Cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca(2+)-pump, reduces Ca(2+)-dependent K+ currents in guinea-pig smooth muscle cells

Chicken heterophils extracellular traps act as early effectors against cyclopiazonic acid dependent upon NADPH oxidase, ROS and glycolysis

Cyclopiazonic acid (CPA) is a secondary metabolite produced by Aspergillus and Penicillium, which is present in contaminated crops and food, causing severe toxicity to humans and animals. Heterophil extracellular traps (HETs) are a novel host innate immune mechanism of chicken heterophils against pathogen infection. However, whether CPA can cause immunotoxicity of heterophils on HETs release remains unclear. Here, we attempt to detect the effects of CPA on HETs release, and further investigate the molecular mechanisms underlying these processes. We exposed heterophils to 2.5, 5, 10 μM CPA for 90 min. The results showed that CPA induced the release of HETs in heterophils, consisting of DNA-modified citrullinated histone 3 and elastase. The quantitative analysis of HETs content was positively correlated with CPA concentration. CPA also promoted reactive oxygen species production and phosphorylation of ERK_1/2 and p38. In addition, CPA-triggered HETs formation was reduced by NADPH oxidase, ERK_1/2, and p38 signaling pathway and glycolysis inhibitors, indicating that CPA-induced HETs were related to the production of ROS dependent on NADPH oxidase, ERK_1/2, and p38 signaling pathways, as well as glycolysis. Our study describes the underlying mechanism of CPA-induced HETs release, which may provide a further understanding of the immunotoxicology of CPA poisoning.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

Adenosine A2A Receptor Activation Enhances Blood-Tumor Barrier Permeability in a Rodent Glioma Model

The blood-tumor barrier (BTB) limits the entry of effective chemotherapeutic agents into the brain for treatment of malignant tumors like glioblastoma. Poor drug entry across the BTB allows infiltrative glioma stem cells to evade therapy and develop treatment resistance. Regadenoson, an FDA-approved adenosine A2A receptor (A2AR) agonist, has been shown to increase drug delivery across the blood-brain barrier in non-tumor-bearing rodents without a defined mechanism of enhancing BTB permeability. Here, we characterize the time-dependent impact of regadenoson on brain endothelial cell interactions and paracellular transport, using mouse and rat brain endothelial cells and tumor models. In vitro, A2AR activation leads to disorganization of cytoskeletal actin filaments by 30 minutes, downregulation of junctional protein expression by 4 hours, and reestablishment of endothelial cell integrity by 8 hours. In rats bearing intracranial gliomas, regadenoson treatment results in increase of intratumoral temozolomide concentrations, yet no increased survival noted with combined temozolomide therapy. These findings demonstrate regadenoson's ability to induce brain endothelial structural changes among glioma to increase BTB permeability. The use of vasoactive mediators, like regadenoson, which transiently influences paracellular transport, should further be explored to evaluate their potential to enhance central nervous system treatment delivery to aggressive brain tumors. IMPLICATIONS: This study provides insight on the use of a vasoactive agent to increase exposure of the BTB to chemotherapy with intention to improve glioma treatment efficacy.

Review Article: Safety Assessment of the Mycotoxin Cyclopiazonic Acid

Cyclopiazonic acid (CPA) is an indol-tetramic acid mycotoxin and is produced by the nearly ubiquitous molds, Aspergillus and Penicillium. CPA produced by these molds has been identified in a number of food sources (including, but not limited to, grain, legumes, meat, milk, and cheese) and from parasitic infections of man and other animals. Few incidents of CPA mycotoxicoses have been reported because of the benign nature of the intoxication, the small amounts present, and its effects may be disguised with concurrent aflatoxicosis (some toxicity data may have been generated using aflatoxin-contaminated CPA). CPA is absorbed in the gastrointestinal tract and following oral administration; it has a half-life of approximately 30 hours and is excreted largely unchanged in the urine and feces. Cyclopiazonic acid is not considered to be a potent acute toxin as its oral LD50 in rodents is in the range of 30 to 70 mg/kg. Multiple dose studies also show a range of effects in several species and among mammalian models, the pig appears to be the most sensitive with a no-observable-effect level (NOEL) in the range of 1.0 mg/kg/day. The preponderance of evidence from the rat and other test animals supports this dose as a defensible estimate of a no effect level. The target organs of CPA toxicity appear to be muscle, hepatic tissue, and spleen, with a localization in the former, although a more apparent toxic change in the latter two. The toxicity and symptoms of CPA poisoning can be attributed to its ability to alter normal intracellular calcium flux through its inhibition of the reticular form of the Ca2+-ATPase pump. CPA was not teratogenic in mice. CPA is not considered a carcinogen and the weight of evidence militates against its characterization as a mutagen. Despite CPA-induced pathological changes ascribed to the spleen or bursa of Fabricius, there does not appear to be an effect on the immune system. In vitro studies imply a potential immunomodulatory effect of CPA, but in all of those reports very high concentrations of CPA were required and none of these findings have been supported with in vivo studies. Therefore, based on a NOEL of 1 mg/kg/day and accounting for species variation, an appropriate acceptable daily intake (ADI) would be approximately 10 μg/kg/day or 700 μg/day. In the context of human exposure, if the uppermost limit of CPA found in cheese is 4 μg/g and the average individual consumes 50 g of cheese daily, this allows an intake of 200 μg, less than one third of a traditionally established ADI.

