Histamine evokes greater increases in phosphatidylinositol metabolism and catecholamine secretion in epinephrine-containing than in norepinephrine-containing chromaffin cells

Carbon-Fiber Nanoelectrodes for Real-Time Discrimination of Vesicle Cargo in the Native Cellular Environment

Carbon-fiber microelectrodes have proven to be an indispensable tool for monitoring exocytosis events using amperometry. When positioned adjacent to a cell, a traditional microdisc electrode is well suited for quantification of discrete exocytotic release events. However, the size of the electrode does not allow for intracellular electrochemical measurements, and the amperometric approach cannot distinguish between the catecholamines that are released. In this work, carbon nanoelectrodes were developed to permit selective electrochemical sampling of nanoscale vesicles in the cell cytosol. Classical voltammetric techniques and electron microscopy were used to characterize the nanoelectrodes, which were ∼5 μm long and sharpened to a nanometer-scale tip that could be wholly inserted into individual neuroendocrine cells. The nanoelectrodes were coupled with fast-scan cyclic voltammetry to distinguish secretory granules containing epinephrine from other catecholamine-containing granules encountered in the native cellular environment. Both vesicle subtypes were encountered in most cells, despite prior demonstration of populations of chromaffin cells that preferentially release one of these catecholamines. There was substantial cell-to-cell variability in relative epinephrine content, and vesicles containing epinephrine generally stored more catecholamine than the other vesicles. The carbon nanoelectrode technology thus enabled analysis of picoliter-scale biological volumes, revealing key differences between chromaffin cells at the level of the dense-core granule.

Profiles of secreted neuropeptides and catecholamines illustrate similarities and differences in response to stimulation by distinct secretagogues

The goal of this study was to define profiles of secreted neuropeptide and catecholamine neurotransmitters that undergo co-release from sympathoadrenal chromaffin cells upon stimulation by distinct secretagogues. Chromaffin cells of the adrenal medulla participate in the dynamic responses to stress, especially that of 'fight and flight', and, thus, analyses of the co-release of multiple neurotransmitters is necessary to gain knowledge of how the stress response regulates cell-cell communication among physiological systems. Results of this study demonstrated that six different secretagogues stimulated the co-release of the neuropeptides Met-enkephalin, galanin, NPY, and VIP with the catecholamines dopamine, norepinephrine, and epinephrine. Importantly, the quantitative profiles of the secreted neurotransmitters showed similarities and differences upon stimulation by the different secretagogues evaluated, composed of KCl depolarization, nicotine, carbachol, PACAP, bradykinin, and histamine. The rank-orders of the secreted profiles of the neurotransmitters were generally similar among these secretagogues, but differences in the secreted amounts of each neurotransmitter occurred with different secretagogues. Epinephrine among the catecholamines showed the highest level of secretion. (Met)enkephalin showed the largest levels of secretion compared to the other neuropeptides examined. Levels of secreted catecholamines were greater than that of the neuropeptides. These data support the hypothesis that profiles of secreted neuropeptide and catecholamine neurotransmitters show similarities and differences upon stimulation by distinct secretagogues. These results illustrate the co-release of concerted neurotransmitter profiles that participate in the stress response of the sympathoadrenal nervous system.

Spontaneous and electrically-evoked catecholamine secretion from long-term cultures of bovine adrenal chromaffin cells

Catecholamine release was measured from bovine adrenal medullary chromaffin cell (CC) cultures maintained over a period of three months. Cells were plated over simple biocompatible cell platforms with electrical stimulation capability and at specified times transferred to an acrylic superfusion chamber designed to allow controlled flow of superfusate over the culture. Catecholamine release was measured from the superfusates using fast cyclic voltammetry before, during and after electrical stimulation of the cells. Immunocytochemical staining of CC cultures revealed that they were composed of epinephrine (EP) and/or norepinephrine (NE) type cells. Both spontaneous and evoked-release of catecholamines from CCs were observed throughout the testing period. EP predominated during spontaneous release, whereas NE was more prevalent during electrically-evoked release. Electrical stimulation for 20 s, increased total catecholamine release by 60-130% (measured over a period of 500 s) compared to that observed for an equivalent 20 s period of spontaneous release. Stimulus intensity was correlated with the amount of evoked release, up to a plateau which was observed near the highest intensities. Shorter intervals between stimulation trials did not significantly affect the initial amount of release, and the amount of evoked release was relatively stable over time and did not decrease significantly with age of the culture. The present study demonstrates long-term survival of CC cultures in vitro and describes a technique useful for rapid assessment of cell functionality and release properties of cultured monoaminergic cell types that later can be transplanted for neurotransmitter replacement following injury or disease.

