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

Modulation of ouabain-insensitive Na(+)-ATPase activity in the renal proximal tubule by Mg(2+), MgATP and furosemide

In addition to the (Na(+)+K(+))ATPase another P-ATPase, the ouabain-insensitive Na(+)-ATPase has been observed in several tissues. In the present paper, the effects of ligands, such as Mg(2+), MgATP and furosemide on the Na(+)-ATPase and its modulation by pH were studied in the proximal renal tubule of pig. The principal kinetics parameters of the Na(+)-ATPase at pH 7.0 are: (a) K(0.5) for Na(+)=8.9+/-2.2mM; (b) K(0.5) for MgATP=1.8+/-0.4mM; (c) two sites for free Mg(2+): one stimulatory (K(0.5)=0.20+/-0.06 mM) and other inhibitory (I(0.5)=1.1+/-0.4 mM); and (d) I(0.5) for furosemide=1.1+/-0.2 mM. Acidification of the reaction medium to pH 6.2 decreases the apparent affinity for Na(+) (K(0.5)=19.5+/-0.4) and MgATP (K(0.5)=3.4+/-0.3 mM) but increases the apparent affinity for furosemide (0.18+/-0.02 mM) and Mg(2+) (0.05+/-0.02 mM). Alkalization of the reaction medium to pH 7.8 decreases the apparent affinity for Na(+) (K(0.5)=18.7+/-1.5 mM) and furosemide (I(0.5)=3.04+/-0.57 mM) but does not change the apparent affinity to MgATP and Mg(2+). The data presented in this paper indicate that the modulation of the Na(+)-ATPase by pH is the result of different modifications in several steps of its catalytical cycle. Furthermore, they suggest that changes in the concentration of natural ligands such as Mg(2+) and MgATP complex may play an important role in the Na(+)-ATPase physiological regulatory mechanisms.

Cytochemical localization of Ca(2+)-ATPases and demonstration of ATP-dependent calcium sequestration in giant smooth muscle fibres of Beroe

A cytochemical analysis of the mechanisms underlying cytosolic calcium regulation was undertaken in the giant smooth muscle fibres of the marine invertebrate Beroe. The ability of the sarcoplasmic reticulum to accumulate Ca2+ was demonstrated on living skinned single cells. In the presence of oxalate, and physiological concentrations of Ca2+, calcium oxalate crystals were formed in the lumen of tubules and cisternae of the sarcoplasmic reticulum. The subcellular distribution of Ca(2+)-ATPase was studied with a cytochemical technique; a dense precipitate resulting from Ca(2+)-ATPase activity was found on the plasma membrane, on the membranes of tubules and cisternae of the sarcoplasmic reticulum, and in mitochondria.

Calcium-sequestering organelles of Dictyostelium discoideum: changes in element content during early development as measured by electron probe X-ray microanalysis

Starving Dictyostelium discoideum amoebae aggregate within a few hours by chemotaxis towards the attractant cAMP to form a multicellular organism. The differentiating cells possess rapid and efficient calcium buffering and sequestration systems which enable them to restrict changes in the cytosolic free calcium concentration temporally and spatially during their chemotactic reaction and allow the continuous accumulation of Ca2+ during development. In order to identify and to characterize calcium storage compartments, we analyzed the element content of amoebae at three consecutive stages of differentiation. Determination of the element distribution was done using energy-dispersive X-ray microanalysis of freeze-dried cryosections of rapid-frozen cells. Amoebae were frozen in the vegetative and aggregation-competent state and after formation of aggregates. Aggregation-competent as well as aggregated cells contained mass dense granules with large amounts of calcium together with phosphorous and either potassium or magnesium: in aggregation-competent cells calcium was colocalized with potassium, whereas in aggregated cells the mass dense granules contained calcium and magnesium. Although mass dense granules were also present in undifferentiated, vegetative cells, they contained only low amounts of phosphorous and potassium together with little Ca and Mg. We conclude that during their differentiation D. discoideum cells use an intracellular storage compartment to sequester Ca and other cations constantly throughout development.