Inhibitory effects of ginsenoside-rb2 on nicotinic stimulation-evoked catecholamine secretion

The aim of the present study was to investigate whether ginsenoside-Rb2 (Rb2) can affect the secretion of catecholamines (CA) in the perfused model of the rat adrenal medulla. Rb2 (3~30 µM), perfused into an adrenal vein for 90 min, inhibited ACh (5.32 mM)-evoked CA secretory response in a dose- and time-dependent fashion. Rb2 (10 µM) also time-dependently inhibited the CA secretion evoked by DMPP (100 µM, a selective neuronal nicotinic receptor agonist) and high K(+) (56 mM, a direct membrane depolarizer). Rb2 itself did not affect basal CA secretion (data not shown). Also, in the presence of Rb2 (50 µg/mL), the secretory responses of CA evoked by veratridine (a selective Na(+) channel activator (50 µM), Bay-K-8644 (an L-type dihydropyridine Ca(2+) channel activator, 10 µM), and cyclopiazonic acid (a cytoplasmic Ca(2+)-ATPase inhibitor, 10 µM) were significantly reduced, respectively. Interestingly, in the simultaneous presence of Rb2 (10 µM) and L-NAME (an inhibitor of NO synthase, 30 µM), the inhibitory responses of Rb2 on ACh-evoked CA secretory response was considerably recovered to the extent of the corresponding control secretion compared with the inhibitory effect of Rb2-treatment alone. Practically, the level of NO released from adrenal medulla after the treatment of Rb2 (10 µM) was greatly elevated compared to the corresponding basal released level. Collectively, these results demonstrate that Rb2 inhibits the CA secretory responses evoked by nicotinic stimulation as well as by direct membrane-depolarization from the isolated perfused rat adrenal medulla. It seems that this inhibitory effect of Rb2 is mediated by inhibiting both the influx of Ca(2+) and Na(+) into the adrenomedullary chromaffin cells and also by suppressing the release of Ca(2+) from the cytoplasmic calcium store, at least partly through the increased NO production due to the activation of nitric oxide synthase, which is relevant to neuronal nicotinic receptor blockade.

Stochasticity intrinsic to coupled-clock mechanisms underlies beat-to-beat variability of spontaneous action potential firing in sinoatrial node pacemaker cells

Recent evidence indicates that the spontaneous action potential (AP) of isolated sinoatrial node cells (SANCs) is regulated by a system of stochastic mechanisms embodied within two clocks: ryanodine receptors of the "Ca(2+) clock" within the sarcoplasmic reticulum, spontaneously activate during diastole and discharge local Ca(2+) releases (LCRs) beneath the cell surface membrane; clock crosstalk occurs as LCRs activate an inward Na(+)/Ca(2+) exchanger current (INCX), which together with If and decay of K(+) channels prompts the "M clock," the ensemble of sarcolemmal-electrogenic molecules, to generate APs. Prolongation of the average LCR period accompanies prolongation of the average AP beating interval (BI). Moreover, the prolongation of the average AP BI accompanies increased AP BI variability. We hypothesized that both the average AP BI and AP BI variability are dependent upon stochasticity of clock mechanisms reported by the variability of LCR period. We perturbed the coupled-clock system by directly inhibiting the M clock by ivabradine (IVA) or the Ca(2+) clock by cyclopiazonic acid (CPA). When either clock is perturbed by IVA (3, 10 and 30 μM), which has no direct effect on Ca(2+) cycling, or CPA (0.5 and 5 μM), which has no direct effect on the M clock ion channels, the clock system failed to achieve the basal AP BI and both AP BI and AP BI variability increased. The changes in average LCR period and its variability in response to perturbations of the coupled-clock system were correlated with changes in AP beating interval and AP beating interval variability. We conclude that the stochasticity within the coupled-clock system affects and is affected by the AP BI firing rate and rhythm via modulation of the effectiveness of clock coupling.

Extracellular gentamicin reduces the activity of connexin hemichannels and interferes with purinergic Ca(2+) signaling in HeLa cells

Gap junction channels (GJCs) and hemichannels (HCs) are composed of protein subunits termed connexins (Cxs) and are permeable to ions and small molecules. In most organs, GJCs communicate the cytoplasm of adjacent cells, while HCs communicate the intra and extracellular compartments. In this way, both channel types coordinate physiological responses of cell communities. Cx mutations explain several genetic diseases, including about 50% of autosomal recessive non-syndromic hearing loss. However, the possible involvement of Cxs in the etiology of acquired hearing loss remains virtually unknown. Factors that induce post-lingual hearing loss are diverse, exposure to gentamicin an aminoglycoside antibiotic, being the most common. Gentamicin has been proposed to block GJCs, but its effect on HCs remains unknown. In this work, the effect of gentamicin on the functional state of HCs was studied and its effect on GJCs was reevaluated in HeLa cells stably transfected with Cxs. We focused on Cx26 because it is the main Cx expressed in the cochlea of mammals where it participates in purinergic signaling pathways. We found that gentamicin applied extracellularly reduces the activity of HCs, while dye transfer across GJCs was not affected. HCs were also blocked by streptomycin, another aminoglycoside antibiotic. Gentamicin also reduced the adenosine triphosphate release and the HC-dependent oscillations of cytosolic free-Ca²⁺ signal. Moreover, gentamicin drastically reduced the Cx26 HC-mediated membrane currents in Xenopus laevis oocytes. Therefore, the extracellular gentamicin-induced inhibition of Cx HCs may adversely affect autocrine and paracrine signaling, including the purinergic one, which might partially explain its ototoxic effects.

Mechanisms of the sarcoplasmic reticulum Ca2+ release induced by P2X receptor activation in mesenteric artery myocytes

BACKGROUND

ATP is one of the principal sympathetic neurotransmitters which contracts vascular smooth muscle cells (SMCs) via activation of ionotropic P2X receptors (P2XRs). We have recently demonstrated that contraction of the guinea pig small mesenteric arteries evoked by stimulation of P2XRs is sensitive to inhibitors of IP3 receptors (IP3Rs). Here we analyzed contribution of IP3Rs and ryanodine receptors (RyRs) to [Ca(2+)]i transients induced by P2XR agonist αβ-meATP (10 μM) in single SMCs from these vessels.

METHODS

The effects of inhibition of L-type Ca(2+) channels (VGCCs), RyRs and IP3Rs (5 μM nicardipine, 100 μM tetracaine and 30 μM 2-APB, respectively) on αβ-meATP-induced [Ca(2+)]i transients were analyzed using fast x-y confocal Ca(2+) imaging.

RESULTS

The effect of IP3R inhibition on the [Ca(2+)]i transient was significantly stronger (67 ± 7%) than that of RyR inhibition (40 ± 5%) and was attenuated by block of VGCCs. The latter indicates that activation of VGCCs is linked to IP3R-mediated Ca(2+) release. Immunostaining of RyRs and IP3Rs revealed that RyRs are located mainly in deeper sarcoplasmic reticulum (SR) while sub-plasma membrane (PM) SR elements are enriched with type 1 IP3Rs. This structural peculiarity makes IP3Rs more accessible to Ca(2+) entering the cell via VGCCs. Thus, IP3Rs may serve as an "intermediate amplifier" between voltage-gated Ca(2+) entry and RyR-mediated Ca(2+) release.

