Electron cytochemistry of calcium uptake in the fragmented sarcoplasmic reticulum

Electrogenic calcium transport in plasma membrane of rat pancreatic acinar cells

ATP-dependent 45Ca2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na+,K+)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca2+ uptake was maximal with 10 and 2 mumol/liter free Ca2+, and half-maximal with 0.9 and 0.5 mumol/liter free Ca2+. It was maximal at 3 and 0.2 mmol/liter free Mg2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg2+ was replaced by Mn2+ or Zn2+ ATP-dependent Ca2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn2+ could replace Mg2+ for Ca2+ uptake by 20%. Other divalent cations such as Ba2+ and Sr2+ could not replace Mg2+ in Ca2+ uptake. Ca2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca2+ uptake. The sequence was K+ greater than Rb+ greater than Na+ greater than Li+ greater than choline+ in plasma membranes and Rb+ greater than or equal to K+ greater than or equal to Na+ greater than Li+ greater than choline+ for rough endoplasmic reticulum. The anion sequence was Cl greater than or equal to Br greater than or equal to 1 greater than SCN greater than NO3 greater than isethionate greater than cyclamate greater than gluconate greater than SO2(4) greater than or equal to glutarate and Cl- greater than Br greater than gluconate greater than SO2(4) greater than NO3 greater than 1 greater than cyclamate greater than or equal to SCN, respectively. Ca2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K+ and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca2+ from the cell that might be involved in the regulation of the cytosolic Ca2+ level.

Uptake of calcium by the endoplasmic reticulum of the frog photoreceptor

We studied retinal photoreceptors of Rana pipiens by using techniques designed to investigate calcium localization. Particularly useful were methods in which intracellular sites of calcium uptake were detected by incubation of saponin-treated isolated retinas in calcium-containing media, with oxalate present as a trapping agent. With these procedures, cell compartments accumulate deposits, which can be shown to contain calcium by x-ray microanalysis. Calcium accumulation was prominent in the rough endoplasmic reticulum in the myoid region. In addition, deposits were observed in agranular reticulum and in certain Golgi-associated compartments of the myoid region, in mitochondria, in axonal reticulum, and in agranular reticulum of presynaptic terminals. Calcium was also detected in the endoplasmic reticulum of retinas fixed directly upon isolation, by a freeze-substitution method. The factors influencing accumulation of calcium in the endoplasmic reticulum were evaluated by a semiquantitative approach based on determining the relative frequency of calcium oxalate crystals under varying conditions. Calcium accumulation was markedly enhanced by ATP. Studies with a nonhydrolyzable ATP analogue (adenylyl- imidodiphosphate ) and with inhibitors of the sarcoplasmic reticulum Ca2+-Mg2+ ATPase (mersalyl and tetracaine) indicated that this ATP-dependent calcium uptake reflects an energy-dependent process roughly comparable to that in the sarcoplasmic reticulum.

Characterization of calcium uptake into rough endoplasmic reticulum of rat pancreas

ATP-dependent Ca2+ uptake into isolated pancreatic acinar cells with permeabilized plasma membranes, as well as into isolated endoplasmic reticulum prepared from these cells, was measured using a Ca2+ -specific electrode and 45Ca2+. Endoplasmic reticulum was purified on an isopycnic Percoll gradient and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the rough endoplasmic reticulum RNA was enriched threefold and the typical marker for the plasma membrane Na+,K+(Mg2+)ATPase was decreased 20-fold. When different fractions of the Percoll gradient were compared, 45Ca2+ uptake correlated with the RNA content and not with the Na+,K+(Mg2+)ATPase activity. The characteristics of nonmitochondrial Ca2+ uptake into leaky isolated cells and 45Ca2+ uptake into isolated endoplasmic reticulum were very similar: Calcium uptake was maximal at 0.3 and 0.2 mmol/liter free Mg2+, at 1 and 1 mmol/liter ATP, at pH 6.0 and 6.5, and free Ca2+ concentration of 2 and 2 mumol/liter, respectively. Calcium uptake decreased at higher free Ca2+ concentration. 45Ca2+ uptake was dependent on monovalent cations (Rb+ greater than K+ greater than Na+ greater than Li+ greater than choline+) and different anions (Cl- greater than Br- greater than SO4(2-) greater than NO3- greater than I- greater than cyclamate- greater than SCN-) in both preparations. Twenty mmol/liter oxalate enhanced 45Ca2+ uptake in permeabilized cells 10-fold and in vesicles of endoplasmic reticulum, fivefold. Calcium oxalate precipitates in the endoplasmic reticulum of both preparations could be demonstrated by electron microscopy. The nonmitochondrial Ca2+ pool in permeabilized cells characterized in this study has been previously shown to regulate the cytosolic free Ca2+ concentration to 0.4 mumol/liter. Our results provide firm evidence that the endoplasmic reticulum plays an important role in the regulation of the cytosolic free Ca2+ concentration in pancreatic acinar cells.

