Electron probe microanalysis of the subcellular compartments of bovine adrenal chromaffin cells. Comparison of chromaffin granules in situ and in vitro

Polyphosphate is a novel pro-inflammatory regulator of mast cells and is located in acidocalcisomes

Polyphosphate (polyP) is a pro-inflammatory agent and a potent modulator of the human blood-clotting system. The presence of polyP of 60 phosphate units was identified in rat basophilic leukemia (RBL-2H3) mast cells using specific enzymatic assays, urea-polyacrylamide gel electrophoresis of cell extracts, and staining of cells with 4,6-diamidino-2-phenylindole (DAPI), and the polyP-binding domain of Escherichia coli exopolyphosphatase. PolyP co-localizes with serotonin- but not with histamine-containing granules. PolyP levels greatly decreased in mast cells stimulated to degranulate by IgE. Mast cell granules were isolated and found to be acidic and decrease their polyP content upon alkalinization. In agreement with these results, when RBL-2H3 mast cells were loaded with the fluorescent calcium indicator fura-2 acetoxymethyl ester to measure their intracellular Ca(2+) concentration ([Ca(2+)](i)), they were shown to possess a significant amount of Ca(2+) stored in an acidic compartment different from lysosomes. PolyP derived from RBL-2H3 mast cells stimulated bradykinin formation, and it was also detected in human basophils. All of these characteristics of mast cell granules, together with their known elemental composition, and high density, are similar to those of acidocalcisomes. The results suggest that mast cells polyP could be an important mediator of their pro-inflammatory and pro-coagulant activities.

The optimal activity of a pseudozymogen form of recombinant matriptase under the mildly acidic pH and low ionic strength conditions

Matriptase is a transmembrane serine protease that is strongly expressed in epithelial cells. The single-chain zymogen of matriptase is considered to have inherent activity, leading to its own activation (i.e. conversion to the disulphide-linked-two-chain form by cleavage after Thr-Lys-Gln-Ala-Arg614). Also, there is growing evidence that the activation of zymogen occurs at the cell surface and in relation to the acidification and lowering of ionic strength within cell-surface microenvironments. The present study aimed to provide evidence for the involvement of zymogen activity in its activation in physiologically relevant cellular contexts. For this purpose, the activity of a pseudozymogen form of recombinant matriptase (HL-matriptase zymogen) was examined using acetyl-l-Lys-l-Thr-l-Lys-l-Gln-l-Leu-l-Arg-4-methyl-coumaryl-7-amide as a substrate. HL-matriptase zymogen exhibited optimal activity toward the substrate pH approximately 6.0. The substrate hydrolysis at the pH value was hardly detected when NaCl was present at a concentration of 145 mM. In a buffer of pH 6.0 containing 5 mM NaCl, the activity of HL-matriptase zymogen was only approximately 30-times lower than that of the respective two-chain form. These findings suggest that the in vivo activation of matriptase zymogen occurs via a mechanism involving the zymogen activity.

Lessons from tomographic studies of the mammalian Golgi

Basic structure studies of the biosynthetic machinery of the cell by electron microscopy (EM) have underpinned much of our fundamental knowledge in the areas of molecular cell biology and membrane traffic. Driven by our collective desire to understand how changes in the complex and dynamic structure of this enigmatic organelle relate to its pivotal roles in the cell, the comparatively high-resolution glimpses of the Golgi and other compartments of the secretory pathway offered to us through EM have helped to inspire the development and application of some of our most informative, complimentary (molecular, biochemical and genetic) approaches. Even so, no one has yet even come close to relating the basic molecular mechanisms of transport, through and from the Golgi, to its ultrastructure, to everybody's satisfaction. Over the past decade, EM tomography has afforded new insights into structure-function relationships of the Golgi and provoked a re-evaluation of older paradigms. By providing a set of tools for structurally dissecting cells at high-resolution in three-dimensions (3D), EM tomography has emerged as a method for studying molecular cell biology in situ. As we move rapidly toward the establishment of molecular atlases of organelles through advances in proteomics and genomics, tomographic studies of the Golgi offer the tantalizing possibility that one day, we will be able to map the spatio-temporal coordinates of Golgi-related proteins and lipids accurately in the context of 4D cellular space.

Dinucleoside polyphosphates-friend or foe?

