Cholinergic stimulation of inositol phosphate formation in bovine adrenal chromaffin cells: distinct nicotinic and muscarinic mechanisms

Quantitative determination of nitric oxide from tissue samples using liquid chromatography-Mass spectrometry

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We report a method to measure NO by reacting it with carboxy-PTIO to form carboxy-PTI.

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The carboxy-PTI is quantified by liquid chromatography – mass spectrometry (LCMS).

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This method can quantitate NO concentrations ranging from 5 nM to 1 μM.

Ever since it was found to mediate the endothelium-dependent dilation of blood vessels, nitric oxide (NO) has generated enormous research interest throughout the biological sciences. Over thirty years of research has identified NO as a ubiquitous and versatile regulatory factor utilized by both vertebrates and invertebrates. The short lifetime and low concentration of NO make quantitation difficult. Here we report a method for measuring NO using the selective reaction with 2-​(4-​carboxyphenyl)-​4,​5-​dihydro-​4,​4,​5,​5-​tetramethyl-1H-​imidazolyl-​1-​oxy-​3-​oxide (carboxy-PTIO) to form carboxy-PTI. We used tandem mass spectrometry to verify the validity of this reaction, and liquid chromatography – mass spectrometry to quantitate the amount of carboxy-PTI formed. Using diethylamine nonoate as a NO donor we demonstrate this method can quantitate NO concentrations with a detection limit of 5 nM. We successfully determined the amount of NO generated endogenously by frog heart/aorta when stimulated by carbachol, a non-selective acetylcholine receptor agonist. Based on these results, we suggest that this technique can be useful for the quantitative determination of NO in biological samples.•

We report a method to measure NO by reacting it with carboxy-PTIO to form carboxy-PTI.

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The carboxy-PTI is quantified by liquid chromatography mass spectrometry (LCMS).

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This method can quantitate NO concentrations ranging from 5 nM to 1 µM

Mechanisms and roles of muscarinic activation in guinea-pig adrenal medullary cells

Muscarinic receptors are expressed in the adrenal medullary (AM) cells of various mammals, but their physiological roles are controversial. Therefore, the ionic mechanism for muscarinic receptor-mediated depolarization and the role of muscarinic receptors in neuronal transmission were investigated in dissociated guinea-pig AM cells and in the perfused guinea-pig adrenal gland. Bath application of muscarine induced an inward current at -60 mV. This inward current was partially suppressed by quinine with an IC(50) of 6.1 μM. The quinine-insensitive component of muscarine-induced currents changed the polarity at -78 mV and was inhibited by bupivacaine, a TWIK-related acid-sensitive K(+) (TASK) channel inhibitor. Conversely, the current-voltage relationship for the bupivacaine-insensitive component of muscarine currents showed a reversal potential of -5 mV and a negative slope below -40 mV. External application of La(3+) had a double action on muscarine currents of both enhancement and suppression. Immunoblotting and immunocytochemistry revealed expression of TASK1 channels and cononical transient receptor potential channels 1, 4, 5, and 7 in guinea-pig AM cells. Retrograde application of atropine reversibly suppressed transsynaptically evoked catecholamine secretion from the adrenal gland. The results indicate that muscarinic receptor stimulation in guinea-pig AM cells induces depolarization through inhibition of TASK channels and activation of nonselective cation channels and that muscarinic receptors are involved in neuronal transmission from the splanchnic nerve.

Nicotinic acetylcholine receptor-mediated calcium signaling in the nervous system

Based on the composition of the five subunits forming functional neuronal nicotinic acetylcholine receptors (nAChRs), they are grouped into either heteromeric (comprising both alpha and beta subunits) or homomeric (comprising only alpha subunits) receptors. The nAChRs are known to be differentially permeable to calcium ions, with the alpha7 nAChR subtype having one of the highest permeabilities to calcium. Calcium influx through nAChRs, particularly through the alpha-bungarotoxin-sensitive alpha7-containing nAChRs, is a very efficient way to raise cytoplasmic calcium levels. The activation of nAChRs can mediate three types of cytoplasmic calcium signals: (1) direct calcium influx through the nAChRs, (2) indirect calcium influx through voltage-dependent calcium channels (VDCCs) which are activated by the nAChR-mediated depolarization, and (3) calcium-induced calcium release (CICR) (triggered by the first two sources) from the endoplasmic reticulum (ER) through the ryanodine receptors and inositol (1,4,5)-triphosphate receptors (IP(3)Rs). Downstream signaling events mediated by nAChR-mediated calcium responses can be grouped into instantaneous effects (such as neurotransmitter release, which can occur in milliseconds after nAChR activation), short-term effects (such as the recovery of nAChR desensitization through cellular signaling cascades), and long-term effects (such as neuroprotection via gene expression). In addition, nAChR activity can be regulated by cytoplasmic calcium levels, suggesting a complex reciprocal relationship. Further advances in imaging techniques, animal models, and more potent and subtype-selective ligands for neuronal nAChRs would help in understanding the neuronal nAChR-mediated calcium signaling, and lead to the development of improved therapeutic treatments.

Muscarinic excitation-secretion coupling in chromaffin cells

Excitation-secretion coupling in adrenomedullary chromaffin cells physiologically commences when acetylcholine molecules released from splanchnic nerve terminals bind to cholinergic receptors located at the cell's plasma membrane. While nicotinic acetylcholine receptors ensure a rapid and efficacious transmission of preganglionic impulses, muscarinic acetylcholine receptors are considered to play a subsidiary role mostly by facilitating the nicotinic responses. Nevertheless, the variety of effects brought about by muscarinic stimulation in chromaffin cells (release of intracellular Ca2+, activation of Ca2+ entry through non-selective cation channels and voltage-dependent Ca2+ channels, impairment and/or enhancement of action potential firing, etc.) and the long-lasting nature of many of them suggests that muscarinic receptors might contribute to the fine tuning of the catecholamine secretory response upon graded preganglionic stimulation and prolonged periods of time. Such a variety of effects probably reflects not only the diversity of muscarinic receptors expressed in chromaffin cells but also the existence of differences among the animal species employed in the reported investigations. Accordingly, we first review on an animal species-based approach the most relevant features of the muscarinic response in chromaffin cells from a set of mammals, and finally present a unified picture of the mechanisms of muscarinic excitation-secretion coupling in chromaffin cells.

Key role of the nicotinic receptor in neurotransmitter exocytosis in human chromaffin cells

The whole-cell secretory response evoked by acetylcholine (ACh) in human chromaffin cells was examined using a new protocol based on quickly switching from the voltage-clamp to the current-clamp (CC) configuration of the patch-clamp technique. Our experiments revealed that Ca(2+) entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4 +/- 27.3 fF) evoked by 200 ms pulses of ACh, as Ca(2+) flux through voltage-dependent Ca(2+) channels at depolarized membrane potentials. The nicotinic current triggered a depolarization event with a peak at +49.3 mV and a 'plateau' phase that ended at -23.9 mV, which was blocked by 10 mumol/L mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 +/- 3.6 pA, but the membrane potential depolarization still remained at the 'plateau' stage until withdrawal of the agonist. Perfusion with 200 mumol/L Cd(2+) during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse, indicating that Ca(2+) entry through Ca(2+) channels contributed to the membrane potential depolarization provoked by prolonged ACh pulses. These findings also reflect that voltage-dependent Ca(2+) channels were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization. Finally, muscarinic receptor activation triggered a delayed exocytotic process after prolonged ACh stimulation, dependent on Ca(2+) mobilization from the endoplasmic reticulum. In summary, we show here that nicotinic and muscarinic receptors contribute to the exocytosis of neurotransmitters in human chromaffin cells, and that the nicotinic receptor plays a key role in several stages of the stimulus-secretion coupling process in these cells.

Involvement of protein kinase C-epsilon in activity-dependent potentiation of large dense-core vesicle exocytosis in chromaffin cells

Neurotransmitter release is modulated in an activity-dependent manner. We showed previously that repetitive stimulation of nicotinic acetylcholine receptor (nAChR) induced activity-dependent potentiation (ADP) of large dense-core vesicle (LDCV) exocytosis in chromaffin cells. Here we report that protein kinase C (PKC)-epsilon is critically involved in ADP. Stimulation of nAChR induced activation of PKC-epsilon, and inhibition of PKC-epsilon by expression of the dominant-negative mutant of PKC-epsilon (DN-PKC-epsilon) or short interfering (siRNA) against PKC-epsilon abolished ADP via decreasing the frequency and quantal size of fused vesicles without affecting basal exocytosis, suggesting that PKC-epsilon is specifically involved in ADP. Electron microscopy revealed that inhibition of PKC-epsilon disrupts activity-induced vesicle translocation required for ADP. We also suggest the involvement of myristoylated alanine-rich C kinase substrate (MARCKS), which is known as a downstream target of PKC-epsilon, in ADP of LDCV exocytosis. The level of phospho-MARCKS correlated with the time course of ADP and was reduced by transfection with DN-PKC-epsilon. Actin filament disassembly induced by MARCKS phosphorylation was also significantly blocked by transfection of DN-PKC-epsilon. Furthermore, knockdown of MARCKS by siRNA resulted in inhibition of ADP and reduction of the number of fused vesicles. Together, we provide evidence that ADP of LDCV exocytosis is regulated by PKC-epsilon and its downstream target MARCKS via modulating vesicle translocation.

