Phorbol esters cause preferential secretion of norepinephrine from bovine chromaffin cells

Potential aversive compounds in leafy spurge for ruminants and rats

Several wild and domestic ruminant species and horses apparently will not consume leafy spurge (Euphorbia esula) while grazing range and pasture lands. It has been demonstrated that leafy spurge can elicit conditioned food aversions in cattle and sheep, and the aversion-eliciting capacity of leafy spurge may account for why cattle seldom graze this nutritious plant and why sheep may not readily consume it at some locations. The identity of the aversive compound(s) in leafy spurge is unknown, but several different diterpenoid ingenol esters have been isolated from its tissues, and we suspect that one or more ingenol esters may be aversion-eliciting compounds in leafy spurge. The objectives of this study were to determine whether or not leafy spurge is aversive to laboratory rats and if a crude acetone extract of leafy spurge, presumably containing ingenol esters and other phytochemicals, could generate an aversive response in sheep and laboratory rats. An additional objective was to determine whether or not a particular ingenol monobenzoate, which may be similar to ingenol esters in leafy spurge, might also elicit an aversive response from rats. Rats exhibited food aversions associated with leafy spurge (P < 0.05). An acetone extract of leafy spurge induced conditioned food aversions in both sheep and rats (P < 0.01). The ingenol 3-monobenzoate also induced conditioned food aversions in rats (P < 0.01). Our interpretation of these data is that rats can be used as a model for cattle and sheep with respect to their aversion to leafy spurge ingestion. Additionally, we suggest that one or more ingenol esters may be aversion-inducing agents in leafy spurge. However, others may exist in leafy spurge that are also aversive or are the only or prime aversive chemicals.

Gabapentin inhibits catecholamine release from adrenal chromaffin cells

BACKGROUND

Gabapentin is most commonly prescribed for chronic pain, but acute perioperative effects, including preemptive analgesia and hemodynamic stabilization, have been reported. Adrenal chromaffin cells are a widely used model to investigate neurosecretion, and adrenal catecholamines play important physiologic roles and contribute to the acute stress response. However, the effects of gabapentin on adrenal catecholamine release have never been tested.

METHODS

Primary cultures of bovine adrenal chromaffin cells were treated with gabapentin or vehicle for 18-24 h. The authors quantified catecholamine secretion from dishes of cells using high-performance liquid chromatography and resolved exocytosis of individual secretory vesicles from single cells using carbon fiber amperometry. Voltage-gated calcium channel currents were recorded using patch clamp electrophysiology and intracellular [Ca2+] using fluorescent imaging.

RESULTS

Gabapentin produced statistically significant reductions in catecholamine secretion evoked by cholinergic agonists (24 ± 3%, n = 12) or KCl (16 ± 4%, n = 8) (mean ± SEM) but did not inhibit Ca2+ entry or calcium channel currents. Amperometry (n = 51 cells) revealed that gabapentin inhibited the number of vesicles released upon stimulation, with no change in quantal size or kinetics of these unitary events.

CONCLUSIONS

The authors show Ca2+ entry was not inhibited by gabapentin but was less effective at triggering vesicle fusion. The work also demonstrates that chromaffin cells are a useful model for additional investigation of the cellular mechanism(s) by which gabapentin controls neurosecretion. In addition, it identifies altered adrenal catecholamine release as a potential contributor to some of the beneficial perioperative effects of gabapentin.

Noradrenergic, but not Adrenergic Chromaffin Cells in the Adrenal Gland Express Neuromodulin (GAP-43)

Neuroendocrine chromaffin cells of the adrenal gland express certain molecular markers either transiently during development or permanently. In the present study, the expression of neuromodulin (GAP-43), a neuronal protein often associated with neurite outgrowth, was examined in adult adrenals. Neuromodulin was detected by Western blot analysis in extracts of both rat adrenals and cultured bovine chromaffin cells, and was localized in situ in a subpopulation of chromaffin cells, as well as in nerve fibres and Schwann cells. The use of anti-tyrosine hydroxylase or anti-phenylethanolamine N-methyltransferase antibodies in combination with anti-neuromodulin antibodies in double immunofluorescent labelling of cryostat sections of rat glands demonstrated that neuromodulin is expressed by noradrenergic, and not by adrenergic chromaffin cells. The results provide further evidence that neuromodulin is not limited to neurons; it is also expressed in a subpopulation of neuroendocrine chromaffin cells. Neuromodulin may play a role in the development of the adrenal medulla or in the specific regulation of noradrenalin secretion from chromaffin cells.