Despite being known for over 30 years, the functions of the dinucleoside polyphosphates, such as diadenosine 5',5"'-P(1), P(4)-tetraphosphate (Ap(4)A) and diadenosine 5',5"'-P(1), P(3)-triphosphate (Ap(3)A), are still unclear. On the one hand, they may have important signalling functions, both inside and outside the cell (friend), while on the other hand, they may simply be the unavoidable by-products of certain biochemical reactions, which, if allowed to accumulate, would be potentially toxic through their structural similarity to ATP and other essential mononucleotides (foe). Here, the occurrence, synthesis, degradation, and proposed functions of these compounds are briefly reviewed, along with some new data and recent evidence supporting roles for Ap(3)A and Ap(4)A in the cellular decision making processes leading to proliferation, quiescence, differentiation, and apoptosis. Hypotheses are forwarded for the involvement of Ap(4)A in the intra-S phase DNA damage checkpoint and for Ap(3)A and the pFhit (fragile histidine triad gene product) protein in tumour suppression. It is concluded that the roles of friend and foe are not incompatible, but are distinguished by the concentration range of nucleotide achieved under different circumstances.

Kinetic parameters for dimeric and tetrameric forms of bovine dopamine beta-monooxygenase and their relationship to non-Michaelis-Menten behavior

Bovine dopamine beta-monooxygenase has been assayed over a 10,000-fold range in protein concentration, to approximate conditions where the enzyme was shown to be a dimer or tetramer. Michaelis-Menten kinetics are observed with k(cat) and k(cat)/Km for dissociated enzyme reduced 30% and 200-300% relative to tetramer. Addition of chloride ions to very dilute enzyme or the use of intermediate enzyme concentrations causes non-Michaelis-Menten behavior, attributed to an equilibration between dimer and tetramer. This is not expected to contribute to activity within the chromaffin vesicle, where enzyme and chloride ions are at high levels.

L- and T-type voltage-gated Ca2+ currents in adrenal medulla endothelial cells

We investigated voltage-dependent Ca2+ channels of bovine adrenal medulla endothelial cells with the whole cell version of the patch-clamp technique. Depolarization elicited an inward current that was carried by Ca2+ and was composed of a transient (T) current, present in all the cells tested, and a sustained (L) current, present in 65% of them. We separated these currents and measured their individual kinetic and gating properties. The activation threshold for T current was approximately -50 mV, and its maximum amplitude was -49.8 +/- 4.8 pA (means +/- SE, n = 19) at 0 mV. The time constant was 10.2 +/- 1.5 ms (n = 4) for activation and 18.4 +/- 2.8 ms (n = 4) for inactivation. The L current activated at -40 mV, and it reached a plateau at -20.1 +/- 2.3 pA (n = 6). Its activation time course was a single exponential with an activation time contant of 26.8 +/- 2.3 ms (n = 4). Current-voltage curves, kinetics, gating, response to BAY K 8644, nifedipine, amiloride, and different selectivity for Ba2+ and Ca2+ indicated that the underlying channels for the observed currents are only of the T- and L-types that resemble those of the endocrine secretory cells.

Stimulus-secretion coupling in neurohypophysial nerve endings: a role for intravesicular sodium?

It is generally accepted that Ca is essentially involved in regulated secretion, but the role of this cation, as well as others such as Na, is not well understood. An illustrative example occurs in neurohypophysial secretion, where an experimentally induced increase in the cytosolic concentration of Na+ can induce continuous neuropeptide release. In contrast, an increase in cytosolic Ca2+ will have only a transient stimulatory effect. The secretion-promoting targets for Ca2+ are not known; they may be cytosolic, as is usually assumed, but they may also be intravesicular, especially in view of evidence that Ca-rich secretory vesicles are preferentially secreted. In the present work, we have investigated the movements of these cations into and out of secretory vesicles during stimulus-secretion coupling. Isolated rat neurohypophysial nerve endings were stimulated by potassium (55 mM) depolarization, and at 6 min (peak secretion) and 20 min after the onset of stimulation, the elemental content of individual secretory vesicles was measured by quantitative x-ray microanalysis. A depolarization-induced transient increase in intravesicular Na+ concentration was found to coincide with the onset of secretion. Moreover, only a predicted small fraction of peripheral vesicles-presumably the docked ones-were Na+-loaded. The low sulfur concentration of Na+-rich vesicles most likely resulted from vesicle swelling. The results suggest that high intravesicular Na+ concentrations in docked vesicles, occurring by Na+/Ca2+ exchange or by transient fusion pore opening, is a proximal event in exocytosis.