Nicotinic acetylcholine receptors and the regulation of neuronal signalling

Neuronal nicotinic acetylcholine (nACh) receptors in the brain are more commonly associated with modulatory events than mediation of synaptic transmission. nACh receptors have a high permeability for Ca(2+), and Ca(2+) signals are pivotal in shaping nACh receptor-mediated neuromodulatory effects. In this review, we consider the mechanisms through which nACh receptors convert rapid ionic signals into sustained, wide-ranging phenomena. The complex Ca(2+) responses that are generated after activation of nACh receptors can transmit information beyond the initial domain and facilitate the interface with many intracellular processes. These mechanisms underlie the diverse repertoire of neuronal activities of nicotine in the brain, from the enhancement of learning and memory, to addiction and neuroprotection.

Betagamma subunits mediate the NPY enhancement of ATP-stimulated inositol phosphate formation

Neuropeptide Y (NPY) enhances ATP-stimulated inositol phosphate (InsP) formation in bovine chromaffin cells through an unknown mechanism. Chromaffin cells were transduced with the carboxyl terminus of beta-adrenergic receptor kinase 1 (betaARK1CT), a Gbetagamma subunits scavenger, using a recombinant adenovirus system. The adenovirus also expresses a green fluorescent protein (GFP) which serves as an index of transduction. Flow cytometry showed that up to 80% of chromaffin cells were transduced by the virus. There was a direct correlation between the betaARK1CT inhibition of the NPY enhancement of ATP-stimulated InsP formation and the percent of cells expressing GFP ( r2=0.9993 ). These results demonstrate that Gbetagamma subunits are required for the NPY enhancement of ATP-stimulated InsP formation in bovine chromaffin cells.

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.

Phospholipase C-mediated signalling is not required for histamine-induced catecholamine secretion from bovine chromaffin cells

A possible role for signalling through phospholipase C in histamine-induced catecholamine secretion from bovine adrenal chromaffin cells has been investigated. Secretion evoked by histamine over 10 min was not prevented by inhibiting inositol-1,4,5-trisphosphate receptors with 2-APB, by blocking ryanodine receptors with a combination of ryanodine and caffeine, or by depleting intracellular Ca(2+) stores by pretreatment with thapsigargin. Inhibition of protein kinase C with Ro31-8220 also failed to reduce secretion. Inhibition of phospholipase C with ET-18-OCH(3) reduced both histamine- and K(+) -induced inositol phosphate responses by 70-80% without reducing their secretory responses. Stimulating phospholipase C with Pasteurella multocida toxin did not evoke secretion or enhance the secretory response to histamine. The secretory response to histamine was little affected by tetrodotoxin or by substituting extracellular Na(+) with N -methyl-d-glucamine(+) or choline(+), or by substituting external Cl(-) with nitrate(-). Blocking various K(+) channels with apamin, charybdotoxin, Ba(2+), tetraethylammonium, 4-aminopyridine, tertiapin or glibenclamide failed to reduce the ability of histamine to evoke secretion. These results indicate that histamine evokes secretion by a mechanism that does not require inositol-1,4,5-trisphosphate-mediated mobilization of stored Ca(2+), diacylglycerol-mediated activation of protein kinase C, or activation of phospholipase C. The results are consistent with histamine acting by depolarizing chromaffin cells through a phospholipase C-independent mechanism.

Visualization of IP(3) dynamics reveals a novel AMPA receptor-triggered IP(3) production pathway mediated by voltage-dependent Ca(2+) influx in Purkinje cells

IP(3) signaling in Purkinje cells is involved in the regulation of cell functions including LTD. We have used a GFP-tagged pleckstrin homology domain to visualize IP(3) dynamics in Purkinje cells. Surprisingly, IP(3) production was observed in response not only to mGluR activation, but also to AMPA receptor activation in Purkinje cells in culture. AMPA-induced IP(3) production was mediated by depolarization-induced Ca(2+) influx because it was mimicked by depolarization and was blocked by inhibition of the P-type Ca(2+) channel. Furthermore, trains of complex spikes, elicited by climbing fiber stimulation (1 Hz), induced IP(3) production in Purkinje cells in cerebellar slices. These results revealed a novel IP(3) signaling pathway in Purkinje cells that can be elicited by synaptic inputs from climbing fibers.

Ca(2+)-dependent K(+) current and exocytosis in responses to caffeine and muscarine in voltage-clamped guinea-pig adrenal chromaffin cells

We characterized changes in membrane currents and the cytosolic Ca(2+) concentration, [Ca(2+)](i), in response to caffeine, and compared them with those in response to muscarine using the perforated patch-clamp technique and fura-2 microfluorimetry in guinea-pig adrenal chromaffin cells. Catecholamine release from single voltage-clamped cells was monitored with amperometry using carbon microelectrodes. Caffeine produced a transient outward current (I(out)) at holding potentials over - 60 mV, increasing in amplitude with increasing the potentials. It also evoked a rapid increase of [Ca(2+)](i) at all potentials examined. The current-voltage relation revealed that the activation of K(+) channels was responsible for the I(out) evoked by caffeine. Both current and [Ca(2+)](i) responses were reversibly abolished by cyclopiazonic acid, an inhibitor of Ca(2+)-pump ATPase. At - 30 mV, the caffeine-induced I(out), but not [Ca(2+)](i), was partly inhibited by either charybdotoxin or apamin. In the majority of cells tested, caffeine induced a larger I(out) but a smaller [Ca(2+)](i) increase than muscarine. Caffeine and muscarine increased catecholamine release from voltage-clamped single cells concomitant with the transient increase of [Ca(2+)](i), and there was a positive correlation between them. These results indicate that caffeine activates Ca(2+)-dependent K(+) channels and catecholamine secretion due to the release of Ca(2+) from internal stores in voltage-clamped adrenal chromaffin cells of the guinea-pig. There seems to be a spatial difference between [Ca(2+)](i) increased by Ca(2+) release from caffeine-sensitive stores and that released from muscarine (inositol 1,4,5-trisphosphate)-sensitive ones.

Interaction of SK(Ca) channels and L-type Ca(2+) channels in catecholamine secretion in the rat adrenal gland

We elucidated the interaction of small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels and L-type Ca(2+) channels in muscarinic receptor-mediated control of catecholamine secretion in the isolated perfused rat adrenal gland. The muscarinic agonist methacholine (10-300 microM) produced concentration-dependent increases in adrenal output of epinephrine and norepinephrine. The SK(Ca) channel blocker apamin (1 microM) enhanced the methacholine-induced catecholamine responses. The facilitatory effect of apamin on the methacholine-induced catecholamine responses was not observed during treatment with the L-type Ca(2+) channel blocker nifedipine (3 microM) or Ca(2+)-free solution. Nifedipine did not affect the methacholine-induced catecholamine responses, but it inhibited the responses during treatment with apamin. The L-type Ca(2+) channel activator Bay k 8644 (1 microM) enhanced the methacholine-induced catecholamine responses, whereas the enhancement of the methacholine-induced epinephrine and norepinephrine responses were prevented and attenuated by apamin, respectively. These results suggest that SK(Ca) channels are activated by muscarinic receptor stimulation, which inhibits the opening of L-type Ca(2+) channels and thereby attenuates adrenal catecholamine secretion.

