Bovine adrenal medulla: subcellular distribution of newly synthesised catecholamines, nucleotides and chromogranins

María Teresa Miras Portugal: a pioneer for vesicular nucleotide storage

Chromaffin granules are secretory granules present in adrenal medulla chromaffin cells. They contain high contents of catecholamines and nucleotides and have been regarded as a model system for the study of vesicular transmitter uptake and release. In 1988, Dr. María Teresa Miras Portugal, when studying catecholamine biosynthesis, detected a novel group of nucleotides, the diadenosine polyphosphates, in the adrenal chromaffin granules. Based on this finding, she unraveled the existence of diadenosine polyphosphate-mediated chemical transmission, leading to a paradigm shift in the field of purinergic signaling. She is also a pioneer in the studies on vesicular nucleotide storage. First, María Teresa and her group characterized nucleotide transport in chromaffin granules and synaptic vesicles using fluorescent nucleotide derivatives such as 1, N6-ethenoadenosine triphosphates. Then, they revealed the presence of a hypothetical vesicular nucleotide transporter with unique properties in terms of substrate specificity. In this article, we will describe her contributions to vesicular nucleotide storage and the foundations she laid for future studies.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

A nibbling mechanism for clathrin-mediated retrieval of secretory granule membrane after exocytosis

Clathrin-mediated endocytosis is the major pathway for recycling of granule membrane components after strong stimulation and high exocytotic rates. It resembles "classical" receptor-mediated endocytosis but has a trigger that is unique to secretion, the sudden appearance of the secretory granule membrane in the plasma membrane. The spatial localization, the relationship to individual fusion events, the nature of the cargo, and the timing and nature of the nucleation events are unknown. Furthermore, a size mismatch between chromaffin granules (∼300-nm diameter) and typical clathrin-coated vesicles (∼90 nm) makes it unlikely that clathrin-mediated endocytosis internalizes as a unit the entire fused granule membrane. We have used a combination of total internal reflection fluorescence microscopy of transiently expressed proteins and time-resolved quantitative confocal imaging of endogenous proteins along with a fluid-phase marker to address these issues. We demonstrate that the fused granule membrane remains a distinct entity and serves as a nucleation site for clathrin- and dynamin-mediated endocytosis that internalizes granule membrane components in small increments.

Bovine chromaffin cells: studies on the biosynthesis of phospholipids in chromaffin granules

We have studied the biosynthesis of chromaffin granules by labelling primary cultures of bovine chromaffin cells with either [35S]methionine or various precursors for lipids. After labelling the cells were subjected to subcellular fractionation including density gradient centrifugation. After [35S]methionine significant label (mainly represented by labelled chromogranin A) was found in the soluble proteins of chromaffin granules, whereas the membranes were relatively little labelled. However incorporation into membrane bound dopamine beta-hydroxylase and cytochrome b-561 could be demonstrated. Neither of the used lipid precursors ([3H]glycerol, [3H]choline, [3H]palmitic acid or [3H]arachidonic acid) was incorporated to any significant extent into the soluble components of chromaffin granules. Thus there is no evidence that this secretory compartment contains any lipids or acylated proteins. Incorporation of lipid precursors into the membranes of chromaffin granules was apparently low. After short chases labelled lysolecithin was not present in these organelles. However with prolonged chase times labelled lysolecithin, apparently appeared in chromaffin granules irrespective of whether the cells were stimulated or not. We can conclude that the reusable membranes of chromaffin granules have a very low lipid turnover. Lysolecithin is not transferred into these organelles during biosynthesis but is formed in them during their long life span. This formation of lysolecithin is independent of stimulation of these cells and therefore unlikely to be involved in exocytosis.

Comparative incorporation of proenkephalin-derived peptides, chromogranin A, and dopamine beta-hydroxylase into chromaffin vesicles

The incorporation of enkephalin-containing peptides (ECPs) derived from proenkephalin into chromaffin vesicles was examined in primary cultures of adrenal medullary chromaffin cells. Cells were pulse-labeled with [35S]methionine and chased for periods up to 24 h. Chromaffin vesicles in cell homogenates were then fractionated by density gradient centrifugation and the presence of [35S]Met-enkephalin sequences in gradient fractions determined. 35S-ECPs were incorporated into particles suggestive of immature vesicles within 1-2 h after radiolabeling. Vesicle maturation, measured by co-equilibration of 35S-ECPs and total ECPs in the gradients, was complete within 9-12 h and was unaffected by treatments that increase proenkephalin synthesis. Incorporation of [35S]chromogranin A into chromaffin vesicles followed a similar time course, but 35S-labeled dopamine beta-hydroxylase was much more slowly incorporated, possibly reflecting differences in incorporation of membrane and soluble components. In summary, the data demonstrate that ECPs are rapidly sequestered in immature chromaffin vesicles, a process unaltered by changing rates of proenkephalin synthesis.

