Cyclic GMP-dependent protein kinase activation mediates inhibition of catecholamines secretion and Ca2+ influx in bovine chromaffin cells

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

Intracellular cGMP may promote Ca2+-dependent and Ca2+-independent release of catecholamines from sympathetic nerve terminals

OBJECTIVE

This study examined the hypothesis that intracellular cGMP stimulates the release of catecholamines from sympathetic nerve terminals (SNTs) in conscious rats.

METHODS

Conscious rats were prepared to determine the effects of intravenously-administered agents on heart rate (HR) and mean arterial blood pressure (MAP).

RESULTS

Bolus intravenous injections of the membrane-permeable cGMP analogue, 8-(4-chlorophenylthio)-cGMP (8-CPT-cGMP), elicited immediate and pronounced increases in HR before any changes in MAP were observed. In contrast, injections of cGMP did not elicit changes in HR or MAP. The 8-CPT-cGMP-induced tachycardia was markedly diminished by (1) the beta(1,2)-adrenoceptor antagonist, propranolol, (2) the ganglion blocking agent, chlorisondamine, and (3) bretylium, which blocks Ca2+-dependent mobilization of vesicular stores of catecholamines from SNTs. 8-CPT-cGMP also elicited minor falls in MAP in propranolol-treated rats but elicited pronounced falls in MAP in rats treated with chlorisondamine, bretylium, or combined administration of bretylium and the muscarinic receptor antagonist, methyl-atropine.

CONCLUSIONS

These findings suggest that (1) intracellular cGMP elicits the release of Ca2+-sensitive and Ca2+-insensitive stores of catecholamines from SNTs in conscious rats, and (2) cGMP-mediated release of catecholamines from SNTs antagonizes cGMP-mediated relaxation of vascular smooth muscle in resistance arteries. Taken together, these findings support the concept that increases in intracellular cGMP levels by atrial natriuretic peptide and endothelium- and cardiac-derived nitric oxide regulate sympathetic control of the heart and the microvasculature of conscious rats via cGMP-dependent release of catecholamines.

Nitric oxide donors induce calcium-mobilisation from internal stores but do not stimulate catecholamine secretion by bovine chromaffin cells in resting conditions

The potential role of nitric oxide (NO) donors and peroxynitrites on both basal catecholamine (CA) secretion and modulation of calcium levels has been investigated in primary cultures of bovine chromaffin cells. NO donors did not modulate catecholamine secretion, while peroxynitrites induced a time dose-dependent increase in basal CA secretion. Two facts may explain the lack of these compounds on basal CA secretion. NO donors induce, on the one hand, an increase in intracellular calcium levels by depletion of internal IP3-stores from endoplasmic reticulum. On the other hand, a small calcium influx through N-type voltage-dependent calcium channels (VDCC), which seem not to be coupled to exocytosis of adrenaline and noradrenaline in chromaffin cells. Both effects, calcium-mobilisation from internal stores and calcium entry through N-type VDCC are mediated by cGMP synthesis. In contrast, peroxynitrites induce an increase in basal CA secretion by both a decrease of intracellular catecholamine content and a toxic effect on cellular membrane. All these results, taken together, could explain contradictory results in the literature on the role of NO on basal catecholamine secretion and on modulation of intracellular calcium in chromaffin cells.

Butyl benzyl phthalate blocks Ca2+ signaling and catecholamine secretion coupled with nicotinic acetylcholine receptors in bovine adrenal chromaffin cells