Latrotoxin stimulates secretion in permeabilized cells by regulating an intracellular Ca2+ - and ATP-dependent event: a role for protein kinase C

alpha-Latrotoxin, a component of black widow spider venom, stimulates transmitter release from nerve terminals and intact chromaffin cells and enhances secretion from permeabilized chromaffin cells already maximally stimulated by Ca(2+). In this study we demonstrate that chromaffin cells contain a protein antigenically similar to the cloned Ca(2+)-independent receptor for alpha-latrotoxin. Although this receptor has homology to the secretin family of G-protein-linked receptors, pertussis toxin has no effect on the ability of alpha-latrotoxin to enhance secretion, suggesting that neither G(i) nor G(o) is involved in the response. Furthermore, in the absence of Ca(2+), alpha-latrotoxin does not stimulate polyphosphoinositide-specific phospholipase C. alpha-Latrotoxin specifically enhances ATP-dependent secretion in permeabilized cells. An in situ assay for protein kinase C reveals that alpha-latrotoxin augments the activation of protein kinase C by Ca(2+), and use of protein kinase inhibitors demonstrates that this activation is important for the toxin's enhancing effect. This enhancement of secretion requires Ca(2+) concentrations above 3 microm and is not supported by Ba(2+) or nonhydrolyzable guanine nucleotides, which do not stimulate protein kinase C. We conclude that alpha-latrotoxin stimulates secretion in permeabilized cells by regulating a Ca(2+)- and ATP-dependent event involving protein kinase C.

Role of potassium channels in catecholamine secretion in the rat adrenal gland

We elucidated the functional contribution of K(+) channels to cholinergic control of catecholamine secretion in the perfused rat adrenal gland. The small-conductance Ca(2+)-activated K(+) (SK(Ca))-channel blocker apamin (10-100 nM) enhanced the transmural electrical stimulation (ES; 1-10 Hz)- and 1, 1-dimethyl-4-phenyl-piperazinium (DMPP; 5-40 microM)-induced increases in norepinephrine (NE) output, whereas it did not affect the epinephrine (Epi) responses. Apamin enhanced the catecholamine responses induced by acetylcholine (6-200 microM) and methacholine (10-300 microM). The putative large-conductance Ca(2+)-activated K(+) channel blocker charybdotoxin (10-100 nM) enhanced the catecholamine responses induced by ES, but not the responses induced by cholinergic agonists. Neither the K(A) channel blocker mast cell degranulating peptide (100-1000 nM) nor the K(V) channel blocker margatoxin (10-100 nM) affected the catecholamine responses. These results suggest that SK(Ca) channels play an inhibitory role in adrenal catecholamine secretion mediated by muscarinic receptors and also in the nicotinic receptor-mediated secretion of NE, but not of Epi. Charybdotoxin-sensitive Ca(2+)-activated K(+) channels may control the secretion at the presynaptic site.

A pleckstrin homology domain specific for phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P2) and fused to green fluorescent protein identifies plasma membrane PtdIns-4,5-P2 as being important in exocytosis

Kinetically distinct steps can be distinguished in the secretory response from neuroendocrine cells with slow ATP-dependent priming steps preceding the triggering of exocytosis by Ca(2+). One of these priming steps involves the maintenance of phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P(2)) through lipid kinases and is responsible for at least 70% of the ATP-dependent secretion observed in digitonin-permeabilized chromaffin cells. PtdIns-4,5-P(2) is usually thought to reside on the plasma membrane. However, because phosphatidylinositol 4-kinase is an integral chromaffin granule membrane protein, PtdIns-4,5-P(2) important in exocytosis may reside on the chromaffin granule membrane. In the present study we have investigated the localization of PtdIns-4,5-P(2) that is involved in exocytosis by transiently expressing in chromaffin cells a pleckstrin homology (PH) domain that specifically binds PtdIns-4, 5-P(2) and is fused to green fluorescent protein (GFP). The PH-GFP protein predominantly associated with the plasma membrane in chromaffin cells without any detectable association with chromaffin granules. Rhodamine-neomycin, which also binds to PtdIns-4,5-P(2), showed a similar subcellular localization. The transiently expressed PH-GFP inhibited exocytosis as measured by both biochemical and electrophysiological techniques. The results indicate that the inhibition was at a step after Ca(2+) entry and suggest that plasma membrane PtdIns-4,5-P(2) is important for exocytosis. Expression of PH-GFP also reduced calcium currents, raising the possibility that PtdIns-4,5-P(2) in some manner alters calcium channel function in chromaffin cells.

Alpha-latrotoxin and its receptors CIRL (latrophilin) and neurexin 1 alpha mediate effects on secretion through multiple mechanisms

Alpha-Latrotoxin and its plasma membrane receptors cause a number of distinct effects in secretory cells. First, by tethering alpha-latrotoxin to the plasma membrane, CIRL/latrophilin and neurexin 1 alpha facilitate alpha-latrotoxin-induced channel formation. The stimulation of secretion by alpha-latrotoxin in neuroendocrine cells is a consequence of Ca(2+) influx through these alpha-latrotoxin-induced channels. In addition to channel formation, alpha-latrotoxin enhances secretion in permeabilized cells through interaction with the plasma membrane receptor CIRL/latrophilin. Finally, overexpression of CIRL/latrophilin inhibits Ca(2+)-dependent secretion in permeabilized chromaffin cells in the absence of alpha-latrotoxin. This effect represents a 'constitutive' action of the G-protein coupled receptor to specifically inhibit an ATP-dependent priming step in the secretory pathway. The effect suggests that the receptor may have an important modulatory role in synaptic transmission.

Cotinine and nicotine inhibit each other's calcium responses in bovine chromaffin cells

Cotinine is the major metabolite of nicotine. It has some biological activity, but its pathophysiological effects are largely unclear. We studied whether cotinine initiates calcium transients or affects those induced by nicotine. In bovine adrenal chromaffin cells labeled with the fluorescent calcium indicator Fura 2, cotinine (0. 32-3.2 mM) concentration-dependently increased the intracellular Ca(2+) concentration ([Ca(2+)](i)). The effect was abolished by omitting extracellular Ca(2+) during the stimulations. Also nicotinic receptor channel blockers hexamethonium (10 microM-1 mM) and chlorisondamine (100 microM), as well as a competitive nicotinic receptor antagonist dihydro-beta-erythroidine (10-100 microM), inhibited the response. Cotinine (0.32-3.2 mM) preincubation for 2 min inhibited both the nicotine-induced and the cotinine-induced increases in [Ca(2+)](i). Also nicotine (3.2-10 microM) inhibited the cotinine-induced increase in [Ca(2+)](i). Tetrodotoxin (1 microM) and thapsigargin (1 microM) pretreatments did not affect the responses to cotinine, while 300 nM nimodipine partially inhibited the cotinine-induced increase in [Ca(2+)](i). The results indicate that cotinine has nicotine-like effects on chromaffin cells. It may also desensitize the nicotinic cholinergic receptors, possibly by acting as a low-affinity agonist at these receptors.

Neuronal control of catecholamine secretion from chromaffin cells in the rainbow trout (Oncorhynchus mykiss)

The goal of the present investigation was to assess the relative involvement of nicotinic and muscarinic cholinergic receptors in the neuronal control of catecholamine secretion from the chromaffin tissue of rainbow trout (Oncorhynchus mykiss). This was accomplished by first developing and validating a nerve-stimulating technique able specifically to activate the nerve fibres innervating the chromaffin cells in order to elicit secretion of catecholamines. Using an in situ saline-perfused posterior cardinal vein preparation, it was demonstrated that whole-body field stimulation caused specific voltage-dependent neuronal stimulation of adrenaline and noradrenaline secretion. The contribution of non-specific depolarization was negligible. Several experimental results confirmed the specificity of the field stimulation technique. First, pre-treatment with neostigmine (an anticholinesterase) prolonged and more than doubled the amount of adrenaline secreted in response to electrical stimulation. Second, pre-treatment with the nicotinic receptor antagonist hexamethonium inhibited the electrically evoked secretion of adrenaline and noradrenaline. Third, perfusion with Na+-free saline or removal of the spinal cord abolished secretion of both catecholamines in response to the electrical stimulus. By using the field stimulation technique, this study is the first to demonstrate conclusively a role for muscarinic receptors in catecholamine secretion from trout chromaffin cells. Specifically, muscarinic cholinergic stimulation enhances nicotinic-evoked secretion of catecholamines and, under intense stimulation, may directly cause secretion. The results of the present study suggest the presence of muscarinic receptors on rainbow trout chromaffin cells with a functional role in the cholinergic control of catecholamine secretion.

Regulation of inositol lipid-specific phospholipase cdelta by changes in Ca2+ ion concentrations

Studies of inositol lipid-specific phospholipase C (PLC) have elucidated the main regulatory pathways for PLCbeta and PLCgamma but the regulation of PLCdelta isoenzymes still remains obscure. Here we demonstrate that an increase in Ca2+ ion concentration within the physiological range (0.1-10 microM) is sufficient to stimulate PLCdelta1, but not PLCgamma1 and PLCbeta1, to hydrolyse cellular inositol lipids present in permeabilized cells. The activity of PLCdelta1 is further enhanced in the presence of phosphatidylinositol transfer protein (PI-TP). Both full activation by Ca2+ ions and stimulation in the presence of PI-TP require an intact PH domain involved in the membrane attachment of PLCdelta1. The physiological implication of this study is that PLCdelta1 could correspond to a previously uncharacterized PLC responsible for Ca2+ ion-stimulated inositol lipid hydrolysis observed in many cellular systems.