A physiological view of the central and peripheral mechanisms that regulate the release of catecholamines at the adrenal medulla

Here we review the tight neural control of the differential secretion into the circulation, of the adrenal medullary hormones adrenaline and noradrenaline. One or the other catecholamines are differentially released on various stress conditions. This is specifically controlled by central nervous system nuclei at the cortex, hypothalamus and spinal cord. Different firing patterns of splanchnic nerves and nicotinic or muscarinic receptors cause the selective release of noradrenaline or adrenaline, to adapt the body to the 'fight or flight' reaction, or during severe hypoglycaemia, haemorrhage, cold, acute myocardial infarction or other severe stressful conflicts. Endogenously acetylcholine (ACh) released at the splanchnic nerve-chromaffin cell synapse, acting on muscarinic and nicotinic receptors, causes membrane depolarization and action potentials (AP) in chromaffin cells. These changes vary with the animal species, the cell preparation (intact bisected adrenal, adrenal slices, or isolated fresh or cultured cells) or the recording technique (intracellular microelectrodes, patch-clamp, perforated-patch, cell-attached). Conflicting results leave many open questions concerning the actions of ACh on chromaffin cell excitability. The use of adrenal slices and field electrical stimulation will surely provide new insights into these mechanisms. Chromaffin cells have been thoroughly used as models to study the relationship between Ca2+ entry, cytosolic Ca2+ signals, exocytosis and endocytosis, using patch-clamp and amperometric techniques. Cells have been stimulated with single depolarizing pulses (DPs), DP trains and with simulated AP waveforms. These approaches have provided useful information but we have no data on APs generated by pulsatile secretory quanta of ACh, trying to mimic the intermittent and repetitive splanchnic nerve discharge of the neurotransmitter. We present some recent experiments using ultrashort ACh pulses (25 ms), that cause non-desensitizing repetitive APs with each ACh pulse, at low ACh concentrations (30 microM). Ultrashort pulses of a high ACh concentration (1000 microM) causes a single AP followed by a prolonged depolarization. It could be interesting trying to correlate these 'patterns of splanchnic nerve discharge' with Ca2+ signals and exocytosis. This, together with the use of adrenal slices and transmural electrical stimulation of splanchnic nerves will provide new physiologically sound data on the regulation of adrenal medullary secretion.

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

Capsaicin inhibits catecholamine secretion and synthesis by blocking Na+ and Ca2+ influx through a vanilloid receptor-independent pathway in bovine adrenal medullary cells

We report here the effects of capsaicin, a flavoring ingredient in the hot pepper Capsicum family, on catecholamine secretion and synthesis in cultured bovine adrenal medullary cells. Capsaicin inhibited catecholamine secretion (IC(50)=9.5, 11.8, and 62 microM) stimulated by carbachol, an agonist of the nicotinic acetylcholine receptor, by veratridine, an activator of voltage-dependent Na(+) channels, and by high K(+), an activator of voltage-dependent Ca(2+) channels, respectively. Capsaicin also suppressed carbachol-induced (22)Na(+) influx (IC(50)=5.0 microM) and (45)Ca(2+) influx (IC(50)=24.4 muM), veratridine-induced (22)Na(+) influx (IC(50)=2.4 microM) and (45)Ca(2+) influx (IC(50)=1.1 microM), and high K(+)-induced (45)Ca(2+) influx (IC(50)=5.8 microM). The reduction in catecholamine secretion caused by capsaicin was not overcome by increasing the concentration of carbachol. Furthermore, capsazepine (10 microM), a competitive antagonist for the transient receptor potential vanilloid 1, and ruthenium red (30 microM), a nonselective cation channel antagonist, did not block the inhibition by capsaicin of catecholamine secretion. Capsaicin also suppressed both basal and carbachol-stimulated (14)C-catecholamine synthesis (IC(50)=10.6 and 26.4 microM, respectively) from [(14)C] tyrosine but not from L: -3, 4-dihydroxyphenyl [3-(14)C] alanine ([(14)C] DOPA) as well as tyrosine hydroxylase activity (IC(50)=8.4 and 39.0 microM, respectively). The present findings suggest that capsaicin inhibits catecholamine secretion and synthesis via suppression of Na(+) and Ca(2+) influx through a vanilloid receptor-independent pathway.

Fine ultrastructure of chromaffin granules in rat adrenal medulla indicative of a vesicle-mediated secretory process

Observation by transmission electron microscopy, coupled with morphometric analysis and estimation procedure, revealed unique ultrastructural features in 25.94% of noradrenaline (NA)-containing granules and 16.85% of adrenaline (A)-containing granules in the rat adrenal medulla. These consisted of evaginations of the granule limiting membrane to form budding structures having different morphology and extension. In 14.8% of NA granules and 12.0% of A granules, outpouches were relatively short, looked like small blebs emerging from the granule surface and generally contained electron-dense material. A proportion of 11.2% of NA granules and 4.9% of A granules revealed the most striking ultrastructural features. These secretory organelles presented thin, elongated, tail-like or stem-like appendages, which were variably filled by chromaffin substance and terminated with spherical expansions of different electron density. A cohort of vesicles of variable size (30-150 nm in diameter) and content was found either close to them or in the intergranular cytosol. Examination of adrenal medullary cells fixed by zinc iodide-osmium tetroxide (ZIO) revealed fine electron dense precipitates in chromaffin granules, budding structures as well as cytoplasmic vesicles. These data indicate that a common constituent is revealed by the ZIO histochemical reaction in chromaffin cells. As catecholic compounds are the main tissue targets of ZIO complexes, catecholamines are good candidates to be responsible for the observed ZIO reactivity. This study adds further to the hypothesis that release of secretory material from chromaffin granules may be accomplished by a vesiclular transport mechanism typical of piecemeal degranulation.

