Calcium signalling and the triggering of secretion in adrenal chromaffin cells

Nicotine and vascular dysfunction

Cigarette smoking is the single most important risk factor for the development of cardiovascular diseases (CVDs). However, the role of nicotine, the addictive component of all tobacco products, in the development of CVD is incompletely understood. Although increased public awareness of the harms of cigarette smoking has successfully led to a decline in its prevalence, the use of electronic cigarettes (e-cig) or electronic nicotine delivery system has increased dramatically in recent years because of the perception that these products are safe. This review summarizes our current knowledge of the expression and function of the nicotinic acetylcholine receptors in the cardiovascular system and the impact of nicotine exposure on cardiovascular health, with a focus on nicotine-induced vascular dysfunction. Nicotine alters vasoreactivity through endothelium-dependent and/or endothelium-independent mechanisms, leading to clinical manifestations in both cigarette smokers and e-cig users. In addition, nicotine induces vascular remodelling through its effects on proliferation, migration and matrix production of both vascular endothelial and vascular smooth muscle cells. The purpose of this review is to identify critical knowledge gaps regarding the effects of nicotine on the vasculature and to stimulate continued nicotine research.

Monitoring the Secretory Behavior of the Rat Adrenal Medulla by High-Performance Liquid Chromatography-Based Catecholamine Assay from Slice Supernatants

Catecholamine (CA) secretion from the adrenal medullary tissue is a key step of the adaptive response triggered by an organism to cope with stress. Whereas molecular and cellular secretory processes have been extensively studied at the single chromaffin cell level, data available for the whole gland level are much scarcer. We tackled this issue in rat by developing an easy to implement experimental strategy combining the adrenal acute slice supernatant collection with a high-performance liquid chromatography-based epinephrine and norepinephrine (NE) assay. This technique affords a convenient method for measuring basal and stimulated CA release from single acute slices, allowing thus to individually address the secretory function of the left and right glands. Our data point that the two glands are equally competent to secrete epinephrine and NE, exhibiting an equivalent epinephrine:NE ratio, both at rest and in response to a cholinergic stimulation. Nicotine is, however, more efficient than acetylcholine to evoke NE release. A pharmacological challenge with hexamethonium, an α3-containing nicotinic acetylcholine receptor antagonist, disclosed that epinephrine- and NE-secreting chromaffin cells distinctly expressed α3 nicotinic receptors, with a dominant contribution in NE cells. As such, beyond the novelty of CA assays from acute slice supernatants, our study contributes at refining the secretory behavior of the rat adrenal medullary tissue, and opens new perspectives for monitoring the release of other hormones and transmitters, especially those involved in the stress response.

Functional characterization of alpha9-containing cholinergic nicotinic receptors in the rat adrenal medulla: implication in stress-induced functional plasticity

An increase in circulating adrenal catecholamine levels constitutes one of the mechanisms whereby organisms cope with stress. Accordingly, stimulus-secretion coupling within the stressed adrenal medullary tissue undergoes persistent remodeling. In particular, cholinergic synaptic neurotransmission between splanchnic nerve terminals and chromaffin cells is upregulated in stressed rats. Since synaptic transmission is mainly supported by activation of postsynaptic neuronal acetylcholine nicotinic receptors (nAChRs), we focused our study on the role of alpha9-containing nAChRs, which have been recently described in chromaffin cells. Taking advantage of their specific blockade by the alpha-conotoxin RgIA (alpha-RgIA), we unveil novel functional roles for these receptors in the stimulus-secretion coupling of the medulla. First, we show that in rat acute adrenal slices, alpha9-containing nAChRs codistribute with synaptophysin and significantly contribute to EPSCs. Second, we show that these receptors are involved in the tonic inhibitory control exerted by cholinergic activity on gap junctional coupling between chromaffin cells, as evidenced by an increased Lucifer yellow diffusion within the medulla in alpha-RgIA-treated slices. Third, we unexpectedly found that alpha9-containing nAChRs dominantly (>70%) contribute to acetylcholine-induced current in cold-stressed rats, whereas alpha3 nAChRs are the main contributing channels in unstressed animals. Consistently, expression levels of alpha9 nAChR transcript and protein are overexpressed in cold-stressed rats. As a functional relevance, we propose that upregulation of alpha9-containing nAChR channels and ensuing dominant contribution in cholinergic signaling may be one of the mechanisms whereby adrenal medullary tissue appropriately adapts to increased splanchnic nerve electrical discharges occurring in stressful situations.