CONCLUSIONS

P2X receptor activation in mesenteric artery SMCs recruits IP3Rs-mediated Ca(2+) release from sub-PM SR, which is facilitated by activation of VGCCs. Sensitivity of IP3R-mediated release to VGCC antagonists in vascular SMCs makes this mechanism of special therapeutic significance.

Mechanisms of [Ca2+]i elevation following P2X receptor activation in the guinea-pig small mesenteric artery myocytes

BACKGROUND

There is growing evidence suggesting involvement of L-type voltage-gated Ca2+ channels (VGCCs) in purinergic signaling mechanisms. However, detailed interplay between VGCCs and P2X receptors in intracellular Ca2+ mobilization is not well understood. This study examined relative contribution of the Ca2+ entry mechanisms and induced by this entry Ca2+ release from the intracellular stores engaged by activation of P2X receptors in smooth muscle cells (SMCs) from the guinea-pig small mesenteric arteries.

METHODS

P2X receptors were stimulated by the brief local application of αβ-meATP and changes in [Ca2+]i were monitored in fluo-3 loaded SMCs using fast x-y confocal Ca2+ imaging. The effects of the block of L-type VGCCs and/or depletion of the intracellular Ca2+ stores on αβ-meATP-induced [Ca2+]i transients were analyzed.

RESULTS

Our analysis revealed that Ca2+ entry via L-type VGCCs is augmented by the Ca2+-induced Ca2+ release significantly more than Ca2+ entry via P2X receptors, even though net Ca2+ influxes provided by the two mechanisms are not significantly different.

CONCLUSIONS

Thus, arterial SMCs upon P2X receptor activation employ an effective mechanism of the Ca2+ signal amplification, the major component of which is the Ca2+ release from the SR activated by Ca2+ influx via L-type VGCCs. This signaling pathway is engaged by depolarization of the myocyte membrane resulting from activation of P2X receptors, which, being Ca2+ permeable, per se form less effective Ca2+ signaling pathway. This study, therefore, rescales potential targets for therapeutic intervention in purinergic control of vascular tone.

Functional characterization of acetylcholine receptors and calcium signaling in rat testicular capsule contraction

The motor activity of mammalian testicular capsule (TC) contributes to male fertility and infertility, but the acetylcholine receptors related to the contractions induced by cholinergic drugs are poorly known. Indeed to characterize the acetylcholine receptors and cellular signaling by Ca(2+) involved in TC motor activity of rats, the potency of agonists (pD₂) and antagonists (pA₂) of acetylcholine receptors, and effects of Ca(2+) cellular transport blockers on the cholinergic contractions were evaluated. pD₂ values of acetylcholine (5.98) were ten-fold higher than that of carbachol (4.99). Efficacy (Emax) of acetylcholine and carbachol to induce contractions corresponded to 95% and 97% of Emax for KCl, but Emax for nicotine was very low (8% of Emax for KCl). Further, physostigmine did not affect the acetylcholine potency. Contractions induced by acetylcholine or carbachol were antagonized by muscarinic but not nicotinic antagonist. The order of pA₂ values obtained for muscarinic antagonists, namely atropine>4-DAMP>AF-DX116>pirenzepine, corresponded to a typical profile of M3 receptors. Contractions induced by acetylcholine or carbachol were inhibited by blockers of Ca(2+) influx through voltage-dependent calcium channels (nifedipine and Ni(2+)), Ca(2+) reuptake by sarco-endoplasmic reticulum (cyclopiazonic acid) and mitochondria (FCCP). The protein kinase C (PKC) inhibitor chelerythrine only affected the acetylcholine-induced contraction. These results suggest that TC motor activity of rats are mediated mainly by M₃ receptors followed by the increase of cytosolic Ca(2+) concentration regulated by voltage-dependent calcium channels, sarco-endoplasmic reticulum and mitochondria. Furthermore, the differential effects of chelerythrine in the acetylcholine or carbachol-induced contractions are discussed.

Inhibitory Effects of Total Ginseng Saponin on Catecholamine Secretion from the Perfused Adrenal Medulla of SHRs

There seems to be some controversy about the effect of total ginseng saponin (TGS) on the secretion of catecholamines (CA) from the adrenal gland. Therefore, the present study aimed to determine whether TGS can affect the CA release in the perfused model of the adrenal medulla isolated from spontaneously hypertensive rats (SHRs). TGS (15-150 μg/mL), perfused into an adrenal vein for 90 min, inhibited the CA secretory responses evoked by acetylcholine (ACh, 5.32 mM) and high K⁺ (56 mM, a direct membrane depolarizer) in a dose- and time-dependent fashion. TGS (50 μg/mL) also time-dependently inhibited the CA secretion evoked by 1.1-dimethyl-4 -phenyl piperazinium iodide (DMPP; 100 μM, a selective neuronal nicotinic receptor agonist) and McN-A-343 (100 μM, a selective muscarinic M₁ receptor agonist). TGS itself did not affect basal CA secretion (data not shown). Also, in the presence of TGS (50 μg/mL), the secretory responses of CA evoked by veratridine (a selective Na⁺ channel activator (50 μM), Bay-K-8644 (an L-type dihydropyridine Ca²⁺ channel activator, 10 μM), and cyclopiazonic acid (a cytoplasmic Ca²⁺-ATPase inhibitor, 10 μM) were significantly reduced, respectively. Interestingly, in the simultaneous presence of TGS (50 μg/mL) and Nω-nitro-L-arginine methyl ester hydrochloride [an inhibitor of nitric oxide (NO) synthase, 30 μM], the inhibitory responses of TGS on the CA secretion evoked by ACh, high K⁺, DMPP, McN-A-343, Bay-K-8644, cyclopiazonic acid, and veratridine were considerably recovered to the extent of the corresponding control secretion compared with the inhibitory effect of TGS-treatment alone. Practically, the level of NO released from adrenal medulla after the treatment of TGS (150 μg/mL) was greatly elevated compared to the corresponding basal released level. Taken together, these results demonstrate that TGS inhibits the CA secretory responses evoked by stimulation of cholinergic (both muscarinic and nicotinic) receptors as well as by direct membrane-depolarization from the isolated perfused adrenal medulla of the SHRs. It seems that this inhibitory effect of TGS is mediated by inhibiting both the influx of Ca²⁺ and Na⁺ into the adrenomedullary chromaffin cells and also by suppressing the release of Ca²⁺ from the cytoplasmic calcium store, at least partly through the increased NO production due to the activation of nitric oxide synthase, which is relevant to neuronal nicotinic receptor blockade, without the enhancement effect on the CA release. Based on these effects, it is also thought that there are some species differences in the adrenomedullary CA secretion between the rabbit and SHR.