Submillimolar levels of calcium regulates DNA structure at the dinucleotide repeat (TG/AC)n

Submillimolar levels of calcium, similar to the physiological total (bound + free) intranuclear concentration (0.01-1 mM), induced a conformational change within d(TG/AC)n, one of the frequent dinucleotide repeats of the mammalian genome. This change is calcium-specific, because no other tested cation induced it and it was detected as a concentration-dependent transition from B- to a non-B-DNA conformation expanding from 3' end toward the 5' of the repeat. Genomic footprinting of various rat brain regions revealed the existence of similar non-B-DNA conformation within a d(TG/AC)28 repeat of the endogenous enkephalin gene only in enkephalin-expressing caudate nucleus and not in the nonexpressing thalamus. Binding assays demonstrated that DNA could bind calcium and can compete with calmodulin for calcium.

Simultaneous detection of catecholamine exocytosis and Ca2+ release from single bovine chromaffin cells using a dual microsensor

A dual microsensor with a 5 microns radius was fabricated to detect simultaneously Ca2+ and catecholamines following their secretion from individual biological cells. Detection of Ca2+ was based on changes in fluorescence as a result of its binding with a surface-attached dye, and catecholamines were detected by amperometry. The fluorescent dye employed, calcium green-1 dextran, is a selective chelator for Ca2+. It was attached to the tip of a carbon fiber electrode by cross-linking with 5% glutaraldehyde. The dual microsensor has a subsecond response time for both Ca2+ and catecholamine concentration changes. Ca2+ concentrations of 100 nM can be detected, while the detection limit for catecholamine is in the micromolar range. The utility of the dual microsensor was evaluated at the surface of bovine adrenal medullary cells. Release of catecholamines by exocytosis was evoked by transient application of histamine. This was detected by amperometry, and it was found to be accompanied by Ca2+ release, as measured by fluorescence from the same sensor.

In situ compositional analysis of acidocalcisomes in Trypanosoma cruzi

We measured the elemental content of different compartments in Trypanosoma cruzi epimastigotes using quick freezing, ultracryomicrotomy, and electron probe microanalysis. Vacuoles identified by high electron density contained (in units of mmol/kg dry weight +/- S.E.) large amounts of phosphorus (1390 +/- 13), magnesium (646 +/- 19), calcium (171 +/- 5), sodium (161 +/- 18), and zinc (148 +/- 6). No other compartment had appreciable calcium or zinc content. Iron (128 +/- 16 mmol/kg) was detected only in vacuoles distinct from the electron-dense vacuoles and other organelles. Incubation of cells for 70 min in culture medium in the presence of ionomycin plus nigericin led to a very significant 3- or 2-fold increase in potassium in the electron-dense vacuoles and the iron-rich vacuoles, respectively, with no significant change in the other elements investigated. This indicated the acidic nature of the vacuoles and demonstrated that the electron-dense vacuoles correspond to what were described previously as acidocalcisomes, i.e. acidic compartments rich in Ca2+. The acidocalcisomes were investigated by separation of epimastigote fractions on Percoll gradients in combination with Triton WR-1339 treatment. This detergent caused a rapid vacuolation; these vacuoles were shown by electron microscopy to be largely transparent, with a diffuse matrix. Percoll gradient fractionation demonstrated decreases in the density of various organelle markers in detergent-treated cells compared with controls. Large decreases in the density of the acidocalcisome and the mitochondrion were seen, as well as smaller decreases in the density of the other markers. Conventional electron microscopy of epimastigotes loaded with gold-labeled transferrin indicated that the endosomal system was separate from vacuoles that probably corresponded to the calcium-containing organelles detected by electron probe microanalysis. The combined results provide evidence that acidocalcisomes are organelles different from lysosomes or other organelles previously described in these parasites.