Extracellular calcium acts as a "third messenger" to regulate enzyme and alkaline secretion

It is generally assumed that the functional consequences of stimulation with Ca²⁺-mobilizing agonists are derived exclusively from the second messenger action of intracellular Ca²⁺, acting on targets inside the cells. However, during Ca²⁺ signaling events, Ca²⁺ moves in and out of the cell, causing changes not only in intracellular Ca²⁺, but also in local extracellular Ca²⁺. The fact that numerous cell types possess an extracellular Ca²⁺ “sensor” raises the question of whether these dynamic changes in external [Ca²⁺] may serve some sort of messenger function. We found that in intact gastric mucosa, the changes in extracellular [Ca²⁺] secondary to carbachol-induced increases in intracellular [Ca²⁺] were sufficient and necessary to elicit alkaline secretion and pepsinogen secretion, independent of intracellular [Ca²⁺] changes. These findings suggest that extracellular Ca²⁺ can act as a “third messenger” via Ca²⁺ sensor(s) to regulate specific subsets of tissue function previously assumed to be under the direct control of intracellular Ca²⁺.

Facilitation of Ca(2+)-dependent exocytosis by Rac1-GTPase in bovine chromaffin cells

Rho family GTPases are primary mediators of cytoskeletal reorganization, although they have also been reported to regulate cell secretion. Yet, the extent to which Rho family GTPases are activated by secretory stimuli in neural and neuroendocrine cells remains unknown. In bovine adrenal chromaffin cells, we found Rac1, but not Cdc42, to be rapidly and selectively activated by secretory stimuli using an assay selective for the activated GTPases. To examine effects of activated Rac1 on secretion, constitutively active mutants of Rac1 (Rac1-V12, Rac1-L61) were transiently expressed in adrenal chromaffin cells. These mutants facilitated secretory responses elicited from populations of intact and digitonin-permeabilized cells as well as from cells under whole cell patch clamp. A dominant negative Rac1 mutant (Rac1-N17) produced no effect on secretion. Expression of RhoGDI, a negative regulator of Rac1, inhibited secretory responses while overexpression of effectors of Rac1, notably, p21-activated kinase (Pak1) and actin depolymerization factor (ADF) promoted evoked secretion. In addition, expression of effector domain mutants of Rac1-V12 that exhibit reduced activation of the cytoskeletal regulators Pak1 and Partner of Rac1 (POR1) resulted in a loss of Rac1-V12-mediated enhancement of evoked secretion. These findings suggest that Rac1, in part, functions to modulate secretion through actions on the cytoskeleton. Consistent with this hypothesis, the actin modifying drugs phalloidin and jasplakinolide enhanced secretion, while latrunculin-A inhibited secretion and eliminated the secretory effects of Rac1-V12. In summary, Rac1 was activated by secretory stimuli and modulated the secretory pathway downstream of Ca2+ influx, partly through regulation of cytoskeletal organization.

Phosphotyrosine phosphatase activity associated with c-Src in large multimeric complexes isolated from adrenal medullary chromaffin cells

Chromaffin cells, which secrete catecholamines in response to acetylcholine, express high levels of the Src-family tyrosine kinases. These kinases contain protein-protein interaction domains which bind signal transduction proteins that participate in a variety of cellular processes. To determine if signalling proteins bind c-Src in chromaffin cells, we examined c-Src immunocomplexes for co-precipitating proteins. We discovered a phosphotyrosine phosphatase (PTPase; EC 3.1.3.48) activity which associates with specific subcellular pools of c-Src in vivo and which preferentially binds the SH2 (Src homology 2) domain of c-Src in vitro. Known PTPases were not identified by blotting of c-Src immunocomplexes with a panel of anti-PTPase antibodies, suggesting that the PTPase may be a novel family member. The c-Src-PTPase complex is enriched in the plasma membrane fraction and exists in several large complexes, as revealed by gel-filtration analysis. This PTPase activity is altered rapidly following stimulation by secretagogues, decreasing within 30 s and returning to basal levels by 60 s of stimulation. Both the subcellular localization and rapid activity changes suggest that the c-Src-associated PTPase may function in early signalling events emanating from the nicotinic acetylcholine receptor. In support of this is the co-precipitation of a PTPase activity with the nicotinic acetylcholine receptor and co-chromatography of this receptor with one or the c-Src-PTPase complexes.

