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

The intravesicular cocktail and its role in the regulation of exocytosis

The accumulation of neurotransmitters within secretory vesicles (SVs) far exceeds the theoretical tonic concentrations in the cytosol, a phenomenon that has captivated the attention of scientists for decades. For instance, chromaffin granules can accumulate close to molar concentrations of catecholamines, along with many other products like ATP, calcium, peptides, chromogranins, ascorbate, and other nucleotides. In this short review, we will summarize the interactions that are currently believed to occur between the elements that make up the vesicular cocktail in the acidic environment of SVs, and how they permit the accumulation of such high concentrations of certain components. In addition, we will examine how the vesicular cocktail regulates the exocytosis of neurotransmitters. In this review, we have highlighted the mechanisms that permit the storage of neurotransmitters and hormones inside secretory vesicles. We also have proposed a novel model based in the intravesicular interactions of the main components of this inner cocktail - catecholamines, ATP, and chromogranins - to allow the accumulation of near molar concentrations of transmitters in secretory vesicles. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015).

The chromaffin vesicle: advances in understanding the composition of a versatile, multifunctional secretory organelle

Chromaffin vesicles (CV) are highly sophisticated secretory organelles synthesized in adrenal medullary chromaffin cells. They contain a complex mixture of structural proteins, catecholamine neurotransmitters, peptide hormones, and the relative processing enzymes, as well as protease inhibitors. In addition, CV store ATP, ascorbic acid, and calcium. During the last decades, extensive studies have contributed to increase our understanding of the molecular composition of CV. Yet, the recent development of biochemical and imaging procedures has greatly increased the list of CV-soluble constituents and opened new horizons as to the complexity of CV involvement in acute stress responses. Thus, a coherent picture of CV molecular composition is still to be drawn. This review article will provide a detailed account of the content of CV soluble molecules as it emerges from the most recent analytical studies. Moreover, this review article will attempt at focussing on the physiological and pathophysiological implications of the products released by CV.

The bovine central adrenomedullary vein: a target for endothelins

This study reports on morphological and contractile properties of the bovine central adrenomedullary vein (bCAMV). Up to several layers of circularly orientated smooth muscle cells (SMC) were observed, however, without forming a continuous, closed sheath. Discrete bundles of eccentrically arranged, longitudinal SMC were also conspicuous. Chromaffin cells were in most cases located outside the SMC layers, while sometimes being in close apposition to the endothelium in areas without SMC. Circularly mounted preparations of the endothelium-denuded vessel responded selectively to high K+, endothelins (ETs) and neuropeptide Y (NPY). The threshold for ET-1 was 0.13 nM and the half maximally effective concentration (EC50) was 3 +/- 1 nM (n = 9). The order of potencies was ET-1 > or = ET-2 >> ET-3, suggesting a vascular receptor (ETA). Concentrations at and above EC50 frequently developed long-lasting oscillations during the spontaneous relaxation of the ET-1 evoked tension. This response was partly (21%) independent of extracellular Ca2+. A marked tachyphylaxis developed to ET-1 (3-30 nM), resulting, on the other hand, in facilitation of the subsequent constrictor responses to high K+ and NPY. Propranolol and phentolamine alone, or in combination, were without effects on the basal tension and on the above-mentioned responses to high K+, ET-1 or NPY, making a contribution from adrenoceptor activation unlikely. No response was obtained with exogenous catecholamines, acetylcholine or serotonin, nor with a series of peptides known to occur in the adrenal medulla. This study shows that bCAMV is not a passive capacitance vessel but appears unique among mammalian veins in being selectively regulated by ETs.

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.

Galanin and neuropeptide Y in chromaffin granules from the guinea-pig

We have examined the subcellular distribution of galanin-like immunoreactivity, neuropeptide Y-like immunoreactivity and the catecholamines noradrenaline and adrenaline in the adrenal medulla from guinea-pigs. By differential centrifugation of the adrenal medulla homogenate the neuropeptides as well as the catecholamines sedimented in a 10,000 g pellet. This pellet was resuspended and further examined in discontinuous and continuous density gradients. In the discontinuous gradient the catecholamines peaked in the heavy bottom fraction, assumed to contain chromaffin granules. Galanin-like immunoreactivity and neuropeptide Y-like immunoreactivity were also enriched in this fraction. However, both neuropeptides showed high levels of sedimentable material also in a fraction of intermediate density. In the continuous density gradient, the sum of sedimentable and soluble catecholamines showed peak values in two fractions corresponding to 1.07 and 1.47 M sucrose, respectively. The NA peak in the denser fraction was more pronounced than the corresponding A peak. Galanin-like immunoreactivity showed only one peak, in the fraction corresponding to 1.07 M sucrose. The data suggest that galanin-like immunoreactivity and neuropeptide Y-like immunoreactivity are partly stored with catecholamines in chromaffin granules. However, galanin-like immunoreactivity and neuropeptide Y-like immunoreactivity was also found in fractions lighter than those containing the bulk of the catecholamines.