Electron probe analysis and biochemical characterization of electron-dense granules secreted by Entamoeba histolytica

The interaction of Entamoeba histolytica with collagen induces the intracellular formation and release of electron-dense granules (EDGs) containing collagenase activity which are important in the pathogenicity of this parasite. Purified EDGs contain at least 25 polypeptides with acidic pIs, nine gelatinase activities, small molecules, including inorganic phosphate (Pi), pyrophosphate (PP) and other elements, including Na, Mg, S, Cl, K, Ca and Fe as measured by scanning transmission electron microscopy. Six of these polypeptides with apparent molecular weights of 108, 106, 104, 97, 68 and 59 kDa and two protease activities with apparent molecular weights of 40 and 85 kDa were detected exclusively in the EDGs and were not observed in total trophozoite extracts. Actin was also detected in the EDGs. Therefore, EDGs are a complex of mainly cationic proteins, which contains numerous proteolytic activities, actin and small molecules such as P(i), PP and cations.

Agonist-releasable intracellular calcium stores and the phenomenon of store-dependent calcium entry. A novel hypothesis based on calcium stores in organelles of the endo- and exocytotic apparatus

Store-dependent calcium entry represents a little characterized calcium permeation pathway that is present in a variety of cell types. It is activated in an unknown way by depletion of intracellular calcium stores, for example in the course of phospholipase C stimulation. Current hypotheses propose that depleted calcium stores signal their filling state to this permeation pathway either by direct, protein-mediated interaction or by release of a small, diffusible messenger. The further characterization of store-dependent calcium entry will benefit from progress in the identification of the intracellular calcium storing compartments. Recent findings reviewed here suggest that these compartments include parts of the organelle system that is involved in endo- and exocytosis. This commentary describes a novel model of store-dependent calcium entry based on calcium stores belonging to the endo- and exocytotic organelle system. Such calcium stores could establish a tubule-like connection with the extracellular space, in analogy to the cellular compartments that contain the insulin-sensitive glucose transporter or the gastric proton pump. This connection will provide a pathway for store-dependent calcium entry. Under store depletion, extracellular calcium will permeate through the tubule-like connection into the store lumen and from there into the cytosol. The consequences of this model for the development of drugs modulating store-dependent calcium entry are discussed.

Role of [Na+]i and [Ca2+]i in nicotine-induced norepinephrine release from bovine adrenal chromaffin cells

Intracellular free sodium ([Na+]i) and calcium ([Ca2+]i) concentrations were determined by sodium-binding benzofuran isophthalate (SBFI) and fura 2 microfluorimetry, respectively, in bovine adrenal chromaffin cells (BCC). Validation of SBFI microfluorimetry by in vitro and in vivo calibration revealed a reliable assessment of [Na+]i within a range of 1-30 mM in single BCC. Nicotine (0.1-10 microM) induced concentration-dependent increases of both [Na+]i (from 3.3 +/- 0.1 to 25.6 +/- 0.4 mM, n = 76, P < 0.001) and [Ca2+]i (from 64 +/- 1 to 467 +/- 16 nM, n = 87, P < 0.001), which were accompanied by an increase in [3H]norepinephrine (NE) release. Consistent with an exocytotic release mechanism, nicotine-induced increments of [Ca2+]i and [3H]NE release were reduced under calcium-free conditions and by gadolinium chloride (40 microM), whereas [Na+]i was not affected. In contrast, a parallel attenuation of nicotine-evoked changes in [Na+]i, [Ca2+]i, and [3H]NE release was observed during reduction of the extracellular sodium concentration. The nicotine-evoked responses were neutralized by the nicotinic receptor antagonist hexamethonium (100 microM) but not by blockade of voltage-dependent sodium channels (1 microM tetrodotoxin). In conclusion, the nicotine-induced exocytotic release of [3H]NE is triggered by an increase in [Ca2+]i, which is facilitated by sodium influx through the nicotinic receptor ionophore.

Calcium loading of secretory granules in stimulated neurohypophysial nerve endings

The total calcium content of secretory granules, Cag, was evaluated in isolated neurohypophysial nerve endings. The Cag in the resting state, as measured by X-ray microanalysis, is relatively high with an average of 7.4 +/- 0.6 mmol/kg wet weight. Following a depolarizing potassium challenge, a subpopulation of granules with even higher Cag could be detected, dispersed over a wider range of concentrations (up to 70 mmol/kg wet weight). After subsequent rinsing in physiological saline, Cag decreased to control values. This could have resulted from Ca2+ extrusion, or from preferential secretion of calcium-enriched granules. Our data can be interpreted in favor of the second explanation since no decrease in Cag was observed when secretion was blocked by a hyperosmotic saline. The effect of hyperosmotic conditions on isolated nerve endings was further studied by monitoring free cytoplasmic Ca2+ with the calcium-sensitive dye Fura-2 and by conventional electron microscopy. It was demonstrated that hyperosmotic treatment alone did not increase basal cytosolic Ca2+ concentrations but did significantly reduce the potassium-induced cytosolic rise in Ca2+. Electron microscopy of nerve endings in hyperosmotic conditions showed numerous exocytotic figures at various stages. The observed changes in Cag are in accord with a published hypothesis which proposes that intragranular calcium is a significant variable in regulated secretion.

