Calcium signalling in bovine adrenal chromaffin cells: additive effects of histamine and nicotine

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.

Coincident elevation of cAMP and calcium influx by PACAP-27 synergistically regulates vasoactive intestinal polypeptide gene transcription through a novel PKA-independent signaling pathway

Pituitary adenylate cyclase-activating polypeptide (PACAP) causes calcium influx, intracellular calcium release, and elevation of cAMP in chromaffin cells. Calcium influx is required for PACAP-stimulated secretion of catecholamines and neuropeptides. The role of cAMP elevation in the action of PACAP at either sympathetic or adrenomedullary synapses, however, is unknown. Here, we show that PACAP-27-induced calcium influx through voltage-sensitive calcium channels (VSCCs), together with elevation of intracellular cAMP, was sufficient to stimulate vasoactive intestinal polypeptide (VIP) biosynthesis at least 40-fold. Combined treatment of chromaffin cells with 40 mm KCl, which elevates intracellular calcium, and 25 micrometer forskolin, which elevates intracellular cAMP, caused an increase in VIP peptide and mRNA much greater than that elicited by either agent alone, and comparable to the increase caused by 10-100 nm PACAP-27. Elevation of VIP mRNA by either KCl plus forskolin, or PACAP, (1) was independent of new protein synthesis, (2) was blocked by inhibition of calcium influx through voltage-sensitive calcium channels, (3) was calcineurin dependent, and (4) was dependent on MAP kinase activation but not activation of protein kinase A. The degree of activation of two different second-messenger pathways, calcium influx and cAMP elevation, appears to determine the magnitude of transcriptional activation of the VIP gene in chromaffin cells. Maximal stimulation of VIP biosynthesis by PACAP appears to require the coincident activation of both of these pathways.

Pathophysiological effects of nicotine on the pancreas: an update

Epidemiological evidence strongly suggests an association between cigarette smoking and pancreatic diseases. It is well recognized that nicotine, a major component in cigarette smoke, is an addictive agent and, therefore, reinforces smoking behavior. The current review update focuses on the genetics of nicotine dependence and its role on the development of pancreatic diseases. The role of smoking and nicotine in pancreatitis and pancreatic cancer development is also discussed. Exposure of laboratory animals to nicotine clearly supports the notion that nicotine can induce pancreatic injury. The mechanism by which nicotine induces such effects is perhaps mediated via signal transduction pathways in the pancreatic acinar cell, leading to enhanced levels of intracellular calcium release, resulting in cytotoxicity and eventual cell death. The induction of pancreatic injury by nicotine may also involve activation and expression of protooncogene, H-ras, which can increase cytosolic calcium via second messenger pathways. Development of pancreatic carcinoma in cigarette smokers as observed in human populations may be the result of activation and mutation of the H-ras gene. A possible pathogenetic mechanism of nicotine in the pancreas activating multiple signal transduction pathways is schematically summarized in Figure 1.

Neurotransmitter release from bovine adrenal chromaffin cells is modulated by capacitative Ca(2+)entry driven by depleted internal Ca(2+)stores

Two potential mechanisms by which the intracellular Ca(2 stores might modulate catecholamine release from bovine adrenal chromaffin cells were investigated: (i) that the cytosolic Ca(2+)transient caused by Ca(2+)release from the intracellular stores recruits additional chromaffin granules to a readily releasable pool that results in augmented catecholamine release when this is subsequently evoked, and (ii) that the Ca(2+)influx that follows depletion of intracellular stores (i.e. store-operated Ca(2+)entry) triggers release per se thereby augmenting evoked catecholamine release. When histamine or caffeine were applied in Ca(2+)-free perfusion media, a transient elevation of intracellular free Ca(2+)occurred owing to mobilization of Ca(2+)from the stores. When Ca(2+)was later readmitted to the perfusing fluid there followed a prompt and maintained rise in intracellular Ca(2+)concentrations of magnitude related to the degree of store mobilization. In parallel experiments, increased catecholamine secretion was measured under the conditions when Ca(2+)influx following store-mobilization occurred. Furthermore, the size of the catecholamine release increment correlated with the degree of Ca(2+)influx. Store-operated Ca(2+)entry evoked by mobilization with histamine and/or caffeine did not augment nicotine-evoked secretion per se; that is, it augmented evoked catecholamine release only to the extent that it increased basal catecholamine release. The nicotine-evoked catecholamine release was sensitive to cytosolic BAPTA, which, at the concentration used (50 microM BAPTA-AM), reduced release by approximately 25%. However, the increment in basal catecholamine release which followed Ca(2+)influx triggered by Ca(2+)store mobilization was not reduced by intracellular BAPTA. This finding is inconsistent with the hypothesis that the elevated cytosolic Ca(2+)from store mobilization recruits additional vesicles of catecholamine to the sub-plasmalemmal release sites to augment subsequently evoked secretion. This position is supported by the observation that histamine (10 microM) in Ca(2+)-free medium caused a pronounced elevation of cytosolic free Ca(2+), but this caused no greater catecholamine release when Ca(2+)was re-introduced than did prior exposure to Ca(2+)-free medium alone, which caused no elevation of cytosolic free Ca(2+). It is concluded that intracellular Ca(2+)stores can modulate secretion of catecholamines from bovine chromaffin cells by permitting Ca(2+)influx through a store-operated entry pathway. The results do not support the notion that the Ca(2+)released from intracellular stores plays a significant role in the recruitment of vesicles into the ready-release pool under the experimental conditions reported here.