Synthesis of chromogranin A, dopamine beta-hydroxylase, and chromaffin vesicles

Primary cultures of bovine adrenal medullary cells synthesize chromogranin A (CgA) and dopamine beta-hydroxylase (DBH) and incorporate them into chromaffin vesicles. The incorporation of L-[35S]methionine into CgA, DBH, and total protein was approximately linear for 8 h at methionine concentrations of 12.5, 25, and 50 microM. Newly synthesized CgA and DBH were initially incorporated into vesicles of low buoyant density that matured over 24 h into vesicles having the greater buoyant density of chromaffin vesicles. Approximately 10% of the newly synthesized CgA is released constitutively within 4 h of formation, approximately 30-40% appears to be degraded, and the remainder is incorporated into chromaffin vesicles, which can secrete CgA in response to nicotinic stimulation. Newly synthesized DBH follows a similar course. Once incorporated into chromaffin vesicles, the newly synthesized CgA and DBH appear to be stable for 2-3 days and then decline with a half-time of 3-4 days. Primary cultures of bovine adrenal medullary cells are a good model system for studying factors regulating CgA and DBH synthesis and the formation of chromaffin vesicles.

The recycling of a secretory granule membrane protein

We have used N-hydroxysuccinimido-d-biotin as a reagent for labeling proteins exposed at the surface of cultured bovine adrenal chromaffin cells during Ba2+-stimulated secretion. A specific secretory granule membrane constituent, dopamine-beta-hydroxylase (DBH), has been investigated using immunoprecipitation followed by electrophoresis. Within 30 min of stimulation, exposed DBH had been cleared from the cell surface. Nevertheless, quantitation of labeled DBH using [125I] streptavidin suggested that it remained undegraded over a period of 24 h, a time during which secretory granule stores of catecholamines were being replenished. Subcellular fractionation of the cultured cells suggested that, after 3 or 4 h, the biotinylated DBH, which was still membrane-bound, was located in particulate material that also contained cytochrome b561, another major secretory granule membrane component.

Protein-carboxyl methylation in adrenal medullary cells

1. The protein-carboxyl methylating system has been studied in adrenal medullary cells either using disrupted cell components or with intact cells. Whereas the enzyme protein-carboxyl methylase (PCM) is cytosolic, the majority of its substrates is on or within chromaffin granules. With intact granules, methylation of surface proteins results in solubilization of membrane proteins. 2. Membrane PCM substrates have been identified as two proteins with apparent molecular weights of 55,000 and 32,000. Among the substrates located inside the granules, the chromogranins are excellent substrates, while dopamine beta-hydroxylase is poorly methylated. 3. Under physiological conditions, stimulation of the splanchnic nerve results in an increase in adrenal medullary protein-methyl ester formation as well as in an augmented methanol production. With adrenal medullary cells in culture, carboxyl-methylated chromogranin A is detected in mature chromaffin granules between 3 and 6 hr after labeling. Methylated chromogranins are secreted concomitantly with catecholamines following cholinergic stimulation. 4. These data coupled with those of Chelsky et al. (J. Biol. Chem. 262:4303-4309, 1987) on lamin B suggest that PCM methylates residues other than D-aspartyl and L-isoaspartyl in proteins. They further suggest that methylation may occur on nascent peptide chains before they are injected into the rough endoplasmic reticulum.