Deprivation of Na+, Ca2+ and Mg2+ from the extracellular solution increases cytosolic Ca2+ and stimulates catecholamine secretion from cultured bovine adrenal chromaffin cells

We report here that exposing cultured chromaffin cells to a low ionic strength medium (with sucrose in place of NaCl to maintain osmolarity) can induce a marked elevation in cytosolic Ca2+ concentration ([Ca2+]i) and catecholamine (CA) release. To determine the underlying mechanism, we first studied the effects of low [Na+]o on single cell [Ca2+]i (using fluo-3 as Ca2+ indicator) and CA release from many cells. In a Mg2+ and Ca2+-deficient medium, lowering the external concentration of Na2+ ([Na+]o) evoked CA secretion preceded by a transitory [Ca2+]i rise, the amplitude of which was inversely related to [Na+]o. By contrast, in the presence of either [Ca2+]o (2 mM) and [Mg2+]o (1.4 mM) or [Mg2+]o alone (3.4 mM), lowering the ionic strength was without effect. Furthermore, in a physiologic [Na+]o, [Ca2+]o and [Mg2+]o medium, two or three consecutive applications of the cholinergic agonist oxotremorine-M (oxo-M) consistently evoked a substantial [Ca2+]i, rise. By contrast, consecutive applications of oxo-M in a Ca2+-deficient medium failed to evoke a rise in [Ca2+]i after the first exposure to the agonist. To clarify the underlying mechanism, we measured and compared the effects of low [Na+]o and the cholinergic agonists nicotine and oxo-M on changes in [Ca2+]i; we studied the effects of these agonists on both membrane potential, Vm (under current clamp conditions), and [Ca2+]i by single cell microfluorimetry (indo-1 as Ca2+ indicator). We observed that, in the presence of [Ca2+]o and [Mg2+]o, lowering [Na+]o had no effect on Vm. In a Ca2+-deficient medium, lowering [Na+]o depolarized the membrane from ca. -60 to -10 mV. As expected, we found that nicotine (10 microM) depolarized the membrane (from ca. -60 to -20 mV) and simultaneously evoked a substantial [Ca2+]i rise that was [Ca2+]o-dependent. However, contrary to our expectations, we found that the muscarinic agonist oxo-M (50 microM) also depolarized the membrane and induced an elevation in [Ca2+]i. Furthermore, both signals were blocked by D-tubocurarine, insinuating the nicotinic character of oxo-M in adrenal chromaffin cells from bovine. These results suggest that both nicotine and oxo-M stimulate Ca2+ entry, probably through voltage-gated Ca2+-channels. We also show here that oxo-M (and not low [Na+]o) stimulates phosphoinositide turnover.

Interaction between G protein-operated receptors eliciting secretion in rat adrenals. A possible role of protein kinase C

Catecholamine release induced by angiotensin II, histamine, bradykinin and methacholine from the rat adrenal gland perfused in vitro was studied under conditions in which the activity of protein kinase C (PKC) was modified. Perfusion of glands with 10 nM bradykinin abolished, in a reversible way, the secretion induced by short pulses of angiotensin II, histamine and methacholine but did not modify the release evoked by 23.6 mM KCl (high K+). Perfusion with histamine or methacholine (30 microM) inhibited the secretion induced by the other agents by 30-50%, whereas incubation with angiotensin II (100 nM) caused little or no reduction in the release evoked by the other agents. The treatment of glands with 1 nM of the PKC activator phorbol 12,13-dibutyrate (PDBu) suppressed the responses induced by angiotensin II, histamine and methacholine, did not affect those evoked by bradykinin, and potentiated the secretion evoked by high K+. The adenylate cyclase stimulator forskolin (1 microM) did not affect the basal secretion but strongly potentiated the release evoked by all secretagogues used, suggesting a role for protein kinase A (PKA) downstream of the receptor. The PKC inhibitor Ro-31-8220 partially reversed the inhibitory effect of bradykinin. Our results suggest that angiotensin II, histamine and muscarinic receptors share some common transduction mechanism that is regulated by PKC. PKC activity was enhanced by these agents PDBu >> bradykinin = histamine > methacholine = angiotensin II. Bradykinin receptor transduction does not appear to be regulated by PKC.

Muscarinic and nicotinic receptors raise intracellular Ca2+ levels in rat carotid body type I cells

1. The effects of cholinergic agonists upon intracellular free Ca2+ levels ([Ca2+]i) have been studied in enzymically isolated rat carotid body single type I cells, using indo-1. 2. Acetylcholine (ACh) dose-dependently increased [Ca2+]i in 55% of cells studied (EC50 = 13 microM). These [Ca2+]i rises were partially inhibited by atropine or mecamylamine. 3. Specific nicotinic and muscarinic agonists also elevated [Ca2+]i in a dose-dependent manner (nicotine, EC50 = 15 microM; methacholine, EC50 = 20 microM). 4. While the majority of the ACh-sensitive cells responded to both classes of cholinergic agonist, 29% responded exclusively to nicotinic stimulation and 9% responded exclusively to muscarinic stimulation. 5. In the presence of nicotinic agonists, Ca2+i responses were transient. In the presence of muscarinic agonists, Ca2+i responses consisted of an initial rise, which then declined to a lower plateau level. 6. Nicotinic responses were rapidly abolished in Ca(2+)-free medium, suggesting that they are dependent on Ca2+ influx. 7. The plateau component of the muscarinic-activated response was also abolished in Ca(2+)-free conditions. The rapid initial [Ca2+]i rise, however, could still be evoked after several minutes in Ca(2+)-free medium. Muscarine also increased Mn2+ quenching of intracellular fura-2 fluorescence. These data suggest that the full muscarinic response depends on both Ca2+ release from intracellular stores and Ca2+o influx. 8. The results indicate that, in rat carotid body type I cells, both nicotinic and muscarinic acetylcholine receptors increase [Ca2+]i, but achieve this via different mechanisms. ACh may therefore play a role in carotid body function by modulating Ca2+i in the chemosensory type I cells.

Characteristics of cytosolic Ca2+ elevation induced by muscarinic receptor activation in single adrenal chromaffin cells of the guinea pig

In Fura-2 loaded-single guinea pig adrenal chromaffin cells, muscarine, nicotine and KCl all caused an early peak rise in intracellular Ca concentration ([Ca2+]i) followed by a sustained rise. In Ca(2+)-free solution, muscarine, but neither nicotine nor KCl, caused a transient increase in [Ca2+]i, which was partially reduced by preceding application of caffeine or by treatment with ryanodine plus caffeine. In voltage-clamped cells at a holding potential of -60 mV, the muscarine-induced [Ca2+]i rise, especially its sustained phase, decreased in magnitude. Intracellular application of inositol 1,4,5-trisphosphate caused a transient increase in [Ca2+]i and inhibited the following [Ca2+]i response to muscarine without affecting responses to nicotine and a depolarizing pulse. Muscarine evoked membrane depolarization following brief hyperpolarization in most cells tested. There was a significant positive correlation between the amplitude of the depolarization and the magnitude of the sustained rise in [Ca2+]i. Muscarine-induced sustained [Ca2+]i rise was much greater in the current-clamp mode than that in the voltage-clamp mode. The sustained phase of [Ca2+]i rise and Mn2+ influx in response to muscarine were suppressed by a voltage-dependent Ca2+ channel blocker, methoxyverapamil. These results suggest that stimulation of muscarinic receptors causes not only extracellular Ca2+ entry, but also Ca2+ mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Voltage-dependent Ca(2+)-channels may function as one of the Ca2+ entry pathways activated by muscarinic receptor in guinea pig adrenal chromaffin cells.