Correlation between secretagogue-induced Ca2+ influx, intracellular Ca2+ levels and secretion of catecholamines in cultured adrenal chromaffin cells

Catecholamine secretion induced by various secretagogues in cultured bovine chromaffin cells has been correlated with Ca2+ influx and intracellular Ca2+ concentrations. Nicotine and high K+ caused prompt secretion of catecholamines from cells. Coincidently, both secretagogues evoked 45[Ca2+] influx with a parallel increase in free intracellular Ca2+ concentration, as determined by Quin 2 fluorescence. However, the rate of return of Ca2+ level to baseline after nicotine stimulation was more rapid than after K+ stimulation. In comparison, stimulation with veratridine produced a slow and prolonged Ca2+ influx accompanied by lower levels of intracellular Ca2+ than those observed after nicotine or K+ stimulation. Yet, during 15 min of stimulation, veratridine induced a substantial catecholamine release, which was larger than that obtained after nicotine or K+ stimulations. The Ca2+ ionophore A23187 (1 microM) induced a pronounced increase in intracellular Ca2+ levels, but did not evoke any significant catecholamine release. Finally, addition of the Ca2+ channel blocker verapamil following stimulation, at a time when intracellular Ca2+ concentration was at its peak level, did not affect the rate of decline in intracellular free Ca2+ concentration but promptly blocked Ca2+ uptake and catecholamine secretion. These findings suggest that the rate of Ca2+ influx, rather than the absolute level of intracellular Ca2+ concentration, determines the rate and extent of catecholamine release.

Staurosporine affects calcium homeostasis in cultured bovine adrenal chromaffin cells

These studies show that the potent, non-specific, protein kinase inhibitor, staurosporine, disrupts Ca2+ homeostasis in cultured bovine adrenal chromaffin cells. Staurosporine treatment reduces basal and A23187-stimulated catecholamine release from chromaffin cells, but does not inhibit activated Ca2+ influx. Furthermore, pretreatment with staurosporine also reduces Ca(2+)-stimulated catecholamine release from digitonin-permeabilized cells (t1/2, 40.6 min; IC50, 66.0 nm). However, staurosporine does not inhibit the rise in intracellular Ca2+ ([Ca2+]i) in response to nicotine stimulation as measured by fura-2 photometry. These studies demonstrate that staurosporine interferes with the secretory process at some step at or after the rise in [Ca2+]i in adrenal chromaffin cells. Examination of the effects of staurosporine on 45Ca2+ movement shows that staurosporine produces a slowly developing basal 45Ca2+ accumulation; after 30 min no significant change is observed, but by 120 min, 45Ca2+ accumulation is increased by 29.5%. Thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), inhibitors of Ca(2+) ATPases, were used to determine whether staurosporine induced 45Ca2+ accumulation results from sequestration of 45Ca2+ within intracellular stores. While thapsigargin has no significant effect, concomitant treatment with tBHQ prevents the increase in 45Ca2+ uptake associated with staurosporine treatment. Therefore, the tBHQ-sensitive Ca2+ store, but not the thapsigargin/inositol 1,4,5-triphosphate-sensitive Ca2+ store, appears to be staurosporine-sensitive. Overall, these studies indicate that staurosporine reduces catecholamine release by interfering with Ca2+ homeostasis. Furthermore, this work suggests that a staurosporine-sensitive phosphoprotein(s) is involved with the regulation of Ca2+ homeostasis in bovine adrenal chromaffin cells.

Intracellular mechanism of Pb(2+)-induced norepinephrine release from bovine chromaffin cells

The intracellular mechanism of Pb(2+)-induced release of norepinephrine (NE) was investigated in comparison with Ca2+ in bovine chromaffin cells permeabilized with staphylococcal alpha-toxin. Pb2+ activated NE release at considerably lower concentrations [concentration of free metal giving half maximal metal-dependent release (K0.5) 4.6 nM] than Ca2+ (K0.5 2.4 microM). The release of NE was associated with the release of dopamine-beta-hydroxylase but not lactate dehydrogenase. The maximal secretory responses produced by Pb2+ and Ca2+ were similar and nonadditive. Pb(2+)- and Ca(2+)-dependent releases showed a similar requirement for MgATP and were equally enhanced by protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA) but not by kinase A activator 8-bromoadenosine 3',5'-cyclic monophosphate free base. The protein kinase C inhibitor staurosporine blocked the TPA-stimulated component of secretion but had no effect on the NE release in the absence of TPA. Calmidazolium, an inhibitor of calmodulin, inhibited the secretion evoked by both metals to similar extent. Agents interacting with microtubules (colchicine and vinblastine) or microfilaments (cytochalasin B and phalloidin) had no effect on secretion induced by either metal cation. These observations indicate that both Pb2+ and Ca2+ act at a common site and activate the exocytotic release of NE by an analogous mechanism.

