Rotational motion of Ca-ATPase monitored by electron spin echoes

Electron spin echoes are used to study the dynamics of different aggregational forms of spin-labeled Ca-ATPase in the sarcoplasmic reticulum membrane. The 2D-ESE measurements are sensitive to motions on the microsecond time scale. The motional information is extracted from the variation of the echo decays across the CW-ESR absorption spectrum. The motional contribution to the decays is described by assuming that the Ca-ATPase molecule is perfectly oriented along the normal to the membrane surface and only undergoes rotational motion about its long axis. The echo-amplitude decays have been evaluated in the time domain by solving the Bloch equations for the stochastic spin Hamiltonian on making use of stochastic trajectories for the orientational behavior of the spin-labeled protein. This approach provides a useful insight into the information provided by the 2D-ESE measurements and affords a direct comparison of the results obtained with different experimental techniques. It is shown that the 2D-ESE technique monitors the orientational motions of dimers or larger aggregates of Ca-ATPase molecules whose rotational correlation times vary between 200 microseconds and 1 ms for the temperature range between 37 and 4 degrees C.

Messenger RNA expression and immunological quantification of phospholamban and SR-Ca(2+)-ATPase in failing and nonfailing human hearts

OBJECTIVES

Human heart failure is associated with prolonged relaxation and prolonged Ca2+ transients which indicates an impaired function of the sarcoplasmic reticulum (SR) and may be detrimental for cardiac function. Controversy exists whether the altered SR function is accompanied by changes in the expression of phospholamban (PLB) and cardiac SR-Ca(2+)-ATPase (SERCA2) on mRNA and/or protein levels.

METHODS

We determined mRNA and/or protein levels for PLB and SERCA2 in the same left ventricular tissue of patients undergoing heart transplantation due to idiopathic dilated cardiomyopathy (IDC) or ischemic cardiomyopathy (ICM) in comparison to left ventricular tissue from nonfailing human hearts (NF). Total protein extracts were prepared and subjected to SDS gel electrophoresis. PLB and SERCA2 were identified with specific antibodies. Total RNA was isolated and hybridized with 32P-labeled cDNAs for human PLB and rat SERCA2.

RESULTS

Hybridization revealed the three expected mRNAs with the PLB probe (3.3 kb, 1.9 kb and 0.6 kb) and a single band with the SERCA2 probe (4.5 kb). Determination of respective proteins by immunoblotting revealed unchanged protein levels for PLB and SERCA2, whereas the mRNA levels for PLB and SERCA2 were reduced by about 30% and 50%, respectively.

CONCLUSIONS

These data show the level of SERCA2 and PLB protein and mRNA in the same hearts. The reduced mRNA level of SERCA2 and PLB is in accordance with previous data. However, the unchanged protein levels may indicate that the diminished RNA expression is not accompanied by a corresponding decrease for these proteins in human heart failure. These data also show that the altered SR function in human heart failure cannot be explained by altered protein levels of PLB and SERCA2. Furthermore, it is suggested that extrapolations from cardiac mRNA levels to protein expression may be misleading.

Changes in the distribution of a calcium-dependent ATPase during demyelination and remyelination in the central nervous system

A calcium-adenosine triphosphatase (Ca(2+)-ATPase) activity expressed by CNS nerve fibres has been examined during demyelination and remyelination in rats, 21-26 days after an intraspinal injection of ethidium bromide. The Ca(2+)-ATPase distribution was determined cytochemically, using a technique believed primarily to reflect the presence of ecto-ATPases. We confirm that in normal nerve fibres Ca(2+)-ATPase activity was present on the external surface of the myelin sheath, and on the axolemma at the nodes of Ranvier. Labelling of the internodal axolemma was restricted to small, scattered, punctate regions. However, following demyelination the Ca(2+)-ATPase activity was expressed continuously along both the exposed, previously internodal axolemma of entirely naked axons, and it was particularly prominent at sites of contact between axons and glial-cell processes. During remyelination (which in this lesion is accomplished predominantly by Schwann cells) the proportion of the axonal surface exhibiting Ca(2+)-ATPase activity decreased in concert with the progressive thickening of the new myelin sheath. The non-myelin forming plasmalemma of Schwann cells was positive for the Ca(2+)-ATPase activity, but activity was abruptly lost at the site of compaction between the inner and outer leaflets of the forming myelin sheath. Ecto-ATPase activity is a property of some cell adhesion molecules, and it follows that the changes observed in the distribution of ATPase activity in this study may reflect changes in the axolemma which are important for the successful repair of the lesion by remyelination. The ATPase activity may, for example, reflect the changing distribution of molecules important in aiding axo-glial recognition and the establishment of axo-glial contacts.

Intracellular calcium stores and inositol 1,4,5-trisphosphate receptor in rat liver cells

The D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor was localized by immunofluorescence experiments in situ in liver cryosections. Two anti-Ins(1,4,5)P3 receptor antibodies (against the 14 C-terminal residues of the type 1 receptor or against the entire cerebellar receptor) weakly decorated the whole cytoplasm, and a more intense labelling was observed at the periphery of the hepatocytes, particularly beneath the canalicular and the sinusoidal domains of the plasma membrane (PM). Antibodies against calreticulin, the Ca2+ pump (SERCA2b) or endoplasmic reticulum (ER) membranes homogeneously labelled the cytoplasm and the subplasmalemmal area. These data indicate that the ER can be divided into at least two specialized subregions: one is located throughout most of the cytoplasm and contains markers of the rough ER (RER), calreticulin, SERCA2b and a low density of Ins(1,4,5)P3 receptor, and the other is confined to the periphery of the cells and contains calreticulin, Ca2+ pump, RER markers and a high density of Ins(1,4,5)P3 receptor. A membrane fraction enriched in Ins(1,4,5)P3 receptor and in markers of the PM was immuno-adsorbed with the antibody against the C-terminal end of the Ins(1,4,5)P3 receptor and pelleted with Sepharose protein A. The immuno-isolated material was enriched in Ins(1,4,5)P3 receptor, but none of the markers of the ER or of the PM could be detected. This suggests that the Ins(1,4,5)P3 receptor is localized on discrete domains of the ER membrane beneath the canalicular and the sinusoidal membranes, where it was found at higher densities than the other markers.

Cytochemical localization of ecto-ATPases in rat neurohypophysis

We studied the distribution of Ca(2+)- or Mg(2+)-dependent ATPase activity in rat neurohypophysis using the lead cytochemical method of Ando et al. In electron microscopy, precipitates were found lining the outer surface of the plasma membrane surrounding nerve endings and pituicytes. These precipitates were believed to represent the activity of ecto-ATPases (as opposed to Ca pump ATPases) for the following reasons: there was equal activation by Ca2+ in the absence of Mg2+ or Mg2+ in the absence of Ca2+; the effects of the two ions were not additive; there was activation by ATP or GTP; and there was resistance to glutaraldehyde fixation, to high (10 mM) Ca2+ concentrations, and to various inhibitors such as NEM, vanadate, oligomycin, quercetin, p-chloromercuribenzoate, ouabain, and levamisole. Cytosolic activity observed in certain nerve endings in the same conditions of incubation but more sensitive to NEM is also described and discussed.

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.

Morphometric and metabolic profile of the distal segment of the internal mammary artery: caution on its use for coronary anastomoses

OBJECTIVE

Elastic arteries were found to be less prone to intimal hyperplasia than muscular arteries. The internal mammary artery (IMA), which is elastic in its proximal segment, presents a gradual decrease of media elastic fibers along its downstream course. Metabolic and morphometric studies of the distal end of the IMA with regard to its local susceptibility to develop intimal changes were undertaken in order to evaluate the reliability of its use as an anastomotic site for bypass grafting.

METHODS

Twenty distal segments of IMA were harvested from patients who had undergone myocardial revascularization. Histologic, enzyme-histochemical and morphometric studies were undertaken on these arterial segments.