Calcium-activated potassium channels in adrenal chromaffin cells

Rat chromaffin cells express an interesting diversity of Ca(2+)-dependent K+ channels, including a voltage-independent, small-conductance, apamin-sensitive SK channel and two variants of voltage-dependent, large-conductance BK channels. The two BK channel variants are differentially segregated among chromaffin cells, such that BK current is completely inactivating in about 75-80% of rat chromaffin cells, while the remainder express a mix of inactivating and non-inactivating current or mostly non-inactivating BKs current. The single-channel conductance of BKi channels is identical to that of BKs channels. Although rates of current activation are similar in the two variants, the deactivation kinetics of the two channels also differ. Furthermore, BKi channels are somewhat less sensitive to scorpion toxins than BKs channels. The slow component of BKi channel deactivation may be an important determinant of the functional role of these channels. During blockade of SK current, cells with BKi current fire tonically during sustained depolarizing current injection, whereas cells with BKs current tend to fire only a few action potentials before becoming quiescent. The ability to repetitively fire requires functional BKi channels, since partial blockade of BKi channels by CTX makes a BKi cell behave much like a BKs cell. In contrast, the physiological significance of BKi inactivation may arise from the ability of secretagogue-induced [Ca2+]i elevations to regulate the availability of BKi channels during subsequent action potentials (Herrington et al., 1995). By reducing the number of BK channels available for repolarization, the time course of action potentials may be prolonged. This possibility remains to be tested directly. These results raise a number of interesting questions pertinent to the control of secretion in rat adrenal chromaffin cells. An interesting hypothesis is that cells with a particular kind of BK current may reflect particular subpopulations of chromaffin cells. These subpopulations might differ either in the nature of the material secreted from the cell (e.g., Douglass and Poisner, 1965) or in the responsiveness to particular secretagogues. The differences in electrical behavior between cells with BKi and BKs current suggest that the pattern of secretion that might be elicited by a single type of stimulus could differ. For BKi cells, secretion may occur in a tonic fashion during sustained depolarization, while secretion from cells with BKs current may be more phasic. In the absence of specific structural information about the domains responsible for inactivation of BKi channels, our understanding of the mechanism of inactivation remains indirect. BKi inactivation shares many features with N-terminal inactivation of voltage-dependent K+ channels. However, there are provocative differences between the two types of inactivation which require us to propose that the native inactivation domain of BKi channels may occlude access of permeant ions to the BK channel permeation pathway in a position at some distance from the actual mouth of the channel. Further understanding of the structural and mechanistic basis of inactivation of BKi channels promises to provide new insights into both the cytoplasmic topology of BK channels and the Ca(2+)- and voltage-dependent steps involved in channel activation.

Activation of phospholipase C in SH-SY5Y neuroblastoma cells by potassium-induced calcium entry

1. We used SH-SY5Y human neuroblastoma cells to investigate whether depolarization with high K+ could stimulate inositol (1,4,5)trisphosphate (Ins(1,4,5)P3) formation and, if so, the mechanism involved. 2. Ins(1,4,5)P3 was measured by a specific radioreceptor mass assay, whilst [Ca2+]i was measured fluorimetrically with the Ca2+ indicator dye, Fura-2. 3. Depolarization with K+ caused a time- and dose-dependent increase in [Ca2+]i (peak at 27 s, EC50 of 50.0 +/- 9.0 mM) and Ins(1,4,5)P3 formation (peak at 30 s, EC50 of 47.4 +/- 1.1 mM). 4. Both the K(+)-induced Ins(1,4,5)P3 formation and increase in [Ca2+]i were inhibited dose-dependently by the L-type voltage-sensitive Ca2+ channel closer, (R+)-BayK8644, with IC50 values of 53.4 nM and 87.9 nM respectively. 5. These data show a close temporal and dose-response relationship between Ca2+ entry via L-type voltage-sensitive Ca2+ channels and Ins(1,4,5)P3 formation following depolarization with K+, indicating that Ca2+ influx can activate phospholipase C in SH-SY5Y cells.

Veratridine-induced oscillations of cytosolic calcium and membrane potential in bovine chromaffin cells

1. Veratridine (VTD) induced large oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential (Vm) in otherwise silent bovine chromaffin cells loaded with fura-2. 2. Depletion of the intracellular Ca2+ stores by thapsigargin or ryanodine did not affect these oscillations. Caffeine had a complex effect, decreasing them in cells with high activity but increasing them in cells with low activity. 3. The [Ca2+]i oscillations required extracellular Ca2+ and Na+ and were blocked by Ni2+ or tetrodotoxin. They were antagonized by high external concentrations of Mg2+ and/or Ca2+. 4. The oscillations of Vm had three phases: (i) slow depolarization (20 mV in 10-40 s); (ii) further fast depolarization (30 mV in 1 s); and (iii) rapid (5 s) repolarization. [Ca2+]i decreased during (i), increased quickly during (ii) with a 1 s delay with regard to the peak depolarization, and decreased during (iii). 5. Slight depolarizations increased the frequency of the oscillations whereas large depolarizations decreased it. 6. The Ca(2+)-dependent K+ channel blocker apamin increased the duration and decreased the frequency of the oscillations. 7. We propose the following mechanism for the oscillations: (i) the membrane depolarizes slowly by a decrease of potassium conductance (gK), perhaps due to a gradual decrease of [Ca2+]i; (ii) the threshold for activation of Na+ channels (decreased by VTD) is reached, producing further depolarization and recruiting Ca2+ channels, and inactivation of both Ca2+ and VTD-poisoned Na+ channels is slow; and (iii) gK increases, aided by activation of Ca(2+)-dependent K+ channels by the increased [Ca2+]i, and the membrane repolarizes. The contribution of the Na+ channels seems essential for the generation of the oscillations. 8. Bovine chromaffin cells have the machinery required for [Ca2+]i oscillations even though the more physiological stimulus tested here (high K+, field electrical stimulation, nicotinic or muscarinic agonists) produced mainly non-oscillatory responses.

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.

The sigma compounds 1,3-di-o-tolylguanidine and N-allylnormetazocine inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells

Muscarine stimulated a concentration-dependent accumulation of [3H]inositol phosphates in bovine adrenal medullary cells preloaded with [3H]inositol. This muscarinic activation of inositol phospholipid metabolism was fully inhibited by the sigma-ligand 1,3-di-o-tolylguanidine (DTG) with an IC50 of approximately 45 microM. Higher concentrations (100 microM) of (+) N-allylnormetazocine (SKF-10047) also partially inhibited this response. A concentration of DTG sufficient to fully inhibit the muscarinic response also produced a significant partial inhibition of [3H]inositol phosphate accumulation in response to histamine but not to angiotensin II. These data demonstrate that sigma-compounds inhibit agonist-stimulated inositol phospholipid metabolism in bovine adrenal medullary cells, with a degree of selectivity towards the muscarinic response.

An ibotenate-selective metabotropic glutamate receptor mediates protein phosphorylation in cultured hippocampal pyramidal neurons

Previous results showed that within 30 s after glutamate stimulation of cultured rat hippocampal pyramidal neurons there occurred an elevation of Ca2+ and diacylglycerol, and the phosphorylation of three acidic protein kinase C substrates, i.e., an 87-kDa protein known as myristoylated alanine-rich C kinase substrate and a 120- and a 48-kDa protein. In addition, it was suggested that a metabotropic-type glutamate receptor might be responsible for the phosphorylation observed. This work examines the ability of metabotropic and inotropic glutamate receptor agonists to quickly activate phospholipases in 1.26 mM versus 50 nM extracellular Ca2+ by measuring the generation of inositol phosphates. NMDA, quisqualate, and trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid did not stimulate the generation of inositol phosphates in the presence of normal or low extracellular Ca2+ in pyramidal neurons. Kainate stimulated the production of inositol phosphates in the presence of 1.26 mM extracellular Ca2+ but not in 50 nM extracellular Ca2+. Other than glutamate, only ibotenate was able to stimulate the generation of inositol phosphatases in both normal and low extracellular Ca2+. The maximal response to ibotenate was approximately equal to that of glutamate, when pyramidal neurons were stimulated in 50 nM extracellular Ca2+. The generation of inositol phosphates by glutamate and ibotenate could be partially blocked (50-60% reduction) by pretreatment of neurons with pertussis toxin (250 ng/ml), suggesting that a GTP-binding protein might be involved. In addition, ibotenate stimulated the immediate phosphorylation of the same three protein kinase C substrates as glutamate. The NMDA receptor blocker MK-801 had no effect on this phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin- and sarafotoxin-induced phosphoinositide hydrolysis in cultured canine tracheal smooth muscle cells