State of translocated Ca2+ by sarcoplasmic reticulum inferred from kinetic analysis of calcium oxalate precipitation

Uptake of Ca2+ by sarcoplasmic reticulum in the presence of oxalate displays biphasic kinetics. An initial phase of normal uptake is followed by a second phase coincident with precipitation of calcium oxalate inside the vesicles. The precipitation rate induced by actively transported Ca2+ is depressed by increasing the added Ca2+ concentration. This correlates linearly with the reciprocal of precipitation rate. Therefore, a maximal limit rate could be extrapolated at zero Ca2+ (V0). The rate of precipitation, also a function of added amount protein, gives a linear correlation in a double reciprocal plot. Thus, it was possible to estimate the maximal precipitation rate occurring at infinite protein concentration (V infinity). With the combined extrapolated values a maximal expected precipitation rate could be calculated (V infinity 0). Kinetics of calcium oxalate precipitation was studied in the absence of calcium uptake and empirical equations relating the rate of precipitation with the added Ca2+ were established. Entering V infinity 0 in the equations, an internal free Ca2+ concentration of approx. 2.5 mM was estimated. Additionally, it is shown that the ionophore X-537A does not suppress the Ca2+ uptake, if added during the oxalate-dependent phase, albeit the uptake proceeds at a slower rate after the release of approx. 70 nmol Ca2+/mg protein. This amount presumably equals the internal free Ca2+ not sequestered by oxalate, producing a maximal concentration approx. 14 mM. Taking into account low affinity binding of internal binding sites and the transmembrane Ca2+ gradients built up during the uptake of Ca2+, values of free Ca2+ ranging from 3 to 6 mM, approaching those estimated by the precipitation analysis, could be estimated.

Localization of ATP-dependent calcium transport activity in mouse pancreatic microsomes

Electron-dense deposits representing calcium oxalate crystals which result from ATP-dependent calcium uptake have been localized within vesicles of of a heavy microsomal fraction prepared from mouse pancreatic acini. In the absence of either ATP or oxalate, no electron-dense deposits could be observed. By subfractionation of microsomes on discontinuous sucrose gradients, it could be shown that the highest energy-dependent calcium transport activity was associated with the rough endoplasmic reticulum. In rough microsomes, the 45Ca2+-uptake measured was 7 times greater than that of smooth microsomes in the presence of ATP and oxalate and about 3 times greater in he presence of ATP alone. When ribosomes were released from the rough endoplasmic reticulum vesicles by treatment with KCl in the presence of puromycin, the stripped microsomes showed a 40% increase in the specific 45Ca2+-uptake activity measured in he presence of ATP and oxalate and an increase of 80 to 90% in the presence of ATP alone. From these results it can be concluded that the calcium transport activity of microsomes prepared from mouse pancreatic acini is located predominantly in the rough endoplasmic reticulum membrane.

Studies of platelets with heavy metal impregnation techniques

Various methods of heavy metal impregnations were performed on human platelets. The optimal technique consisted of glutaraldehyde fixation, incubation in warm uranyl acetate at a pH of 3.5, followed by a double solution of lead and copper, and finally overnight immersion in cold osmium tetroxide. Semi-thin sections, viewed at 90 kV, revealed three types of platelets: (1) 'reticular' cells, with a prominent tubular network and very dark granules in a pale cytoplasm; (2) 'dark' cells, with an electron-dense cytoplasm; and (3) 'pale' cells, with microvesicles and non-staining granules. Pre-treatments with EGTA, aspirin and various platelet activators altered the appearances and proportions of the three cell types. A cell-partitioning two-phase polymer system showed that the sub-grouping is related to surface membrane properties, the cells retained in the top phase being exclusively type 2 'dark' cells. The changes in cell type distribution produced by activation show that metal impregnation may be a useful method for studying structure-function correlations in platelets.

Differentiation between Ca2+ transport and ATP-induced Ca2+ binding by sarcoplasmic reticulum

The Ca2+ actively accumulated by sarcoplasmic reticulum isolated from skeletal muscle is composed of two fractions; one represented by intravesicular free Ca2+ and another represented by Ca2+ selectively bound to the membranes. Both of these Ca2+ fractions depend on ATP, although it is not clear whether ATP hydrolysis is essential for accumulation of the second Ca2+ fraction. The existence of the membrane-bound Ca2+ induced by ATP is clearly shown in experiments in which the Ca2+ retention by sarcoplasmic reticulum is measured in the presence and in the absence of X-537A, a Ca2+ ionophore, which makes the membrane permeable to Ca2+. Thus, in the presence of X-537A all Ca2+ accumulated due to ATP is bound to the membranes. This membrane-bound Ca2+ represents about 30 nmol/mg protein in the range of external pCa values of 7 to 3.5. The magnitude of this Ca2+ fraction is slightly higher whether or not the experiments are performed in the presence of oxalate, which greatly increased the intravesicular Ca2+ accumulation. Furthermore, taking advantage of the impermeability of sarcoplasmic reticulum to EGTA, it is possible to show the existence of the membrane-bound Ca2+ as a distinct fraction from that which exists intravesicularly.