RESULTS

Histologic examinations indicated an elastomuscular structure in 13 patients, a muscular structure in 6 and an elastic structure in 1. Of the 20 IMAs, none was found to have intimal thickening of greater than 25% of the diameter of the lumen. The enzyme-histochemical profile of the proliferating cells found in the intimal thickening differed from normal contractile smooth muscle medial cells in the loss of myosin and mitochondrial ATPase, plasma membrane 5' nucleotidase, moderately decreased aerobic dehydrogenase and increased lactate dehydrogenase activity and ribonucleoprotein-linked pyroninophilia. Lysosomal beta-glucuronidase and sulfatase were strongly active. This enzyme behavior is unfavorable to contractile function and favorable to cell proliferation and lipid accumulation, two events strongly involved in the atherogenic process.

CONCLUSION

Intimal proliferative changes were observed in the distal segment of the IMA. Although there was no histologic evidence of atherosclerotic plaque, the enzyme-histochemical profile of this intimal thickening was favorable to cell proliferation and lipid accumulation. These findings suggest that it may be beneficial to avoid coronary anastomoses with the distal end of the IMA and to use a more proximal/elastic segment.

Thapsigargin discriminates strongly between Ca(2+)-ATPase phosphorylated intermediates with different subcellular distributions in bovine adrenal chromaffin cells

We studied the effects of thapsigargin on the formation of the phosphorylated intermediates (E approximately Ps) of endoplasmic reticulum Ca(2+)-ATPases in microsomes from bovine adrenal medulla. When submicrosomal fractions were separated on a sucrose gradient, two components of 100 kDa Ca(2+)-ATPase E approximately P displaying distinct subcellular distributions were resolved. The first component was defined by Ca(2+)-induced protection against thapsigargin inhibition. The second component did not display such protection, with a 3 orders of magnitude difference in thapsigargin inhibitory potency towards the 2 components. In the absence of Ca2+, both E approximately P components were highly sensitive to thapsigargin inhibition, revealing the presence of high-affinity thapsigargin-binding sites characteristic of SERCA ATPases. These data demonstrate a new level of molecular heterogeneity among Ca(2+)-ATPases of endoplasmic reticulum, and provide the first evidence of differential subcellular localization of individual Ca2+ pump subtypes in cells of neural origin.

Molecular cloning of an intracellular P-type ATPase from Dictyostelium that is up-regulated in calcium-adapted cells

Results from a number of laboratories suggest that intracellular Ca2+ is involved in the regulation of Dictyostelium discoideum growth and development. To learn more about the regulation and function of intracellular Ca2+ in this organism, we have cloned and sequenced cDNAs that encode a putative P-type Ca2+ ATPase designated patA. The deduced protein product of this gene (PAT1) has a calculated molecular mass of 120,718 daltons. It exhibits about 46% amino acid identity with Ca2+ ATPases of the plasma membrane Ca2+ ATPase family and lower identity with sarco(endo)plasmic reticulum Ca2+ ATPase family members and monovalent cation pumps. However, PAT1 lacks the highly conserved calmodulin-binding domain present in the C-terminal region of most plasma membrane Ca2+ ATPase-type enzymes. When Dictyostelium amoebae are adapted to grow in the presence of 80 mM CaCl2, both the patA message and protein product are up-regulated substantially. These cells also exhibit an increase in the rate and magnitude of intracellular P-type Ca2+ uptake activity. Immunofluorescence analysis indicates that PAT1 colocalizes with bound calmodulin to intracellular membranes, probably components of the contractile vacuole complex. The presence of PAT1 on the contractile vacuole suggests that in Dictyostelium this organelle might function in Ca2+ homeostasis as well as in water regulation.

The hydrophilic domain of phospholamban inhibits the Ca(2+)-ATPase--the importance of the method of assay

The peptide MEKVQYLTRSAIRRASTIEMPQQAR-Cys representing residues 1-25 of phospholamban (PLN) decreases by 40% the maximal state rate of ATP hydrolysis by the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum (SR), measured at saturating concentrations of Ca2+. The pattern of Ca2+ uptake by SR vesicles in the presence of oxalate is complex, with an initial fast phase being followed by a lag phase and a second, slower phase of Ca2+ accumulation. PLN(1-25) reduces the rate of the slower phase of Ca2+ accumulation by 30%. However, if the level of accumulation of Ca2+ is measured after 2 min., the effect of PLN(1-25) is much less marked. It is concluded that PLN(1-25) inhibits the ATPase, but that the effects of this inhibition are not apparent under some assay conditions.

In situ hybridization and monoclonal antibody analysis of plasma membrane Ca-pump mRNA and protein in submandibular glands of rabbit, rat and man

The degree of supersaturation of saliva with calcium (Ca) is related to the mineral phase of enamel in erupted teeth, the incidence of caries, and the formation of calculus. The mechanisms for regulating salivary Ca concentration are therefore of relevance to dentistry. Sections of rabbit, rat and human submandibular gland (SMG) were processed for immuno-histochemistry with a specific anti-plasma membrane Ca-pump antibody, 5F10. Western blots confirm that the molecular weight of the proteins identified by our antibody (135 kDa) is consistent with an appropriate molecular weight for PMCA antigen (135-150 kDa). Tissue sections were also processed for in situ hybridization to study the distribution of the PMCA mRNA isoforms. In mammals, the PMCA1 gene is reported to code for a PMCA protein with a role in maintaining the intracellular Ca levels in both epithelial and non-epithelial cells. Other genes including the PMCA2 and PMCA4 genes may code for PMCA proteins specific to Ca transporting tissues. Our studies demonstrate cytoplasmic labeling of PMCA mRNA with hPMCA-1 and hPMCA-4 specific cDNA probes in humans, and rPMCA-1 and rPMCA-2 specific oligonucleotide probes in rats. Labeling of PMCA protein and all mRNA isoforms was found in the cytoplasm of the interlobular and intralobular ducts (except for intercalated ducts). The demonstrated presence of PMCA in SMGs of rabbit, rat, and man, may suggest a role for PMCA in the regulation of intracellular Ca and in a mechanism for regulating and maintaining the high concentration of Ca in salvia.

Histochemistry of sarcoplasmic reticulum Ca-ATPase using dysprosium as capturing reagent

This report describes the development of a histochemical method for the demonstration of sarcoplasmic reticulum Ca-ATPase activity in cross-sections of skeletal muscle. The demonstration of sarcoplasmic reticulum Ca-ATPase activity is complicated by the fact that capturing reagents for phosphate inhibit the enzyme. We present a minimal model for heavy-metal-phosphate precipitation reactions which gives a theoretical description of the effect of enzyme inhibition on the rate of phosphate precipitation in the section. The model indicates that the choice of capturing reagent is crucial: whether or not ATPase activity can be demonstrated depends mainly on the inhibition constant and the solubility product of the phosphate salt of the capturing reagent (but also on a fairly large number of other factors). All lanthanides tested can be used to demonstrate sarcoplasmic reticulum Ca-ATPase activity, but dysprosium results in the highest staining intensity. This suggests that dysprosium inhibits sarcoplasmic reticulum Ca-ATPase to a lesser degree than the other lanthanides and/or the solubility product of its phosphate salt is smaller. As an example, the method is used to investigate the effect of thyroid hormone on sarcoplasmic reticulum Ca-ATPase activity in individual fibres of the rat soleus muscle.

Endoplasmic reticulum Ca(2+)-ATPase in microsomal vesicles isolated from bovine retinae

Active Ca2+ transport was measured in microsomal vesicles prepared from bovine retinae and was compared with that in disk membranes of the photoreceptor cells of the same retina. The 45Ca uptake was dependent on the presence of Mg(2+)-ATP and was inhibited by vanadate or when GTP substituted for ATP. The dependence of calcium uptake on the external free Ca2+ concentration gave a KM = 13 microM or a KM = 0.1 microM for disks and microsomal vesicles, respectively. A phosphorylated intermediate (E-P) of Ca(2+)-ATPase of about 100 kDa was isolated in microsomal vesicles. The E-P formation was strongly inhibited by thapsigargin and partially by 2,5-di-(-butyl)benzohydroquinone. Digestion of disks or microsomes with calpain had no effect on the phosphorylated intermediate, while digestion with trypsin produced two fragments of approximately 55 kDa and 35 kDa. These results suggest that bovine retinal microsomes contain a calcium pump belonging to the SERCA family.