Regulatory volume decrease in Ehrlich ascites tumor cells is not mediated by a rise in intracellular calcium

Ehrlich ascites tumor cells suspended in hyposmotic solution initially swell and then shrink back towards normal volume, a process known as regulatory volume decrease (RVD). RVD is characterized by a specific loss of KCl, although the mechanism for this is currently unknown. The hypothesis that a rise in intracellular calcium ([Ca2+]i) activates calcium-sensitive ion conductances to initiate RVD was investigated. The results indicate that in the Ehrlich cell no rise in [Ca2+]i occurs when the extracellular osmolality is reduced from 300 mosM to 180 mosM. These findings were substantiated by the lack of sensitivity of RVD to the Ca(2+)-sensitive K+ channel blockers charybdotoxin (CTX) and nifedipine. In contrast, the ionophore ionomycin induced a cell shrinkage that was sensitive to CTX and nifedipine indicating that a rise in [Ca2+]i could play a role in cell volume reduction but that this occurred by a mechanism different from that observed in RVD. The conclusion from these experiments is that Ca2+ does not act as a second messenger for RVD in the Ehrlich cell.

ATP-driven Ca2+/H+ antiport in acid vesicles from Dictyostelium

Amoebae of the cellular slime mold Dictyostelium discoideum possess an extensive and dynamic endomembrane system that includes many types of acidic vacuoles. A light membrane fraction from Dictyostelium, rich in vacuolar-type H(+)-ATPase, has been described [Padh, H., Lavasa, M. & Steck, T.L. (1989) J. Cell Biol. 108, 865-874]. Here, we show that this "acidosomal" fraction also contains a high-affinity vanadate-sensitive Ca2+ uptake activity that is stimulated by the pH gradient formed by the H(+)-ATPase. We attribute this Ca2+ uptake to the presence of a H(+)-countertransporting Ca(2+)-ATPase, pumping Ca2+ into an acidic compartment.

Effects of ryanodine on calcium sequestration in the rat liver

Ryanodine, a highly toxic alkaloid known to react specifically with the Ca2+ release channels in sarcoplasmic reticulum (SR), was employed to study Ca2+ sequestration in the liver. Ryanodine at a 200 microM concentration increased cytosolic free Ca2+ levels and phosphorylase a activity in isolated hepatocytes. These effects may involve microsomal Ca2+ sequestration, because ryanodine, in the presence of inhibitors of mitochondrial Ca2+ uptake, at concentrations of 1 nM, 1 microM, 50 microM and 100 microM decreased 45Ca2+ retention in permeabilized hepatocytes. This inhibition of Ca2+ retention by ryanodine was not due to inhibition of the microsomal Ca(2+)-ATPase. Dantrolene, a compound shown previously to inhibit ryanodine binding in the liver, also decreased 45Ca2+ retention in permeabilized hepatocytes, and activated phosphorylase a. These results show that ryanodine administration alters calcium sequestration in liver. The possibility of the existence of a ryanodine-sensitive Ca(2+)-release channel in liver is discussed.

Calcium sequestration in the liver

Hepatic parenchymal cells maintain intracellular total and cytosolic free Ca2+ levels by: entry of Ca2+ through channels, extrusion of Ca2+ by an outwardly directed Ca2+ pump, and controlled sequestration into intracellular pools. The mechanism of Ca2+ inflow is poorly characterized. The plasma membrane Ca2+ channels seem to share some of the characteristics of Ca2+ channels in excitable cells, but also differ from them. The outwardly directed plasma membrane Ca2(+)-ATPase is a calmodulin independent, P-type enzyme. Ca2+ uptake into the endoplasmic reticulum is due to the activity of a different Ca2(+)-ATPase, which is similar in molecular weight and shares antigenic determinants with the sarcoplasmic reticulum enzyme. In addition, mitochondria and nuclei also take up calcium. The exact mechanism by which Ca2+ is released from intracellular organelles is not well known. Several mechanisms for Ca2+ release from the endoplasmic reticulum were reported, including IP3 and GTP-induced. The most effective identified way of eliciting Ca2+ release from microsomal fraction is by the oxidation of critical -SH groups. This mechanism is likely to be involved in the rise of cytosolic Ca2+ observed in many situations of hepatocellular injury. In addition to being sequestered into subcellular organelles, some of the intracellular Ca2+ is bound to specific Ca2+ binding proteins. Both calmodulin and members of the annexin family were identified in the liver. Stimulation of the liver with gluconeogenic hormones results in increased Ca2+ entry into the cell, the release of Ca2+ from intracellular pools, and an oscillatory increase in free cytosolic Ca2+ levels. Extensive research is still needed for the elucidation of the exact mechanisms by which these events occur.