Composition of the aqueous phase of chromaffin granules

Untapped Potential: Real-Time Measurements of Opioid Exocytosis at Single Cells

The endogenous opioid system is commonly targeted in pain treatment, but the fundamental nature of neuropeptide release remains poorly understood due to a lack of methods for direct detection of specific opioid neuropeptides in situ. These peptides are concentrated in, and released from, large dense-core vesicles in chromaffin cells. Although catecholamine release from these neuroendocrine cells is well characterized, the direct quantification of opioid peptide exocytosis events has not previously been achieved. In this work, a planar carbon-fiber microelectrode served as a "postsynaptic" sensor for probing catecholamine and neuropeptide release dynamics via amperometric monitoring. A constant potential of 500 mV was employed for quantification of catecholamine release, and a higher potential of 1000 mV was used to drive oxidation of tyrosine, the N-terminal amino acid in the opioid neuropeptides released from chromaffin cells. By discriminating the results collected at the two potentials, the data reveal unique kinetics for these two neurochemical classes at the single-vesicle level. The amplitude of the peptidergic signals decreased with repeat stimulation, as the halfwidth of these signals simultaneously increased. By contrast, the amplitude of catecholamine release events increased with repeat stimulation, but the halfwidth of each event did not vary. The chromogranin dense core was identified as an important mechanistic handle by which separate classes of transmitter can be kinetically modulated when released from the same population of vesicles. Overall, the data provide unprecedented insight into key differences between catecholamine and opioid neuropeptide release from isolated chromaffin cells.

Protein mobility within secretory granules

We investigated the basis for previous observations that fluorescent-labeled neuropeptide Y (NPY) is usually released within 200 ms after fusion, whereas labeled tissue plasminogen activator (tPA) is often discharged over many seconds. We found that tPA and NPY are endogenously expressed in small and different subpopulations of bovine chromaffin cells in culture. We measured the mobility of these proteins (tagged with fluorophore) within the lumen of individual secretory granules in living chromaffin cells, and related their mobilities to postfusion release kinetics. A method was developed that is not limited by standard optical resolution, in which a bright flash of strongly decaying evanescent field (∼64 nm exponential decay constant) produced by total internal reflection (TIR) selectively bleaches cerulean-labeled protein proximal to the glass coverslip within individual granules. Fluorescence recovery occurred as unbleached protein from distal regions within the 300 nm granule diffused into the bleached proximal regions. The fractional bleaching of tPA-cerulean (tPA-cer) was greater when subsequently probed with TIR excitation than with epifluorescence, indicating that tPA-cer mobility was low. The almost equal NPY-cer bleaching when probed with TIR and epifluorescence indicated that NPY-cer equilibrated within the 300 ms bleach pulse, and therefore had a greater mobility than tPA-cer. TIR-fluorescence recovery after photobleaching revealed a significant recovery of tPA-cer (but not NPY-cer) fluorescence within several hundred milliseconds after bleaching. Numerical simulations, which take into account bleach duration, granule diameter, and the limited number of fluorophores in a granule, are consistent with tPA-cer being 100% mobile, with a diffusion coefficient of 2 × 10(-10) cm(2)/s (∼1/3000 of that for a protein of similar size in aqueous solution). However, the low diffusive mobility of tPA cannot alone explain its slow postfusion release. In the accompanying study, we suggest that, additionally, tPA itself stabilizes the fusion pore with dimensions that restrict its own exit.

The assisted self-association of ATP4- by a poly(amino acid) [poly(Lys)] and its significance for cell organelles that contain high concentrations of nucleotides

The occurrence of high concentrations of ATP in certain cell organelles prompted us to study the self-association of ATP via the concentration dependence of the 1H-NMR chemical shifts for H2, H8 and H1' in D2O at pD 8.4 (25 degrees C) in the range 0.0025-0.4 M in the presence and absence of poly(alpha, L-lysine), where [Lys units] was 0.4 M. The experiment in the presence of poly(Lys) was repeated at pD 12.1. At pD 8.4, the poly(amino acid) is protonated, i.e. poly(H.Lys)n+, whereas at pD 12.1 only approximately 10% of the epsilon-amino groups are positively charged. The results in all three systems are consistent with the isodesmic model of indefinite non-cooperative stacking. The stacking tendency follows the series: ATP4- (K = 1.3 M-1; pD 8.4) < ATP4-/poly(H.Lys)n+ (K = 11.5 M-1; pD 8.4) > ATP4-/90% poly(Lys)/10% poly(H.Lys)n+ (K = 3.1 M-1; pD 12.1). It is evident that poly(H.Lys)n+ assists the association of ATP by a factor of approximately 10, and it is suggested that, via its positively charged epsilon-ammonium groups, poly(H.Lys)n+ acts as a matrix by aligning ATP4- ions via ionic interactions with the negatively charged phosphate residues. The intragranular concentrations of various constituents of several storage or secretory cell organelles, as reported in the literature, are tabulated. The chromaffin granules of the adrenal medulla and the dense granules of blood platelets contain particularly high concentrations of nucleotides ([ATP] is approximately 0.14 M in the chromaffin granules and 0.5 M in the dense granules of rabbit blood platelets) and amines, such as epinephrine or 5-hydroxytryptamine. These granules, and probably also the storage vesicles of certain neurons (which seem to have a similar composition), appear, if the total concentrations of the various solutes are considered, to be osmotically unstable, which means that the intragranular solutes must be associated. This aggregation is discussed, especially with regard to the nucleotides.