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: osmotically active fragments and their susceptibility to proteolysis during lysis of the bovine chromaffin granules

Osmotically active fragments of chromogranin A (Chr A) were studied in lysates from bovine chromaffin granules (CG) disrupted in the presence or absence of inhibitors of endogenous proteolytic activities. The effects of various methods of lysis were examined by micro-osmometry, PAGE-SDS electrophoretic techniques and immunoblots with polyclonal anti-Chr A sera. Osmotically active 'small' Chr A fragments (below 30 kDa) were conspicuous in lysates containing cocktails of leupeptin, pepstatin A, pHMB, PMSF and aprotinin. The osmotically inactive native Chr A in the 68-100 kDa range and the osmotically active fragments below 47 kDa were degraded in lysates at neutral or acid pH in the absence of inhibitors. However, degradation of the native Chr A and intermediates below 47 kDa could be prevented by extraction directly from intact CG, notably in cold or boiling distilled water. On the other hand, the main product after large-scale extraction of CG in 1 M acetic acid (pH 1.9, 100 degrees C) was a novel, osmotically active fragment (22 kDa), immunostaining only for the N-terminal sequence (Chr A1-40). The heat-stable fraction (Mr,n 23 kDa) exhibited concentration-independent colloid osmotic pressures even in the absence of phosphate, a property which may distinguish this N-terminal-containing fragment from the larger intermediates, probably containing the pancreastatin sequence, and other regions at the C-terminal side of the prohormone molecule. The functional roles of these osmotically active intermediates in the processing of Chr A are not yet known.

Catecholamines (CA) and adenosine triphosphate (ATP) are separately stored in bovine adrenal medulla, both in ionic linkage to granule sites, and not as a non-diffusible CA-ATP-protein complex

On superfusion of chromaffin granules from bovine adrenals with isotonic sodium and potassium salts, catecholamines and ATP were released in parallel and both in accordance with ion exchange kinetics. An artificial model was prepared by mixing a cationic (IRC-50) and an anionic (IR-4B) ion exchanger with COO- and NH+3 groups, respectively, as binding sites. This mixed ion exchanger showed in its storage and release of CA+ and ATP- striking similarities to the chromaffin granules. Within the pH range given for the interior of the granules--5.5-6--the artificial model even stored and released CA+ and ATP- within the same molar ratio as observed for the granules. We hypothesize that the chromaffin granule matrix in its storage and release functions operates as an amphoteric ion exchanger with COO- and NH+3 groups as the binding sites.

Effects of reserpine and tetrabenazine on catecholamine and ATP storage in cultured bovine adrenal medullary chromaffin cells

The in vivo storage relationship between catecholamines and ATP in chromaffin vesicles of cultured bovine adrenal medulla cells was investigated using drugs that block vesicular catecholamine uptake. Three-day treatments with reserpine and tetrabenazine causing 85-90% depletion of catecholamines resulted in 41-46% reductions in cellular ATP content. Subcellular fractionation of reserpine-treated cells indicated that the ATP is lost from the chromaffin vesicle pool. This was confirmed in experiments using metabolic inhibitors to differentiate the vesicular and extravesicular ATP pools. The vesicular ATP loss was not proportional to that of catecholamines, resulting in a reduction by 50% in the chromaffin vesicle mole ratio of catecholamines to ATP after 48 h of treatment. In metabolic labeling studies, it was found that reserpine treatment reduced the incorporation of [3H]adenosine into vesicular ATP selectively, but it reduced the incorporation of 32Pi into both the vesicular and extravesicular pools. The reduction of the [3H]adenosine incorporation was not due to diminished vesicular nucleotide uptake resulting from low catecholamine levels, because when the catecholamines were depleted by tetrabenazine pretreatment followed by removal of the drug before labeling, no reduction in [3H]adenosine incorporation was observed. When present during the labeling, tetrabenazine was found to be a reversible inhibitor of plasma membrane adenosine uptake. The observed loss of adenine nucleotides from catecholamine-depleted chromaffin vesicles in vivo provides evidence that interactions between ATP and catecholamines are important in the vesicular storage of high concentration of these compounds.

