters that can operate in both forward and reverse modes. Under normal conditions, the Na(+)-Ca2+ antiporter apparently operates in the reverse mode as a Ca2+ influx pathway. The increase in Na(+)-dependent Ca2+ efflux evoked by adrenalectomy in plasma membrane vesicles could explain the recent results we obtained in hepatocytes isolated from adx rats, showing increased calcium influx, increased Ca2+i, increased intracellular calcium sequestration, and increased plasmalemmal calcium cycling.

Effects of chelating agents on the Ca2+-stimulated ATPase of rat liver plasma membranes

Using strictly controlled ionic conditions we have demonstrated, in agreement with previous findings (Lotersztajn et al. (1981) J. Biol. Chem. 256, 11209-11215; Lotersztajn, S. and Pecker, F. (1982) J. Biol. Chem. 257, 6638-6641) a Ca2+-stimulated ATPase in rat liver plasma membranes which is detectable at low free Mg2+ concentrations (normally fulfilled by endogenous levels) but not at free Mg2+ concentrations greater than about 10(-5) M. The findings reported here also suggest that this (Ca2+ + Mg2+)-ATPase is activated by EGTA or one of its liganded species. Furthermore, this is probably an intrinsic property of the enzyme as it was found to be independent of the isolation technique. The stimulation by EGTA appears to be a function both of free Ca2+ concentration and of one or more liganded species of EGTA and it is also inhibited at high free Mg2+ concentrations (approx. 10(-5) M). The specificity of the EGTA effect on ATPase activity is studied with respect to other, widely used, chelating agents namely HEEDTA, EDTA and CDTA. Of these, only CDTA shares the effect, although the concentration dependence of the activation is different from EGTA, suggesting that there is some degree of structural specificity involved rather than a generalised effect of complexed Ca2+.

The kinetics of calcium influx into rat liver slices

The unidirectional influx of calcium across rat liver slices is a carrier-mediated process which displays saturation kinetics. The presence of Mg2+ in the incubation medium competitively inhibits calcium influx into rat liver slices. Metabolic inhibitors such as ouabain and 2,4-dinitrophenol at a concentration of 1 X 10(-4) and 2.5 X 10(-4) M respectively, inhibited significantly (P less than 0.001) calcium influx across the liver slices. Calcium influx is dependent on the presence of sodium in the extracellular medium, and is significantly reduced (P less than 0.01) when sodium concentration in the preincubation solution is reduced to zero.

Effect of Epomediol on the ATPase and adenylate cyclase activities in plasmamembranes isolated from rat liver

The effect of Epomediol (Clesidren (R)) on the membrane ATPase and Adenylate Cyclase activities has been investigated using highly purified rat liver plasmamembranes. The drug, even at micromolar concentration, showed a significant stimulatory effect on Ca++/Mg++- ATPase, Ca++- ATPase and Adenylate Cyclase activities. In particular the effect on Ca++- ATPase activity suggests that the drug under study may favourably modulate the calcium intracellular homeostasis.

Effect of adrenalectomy on cellular calcium metabolism and on the response to adrenergic stimulation of hepatocytes isolated from male and female rats

The effects of adrenalectomy on cell calcium metabolism and on the effects of epinephrine on cAMP, phosphorylase a activity, and calcium efflux were studied in hepatocytes isolated from adult male and female rats. Adrenalectomy increased the total calcium of hepatocytes, all exchangeable calcium pools, and all calcium fluxes between the cellular pools in both sexes. After adrenalectomy, basal cAMP was elevated, phosphorylase a + b was decreased, but basal phosphorylase a activity was not changed. In adrenalectomized males and at all concentrations of epinephrine studied (1.10(-8)-1.10(-5)M) stimulation of calcium efflux was decreased and cAMP accumulation was enhanced, while the resulting phosphorylase a activation was depressed. In hepatocytes from adrenalectomized females there was a similar increase in cAMP accumulation induced by epinephrine, and a decrease in the stimulation of calcium efflux; however, the depression in phosphorylase a activation was much less and was significant only at 1.8(-8) and 1.10(-5)M epinephrine. In the male, while activation of phosphorylase a shifted from a pure alpha-adrenergic response mediated by calcium to one also involving a cAMP-mediated beta-adrenergic response, the contribution of the attenuated calcium signal was still significant. Hepatocytes from female rats did not show a comparable alpha- to beta-shift, since the relative contribution of calcium and cAMP to phosphorylase activation was similar in sham-operated and adrenalectomized animals.