The endothelins (ETs) and sarafotoxin are two structurally related classes of potently contractile peptides. To understand the mechanism of action of ETs, we have examined the effect of ETs and sarafotoxin on phosphoinositide (PI) hydrolysis in cultured canine tracheal smooth muscle cells (TSMCs). ET-1, ET-2, ET-3, and sarafotoxin caused dose-dependent accumulation of inositol phosphatase (IPs) and tracheal smooth muscle contraction. BQ-123, an ETA receptor antagonist, had a high affinity to block the ET-1-induced IP accumulation and tracheal smooth muscle contraction with pKB values of 7.3 and 7.4, respectively. Pretreatment of TSMCs with cholera toxin impaired the ability of ET-1 and ET-2 to stimulate IP formation, whereas there was no effect by treatment with pertussis toxin. Stimulation of PI turnover by these peptides required the presence of extracellular Ca2+ and was blocked by treatment with EGTA. The addition of Ca2+ (3-620 nM) to digitonin-permeabilized TSMCs directly stimulated IP accumulation. A further Ca(2+)-dependent increase in IP formation was obtained by inclusion of either GTPrS or ET-1. The combined presence of GTPrS and ET-1 elicited an additive effect on IP formation. Short-term exposure to phorbol 12-myristate 13-acetate (PMA, 1 microM) abolished the stimulation of PI hydrolysis induced by these peptides. The inhibitory effect of PMA on ET-induced response was reversed by staurosporine, a protein kinase C (PKC) inhibitor, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Prolonged incubation of TSMCs with PMA resulted in a recovery of receptor responsiveness that may be due to downregulation of PKC.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotine-induced intracellular calcium changes are not antagonized by alpha-bungarotoxin in adrenal medullary cells

The snake toxin alpha-bungarotoxin distinguishes between neuronal nicotinic receptor subtypes. In chick ciliary ganglion neurons, activation of alpha-bungarotoxin-sensitive nicotinic receptors has been proposed to produce elevations in intracellular calcium levels. In the present study we show that prolonged treatment with alpha-bungarotoxin did not affect the nicotine-evoked calcium response in suspended chromaffin cells. On the other hand, the classical nicotinic receptor blocker d-tubocurarine potently blocked nicotinic receptor mediated effects. The degree of inhibition of the nicotinic response observed with d-tubocurarine was not modified by prior treatment with alpha-bungarotoxin. These results suggest that nicotinic alpha-bungarotoxin receptors are not primarily involved in nicotine-mediated increases in intracellular calcium in bovine adrenal medullary cells.

5-Hydroxytryptamine receptor-mediated phosphoinositide hydrolysis in canine cultured tracheal smooth muscle cells

1. 5-Hydroxytryptamine (5-HT) has been shown to induce contraction of tracheal smooth muscle. However, the mechanisms of action of 5-HT are not known. We therefore investigated the effects of 5-HT on phospholipase C (PLC)-mediated phosphoinositide (PI) hydrolysis and its regulation in canine cultured tracheal smooth muscle cells (TSMCs) labelled with [3H]-inositol. 5-HT-induced inositol phosphates (IPs) accumulation was time- and dose-dependent with a half-maximal response (EC50) and a maximal response at 0.38 +/- 0.05 and 10 microM, respectively. 2. Ketanserin and mianserin (10 and 100 nM), 5-HT2 receptor antagonists, were equipotent in blocking the 5-HT-induced IPs accumulation with pKB values of 8.46 and 8.21, respectively. In contrast, the dose-response curves of 5-HT-induced IPs accumulation were not shifted until the concentrations of NAN-190 and metoclopramide (5-HT1A and 5-HT3 receptor antagonists, respectively) were increased up to 10 microM. 3. Pretreatment of TSMCs with pertussis toxin or cholera toxin did not inhibit the 5-HT-induced IPs accumulation, but partially inhibited the AlF(4-)-induced IPs response. 4. Stimulation of IPs accumulation by 5-HT required the presence of external Ca2+ and was blocked by EGTA. The addition of Ca2+ (3-620 nM) to digitonin-permeabilized TSMCs directly stimulated IPs accumulation. A further Ca(2+)-dependent increase in IPs accumulation was obtained by inclusion of either guanosine 5'-O-(3-thiotriphoshate) (GTP gamma S) or 5-HT. The combination of GTP gamma S and 5-HT elicited an additive effect on IPs accumulation. 5. Treatment with phorbol 12-myristate 13-acetate (PMA, 1 microM, 30 min) abolished the 5-HT-induced IPs accumulation. The concentrations of PMA that gave a half-maximal and maximal inhibition of 5-HT-induced IPs accumulation were 2.2 +/- 0.4 nM and 1 microM, n = 3, respectively. The protein kinase C (PKC) activator, 4 alpha-phorbol 12,13-didecanoate, at 1 microM, did not influence this response. The inhibitory effect of PMA was reversed by staurosporine, a PKC inhibitor, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. 6. The site of this inhibition was further investigated by examining the effect of PMA on AlF(4-)-induced IPs accumulation in canine TSMCs. AlF(4-)-stimulated IPs accumulation was inhibited by PMA treatment, suggesting that the effect of PMA is distal to the 5-HT receptor. 7. Acetylcholine-induced IPs accumulation was completely inhibited by atropine, but not affected by ketanserin or mianserin, suggesting that 5-HT-induced IPs accumulation is not due to release of acetylcholine.8. These results demonstrate that 5-HT directly stimulates PLC-mediated PI hydrolysis via a pertussis toxin- and cholera toxin-insensitive GTP binding protein in canine TSMCs and that this coupling process is negatively regulated by PKC. 5-HT2 receptors may be predominantly mediating IPs accumulation and presumably IP-induced Ca2+ release may function as the transducing mechanism for 5-HT stimulated contraction of tracheal smooth muscle.

Bradykinin-stimulated phosphoinositide metabolism in cultured canine tracheal smooth muscle cells

1. Stimulation of bradykinin (BK) receptors coupled to phosphoinositide (PI) hydrolysis was investigated in canine cultured tracheal smooth muscle cells (TSMCs). BK, kallidin, and des-Arg9-BK, stimulated [3H]-inositol phosphates (IPs) accumulation in a dose-dependent manner with half-maximal responses (EC50) at 20 +/- 5, 13 +/- 4, and 2.3 +/- 0.7 nM, (n = 5), respectively. 2. D-Arg[Hyp3, D-Phe7]-BK and D-Arg[Hyp3, Thi5,8, D-Phe7]-BK, B2 receptor antagonists, were equipotent in blocking the BK-induced IPs accumulation with pKB = 7.1 and 7.3, respectively. 3. Short-term exposure of TSMCs to phorbol 12-myristate 13-acetate (PMA, 1 microM) attenuated BK-stimulated IPs accumulation. The concentrations of PMA that gave half-maximal and maximal inhibition of BK-induced IPs accumulation were 15 +/- 4 nM and 1 microM, n = 3, respectively. The inhibitory effect of PMA on BK-induced response was reversed by staurosporine, a protein kinase C (PKC) inhibitor, suggesting that the inhibitory effect of PMA was mediated through the activation of PKC. 4. Prolonged incubation of TSMCs with PMA for 24 h, resulted in a recovery of receptor responsiveness which may be due to down-regulation of PKC. The inactive phorbol ester, 4 alpha-phorbol 12, 13-didecanoate at 1 microM, did not inhibit this response. 5. The site of this inhibition was further investigated by examining the effect of PMA on AlF(4-)-induced IPs accumulation in canine TSMCs. AlF(4-)-stimulated IPs accumulation was inhibited by PMA treatment, suggesting that the G protein(s) can be directly activated by AlF4-, which is uncoupled from phospholipase C by PMA treatment. 6. Incubation of TSMCs in the absence of external Ca2+ or upon removal of Ca2+ by addition of EGTA, caused a decrease in IPs accumulation without changing the basal levels. Addition of Ca2+ (3-620 nM) to digitonin-permeabilized TSMCs stimulated IPs accumulation was obtained by inclusion of either guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or BK. The combination of GTP gamma S and BK caused an additive effect on IPs accumulation.7. Pretreatment of TSMCs with cholera toxin enhanced BK-stimulated IPs accumulation, whereas there was no effect with pertussis toxin.8. These data suggest that BK-stimulated PI metabolism is mediated by the activation of BK B2 receptors coupling to a G protein which is not blocked by cholera toxin or pertussis toxin treatment and dependent on external Ca2+. The transduction mechanism of BK coupled to PI hydrolysis is sensitive to feedback regulation by PKC.