Angiotensin II causes calcium entry into bovine adrenal chromaffin cells via pathway(s) activated by depletion of intracellular calcium stores

The characteristics and properties of the increase in cytosolic [Ca2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to angiotensin 11 have been investigated. In fura-2 loaded cells exposure to a maximally effective concentration of angiotensin II (100 nM) caused a rapid, but transient increase in cytosolic [Ca2+] followed by a lower plateau that was sustained as long as external Ca2+ was present. In the absence of external Ca2+ only the initial brief transient was observed. In cells previously treated with thapsigargin in Ca2+-free medium to deplete the internal Ca2+ stores, angiotensin II caused no increase in cytosolic [Ca2+] when external Ca2+ was absent. Reintroduction of external Ca2+ to thapsigargin-treated, store-depleted cells caused a sustained increase in cytosolic [Ca2+] that was not further increased upon exposure to angiotensin II. Analysis of the data suggests that in bovine chromaffin cells angiotensin II causes Ca2+ entry via a pathway(s) activated as a consequence of internal store mobilization, and entry through this pathway(s) forms the majority of the sustained Ca2+ influx evoked by angiotensin II.

The involvement of specific phospholipase C isozymes in catecholamine release from digitonin permeabilized bovine adrenal medullary chromaffin cells

The role of phospholipase C (PLC) in exocytosis has been investigated using digitonin permeabilized, [(3)H]noradrenaline ([(3)H]NA) loaded, bovine adrenal medullary chromaffin cells. The PLC inhibitor U-73122 caused a concentration-dependent suppression of Ca(2+)-evoked [(3)H]NA release but increased basal release (that occurring in the absence of Ca(2+)). Preincubation with antibodies against PLCgamma1 or PLCbeta3 (but not PLCdelta1, delta2, beta1 and beta2) also inhibited [(3)H]NA release evoked by Ca(2+) and increased basal release, indicating that only specific PLC isozymes are involved in these actions. Interestingly, PLCgamma1 (but not PLCbeta3) antibodies inhibited the ability of Ca(2+) to increase PLC activity in these permeabilized cells. These data therefore suggest that PLCgamma1 activity may have a specific role in regulating the exocytotic response from the adrenal chromaffin cell.

Role of brain thromboxane A2 in the release of noradrenaline and adrenaline from adrenal medulla in rats

Plasma noradrenaline reflects the release from adrenal medulla and sympathetic nerves; however, the exact mechanisms of adrenal noradrenaline release remain to be elucidated. The present study was designed to characterize the source of plasma noradrenaline induced by centrally administered vasopressin and corticotropin-releasing hormone (CRH) in urethane-anesthetized rats. Intracerebroventricularly administered vasopressin (0.2 nmol/animal) and CRH (1.5 nmol/animal) elevated plasma levels of noradrenaline and adrenaline. Intracerebroventricularly administered indomethacin [1.2 micromol (500 microg)/animal] (an inhibitor of cyclooxygenase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin and CRH. Intracerebroventricularly administered furegrelate [1.8 micromol (500 microg)/animal] (an inhibitor of thromboxane A(2) synthase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin, while the reagent only attenuated the elevation of plasma adrenaline evoked by CRH. Acute bilateral adrenalectomy abolished the elevation of both noradrenaline and adrenaline induced by vasopressin, while the procedure reduced only the elevation of adrenaline induced by CRH. These results suggest that the release of noradrenaline from adrenal medulla and sympathetic nerves is mediated by different central mechanisms. The vasopressin-induced noradrenaline release from adrenal medulla is mediated by brain thromboxane A(2)-mediated mechanisms, while the CRH-induced noradrenaline release from sympathetic nerves is mediated by brain prostanoid (other than thromboxane A(2))-mediated mechanisms. The vasopressin- and CRH-induced adrenaline release from adrenal medulla is also mediated by brain thromboxane A(2)-mediated mechanisms in rats.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Distribution of gamma-aminobutyric acid receptors in cultured adrenergic and noradrenergic bovine chromaffin cells