Membrane recycling after exocytosis: an ultrastructural study of cultured chromaffin cells

When exocytosis of granule contents is induced by nicotine stimulation, glycoprotein III (a chromaffin granule membrane constituent) is exposed on the surface of cultured chromaffin cells, where it may be labeled with an immunocytochemical tracer. The subsequent fate of this glycoprotein after endocytosis was followed at the ultrastructural level using immunogold methods and was analyzed by morphometry. After stimulation exocytosis membranes newly inserted into the plasma membrane labeled with gold particles for glycoprotein III were found to be endocytosed via coated vesicles and finally found in organelles devoid of chromogranin A, the major secretory granule protein. At intervals between 30 min and 24 h after cell stimulation and immunolabeling, most labeled structures were identified by two different morphological approaches as prelysosomes and lysosomes. In contrast with results obtained on freshly isolated chromaffin cells, it is thus concluded that in cultured cells granule membrane recycling into new granules does not occur. It is suggested that the fate of granule membrane endocytosed after cell stimulation may be influenced by the external conditions to which cells are previously exposed.

Secretory proteins from adrenal medullary cells are carboxyl-methylated in vivo and released under their methylated form by acetylcholine

The carboxyl methylation of secretory proteins in vivo was investigated in bovine adrenal medullary cells in culture. Chromogranin A, the major intragranular secretory protein in adrenal medullary cells, and other secretory proteins were found to be carboxyl-methylated within secretory vesicles. The in vivo labeling pattern using [methyl-3H]methionine and the in vitro labeling pattern using S-adenosyl-[methyl-14C]methionine of intravesicular secretory proteins were similar. The detection of methylated chromogranin A in mature secretory vesicles required 3-6 h, a time consistent with the synthesis and storage of secretory proteins in this tissue. Carboxyl-methylated chromogranin A was secreted from medullary cells by exocytosis via activation of nicotinic cholinergic receptor and recovered still under the methylated form in the incubation medium. Since protein-carboxyl-methylase is cytosolic, these results suggest that methylation of secretory proteins is a cotranslational phenomenon.

Bovine chromaffin granules: immunological studies with antisera against neuropeptide Y, [Met]enkephalin and bombesin

Antisera against neuropeptide Y, [Met]enkephalin and bombesin were used for characterizing the immunoreactive material in subcellular fractions of bovine adrenal medulla. Neuropeptide Y was identified by high performance liquid chromatography and by immunoblotting. Subcellular fractionation established that neuropeptide Y is present in chromaffin granules. During stimulation of the adrenal it is released concomitantly with catecholamines. The soluble proteins of chromaffin granules contain 1.9 micrograms neuropeptide Y/mg protein which gives 429 copies of neuropeptide Y for a single granule. In two-dimensional immunoblots two peptides of the same molecular size, but with differing pI (6.4 and 7.3) react with the antiserum against neuropeptide Y. There was no evidence for the presence of a larger neuropeptide Y precursor in chromaffin granules. On the other hand, larger enkephalin-containing peptides could be detected by immunoblotting. The subcellular distribution of these enkephalin precursors differed. The larger peptides (23.3 and 18.2 kD) were more concentrated in lighter granules when compared to the smaller precursors (12.6 and 8.6 kD) which is consistent with proteolytic processing of these peptides during granule maturation. An antiserum against bombesin reacts in immunoblots with the chromogranin B family. This study further illustrates that chromaffin granules contain a complex mixture of neuropeptide-immunoreactive material. The combination of immunoblotting with subcellular fractionation appears as a useful tool to characterize these peptides and their precursors.

Exocytotic exposure and recycling of membrane antigens of chromaffin granules: ultrastructural evaluation after immunolabeling

The exocytotic exposure and retrieval of an antigen of chromaffin granule membranes were studied with chromaffin cells isolated from bovine adrenal medulla. Cells were incubated with an antiserum against glycoprotein III followed by fluorescein- or gold-labeled anti-IgG. Immunofluorescence on the cell surface was present in a patchy distribution irrespective of whether bivalent antibodies or Fab fragments were used. During subsequent incubation these fluorescent membrane patches were internalized within 45 min. At the ultrastructural level immunogold-labeled patches were present on the surface of stimulated cells. During incubation (5 min to 6 h) these immunolabeled membrane patches became coated, giving rise to coated vesicles and finally to smooth vesicles. These latter vesicles were found spread throughout the cytoplasm including the Golgi region, but Golgi stacks did not become labeled. Part of the immunolabel was transferred to multivesicular bodies, which probably represent a lysosomal pathway. 30 min after incubation immunolabel was also found in electron-dense vesicles apparently representing newly formed chromaffin granules. After 6 h of incubation immunolabel was found in vesicles indistinguishable from mature chromaffin granules. These results provide direct evidence that after exocytosis membranes of chromaffin granules are selectively retrieved from the plasma membrane and are partly recycled to newly formed chromaffin granules, providing a shuttle service from the Golgi region to the plasma membrane.