Quantification in crude homogenates of rat myocardial Na+, K(+)- and Ca(2+)-ATPase by K+ and Ca(2+)-dependent pNPPase. Age-dependent changes

Assays for complete quantification of Na+, K(+)- and Ca(2+)-ATPase in crude homogenates of rat ventricular myocardium by determination of K(+)- and Ca(2+)-dependent p-nitrophenyl phosphatase (pNPPase) activities were evaluated and optimized. Using these assays the total K(+)- and Ca(2+)-dependent pNPPase activities in ventricular myocardium of 11-12 week-old rats were found to be 2.98 +/- 0.10 and 0.29 +/- 0.02 mumol x min-1 x g-1 wet wt. (mean +/- SEM) (n = 5), respectively. Coefficient of variance of interindividual determinations was 7 and 12%, respectively. The total Na+, K(+)- and Ca(2+)-ATPase concentrations were estimated to 2 and 10 nmol x g-1 wet wt., respectively. Evaluation of a putative developmental variation revealed a biphasic age-related change in the rat myocardial Ca(2+)-dependent pNPPase activity with an increase from birth to around the third week of life followed by a decrease. By contrast, the K(+)-dependent pNPPase activity of the rat myocardium showed a decrease from birth to adulthood. It was excluded that the changes were simple outcome of variations in water and protein content of myocardium. Expressed per heart, the K(+)- and Ca(2+)-dependent pNPPase activity gradually increased to a plateau. The present assay for Na+, K(+)-ATPase quantification has the advantage over [3H] ouabain binding of being applicable on the ouabain-resistant rat myocardium, and is more simple and rapid than measurements of K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase) in crude tissue homogenates. Furthermore, with few modifications the pNPPase assay allows quantification of Ca(2+)-ATPase on crude myocardial homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional coupling between glycolysis and sarcoplasmic reticulum Ca2+ transport

To investigate whether the energy derived from glycolysis is functionally coupled to Ca2+ active transport in sarcoplasmic reticulum (SR), we determined whether glycolytic enzymes were associated with SR membranes and whether metabolism through these enzymes was capable of supporting 45Ca transport. Sealed right-side-out SR vesicles were isolated by step sucrose gradient from rabbit skeletal and cardiac muscle. Intravesicular 45Ca transport was measured after the addition of glycolytic substrates and cofactors specific for each of the glycolytic reactions being studied or after the addition of exogenous ATP and was expressed as transport sensitive to the specific Ca(2+)-ATPase inhibitor thapsigargin. We found that the entire chain of glycolytic enzymes from aldolase onward, including aldolase, GAPDH, phosphoglycerate kinase (PGK), phosphoglyceromutase, enolase, and pyruvate kinase (PK), was associated with SR vesicles from both cardiac and skeletal muscle. Iodoacetic acid, an inhibitor of GAPDH, eliminated 45Ca transport supported by fructose-1,6-diphosphate, the substrate for aldolase, but transport was completely restored by phosphoenolpyruvate (the substrate for PK), indicating that both of the ATP-producing glycolytic enzymes, GAPDH/PGK and PK, were associated with the SR and functionally capable of providing ATP for the Ca2+ pump. Addition of a soluble hexokinase ATP trap eliminated 45Ca transport fueled by exogenous ATP but had markedly less effect on 45Ca transport supported by endogenously produced ATP (via glycolysis). Similarly, at very low concentrations of ATP and ADP (10 to 50 nmol/L), ATP that was produced endogenously from ADP and phosphoenolpyruvate supported 15-fold more 45Ca transport than ATP that was supplied exogenously at the same concentration. These results are consistent with functional coupling of glycolytic ATP to Ca2+ transport and support the hypothesis that ATP generated by SR-associated glycolytic enzymes may play an important role in cellular Ca2+ homeostasis by driving the SR Ca2+ pump.

Renaissance of cytochemical localization of membrane ATPases in the myocardium

ATPases of cardiac cells are known to be among the most important enzymes to maintain the fluxes of vital cations by hydrolysis of the terminal high-energy phosphate of ATP. Biochemically the activities of Ca(2+)-pump ATPase, Ca2+/Mg(2+)-ecto ATPase, Na+,K(+)-ATPase and Mg(2+)-ATPase are determined in homogenates and isolated membranes as well as in myofibrillar and mitochondrial fractions of various purities. Such techniques permit estimation of enzyme activities in vitro under optimal conditions without precise enzyme topography. On the other hand, cytochemical methods demonstrate enzyme activity in situ, but not under optimal conditions. Until recently several cytochemical methods have been employed for each enzyme in order to protect its specific activity and precise localization but the results are difficult to interpret. To obtain more consistent data from biochemical and cytochemical point of view, we modified cytochemical methods in which unified conditions for each ATPase were used. The fixative solution (1% paraformaldehyde-0.2% glutaraldehyde in 0.1 M Tris Base buffer, pH 7.4), the same cationic concentrations of basic components in the incubation medium (0.1 M Tris Base, 2 mM Pb(NO2)3, 5 mM MgSO4, 5 mM ATP) and selective stimulators or inhibitors were employed. The results reveal improved localization of Ca(2+)-pump ATPase, Na(+)-K+ ATPase and Ca2+/Mg(2+)-ecto ATPase in the cardiac membrane.

Characterisation of endoplasmic reticulum and plasma membrane Ca(2+)-ATPases in pancreatic beta-cells and in islets of Langerhans

We have investigated the plasma membrane (PMCA) and endoplasmic reticulum (SERCA) Ca(2+)-ATPases involved in active transport of Ca2+ in pancreatic beta-cell lines (MIN6, HIT T15, RINm5F) and in islets of Langerhans. Under selective membrane phosphorylation conditions (at low ATP concentration, in the presence of Ca2+ and La3+ and in the absence of Mg2+ at 4 degrees C) the only labelled proteins are the phosphoenzyme intermediates of the Ca(2+)-ATPases. Under these conditions, beta-cell membranes incorporated 32P from [gamma-32P]ATP into two proteins with molecular mass on acidic SDS-polyacrylamide gels of around 115 and 150 kDa. The 150 kDa band was identified as PMCA (i) by reaction with a monoclonal anti-human erythrocyte plasma membrane Ca(2+)-ATPase antibody; (ii) by its typical tryptic cleavage pattern which generated an 80 kDa band; (iii) by lack of inhibition of its autophosphorylation by SERCA-specific inhibitors. The 115 kDa band was identified as SERCA (i) by reaction with a polyclonal anti-rat fast skeletal muscle Ca(2+)-ATPase antibody; (ii) by the concentration-dependent inhibition of its autophosphorylation by thapsigargin and 2,5-di(t-butyl)-1,4-benzohydroquinone (tBHQ), which are specific inhibitors of SERCA. The 115 kDa band was further characterised as the SERCA-2b isoform by reaction with a polyclonal rabbit antibody against the 12 C-terminal amino acids of SERCA-2b.

A continuous spectrophotometric assay for simultaneous measurement of calcium uptake and ATP hydrolysis in sarcoplasmic reticulum

A continuous, spectrophotometric assay to simultaneously measure Ca uptake and ATP hydrolysis has been developed, in order to assess the function of the Ca-ATPase in skeletal and cardiac sarcoplasmic reticulum (SR) vesicles. The absorbance of Fura Red was measured continuously at 490 nm, in EGTA-buffered solutions containing initial free ionized calcium concentrations of 300 nM, 500 nM, 790 nM, and 2 microM, during assays of oxalate-facilitated or phosphate-facilitated active calcium uptake in skeletal SR. Simultaneous measurement of ATP hydrolysis during the measurement of phosphate-facilitated Ca uptake was accomplished by measuring the disappearance of NADH at 340 nm, coupled to the hydrolysis of ATP by an enzyme-linked, continuous ATPase assay. This new method, unlike the standard 45Ca-filtration assay, measures calcium uptake in real time and eliminates the need for radioactivity. Moreover, the rates of calcium uptake and ATP hydrolysis are measured simultaneously, allowing the direct quantitative comparison of the two parameters. This assay will facilitate the characterization of Ca-ATPase function and malfunction in skeletal and cardiac SR and advances the methodology for comparison of normal and physically, chemically, or biologically altered Ca-ATPase.