The formation of intravesicular calcium phosphate deposits in microsomes of smooth muscle. A comparison with sarcoplasmic reticulum of skeletal muscle

The calcium uptake in the microsomial fraction isolated from the smooth muscle of the antrum of the pig stomach is stimulated by phosphate. The microsomial vesicles which are loaded with calcium phosphate can be purified by differential centrifugation. A purification of 36 times in terms of calcium content was reached. Electron microscopy of the freshly prepared material revealed calcium phosphate deposits in the form of needles of crystalline calcium phosphate. This structure differs from that of the deposits which appear in the fragmented sarcoplasmic reticulum of skeletal muscle. Their morphology is that of non-crystalline calcium phosphate. However, on standing these deposits convert slowly into crystalline calcium phosphate. This difference reflects different kinetics of crystallization of the precipitates in the two preparations. After negative staining of the calcium phosphate loaded microsomes of skeletal and of smooth muscle, only few deposits are preserved because a release of calcium occurs as a consequence of the action of the stain and also of the dilution and warming up of the suspension. Smooth muscle microsomes partially purified by loading with calcium phosphate were studied by freeze etching and rotary replication. Membrane fragments displaying subunit intramembrane particles similar to those observed in sarcoplasmic reticulum of skeletal muscle could be identified. However, in the smooth muscle microsomes the intramembrane particles were much less densely packed. Part of these particles could correspond to calcium transport sites.

Electron cytochemistry of oxalate-stimulated calcium uptake in microsomes from the smooth muscle of the pig stomach

A microsomal fraction was isolated from the smooth muscle of the antrum of the pig stomach by differential centrifugation. Electron microscopy of the negatively stained material showed that this fraction is heterogeneous in composition. The microsomes accumulated calcium in the presence of ATP, magnesium and oxalate. The amount of calcium taken up per mg protein was in the same range as observed for other smooth muscle microsomal preparations. Although this amount is much smaller than that in the microsomal fraction of skeletal muscle, calcium oxalate crystals were formed in some vesicles, as occurs in the skeletal muscle fragmented sarcoplasmic reticulum. Through the presence of the calcium oxalate crystals, many of these vesicles acquired sufficient mass and density to allow them to be isolated by centrifugation. A purification of about 40 fold in terms of calcium content was reached.

Effect of 1,25-dihydroxycholecalciferol on impaired calcium transport by the sarcoplasmic reticulum in experimental uremia

In the fragmented sarcoplasmic reticulum from skeletal muscle of rabbits with experimental uremia, defective calcium ion transport is found. An impairment of all parameters is observed (initial rate of uptake, storing capacity with and without oxalate, and concentrating ability). In vivo administration of 1,25-dihydroxycholecalciferol (1,25-(OH)2-vitamin D3)(2 X 27 ng X kg of body wt-1 X day-1 and 6 X 27 ng X kg-1 X day-1, respectively) improved the kinetic parameters. The low dose improved storing capacity, and the higher dose, in addition to the storing capacity, also corrected concentrating ability and the initial rate of uptake. It is concluded that active calcium transport in the sarcoplasmic reticulum is impaired by uremia and that this defect is responsive to the administration of 1,25-(OH)2-vitamin D3.

Detection of ion shifts in proximal tubule cells of the rat kidney using x-ray microanalysis

The present study represents an attempt to combine scanning transmissions electron microscopy and x-ray microanalysis on thin sections at the intracellular level using kidney tubules in both normal and altered conditions. The results show that semi-quantitative estimates made from X-ray microanalysis can predict the pattern of electrolyte change both in direction and degree; these estimated agree with previously determined chemical analysis of identically treated tissue. Before these promising results can be extended to quantitative intracellular compartment analysis, further work is needed in several areas including preparation of adequate standards, improved ultracryomicrotomy techniques, improvement of contrast of ultrathin frozen sections and standardization in computing background measurements.

A simplified method of specimen preparation for X-ray microanalysis of muscle and blood cells

The effect of various steps in the preparation of muscle and blood cells for electron microscopic X-ray microanalysis, was studied with reference to calcium, zinc and copper levels. It was found that these elements are readily detectable in material fixed in glutaraldehyde and embedded directly in Epon 812. Alcohol dehydration, buffer rinses and post-osmication caused marked depletion of all elements examined.