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.

Chromogranin: a newly recognized marker for endocrine cells of the human gastrointestinal tract

Existing methods for the histochemical demonstration of gastrointestinal cells are somewhat limited. Chromogranin represents a family of proteins that coexist with catecholamines in the secretory vesicles of adrenal medulla cells. In the present study, immunocytochemistry was used to test whether chromogranin is a marker for gut endocrine cells. Serial sections of each area of human gut were immunostained for chromogranin and for the amine and each of the peptides known to be present in mucosal endocrine cells. Chromogranin was immunostained in large numbers of endocrine cells in all tissues examined. All identified endocrine cell types were found, in serial sections or by sequential silver impregnations, to be chromogranin immunoreactive. However, the possibility exists that some chromogranin-immunoreactive cells contain a yet to be discovered endocrine substance. Immunostaining of chromogranin thus appears to provide a means for demonstrating all gastrointestinal mucosal endocrine cells identifiable by the methods described in this study.

Ultrastructural localization of chromogranin: a potential marker for the electron microscopical recognition of endocrine cell secretory granules

Using a monoclonal antibody (LK2H10) directed against human chromogranin, we have been able to localize this soluble glycoprotein to the matrix of secretory granules from a wide variety of endocrine cells. In the gut, enterochromaffin, enteroglucagon, glucose-dependent insulinotropic peptide, gastrin, and neurotensin-containing cells exhibit chromogranin immunoreactivity. In our system, chromogranin-immunoreactive material was restricted to the halo of human pancreatic glucagon-containing secretory granules within A-cells. Chromogranin immunoreactivity was also localized to secretory granules in phaeochromocytomas, gastrinomas, medullary carcinomas of the thyroid and a carotid body tumour (chemodectoma). Chromogranin is proposed as a potential marker for the ultrastructural recognition of endocrine cell secretory granules.

Evidence for a divalent cation dependent catecholamine storage complex in chromaffin granules

Chromaffin granules, the secretory vesicles of the adrenal medulla, are stable in isotonic sucrose solutions at room temperature; however, when low concentrations of the ionophore A23187 are added, rapid lysis ensues which is dependent on the presence of a divalent cation chelator and is prevented by the addition of either Ca2+ or Mg2+. As little as 10 microM Ca2+ totally inhibit lysis of chromaffin granules by A23187, while 28 mM KCl have no effect. Lysis by A23187 at 4.7 microM is almost 100% in the presence of EDTA in isotonic sucrose in 1 h and can be suppressed by raising the osmotic strength of the medium with half maximal inhibition at 0.57 M sucrose, demonstrating that A23187 causes osmotic lysis of chromaffin granules as a consequence of the withdrawal of divalent cations from the core solution. Our results strongly suggest that divalent cations are involved in the formation of a ionic complex in the core solution which lowers its effective osmotic pressure.

Temperature-induced lysis of chromaffin granules provides evidence against the two-pool hypothesis of catecholamine storage

The temperature-dependent release of core constituents from isolated chromaffin granules in isotonic sucrose has been a controversial and puzzling phenomenon that has been interpreted either as selective catecholamine efflux from different catecholamine pools or as temperature-dependent lysis. We have analysed the kinetics, temperature dependence and physical basis of this process. Our results demonstrate that, upon increasing the ambient temperature, chromaffin granules show a shift in their osmotic fragility to higher osmolarities, which is linearly dependent on temperature and leads to measurable lysis in 0.26 M buffered sucrose at temperatures above 12 degrees C. It is possible to demonstrate both protein and dopamine beta-hydroxylase release when lysis as a function of temperature is measured in 0.26 M buffered sucrose. Real time measurements of the lysis kinetics were recorded on cassettes and analysed by a computer program for exponential decay kinetics. It is shown that the temperature-dependent lysis proceeds in two separate phases, the fast one of which is associated with temperature-dependent shift in the osmotic fragility curve. It has no characteristics of any exponential decay kinetics. The slow phase, when followed over several hours, leads to complete lysis of the granules in a sigmoidal time course at 30 degrees C. We conclude from the absence of exponentiality that there is no basis on which to assume the existence of different catecholamine pools. The fast phase of temperature-dependent lysis can be best explained as a simple temperature-dependent increase of the granule core solution's osmotic pressure, while the slow phase is probably caused by sucrose permeation into the granules. On the basis of these results, we warn against any efflux experiments measuring the temperature-dependent transmitter release from secretory vesicles with highly concentrated core solutions.