Tissue distribution of the four gene products of the plasma membrane Ca2+ pump. A study using specific antibodies

Antibodies against the four isoforms of the human plasma membrane Ca(2+)-ATPase (PMCA) were raised using an N-terminal sequence of the pump as epitope. The antibodies against PMCA isoforms 1, 2, and 3 were not species-specific, e.g. they also recognized the corresponding proteins in rat, whereas that against the human PMCA isoform 4 failed to do so. The tissue distribution of the four isoforms was estimated by Western blot analysis. Two, PMCA1 and PMCA4, were expressed in all tissues tested (with the exception of the choroid plexus, where the former was not detected). In most tissues the signal from the PMCA1 protein exceeded that of PMCA4, the exception being the erythrocyte. The PMCA2 and PMCA3 proteins were only found in neuronal tissues; the PMCA2 protein was present in high concentrations in the cerebellum and in the cerebral cortex. At variance with previous results on mRNA (e.g. the kidney) no other tissues contained the PMCA2 protein. PMCA3 was the other tissue-specific isoform; in agreement with results in the rat, the protein was found in human neuronal tissues, particularly in the choroid plexus, but was practically absent in all other tissues tested.

Localization and identification of Ca2+ATPases in highly purified human platelet plasma and intracellular membranes. Evidence that the monoclonal antibody PL/IM 430 recognizes the SERCA 3 Ca2+ATPase in human platelets

The Ca2+ATPase activities of highly purified human platelet membranes prepared by high-voltage free-flow electrophoresis have been analysed by using [gamma-32P]ATP hydrolysis, recognition by antibodies and phosphoenzyme-complex formation. The Ca2+ATPase activity present in mixed membranes was found to be predominantly associated with intracellular membranes after subfractionation, with only a low level of activity associated with plasma membranes. The intracellular-membrane Ca2+ATPase activity was inhibited totally with thapsigargin (Tg), whereas the plasma-membrane Ca2+ATPase was not significantly affected, suggesting that the latter does not belong to the SERCA (sarco-endoplasmic-reticulum Ca2+ATPase) class. A monoclonal antibody, 5F10, raised to the red-cell membrane Ca2+ATPase [Cheng, Magocsi, Cooper, Penniston and Borke (1993) Cell Physiol. Biochem. 4, 31-43] recognized two bands at 135 and 150 kDa in mixed membranes and plasma membranes, and the corresponding bands in red-blood-cell membranes, confirming the Ca2+ATPase to be of the PMCA (plasma-membrane Ca2+ATPase) type. No recognition of any band was detected in intracellular membranes. Identification of the intracellular-membrane Ca2+ATPase activity was carried out with polyclonal antibodies with known specificity towards SERCA 2b (S.2b) and SERCA 3 (N89), and a monoclonal antibody, PL/IM 430, raised against platelet intracellular membranes. All of these antibodies recognized the 100 kDa Ca2+ATPase in mixed membranes and intracellular membranes, with little or no recognition of the activity in the plasma membranes. In some membrane preparations the antibody PL/IM 430 and antiserum N89 recognized similar degradation products, of 74, 70 and 40 kDa, in the intracellular-membrane fraction. The Ca2+ATPase recognized by PL/IM 430 was immunoprecipitated, and the immunoprecipitated protein was specifically recognized by the antiserum N89, but not by S.2b. Analysis of the phosphoenzyme-complex formation revealed potent phosphorylation of the 100 and 74 kDa peptides, both recognized by PL/IM 430 and N89. These studies report the presence of a PMCA in a purified plasma-membrane fraction from human platelets, and that the antibody PL/IM 430 recognizes the SERCA 3 Ca2+ATPase in intracellular membranes.

ATPase activities of rabbit and bovine lens epithelial microsomes: a continuous fluorimetric assay study

Purine nucleoside phosphorylase (EC 2.4.2.1) catalyzes the irreversible phosphorolysis of 7-methylguanosine (m7Guo), a fluorescent guanosine analogue. Using purine nucleoside phosphorylase and m7Guo, a continuous fluorimetric assay for microsomal ATPases from rabbit and bovine lens is described. The decrease in m7Guo fluorescence intensity at 380 nm, which represents the hydrolysis of ATP, is linear with time up to exhausting all m7Guo. The rate of the fluorescence decrease depends on the sample protein concentration. In the presence of ATPase inhibitors, ion-specific ATPase activities in the lens were determined from the difference of the fluorescence decay rates. Using the fluorimetric assay thapsigargin-sensitive Ca-ATPase in the bovine lens epithelium has been characterized. The fluorimetric assay provides a number of advantages over previous membrane ATPase assays.

Immuno-localization of the calcitriol receptor, calbindin-D28k and the plasma membrane calcium pump in the human eye

Using immunohistochemical methods, we detected epitopes of the calcitriol receptor, the ATP-dependent plasma membrane calcium pump and the 28kD vitamin D-dependent calcium-binding protein in sections of the human eye. In retinal photoreceptors, vitamin D receptor, plasma membrane calcium pump and calcium-binding protein epitopes were detected in the outer nuclear layer. Epitopes for the vitamin D receptor and the calcium-binding protein were present in the inner and outer segments of the photoreceptors, where visual transduction occurs. All three proteins were detected in some cells of the ganglion cell layer, the inner nuclear layer, and the retinal pigment epithelium. Epitopes for these proteins also were noted in the ciliary body epithelium. VDR epitopes were seen in lens epithelium. Some immunostaining for VDR, PMCA and calbindin-D28k also was present in the endothelium and in the basal epithelium of the cornea. The presence of these proteins in several tissues of the human eye suggests that the proteins may play a role in the cellular physiology of the eye. Their exact functions in the eye remain undetermined.

Use of thermal analysis to distinguish magnesium and calcium stimulated ATPase activity in isolated transverse tubules from skeletal muscle

The presence of calcium stimulated adenosine triphosphatase (Ca2+,Mg(2+)-ATPase) activity in isolated transverse tubule (t-tubule) membranes is distinguished from magnesium adenosine triphosphatase (Mg(2+)-ATPase) activity on the basis of differing thermal stabilities. The Mg(2+)-ATPase is the major protein component of the t-tubule membrane, and it can be difficult to discriminate between the low levels of Ca2+ stimulated ATPase activity found in isolates of t-tubules compared to the much higher Mg(2+)-ATPase activity. Thermal analysis reveals different inactivation temperatures (Ti) for the proteins responsible for ATP dependent calcium transport (Ti = 49 degrees C) and Mg(2+)-ATPase activity (Ti = 57 degrees C) in isolated t-tubule membranes. The differential scanning calorimetry profile of t-tubule membranes consists of three major components with transition temperatures (Tm) of 51 degrees C, 57 degrees C and 63 degrees C. Denaturation of the component with Tm = 57 degrees C correlates with inactivation of Mg(2+)-ATPase activity, and denaturation of the Tm = 51 degrees C component correlates with the inactivation of Ca2+,Mg(2+)-ATPase activity and calcium transport. The functions of the t-tubule membrane component or components that denature with Tm = 63 degrees C have yet to be identified. The lack of stimulation of calcium transport in isolated t-tubules by oxalate, the impermeability of isolated t-tubules to oxalate, and experiments performed on t-tubules with defined amounts of sarcoplasmic reticulum (SR) added suggest that contamination of the isolated t-tubules by SR is unlikely to account for the level of Ca2+,Mg(2+)-ATPase activity detected. The presence of a Ca2+,Mg(2+)-ATPase in the t-tubule membrane would provide a mechanism that may be involved in the partial removal of calcium that is accumulated in the junctional space during muscle relaxation or calcium that is released from the terminal cisternae of sarcoplasmic reticulum during excitation-contraction coupling.

Immunolocalization of calcitriol receptor, plasma membrane calcium pump and calbindin-D28k in the cornea and ciliary body of the rat eye

Epitopes of the calcitriol receptor, the ATP-dependent plasma membrane calcium pump (PMCA) and the 28-kD vitamin-D-dependent Ca-binding protein (calbindin-D28k) were detected in sections of the albino rat eye using light microscopy and immunohistochemistry of paraffin-embedded tissues. Calcitriol receptor and calbindin-D28k epitopes were detected in both the inner and outer layers of the ciliary body epithelium. PMCA was present in both epithelial cell layers of the ciliary body but was most prominent in the inner layer. Corneal epithelium and endothelium also contained epitopes for calcitriol receptor, PMCA and calbindin-D28k. These Ca-regulatory proteins may play a role in the cellular physiology of the albino rat eye by maintaining appropriate intracellular and aqueous humor Ca concentrations.