Structural characterization of adrenal chromogranin A and parathyroid secretory protein-I as homologs

We have isolated and purified adrenal chromogranin A (Ch A) for the purpose of making structural comparisons to parathyroid secretory protein-I (SP-I), because our earlier data indicated these two molecules may be the same protein. An improved purification step, using high-performance liquid chromatography (HPLC), has enabled us to demonstrate that both SP-I and Ch A consists of two species, one of approximately 72,000 Da and one of approximately 66,000 Da. The amino acid composition is the same for all four species. The difference in molecular mass is assumed to be due to carbohydrate content. Cyanogen bromide digestion of each of the four samples, followed by HPLC separation of the generated peptides, resulted in a chromatographic profile that was the same for each digest. Amino acid analysis of the eight peptide fragments obtained from each digest indicates that both species of Ch A and both species of SP-I yielded the same peptide mixtures following this cleavage reaction. One large (approximately 50,000 Da) CNBr peptide was obtained and seven smaller ones, one of which contains cysteine. The large fragment behaved similarly to the intact molecule in a radioimmunoassay. HPLC separation of tryptic digests of Ch A (72,000 Da) and SP-I (72,000 Da) also resulted in elution profiles that were very similar to each other. Amino acid analysis revealed 23 peptides common to each digest. Ch A contained four peptides ranging in size from 4 to 30 residues that were not observed in the SP-I digest. SP-I contained two peptides, each with about 30 residues, that were not found in the Ch A digest. Nothing unusual was noted in any of the uncommon peptides. Thus, both a chemical and an enzymatic digestion of these molecules followed by analysis of the peptides generated, indicates that SP-I and Ch A are nearly identical homologs.

Exocytotic exposure and retrieval of membrane antigens of chromaffin granules: quantitative evaluation of immunofluorescence on the surface of chromaffin cells

The exocytotic exposure of antigens of chromaffin granule membranes was studied with chromaffin cells isolated from bovine adrenal medulla. Antigens on the cell surface were visualized by indirect membrane immunofluorescence employing antisera against glycoprotein III and dopamine beta-hydroxylase. With unstimulated cells, only weak immunofluorescence on the cell surface was observed, whereas stimulated cells (with carbachol or Ba2+) exhibited much stronger reactions. In all cases the staining appeared as dots and patches. To quantitatively prove these observations, we analyzed the immunostained cells using a fluorescence-activated cell sorter. After stimulation, the average fluorescence intensity of the cell population was enhanced. This increase correlated with the degree of catecholamine secretion. The fluorescence intensity of stimulated cells varied over a broad range indicating that individual cells reacted variably to the secretagogues. When stimulated cells were incubated at 37 degrees C for up to 45 min after stimulation, a decrease of membrane immunofluorescence approaching that of unstimulated control cells was observed. Apparently, the membranes of chromaffin granules, which had been incorporated into the plasma membrane, were retrieved by a specific and relatively fast process. This retrieval of the antigen from the cell surface was blocked by sodium azide, but not influenced by colchicine, cytochalasin B, and trifluoperazine. The quantitative methods established in this paper should prove useful for further study of the kinetics of the exo-endocytotic cycle in secretory tissues.

Immunofluorescent patterns of clathrin and dopamine beta-hydroxylase in chromaffin cells in culture

Bovine chromaffin cells maintained in culture for eight days were loaded with [3H]noradrenaline and then stimulated by a depolarizing concentration (56 mM) of K+. Control and stimulated cells were fixed in 3.7% formaldehyde, treated with acetone or Triton X-100, and then exposed to antibodies raised against dopamine beta-hydroxylase (a secretory granule marker) and clathrin, and purified by affinity chromatography. The cellular distribution of the correspondent antigens was investigated by indirect immunofluorescence. Cells treated with anti-dopamine beta-hydroxylase exhibited a granular pattern of fluorescence in the cytosol of the cell body, neurites, and terminal cones. Chromaffin cells exposed to anti-clathrin also showed a punctate pattern of fluorescence staining. However, in this case, the fluorescent dots were smaller than those observed with anti-dopamine beta-hydroxylase, and they were differently distributed. The speckled anti-clathrin fluorescence was preferentially condensed in the juxtanuclear region of the cell bodies, suggesting the possibility that clathrin was concentrated at the level of the Golgi apparatus. The stimulation of cultured chromaffin cells by 10 pulses of 56 mM K+ produced 91 +/- 2% (n = 5) depletion in the [3H]noradrenaline cell content and a concomitant displacement of the dopamine beta-hydroxylase fluorescence to the periphery of the cells. Four days after cell stimulation the dopamine beta-hydroxylase fluorescence was similar to that observed in control cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity and properties of the nucleotide carrier in chromaffin granules from bovine adrenal medulla