Influence of Fimasartan (a Novel AT(1) Receptor Blocker) on Catecholamine Release in the Adrenal Medulla of Spontaneously Hypertensive Rats

The aim of this study was to determine whether fimasartan, a newly developed AT₁ receptor blocker, can affect the CA release in the isolated perfused model of the adrenal medulla of spontaneously hypertensive rats (SHRs). Fimasartan (5~50 µM) perfused into an adrenal vein for 90 min produced dose- and time-dependently inhibited the CA secretory responses evoked by ACh (5.32 mM), high K⁺ (56 mM, a direct membrane depolarizer), DMPP (100 µM) and McN-A-343 (100 µM). Fimasartan failed to affect basal CA output. Furthermore, in adrenal glands loaded with fimasartan (15 µM), the CA secretory responses evoked by Bay-K-8644 (10 µM, an activator of L-type Ca²⁺ channels), cyclopiazonic acid (10 µM, an inhibitor of cytoplasmic Ca²⁺-ATPase), and veratridine (100 µM, an activator of Na⁺ channels) as well as by angiotensin II (Ang II, 100 nM), were markedly inhibited. In simultaneous presence of fimasartan (15 µM) and L-NAME (30 µM, an inhibitor of NO synthase), the CA secretory responses evoked by ACh, high K⁺, DMPP, Ang II, Bay-K-8644, and veratridine was not affected in comparison of data obtained from treatment with fimasartan (15 µM) alone. Also there was no difference in NO release between before and after treatment with fimasartan (15 µM). Collectively, these experimental results suggest that fimasartan inhibits the CA secretion evoked by Ang II, and cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization from the rat adrenal medulla. It seems that this inhibitory effect of fimasartan may be mediated by blocking the influx of both Na⁺ and Ca²⁺ through their ion channels into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca²⁺ release from the cytoplasmic calcium store, which is relevant to AT₁ receptor blockade without NO release.

Group I mGluRs evoke K-ATP current by intracellular Ca2+ mobilization in rat subthalamus neurons

We reported previously that Ca(2+) influx through N-methly-d-aspartate-gated channels evokes ATP-sensitive K(+) (K-ATP) currents in rat subthalamic nucleus (STN) neurons. By using whole-cell patch clamp recordings in brain slices, we investigated the ability of (RS)-3,5-dihydroxyphenylglycine (DHPG), a group I metabotropic glutamate receptor (mGluR) agonist, to evoke K-ATP currents. DHPG (20 µM) evoked outward current at -70 mV and was associated with a positive slope conductance of 2.7 nS. The sulfonylurea agent tolbutamide (100 µM) converted the positive slope to negative slope conductance, indicating mediation by K-ATP channels (ATP-sensitive K+ channels). Currents evoked by DHPG were significantly reduced by a combination of mGluR1 and mGluR5 negative allosteric modulators. DHPG-evoked outward current was blocked by cyclopiazonic acid and thapsigargin and mimicked by caffeine, suggesting mediation by release of intracellular Ca(2+). DHPG outward current was also blocked by ryanodine and 2-aminoethoxydiphenylborane, suggesting mediation by ryanodine- and inositol 1,4,5-triphosphate-sensitive Ca(2+) release. The nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester and inhibitors of protein kinase G activity also suppressed DHPG-induced outward current. Voltage recordings showed that tolbutamide prolonged depolarizing plateau potentials and increased the spontaneous firing rate of STN neurons recorded in the presence of DHPG. These results show that group I mGluR stimulation generates K-ATP current by a nitric oxide- and protein kinase G-dependent process that is mediated by release of Ca(2+) from intracellular stores. Because burst firing is linked to symptoms of Parkinson's disease, we suggest that K-ATP channels might provide a physiologically important inhibitory influence on STN neuronal activity.

Ca2+ entry following P2X receptor activation induces IP3 receptor-mediated Ca2+ release in myocytes from small renal arteries

BACKGROUND AND PURPOSE

P2X receptors mediate sympathetic control and autoregulation of the renal circulation triggering contraction of renal vascular smooth muscle cells (RVSMCs) via an elevation of intracellular Ca(2+) concentration ([Ca(2+) ](i) ). Although it is well-appreciated that the myocyte Ca(2+) signalling system is composed of microdomains, little is known about the structure of the [Ca(2+) ](i) responses induced by P2X receptor stimulation in vascular myocytes.

EXPERIMENTAL APPROACHES

Using confocal microscopy, perforated-patch electrical recordings, immuno-/organelle-specific staining, flash photolysis and RT-PCR analysis we explored, at the subcellular level, the Ca(2+) signalling system engaged in RVSMCs on stimulation of P2X receptors with the selective agonist αβ-methylene ATP (αβ-meATP).

KEY RESULTS

RT-PCR analysis of single RVSMCs showed the presence of genes encoding inositol 1,4,5-trisphosphate receptor type 1(IP(3) R1) and ryanodine receptor type 2 (RyR2). The amplitude of the [Ca(2+) ](i) transients depended on αβ-meATP concentration. Depolarization induced by 10 µmol·L(-1) αβ-meATP triggered an abrupt Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum enriched with IP(3) Rs but poor in RyRs. Depletion of calcium stores, block of voltage-gated Ca(2+) channels (VGCCs) or IP(3) Rs suppressed the sub-plasmalemmal [Ca(2+) ](i) upstroke significantly more than block of RyRs. The effect of calcium store depletion or IP(3) R inhibition on the sub-plasmalemmal [Ca(2+) ](i) upstroke was attenuated following block of VGCCs.

CONCLUSIONS AND IMPLICATIONS

Depolarization of RVSMCs following P2X receptor activation induces IP(3) R-mediated Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum, which is activated mainly by Ca(2+) influx through VGCCs. This mechanism provides convergence of signalling pathways engaged in electromechanical and pharmacomechanical coupling in renal vascular myocytes.