Localization of plasma membrane Ca2+ pump mRNA and protein in human ameloblasts by in situ hybridization and immunohistochemistry

The distribution of the plasma membrane Ca(2+)-pump (PMCA) proteins in human ameloblasts was examined immunohistochemically using monoclonal antibodies JA8 and 5F10. Further, the distribution of mRNA transcripts derived from two PMCA genes, PMCA-1 and PMCA-4 was examined using in situ hybridization. In rats, the PMCA-1 gene is purported to code for PMCA proteins with a role in maintaining the intracellular Ca2+ levels in nonepithelial cells. Other genes including the PMCA-4 gene may code for PMCA proteins characteristic of Ca2+ transporting epithelia. The present results show immunohistochemical staining in the Tomes processes and plasma membranes of human ameloblasts. Our studies also demonstrate a gradation of expression of the PMCA-1 and PMCA-4 mRNA transcripts which parallels the onset and progression of enamel mineralization. These studies suggest that PMCA proteins in human ameloblasts may function both in intracellular Ca2+ homeostasis and in regulating the vectorial Ca2+ influx into mineralizing enamel.

The influence of nifedipine and verapamil on the ischaemia-induced changes in canine sarcolemmal membranes

Ischaemic injury was produced in the dog heart by occluding the left anterior descending coronary artery just below the second diagonal branch for the duration of 3 h. Nifedipine 5.0 mg/kg and Verapamil 5.0 mg/kg were injected intravenously 10 min before the occlusion. Sarcolemmal membrane vesicles were prepared from an ischaemic and non-ischaemic hearts. Without treatment with Nifedipine or Verapamil the total amount of sarcolemmal phospholipid obtained from ischaemic hearts, as well as the amounts of phosphatidylcholine and phosphatidylethanolamine were significantly decreased as compared with those from non-ischaemic hearts. Calcium channel blocker pretreatment abolished the decrease in the total amount of sarcolemmal phospholipids, as well as phosphatidylcholine and phosphatidylethanolamine content induced by ischaemia. Thus, our work indicates that Nifedipine and Verapamil, suppressed the ischaemia-induced increase in phospholipid breakdown of cardiac sarcolemma.

Targeted disruption of the Dictyostelium RMLC gene produces cells defective in cytokinesis and development

Conventional myosin has two different light chains bound to the neck region of the molecule. It has been suggested that the light chains contribute to myosin function by providing structural support to the neck region, therefore amplifying the conformational changes in the head following ATP hydrolysis (Rayment et al., 1993). The regulatory light chain is also believed to be important in regulating the actin-activated ATPase and myosin motor function as assayed by an in vitro motility assay (Griffith et al., 1987). Despite extensive in vitro biochemical study, little is known regarding RMLC function and its regulatory role in vivo. To better understand the importance and contribution of RMLC in vivo, we engineered Dictyostelium cell lines with a disrupted RMLC gene. Homologous recombination between the introduced gene disruption vector and the chromosomal RMLC locus (mlcR) resulted in disruption of the RMLC-coding region, leading to cells devoid of both the RMLC transcript and the 18-kD RMLC polypeptide. RMLC-deficient cells failed to divide in suspension, becoming large and multinucleate, and could not complete development following starvation. These results, similar to those from myosin heavy chain mutants (DeLozanne et al., 1987; Manstein et al., 1989), suggest the RMLC subunit is required for normal cytokinesis and cell motility. In contrast to the myosin heavy chain mutants, however, the mlcR cells are able to cap cell surface receptors following concanavilin A treatment. By immunofluorescence microscopy, RMLC null cells exhibited myosin localization patterns different from that of wild-type cells. The myosin localization in RMLC null cells also varied depending upon whether the cells were cultured in suspension or on a solid substrate. In vitro, purified RMLC- myosin assembled to form thick filaments comparable to wild-type myosin, but the filaments then exhibit abnormal disassembly properties. These results indicate that in vivo RMLC is necessary for myosin function.

Characterization of the sarcoplasmic reticulum proteins in the thermogenic muscles of fish

Marlins, sailfish, spearfishes, and swordfish have extraocular muscles that are modified into thermogenic organs beneath the brain. The modified muscle cells, called heater cells, lack organized myofibrils and are densely packed with sarcoplasmic reticulum (SR), transverse (T) tubules, and mitochondria. Thermogenesis in the modified extraocular muscle fibers is hypothesized to be associated with increased energy turnover due to Ca2+ cycling at the SR. In this study, the proteins associated with sequestering and releasing Ca2+ from the SR (ryanodine receptor, Ca2+ ATPase, calsequestrin) of striated muscle cells were characterized in the heater SR using immunoblot and immunofluorescent techniques. Immunoblot analysis with a monoclonal antibody that recognizes both isoforms of nonmammalian RYRs indicates that the fish heater cells express only the alpha RYR isoform. The calcium dependency of [3H]ryanodine binding to the RYR isoform expressed in heater indicates functional identity with the non-mammalian alpha RYR isoform. Fluorescent labeling demonstrates that the RYR is localized in an anastomosing network throughout the heater cell cytoplasm. Measurements of oxalate supported 45Ca2+ uptake, Ca2+ ATPase activity, and [32P]phosphoenzyme formation demonstrate that the SR contains a high capacity for Ca2+ uptake via an ATP dependent enzyme. Immunoblot analysis of calsequestrin revealed a significant amount of the Ca2+ binding protein in the heater cell SR. The present study provides the first direct evidence that the heater SR system contains the proteins necessary for Ca2+ release, re-uptake and sequestration, thus supporting the hypothesis that thermogenesis in the modified muscle cells is achieved via an ATP-dependent cycling of Ca2+ between the SR and cytosolic compartments.

Ca(2+)-ATPase distributes differently in cardiac sarcolemma than dihydropyridine receptor alpha 1 subunit and Na+/Ca2+ exchanger

We have investigated the distribution of the sarcolemmal Ca2+ transporters in hamster and dog ventricular myocytes by immunocytochemical and membrane fractionation techniques. The data suggest that the DHP receptor alpha 1 subunit and the Na+/Ca2+ exchanger are present in surface sarcolemma as well as T-tubule membranes located at the cardiac dyads. Compared with these Ca2+ transporters, the sarcolemmal Ca(2+)-ATPase is much less abundant in the latter fraction. Thus the sarcolemmal Ca(2+)-ATPase seems to be located predominantly in surface sarcolemma.

Overexpression and characterization of a gene for a Ca(2+)-ATPase of the endoplasmic reticulum in Trypanosoma brucei

Procyclic forms of Trypanosoma brucei were stably transformed with an expression vector containing a gene for a P-type ATPase (tba1) cloned from T. brucei genomic DNA. Transformation with this gene resulted specifically in a 4-5-fold increase in the cellular Ca(2+)-ATPase activity. Subcellular fractionation studies revealed this increase to be enriched in the microsomal fraction. There was no detectable change in the plasma membrane Ca(2+)-ATPase activity of the transformants. Western blot analysis of subcellular fractions using antibodies raised against the recombinant tba1 gene product also demonstrated a significant enrichment of a protein with a M(r) of 115,000 in the microsomal fraction of transformed cells. This protein was not detected in purified plasma membranes. Significantly, the increased Ca(2+)-ATPase activity possessed a high affinity for Ca2+. The activity was sensitive to the classical P-type ATPase inhibitor vanadate, anti-tba1 antibodies, as well as low concentrations of thapsigargin, a specific inhibitor of endoplasmic reticulum Ca(2+)-ATPases. Taken together, these data demonstrated that the tba1 gene codes for a high affinity Ca(2+)-ATPase of the endoplasmic reticulum, with properties similar to those reported for the sarcoplasmic/endoplasmic reticulum family of Ca2+ pumps from higher eukaryotes. In addition, these results have identified the tba1 gene product as potentially important element, in conjunction with the mitochondrial membrane potential and the plasma membrane Ca2+ pump, in the pathways of cellular Ca2+ homeostasis in these protozoans.