1. The influence of various substances on the uptake of [3H]ATP and [14C]-noradrenaline into isolated bovine chromaffin granules was investigated. The carrier-mediated [3H]ATP uptake is specifically inhibited by SO42-, PO43- and phosphoenolpyruvate. Compounds with carboxylic acid or sulphonic acid groups had no significant inhibitory effects on either uptake. 2. 35SO42-, 32PO43- and phosphoenol[14C]pyruvate are taken up into chromaffin granules by a temperature-dependent process that is inhibited by atractyloside, uncouplers of oxidative phosphorylation and lipid-permeant anions. The apparent Km of 35SO42- uptake is 0.4 mM. 3. These results indicate that the nucleotide carrier in chromaffin granules has a broad specificity, transporting compounds with two strong negative charges. 4. Amino acid probes influence the uptake of ATP and catecholamines differently. Pyridoxal phosphate inhibits both uptake processes, 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid preferentially blocks ATP uptake, whereas phenylglyoxal blocks only ATP transport. It is suggested that the nucleotide carrier possesses arginine residues in a functionally important position. 5. The significance of these results obtained on isolated granules for the function of chromaffin granules within the cell is discussed.

Exposure of an antigen of chromaffin granules on cell surface during exocytosis

The synthesis rate of the membrane proteins of the catecholamine-storing vesicles (chromaffin granules) of the adrenal medulla is lower than that of the secretory proteins of the contents. Based on these results we proposed that after exocytosis the membranes of chromaffin granules are retrieved and are re-used for several secretion cycles (see also ref. 4). This concept of re-use of granule membranes has been further strengthened by the finding that exogenous markers which are taken up by secretory cells during stimulation can be traced to the Golgi region and to immature secretory organelles. However, one basic question remains: are the membranes of secretory organelles specifically and completely removed from the plasma membrane and if so, how fast is this process? By using an antiserum against a membrane glycoprotein of chromaffin granules we have now obtained quantitative data which demonstrate that during exocytosis this antigen becomes exposed on the cell surface and disappears again to a large degree within 30 min.

Similarity of secretory protein I from parathyroid gland to chromogranin A from adrenal medulla

We have compared the amino acid and carbohydrate compositions, partial amino acid sequences, immunological crossreactivity, and physical properties of secretory protein I of the parathyroid gland and chromogranin A of adrenal gland. This comparison indicates that these proteins are similar molecules. Because secretory protein I is present in secretory granules containing parathormone and is cosecreted with the hormone, and because chromogranin A is contained within chromaffin granules and, likewise, is secreted with the catecholamines, the present observations raise the possibility that this class of protein plays a general role in hormone secretion or storage mechanisms.

Phosphoryl group transfer by a fraction of the soluble proteins of catecholamine storage vesicles

The terminal phosphate group of ATP was transferred to ADP by an enzyme present in the soluble core proteins of adrenal medulla catecholamine storage vesicles. It was purified 10-30-fold by DEAE Sephadex chromatography (Fraction I). The enzyme required divalent metal ions for activation; Mn2+ was almost as effective as Mg2+, but Ca2+ was only a weak activator. Activation by Mg2+ took place over a very narrow concentration range (0.5-3 mM). The specificity of the enzyme activity to nucleoside triphosphates was broad, to the nucleoside diphosphates narrow, favouring adenosine diphosphate. In dependence on the pH the activity increased from pH 4 to pH 7 and remained constantly high between pH 7 and 9. The Arrhenius plot was linear between 5 and 70 degrees C, with an activation energy of 11.1 kcal/mol. The phosphoryl group transfer reaction depended on the function of thiol groups; p-hydroxymercuribenzoate inhibited 50% of the enzyme activity; dithioerythritol reactivated it completely. Gel electrophoresis revealed that in Fraction I, a protein of molecular weight about 45,000, was enriched compared with the total proteins. The enzyme-enriched Fraction I differed significantly in its relative amino acid composition from that of the total soluble proteins; in general, the acidic amino acids were reduced and the more basic acids enhanced.