Turnover and storage of newly synthesized adenine nucleotides in bovine adrenal medullary cell cultures

The adenine nucleotide stores of cultured adrenal medullary cells were radiolabeled by incubating the cells with 32Pi and [3H]adenosine and the turnover, subcellular distribution, and secretion of the nucleotides were examined. ATP represented 84-88% of the labeled adenine nucleotides, ADP 11-13%, and AMP 1-3%. The turnover of 32P-adenine nucleotides and 3H-nucleotides was biphasic and virtually identical; there was an initial fast phase with a t1/2 of 3.5-4.5 h and a slow phase with a half-life varying from 7 to 17 days, depending upon the particular cell preparation. The t1/2 of the slow phase for labeled adenine nucleotides was the same as that for the turnover of labeled catecholamines. The subcellular distribution of labeled adenine nucleotides provides evidence that there are at least two pools of adenine nucleotides which make up the component with the long half-life. One pool, which contains the bulk of endogenous nucleotides (75% of the total), is present within the chromaffin vesicles; the subcellular localization of the second pool has not been identified. The studies also show that [3H]ATP and [32P]ATP are distributed differently within the cell; 3 days after labeling 75% of the [32P]ATP was present in chromaffin vesicles while only 35% of the [3H]ATP was present in chromaffin vesicles. Evidence for two pools of ATP with long half-lives and for the differential distribution of [32P]ATP and [3H]ATP was also obtained from secretion studies. Stimulation of cell cultures with nicotine or scorpion venom 24 h after labeling with [3H]adenosine and 32Pi released relatively twice as much catecholamine as 32P-labeled compounds and relatively three times as much catecholamine as 3H-labeled compounds.

Effects of calcium and limited proteolysis on membrane-bound and releasable dopamine beta-hydroxylase in adrenomedullary catecholamine granules

Bovine chromaffin granules were shown to contain two potent proteolytic systems resulting in limited proteolysis of granule proteins at pH 6.0 in the cold in the presence of inhibitors of serine and thiol proteases. Calcium, whether added during lysis or remaining bound to the lysate protein during dialysis in non-chelating solutions, enhanced recoveries of total immunoreactive dopamine beta-hydroxylase (14% of total protein) and soluble enzyme (9% of lysate protein) due to degradation of of chromogranins. A pepstatin A-blockable, catepsin D-like proteolytic system converting membrane-bound enzyme to its soluble counterpart at pH 6.0 was detected in the granule membrane fraction.

Subcellular storage and axonal transport of neuropeptide Y (NPY) in relation to catecholamines in the cat

The subcellular storage of neuropeptide Y-like immunoreactivity (NPY-LI) in peripheral sympathetic neurons and adrenal gland as well as its axonal transport in the sciatic nerve was studied in relation to catecholamines in the cat. In the subcellular fractions from different parts of sympathetic neurons, i.e. cell bodies (coeliac ganglia), axons (sciatic nerve) and terminals (spleen), the NPY-LI was found together with noradrenaline (NA) in heavy fractions assumed to contain large dense-cored vesicles. In addition, minor lighter fractions in the coeliac ganglion contained NPY-LI. The molar ratio between vesicular NA and NPY was high in the terminal regions (150 to 1) and much lower in axons and cell bodies (10 to 1), thus reflecting the different mechanisms of resupply for classical transmitter and peptide. In the adrenal gland the NPY-LI was mainly located in the catecholamine-storing chromaffin-granule fraction and also to a smaller extent in lighter fractions. Using reversed-phase HPLC, one molecular form of NPY-LI corresponding to porcine NPY was found in the coeliac ganglion, while the adrenal medulla also contained minor peaks with NPY-LI in addition to the main form, which co-eluted with porcine NPY. NA was stored both in light and heavy fractions in the spleen, while it was mainly found in heavier fractions in the sciatic nerve. In the coeliac ganglion, most of the noradrenaline was present in a non-particulate form. The anterograde transport rate for NPY-LI in the sciatic nerve was estimated to be about 9 mm h-1. A minor retrograde transport of NPY-LI was also detected. In conclusion, the present data suggest that NPY, a peptide with sympathoactive actions, is co-stored with NA in heavy fractions corresponding to large dense-cored vesicles, while light fractions with small dense-cored vesicles probably contain NA but not NPY-LI. The main resupply of NPY to terminals is, in contrast to NA, most likely by axonal transport, which implicates differences in the storage, turnover and release of these co-existing substances in the sympathoadrenal system.