Fluorescence imaging techniques for recording cytosolic [Ca(2+)](i) from single chromaffin cells were used to characterize and discriminate between cell subpopulations containing gamma-aminobutyric acid (GABA)(A) and GABA(B) receptor subtypes. By combining this methodology with the immunoidentification of individual chromaffin cells using specific antibodies against tyrosine hydroxylase (TH), phenyl-etanolamine-N-methyl transferase (PNMT), and glutamic acid decarboxylase (GAD) linked to different fluorescent probes, we have been able to ascribe single-cell calcium responses to identified adrenergic and noradrenergic chromaffin cells. GAD enzyme is present in 30% of the chromaffin cell population, located primarily in adrenergic cells; 86% of GAD(+) cells were also PNMT(+). GAD expression was not correlated with the presence of GABA receptors. GABA-responsive cells were found with equal frequency in the GAD(+) and GAD(-) groups. However, the expression of GABA receptors was correlated with the adrenergic phenotype. [Ca(2+)](i) responses to GABA were found more frequently in adrenergic than in noradrenergic cells. GABA(A) receptors are more evenly distributed; about 90% of GABA-responsive cells have them. GABA(B) receptors have a more restricted distribution (present in 45% of responding cells). The coexpression of both GABA(A) and GABA(B) subtypes is the rule; only a minor subpopulation (about 12%) displays exclusively GABA(B) receptors. GABA receptor subtypes are distributed in a similar way when chromaffin cells are separated according to GAD(+)/GAD(-) or PNMT(+)/PNMT(-) classifications, with only minor differences. These data indicate that the intrinsic GABAergic system in the adrenal medulla is not designed as a paracrine model in which a group of cells specializes in transmitter synthesis and a different group serves as a specific target.

ERG K+ channel blockade enhances firing and epinephrine secretion in rat chromaffin cells: the missing link to LQT2-related sudden death?

The ether-a-go-go-related genes (erg) are expressed in tissues other than heart and brain, in which human erg (HERG) K+ channels are known to regulate the repolarization of heart action potentials and neuronal spike-frequency accommodation. We provide evidence that erg1 transcripts and ERG proteins are present in rat chromaffin cells in which we could isolate a K+ current that was biophysically and pharmacologically similar to the ERG current. Firing frequency and catecholamine release were analyzed at the single-cell level by means of perforated patch-clamp and carbon fiber electrochemical detection. It was found that the blocking of ERG, KATP, and KCa channels led to hyperexcitability and an increase in catecholamine release. Combined immunocytochemical experiments with antibodies directed against phenylethanolamine N-methyltransferase and ERG channels suggested expression of these channels in epinephrine- but not in norepinephrine-containing cells. It is concluded that, in addition to being crucial in regulating the QT period in the heart, ERG channels play a role in modulating epinephrine, a fundamental neurotransmitter shaping cardiac function. This finding suggests that the sudden death phenotype associated with LQT2 syndrome mutations may be the result of an emotionally triggered increase in epinephrine in a long-QT running heart.

Adrenal medullary catecholamine secretion patterns in rats evoked by reflex and direct neural stimulation

Epinephrine (EPI) and norepinephrine (NE), secretion patterns evoked by reflex (to hypotension and hypoglycemia) and direct neural stimulation of the adrenal medulla were measured in pentobarbital anesthetized male Sprague-Dawley rats. Secretion rates were determined by collecting adrenal venous blood. Baseline catecholamine secretion was similar in innervated and denervated glands indicating that there was little tonic sympathetic neural drive to the medulla. Both hydralazine-induced hypotension and insulin-induced hypoglycemia significantly increased catecholamine secretion with the secretion of EPI predominating. Similarly, in response to stimulation of the splanchnic nerve, frequency-related increments in EPI and NE were elicited with EPI release being greater than NE at all frequencies. However, the magnitude of the increase in secretion during splanchnic stimulation at frequencies above 1 hz greatly exceeded the release achieved by reflex stimulation. The results indicate that despite the fact that the stimuli of hypotension and hypoglycemia are integrated by different centers in the brain, the pattern of adrenal release is similar.

Differential secretion of adrenaline and noradrenaline in response to various secretagogues from bovine chromaffin cells

1. Differential secretion of adrenaline (Adr) and noradrenaline (NA) in response to various secretagogues was studied in bovine adrenal chromaffin cells. 2. Acetylcholine (ACh; 3-300 mumol/L), 1,1-dimethyl-4-phenyl-piperazinum (DMPP; 1-100 mumol/L), high K+ (20-60 mmol/L), calcimycin (1-100 mumol/L), histamine (0.3-30 mumol/L) and angiotensin (Ang)II (0.3-30 mumol/L) induced the secretion of a 1.3-2-fold greater percentage of NA stores than Adr stores in intact cells. 3. In beta-escin-permeabilized cells, Ca2+ (0.1-30 mumol/L) induced a greater secretion of Adr and NA in the presence of MgATP (2 mmol/L) than in the absence of MgATP. The percentage of NA secreted was 1.4- and 1.5-fold greater than that of Adr in the presence and absence of MgATP, respectively. 4. The ATP-independent phase of the Ca(2+)-dependent exocytosis is thought to be associated with the final step that ultimately leads to fusion, while the ATP-dependent phase is thought to be associated with the vesicle priming reaction. Therefore, the preferential secretion of NA in response to ACh, DMPP, high K+, calcimycin, histamine and AngII may be due, at least in part, to the greater effectiveness of Ca2+ in producing exocytosis in NA-containing cells.