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.

Release of radioactive purines from cat nictitating membrane labeled with 3H-adenine

Cat nictitating membranes were incubated with 1-2 x 10(-7) M 3H-adenine or 3H-adenosine for 1 h. A tissuebath ratio of about 15 was found for both compounds in intact and denervated membranes. In intact nictitating membranes sympathetic nerve stimulation (4 Hz, 5 min) caused a net release of purines (0.66 +/- 17% of the tissue content), which was reduced by alpha-blockade. Noradrenaline (1-3 microM) or tyramine 60 microM), which produced the same contractile response as did nerve stimulation, increased purine release to the same extent as did nerve stimulation. The effect of either agent was reduced or abolished by phentolamine. Purine release could also be induced by acetylcholine and ATP. This release was not altered after surgical denervation. There was an excellent correlation between the contractile response and the purine release induced by nerve stimulation, noradrenaline, tyramine and acetylcholine. However, ATP caused a larger release of 3H-purines than expected from the contractile responses, possibly indicating displacement. The results indicate that most if not all of the 3H-purines released by nerve stimulation in the cat nictitating membrane are derived from postjunctional elements.

Partial characterization of a phosphoryl group transferring enzyme in the membrane of catecholamine storage vesicles

Phosphoryl group transfer from ATP to ADP occurred in the isolated membrane of catecholamine storage vesicles. The reaction was accelerated by extraction of the membranes with 50% (v/v) acetone and by treatment with 1% (v/v) Triton X-100. The phosphoryl group transfer reaction was activated by Mg2+ and by Mn2+. The activation profile differed from that obtained for the ATPase activity. The Michaelis-Menten kinetics of the phosphoryl transfer reaction were not entirely linear. From the linear parts of the double reciprocal plots KmATP approximately equal to 1 mM and KmADP approximately equal to 0.4 mM was obtained. All lines of the double reciprocal plots intersected indicating a sequential reaction mechanism. The reaction exhibited a narrow specificity for nucleoside diphospate and a broader one for nucleoside triphosphate indicating that ADP was the true substrate. The transfer reaction was slightly inhibited by AMP, orthophosphate and P1, P5-di(adenosine-5')pentaphosphate. The thiol reagents, N-ethylmaleimide and para-chloromercuribenzoate (PCMB), affected the ATPase activity and the phosphoryl transfer activity differently: with the blockade of 2.4 essential thiol equivalents by N-ethylmaleimide the ATPase was inhibited 50% and net uptake of catecholamine ceased, while the phosphoryl transfer remained unimpaired. PCMB affected both, the ATPase activity and phosphoryl transfer reaction. Treatment of the membranes with dithioerythritol prevented the PCMB-induced inhibition of the phosphoryl transfer, but was ineffective in protecting the ATPase activity, indicating that different thiol groups must be involved in the both enzymatic activities.

Distribution and metabolic fate of adenosine nucleotides in the membrane of storage vesicles from bovine adrenal medulla

The reactions of adenosine 14C-and gamma 32P-labelled ATP with isolated membranes from catecholamine storage vesicles of the bovine adrenal medulla were studied. In presence of Mg2+ about twice as much of 32P-radioactivity combined with the membrane as 14C-adenosine compounds at 31 degrees C and also at 0 degrees C, while in the absence of Mg2+ the amounts of 14C and 32P incorporated were similar for both substances. Autoradiography of the SDS-polyacrylamide gel after electrophoresis of the 32P-ATP-treated membrane protein showed two distinct zones corresponding to protein bands. Sonication released twice as much 32P-ATP as 14C-ATP from the space within the membrane particles indicating that at least half of the ATP present in space did not contain its original terminal phosphate group. About 40--45% of the 32P-radioactivity was incorporated in the membrane lipids, whereas only small amounts of 14C-radioactivity were extracted with lipids. About 1/3 of the incorporated 14C-radioactivity was not extractable with acids. The same amount remained in the 32P-ATP treated preparation acid-stably bound after extraction of the lipids and hus must be firmly bound ATP. When the reaction of the membrane preparation with labelled ATP was performed at 0 degrees C the fractions of the acid-stably bound 32P- and 14C-radioactivity increased. About 1 nmole/mg of protein (10--15%) of the bound 32P-radioactivity was exchangeable against unlabelled ATP, while only a very small fraction (less than 0.5 nmol/mg protein) of the 14C-radioactivity was exchanged against unlabelled ATP. Preincubation of the membrane particles with ATP-Mg2+ at 0 degrees C induced 30% inhibition of the ATPase activity and abolition of the net uptake of catecholamines. Different Km values obtained from initial velocity studies of ATPase activity and the overall-incorporation of 32P-radioactivity indicated that a direct correlation between these processes did not exist. Different strong inhibitory effects exerted by ADP on the ATPase activity and net uptake of catecholamine at the one hand and the overall 32P-and 14C-incorporation at the other hand supported that view. It is concluded that small fractions of the observed 32P-and 14C-incorporation can be involved in the ATP hydrolyzing reaction.