Muscarinic regulation of cytosolic free calcium in canine tracheal smooth muscle cells: Ca2+ requirement for phospholipase C activation

1. The relationship between muscarinic receptor-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown and the increase of intracellular Ca2+ ([Ca2+])i has been examined in canine cultured tracheal smooth muscle cells (TSMCs). 2. Addition of acetylcholine (ACh) and carbachol led to a 2-3 fold increase in [Ca2+]i over the resting level as determined by fura-2, with half-maximal stimulation (EC50) obtained at concentrations of 97 and 340 nM, respectively. Addition of the partial agonist, bethanechol, showed a smaller increase in PIP2 turnover and [Ca2+]i than did ACh or carbachol. 3. Addition of ACh or carbachol to TSMCs that had been prelabelled with [3H]-inositol led to the rapid (5-15 s) release of inositol mono, bis and trisphosphates IP1, IP2 and IP3. The time course of IP3 accumulation is correlated with the time course of the peak rise in [Ca2+]i. 4. Inclusion of EGTA lowered the resting [Ca2+]i and markedly reduced the extent of the agonist-induced rise in [Ca2+]i. When assayed under conditions similar to those used for the [Ca2+]i measurements, EGTA reduced the muscarinic agonist-stimulated inositol phosphates (IPs) accumulation. Conversely, ionomycin could stimulate IPs accumulation and elevate [Ca2+]i. The addition of Ca2+ (2.7-617 nM) to digitonin-permeabilized TSMCs directly stimulated IPs accumulation. 5. Both Ca2+ and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) stimulated the formation of IPs in digitonin-permeabilized TSMCs prelabelled with [3H]-inositol. A further calcium-dependent increase in IPs accumulation was obtained by inclusion of either GTP gamma S or carbachol. The combined presence of carbachol and GTP gamma S elicited a synergistic effect on IPs accumulation, with half-maximal stimulation observed at approximately 8 nM free Ca2+.6. These results indicate that (i) the magnitude of the initial rise in [Ca2+], is directly related to the production of IPs and (ii) the phospholipase C-mediated PIP2 breakdown in TSMCs is sensitive to regulation by physiologically relevant concentrations of free Ca2+ ([Ca2+]f).

Methoxyverapamil reduction of nicotine-induced catecholamine release involves inhibition of nicotinic acetylcholine receptor currents

The mechanism by which the putative Ca2+ channel blocker methoxyverapamil (D600) inhibits nicotine-induced catecholamine release was investigated in bovine adrenal chromaffin cells and in neurons from paravertebral sympathetic ganglia of chick embryos. We found D600 to prevent catecholamine release evoked by 30 s applications of nicotine with a significantly higher potency than the release induced either by 30 s K+ depolarizations or by electrical field stimulation of sympathetic neurons. Like the use-dependent action of D600 upon Ca2+ channels, the magnitude of inhibition of the K(+)-evoked secretion depended on the duration of stimulation (10 s to 5 min). Data on catecholamine release were supplemented by patch-clamp recordings. We found whole-cell currents in chromaffin cells evoked by (extrapolated) 0.5 s applications of nicotine to be significantly more sensitive to D600 than Ca2+ currents induced by a 0.5 s depolarization from -80 to 0 mV. In both instances, the potency of D600 depended on the duration of the (nicotinic and depolarizing) stimuli. Our data suggest that D600 inhibits nicotine-induced catecholamine release by reducing nicotinic acetylcholine receptor currents rather than voltage-gated Ca2+ currents. Hence, in chromaffin cells as well as in sympathetic neuronal preparations, D600 does not appear to be a suitable tool to investigate the part voltage-activated Ca2+ currents play in cellular events induced by nicotine.

Two types of prostacyclin receptor coupling to stimulation of adenylate cyclase and phosphatidylinositol hydrolysis in a cultured mast cell line, BNu-2cl3 cells

Prostacyclin (PGI2)-mediated signal transduction was examined in interleukin 3 (IL-3)-dependent BNu-2cl3 mast cells. Iloprost, a stable PGI2 analogue, induced the accumulation of intracellular cAMP and IP3, and an increase in the intracellular Ca2+ concentration. Pretreatment of the cells with a protein kinase C activator, 12-O-tetradecanoyl phorbol 13-acetate, suppressed the iloprost-induced IP3 accumulation and Ca2+ mobilization, but inversely potentiated the cAMP accumulation, suggesting that neither of these signal transduction pathways of iloprosts is the result of a secondary effect of activation of the other. Removal of IL-3 from the culture medium reduced the iloprost-induced IP3 accumulation and Ca2+ mobilization, while it had no effect on the iloprost-induced cAMP accumulation at all. These results taken together suggest that BNu-2cl3 cells express two types of PGI2 receptor; one couples to stimulation of adenylate cyclase, its expression being independent of IL-3, while the other couples to phosphatidylinositol hydrolysis, its expression being dependent on IL-3.

Characteristics of inositol polyphosphate metabolism in cultured adrenal chromaffin cells

1. Nicotine, high K+ and maitotoxin caused the inositol polyphosphate accumulation concomitant with 45Ca2+ uptake. 2. Angiotensin II (Ang II) and ATP induced the inositol polyphosphate accumulation without 45Ca2+ uptake. 3. Nifedipine-treatment and Ca(2+)-deprivation inhibited the high K(+)-induced inositol polyphosphate accumulation but failed to inhibit the Ang II-induced inositol polyphosphate accumulation. 4. 12-O-tetradecanoylphorbol-13-acetate inhibited the Ang II-induced inositol polyphosphate accumulation but failed to inhibit the high K(+)-induced one. 5. These results suggest that the formation of inositol polyphosphates may be regulated by two mechanisms, i.e. Ca2+ uptake-dependent mechanisms represented by high K+, and Ca2+ uptake-independent mechanisms represented by Ang II.

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.

Cholinergic regulation of rat preproenkephalin RNA in the adrenal medulla

Expression of the rat preproenkephalin (ppENK) gene involves transsynaptic cholinergic mechanisms. We evaluated the effects of cholinergic agonist treatments in vivo on the expression of adrenomedullary ppENK RNA. Cholinergic treatment with nicotinic + muscarinic receptor agonists resulted in a synergistic 100-fold rise in steady-state ppENK messenger RNA levels, but only a 30- to 35-fold rise in initiation of steady-state ppENK RNA transcripts. The levels of initiated ppENK steady-state RNA peaked at two days, whereas mature (1.45 kb) ppENK mRNA levels continued to rise, peaking at four days. This suggested that other transcriptional (attenuation or alternative splicing) or post-transcriptional (RNA stabilization) regulatory mechanisms must be operative. As multiple ppENK RNA start sites exist, we examined how usage of multiple sites was altered by cholinergic treatments. The predominant start site changed from E2 in the basal state, to E4 after primary cholinergic stimulation, to E3 after re-treatment. This represents novel example of differential usage of multiple RNA initiation start sites in vivo. Differences in initiated and mature transcripts are consistent with at least four mechanisms involved in control of cholinergic-induced ppENK RNA expression: (i) simply new initiation of RNA transcripts, (ii) differential usage of the multiple RNA start sites, (iii) stabilization of mRNA transcripts, and (iv) attenuation and/or alternative RNA splicing of RNA transcripts.

Neomycin blocks dihydropyridine-insensitive Ca2+ influx in bovine adrenal chromaffin cells

There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Although recent electrophysiological work indicates that the dihydropyridine-resistant pathway is partially mediated by omega-conotoxin-sensitive and -insensitive Ca2+ channels, the pharmacological sensitivity of the latter channels remains elusive. We have now found that combined incubations with nitrendipine (1 microM) and neomycin (0.5 mM) reduced high K+ (50 mM)-evoked intracellular Ca2+ concentration ([Ca2+]i) transients to a larger extent than each drug separately. [Ca2+]i was measured using the fluorescent intracellular Ca2+ indicator fura-2. Neomycin (0.05-2 mM) reduced high K(+)-evoked 45Ca2+ uptake in a dose-dependent manner (IC50 = 0.09 mM). In the presence of nitrendipine (1 microM), the minimal neomycin concentration necessary for total blockade of 45Ca2+ uptake was reduced to 0.3 mM. Moreover, in the absence of nitrendipine the 45Ca2+ uptake remaining in 0.3 mM neomycin (26% of maximum) was similar to the fractional inhibition by nitrendipine alone (29%). Neomycin (0.05-2 mM) inhibited the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) much more extensively at 2 mM than at 0.3 mM (percent inhibition = 59% and 15%, respectively). Neomycin (0.05-2 mM) blocked high K(+)-evoked noradrenaline and adrenaline release in a dose-dependent fashion (IC50 = 0.8-1.1 mM), the blockade efficiency being enhanced in the presence of 1 microM nitrendipine (IC50 = 0.17-0.19 mM). It is concluded that neomycin (< or = 0.3 mM) blocks preferentially the dihydropyridine-insensitive Ca2+ influx pathway of the chromaffin cell. Moreover, both the dihydropyridine-sensitive and the dihydropyridine-resistant, neomycin-sensitive Ca2+ influx pathways contribute strongly to depolarization-evoked catecholamine secretion.