Ca(2+) influx stimulated phospholipase C activity in bovine adrenal chromaffin cells: responses to K(+) depolarization and histamine

The role of Ca(2+) influx in activating phospholipase C in bovine adrenal chromaffin cells has been investigated. Phospholipase C activity in response to K(+) depolarization (56 mM) was blocked by the L-type Ca(2+) channel antagonist nifedipine and partially inhibited by the omega-conotoxins GVIA and MVIIC. In contrast, phospholipase C activity in response to histamine receptor activation was unaffected by omega-conotoxin GVIA and partially inhibited by omega-conotoxin MVIIC or nifedipine. This response was however markedly inhibited by the non-selective Ca(2+) channel antagonists La(3+) or 1-[beta-[3-(4-Methoxyphenyl)propoxy]-4-methoyphenethyl]-H-imidazol e (SKF-96365). Despite this Ca(2+) dependence phospholipase C activity was not increased during periods of "capacitative" Ca(2+) inflow generated by histamine-, caffeine- or thapsigargin-mediated depletion of internal Ca(2+) stores. Thus, while Ca(2+) influx in response to K(+) depolarization or G-protein receptor activation can increase phospholipase C activity in these cells, in the latter case it appears to be ineffective unless there is concurrent agonist occupation of the receptor.

Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and noradrenergic chromaffin cells, with minor populations of small intensely fluorescent cells and ganglionic neurones. Different molecular stimuli evoke distinct secretory events in the gland, involving the release of either adrenaline or noradrenaline together with various neuroactive peptides. The nature of the secretory response can be controlled at a central level or regulated locally within the gland. Specific innervation patterns to the different types of chromaffin cell have been implicated in central regulatory mechanisms, while several explanations for regulating secretion locally have been proposed. The differential distribution of various types of receptors between cell phenotypes, such as muscarinic or nicotinic acetylcholine receptors, histamine receptors, angiotensin receptors and different classes of opiate receptors between the two principal chromaffin cell populations could be involved in local control. In addition exocytosis parameters could be modulated differently in adrenergic and noradrenergic cells by phenotype-specific mechanisms, possibly involving molecules like Growth Associated Protein-43, Synaptosomal Associated Protein-25 isoforms or the p11 annexin subunit. The distribution of the various types of calcium channels is also known to vary between chromaffin cell subtypes. This short review examines possible ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between chromaffin cell phenotypes. Data reviewed here suggest that the adrenal medulla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secretory response that could in part reflect its local history.

Activation of tyrosine hydroxylase by histamine in bovine chromaffin cells

Acute activation of tyrosine hydroxylase by histamine has been studied in cultured bovine chromaffin cells. Tyrosine hydroxylase activity was determined in situ by measuring 14CO2 release following the hydroxylation and rapid decarboxylation of 14C-tyrosine offered to the cells. Histamine increased tyrosine hydroxylase activity 2-fold over 10 min with an EC50 of 0.3 microM and maximal response at 10 microM. Tyrosine hydroxylase activation was detectable within 1-2 min and maintained for at least 10 min. The effect of histamine was fully blocked by the H1 antagonist mepyramine, but unaffected by H2 (cimetidine) and H3 (thioperamide) antagonists. It was mimicked by Nalpha-methylhistamine and the H1 agonist 2-thiazolylethylamine, but not by H2 (dimaprit) or H3 (R)alpha-methylhistamine) agonists. The response to histamine was reduced by 70% by removing extracellular Ca2+ and abolished by removing extracellular Ca2+ and chelating intracellular Ca2+ with BAPTA. Tyrosine hydroxylase activation by histamine was unaffected by the protein kinase C inhibitor Ro 31-8220 but was completely blocked by the protein kinase A inhibitor H89. The results indicate that histamine activates tyrosine hydroxylase and that this effect is mediated through H1 receptors by a mechanism that depends on both extracellular and intracellular Ca2+ and that requires protein kinase A.

Segregation of nitric oxide synthase expression and calcium response to nitric oxide in adrenergic and noradrenergic bovine chromaffin cells

Previous work has demonstrated that nitric oxide can be an important intracellular messenger in the regulation of neurosecretion in chromaffin cells. Since standard chromaffin cell cultures are mixed populations of noradrenaline and adrenaline producing cells, it would seem important to understand the functional differences between these individual components. The use of fluorescence imaging techniques for the recording of cytosolic calcium from single chromaffin cells together with the immunoidentification of individual cells with specific antibodies against tyrosine hydroxylase, N-phenyl ethanolamine methyl transferase and nitric oxide synthase, has allowed us to measure single-cell calcium responses in identified adrenergic, noradrenergic and nitrergic chromaffin cells, thus helping us to clarify the differential role of nitric oxide in the function of these chromaffin cell types. 53 +/- 2% of chromaffin cells were able to synthesize nitric oxide (nitric oxidesynthase-positive cells), these cells being mainly noradrenergic (82 +/-2%). Results indicate that nitric oxide donors such as sodium nitroprusside, molsidomine and isosorbide dinitrate evoke [Ca2+]i increases in a 62 +/- 4% of chromaffin cells, the response to nitric oxide donors being between 30 and 50% of that of 20 microM nicotine. Cells responding to nitric oxide donors were mainly adrenergic (68 +/- 5%) although 45 +/- 9% of noradrenergic cells also gave [Ca2+]i increasing responses. The distribution of nitric oxide responding cells between nitric oxide synthase-positive and negative was very similar in the whole population (63 +/- 5 and 60 +/- 7%, respectively), but these differences were more prominent when considering the distribution of nitric oxide response between noradrenergic and adrenergic nitric oxide synthase-positive cells; while 73 6% of adrenergic nitric oxide synthase-positive cells evoke [Ca2+]i increases by nitric oxide stimulation, only 35 +/- 11% of noradrenergic nitric oxide synthase-positive cells respond. Taken together these results seem to indicate that (i) nitric oxide could act within adrenal medulla as both an intracellular and intercellular messenger; and (ii) noradrenergic cells seem to be specialized in nitric oxide synthesis while adrenergic cells with an endocrine function could mainly act as a target of neurosecretory action of this second messenger.