Preservation of sarcoplasmic reticulum Ca2(+)-sequestering function in homogenates of different type composition following sprint activity

To examine the effect of exercise on sarcoplasmic reticulum function in muscle tissue of different fibre composition, adult male Wistar rats weighing 388 +/- 23 g (x +/- SE) ran intermittently on a treadmill until fatigue. Fatigue was induced by 15-20 min of running performed at 52 m/min on an 8 degrees incline in periods of 2.5 min of exercise separated by 2 min of recovery. Analysis of sarcoplasmic reticulum Ca2+ ATPase activity determined in homogenates indicated no difference (p > 0.05) between age-matched control and exercised tissue for the soleus (SOL; 0.121 +/- 0.012 vs. 0.156 +/- 0.018 mumol.mg-1 protein.min-1), red gastrocnemius (RG; 0.381 +/- 0.022 vs. 0.354 +/- 0.022), or white gastrocnemius (WG; 0.526 +/- 0.05 vs. 0.471 +/- 0.031). Similarly, both total ATPase and Mg2+ ATPase activities were unaffected by the exercise in any of the tissues examined. Exercise also failed to alter sarcoplasmic reticulum Ca2+ uptake in homogenates of the SOL (1.43 +/- 0.15 vs. 1.38 +/- 0.19 nmol.mg-1 protein.min-1), RG (3.74 +/- 0.29 vs. 3.59 +/- 0.24), and WG (5.98 +/- 0.48 vs. 5.41 +/- 0.50). At fatigue, glycogen depletion was similar in all tissue types and amounted to 65.1% in the SOL (172 +/- 9 vs. 60 +/- 16 mmol.glucosyl units-1.kg-1 dry weight), 74.4% in RG (164 +/- 8 vs. 42 +/- 6), and 79% in the WG (167 +/- 9 vs. 35 +/- 9). It is concluded that exercise by itself does not alter sarcoplasmic reticulum Ca2(+)-sequestering function in tissues of primarily different fibre composition when determined in homogenates in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of palmitoyl carnitine and related metabolites on the avian Ca(2+)-ATPase and Ca2+ release channel

1. In birds, prolonged cold exposure induces the development of a non-shivering thermogenesis (NST) of muscular origin that may result from an increase in ATP-dependent cycling of Ca2+ between the sarcoplasmic reticulum (SR) and the cytosol. 2. Because fatty acids are thought to play a significant role in NST, we investigated the effects of palmitic acid and related metabolites on skeletal SR Ca2+ uptake and release in ducklings. 3. Ca(2+)-ATPase activity, 45Ca2+ release and [3H]ryanodine-binding measurements indicated that palmitic acid was without effect on the Ca(2+)-ATPase and Ca2+ release channel. Palmitoyl carnitine and palmitoyl coenzyme A inhibited the Ca(2+)-ATPase at concentrations > 20 microM whereas both activated the Ca2+ release channel at concentrations < or = 20 microM in a dose-dependent manner. 4. Palmitoyl carnitine stimulated [3H]ryanodine binding to skeletal but not cardiac SR vesicles. Induction of 45Ca2+ release was observed with long-chain (C > or = 14) but not with short-chain acyl carnitines (C < or = 12). 5. Long-chain acyl carnitines accumulated significantly in duckling skeletal muscle during cold acclimation. Accordingly, these results suggest that long-chain acyl metabolites may modulate SR Ca2+ cycling and its associated thermogenesis in vivo.

Effect of inhibition of sarcoplasmic Ca(2+)-ATPase on vasoconstriction and cytosolic Ca2+ in aortic smooth muscle from spontaneously hypertensive and normotensive rats

To evaluate the influence of the sarcoplasmic Ca(2+)-ATPase, isometric vasoconstrictions of aortic strips from spontaneously hypertensive rats from the Münster strain (SHR) and normotensive Wistar-Kyoto rats (WKY) were measured after inhibition of Ca(2+)-ATPase by thapsigargin. Inhibition of Ca(2+)-ATPase by thapsigargin caused a biphasic contractile response of the aorta in both SHR and WKY (maximum increase of tension: 1.7 +/- 0.3 x 10(-3) Newton and 2.1 +/- 0.3 x 10(-3) Newton, respectively; mean +/- SE). The second peak of the contractile response was abolished in the absence of external calcium or by inhibition of transplasmamembrane calcium influx by nifedipine, indicating that the second peak occurs as a consequence of calcium influx from the extracellular space. The initial peak of the contractile response after thapsigargin administration was abolished in the presence of an intracellular calcium antagonist, 8-(diethylamino-)-octyl-3,4,5-trimethoxybenzoate (TMB-8), indicating that the initial response was due to calcium release from intracellular stores. Measurements using the fluorescent dye fura2 showed that thapsigargin increased the cytosolic free calcium concentration ([Ca2+]i) in SHR by 72.6 +/- 7.3 nmol/l (n = 34) and in WKY by 53.3 +/- 6.6 nmol/l (n = 39), showing no significant differences between the two strains. The inhibition of Ca(2+)-ATPase increases [Ca2+]i and causes vasoconstriction. The vasoconstriction produced by thapsigargin is not significantly different between SHR and WKY.

Ultrastructural, histochemical, and morphometric analysis of skeletal muscle in a murine model of type I diabetes

BACKGROUND

Since peripheral nerves are damaged in diabetes mellitus, morphological changes occur within the diabetic muscle in response to the diabetic neuropathy. The aim of this study was to examine the extensor digitorum longus (EDL) from a 42-day streptozotocin-induced diabetic Swiss Webster mouse (STZ) and compare the muscle morphology and histochemistry to age-matched, nondiabetic controls.

METHODS

The EDL was evaluated using electron microscopy in order to investigate the morphological integrity of the myofibers and neuromuscular junctions. Histochemical analysis was completed using the myofibrillar CA(++)-ATPase reaction of Doriguzzi et al. (1983. Histochemistry, 79:289-294) for use in computer-assisted morphometric analysis of fiber size using Bioquant System 4 software.

RESULTS

Ultrastructural analysis of the diabetic EDL (N = 5, 225 myofibers/animal) showed a significant number of abnormal myofibers, exhibiting various degrees of degeneration, signs of denervation, and necrosis. The STZ myofibers exhibited excessive lipid accumulations and abnormal mitochondrial arrangements. Histochemical analysis of the STZ EDL revealed a significant shift in fiber type profile (53.6% type 2A and 46.4% type 2B- STZ myofibers; 47.5% type 2A, 52.5% type 2B nondiabetic controls). Morphometric analysis of myofiber size by fiber type (200 myofibers/muscle/fiber type) indicated a significant decrease in myofiber size for both type 2A and type 2B fibers in the STZ diabetic mouse.

CONCLUSION

The degeneration and necrosis of myofibers concomitant with the sever atrophy of both the type 2A and 2B myofibers in the STZ muscle could account for the functional alterations seen in diabetic muscle.

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)

Several compartments of Saccharomyces cerevisiae are equipped with Ca(2+)-ATPase(s)

Sucrose density fractionation of yeast membranes revealed two major and two minor peaks of 45Ca2+ transport activity which all co-migrate with marker enzymes of the endoplasmic reticulum, Golgi and membranes associated with these compartments as well as with ATPase activity measured when all other known ATPase are inhibited. Co-migration of 45Ca2+ transport and ATPase activities was also found after removal of plasma membranes by concanavalin A treatment. SDS-PAGE at pH 6.3 shows the Ca(2+)-dependent formation of acyl phosphate polypeptides of about 110 and 200 kDa. It is concluded that several compartments or sub-compartments of yeast are equipped with Ca(2+)-ATPase(s). It is proposed that these compartments are derived from the protein secretory apparatus of yeast.

In situ mRNA distribution of sarco(endo)plasmic reticulum Ca(2+)-ATPase isoforms during ontogeny in the rat

The Sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) plays a crucial role in maintaining the Ca2+ homeostasis, which itself, controls various essential cellular function. The existence of several SERCA isoforms, encoded by three different genes and produced by alternative splicing of pre-mRNA transcripts, has been established by cDNA cloning. However, the temporo-spatial evolution of their expression during ontogeny was unknown. We have used in situ hybridization to determine the cellular distribution of three of these mRNA isoforms, SERCA 2a, SERCA 2b and SERCA 3 during rat ontogeny and focused our study on the cardiovascular system. We demonstrate that early in embryogenesis, SERCA 3 mRNA is highly expressed in the heart tube and is also present in the yolk sac. In 14-16 days embryos, SERCA 3 mRNA has disappeared from the heart but is expressed in the aorta and in discrete foci of the liver. Later on, its expression in the cardiovascular system is restricted to the arterial endothelium. SERCA 2a mRNA is coexpressed with SERCA 3 mRNA in the heart tube and remains expressed in the cardiomyocytes throughout life. It is transiently expressed in skeletal muscle at the onset of differentiation. In early foetal life, SERCA 2b is expressed in the mesenteric area and thereafter in all cell types at various levels. Our data indicate that (i) expression of SERCA 2b is neither tissue-specific nor developmentally regulated (ii) expression of SERCA 2a and SERCA 3 isoforms is regulated in a cell specific manner during development and suggest that the SERCA 3 gene plays a role in controlling the function of endothelial cells during vasculogenesis.