Effect of P2Y agonists on adenosine transport in cultured chromaffin cells

Adenosine transport in cultured chromaffin cells was inhibited by purinergic P2y-receptor agonists without significant changes in the affinity constant, the values being between 1 +/- 0.4 and 1.6 +/- 0.6 microM. The Vmax parameter was modified significantly, being 40 +/- 1.0, 26 +/- 5.0, 32 +/- 3.0, and 22 +/- 4.7 pmol/10(6) cells/min for control, adenosine-5'-O-(2-thiodiphosphate), 5'-adenylylimidodiphosphate, and P1,P4-di(adenosine-5'-) tetraphosphate (Ap4A) (100 microM for every effector), respectively. Ap4A, a physiological ligand for P2y receptors in chromaffin cells, showed the highest inhibitory effect (45%). This transport inhibition is explained by an increase in the cytosolic Ca2+ concentration ([Ca2+]i) and the activation of protein kinase C (PKC). Experiments of [Ca2+]i measurement with the fura-2 technique showed that P2y agonists, as well as bradykinin, were able to increase [Ca2+]i, this effect being independent of the presence of extracellular Ca2+. The peptide bradykinin, determined to be coupled to phosphatidylinositol hydrolysis and internal Ca2+ mobilization in chromaffin cells, exhibited a behavior similar to that of P2y agonists in adenosine transport inhibition (39%). P2y agonists and bradykinin increased PKC activity associated with the membrane fraction (about 50% increase in particulate PKC activity with respect to controls). The present studies suggest that adenosine transport is regulated by P2y-purinergic receptors mediated via Ca2+ mobilization and PKC activation.

Effects of protein kinase C on the muscarinic excitation of rat adrenal chromaffin cells

The role of protein kinase C (PKC) in the muscarinic excitation of chromaffin cells freshly isolated from rat adrenal medullae was examined by the patch-clamp recording method. Acetylcholine and McN-A-343, a M1-receptor agonist, depolarized the cell and induced action potentials. Phorbol 12,13-dibutyrate (PDBu), an activator of PKC, increased acetylcholine-induced firing concomitant with a persistent depolarization. Under voltage-clamp recording, both McN-A-343 and PDBu decreased the cesium-sensitive K+ current, which was induced by shifting the membrane potential between -140 mV and -40 mV. These results suggested that the stimulation of muscarinic M1-receptors by cholinergic drugs activated phospholipase C to degrade phosphoinositide, consequently producing diacylglycerol, and diacylglycerol activates PKC to induce excitation of adrenal chromaffin cells.

Inhibition of glutamate release: a potential mechanism of action for the anticonvulsant U-54494A in the guinea pig hippocampus

U-54494A, a 1,2-diamine, is a potent inhibitor of glutamate release in a synaptosomal preparation that is highly enriched with hippocampal mossy fiber (MF) nerve endings. At a concentration of 100 microM, U-54494A significantly reduced the availability of cytosolic free calcium (Ca2+) in depolarized MF-enriched synaptosomes by 30% and inhibited the K(+)-evoked release of endogenous glutamate by 85%. The extent to which glutamate release was inhibited allows us to conclude that U-54494A acts directly on the MF subpopulation of glutamatergic nerve endings in the guinea pig hippocampus. In addition, this anticonvulsant effectively countered the presynaptic facilitation of K(+)-evoked glutamate release that is induced by kainic acid (KA). Thus, while KA (1 mM) by itself nearly doubled the rate of K(+)-evoked glutamate release, there was no net increase in the presence of both KA and U-54494A (100 microM). However, the opposed effects of these two compounds on glutamate release do not appear to be due to a direct interaction. In the presence of U-54494A (100 microM), KA (1 mM) significantly enhanced the K(+)-evoked release of glutamate. Finally, it is demonstrated that the KA-induced enhancement of glutamate release does not require the depolarization-induced entry of extracellular Ca2+.

Glutamate and quisqualate regulate expression of metabotropic glutamate receptor mRNA in cultured cerebellar granule cells

Metabotropic glutamate receptor (type 1; mGluR1) is expressed predominantly in the hippocampus and the cerebellum. Using cultured cerebellar granule cells, we investigated the regulation of the mGluR1 mRNA expression. Levels of mGluR1 mRNA were decreased to less than half by high potassium stimulation and by glutamate and quisqualate. Although these glutamate receptor agonists tested are also known to cause neuronal cell death in culture, the effect of cell death cannot explain the observed reduction in mGluR1 mRNA because of the following reasons: (a) antagonists of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors inhibited cell death, but not the reduction of the level of mGluR1 mRNA; (b) mGluR1 mRNA returned to its initial level 48 h after the agonist application; and (c) the mRNA level of one of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptors (GluR1) was not altered by these conditions. Therefore, we conclude that the glutamate or quisqualate stimulation can specifically inhibit the expression of mGluR1 mRNA. The dose response of quisqualate for the reduction in mGluR1 mRNA is consistent with that for inositol phosphate formation stimulated through the cloned mGluR1. The mRNA reduction did not require extracellular calcium. Desensitization of mGluR1 with phorbol ester abolished the mRNA reduction. These results suggest that the reduction in mGluR1 mRNA is mediated by the activation of the metabotropic receptor itself.

Characterization of a triacylglycerol lipase that liberates arachidonic acid from bovine chromaffin cells during secretion

Primary cultures of chromaffin cells from bovine adrenal medullae were used as a model to study lipolytic events during stimulus-secretion coupling. It has been shown that chromaffin cells liberate arachidonic acid in addition to their main secretion product, the catecholamines. To understand more about the mechanism of arachidonic acid liberation, chromaffin cells were labeled with radioactive arachidonic acid, stimulated, and then analyzed for changes in lipid composition. After stimulation with 10(-4) M acetylcholine, the radioactivity of triacylglycerols decreased to the same extent that the free arachidonic acid level rose. This finding suggests that in bovine chromaffin cells a stimulation-dependent triacylglycerol lipase (triacylglycerol hydrolase; EC 3.1.1.3) is involved in arachidonic acid liberation. Further work was performed on detection, characterization, and isolation of this enzyme. Triacylglycerol lipase activity was found in whole cell homogenates and in plasma membrane fractions isolated from adrenal medullary tissue. The plasma membrane lipase showed a pH optimum of 4.3. The apparent Michaelis constant was determined as 3.3 x 10(-4) mol/L. Ca2+ did not influence the enzymatic activity. To differentiate the plasma membrane triacylglycerol lipase from the previously described plasma membrane diacylglycerol lipase of chromaffin cells, the influence of RG 80267, a specific diacylglycerol lipase inhibitor, was examined. RG 80267 (50 microM) inhibited the triacylglycerol lipase by only 24%, although diacylglycerol lipase was totally inhibited with only 20 microM RG 80267. The pH optimum of homogenate lipase was broad, lying between 4 and 7. Starting from the soluble fraction of whole cell homogenates, the triacylglycerol lipase was partially purified by ultracentrifugation and size-exclusion chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Nicotinic agonists, phorbol esters, and growth factors activate two extracellular signal-regulated kinases, ERK1 and ERK2, in bovine chromaffin cells

Treatment of bovine chromaffin cells with nicotinic agonists, phorbol esters, and growth factors increases protein kinase activity toward microtubule-associated protein-2 and myelin basic protein (MBP) in vitro. To characterize the kinases that are activated by these agents, we separated chromaffin cell proteins by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels into which MBP had been incorporated, allowed the proteins to renature, and then assayed MBP kinase activity by incubating the gels with [gamma-32P]ATP. Chromaffin cells contain a family of kinases that phosphorylate MBP in vitro. Two of these kinases, of M(r) 46,000 and 42,000 (PK46 and PK42), were activated by treatment of the cells with dimethylphenylpiperazinium (DMPP), phorbol 12,13-dibutyrate (PDBu), or insulin-like growth factor I (IGF-I). Activation of PK46 and PK42 by DMPP was dependent on extracellular Ca2+, whereas the effects of PDBu and IGF-I were Ca2+ independent. Down-regulation of protein kinase C by incubation of the cells with PDBu abolished the activation of PK46 and PK42 by DMPP, PDBu, and IGF-I. Staurosporine, a protein kinase C inhibitor, prevented the activation of PK46 and PK42 by DMPP and PDBu but did not block the activation of these kinases by IGF-I. Immunoblotting experiments with antiphosphotyrosine (anti-PTyr) antibodies demonstrated that agents that increased the kinase activities of PK46 and PK42 also increased the apparent PTyr content of M(r) 46,000 and 42,000 proteins. PK46 and PK42 comigrated with proteins that reacted with antibodies against extracellular signal-regulated kinases (ERKs). Thus, PK46 and PK42 appear to be the bovine homologues of ERK1 and ERK2.(ABSTRACT TRUNCATED AT 250 WORDS)