Butyl benzyl phthalate (BBP), a plasticizer and an environmental pollutant, exerts genomic estrogenic-like effects via estrogen receptors. In addition to exerting genomic effects via intracellular steroid receptors, estrogen exerts non-genomic effects through interactions with membrane ion channels to lead the rapid alteration of neuronal excitability. Estradiol is known as to have modulating role on nicotinic acetylcholine receptors (nAChR). We investigated the possibility of BBP exerting non-genomic estrogenic-like effects on nAChR in bovine adrenal chromaffin cells. Our results show that BBP inhibited Ca2+ signaling induced by the nicotinic ligands carbachol, 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and epibatidine (IC50 levels of 4.3, 4.1, 5.4 microM, respectively) as well as high K+ solution (IC50 50.9 microM). Additionally, in the electrophysiological observations, BBP blocked the inward current coupled with nAChR under the stimulation of carbachol. We, therefore, suggest that nAChR and voltage-gated Ca2+ channels are major and minor sites, respectively, of BBP action on the plasma membrane. The inhibitory effect of BBP on nAChR was found to be both noncompetitive and reversible, remaining unchanged as nAChR ligand concentration increased and decreased after washing. BBP was 10 times more potent than estradiol in inhibiting nAChR-coupled Ca2+ signals. We conclude that BBP exerts a novel rapidly inhibitory effect on nAChR.

Neuronal nitric oxide synthase modulates basal catecholamine secretion in bovine chromaffin cells

The role of endogenously produced nitric oxide (NO) in the regulation of basal catecholamine (CA) secretion was studied in chromaffin cells. Treatment of chromaffin cells with nitric oxide synthase (NOS) inhibitors produced a dose-dependent increase in basal catecholamine secretion, which paralleled their ability to inhibit NOS activity. This inhibitory profile was similar to that found in neurons, suggesting the constitutive expression of neuronal NOS (nNOS) in these cells, which was confirmed by Western blot analysis. A study of the kinetics and pharmacology of nNOS activity expressed in chromaffin cells in culture indicated that NOS activity is calcium-dependent, increases with time, and is highly dependent on both intracellular concentrations of L-arginine (K(m) approximately 4 microM, V(max) = 908 +/- 60 pmol/hr x 10(6) cells) and transport of L-arginine into the cells (exhibiting two affinity constants of k(1) = 3.2 +/- 0.3 microM and k(2) = 126 +/- 5.5 microM). The effects of NOS inhibitors on CA secretion were mediated by the L-arginine-NO-cGMP pathway, insofar as exogenous L-arginine was able to partially block the increase in CA secretion evoked by them, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ), a specific inhibitor of guanylate cyclase, and zaprinast, an inhibitor of the cGMP phosphodiesterase, were able to increase and inhibit, respectively, basal CA secretion in a dose-dependent manner. These results suggest that chromaffin cells exhibit a tonic production of NO by nNOS that keeps the basal CA secretion at low levels, and this could be necessary for maintaining a normotensive state.

Stimulatory effect of paeoniflorin on the release of noradrenaline from ileal synaptosomes of guinea-pig in-vitro

The effect of paeoniflorin (an active principle of Paeoniae Radix, commonly used in traditional Chinese medicine) on the release of noradrenaline (norepineprhine) from nerve terminals was investigated using guinea-pig isolated ileal synaptosomes. Release was determined as the amount of noradrenaline, quantified by high-performance liquid chromatography-electrochemical detection, from samples incubated with paeoniflorin or vehicle. Paeoniflorin stimulated the release of noradrenaline in a concentration-dependent manner without an effect on the level of lactate dehydrogenase in the bathing medium. Tetrodotoxin abolished the action of paeoniflorin at concentrations sufficient to block sodium channels. The depolarizing effect of paeoniflorin on the membrane potential was also illustrated by a concentration-dependent increase in the fluorescence of bisoxonol. Moreover, the effect of paeoniflorin on bisoxonol fluorescence in ileal synaptosomes seems more potent than that of 4-aminopyridine. That paeoniflorin causes influx of calcium ions via the depolarization of nerve terminals could be considered. The noradrenaline-releasing action of paeoniflorin was abolished by removal of calcium chloride from the bathing medium. This action of paeoniflorin was also attenuated by Rp-cAMP atconcentrations sufficientto inhibitthe action of cyclicAMP. Therefore, paeoniflorin could induce a calcium-dependent and cyclic-AMP-related release of noradrenaline from sympathetic nerve terminals of guinea-pig ileum. Guanethidine inhibited the noradrenaline-releasing action of paeoniflorin in a concentration-dependent manner. The effect of paeoniflorin on the increase of bisoxonol fluorescence was not modified by atropine. Release of noradrenaline by paeoniflorin from noradrenergic nerve terminals was characterized. These findings suggest that paeoniflorin can stimulate tetrodotoxin-sensitive depolarization of membranes to result in a calcium-dependent and cyclic-AMP-related release of noradrenaline from noradrenergic nerve terminals.