Evidence for a K+ channel in bovine chromaffin granule membranes: single-channel properties and possible bioenergetic significance

A K+ channel was incorporated into voltage-clamped planar lipid bilayers from bovine chromaffin granules and resealed granule membranes ("ghosts"). It was not incorporated from plasma membrane-rich fractions from the adrenal medulla. The channel had a conductance of approximately 400 pS in symmetric 450 mM KCl, with the permeability sequence K+ > Rb+ > Cs+ > Na+ > Li+, and was insensitive to both Ca2+ and charybdotoxin. It exhibited complex gating kinetics, consistent with the presence of multiple open and closed states, and its gating was voltage-dependent. The channels appeared to incorporate into bilayers with the same orientation, and were blocked from one side (the side of vesicle addition) by 0.2-1 mM TEA+. The block was slightly voltage-dependent. Acidification of resealed granule membranes in response to external ATP (which activated the vacuolar-type ATPase) was significantly reduced in the presence of 1 mM intralumenal TEACl (with 9 mM KCl), and parallel measurements with the potential-sensitive dye Oxonol V showed that such vesicles tended to develop higher internal-positive membrane potentials than control vesicles containing only 10 mM KCl. 1 mM TEA+ had no effect on proton-pumping activity when applied externally, and did not directly affect either the proton-pumping or ATP hydrolytic activity of the partially-purified ATPase. These results suggest that chromaffin granule membranes contain a TEA(+)-sensitive K+ channel which may have a role in regulating the vesicle membrane potential.

Water distributions of hydrated biological specimens by valence electron energy loss spectroscopy

A technique has been developed for measuring the water distribution in thin frozen hydrated biological specimens by means of electron energy loss spectroscopy (EELS). The method depends on the quantification of subtle changes in the valence electron excitation spectrum as a function of composition. It involves determining the single-scattering intensities, calculating oscillator strengths and applying a multiple-least-squares fitting procedure to reference spectra for water and the organic constituents. The direct EELS approach has an important advantage over other indirect methods that are based on X-ray generation or elastic scattering measurements since these are applied to freeze-dried specimens where differential shrinkage between compartments may produce errors. Precision and accuracy of the EELS method have been tested on cryosectioned solution of bovine serum albumin; data have also been obtained from cryosections of rapidly frozen erythrocytes. Results suggest that a precision of better than +/- 5% (s.d.) is attainable from a single measurement and the accuracy may be as high as +/- 2% if repeated measurements are made. The lateral spatial resolution of the water determinations is limited by radiation damage to approximately 100 nm which is of the same order as the specimen thickness.

Identification of a yeast artificial chromosome (YAC) spanning the synovial sarcoma-specific t(X;18)(p11.2;q11.2) breakpoint

A somatic cell hybrid containing the synovial sarcoma-associated t(X;18)(p11.2;q11.2) derivative (der(X)) chromosome was used to characterize the translocation breakpoint region on the X chromosome. By using Southern hybridization of DNA from this der(X) hybrid in conjunction with Xp-region specific radiation reduced cell hybrids and probes, it was found that this breakpoint maps within the ornithine aminotransferase (OAT) L1 cluster. A yeast artificial chromosome (YAC) clone (OAT YAC2) which hybridizes to a human OAT cDNA probe and is known to contain part of the OATL1 cluster was selected and used to confirm these results both by fluorescence in situ hybridization on synovial sarcoma patient material and by hybridization of its end-clones to the der(X) containing hybrid cells. It was found that indeed the human Xp sequences contained within this YAC are split as a consequence of the (X;18) translocation. Therefore, we conclude that OAT YAC2 spans the synovial sarcoma-specific translocation breakpoint and, as such, may serve as an ideal starting point from which the gene(s) involved in the development of this soft tissue tumor can be isolated.