Ultracytochemical localization of Ca++-ATPase activity in the paraphyseal epithelial cells of the frog, Rana esculenta

Ca++-ATPase activity was studied ultracytochemically (cf. Ando et al. 1981) in the paraphysis cerebri of the frog. An intense reaction was demonstrated on the plasmalemma of the microvilli at the apical pole of paraphyseal cells; in contrast, the basolateral plasmalemma showed only a slight staining. In addition, mitochondria, gap junctions, cilia, and cytoplasmic elements (e.g., microfilaments) displayed Ca++-ATPase activity. Variation of the Ca++-concentration in the incubation medium from 0.1 mM to 100 mM altered the Ca++-ATPase activity of the cell organelles. The substitution of Ca- by Mg-ions resulted in a conspicuous decrease in the enzyme activity, especially on the apical plasmalemma. Ca++-ATPase activity is claimed to be involved in a number of extra- and intracellular functions. In comparison to the epithelium of the adjacent choroid plexus the paraphyseal epithelial cell is thought to be a principal Ca-ion regulator of the cerebrospinal fluid in frogs.

Calcium binding to normal, regenerating and diabetic liver plasma membrane

Calcium binding to liver plasma membrane isolated from diabetic and regenerating livers was significantly reduced (P less than 0.01) when compared to Ca binding to the normal membrane. Procaine at different concentrations inhibited significantly (P less than 0.01) calcium binding to the normal, diabetic and regenerating liver plasma membrane, with the most pronounced inhibition noticed in the normal. Colchicine at a concentration exceeding 5 X 10(-4)M showed a significant decrease (P less than 0.01) in calcium binding to the normal, diabetic and regenerating liver plasma membrane with the maximum inhibition noticed in the regenerating liver membrane.

Perturbations of liver plasma membranes induced by Ca2+ are detected using a fatty acid spin label and adenylate cyclase as membrane probes

Ca2+ decreased the lipid fluidity of rat liver plasma membranes labeled with 5-nitroxide stearate, I(12,3), as indicated by the order parameter (S). These effects form a reversible, saturable process with an association constant of 1 x 10(3) M-1. Arrhenius-type plots of S indicated that the lipid phase separation, present in the external leaflet of native membranes between 28 and 19 degrees C, is perturbed by mM Ca2+ such that the high temperature onset is elevated to 32-34 degrees C. Fluoride-stimulated adenylate cyclase was similarly inhibited by Ca2+ (ID50 = 1 mM) for the enzyme in membrane-bound or solubilized states. The glucagon-stimulated activity was more sensitive to Ca2+ inhibition with an ID50 of 0.2 mM. These inhibitory effects are due neither to perturbations of glucagon binding to its receptor nor to fluidity changes, but are instead attributed to direct Ca2+-enzyme interactions. Such binding desensitizes the enzyme to fluidity alterations induced by temperature elevation or benzyl alcohol addition. With Ca2+, Arrhenius plots of glucagon-stimulated activity indicated breaks at 32 and 16 degrees C, whereas those of fluoride-stimulated activity showed one break at 17 degrees C. Without Ca2+, Arrhenius plots exhibited one break at 28 degrees C for glucagon-stimulated activity, whereas fluoride-stimulated plots were linear. We propose that Ca2+ achieves these effects through asymmetric perturbations of the membrane lipid structure.

Ca2+ transporting activity of membrane fractions isolated from the post-mitochondrial supernatant of rat liver

The post-mitochondrial supernatant of rat liver contains two vesicular fractions which transport Ca2+ actively. The heavier fraction, sedimenting at 17.500 xg, 20 min, is enriched in plasma membrane markers and apparently contains both a Ca2+ pumping ATPase and a Na+/Ca2+ exchanger. These activities have been attributed to the plasma membrane vesicles. The lighter fraction, sedimenting at 100.000 xg, 60 min, is enriched in endoplasmic reticulum markers, and contains only a Ca2+ pumping ATPase, which can be differentiated from that of the heavier fraction on the basis of the sensitivity to vanadate. The Ca2+ pumping activity of endoplasmic reticulum appears to be regulated by both a cAMP-dependent, and a calmodulin-dependent system. The former system involves a heat-stable protein fraction from the cytosol. The regulation by the cAMP and the calmodulin-dependent systems involves the phosphorylation of several proteins in the endoplasmic reticulum membrane.