Osmotic properties of the chromogranins and relation to osmotic pressure in catecholamine storage granules

The soluble proteins (chromogranins) of bovine chromaffin granules have been studied by micro-osmometry with semi-permeable membranes (UM2, PM10 and PM30 with cut-offs greater than 1, greater than 10 and greater than 30 kD, respectively) at 1 = 0.15 and pH 5-8 for protein concentrations up to 20 mg X ml-1. After lysis of chromaffin granules in phosphate buffer pH 6, the released chromogranins behaved as aggregating solutes, consistent with an inconspicuous osmotic pressure contribution from the chromogranins at the protein concentration of the intact granules. Thus, in the presence of phosphate about 90% of the molecules behaved as colloids with Mr = 30,300 at c = o. After lysis in phosphate-free buffers the chromogranins behaved as highly non-ideal solutes in a manner which was incompatible with isotonicity at the protein concentration of the intact granules. About two-thirds of the molecules in the lysates in Na-succinate pH 5-6 and K-acetate pH 6 exhibited Mr = 66,000 and 79,000, respectively. In dilute solutions (less than 12 mg protein X ml-1) and ATP/protein ratios corresponding to those in the intact granules, the UM2 pressures were markedly increased, indicating release of polypeptides with Mr 2000-3000 from aggregates. CaCl2 was without specific effect on the colloid osmotic pressures but reduced the ATP-dependent increase in pressure, suggesting release of molecules twice the size of those released by ATP alone. A model is presented for the contribution of the chromogranins to osmotic pressure regulation in the bovine adrenomedullary catecholamine-storing granules.

Characterization of the monoamine uptake system in catecholamine storage vesicles isolated from a pheochromocytoma taken from a child

The catecholamine storage vesicles of a pheochromocytoma taken from a child have been isolated and characterized. The tumor contained almost exclusively noradrenaline and a large proportion of this amine was vesicle-bound. The noradrenaline-containing vesicles showed great resemblance to bovine chromaffin granules. Their catecholamine and dopamine beta-hydroxylase contents were that of chromaffin granules; their morphology and density were similar to those of the subpopulation of these granules that contain noradrenaline. The pheochromocytoma vesicles contained in their membranes an abundant polypeptide of mol. wt 110,000, which was not apparent in bovine adrenal medulla vesicle membranes. Monoamine uptake by pheochromocytoma noradrenaline vesicles did not differ significantly from that observed in bovine chromaffin granules. The time-course, plateau level and KM for noradrenaline were similar for both types of organelles. Both had an oligomycin-resistant ATPase with similar properties. Investigations using the tetrabenazine derivative [2-3H]dihydrotetrabenazine (2-hydroxy-3-isobutyl-9,10-dimethoxy-1,2,3,4,6, 7-hexahydro-11b-H-benzo[a]quinolizine), which binds specially to the bovine chromaffin granule monoamine carrier indicated that granule membranes from the tumor have a 10-fold increased number of [2-3H]dihydrotetrabenazine binding sites, with no change in dissociation constant. As in the case of bovine chromaffin granules, [2-3H]dihydrotetrabenazine can be totally displaced by noradrenaline and serotonin. To account for the discrepancy observed between the uptake data (which indicated no difference with bovine chromaffin granules) and the [2-3H]dihydrotetrabenazine binding studies (which showed a large excess of binding sites in the tumor membranes), we propose that granules in the investigated tumor contained a large amount of inactive monoamine carrier.

