Functional comparisons between isoforms of the sarcoplasmic or endoplasmic reticulum family of calcium pumps

An investigation on the role of vacuolar-type proton pumps and luminal acidity in calcium sequestration by nonmitochondrial and inositol-1,4,5-trisphosphate-sensitive intracellular calcium stores in clonal insulin-secreting cells

To test whether in RINm5F rat insulinoma cells luminal acidity and the activity of a vacuolar-type proton pump are involved in calcium sequestration by intracellular calcium stores sensitive to inositol 1,4,5-trisphosphate (InsP3) we examined the effects of various proton-conducting ionophores and ammonium chloride, and of bafilomycin, a specific inhibitor of vacuolar proton pumps, on this parameter. Bafilomycin in concentrations up to 1 microM did not affect calcium sequestration by nonmitochondrial, InsP3-sensitive stores at all; 50 microM carbonylcyanide m-chlorophenylhydrazone, 50 microM monensin and 30 mM NH4Cl, which are diverse ways to dissipate transmembrane pH gradients, did not inhibit calcium sequestration. This argues against signficant involvement of internal acidity and vacuolar proton pumps in calcium sequestration by InsP3-sensitive stores in RINm5F cells. The proton-potassium-exchanging ionophore nigericin (20-100 microM), however, inhibited calcium sequestration by nonmitochondrial and InsP3-sensitive stores. This effect was dependent on the presence of potassium and could be reversed by inclusion of carbonylcyanide m-chlorophenylhydrazone or acetate in the incubation medium. Thus, the inhibitory effect of nigericin appears to be based on proton extrusion coupled to potassium influx across the membrane of calcium stores in RINm5F cells, creating an internal alkalinization of these stores. The effect of nigericin implies the continuous maintenance of an outside-to-inside potassium concentration gradient by nonmitochondrial calcium stores in RINm5F cells. This feature will be of potential interest in the identification of InsP3-sensitive calcium-storing organelles.

Comparison of the effects of fluoride on the calcium pumps of cardiac and fast skeletal muscle sarcoplasmic reticulum: evidence for tissue-specific qualitative difference in calcium-induced pump conformation

Comparison of the effects of fluoride (NaF, 1-10 mM) on the catalytic and ion transport functions of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR) vesicles isolated from rabbit cardiac and fast-twitch skeletal muscles revealed similarities as well as striking tissue-specific differences depending on the experimental conditions employed. Short preincubation (3 min at 37 degrees C) of cardiac or fast muscle SR with fluoride in the absence of Ca2+ and ATP prior to initiating enzyme turnover by simultaneous addition of Ca2+ and ATP to the assay medium resulted in a strong inhibitory effect of fluoride on ATP-energized (oxalate-facilitated) Ca2+ uptake and Ca(2+)-ATPase activity. On the other hand, when turnover was initiated by the addition of ATP to SR preincubated with fluoride in the presence of Ca2+ but in the absence of ATP, fluoride caused concentration-dependent stimulation of active Ca2+ uptake by fast muscle SR with no appreciable change in Ca(2+)-dependent phosphoenzyme (EP) formation (from ATP) or Ca(2+)-ATPase activity but inhibition of active Ca2+ uptake by cardiac SR with concomitant inhibition of EP formation and Ca(2+)-ATPase activity. Exposure of cardiac or fast muscle SR to fluoride in the presence of both Ca2+ and ATP resulted in concentration-dependent stimulatory effect of fluoride on Ca2+ uptake with no change in EP formation or Ca(2+)-ATPase activity, this effect diminished substantially at saturating oxalate concentration in the assay. Assessment of the effects of deferoxamine (1 mM) and exogenous aluminum (10 microM) did not indicate a requirement for aluminum in the inhibitory or stimulatory effect of fluoride. These results suggest that (a) the Ca2+ and ATP-deprived (E1/E2) but not the Ca2+ plus ATP-liganded (CaE1ATP) conformation of the SR Ca(2+)-ATPase is susceptible to inhibition by fluoride in both cardiac and fast muscle; (b) the Ca(2+)-bound conformation (CaE1) of the SR Ca(2+)-ATPase is susceptible to inhibition in cardiac muscle but is refractory to fluoride in fast muscle; and (c) the stimulatory effect of fluoride is largely secondary to its ability to mimic the action of oxalate in intravesicular Ca2+ trapping when the fluoride-resistant enzyme is turning over normally. Fluoride inhibited phosphorylation of the Ca(2+)-free enzyme by Pi in cardiac and fast muscle SR indicating that fluoride sensitivity of the phosphorylation site of the SR Ca(2+)-ATPase is similar in cardiac and fast muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Skeletal muscle overload upregulates the sarcoplasmic reticulum slow calcium pump gene

Functional data suggest that the kinetics of force production and relaxation are slowed in hypertrophied skeletal muscle because of chronic overload. The purpose of this study was to determine whether gene expression of the slow/cardiac isoform of the sarcoplasmic reticulum (SR) Ca(2+)-adenosinetriphosphatase (ATPase) pump is upregulated in overloaded fast-twitch plantaris muscles. Increased active muscle loading was induced in rat plantaris muscles bilaterally by surgical removal of gastrocnemius and soleus muscles. Mass of the plantaris muscle was 80% greater 5 wk after surgery than in age-matched unoperated control rats (P < 0.05). Expression of the slow pump mRNA was 135% greater in hypertrophied muscles, as determined from autoradiograms of Northern blots with use of a cDNA probe specific for the slow/cardiac isoform. A monoclonal antibody (7E6) was used to quantify slow Ca2+ pump in SR vesicles with use of Western blot analysis. Densitometry of blots showed that the relative expression of the slow pump protein was 130% greater in hypertrophied plantaris muscles. Expression of the fast SR Ca2+ pump protein isoform, assessed using monoclonal antibody A52, was 25% less in hypertrophied than in control muscles. The Ca2+ uptake rate and ATPase activity of SR vesicles was approximately 15% lower in hypertrophied plantaris muscles (P < 0.05). Differential phospholamban expression could not account for changes in SR Ca2+ handling, because it could not be detected in rat slow- or fast-twitch skeletal muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