Prolonged exposure to YC-1 induces apoptosis in adrenomedullary endothelial and chromaffin cells through a cGMP-independent mechanism

YC-1, a benzyl indazole derivative, is an NO-independent direct activator of soluble guanylyl cyclase (sGC), which presents a synergistic action with NO in stimulating cGMP synthesis. These properties have served to suggest YC-1 as an attractive therapeutic agent by permitting the reduction of nitrovasodilator dosage and regulating endogenous cGMP metabolism. Here we studied the effect of prolonged exposure of adrenomedullary endothelial and chromaffin cells to YC-1. We found that YC-1 increased cGMP in the two types of cells and this action was blocked by the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Cells underwent apoptotic death in association with increased caspase-3-like activity, DNA fragmentation, cytoskeletal disorganization and changes in membrane permeability after prolonged incubation with YC-1. Caspase-3-like protease activity and DNA fragments in the cytoplasm were increased in a dose-dependent manner by 16 h YC-1 treatment. The specific and cell permeable caspase-3-like protease inhibitor DEVD-CHO effectively inhibited YC-1-mediated caspase-3-like activation and DNA fragmentation. Moreover, YC-1 also induced cell shape changes accompanied by actin filament disorganization and alterations in membrane permeability. Cells incubated for 24h with YC-1 showed damaged membranes by binding to nucleic acid of a dye excluded by the intact plasma membrane of live cells. YC-1 also induced a decrease in the intracellular non-specific esterase activity, another indication of cell toxicity. Apoptotic phenomena were not prevented by the presence of ODQ although it effectively inhibited the YC-1-elicited cGMP increases. These findings indicate that YC-1 induces apoptosis by activating caspase-3-like protease through a mechanism independent of sGC activation.

Activation of angiotensin II subtype 2 receptor induces catecholamine release in an extracellular Ca(2+)-dependent manner through a decrease of cyclic guanosine 3',5'-monophosphate production in cultured porcine adrenal medullary chromaffin Cells

We have previously demonstrated that CGP 42112 (AT(2) agonist > or =1 nM) markedly reduces catecholamine biosynthesis through AT(2), which is the major angiotensin II (AngII) receptor subtype in cultured porcine chromaffin cells. Also, we have shown that CGP 42112 (> or =1 nM) induces a reduction in cGMP production in these cells. The present study showed that AngII reduced cGMP production via AT(2) in a manner similar to that found with CGP 42112. AngII (1 nM) significantly increased catecholamine secretion from cultured porcine adrenal medullary chromaffin cells. The stimulation was significantly inhibited by PD 123319 (AT(2) antagonist). The stimulation was moderately, but significantly, attenuated by CV-11974 (AT(1) antagonist, > or =10 nM), suggesting an involvement of AT(1). Moreover, CGP 42112 (> or =10 nM) markedly increased catecholamine release from these cells. The stimulation by CGP 42112 was abolished by PD 123319, whereas CV-11974 had no effect, indicating that this response is also mediated by AT(2). We further examined whether extracellular Ca(2+) is involved in the stimulatory effect of AT(2) on catecholamine secretion. Removal of external Ca(2+) significantly suppressed either AngII plus CV-11974 (100 nM; which simulates specific AT(2) stimulation) or CGP 42112- induced catecholamine secretion. AngII plus CV-11974 or CGP 42112 caused a sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), as determined in fura-2-loaded chromaffin cells in an extracellular Ca(2+)-dependent manner. In the presence of EGTA, the subsequent addition of AngII with CV-11974 and CGP 42112 did not cause any increase in [Ca(2+)](i) levels. Consistent with this finding, CGP 42112 (10 nM to 1 microM) did not alter inositol triphosphate (IP(3)) production, a messenger for mobilization of Ca(2+) from intracellular storage sites. In addition, the intracellular Ca(2+) chelator 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethylester (BAPTA) did not affect CGP 42112-induced catecholamine release. We tested whether a decrease in cGMP was the cause of the stimulatory effect of AT(2) on catecholamine secretion. Pretreatment with 8-bromo-cGMP (1 mM) prevented the stimulatory effect of AngII plus CV-11974 and CGP 42112 on both catecholamine secretion and [Ca(2+)](i). When 8-bromo-cGMP was added after application of AngII plus CV-11974 or CGP 42112, [Ca(2+)](i) induced by these agents was gradually reduced toward the baseline values. Similarly, guanylin completely abolished the AngII- plus CV-11974-induced increase in both NE secretion and [Ca(2+)](i). The Ca(2+) channel blockers, nicardipine and omega-conotoxin G VIA, at 1 microM in both cases, were also effective in inhibiting AT(2) stimulation-induced secretion. On the other hand, neither T-type voltage-dependent Ca(2+) channel blockers, flunarizine, nor Ni(2+) affected catecholamine release caused by AT(2) stimulation. These findings demonstrate that AT(2) stimulation induces catecholamine secretion by mobilizing Ca(2+) through voltage-dependent Ca(2+) channels without affecting intracellular pools and that these effects could be mediated by a decrease in cGMP production.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Nitric oxide-sensitive guanylyl cyclase activity inhibition through cyclic GMP-dependent dephosphorylation