Properties of Ca2+- or Mg2+-dependent ATPase in rat heart sarcolemma

Rat heart sarcolemma was shown to hydrolyze ATP in the presence of Ca2+ or Mg2+; Ka values for Ca2+ and Mg2+ were in the range of 0.58-0.67 and 0.72-0.83 mM, whereas Vmax values were 33-38 and 21-28 mumol Pi/mg per hr, respectively. Both Ca2+ ATPase and Mg2+ ATPase showed low- and high-affinity sites for ATP; the Km value for the low-affinity sites for both enzyme activities was 300-325 microM, whereas Km values for high-affinity sites were 75-85 and 100-108 microM, respectively. The pattern of nucleotide hydrolysis in the presence of Ca2+ was found to be different from that with Mg2+. Although both high concentrations of ADP and Pi inhibited the enzyme activities, Mg2+ ATPase was more sensitive to ADP and less sensitive to Pi in comparison to Ca2+ ATPase. Storage of sarcolemma at about 0 degrees C showed a greater increase in ATP hydrolysis with Ca2+ than with Mg2+. The inhibitory effect of Mg2+ on Ca2+ ATPase, unlike that of Ni2+, Co2+, and Cu2+, was more than that on Mg2+ ATPase. Treatment of membranes with sodium dodecylsulfate or deoxycholate produced a greater reduction in Mg2+ ATPase than in Ca2+ ATPase. These results further support the view that Ca2+ ATPase and Mg2+ ATPase may be two separate enzymes in heart sarcolemma. It is suggested that Ca2+-dependent ATPase may be involved in opening calcium channels for the entry of calcium, whereas Mg2+ ATPase may serve as a Mg2+ pump mechanism for the efflux of magnesium from the cardiac cell.

Stimulation of autophosphorylation of liver cell membrane proteins by calcium and partial hepatectomy

Partial hepatectomy in the rat stimulated the phosphorylation of three proteins (Mr 100,000, 48,000, and 35,000) in the plasma membranes of the proliferatively activated cells of the liver remnant. The autophosphorylation of these plasma membrane proteins began to rise about 8 hours after surgery, peaked at 14 hours, and then returned to the original low level by 24 hours. This increase in autophosphorylation was not evident in isolated plasma membranes from other proliferatively activated liver cells, such as those in fetal liver or hepatomas. The protein kinase responsible for the phosphorylation of the three membrane proteins was activated by calcium but appeared both cyclic-AMP- and calmodulin-independent.

Ca2+ binding sites in plasma membranes of rat liver and hepatoma cells, and effect of concanavalin A on the Ca2+ binding sites and cellular uptake of Ca2+

1. Plasma membranes isolated from rat livers and ascites hepatoma cells (AH-130, AH-7974) were assayed for specific Ca2+ binding sites using 45Ca2+ and a Millipore filtration technique. The presence of higher (Kd = 1.4--1.5 . 10(-5) M) and lower (Kd = 0.9--1.0 . 10(-4) M) affinity sites in both liver and hepatoma membranes was observed. The hepatoma plasma membranes however, showed 1.4--2.1-fold as many Ca2+ binding sites (higher and lower affinity sites) as the liver plasma membranes on the basis of protein. 2. Concanavalin A stimulated the specific Ca2+ binding to liver and hepatoma plasma membranes, showing a maximal stimulation (3--5-fold) at 100 microgram/ml. Succinyl concanavalin A was less effective, whereas wheat germ agglutinin and ricinus lectin were ineffective. 3. Concanavalin A stimulated the Ca2+ uptake by AH-7974 cells. The concanavalin A-mediated stimulation of Ca2+ uptake showed lectin-concentrations and Ca2+-concentration dependencies similar to those in the concanavalin A-mediated stimulation of Ca2+ binding.