Mechanism of cardiovascular actions of the chromogranin A fragment catestatin in vivo

Catestatin (bovine chromogranin A(344-364); RSMRLSFRARGYGFRGPGLQL), reduces catecholamine secretion from chromaffin cells in vitro. We investigated the effects of this peptide on catecholamine release and blood pressure in vivo. Intravenous catestatin reduced pressor responses to activation of sympathetic outflow by electrical stimulation in rats, and the catestatin effect persisted even after adrenergic (alpha plus beta) blockade. Catestatin did not alter plasma norepinephrine levels, but increased plasma epinephrine 11-fold. Catestatin also blunted pressor responses to exogenous neuropeptide Y agonists. A control peptide (chromogranin A(141-160)) did not alter pressor or catecholamine responses to electrical stimulation. Pretreatment with a histamine H1 receptor antagonist blocked both the vasodepressor response to catestatin and the elevation in plasma epinephrine. Catestatin elevated endogenous circulating histamine 21-fold, and exogenous histamine mimicked both the epinephrine elevation and the vasodepressor actions of catestatin. We conclude that catestatin is a potent vasodilator in vivo whose actions appear to be mediated, at least in part, by histamine release and action at H1 receptors.

On-column monitoring of secretion of catecholamines from single bovine adrenal chromaffin cells by capillary electrophoresis

The secretion of catecholamines from individual bovine adrenal medullary cells was quantitatively monitored by capillary electrophoresis with laser-induced native fluorescence detection. By using a physiological balanced-salt solution as the running buffer for CE, the amount of norepinephrine (NE) and epinephrine (E) secreted by their physiological secretagogue, acetylcholine, and the amount remaining in a single cell can be simultaneously quantified. Among the six different glands (from separate cows) studied, a predominance of E-rich cells were found. There was no apparent relationship between the ratio of NE/E released and the original NE/E content in the cell. The secretion process was also monitored dynamically with this method by continuously passing acetylcholine over the cell during stimulation. From the peak width and shape of the released material, one can estimate the time scale of the release process.

Differential expression of alpha-bungarotoxin-sensitive neuronal nicotinic receptors in adrenergic chromaffin cells: a role for transcription factor Egr-1

Adrenomedullary chromaffin cells express at least two subtypes of acetylcholine nicotinic receptors, which differ in their sensitivity to the snake toxin alpha-bungarotoxin. One subtype is involved in the activation step of the catecholamine secretion process and is not blocked by the toxin. The other is alpha-bungarotoxin-sensitive, and its functional role has not yet been defined. The alpha7 subunit is a component of this subtype. Autoradiography of bovine adrenal gland slices with alpha-bungarotoxin indicates that these receptors are restricted to medullary areas adjacent to the adrenal cortex and colocalize with the enzyme phenylethanolamine N-methyl transferase (PNMT), which confers the adrenergic phenotype to chromaffin cells. Transcripts corresponding to the alpha7 subunit also are localized exclusively to adrenergic cells. To identify possible transcriptional regulatory elements of the alpha7 subunit gene involved in the restricted expression of nicotinic receptors, we isolated and characterized its 5' flanking region, revealing putative binding sites for the immediate early gene transcription factor Egr-1, which is known to activate PNMT expression. In reporter gene transfection experiments, Egr-1 increased alpha7 promoter activity by up to sevenfold. Activation was abolished when the most promoter-proximal of the Egr-1 sites was mutated, whereas modification of a close upstream site produced a partial decrease of the Egr-1 response. Because Egr-1 was found to be expressed exclusively in adrenergic cells, we suggest that this transcription factor may be part of a common mechanism involved in the induction of the adrenergic phenotype and the differential expression of alpha-bungarotoxin-sensitive nicotinic receptors in the adrenal gland.