Ethanol-induced changes in membrane ATPases: inhibition by iron chelation

The effect of chronic ethanol intake, with and without an iron chelator, on the activity of rat membrane ATPases was investigated. Using the intragastric feeding model, male Wistar rats (250 g) were fed a liquid diet and ethanol for 1 month. In control pair-fed animals, ethanol was isocalorically replaced by dextrose. In addition to the above groups, two groups of animals (dextrose or ethanol-fed) also received an oral iron chelator (1,2-dimethyl-3-hydroxypyrid-4-one, L1) (25 mg/kg/day for 30 days). The blood ethanol levels were maintained between 150 and 300 mg/dL. Red cells were washed immediately with ice-cold saline, membranes were prepared, and ATPases were measured. The mean Ca2+ pump ATPase in animals fed ethanol was lower than in dextrose-fed controls. In contrast, Na+/K+ pump ATPase was enhanced following chronic ethanol treatment. The addition of L1 to the diet prevented the changes in both the Ca(2+)-ATPase and Na+/K(+)-ATPase in ethanol-fed rats. Although the exact mechanism for the prevention of changes in ATPase activity by L1 is unknown, it is not a result of non-specific interaction between the chelator and membranes. Incubation of purified membranes with different concentrations of L1 for 60 min at 37 degrees had no effect on the activity of the ATPase. In conclusion, chronic intake of ethanol specifically inhibited Ca2+ pump ATPase and enhanced Na+/K(+)-ATPase in rat red blood cell membranes. The iron chelator, L1, corrected both of these ethanol-induced changes.

Cytochemical study of the effect of aluminium on cultured brain microvascular endothelial cells

The cytotoxic effect of aluminium was studied on cultured goat brain microvascular endothelial cells used as an in vitro model of the blood-brain barrier. Confluent monolayers of these cells were exposed for 4 days to aluminium maltol and, for control purposes, to maltol alone, and also to cadmium chloride as a known cytotoxic substance. The localization of plasmalemma-bound enzymatic activities of 5'-nucleotidase and Ca(2+)-ATPase and the distribution of sialic acid residues were studied at the ultrastructural level. It was observed that the reaction for 5'-nucleotidase activity was only insignificantly affected, indicating its resistance to the cytotoxic action of both substances used. On the contrary, the activity of Ca(2+)-ATPase was evidently suppressed, especially in the interendothelial clefts where junctional complexes are presumably to be formed. Aluminium also affects the density of sialic acid residues, as shown by their redistribution, leading to the appearance of relatively long segments of unlabelled apical cell surface. The data obtained suggest that observed changes in the localization of Ca(2+)-ATPase and sialic acid residues can lead ultimately to impairment of the formation and maintenance of intercellular junctions and to disturbances in the negatively charged domains of the endothelial cell surface. Whether these alterations, induced in vitro, contribute to in vivo disturbances of blood-brain barrier function requires further experimental study.

Immunochemical quantification of sarcoplasmic reticulum Ca(2+)-ATPase and calsequestrin in muscle biopsies from patients with myotonia congenita and paramyotonia congenita Eulenburg

A sensitive enzyme-linked immunoadsorbant assay was developed to quantify Ca(2+)-ATPase and calsequestrin from sarcoplasmic reticulum in human muscle biopsies. Tissue levels of Ca(2+)-ATPase and calsequestrin averaged 51.5 +/- 28.1 and 6.4 +/- 1.8 mg/g muscle protein, respectively, in control muscles (means +/- SD, n = 12). The high sensitivity and specificity of the antibodies make the assay a useful tool in the diagnosis of human neuromuscular disorders where defects in sarcoplasmic reticulum function may be expected. The assay was applied to muscle biopsies from patients with myotonia congenita and paramyotonia congenita Eulenburg. The calsequestrin concentration was normal in all patient muscles. The Ca(2+)-ATPase content was also within the normal range but varied considerably with the percentage distribution of slow-twitch fibres. This indicates that the prolonged relaxation observed in the muscles of patients with these disorders is not caused by faulty expression of Ca(2+)-ATPase and calsequestrin.

Low-frequency stimulation of rat fast-twitch muscle enhances the expression of hexokinase II and both the translocation and expression of glucose transporter 4 (GLUT-4)

This study followed changes in the capacities of uptake and phosphorylation of glucose in response to contractile activity in low-frequency stimulated (10Hz, 24 h/d) rat fast-twitch muscle. We investigated the intracellular distribution of GLUT-4, the major glucose transporter isoform in muscle, changes in the amounts of its specific mRNA and total cellular protein, as well as changes in its relative synthesis rate. These analyses were complemented by measurements of total hexokinase activity and hexokinase II (HKII) expression at the levels of mRNA content and protein synthesis. Changes in protein synthesis were determined by in vivo labeling with [35S]methionine. Translocation of GLUT-4 into the sarcolemma was an immediate response to contractile activity, whereas changes in its total amount were observed only with ongoing stimulation (5 d and longer). A twofold increase in GLUT-4 content after 5 d and longer stimulation periods was preceded by elevations of its mRNA and by enhanced [35S]methionine incorporation. Conversely, increases in HKII expression with a rise in total hexokinase activity occurred soon after the onset of stimulation (30-fold elevations of HKII mRNA after 12 h and 20-fold increases in [35S]methionine incorporation after 24 h). With ongoing stimulation, HKII mRNA and synthesis returned to lower levels (fivefold elevations). Nevertheless, hexokinase activity continued to rise, stabilizing at fivefold-elevated levels after 3 d. These observation suggested that posttranscriptional mechanisms contributed to the upregulation of HKII, e.g. stabilization by elevated intracellular glucose and mitochondrial binding of the enzyme. This suggestion was supported by experiments with cessation after 24 h where hexokinase activity continued to increase, although the mRNA content and, especially, the [35S]methionine incorporation decayed steeply. The increase in HKII prior to GLUT-4 suggests that phosphorylation may be rate limiting in glucose utilization of glycolytic fibers under conditions of sustained contractile activity. Taken together, the changes in distribution and content of GLUT-4, as well as in HKII represent early metabolic adaptations. In addition, they are related to the overall process of stimulation-induced fiber type transformation.

Immunocytochemical localization of the plasma membrane calcium pump, calbindin-D28k, and parvalbumin in Purkinje cells of avian and mammalian cerebellum

A monoclonal antibody produced against the human erythrocyte plasma membrane calcium pump (PMCA) was shown to react immunohistochemically with an epitope of the PMCA in avian and mammalian cerebellum. Western blot analysis of purified synaptosomes and homogenates from avian cerebellum revealed major immunoreactive proteins with molecular masses (130 kDa and 138 kDa) similar to those of purified erythrocyte PMCA. Dual-imaging confocal immunofluorescence microscopy of avian cerebellum showed that the PMCA antibody stained the periphery of the soma whereas calbindin-D28k was located in the cytosol. PMCA heavily stained the more distal dendrites of the Purkinje cells and, within the resolution of the fluorescence procedure, colocalized with calbindin-D28k. By using alkaline phosphatase-conjugated second antibody, PMCA was again localized to the peripheral soma, to a segmental pattern in dendrites, and to presumed spiny elements. The soma periphery and dendrites of Purkinje cells of the rat cerebellum were also prominently stained with anti-PMCA antibody and compared to parvalbumin localization. Dendritic depolarization and dendritic spiking behavior are significant Ca(2+)-dependent events of Purkinje cells. The rapid decline of intracellular free Ca2+ after the rapid rise time of Ca2+ transients is considered to be due to sequestration by Ca2+ buffers, uptake by intracellular stores, and Ca2+ extrusion mechanisms, the latter a function of PMCA now shown immunohistochemically to be a prominent feature of Purkinje cell dendrites.