Properties of the calcium-extruding mechanism of liver cells

1. The movements of Ca2+ between liver slices and the suspending medium have been followed with a Ca2+-specific electrode and with measurements of the efflux of 45Ca. 2. A net entry of Ca2+ into the tissue occurred during anaerobiosis. Reoxygenation of the medium resulted in a net extrusion of Ca2+ which was reversed by the addition of a respiratory inhibitor (Amytal), an uncoupler of oxidative phosphorylation (pentachlorophenol) or the divalent-cation-specific ionophore, A23187. 3. The net extrusion of Ca2+ was dependent on the presence of Mg2+ in the medium, but was not prevented by ouabain or by incubation in a Na+-free medium. Extrusion was not prevented by cytochalasin B or colchicine. 4. The undirectional efflux of 45Ca required Mg2+ to attain its maximal rate, and the magnitude was not affected by the presence of ouabain or the absence of Na+ from the medium. 5. It is concluded that extrusion of Ca2+ from liver cells is a metabolically dependent transport process that occurs independently of the exchange of Na+ between tissue and medium.

Hepatic portal vein infusion of glucose and sodium solutions on the control of saline drinking in the rat

1. Rats were prepared under anaesthesia with non-occlusive catheters in hepatic portal vein (HP) and inferior vena cava (VC) and maintained under standard conditions. 2. Each rat received a series (3 day intervals) of 30 min infusions of different solutions or sham into HP or VC. Oral intake of 0.15 M-NaCl and water were measured for 30 min. Significant change in drinking behaviour was assumed when the response to HP infusion differed from both sham and VC infusion. 3. Saline drinking was inhibited by HP infusion of 1 M- or 2M-NaCl, an effect blocked by right vagotomy or by addition of 16 mM-KCl to the infusate. 4. Saline drinking was increased and water drinking decreased by HP infusion of 2 M-glucose but not sucrose or fructose. 5. Saline drinking was decreased by HP infusion of deoxy-D-glucose to inhibit glucose utilization or ouabain to inhibit (Na4-K+) ATPase. 6. Results are consistent with the presence of afferent nerve terminals in hepatic portal vessels which are sensitive to change in NaCl or glucose concentration and which, in response thereto, alter drinking behaviour. The effects of NaCl and glucose on the discharge rate of the nerve terminals may be interpreted in terms of changing activity or electrogenicity of a Na pump but changes in membrane conductance or Na influx cannot be ruled out.

Effects of carbon tetrachloride on rat liver plasmalemmal calcium adenosine triphosphatase

Calcium adenosine triphosphatase (Ca-ATPase) activity in rat liver plasma membrane fractions prepared by zonal centrifugation was studied for sensitivity to carbon tetrachloride (CCl4). Levels of Ca-ATPase activity in such membrane fractions from animals given CCl4 by gastric intubation were no different from those in plasmalemmal fractions obtained from control rats. When the fractions were incubated in vitro, however, this enzyme activity was inhibited by the presence of CCl4 in a dose-dependent manner. Moreover, inhibition of Ca-ATPase could be reversed by the removal of CCl4. These results would explain the observed increase in hepatic calcium content following the administration of CCl4 if the Ca-ATPase were capable of actively extruding calcium and if this extrusion mechanism proved sensitive to CCl4 present on or about the hepatocytes.

An examination of possible mechanisms of angiotensin II-stimulated steroidogenesis

Dog and rat adrenal glomerulosa cells and subcellular fractions have been utilized to evaluate the mechanism of angiotensin II- and angiotensin III-induced aldosterone production. The effects of angiotensin, ACTH, and potassium have been compared on cyclic AMP and cyclic GMP in isolated glomerulosa cells and adenylate cyclase activity in subcellular fractions. The effect of angiotensin II has also been assessed on Na+-K+-activated ATPase of plasma membrane enriched fractions of dog and rat adrenals. We have demonstrated no effect of angiotensin II or angiotensin III on either adenylate cyclase, cyclic AMP, cyclic GMP, or Na+-K+-dependent ATPase activity over a wide range of concentrations. Potassium ion in concentrations that stimulate significant aldosterone production was also without effect. The negative effects of angiotensin and potassium were contrasted against a positive correlation between an ACTH-induced effect on aldosterone production, adenylate cyclase, and cyclic AMP accumulation. These studies have served to demonstrate that neither adenylate cyclase, cyclic AMP, cyclic GMP, or Na+-K+-activated ATPase seem to be directly involved in the mechanism of action of angiotensins on aldosterone production in the rat and dog adrenal glomerulosa.