Different contributions of L- and Q-type Ca2+ channels to Ca2+ signals and secretion in chromaffin cell subtypes

In this study, we investigated the contribution of different subtypes of voltage-dependent Ca2+ channels to changes in cytosolic free Ca2+ ([Ca2+]i) and secretion in noradrenergic and adrenergic bovine chromaffin cells. In single immunocytochemically identified chromaffin cells, [Ca2+]i increased transiently during high K+ depolarization. Furnidipine and BAY K 8644, L-type Ca2+ channel blocker and activator, respectively, affected the [Ca2+]i rise more in noradrenergic than in adrenergic cells. In contrast, the Q-type Ca2+ channel blocker omega-conotoxin MVIIC inhibited the [Ca2+]i rise more in adrenergic cells. omega-Agatoxin IVA (30 nM), which blocks P-type Ca2+ channels, had little effect on the [Ca2+]i signal. The N-type Ca2+ channel blocker omega-conotoxin GVIA similarly inhibited the [Ca2+]i rise in both cell types. The effects of furnidipine, BAY K 8644, and omega-conotoxin MVIIC on K+-evoked norepinephrine and epinephrine release paralleled those effects on [Ca2+]i signals. However, omega-conotoxin GVIA and 30 nM omega-agatoxin IVA did not affect the secretion of either amine. The data suggest that, in the bovine adrenal medulla, the release of epinephrine and norepinephrine are preferentially controlled by Q- and L-type Ca2+ channels, respectively. P- and N-type Ca2+ channels do not seem to control the secretion of either catecholamine.

Selective neural regulation of epinephrine and norepinephrine cells in the adrenal medulla -- cardiovascular implications

The innervation of the adrenal medulla regulates the release of catecholamines from the two, epinephrine (EPI) and norepinephrine (NE), populations of chromaffin cells. Adjustments in the neural output to the adrenal medulla are made by centers in the brain that integrate the sensory input arising from a variety of challenges and the resulting changes in secretion assist in the restoration of homeostasis. Interestingly, the adrenal medullary secretory responses do not simply reflect increments a fixed ratio of EPI to NE as might be expected if release was proportional to the number EPI and NE cells. Instead, the ratio of EPI to NE changes depending on the magnitude and type of stimulus that initiates neural activation of the medulla. The variability in the EPI:NE release ratio implies that the EPI and NE cells can be differentially stimulated. Although the underlying mechanisms are not fully characterized, this review presents an emerging view that the selective control of EPI and NE cells is accounted for, first, by the existence of separate neural circuits between brain centers and the chromaffin cells, and second, through neuromodulation that selectively influences EPI and NE cells. The presence of mechanisms that allow for separate control of the EPI and NE cells may significantly augment the range of cardiovascular and metabolic responses mediated through activation of the adrenal medulla.

Differences in the composition of chromaffin granules in adrenaline and noradrenaline containing cells of bovine adrenal medulla

Several constituents of chromaffin granules were quantitatively determined in noradrenaline and adrenaline cells purified from bovine adrenal medulla. As far as secretory peptides are concerned noradrenaline granules contained slightly more secretogranin II, but much less chromogranin A than adrenaline granules. This can be explained by the dependence of the biosynthesis of chromogranin A on corticosteroids. Proteolytic processing of chromogranin A and secretogranin II was higher in noradrenaline cells which was paralleled by a higher content of the prohormone convertase PC2. Noradrenaline granules also contained a higher concentration of the vesicular monoamine transporter (vMAT2). No differences were found for dopamine beta-hydroxylase, prohormone convertase PC1, carboxypeptidase H and synaptophysin. These results indicate that the secretory cocktail of peptides released from these cells differs significantly between adrenaline and noradrenaline storing cells.

Catecholamine release from fractionated chromaffin cells

Bovine chromaffin cells were separated by density gradient centrifugation into subfractions. After centrifugation on a self-generating Percoll gradient (42.75% isotonic Percoll, 30,000 x g for 22 min at 20 degrees C), the chromaffin cells were found in two clearly distinguishable peaks. The peak with the lower density contained most of the noradrenaline-producing cells (approximately 80%), whereas the adrenaline-producing cells were equally distributed between the two peaks. After collection of suitable fractions from the gradient, cell cultures were obtained, which were enriched with either > 90% adrenaline- or approximately 65% noradrenaline-producing cells. When stimulated by nicotine or carbachol, the dose-response curves of both cell fractions yielded similar EC50s for the release of adrenaline and noradrenaline. On the other hand, the cells of the less dense fraction released 30% more catecholamines (adrenaline as well as noradrenaline) than the cells of the more dense fraction. It is suggested that there are subpopulations among the adrenaline- and noradrenaline-producing cells with differences in receptor-effector coupling.

A difference in the cellular mechanisms of secretion of adrenaline and noradrenaline revealed with lanthanum in bovine chromaffin cells

A comparison of the effectiveness of the trivalent cation, lanthanum (La3+) relative to Ca2+ in causing adrenaline and noradrenaline release from bovine adrenal medullary chromaffin cells has been made. In cells maintained in tissue culture and permeabilised with digitonin, both La3+ and Ca2+ triggered catecholamine release. La3+ was more effective than Ca2+: the EC50 for La3+ was shifted to the left of that for Ca2+ by close to one order of magnitude for both adrenaline and noradrenaline. With respect to adrenaline, the same maximal release was triggered by the two cations, but with respect to noradrenaline, La3+ triggered a significantly greater release than did Ca2+. Mixtures of experimental media containing both La3+ and Ca2+ caused release of adrenaline and noradrenaline in amounts that approximated closely the sum of the releases caused by Ca2+ and La3+ alone. The data strongly imply that either the release mechanisms for adrenaline and for noradrenaline from their respective chromaffin cells are different, or the cellular mechanisms that regulate release from the two cells are different.