Characteristics and determinants of osmotic lysis in chromaffin granules

(1) Using isolated bovine chromaffin granules, we demonstrate that osmotic lysis is not a random process and establish the osmotic pressure dependence of osmotic lysis in chromaffin granules, the so-called osmotic fragility curve. (2) We show by measuring the release of constituents of the granule core and correlating these with changes in spectroscopic parameters (turbidity and endogenous catecholamine fluorescence), that the latter can be safely used to measure lysis. (3) Within a particular granule population, noradrenaline granules lyse at higher osmolarities than adrenaline granules, suggesting a higher core osmolarity of the noradrenaline granules. (4) The size distribution of chromaffin granules as a function of lysis was determined by the use of whole mount electron microscopy. It is shown that the mean size of chromaffin granules decreases as a function of lysis. (5) On the basis of theoretical considerations three alternative models of the sequence of osmotic lysis in chromaffin granules are proposed. The experimental results best support a model which postulates that during partial osmotic lysis, granule membranes reseal into smaller vesicles after graded release of contents. The osmotic fragility would represent several cycles of lysis and resealing and would not be a reflection of the distribution of osmotic pressures in the granule population.

Molecular mobilities and the lowered osmolality of the chromaffin granule aqueous phase

Carbon-13 spin-lattice relaxation times, T1, have been measured in whole adrenal medullary tissue slices, in suspensions of isolated chromaffin granules, in the reconcentrated chromaffin granule lysate, and in various model solutions containing catecholamines. ATP, chromogranins and Ca2+. Reorientational correlation times have been calculated at 10 degrees C using T1 data and nuclear Overhauser enhancements for protonated carbons on both catecholamines and nucleotides. Correlation times in all media are relatively short and characteristic of highly fluid aqueous phases. Adrenalin and ATP exhibit substantial differences in correlation times in all media, however, the ratio tau R (ATP): tau R(catecholamine) ranging from 2.4 in simple 3:1 adrenalin-ATP solutions to 4 in intact chromaffin granules. This difference, as well as the relatively high absolute reorientational mobilities of both components, confirms the importance of labile ionic interactions between ATP and catecholamines, but rules out the presence of high concentrations of base-stacked structures. Participation of the chromogranins in ternary complexes with catecholamines and ATP appears to be of minor importance. Ionic interactions to the protein are not reflected in either 13C T1 values or chemical shifts of arginine or glutamate sidechain resonances, or in the 13C chemical shifts of ATP or catecholamines. Very labile protein-ATP binding appears to be reflected in the correlation time measurements, however, which show selective immobilization of ATP relative to catecholamine in the presence of soluble protein. Osmotic measurements indicate that solutions containing adrenaline, ATP and Ca2+ are highly nonideal, but probably not sufficiently so to account fully for the osmotic stabilization of the chromaffin through their polyelectrolyte properties, exert a significant influence on the intragranular osmolality. The osmotic lowering due to polyion-counterion interactions has been estimated semiquantitatively using a theory developed by Oosawa.

Core structure, internal osmotic pressure and irreversible structural changes of chromaffin granules during osmometer behaviour