Inhibitory effects of olmesartan on catecholamine secretion from the perfused rat adrenal medulla

The present sutdy aimed to determine whether olmesartan, an angiotensin II (Ang II) type 1 (AT(1)) receptor blocker, can influence the CA release from the isolated perfused model of the rat adrenal medulla. Olmesartan (5~50 µM) perfused into an adrenal vein for 90 min produced dose- and time-dependent inhibition of the CA secretory responses evoked by ACh (5.32 mM), high K(+) (56 mM, a direct membrane-depolarizer), DMPP (100 µM) and McN-A-343 (100 µM). Olmesartan did not affect basal CA secretion. Also, in adrenal glands loaded with olmesartan (15 µM), the CA secretory responses evoked by Bay-K-8644 (10 µM, an activator of voltage-dependent L-type Ca(2+) channels), cyclopiazonic acid (10 µM, an inhibitor of cytoplasmic Ca(2+) -ATPase), veratridine (100 µM, an activator of voltage-dependent Na(+) channels), and Ang II (100 nM) were markedly inhibited. However, at high concentrations (150~300 µM), olmesartan rather enhanced the ACh-evoked CA secretion. Taken together, these results show that olmesartan at low concentrations inhibits the CA secretion evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by direct membrane depolarization from the rat adrenal medulla, but at high concentrations it rather potentiates the ACh-evoked CA secretion. It seems that olmesartan has a dual action, acting as both agonist and antagonist at nicotinic receptors of the isolated perfused rat adrenal medulla, which might be dependent on the concentration. It is also thought that this inhibitory effect of olmesartan may be mediated by blocking the influx of both Na(+) and Ca(2+) into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca(2+) release from the cytoplasmic calcium store, which is thought to be relevant to the AT(1) receptor blockade, in addition to its enhancement on the CA secreton.

Polyphenols of Rubus coreanum Inhibit Catecholamine Secretion from the Perfused Adrenal Medulla of SHRs

The present study was attempted to investigate whether polyphenolic compounds isolated from wine, which is brewed from Rubus coreanum Miquel (PCRC), may affect the release of catecholamines (CA) from the isolated perfused adrenal medulla of the spontaneously hypertensive rats (SHRs), and to establish its mechanism of action. PCRC (20~180 microg/ml) perfused into an adrenal vein for 90 min relatively dose-dependently inhibited the CA secretory responses to ACh (5.32 mM), high K(+) (56 mM), DMPP (100 microM) and McN-A-343 (100 microM). PCRC itself did not affect basal CA secretion (data not shown). Also, in the presence of PCRC (60 microg/ml), the CA secretory responses to veratridine (a selective Na(+) channel activator (10 microM), Bay-K-8644 (a L-type dihydropyridine Ca(2+) channel activator, 10 microM), and cyclopiazonic acid (a cytoplasmic Ca(2+) -ATPase inhibitor, 10 microM) were significantly reduced, respectively. In the simultaneous presence of PCRC (60 microg/ml) and L-NAME (an inhibitor of NO synthase, 30 microM), the inhibitory responses of PCRC on the CA secretion evoked by ACh, high K(+), DMPP, and Bay-K-8644 were considerably recovered to the extent of the corresponding control secretion compared with that of PCRC-treatment alone. The level of NO released from adrenal medulla after the treatment of PCRC (60 microg/ml) was greatly elevated compared with the corresponding basal level. Taken together, these results demonstrate that PCRC inhibits the CA secretion from the isolated perfused adrenal medulla of the SHRs evoked by stimulation of cholinergic receptors as well as by direct membrane-depolarization. It seems that this inhibitory effect of PCRC is mediated by blocking the influx of calcium and sodium into the adrenal medullary chromaffin cells of the SHRs as well as by inhibition of Ca(2+) release from the cytoplasmic calcium store at least partly through the increased NO production due to the activation of NO synthase.

Influence of spontaneous calcium events on cell-cycle progression in embryonal carcinoma and adult stem cells

Spontaneous Ca(2+) events have been observed in diverse stem cell lines, including carcinoma and mesenchymal stem cells. Interestingly, during cell cycle progression, cells exhibit Ca(2+) transients during the G(1) to S transition, suggesting that these oscillations may play a role in cell cycle progression. We aimed to study the influence of promoting and blocking calcium oscillations in cell proliferation and cell cycle progression, both in neural progenitor and undifferentiated cells. We also identified which calcium stores are required for maintaining these oscillations. Both in neural progenitor and undifferentiated cells calcium oscillations were restricted to the G1/S transition, suggesting a role for these events in progression of the cell cycle. Maintenance of the oscillations required calcium influx only through inositol 1,4,5-triphosphate receptors (IP(3)Rs) and L-type channels in undifferentiated cells, while neural progenitor cells also utilized ryanodine-sensitive stores. Interestingly, promoting calcium oscillations through IP(3)R agonists increased both proliferation and levels of cell cycle regulators such as cyclins A and E. Conversely, blocking calcium events with IP(3)R antagonists had the opposite effect in both undifferentiated and neural progenitor cells. This suggests that calcium events created by IP(3)Rs may be involved in cell cycle progression and proliferation, possibly due to regulation of cyclin levels, both in undifferentiated cells and in neural progenitor cells.

Effects of losartan on catecholamine release in the isolated rat adrenal gland

The aim of this study was to determine whether losartan, an angiotensin II (Ang II) type 1 (AT(1)) receptor could influence the CA release from the isolated perfused model of the rat adrenal medulla. Losartan (5~50 microM) perfused into an adrenal vein for 90 min produced dose- and time-dependent inhibition of the CA secretory responses evoked by ACh (5.32 mM), high K(+) (56 mM, a direct membrane depolarizer), DMPP (100 microM) and McN-A-343 (100 microM). Losartan failed to affect basal CA output. Furthermore, in adrenal glands loaded with losartan (15 microM) for 90 min, the CA secretory responses evoked by Bay-K-8644 (10 microM, an activator of L-type Ca(2+) channels), cyclopiazonic acid (10 microM, an inhibitor of cytoplasmic Ca(2+)-ATPase), veratridine (100 microM, an activator of Na(+) channels), and Ang II (100 nM) were markedly inhibited. However, at high concentrations (150~300 microM), losartan rather enhanced the CA secretion evoked by ACh. Collectively, these experimental results suggest that losartan at low concentrations inhibits the CA secretion evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) as well as by membrane depolarization from the rat adrenal medulla, but at high concentration it rather inhibits ACh-evoked CA secretion. It seems that losartan has a dual action, acting as both agonist and antagonist to nicotinic receptors of the rat adrenal medulla, which might be dependent on the concentration. It is also thought that this inhibitory effect of losartan may be mediated by blocking the influx of both Na(+) and Ca(2+) into the rat adrenomedullary chromaffin cells as well as by inhibiting the Ca(2+) release from the cytoplasmic calcium store, which is thought to be relevant to the AT(1) receptor blockade, in addition to its enhancement of the CA release.