Spatial localization of agonist-induced Ca2+ entry in bovine adrenal chromaffin cells. Different patterns induced by histamine and angiotensin II, and relationship to catecholamine release

The spatial organization of agonist-induced Ca2+ entry in single bovine adrenal chromaffin cells has been investigated using video-imaging techniques to visualize fura-2 quenching by the Ca2+ surrogate, Mn2+. The potent secretagogue histamine, in addition to releasing Ca2+ from intracellular stores, resulted in a large influx of external Mn2+ that occurred over the entire surface of the cell. The influx of Ca2+ that this mirrors was found to be an obligatory requirement for the triggering of catecholamine release by histamine, which suggests that such a global influx of Ca2+ into the cell probably underlies the ability of this agonist to stimulate a large secretory response. By contrast, the weaker secretagogue angiotensin II, which also acts through the second messenger inositol trisphosphate, produced a localized entry of external Mn2+ in 64% of cells. In these cells, localized Mn2+ entry always occurred at the pole of the cell in which the angiotensin II-induced rise in [Ca2+]i was largest. Since exocytosis in response to angiotensin II has previously been shown to be restricted to this same pole of the cell (Cheek et al. (1989). J. Cell Biol. 109, 1219–1227), these results suggest that localized influx of Ca2+ in response to angiotensin II could underlie the polarized exocytotic response observed with this stimulus. These results directly demonstrate that different agonists can induce different patterns of divalent cation influx in the same cells and, furthermore, suggest how these different patterns can have a direct influence on cellular function.

Desensitization of cell signalling mediated by phosphoinositidase C

The waning of responses to cell-surface receptor activation during persistent stimulation with agonists (desensitization) is a feature common to many forms of transmembrane signalling. However, information is scarce regarding the regulatory processes that modulate the extensive group of receptors linked via phosphoinositidase C to the production of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. This situation is now beginning to change. Recent data indicate (1) that very rapid desensitization, possibly associated with receptor phosphorylation, regulates receptors linked to phosphoinositidase C, (2) that different receptors are desensitized to varying extents, (3) that receptor internalization can mediate desensitization at later times and (4) that signalling can be regulated at additional sites downstream of phosphoinositidase C. As Richard Wojcikiewicz and colleagues discuss here, these diverse regulatory events provide the means by which the breakdown of phosphoinositides and cellular responsiveness to their products are controlled during cell stimulation.

Ionic components of the electrical response of chromaffin cells from the toad (Caudiverbera caudiverbera) adrenal gland

1. Ultra fine tip microelectrodes (300 MOhm) were used to study the electrical properties of the chromaffin cell membrane in situ in the intact toad adrenal gland. 2. In the presence of physiologic [K+]o (2 mM) the resting membrane potential (Vm) was -53 +/- 3.2 mV. Vm depended on [K+]o as predicted by the constant field equation with PNa/PK of 0.16. 3. A small fraction (20%) of the impaled cells exhibited spontaneous electrical activity, though in all the cells examined, the injection of depolarizing current pulses elicited repetitive spikes. 4. Our measurements of the chromaffin cell input resistance (326 +/- 35 MOhm) is substantially smaller than the values reported for bovine isolated chromaffin cells, suggesting that the toad adrenal chromaffin cells might be electrically coupled. 5. Tetraethylammonium (TEA) increased the amplitude and duration of spikes, probably inhibiting outward K+ current. In the presence of tetrodotoxin (TTX) action potentials were abolished, although they reappeared if TEA was added, suggesting the participation of both Na+ and Ca2+ currents in the genesis of spikes. 6. As expected, acetylcholine (ACh) and nicotine depolarized the cells, though they did not always elicit electrical activity. 7. Muscarine (10-100 microM) had no effect on both Vm and on the depolarization induced by ACh or nicotine. Since muscarine inhibits catecholamine (CA) secretion induced by ACh and nicotine, we concluded that the inhibition of CA release by muscarine in the toad probably occurs at a level other than the membrane.

A calcium influx is neither strictly associated with nor necessary for exocytotic membrane fusion in Paramecium cells

Exocytosis of trichocysts in Paramecium cells was generally believed to depend on extracellular Ca, since it is accompanied by a Ca influx and not seen in the absence of Ca. However, by short term removal of Ca we showed recently that only extrusion of secretory contents, but not membrane fusion after stimulation with aminoethyldextran (AED), depends on extracellular Ca. We have now extended these studies to longer times and shown that membrane fusion is stimulated by AED even after 1 min at low Ca (< or = 30 nM). At prolonged times membrane fusion was induced by sole removal of Ca. In the presence of AED, trichocyst contents were slowly extruded followed by resealing of the fused membranes, indicating independency of endocytotic membrane fusion from extracellular Ca (though we observed aberrant resealing). Later on, Ca removal is followed by cell death. By using videomicroscopy, we further provide the first evidence that exocytosis is not necessarily accompanied by an influx of Ca in the presence of the usual high concentrations (1 mM), since local exocytosis at the rear end of the cells is not followed by ciliary reversal which is triggered by Ca influx. We conclude that a Ca influx is neither regularly associated with, nor necessary for, induction of exocytotic membrane fusion in Paramecium cells. As a source for a possible alternative intracellular liberation of calcium during exocytosis, we analyzed the subplasmalemmal alveolar sac system by electron spectroscopic imaging and found indications for Ca redistributions shortly after stimulation.