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.

Histamine causes Ca2+ entry via both a store-operated and a store-independent pathway in bovine adrenal chromaffin cells

The characteristics and properties of the increase in cytosolic [Ca2+] that occurs in bovine adrenal medullary chromaffin cells on exposure to histamine have been investigated. Specifically, these experiments were conducted to determine how much external Ca2+ enters the cell through a (capacitative) Ca2+ entry pathway activated as a consequence of intracellular Ca2+ store mobilization, relative to that which enters independently of store depletion via other channels activated by histamine. In Fura-2 loaded cells continued exposure to histamine (10 microM) 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 (100 nM) in Ca(2+)-free medium to deplete the internal Ca2+ stores, histamine caused no increase in cytosolic [Ca2+] when external Ca2+ was absent. Re-introduction of external Ca2+ to thapsigargin-treated store-depleted cells caused a sustained increase in cytosolic [Ca2+] that was further increased (P < 0.0002) upon exposure to histamine. The histamine-evoked increase was prevented by the H1-receptor antagonist, mepyramine (2 microM). A comparison was made between store-dependent Ca2+ entry consequent upon store mobilization with histamine in Ca(2+)-free medium and plateau phase Ca2+ entry resulting from stimulation with histamine in Ca(2+)-containing medium. The latter was found to be approximately 3 times greater in magnitude than the former (P << 0.0001) at the same concentration of histamine (10 microM). It is concluded that histamine causes Ca2+ entry not only via a capacitative entry pathway secondary to internal store mobilization, but also causes substantial Ca2+ entry through other pathways.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Vesicular monoamine transport inhibitors. Novel action at calcium channels to prevent catecholamine secretion

Vesicular monoamine transport (VMAT) inhibitors, such as reserpine and tetrabenazine, impair vesicular catecholamine storage in chromaffin cells and sympathetic neurons, thereby lowering blood pressure. Here we describe a novel action of VMAT inhibitors-blockade of L-type voltage-gated calcium channels-that may also influence catecholamine release from both PC12 rat pheochromocytoma cells and bovine adrenal chromaffin cells. When given alone, VMAT inhibitors acutely release catecholamines from chromaffin cells in a dose-dependent fashion. However, VMAT inhibitors block catecholamine secretion stimulated by either nicotinic cholinergic agonists or cell membrane depolarization, each of which rely on the opening of L-type channels; the inhibition was more potent after long-term exposure to VMAT inhibitors (IC50 < 100 nmol/L). Reserpine blocked nicotinic-stimulated catecholamine release from neurite-bearing PC12 cells. Reserpine also antagonized catecholamine release triggered by combined membrane depolarization and the dihydropyridine L-type channel agonist Bay K8644, and reserpine blocked cellular uptake of extracellular 45Ca2+ in response to nicotine. Taken together, these results indicate that VMAT inhibitors are also antagonists at L-type voltage-gated calcium channels. Classic L-type channel antagonists (verapamil or nifedipine) also exhibited the reciprocal actions; acutely, they released norepinephrine from chromaffin cells, and chronically, they depleted cellular catecholamine stores, albeit with inferior molar potency to reserpine (IC50 < 1 nmol/L). We conclude that VMAT inhibitors and L-type calcium channel antagonists exert reciprocal inhibitory actions on each other's more classic pharmacological targets. Furthermore, these novel actions are seen at concentrations of these compounds frequently taken to be specific in vitro and likely to occur during antihypertensive treatment in vivo.