The soluble form of guanylyl cyclase (sGC) plays a pivotal role in the transduction of inter- and intracellular signals conveyed by nitric oxide. Here, a feedback inhibitory mechanism triggered by cyclic guanosine-3',5'-monophosphate (cGMP)-dependent protein kinase (PKG) activation is described. Preincubation of chromaffin cells with C-type natriuretic peptide, which increased cGMP levels and activated PKG, or with cGMP-permeant analogue (which also activates PKG), in the presence of a broad-spectrum phosphodiesterase inhibitor, resulted in a decrease in subsequent sodium nitroprusside (SNP)-dependent cGMP elevations. This inhibitory effect was mimicked by activating a protein phosphatase and counteracted by the selective PKG inhibitor KT-5823 and by different protein phosphatase inhibitors. Immunoprecipitation of sGC from cells submitted to different treatments followed by immunodetection with antiphosphoserine antibodies (clone 4A9) showed changes in phosphorylation levels of the beta subunit of sGC, and these changes correlated well with differences in SNP-elicited cGMP accumulations. Pretreatment of cells with several PKG inhibitors or protein phosphatase inhibitors produced an enhancement of SNP-stimulated cGMP rises without changing the SNP concentration required to produce half-maximal or maximal responses. Taken together, these results indicate that the catalytic activity of sGC is closely coupled to the phosphorylation state of its beta subunit and that the tonic activity of PKG or its stimulation regulates sGC activity through dephosphorylation of the beta subunit.

Stimulatory effect of trans-cinnamaldehyde on noradrenaline secretion in guinea-pig ileum myenteric nerve terminals

The effect of trans-cinnamaldehyde (CNMA) on the release of noradrenaline (NA) from nerve terminal was investigated using isolated ileal synaptosomes of guinea-pig. Release was determined as the amount of NA, quantified by h.p.l.c.-electrochemical detection, from samples incubated with CNMA minus that in parallel blanks treated with same volume of vehicle. CNMA stimulated the secretion of NA in a concentration-dependent manner from 5 microM to 50 microM, while the value of lactate dehydrogenase in the incubated medium was not influenced by CNMA. However, trans-cinnamic acid, cinnamoyl chloride and cinnamamide failed to produce similar effect. Specific action of CNMA can thus be considered. Guanethidine inhibited the release of NA by CNMA in a concentration- dependent manner. Saxitoxin attenuated the action of CNMA at concentrations sufficient to block sodium channels. The depolarizing effect of CNMA on the membrane potential was also illustrated by a concentration-dependent increase in the fluorescence of bisoxonol, a potential sensitive dye. The NA releasing action of CNMA was deleted by removal of calcium chloride from the bathing medium. This action of CNMA was also attenuated by Rp-cAMP at concentrations sufficient to inhibit the action of cyclic AMP. These findings suggest that CNMA can depolarize the membrane to result in a calcium-dependent and cyclic AMP-related release of NA from noradrenergic terminals.