Provinol inhibits catecholamine secretion from the rat adrenal medulla

The aim of the present study was to examine the effect of provinol, which is a mixture of polyphenolic compounds from red wine, on the secretion of catecholamines (CA) from isolated perfused rat adrenal medulla, and to elucidate its mechanism of action. Provinol (0.3~3 microg/ml) perfused into an adrenal vein for 90 min dose- and time-dependently inhibited the CA secretory responses evoked by ACh (5.32 mM), high K(+) (a direct membrane-depolarizer, 56 mM), DMPP (a selective neuronal nicotinic N(N) receptor agonist, 100 microM) and McN-A-343 (a selective muscarinic M(1) receptor agonist, 100 microM). Provinol itself did not affect basal CA secretion. Also, in the presence of provinol (1 microg/ml), the secretory responses of CA evoked by Bay-K-8644 (a voltage-dependent L-type dihydropyridine Ca(2+) channel activator, 10 microM), cyclopiazonic acid (a cytoplasmic Ca(2+)-ATPase inhibitor, 10 microM) and veratridine (an activator of voltage-dependent Na(+) channels, 10 microM) were significantly reduced. Interestingly, in the simultaneous presence of provinol (1 microg/ml) plus L-NAME (a selective inhibitor of NO synthase, 30 microM), the CA secretory responses evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclpiazonic acid recovered to the considerable extent of the corresponding control secretion in comparison with the inhibition of provinol-treatment alone. Under the same condition, the level of NO released from adrenal medulla after the treatment of provinol (3 microg/ml) was greatly elevated in comparison to its basal release. Taken together, these data demonstrate that provinol inhibits the CA secretory responses evoked by stimulation of cholinergic (both muscarinic and nicotinic) receptors as well as by direct membrane-depolarization from the perfused rat adrenal medulla. This inhibitory effect of provinol seems to be exerted by inhibiting the influx of both calcium and sodium into the rat adrenal medullary cells along with the blockade of Ca(2+) release from the cytoplasmic calcium store at least partly through the increased NO production due to the activation of nitric oxide synthase.

Resveratrol inhibits nicotinic stimulation-evoked catecholamine release from the adrenal medulla

Resveratrol has been known to possess various potent cardiovascular effects in animal, but there is little information on its functional effect on the secretion of catecholamines (CA) from the perfused model of the adrenal medulla. Therefore, the aim of the present study was to determine the effect of resveratrol on the CA secretion from the isolated perfused model of the normotensive rat adrenal gland, and to elucidate its mechanism of action. Resveratrol (10~100microM) during perfusion into an adrenal vein for 90 min inhibited the CA secretory responses evoked by ACh (5.32 mM), high K(+) (a direct membrane-depolarizer, 56 mM), DMPP (a selective neuronal nicotinic N(n) receptor agonist, 100microM) and McN-A-343 (a selective muscarinic M(1) receptor agonist, 100microM) in both a time- and dose-dependent fashion. Also, in the presence of resveratrol (30microM), the secretory responses of CA evoked by veratridine 8644 (an activator of voltage-dependent Na(+) channels, 100microM), Bay-K-8644 (a L-type dihydropyridine Ca(2+) channel activator, 10microM), and cyclopiazonic acid (a cytoplasmic Ca(2+)-ATPase inhibitor, 10microM) were significantly reduced. In the simultaneous presence of resveratrol (30microM) and L-NAME (an inhibitor of NO synthase, 30microM), the CA secretory evoked by ACh, high K(+) , DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were recovered to a considerable extent of the corresponding control secretion compared with the inhibitory effect of resveratrol alone. Interestingly, the amount of nitric oxide (NO) released from the adrenal medulla was greatly increased in comparison to its basal release. Taken together, these experimental results demonstrate that resveratrol can inhibit the CA secretory responses evoked by stimulation of cholinergic nicotinic receptors, as well as by direct membrane-depolarization in the isolated perfused model of the rat adrenal gland. It seems that this inhibitory effect of resveratrol is exerted by inhibiting an influx of both ions through Na(+) and Ca(2+) channels into the adrenomedullary cells as well as by blocking the release of Ca(2+) from the cytoplasmic calcium store, which are mediated at least partly by the increased NO production due to the activation of NO synthase.

Influence of ketamine on catecholamine secretion in the perfused rat adrenal medulla

The aim of the present study was to examine the effects of ketamine, a dissociative anesthetics, on secretion of catecholamines (CA) secretion evoked by cholinergic stimulation from the perfused model of the isolated rat adrenal gland, and to establish its mechanism of action, and to compare ketamine effect with that of thiopental sodium, which is one of intravenous barbiturate anesthetics. Ketamine (30~300microM), perfused into an adrenal vein for 60 min, dose- and time-dependently inhibited the CA secretory responses evoked by ACh (5.32 mM), high K(+) (a direct membrane-depolarizer, 56 mM), DMPP (a selective neuronal nicotinic NN receptor agonist, 100microM) and McN-A-343 (a selective muscarinic M1 receptor agonist, 100microM). Also, in the presence of ketamine (100microM), the CA secretory responses evoked by veratridine (a voltage-dependent Na(+) channel activator, 100microM), Bay-K-8644 (an L-type dihydropyridine Ca(2+) channel activator, 10microM), and cyclopiazonic acid (a cytoplasmic Ca(2+)-ATPase inhibitor, 10microM) were significantly reduced, respectively. Interestingly, thiopental sodium (100microM) also caused the inhibitory effects on the CA secretory responses evoked by ACh, high K(+) , DMPP, McN-A-343, veratridine, Bay-K-8644, and cyclopiazonic acid. Collectively, these experimental results demonstrate that ketamine inhibits the CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors and the membrane depolarization from the isolated perfused rat adrenal gland. It seems likely that the inhibitory effect of ketamine is mediated by blocking the influx of both Ca(2+) and Na(+) through voltage-dependent Ca(2+) and Na(+) channels into the rat adrenal medullary chromaffin cells as well as by inhibiting Ca(2+) release from the cytoplasmic calcium store, which are relevant to the blockade of cholinergic receptors. It is also thought that, on the basis of concentrations, ketamine causes similar inhibitory effect with thiopental in the CA secretion from the perfused rat adrenal medulla.