In the adrenal medullary cells, catecholamines are stored in and secreted from specialized secretory vesicles, the chromaffin granules. In order to gain some understanding of both functions of chromaffin granules, it is important to characterize their biophysical organization. Using isolated bovine chromaffin granules we have investigated the osmometer behaviour of chromaffin granules by 31P-NMR and fluorescence spectroscopy, by turbidity measurements and by electron-microscopic determination of chromaffin granule size distributions. On the basis of the osmometer model we have formulated equations predicting the behaviour of the native catecholamine fluorescence quenching and of the size of chromaffin granules a a function of osmolarity and have shown experimentally that the granules' behaviour conforms to these. It was possible to estimate the osmotic activity of the chromaffin granule core solution and the mean absolute water space in chromaffin granules from the determination of the size distributions as a function of osmotic pressure. With NMR spectroscopy a selective line-broadening of the alpha-resonances was observed with increasing osmolarities, while the gamma-phosphorus resonances remained virtually unchanged. Possibly there is an increase in core viscosity with osmolarity which affects only the alpha- and beta-phosphorus groups. While suspending chromaffin granules from lower to higher osmolarities causes no lysis, moving them back to their original osmolarity at which they were previously stable lyses them, thereby releasing a maximum of 70% of their releasable protein. This 'hyperosmolar' lysis is independent of preincubation times in the higher osmolarities and of the absolute dilution applied but depends on dilution beyond the 405 to 322 mosM sucrose range. Under the experiment conditions no uptake of sucrose from the medium into the granules could be measured, thereby suggesting that hyperosmolar lysis is a phenomenon not due to solute penetration. Since with NMR and fluorescence spectroscopy no chemical changes in the core composition can be observed, we conclude that hyperosmolar lysis may be caused by irreversible membrane relaxation upon osmotic shrinking.

Transport of Ca2+ and Na+ across the chromaffin-granule membrane

Bovine chromaffin-granule ghosts accumulate 45Ca2+ in a temperature- and osmotic-shock-sensitive process; the uptake is saturable, with Km 38 microM and Vmax. 28 nmol/min per mg at 37 degrees C. Entry occurs by exchange with Ca2+ bound to the inner surface of the membrane. It is inhibited non-competitively by Na+, La3+ and Ruthenium Red (Ki 10.7 mM, 7 microM and 2 microM respectively), and competitively by Mg2+ (ki 0.9 mM). Uptake was not stimulated by ATP. Na+ induces Ca2+ efflux; Ca2+ can re-enter the ghosts by a process of Ca2+/Na+ exchange. La3+ inhibits Ca2+ efflux during Ca2+-exchange, and Ca2+ efflux induced by Na+, suggesting that Ca2+ uptake and efflux, and Ca2+/Na+ exchange, are catalysed by the same protein. Na+ enters ghosts during CA2+ efflux, but the kinetics of its entry are not exactly similar to the kinetics of Ca2+ efflux. Initially 1-2 Na+ enter per Ca2+ lost, but at equilibrium 3-4 Na+ have replaced each Ca2+. There is no evidence that either Ca2+ uptake or efflux by Ca2+/Na+ exchange is electrogenic, suggesting that the stoichiometry of exchange is Ca2+/2Na+. This exchange reaction may have a role in depleting cytoplasmic Ca2+ after depolarization-induced Ca2+ entry through the adrenal medulla plasma membrane; there is some evidence that there may be an additional entry mechanism for Na+ across the granule membrane.

High-molecular-weight catecholamine--ATP aggregates are absent from the chromaffin-granule aqueous phase

Direct n.m.r. studies show that the viscous liquid phase which separates from solutions containing physiological concentrations of noradrenaline, ATP and Ca2+ is not present in the chromaffin-granule interior. This finding is verified for both adrenaline and noradrenaline by using 13C n.m.r. spectra.

Heterogeneity in the adrenomedullary storage of catecholamines, ATP, calcium and releasable dopamine beta-hydroxylase activity

Bovine adrenomedullary granules were separated into two subfractions by isopycnic density centrifugation. A small subfraction (approximately 10% of the total population) was sedimented into 2.2 M sucrose while the main population (80% of the total) was recovered at the interphase between 1.6 and 2.2 M sucrose. The concentrations of catecholamine (CA) and calcium showed marked seasonal variations for both subfractions, with lowest levels in the spring and highest levels in the winter. Throughout the year the concentrations of CA and calcium were 2-3 times higher in the minor subpopulation which also accounted for an abundance of noradrenaline (NA); on average 68% NA of total CA, 6.6 mumol CA and 225 nmol calcium/mg protein. The two subpopulations stored CA in similar ratios to ATP and calcium; i.e. 30 mol CA: 4 mol ATP: 1 mol Ca2+, indicating storage of CA largely independent of an equivalent amount of ATP, at least during winter when CA storage was 3.3 and 9.9 mumol/mg protein in the major and minor subpopulation respectively. The two subpopulations differed significantly in ratio of releasable dopamine beta-hydroxylase (DBH) activity per mole CA due to insignificant differences in specific activity of releasable DBH (0.4 unit/mg protein). The results show: (1) that the adrenomedullary granules are heterogeneous with respect to releasable activity of DBH per mole CA and subject to considerable seasonal variations; (2) a large portion of the NA-storing granules has a high concentration of releasable constituents; (3) both adrenaline (A)- and NA-storage are closely associated with that of calcium and occur in excess of that balanced by equivalent amounts of ATP.