Immunogold labeling of calcium ATPase in sarcoplasmic reticulum of skeletal muscle: use of 1-nm, 5-nm, and 10-nm gold

We evaluated the use of immunogold electron microscopy to study the distribution of calcium ATPase in the sarcoplasmic reticulum membrane of skeletal muscle. We examined (a) 1-nm gold labeling, (b) the effect of gold size on immunolabeling, and (c) the densities of gold particles in areas of maximal labeling in fibers from rat extensor digitorum longus and pig gracilis muscles. The technique allowed unequivocal identification of the calcium ATPase. Gold particles of 1 nm were successfully visualized in unstained or lightly stained sections and the density of labeling was about 20 times greater than with 10-nm gold. The average densities in areas of intense labeling were 2878 +/- 139/microns 2 with 5-nm gold and 4310 +/- 276/microns 2 with 1-nm gold. These numbers are similar to the density of particles in freeze-fracture replicas of sarcoplasmic reticulum. The low density of 10-nm gold suggests that the large gold particles hinder binding of secondary to primary antibodies. The difference between 1- and 5-nm gold is explained by the amounts of gold conjugated to the immunoglobulin. The results suggest that there is a one-to-one relationship between secondary immunoglobulins (1-nm or 5-nm gold conjugates) and oligomeric complexes of calcium ATPase.

Distribution of Ca(2+)-modulating proteins in sarcoplasmic reticulum membranes after denervation

The early response to the loss of motor innervation to the muscle is connected with an altered Ca(2+)-homeostasis. Our study, based on Western blotting, indicates that denervation influenced expression of some sarcoplasmic Ca(2+)-modulating proteins. Evidence has been brought for an increase of the level of calsequestrin and of the putative ryanodine receptor paralleled with a slight decrease of the total amount of Ca(2+)-pump protein. The expression of unchanged Ca(2+)-pump isoform and unaltered quantities of other non-junctional Ca(2+)-binding proteins support the hypothesis that changed cellular Ca2+ homeostasis include also an alteration of Ca(2+)-modulating systems, mainly from the junctional region of sarcoplasmic membranes.

[Na, K-ATPase activity in the erythrocyte membrane preparations and kidneys of spontaneously hypertensive rats]

The enzyme activity in the cortex and medulla of the SHR was 1.5-2.0-fold lower than that of the WKY and Wistar rats. The Na, K-ATPase and Ca-ATPase activities were by 25 and 15% lower in the erythrocyte ghosts of the SHR than in normotensive rats. Removal of protein of the membrane skeleton from the erythrocyte membranes abolished the difference in the enzyme activity of normo- and hypertensive rats. The lower Na, K-ATPase activity in the SHR kidney seems to be an etiological factor in development of primary hypertension.

Morphogenesis of "duct-like" structures in three-dimensional cultures of human cancerous pancreatic duct cells (Capan-1)

Pancreatic duct cells secrete water and ions, bicarbonate in particular. The study of these secretion processes is hindered by the unavailability of human pancreatic tissue. In this study, pancreatic human cells of the Capan-1 cell line were employed to investigate secretion in vitro. These cells are of ductal origin because in standard culture they polarize spontaneously forming domes in the culture dishes, indicating the existence of transepithelial exchange of water and electrolytes. In culture in suspension, Capan-1 cells form hollow spheroids bounded by a cell monolayer in a radial organization. These three-dimensional structures could be maintained in culture for more than 140 days. In young cultures, the cells of these spheroids grew rapidly (mitotic index = 9.2% on Day 2). Their cytologic features were analyzed by immunocytochemical, cytoenzymatic methods, and by electron microscopy. We showed that they are: a) polarized with an apical pole facing the culture medium; b) organized in a monolayer; c) bound by tight junctions and desmosomes; d) characterized by a particular distribution of enzyme systems known to play a role in ion exchanges, with placental-type alkaline phosphatases and carbonic anhydrases IV on their apical membranes and Ca(2+)-ATPases on their basolateral membranes. Crystalline structures were detected histochemically in the closed cavities and in the intercellular spaces of the spheroids. X-ray emission spectroscopy and electron diffraction showed that they consisted of calcium phosphate in an apatite structure. They were assumed to derive from a raised concentration of Ca2+ and phosphate ions under the impermeable monolayer of the spheroids. In addition, numerous cells secreted M1 gastric-type mucins, and acquired the ability to produce colonic-type M3 mucins. These hollow spheroids swelled during the culture period. Taken together these results suggest that the Capan-1 cells organized in these hollow spheroids exchange ions. Their three-dimensional structure resembles that of human pancreatic ducts, and they may therefore represent a useful model system for investigation of Cl- and HCO3- ion exchange processes in the human pancreas.

Differential immunohistochemical localization of inositol 1,4,5-trisphosphate- and ryanodine-sensitive Ca2+ release channels in rat brain

Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive and ryanodine-sensitive intracellular Ca2+ stores is mediated by distinct proteins identified as IP3 receptors (IP3R) and ryanodine receptors (RyR), respectively. We have compared the immunohistochemical localizations of IP3R and RyR in the brain at the light and electron microscopic levels and have also evaluated the distribution of the major brain intracellular Ca(2+)-pumping ATPase. IP3R and RyR occur in overlapping populations of neurons in widespread areas of the brain, but labeling is distinct in a number of areas. For example, IP3R is enriched in cerebellar Purkinje cells and hippocampal CA1 pyramidal cells, while RyR is present at relatively low levels in these cells. RyR is most enriched in the dentate gyrus and CA3/4 areas of the hippocampus, where IP3R levels are low. In the cortex, IP3R is found in pyramidal cell bodies and proximal dendrites, whereas RyR is located predominantly in long, thin apical dendrites of pyramidal cells. In deep cerebellar nuclei, RyR is located in cell bodies that appear devoid of IP3R, whereas IP3R is enriched in terminals surrounding cell bodies. Electron microscopy in the hippocampus reveals RyR in axons, dendritic spines, and dendritic shafts near dendritic spines while IP3R is primarily identified in dendritic shafts and cell bodies. These results suggest that the IP3- and ryanodine-sensitive Ca2+ pools have largely distinct roles in controlling intracellular Ca2+ levels, though in some sites they may interact to varying degrees.

Effect of estrogen in relation to dietary vitamin D3 and calcium on activity of intestinal alkaline phosphatase and Ca-ATPase in immature chicks

The interaction between 17 beta-estradiol (E2), vitamin D3 (D3), and dietary Ca on the activities of Ca-ATPase and alkaline phosphatase (AP) was determined in the intestine of young female chicks. Chicks (n = 36) were assigned to two groups, one of which was transferred to a low Ca (0.2%) diet and the other maintained on a regular diet. One week later, each group was further divided into three subgroups and given daily injections of 0(oil), 0.25, or 0.5 mg E2/kg body wt for 14 days. E2 treatment as well as low dietary Ca significantly increased AP activity (P < 0.05), whereas the highest E2 dose decreased jejunal Ca-ATPase (P < 0.05). In a separate study, day-old chicks (n = 40) fed a purified diet supplemented with or without D3 for 24 days were divided into two subgroups and administered daily injections of either 0 or 0.25 mg estrogen 3-benzoate/kg body wt for 5 days. E2 alone or in combination with D3 failed to change Ca-ATPase activity in either the duodenum or the jejunum. However, E2 enhanced the D3-stimulated AP activity measured in the supernatant of duodenum (D3, P < 0.001; E2, P > 0.05; E2 x D3, P < 0.05) and jejunum (D3, P < 0.001; E2, P > 0.05; E2 x D3, P = 0.06). Daily injections of 0.5 mg E2/kg body wt for 6 days to 6-week-old D3-adequate chicks (n = 16) significantly increased AP activity in jejunum but not in liver and kidney (P < 0.05). In conclusion, E2 treatment enhanced the activity of intestinal AP but not Ca-ATPase. This enhancement was independent of dietary Ca, but was D3-dependent and tissue specific. The results suggest that the pubertal increase in plasma E2 can affect Ca absorption from the intestine by increasing the activity of AP.