Roles of dopaminergic d(1) and d(2) receptors in catecholamine release from the rat adrenal medulla

The aim of the present study was designed to establish comparatively the inhibitory effects of D(1)-like and D(2)-like dopaminergic receptor agonists, SKF81297 and R(-)-TNPA on the release of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused model of the rat adrenal medulla. SKF81297 (30microM) and R-(-)-TNPA (30microM) perfused into an adrenal vein for 60 min, produced great inhibition in the CA secretory responses evoked by ACh (5.32x10(- 3) M), DMPP (10(-4) M), McN-A-343 (10(-4) M), high K(+) (5.6x10(-2) M), Bay-K-8644 (10microM), and cyclopiazonic acid (10microM), respectively. For the release of CA evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid, the following rank order of inhibitory potency was obtained: SKF81297>R-(-)-TNPA. However, R(+)-SCH23390, a selectve D(1)-like dopaminergic receptor antagonist, and S(-)-raclopride, a selectve D(2)-like dopaminergic receptor antagonist, enhanced the CA secretory responses evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid only for 0~4 min. The rank order for the enhancement of CA release evoked by high K(+), McN-A-343 and cyclopiazonic acid was R(+)-SCH23390>S(-)-raclopride. Also, the rank order for ACh, DMPP and Bay-K-8644 was S(-)-raclopride > R(+)-SCH23390. Taken together, these results demonstrate that both SKF81297 and R-(-)-TNPA inhibit the CA release evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors and the membrane depolarization from the isolated perfused rat adrenal gland without affecting the basal release, respectively, but both R(+)-SCH23390 and S(-)-raclopride facilitate the CA release evoked by them. It seems likely that the inhibitory effects of SKF81297 and R-(-)-TNPA are mediated by the activation of D(1)-like and D(2)-like dopaminergic receptors located on the rat adrenomedullary chromaffin cells, respectively, whereas the facilitatory effects of R(+)-SCH23390 and S(-)-raclopride are mediated by the blockade of D(1)-like and D(2)-like dopaminergic receptors, respectively: this action is possibly associated with extra- and intracellular calcium mobilization. Based on these results, it is thought that the presence of dopaminergic D(1) receptors may play an important role in regulation of the rat adrenomedullary CA secretion, in addition to well-known dopaminergic D(2) receptors.

Mechanism of acetylcholine-induced calcium signaling during neuronal differentiation of P19 embryonal carcinoma cells in vitro

Muscarinic (mAChRs) and nicotinic acetylcholine receptors (nAChRs) are involved in various physiological processes, including neuronal development. We provide evidence for expression of functional nicotinic and muscarinic receptors during differentiation of P19 carcinoma embryonic cells, as an in vitro model of early neurogenesis. We have detected expression and activity of alpha(2)-alpha(7), beta(2), beta(4) nAChR and M1-M5 mAChR subtypes during neuronal differentiation. Nicotinic alpha(3) and beta(2) mRNA transcription was induced by addition of retinoic acid to P19 cells. Gene expression of alpha(2), alpha(4)-alpha(7), beta(4) nAChR subunits decreased during initial differentiation and increased again when P19 cells underwent final maturation. Receptor response in terms of nicotinic agonist-evoked Ca(2+) flux was observed in embryonic and neuronal-differentiated cells. Muscarinic receptor response, merely present in undifferentiated P19 cells, increased during neuronal differentiation. The nAChR-induced elevation of intracellular calcium ([Ca(2+)](i)) response in undifferentiated cells was due to Ca(2+) influx. In differentiated P19 neurons the nAChR-induced [Ca(2+)](i) response was reduced following pretreatment with ryanodine, while the mAChR-induced response was unaffected indicating the contribution of Ca(2+) release from ryanodine-sensitive stores to nAChR- but not mAChR-mediated Ca(2+) responses. The presence of functional nAChRs in embryonic cells suggests that these receptors are involved in triggering Ca(2+) waves during initial neuronal differentiation.

Local influence of mitochondrial calcium transport in retinal amacrine cells

Ca2+-dependent synaptic transmission from retinal amacrine cells is thought to be initiated locally at dendritic processes. Hence, understanding the spatial and temporal impact of Ca2+ transport is fundamental to understanding how amacrine cells operate. Here, we provide the first examination of the local effects of mitochondrial Ca2+ transport in neuronal processes. By combining mitochondrial localization with measurements of cytosolic Ca2+, the local impacts of mitochondrial Ca2+ transport for two types of Ca2+ signals were investigated. Disruption of mitochondrial Ca2+ uptake with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) produces cytosolic Ca2+ elevations. The amplitudes of these elevations decline with distance from mitochondria suggesting that they are related to mitochondrial Ca2+ transport. The time course of the FCCP-dependent Ca2+ elevations depend on the availability of ER Ca2+ and we provide evidence that Ca2+ is released primarily via nearby ryanodine receptors. These results indicate that interactions between the ER and mitochondria influence cytosolic Ca2+ in amacrine cell processes and cell bodies. We also demonstrate that the durations of glutamate-dependent Ca2+ elevations are dependent on their proximity to mitochondria in amacrine cell processes. Consistent with this observation, disruption of mitochondrial Ca2+ transport alters the duration of glutamate-dependent Ca2+ elevations near mitochondria but not at sites more than 10 microm away. These results indicate that mitochondria influence local Ca2+-dependent signaling in amacrine cell processes.