Phosphorylation of tyrosine hydroxylase by cGMP-dependent protein kinase in intact bovine chromaffin cells

The phosphorylation of the enzyme tyrosine hydroxylase by the cGMP pathway was investigated in chromaffin cells from the bovine adrenal medulla. The nitric oxide donor, sodium nitroprusside, and the natriuretic peptide, C-type natriuretic peptide, which are able to increase cGMP levels and cGMP-dependent protein kinase activity, produced significant increases in the phosphorylation level of tyrosine hydroxylase in a time- and concentration-dependent manner. The pretreatment of the cells with the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one blocked the effect of sodium nitroprusside. This result indicates that cGMP production by this enzyme mediated this effect. Experiments performed with a cGMP-dependent protein kinase inhibitor, the Rp-isomer of 8-(4-chlorophenylthio)-cyclic guanosine monophosphorothioate, which blocked the effects of both sodium nitroprusside and C-type natriuretic peptide, demonstrated that the phosphorylation increases evoked by both compounds were mediated by the activation of cGMP-dependent protein kinase. In cells incubated with the adenylyl cyclase activator, forskolin, an increase in the phosphorylation level of the tyrosine hydroxylase was also found. When cells were treated simultaneously with forskolin and sodium nitroprusside or C-type natriuretic peptide, an additive effect on tyrosine hydroxylase phosphorylation was not observed. This suggests that cAMP- and cGMP-dependent protein kinases may phosphorylate the same amino acid residues in the enzyme. Western blot analysis of soluble extracts from chromaffin cells detected specific immunoreactivity for two different commercial antibodies raised against cGMP-dependent protein kinase (both Ialpha and Ibeta isoforms). Electrophoretic mobility correlates with that of purified PKG Ialpha. Because the phosphorylation of the tyrosine hydroxylase correlates with increases in its enzymatic activity and thus with augmentation in the cell capacity to synthesize catecholamines, our results indicate that a cGMP-based second messenger pathway participates in catecholamine biosynthesis regulation in chromaffin cells, a mechanism which may be widespread in other catecholamine-synthesizing cells.

Nitric oxide activates Ca2+-activated K+ channels in cultured bovine adrenal chromaffin cells

The effects of sodium nitroprusside (SNP) on Ca2+-dependent K+ (KCa) channels in cultured bovine adrenal chromaffin cells were investigated using single channel recording patch-clamp techniques. KCa channels were activated by application of 100 microM SNP to the extracellular side of cell-attached patches. Methylene blue (300 microM), an inhibitor of soluble guanylate cyclase, or H-8 (1 microM), a protein kinase inhibitor with relative specificity for cGMP-dependent protein kinase, diminished but did not completely abolish the SNP-induced KCa channel activation. Diethylamine/NO complex (DEA/NO), an NO donor, also activated KCa channels in cell-attached patches. Furthermore, application of 100 microM SNP or 100 nM DEA/NO to the intracellular surface of excised inside-out patches also activated KCa channels in the bath solution which contained 1 microM Ca2+. These results indicate that SNP is capable of activating the KCa channel via cGMP-dependent and -independent mechanisms. These studies demonstrate that NO may serve as an important regulatory mechanism for catecholamine secretion in chromaffin cells via the activation of KCa channels.

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

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

Functional coupling of nitric oxide synthase and soluble guanylyl cyclase in controlling catecholamine secretion from bovine chromaffin cells