A reassessment of the modulatory role of cyclic AMP in catecholamine secretion by chromaffin cells

1. The role of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in the regulation of catecholamine (CA) secretion in chromaffin cells remains equivocal from previous studies. 2. In the present study the effect of this cyclic nucleotide on basal CA secretion, as well as on intracellular calcium and membrane potential has been examined. 3. Forskolin and the permeable cyclic AMP analogue, 8-(4-chlorphenylthio)-adenosine-3'-5' monophosphate cyclic (pClpcAMP), increased basal CA secretion in a dose-dependent manner. The EC50s were 0.43 +/- 0.10 microM for forskolin and 39 +/- 9 microM for pClpcAMP. Other agonists with adenylate cyclase activity such as stimulants of adenosine receptors, beta-adrenoceptors, GABAB receptors and intestinal vasoactive peptide (VIP), also increased basal CA secretion in a highly significant manner. However, when they were added together with forskolin, CA secretion was not affected although an additive increase in cyclic AMP levels was produced. 4. Statistical analysis of the correlation between cyclic AMP levels and CA secretion evoked by these cyclic AMP increasing compounds showed that a significant direct correlation between both parameters existed only when low levels of cyclic AMP were produced by secretagogue stimulation. When the increase in intracellular cyclic AMP concentrations exceeded approximately 8 times the basal cyclic AMP levels the correlation was not significant. These results indicate a dual dose-dependent effect of cyclic AMP on basal CA secretion. 5. The stimulatory effect of low cyclic AMP on basal CA secretion was accompanied by an increase in membrane potential and in intracellular calcium concentrations ([Ca2+]j), the latter mainly being due to an increase in intracellular Ca2+ entry through L-type voltage-dependent Ca2" channels.6. The possible mechanisms involved in these cyclic AMP effects are discussed.

Differential stimulation of noradrenaline release by reversal of the Na+/Ca2+ exchanger and depolarization in chromaffin cells

We compared the effectiveness of Ca2+ entering by Na+/Ca2+ exchange with that of Ca2+ entering by channels produced by membrane depolarization with K+ in inducing catecholamine release from bovine adrenal chromaffin cells. The Ca2+ influx through the Na+/Ca2+ exchanger was promoted by reversing the normal inward gradient of Na+ by preincubating the cells with ouabain to increase the intracellular Na+ and then removing Na+ from the external medium. In this way we were able to increase the cytosolic free Ca2+ concentration ([Ca2+]c) by Na+/Ca2+ exchange to 325 +/- 14 nM, which was similar to the rise in [Ca2+]c observed upon depolarization with 35 mM K+ of cells not treated with ouabain. After incubating the cells with ouabain, K+ depolarization raised the [Ca2+]c to 398 +/- 31 nM, and the recovery of [Ca2+]c to resting levels was significantly slower. Reversal of the Na+ gradient caused an approximately 6-fold increase in the release of noradrenaline or adrenaline, whereas K+ depolarization induced a 12-fold increase in noradrenaline release but only a 9-fold increase in adrenaline release. The ratio of noradrenaline to adrenaline release was 1.24 +/- 0.23 upon reversal of the Na+/Ca2+ exchange, whereas it was 1.83 +/- 0.19 for K+ depolarization. Reversal of the Na+/Ca2+ exchange appeared to be as efficient as membrane depolarization in inducing adrenaline release, in that the relation of [Ca2+]c to adrenaline release was the same in both cases. In contrast, we found that for the same average [Ca2+]c, the Ca2+ influx through voltage-gated channels was much more efficient than the Ca2+ entering through the Na+/Ca2+ exchanger in inducing noradrenaline release from chromaffin cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Tetraethylammonium selectively stimulates secretion from noradrenergic bovine chromaffin cells

1. The effects of tetraethylammonium chloride (TEA) on catecholamine secretion from primary cultures of noradrenaline-rich (noradrenergic) and adrenaline-rich (adrenergic) bovine chromaffin cells were studied. TEA stimulated catecholamine secretion from both cell types but was a much more effective secretory stimulus for noradrenergic cells. 2. TEA-induced catecholamine secretion was dependent on extracellular Ca2+, was partially inhibited by nifedipine and by tetrodotoxin, and was potentiated by ouabain. Other K+ channel blocking agents including 4-aminopyridine, glibenclamide, and tolbutamide did not stimulate catecholamine secretion. 3. TEA had no effect on Ca(2+)-induced secretion from digitonin-permeabilized chromaffin cells. 4. TEA presumably evokes secretion by inhibiting K+ channels, depolarizing chromaffin cells, and activating voltage-gated Ca2+ channels in the cells. Noradrenergic cells appear to be more sensitive to K+ channel inhibition than are adrenergic cells. 5. The secretory response of the chromaffin cells to TEA increased with time in culture. 6. In addition to being a more effective secretagogue in noradrenergic cells, TEA was also more effective in stimulating catecholamine synthesis in these cells.