Synthesis of chromogranins and dopamine beta-hydroxylase by perfused bovine adrenal glands

The incorporation of [3H]leucine into chromogranins and into soluble and membrane-bound dopamine beta-hydroxylase (DBH) was studied in isolated perfused adrenal glands. [3H]chromogranins and [3H]DBH were quantitatively determined by immunoprecipitation. The amounts of soluble [3H]DBH formed were about equal to the amounts of membrane-bound [3H]DBH whereas the amounts of [3H]chromogranin were 5- to 20-fold greater than that of soluble [3H]DBH. On continuous sucrose density gradients, [3H]chromogranin was unimodally distributed after 2- to 20-h chase periods and accumulated in a vesicle having a lower buoyant density than mature chromaffin vesicles. At 2 h both membrane and soluble [3H]DBH were both bimodally distributed whereas after a 20-h chase period the distribution of soluble and membrane [3H]DBH was essentially unimodal and paralleled the distribution of [3H]chromogranin. These studies indicate that [3H]chromogranin, soluble [3H]DBH, and membrane [3H]DBH are synthesized concomitantly, but that each is transported into chromaffin vesicles at different rates.

Calcium movements during the release of catecholamines from the adrenal medulla: effects of methoxyverapamil and external cations

1. Cortex-free adrenal glands previously labelled with the isotope (45)Ca have been perfused with Locke or modified Locke solution to assess Ca(2+) movements under different conditions.2. Substitution of Na(+) by either sucrose or choline during perfusion with Ca(2+)-free Locke solution induced a significant and sustained decrease in the (45)Ca efflux. Concomitant with this effect there was an increase in the output of catecholamines from the perfused gland.3. In the presence of Ca(2+) (2.2 mM) in the perfusion fluid, Na(+) omission induced an increase in the (45)Ca efflux. This increase was significantly reduced if 3 x 10(-4)M methoxyverapamil (D-600) was present in the perfusion fluid. However, the increased catecholamine output in response to Na(+) deprivation remained unchanged.4. Excess of Mg(2+) (20 mM) in the extracellular medium blocked the increase in catecholamine output in response to Na(+) omission. However, the decrease in the (45)Ca efflux produced by Na(+) deprivation in the presence of this high concentration of Mg(2+) was similar to that observed in the presence of 1.2 mM-Mg(2+).5. In the absence of Mg(2+) in the extracellular medium, substitution of Na(+) by either sucrose or choline induced a sharp and transient increase in the (45)Ca efflux rate coefficient. This increased (45)Ca efflux, which has similar time course as the enhanced catecholamine output, was not affected by the presence of 3 x 10(-4)M methoxyverapamil.6. In the absence of Mg(2+), the graded substitution of Na(+) in the perfusion medium by sucrose enhanced the efflux of (45)Ca. This increase in the (45)Ca outward movement was linearly related to the logarithm of the extracellular Na(+) concentration.7. After perfusion of glands with Ca(2+)-free Locke solution, the reintroduction of Ca(2+) (2.2 mM) into the perfusion fluid produced an increase in the (45)Ca efflux. This was accompanied by a discharge of catecholamines.8. Although Mg(2+) (20 mM) was effective in blocking catecholamine release, this divalent cation did not modify the increase in the (45)Ca efflux produced by Ca(2+) reintroduction.9. In contrast to these later observations, methoxyverapamil (3 x 10(-4)M) was effective in inhibiting both increases in catecholamine output and (45)Ca efflux in response to Ca(2+) reintroduction.10. It is concluded from these experiments that (a) Ca(2+) movements in the adrenal medulla may involve both Na(+)-Ca(2+) and Ca(2+)-Ca(2+) exchange mechanisms; (b) the omission of Na(+) from the extracellular environment produces not only an increase in the output of catecholamines but it may increase the intracellular levels of Ca(2+) and that this may result in an increased Ca(2+) efflux when Mg(2+) is omitted from the perfusion fluids, and that (c) the competition between Ca(2+) and Mg(2+) during the secretory process may involve an intracellular site.