This study was designed to evaluate whether the enzymes of the nitric oxide/cyclic-GMP pathway, nitric oxide synthase and soluble guanylyl cyclase, are functionally coupled in controlling catecholamine secretion in primary cultures of bovine chromaffin cells. In immunocytochemical studies, 80-85% of the tyrosine hydroxylase-positive chromaffin cells also possessed phenylethanolamine-N-methyltransferase, f1p4cating their capability to synthesize epinephrine. Immunoreactivity for neuronal-type nitric oxide synthase was found in over 90% of all chromaffin cells. Reverse transcription-polymerase chain reaction also demonstrated neuronal-type nitric oxide synthase messenger RNA. Immunoreactivity for soluble guanylyl cyclase was detectable in over 95% of chromaffin cells. Double-labeling immunofluorescence studies co-localized neuronal-type nitric oxide synthase and soluble guanylyl cyclase with tyrosine hydroxylase and phenylethanolamine-N-methyltransferase in the majority of chromaffin cells. Chromaffin cells possessed basal nitric oxide synthase activity which could be stimulated by acetylcholine and inhibited by NG-nitro-L-arginine methyl ester. Activation of soluble guanylyl cyclase by endogenously synthesized nitric oxide or the nitric oxide donor compound sodium nitroprusside was blocked by the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Catecholamine release and the increase in cytosolic Ca2+ concentration evoked by acetylcholine were enhanced by inhibitors of the endogenous nitric oxide/cyclic-GMP pathway such as NG-nitro-L-arginine methyl ester, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and the protein kinase G inhibitor Rp-8-pCPT-cGMPS. These data indicate that chromaffin cells possess an autocrine nitric oxide/cyclic-GMP pathway tonically controlling the inhibition of catecholamine release.

Inhibition of [Ca2+]i transients in rat adrenal chromaffin cells by neuropeptide Y: role for a cGMP-dependent protein kinase-activated K+ conductance

The effects of neuropeptide Y on the intracellular level of Ca2+ ([Ca2+]i) were studied in cultured rat adrenal chromaffin cells loaded with fura-2. A proportion (16%) of cells exhibited spontaneous rhythmic [Ca2+]i oscillations. In silent cells, oscillations could be induced by forskolin and 1,9-dideoxyforskolin. This action of forskolin was not modified by H-89, an inhibitor of protein kinase A. Spontaneous [Ca2+]i fluctuations and [Ca2+]i fluctuations induced by forskolin- and 1,9-dideoxyforskolin were inhibited by neuropeptide Y. Increases in [Ca2+]i induced by 10 and 20 mM KCI but not by 50 mM KCI were diminished by neuropeptide Y. However, neuropeptide Y had no effect on [Ca2+]i increases evoked by (-)BAY K8644 and the inhibitory effect of neuropeptide Y on responses induced by 20 mM KCI was not modified by omega-conotoxin GVIA, consistent with neither L- nor N-type voltage-sensitive Ca2+ channels being affected by neuropeptide Y. Rises in [Ca2+]i provoked by 10 mM tetraethylammonium were not decreased by neuropeptide Y, suggesting that K+ channel blockade reduces the effect of neuropeptide Y. However, [Ca2+]i transients induced by 1 mM tetraethylammonium and charybdotoxin were still inhibited by neuropeptide Y, as were those to 20 mM KCI in the presence of apamin. The actions of neuropeptide Y on [Ca2+]i transients provoked by 20 and 50 mM KCI, 1 mM tetraethylammonium, (-)BAY K8644 and charybdotoxin were mimicked by 8-bromo-cGMP. In contrast, 8-bromo-cAMP did not modify responses to 20 mM KCI or 1 mM tetraethylammonium. The inhibitory effects of neuropeptide Y and 8-bromo-cGMP on increases in [Ca2+]i induced by 1 mM tetraethylammonium were abolished by the Rp-8-pCPT-cGMPS, an inhibitor of protein kinase G, but not by H-89. A rapid, transient increase in cGMP level was found in rat adrenal medullary tissues stimulated with 1 microM neuropeptide Y. Rises in [Ca2+]i produced by DMPP, a nicotinic agonist, but not by muscarine, were decreased by neuropeptide Y. Our data suggest that neuropeptide Y activates a K+ conductance via a protein kinase G-dependent pathway, thereby opposing the depolarizing action of K+ channel blocking agents and the associated rise in [Ca2+]i.