Membrane dopamine beta-hydroxylase: a precursor for the soluble enzyme in the bovine adrenal medulla

Searching for endogenous proteolytic activities converting the membrane form of dopamine beta-hydroxylase (dopamine beta-monooxygenase, DBH) into the soluble and releasable form, DBH was monitored enzymatically and immunologically in aqueous and detergent-solubilized extracts of the adrenomedullary fractions. Degradation of the soluble DBH and acidic chromogranins by activation of endogenous proteases occurred during lysis in H2O. Shifts in the hydrophobicity of the membrane DBH were also apparent. Loss in enzyme protein or activity was, on the other hand, not observed for buffer-dialysed CG (pH 5-6). Limited proteolysis within the membrane phase was, however, indicated by the shift towards dominance of the intermediate hydrophobic DBH in the buffer-dialysed CG. By two-dimensional, crossed immunoelectrophoresis with cationic detergent the microsomal DBH was immunologically identical to the granule-bound enzyme but differed from the latter in molecular heterogeneity and in susceptibility to proteolytic solubilization by endogenous protease activities. DBH in the membranes of the chromaffin granules was proteolytically solubilized at pH 6-8 and the soluble DBH further degraded at pH 5. The results indicate that a post-translational conversion of the amphiphilic DBH into the soluble form, initiated at the level of the microsomes, may continue within the light and the heavy granule fractions which contain several DBH-converting and degrading proteolytic activities with acid optima.

Time course of release of catecholamine and other granular contents from perifused adrenal chromaffin cells of guinea-pig

Experiments were carried out to investigate the time course of the release of catecholamine, dopamine-beta-hydroxylase (DBH) and adenine nucleotides from isolated chromaffin cells of guinea-pig adrenal gland. When the isolated chromaffin cells were incubated with medium containing acetylcholine (ACh) (0.1 mM), veratridine (0.1 mM) or scorpion (Leiurus quinquestriatus) venom, (10 micrograms/ml.), catecholamine was released into the medium. Catecholamine secretion induced by veratridine or scorpion venom was inhibited by tetrodotoxin (1 microM) but not by atropine (0.1 mM) plus hexamethonium (0.1 mM). On the other hand, the secretory response to ACh was abolished by the cholinergic blocking drugs but not by tetrodotoxin. DBH was released together with catecholamine into the medium in which cells were suspended with these drugs. The ratio of catecholamine (n-mole) to DBH activity (n-mole/hr) appearing in the supernatant was 7.08 +/- 0.55, 6.60 +/- 0.27 and 8.91 +/- 0.47 for ACh, veratridine and scorpion venom, respectively. These values were close to that found in the lysate of chromaffin granules obtained from guinea-pig adrenal glands (7.37 +/- 0.39). The application of ACh or veratridine to perifused chromaffin cells was found to cause a parallel increase in catecholamine and DBH secretion in the perifusion medium without corresponding amounts of phenylethanolamine-N-methyltransferase leakage. However, DBH secretion tended to last for a longer period than catecholamine secretion. Adenine nucleotides were released from perifused chromaffin cells together with catecholamine, by ACh and veratridine. ATP added to the perifusion medium was metabolized to ADP and AMP, of which the ratio (ATP, 21.6%; ADP, 34%; AMP, 17.9%) was close to those of adenine nucleotides released from the cells. The secretion of adenine nucleotides induced by both secretagogues ceased much faster than the catecholamine secretion, so that molar ratio of catecholamine to adenine nucleotides was gradually increased during and after stimulation. The results indicate that catecholamine secretion is accompanied with a simultaneous release of DBH and ATP from adrenal chromaffin cells. Therefore, it is suggested that the delayed output of DBH, unlike catecholamine secretion, in perfused adrenal glands results from the presence of a diffusion barrier for this protein. The releasable secretory granules of isolated chromaffin cells are suggested to be heterogeneous with respect to the ratio of catecholamine to ATP.

Adenine nucleotide and phosphoenolpyruvate transport by bovine chromaffin granule "ghosts"

Purified bovine adrenal chromaffin granule "ghosts" (resealed membranes) accumulate nucleotides and phosphoenolpyruvate in a temperature-dependent process. Equilibration takes several hours, the compounds being accumulated to not more than twice their concentration in the medium. This uptake occurs primarily by a non-saturable diffusion process, and rates and extents show little dependence on membrane energization. Uptake rates are inhibited less than 50% by compounds that discharge the "ghost" membrane potential, or by atractylate or low concentrations of Cibacron Blue. The data gave little support to the theory that chromaffin granules have a membrane potential-dependent adenine nucleotide transport process and that nucleotides are in equilibrium with the positive-inside potential across the granule membrane.