Fluxes and distribution of calcium in rat liver cells: kinetic analysis and identification of pools

1. Fluxes and distribution of Ca were studied in the perfused rat liver. Kinetic analysis of (45)Ca exchange revealed three compartments with time constants of 4, 14 and 223 min, pool sizes of 250, 385 and 670 mumole.kg(-1) wet wt. respectively, and one non-exchangeable compartment of 400 mumole.kg(-1).2. (45)Ca uptake by in situ mitochondria followed, as a function of cell loading time, a mono-exponential function with a time constant of 16 min. This suggests that the second compartment may be identified as the intracellular pool of Ca. The calculated cell transmembrane flux of Ca was 28 mumole.kg(-1) min(-1) or 0.17 p-mole.cm(-2).sec(-1).3. The maximum (45)Ca uptake by in situ mitochondria was 2.3 n-mole.mg(-1) of protein which represents, on the basis of 50 mg of mitochondrial protein per g of fresh liver, 115 mumole.kg(-1) or 30% of the cytoplasmic pool. A pool of 10.8 n-mole.mg(-1) protein (or 540 mumole.kg(-1)) of non-exchangeable Ca (at steady state) was probably in the form of Ca phosphate precipitated in the mitochondrial matrix.4. Extracellular Ca pools were studied using competitor of Ca binding (La) or Ca chelators (EGTA). La displaced specifically a homogeneous pool of Ca (tau = 5.1 min) which represented a fraction (55 mumole.kg(-1)) of the rapidly exchangeable Ca (first compartment) without perturbing other external pools. On the other hand, EGTA displaced completely that compartment, and about 85% of the Ca of the third compartment. These results suggest that the first and the major fraction of the third compartments are extracellular. They account for 63% of total exchangeable Ca.5. A model of distribution and exchange of Ca in hepatocytes is proposed.

Calcium-activated ATPase activity in a plasma membrane-rich preparation of rat pancreas

1. Calcium-stimulated ATPase activity was studied in a plasma membrane rich fraction of rat pancreas homogenate. 2. The enzyme is stimulated to the same maximum rate of ATP hydrolysis by either calcium or magnesium, but the apparent requirement for calcium is five-fold lower than for magnesium. 3. Maximum hydrolytic activity of the enzyme is not increased when additional magnesium is added to the optimal amount of calcium. 4. The enzyme does not require Na+ or K+ for its activation by Ca2+ and is not inhibited by ouabain or 2,4-dinitrophenol. 5. Pancreozymin, in concentrations which evoke secretion of zymogen protein, inhibits the calcium-stimulated ATPase. 6. Carbachol and dibutyryl cyclic AMP, in concentrations which increase the release of digestive proteins, do not alter the activity of the calcium-stimulated enzyme. 7. It is suggested that the plasma membrane calcium-activated enzyme, is not involved in the active calcium extrusion, previously reported to occur with use of the various pancreatic secretagogues tested.

Adenylate cyclase from rat-liver plasma membrane: inhibition by mersalyl and other mercurial derivatives

The adenylate cyclase activity from a rat liver plasma membrane preparation was inhibited by low concentrations (1-10 muM) of the mercurial diuretic mersalyl. Complete inhibition was obtained with 0.1 mM mersalyl. Similar effects were observed whether the adenylate cyclase preparation was assayed in the presence of 10 muM GTP, 0.1 muM glucagon, 10 mM NaF or without any addition. The effect of mersalyl was not due to inhibition of the regenerating system present in the incubation medium, since the effect of mersalyl was preserved and even enhanced in its absence. The inhibition brought about by mersalyl was due to both a decrease of the maximal velocity of the reaction and of the affinity of the enzyme for the substrate. It was immediate, and irreversible spontaneously, but it was reversed by the simultaneous additions of 2-mercaptoethanol, in a dose-dependent fashion. Other -SH reagents were found to have an effect equal to, or lower than, that of mersalyl. Mersalyl had no effect upon Mg2+-ATPase, although it inhibited the (Na+-K+) activated ATPase. Since mersalyl is known to be a 'non-penetrant' reagent, it is postulated that a catalytically important, mercurial-sensitive, part of adenylate cyclase is at the surface of the plasma membrane. This view is supported by the following facts: (a) mersalyl acted with a similar dose-response curve upon an intact as well as a detergent-dispersed cyclase preparation while no effect was observed upon a solubilized Mg2+-ATPase preparation; (b) a covalent p-chloromercuribenzoate-Sephadex preparation (but not its supernatant) inhibited the cyclase from intact membranes. It is proposed that mercurial derivatives, by their relative specificity of action (no effect on Mg2+-ATPase), can serve as useful probes in the elucidation of the multicomponent structure of the cyclase system.