Simulatory effect of porcine insulin on noradrenaline secretion in guinea-pig ileum myenteric nerve terminals

1. The effect of insulin on the release of noradrenaline (NA) from nerve terminals was investigated in isolated ileal synaptosomes of guinea-pig. Release was determined as the amount of NA, quantified by h.p.l.c.-electrochemical detection, from samples incubated with insulin minus that in parallel blanks treated with some volume of vehicle. 2. Porcine insulin stimulated the secretion of NA in a concentration-dependent manner from 0.01 i.u. ml-1, while the value of lactate dehydrogenase in the incubated medium was not influenced by insulin. 3. The presence of insulin receptors in this preparation was illustrated by immunoblotting with insulin receptor monoclonal antibodies. 4. The release of NA by insulin was reduced by guanethidine and bretylium and it was markedly lowered in the samples obtained from guinea-pigs that had received an intraperitoneal injection of DSP-4, the noradrenergic neurotoxin. 5. Tetrodotoxin attenuated the action of insulin at concentrations sufficient to block sodium channels. The depolarizing effect of insulin on the membrane potential was also illustrated by a concentration-dependent increase in the fluorescence of bisoxonol, a potential-sensitive dye. 6. The action of insulin was attenuated by removal of calcium chloride from the bathing medium. The induction of calcium ion influx by insulin into the synaptosomes is supported by the inhibitory effects of the calcium channel blockers omega-conotoxin GVIA (for the N-type channels) and nifedipine (for the L-type channels). 7. These findings suggest that insulin can stimulate NA release from noradrenergic terminals via activation of calcium influx.

Activation of NO:cGMP pathway by acetylcholine in bovine chromaffin cells. Possible role of Ca2+ in the down-regulation of cGMP signaling

The production of cyclic GMP (cGMP) induced by acetylcholine and other stimuli was studied in bovine chromaffin cells. Acetylcholine increased intracellular cGMP in a transitory (peak at 2 min) and concentration-dependent manner (estimated half maximal increase, EC50 = 61 +/- 5 microM). NG-nitro-L-arginine methyl ester (NAME) inhibited such a rise in cGMP with a half maximal inhibitory concentration (IC50) of 231 +/- 55 microM. The acetylcholine-induced increase in cGMP was also inhibited by a calmodulin antagonist (calmidazolium, 30 microM) and by the absence of extracellular calcium. Other agents that strongly increased cytosolic calcium concentration ([Ca2+]i) as acetylcholine did, such as the nicotinic-agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP), high-KCl (50 mM), and ionomycin, also caused a rise in cGMP in cultured bovine chromaffin cells. Veratridine, an activator of sodium channels, produced a slowly developing calcium increase and no significant cGMP production. The muscarinic-agonist, muscarine, failed to increase cytosolic calcium, and was the weakest stimulator of cGMP production. cGMP formation, induced by sodium nitroprusside (SNP, 100 microM) and by C-type natriuretic peptide (CNP, 100 nM), was inhibited by 30-40% by increasing [Ca2+]i with ionomycin. This inhibition was abolished by calmidazolium (30 microM) and by the absence of calcium in the extracellular medium. In conclusion, bovine chromaffin cells synthesize nitric oxide (NO) to activate guanylate cyclase in response to several stimuli, which increase [Ca2+]i. Moreover, the increase in [Ca2+]i also stimulates a Ca2+/calmodulin phosphodiesterase, which could down-regulate the levels of cGMP in these cells.

Modulation of the dihydropyridine-insensitive Ca2+ influx by 8-bromo-guanosine-3':5'-monophosphate, cyclic (8-Br-cGMP) in bovine adrenal chromaffin cells

Pretreatment of chromaffin cells with the permeable analogue of cGMP, 8-Br-cGMP (100 microM), leads to a reduction (35%) of depolarization-evoked intracellular calcium concentration ([Ca2+]i) increases. There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Combined incubations with nifedipine 10 microM and 8-Br-cGMP reduced KCl-evoked intracellular Ca2+ concentration to a greater extent that each compound separately. Moreover, 8-Br-cGMP failed to affect the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) under conditions of low depolarization. Neomycin (0.2 mM) and omega-AgaToxin-IVA (AgTx) (1 microM) inhibited the calcium transient to a similar extent, and this inhibition was not enhanced by the presence of 8-Br-cGMP. It is concluded that 8-Br-cGMP modulated the dihydropyridine-insensitive Ca2+ influx pathway in the chromaffin cell.