Radiation damage of fluorinated organic compounds measured by parallel electron energy loss spectroscopy

The sensitivity of fluorinated organic compounds to radiation damage in the electron microscope was measured by electron energy loss spectroscopy (EELS). Five classes of molecules were investigated with fluorine atoms situated on: (1) an aliphatic chain, (2) an aromatic ring, (3) a trifluoromethyl group on an aromatic ring, (4) a trifluoromethyl group on a heterocyclic ring, and (5) a trifluoromethyl group next to a carbonyl group. The damage dose for fluorine loss was found to depend strongly on the position of the fluorine atoms and on specimen temperature. For poly-substituted fluorine on an aliphatic chain, approximately half of the fluorine was retained at doses in excess of 10(7) e/nm2. At room temperature molecules containing trifluoromethyl groups on aromatic or heterocyclic rings were much more sensitive to fluorine loss than compounds having fluorine substituted directly on the rings. This behavior is consistent with the relatively low chemical stability of the trifluoromethyl group in these structures. The rapid loss of fluorine at low electron doses (approximately 10(4) e/nm2) in the trifluoromethyl aromatic compounds was reduced several orders of magnitude by cooling the specimen to liquid-nitrogen temperatures. An understanding of how specific types of fluorinated compounds suffer damage under the electron beam will be important in their potential application as biochemical or pharmacological probes.

Use of the uranaffin reaction in the identification of neuroendocrine granules

This overview emphasizes the utility of the uranaffin reaction in the diagnosis of tumors derived from neuroendocrine cells. The history, cell organelle specificity, tissue specificity, pH requirements, and detailed procedure of the uranaffin reaction is provided. Uranaffin-positive granules are also identified within the NS granules of the stem cell paraneuron (archiparaneuron) of coelenterates, and a hypothetical evolutionary scheme depicting the possible origins of the key biochemical features of the advanced mammalian NS granule is included. The role of nucleotides, a major component of true NS granules, is discussed. A possible intragranular function of ATP as a regulator of osmotic pressure and the extracellular physiologic effects of secreted intragranular nucleotides are discussed in some detail.

The calcium content of cortical granules and the loss of calcium from sea urchin eggs at fertilization

In many species, fertilization triggers a wave of cortical granule exocytosis in the egg that is the consequence of an increase in intracellular free calcium concentration. We have measured the total calcium content of cortical granules from two species of sea urchins by quantitative X-ray microanalysis and spectrometric measurements. Our results show that cortical granules: (1) contain a high concentration of total calcium (around 30 and 95 mM for Paracentrotus lividus and Arbacia lixula, respectively), (2) represent a major cortical storage site of calcium in the egg (5 and 11% of total egg calcium for P. lividus and A. lixula, respectively), and (3) exchange part of their accumulated calcium by an ATP dependent mechanism. In addition we have confirmed that at fertilization, sea urchin eggs lose a sizeable amount of their calcium (7% for P. lividus and 15% for A. lixula). The kinetics and magnitude of the loss suggest that some of this calcium could be provided by cortical granules during exocytosis.

Osmotic lysis of chromaffin granules treated with the ionophores nigericin and A23187 in isotonic sucrose solution at low pH

Bovine chromaffin granules were treated with the ionophores nigericin or A23187 in sucrose solutions with the pH varying from 4.7 to 7.0. Nigericin and A23187 induced osmotic lysis of the granules in sucrose solutions at pH values below 5.8, but not at physiological pH. This effect is explained by a progressive protonation of the acidic chromogranins induced by the ionophore-promoted exchange of internal potassium- and calcium ions for external protons. The results support the view that the interactions between catecholamines and ATP with chromogranins play a significant role in osmotic pressure reduction of the granule interior.

Maturation-related changes in mass and elemental contents of secretory granules as measured by electron-microprobe

The relationship between granule density, protein content, and Ca and S contents were studied in two secretory granule fractions, from parotid glands of the rat, previously shown to constitute different stages in granule maturation. The density of the lighter fraction was between 1.133 and 1.142 g/ml, while that of the heavier fraction was greater than 1.142 g/ml. The mean protein content of the denser granules was 12% greater than that of the lighter granules (P less than 0.03), while the dry-mass elemental concentrations in the two granule fractions were unchanged. These results indicate that protein is added to granules during the maturation process (presumably by vesicular traffic), and that the resulting increase in granule density is not driven simply by decrease in water content and/or increased concentrations of inorganic Ca or S in the granules. The elemental concentration values also indicate that the diffusible elements permeate the granule membrane during the fractionation procedures.