Synthesis, subcellular distribution and turnover of dopamine beta-hydroxylase in organ cultures of sympathetic ganglia and adrenal medullae

The synthesis, subcellular distribution and turnover of dopamine beta-hydroxylase was studied in organ cultures of rat adrenal medullae and superior cervical ganglia. After exposure to [3H]leucine for 1 or 3 h, the tissues were homogenized at various time intervals and the amount of labelled dopamine beta-hydroxylase in different subcellular fractions (cytosol, soluble and membrane-bound fraction of catecholamine storage vesicles) was determined by immunoprecipitation and subsequent electrophoresis. In cultured adrenal medullae, induction of dopamine beta-hydroxylase initiated in vivo by administration of reserpine affected both soluble and membrane-bound pools of dopamine beta-hydroxylase to a similar extent after pulse-labelling for 1 or 3 h. The half-lives of dopamine beta-hydroxylase, which amounted to 6 h for the cytosol, 7.5 h for the soluble vesicular and 32 h for the membrane-bound vesicular pools were not altered by pretreatment with reserpine. In superior cervical ganglia the half-lives of the soluble pools were 2-3 times longer than in the adrenal medulla, whereas the half-life of the membrane-bound fraction was the same as in the adrenal medulla. In both organs the most heavily labelled fraction (both after a pulse of 1 or 3 h) was always that of the vesicular membrane, suggesting that newly-synthesized dopamine beta-hydroxylase is immediately incorporated into the storage vesicles and not via release into the cytosol from the site of synthesis. The fact that the half-life of membrane-bound dopamine beta-hydroxylase is markedly longer than that of the two soluble pools suggests that the single pools are not only independently supplied by newly-synthesized DBH but there is also no appreciable subsequent exchange between soluble and membrane-bound pools.

Non-parallel transport of membrane proteins and content proteins during assembly of the secretory granule in rat parotid gland

The insertion of newly synthesized protein molecules into the membrane of the secretory granule of the rat parotid gland was studied by in vivo labeling with [3-H]-proline and [3-H]leucine. 2 h after the injection of the amino acid into the rat, the membrane fraction isolated from the secretory granules was found to be highly labeled with proline but only slightly labeled with leucine. The ratio of proline label in the granule membrane to that in the granule's secretory content was roughly equivalent to the ratio of total proline in the proteins of these two fractions. In contrast the ratio of leucine label in the membrane to that in the secretory content was much less than would be expected from the relative amount of leucine in both fractions. Separation of the proteins of the granule membrane by gel electrophoresis in presence of sodium dodecylsulfate showed that a considerable amount of these proteins was unlabeled. The labeled proteins could be selectively extracted from the membrane by 0.15 M Nacl solution or by dilute buffer at pH 4.5. These extracted proteins were found to contain a high proportion of proline residues and a negligible amount of leucine residues. In the extract proline constituted 36 mole % of the total amino acids. Proline plus glycine plus glutamic acid constituted more than 80 mole % and leucine constituted about 1 mole% of the total amino acids. Further analyses by gel electrophoresis in presence of sodium dodecylsulfate showed that the fractions of secretory granule membrane and secretory granule content are relatively free of contamination by proteins from other subcellular structures. It is suggested that the proteins which will constitute the mature secretory granule are transported to the site of final assembly by two pathways. The proline-rich proteins are transported to the site of assembly in close coordination with all the exportable proteins. The other membrane proteins arrive by a different pathway. Two alternative mechanisms are suggested to explain the finding that a considerable part of the membrane proteins are not labeled. I. The pathway of the intracellular transport of the unlabeled membrane proteins is similar to that of the secretory proteins but the newly synthesized membrane protein molecules are diluted in a large intermediate pool--the GOLgi complex. II. The proteins that did not get labeled are derived by a process of reutilization, from membranes of granules which have previously discharged their content in the process of secretion.