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.

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.

ATP stimulated catecholamine secretion: response in perfused adrenal glands and a subpopulation of cultured chromaffin cells

Extracellular ATP is shown to induce catecholamine secretion in bovine chromaffin cells. Our data indicate that cells in culture gradually increase their response to ATP, and we have separated freshly isolated cells on a density gradient and found that the lighter cells develop a much stronger response to ATP than do the heavier cells. To see if the ATP sensitivity is physiological, we have perfused intact adrenal glands. ATP induces a greater secretory response from glands than does acetylcholine without causing preferential secretion of norepinephrine or epinephrine. These data show that the response to ATP found in cultured cells is not an artifact of cell culture, and that ATP co-released with catecholamines from the storage vesicles may have a significant physiological role.

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.

Correlation of GAP-43 immunoreactivity with subpopulations of chromaffin cells in rat adrenal medulla

The neuronal growth-associated protein (GAP-43) is widely expressed during embryonic growth and axonal regeneration and has been thought to contribute to synaptic plasticity in adult animals. In contrast to the exclusively presynaptic pattern of GAP-43 immunoreactivity in sympathetic ganglia, GAP-43 intensely and selectively labeled the noradrenergic subpopulation of adult rat adrenal medullary chromaffin cell bodies, a pattern which persisted with adrenal denervation. Adjacent adrenergic islands containing neuropeptide Y and phenylethanolamine-N-methyl transferase immunoreactivity failed to express GAP-43. The immunohistochemical appearance of GAP-43 was qualitatively unchanged in the adrenal medulla of aged and diabetic rats, conditions in which the sympathoadrenal axis is thought to be dysfunctional.

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

Chromaffin cells have H1 histamine receptors. Histamine, acting at these receptors, increases the metabolism of inositol-containing phospholipids and stimulates catecholamine secretion from chromaffin cells. We have investigated the effects of histamine and other agents on the accumulation of inositol monophosphate (InsP1) and catecholamine secretion in purified cultures of norepinephrine-containing and epinephrine-containing bovine chromaffin cells. Histamine-stimulated InsP1 accumulation in epinephrine cells was three times greater than that in norepinephrine cells. In contrast, bradykinin caused roughly equivalent increases in InsP1 accumulation in the two chromaffin cell subtypes. Histamine-stimulated catecholamine secretion was also greater in epinephrine cells than in norepinephrine cells, whereas high K+, bradykinin, phorbol 12,13-dibutyrate, and angiotensin II all caused greater secretion from norepinephrine cells than from epinephrine cells. The density of H1 receptors in epinephrine cells was approximately three times greater than that in norepinephrine cells. The greater density of H1 receptors on epinephrine cells may account for the greater effects of histamine on InsP1 accumulation and catecholamine secretion in these cells.

Difference in the effectiveness of Ca2+ to evoke catecholamine secretion between adrenaline- and noradrenaline-containing cells of bovine adrenal medulla

Differential adrenaline (Ad) and noradrenaline (NA) secretions evoked by secretagogues were investigated using digitonin-permeabilized adrenal chromaffin cells, cultured adrenal chromaffin cells, and perfused adrenal glands of the ox. In digitonin-permeabilized cells, Ca2+ (0.8-160 microM) caused a concentration-dependent increase in catecholamine secretion, which was characterized by a predominance of NA over Ad secretion. Acetylcholine (10-1,000 microM), high K+ (14-56 mM), and bradykinin (0.1-1,000 nM) all were confirmed to induce the release of more NA than Ad at all concentrations used. There was no apparent difference in the ratios of NA/Ad between Ca(2+)-induced catecholamine secretion from digitonin-permeabilized cells and those induced by secretagogues from cultured cells. Qualitatively the same result was obtained in the secretory responses to acetylcholine and high K+ in perfused adrenal glands. These results indicate that the effectiveness of Ca2+ for catecholamine secretion is higher in the secretory apparatus of NA cells than in that of Ad cells of the bovine adrenal medulla. This may be one of the reasons why the secretagogues cause a predominance of NA secretion over Ad secretion in the bovine adrenal medulla.