The lacunar glial zone at the periphery of Aplysia giant neuron: volume of extracellular space and total calcium content of gliagrana

The relative volume of perineuronal extracellular space, the number of gliagrana and their total calcium content have been measured in Aplysia punctata and A. californica, at the periphery of giant neurons R2 and LP1. After chemical fixation, the extracellular space amounts to 26% of the periganglionic glial zone, but this increases to 36% after quick freezing and freeze-substitution. The glial cytoplasm contains gliagrana, membrane-bound granules approximately 0.3 micron in diameter. The number of gliagrana per micron 2 of section, defined as "abundance", was counted in electron micrographs of chemically fixed tissues. The abundance of gliagrana appears to be directly proportional to the volume of the extracellular space when the values are averaged per individual Aplysia. The total calcium concentration of the gliagrana is measured by X-ray microanalysis on sections of ganglia processed by rapid freezing and freeze-substitution in the presence of oxalic acid: it was found to be very high. An individual granule may contain 100 mM Ca in A. californica and 50 mM in A. punctata but in both species the calcium concentration varies along a wide range as if there were different functional states of the granules with respect to this concentration. The total calcium stored in the specific granules of the glial zone was estimated. It was calculated that should the glial calcium store be entirely diluted in the extracellular space of the glial zone, it would raise the calcium concentration of this space by approximately 1 mM (0.1-2.7 mM). These findings are discussed with regard to the hypothesis of glial cells regulating the perineuronal calcium concentration.

Comparison of elemental concentrations in the acinar cells of the human labial salivary gland

Two types of acinar cells were observed in human labial glands by conventional and analytical electron microscopic and light microscopic techniques. The predominant type contained large and prominent secretory granules that were strongly mucicarmine and PAS (with and without diastase) positive. The second type contained small, lacy, secretory granules, and these cells were faintly positive with these stains. The elemental contents of the two types of granules were measured by analytical electron microscopy using digital mapping and spot analysis applied to freeze-dried cryosections prepared from gland slices incubated in vitro under non-stimulated conditions. The large secretory granules had significantly higher Ca, S and Mg concentrations and significantly lower Cl and K concentrations than the small granules. The difference in elemental contents probably reflects differences in the content of secretory macromolecules. Specifically, the S content is thought to reflect the anionic properties of the secretory macromolecules, while the levels of divalent cations are thought to be determined by electroneutrality requirements for macromolecular folding and storage. No differences were found in nuclear or cytoplasmic elemental concentrations between the two cell types.

Interaction of calcium with porcine adrenal chromogranin A (secretory protein-I) and chromogranin B (secretogranin I)

Secretory granules of endocrine cells contain one or more of the acidic secretory proteins chromogranin A (secretory protein-I), chromogranin B (secretogranin I), and secretogranin II (chromogranin C). It has been proposed that these proteins play a role in the packaging of secretory products. In the present study, lysates of purified porcine adrenal chromaffin granules containing chromogranins A and B and a putative chromogranin B fragment bound calcium and formed aggregates in the presence of 10-20 mM calcium at pH 5-6 and at 100 mM or less KCl, NaCl, or norepinephrine. The precipitates contained virtually all of the chromogranin B and the chromogranin B fragment and about one-third of the chromogranin A. The aggregates did not form or were dissociated at the pH and salt concentration of the extracellular fluid. Calcium precipitated purified chromogranin A and chromogranin B from pure solution to the same extent as from the granule lysates. Parathormone, added to the lysates, was incorporated in the precipitates, whereas the acidic secretory protein ovalbumin and norepinephrine were not. These findings suggest that secretory protein-I and secretogranin can exist in situ as aggregates that may include selected secretory products.

Evidence for two conductance/exchange pathways for chloride in rat parotid secretory granules

Electron probe x-ray microanalysis was used to determine that bromide is localized to rat parotid secretory granules at early stages of an in situ Cl/Br washout experiment. Chloride efflux and bromide influx across the secretory granule membrane occurred with a time order of minutes. Since the Cl washout data indicated minimal Cl binding within the granule, and therefore minimal Br binding, the Br localization results suggest the presence of two or more anion conductance/exchange pathways in the granule membrane for the Cl (Br) ion.