The sarco(endo)plasmic reticulum Ca(2+)-ATPase mRNA isoform, SERCA 3, is expressed in endothelial and epithelial cells in various organs

The sarco(endo)plasmic reticulum Ca(2+)-ATPase mRNA isoform, SERCA 3, was previously shown to be expressed in a great variety of muscle and non-muscle tissues [(1989) J. Biol. Chem. 264, 18568] but its cellular localization within these organs was unknown. We have used in situ hybridization and RNase protection techniques to demonstrate that SERCA 3 mRNA is expressed in specific cell types, namely the endothelial and epithelial cells.

Demonstration of two isoforms of the SERCA-2b type Ca2+,Mg(2+)-ATPase in pancreatic endoplasmic reticulum

An antibody raised against a 12 amino acid peptide corresponding to the C-terminal sequence of the SERCA-2b Ca2+,Mg(2+)-ATPase precipitated Ca2+,Mg(2+)-ATPase activity from pancreatic rough ER. Thapsigargin and vanadate inhibited the activity with the same concentration-dependence as for native ER membranes. Partial purification of Ca2+,Mg(2+)-ATPase using Reactive Dye-agarose affinity chromatography resulted in activation of the enzyme, suggesting the presence of an endogenous inhibitor which was detached by binding to the Reactive Dye. Immunoblots and analysis of immunoprecipitated protein revealed two bands of molecular masses approx. 111 kDa and 97 kDa. It is concluded that pancreatic ER Ca2+,Mg(2+)-ATPase is of the SERCA-2b type and consists of two isoforms.

Protein and lipid rotational dynamics in cardiac and skeletal sarcoplasmic reticulum detected by EPR and phosphorescence anisotropy

We have used time-resolved phosphorescence anisotropy and electron paramagnetic resonance (EPR) spectroscopy to detect the rotational dynamics of the Ca-ATPase and its associated lipids in dog cardiac sarcoplasmic reticulum (DCSR), in comparison with rabbit skeletal SR (RSSR), in order to obtain insight into the physical bases for different activities and regulation in the two systems. Protein rotational motions were studied with time-resolved phosphorescence anisotropy (TPA) of erythrosin isothiocyanate (ERITC) and saturation-transfer EPR (ST-EPR) of a maleimide spin-label (MSL). Both labels were attached selectively and rigidly to the Ca-ATPase. Lipid rotational motions were studied with conventional EPR of stearic acid spin-labels. As in previous studies on RSSR, the phosphorescence anisotropy decays of both preparations at 4 degrees C were multiexponential, due to the presence of different oligomeric species. The rotational correlation times for the different rotating species were similar for the two preparations, but the total decay amplitude was substantially less for cardiac SR, indicating that more of the Ca-ATPase molecules are in large aggregates in DCSR. ST-EPR spectra confirmed that the Ca-ATPase is less rotationally mobile in DCSR than in RSSR. Lipid probe mobility and fatty acid composition were very similar in the two preparations, indicating that the large differences observed in protein mobility are not due to differences in lipid fluidity. We conclude that the higher restriction in protein mobility observed by both ST-EPR and TPA is due to more extensive protein-protein interactions in DCSR than in RSSR.

A novel primary Ca(2+)-transport system from Saccharomyces cerevisiae

A novel primary Ca(2+)-transport system in membranes from Saccharomyces cerevisiae is described. Ca2+ transport is strictly dependent on the presence of ATP; other nucleotides like GTP, UTP and CTP do not efficiently (< 10% of the rate of ATP) drive uptake. Transport is inhibited by sodium vanadate with an IC50 of 130 microM, but is insensitive to carbonylcyanide p-trifluoromethoxy-phenylhydrazone, valinomycin, gramicidin or calmodulin. Ca2+ accumulates in a free form and can be readily released by the Ca2+ ionophore A-23187 or by osmotic shock. The apparent Km values of transport activity for free Ca2+ was determined to be 0.11 microM and 5 microM for Mg.ATP, respectively. Taken together the results indicate that the Ca2+ transport described here does not belong to the plasma-membrane-type Ca(2+)-ATPase family but rather to the family of endomembrane-type ATPases. Cell-fractionation studies of crude membranes on sucrose gradient centrifugation have shown that the Ca(2+)-transport activity separates from marker enzymes for endoplasmic reticulum, vacuole, or plasma membrane and migrates with GDPase activity, a marker for the yeast Golgi complex.

Increased frequency of calcium waves in Xenopus laevis oocytes that express a calcium-ATPase

When inositol 1,4,5-triphosphate (IP3) receptors are activated, calcium is released from intracellular stores in excitatory propagating waves that annihilate each other upon collision. The annihilation phenomenon suggests the presence of an underlying refractory period that controls excitability. Enhanced calcium-adenosine triphosphatase (ATPase) activity might alter the refractory period of calcium release. Expression of messenger RNA encoding the avian calcium-ATPase (SERCA1) in Xenopus laevis oocytes increased the frequency of IP3-induced calcium waves and narrowed the width of individual calcium waves. The effect of SERCA1 expression on calcium wave frequency was dependent on the concentration of IP3 and was larger at higher (1 microM) than at lower (0.1 microM) concentrations of IP3. The results demonstrate that calcium pump activity can control IP3-mediated calcium signaling.