Depolymerisation and rearrangement of actin filaments during exocytosis in rat peritoneal mast cells: involvement of ryanodine-sensitive calcium stores

Cytoskeletal F-actin associated with synaptic vesicles and granules plays an important role during Ca(2+)-mediated exocytosis. In the present work, we have used amperometry and confocal fluorescence to study the role of internal Ca(2+) in the rearrangement of F-actin (visualised with phalloidin-Alexa 546) during exocytosis in rat mast cells. The F-actin-depolymerising drug, latrunculin A, and the ryanodine receptor agonists ryanodine and caffeine that, per se did not induce exocytosis, enhanced the exocytotic responses elicited by compound 48/80 (C48/80). They also induced cortical actin depolymerisation in the presence or absence of external Ca(2+). Degranulation induced by C48/80 was accompanied by the formation of a cytoplasmic F-actin network. Depletion of internal Ca(2+) with cyclopiazonic acid inhibited latrunculin potentiation of C48/80-stimulated exocytosis and completely blocked the formation of the cytoplasmic F-actin network. This indicates that the mobilisation of Ca(2+) from ryanodine-sensitive intracellular stores plays an important role in the depolymerisation of the cortical F-actin barrier and possibly in the formation of the internal F-actin network during exocytotic activation of peritoneal mast cells.

The role of Ca2+influx and intracellular Ca2+release in the muscarinic-mediated contraction of mammalian urinary bladder smooth muscle

OBJECTIVE To study the involvement of extracellular Ca2+ and the properties of the intracellular Ca2+ ([Ca2+]i) stores on the carbachol-induced contraction of mammalian urinary bladder smooth muscle strips under polarized and depolarized conditions.

MATERIALS AND METHODS

Strips of bladder were suspended between platinum ring electrodes in a cylindrical organ bath (0.2 mL) and continuously superfused with Krebs' solution at 1 mL/min. The effect of nifedipine, cyclopiazonic acid (CPA), thapsigargin, procaine, ryanodine and caffeine before and during a 10-s application of 100 microm carbachol under polarized conditions were studied. The effect of these drugs was also assessed under depolarized conditions using a protocol that allowed a more detailed assessment of the role of [Ca2+]i stores, consisting of emptying the stores by exposure to Ca2+-free solution, rapidly refilling them by a 10-s application of 81.5 mm Ca2+ (priming), returning to the Ca2+-free solution for 3 min and then applying 100 microm carbachol (10 s) in Ca2+-free solution (store release).

RESULTS

Under polarized conditions, nifedipine and Ca2+ removal almost completely inhibited the carbachol-induced contractions. CPA increased the amplitude and duration of both carbachol- and electrical field stimulation-induced contractions. Although ryanodine had no inhibitory effect, caffeine and procaine significantly inhibited the carbachol-induced contraction. Under depolarized conditions nifedipine blocked both priming and store release contractions. CPA, thapsigargin, procaine and ryanodine significantly increased the priming and inhibited the store release contractions. However, caffeine virtually abolished both priming and store release contractions.

CONCLUSION

These results suggest that in guinea-pig urinary bladder smooth muscle the Ca2+ necessary for contraction enters the cell through voltage-dependent dihydropyridine-sensitive Ca2+ channels and is pumped into an intracellular store that is released by carbachol. Under polarized conditions, the blockade of sarco-endoplasmic reticulum calcium ATP-ase (SERCA) with CPA increases [Ca2+]i and carbachol-induced contractions. The effects of caffeine and procaine suggest that store release involves ryanodine receptors and calcium-induced calcium release. Under depolarized conditions, Ca2+ entry is blocked by nifedipine and the stores diminish. Stored Ca2+ is also greatly reduced by the blockade of SERCA with either CPA or thapsigargin. Procaine, ryanodine and caffeine blocked the store release contractions, suggesting that this involves ryanodine receptors and calcium-induced calcium release.

CCCP enhances catecholamine release from the perfused rat adrenal medulla

The present study was designed to investigate the effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP), a mitochondrial uncoupler, on secretion of catecholamines from the isolated perfused model of the rat adrenal gland and to establish the mechanism of its adrenomedullary secretion. The perfusion of CCCP (3x10(-5) M) into an adrenal vein of for 90 min caused a great increase in catecholamine secretion. Tachyphylaxis to catecholamine-releasing effect of CCCP was not observed by repeated perfusion of it. The net catecholamine-releasing effects of CCCP were depressed by pretreament with pirenzepine (a selective muscarinic M(1)-receptor antagonist), chlorisondamine (a selective neuronal nicotinic receptor antagonist), nicardipine (an L-type Ca2+-channel antagonist), TMB-8 (an intracellular Ca2+-antagonist), and the perfusion of EGTA plus Ca2+-free medium, respectively. In the presence of CCCP (3x10(-5) M), catecholamine secretory responses induced by ACh (5.32x10(-3) M), high K+ (5.6x10(-2) M, a direct membrane depolarizer), DMPP (10(-4) M, (a selective neuronal nicotinic receptor agonist), and McN-A-343 (10(-4) M, (a selective muscarinic M1-receptor agonist) were significantly enhanced. CCCP also significantly enhanced the catecholamine secretory responses evoked by Bay-K-8644 (10(-5) M), L-type Ca2+ channel activator, and cyclopiazonic acid (10(-5) M), an inhibitor of Ca2+-ATPase. Furthermore, the perfusion of FCCP (3x10(-5) M), a similar mitochondrial uncoupler, into an adrenal vein of for 90 min also caused a great increase in catecholamine secretion in a similar pattern with CCCP. Taken together, the results demonstrate that CCCP causes the catecholamine secretion from the perfused rat adrenal medulla in a calcium-dependent fashion. It is thought that this catecholamine secretory enhancement of CCCP may be mediated by both cholinergic receptor stimulation and membrane depolarization, which are relevant to the cytoplasmic Ca2+ increase by stimulation of the Ca2+ influx as well as by the inhibition of Ca2+ uptake into the cytoplasmic Ca2+ stores (both endoplasmic reticulum and mitochondria in chromaffin cells). It also seems that protonophores, such as CCCP, suppress mitochondrial Ca2+ uptake and increase the stimulated secretion of catecholamine by the secretagogues. These results indicate that mitochondria modulate catecholamine secretion by regulating the Ca2+ mobilization for exocytosis.

Arecoline inhibits catecholamine release from perfused rat adrenal gland

AIM

To study the effect of arecoline, an alkaloid isolated from Areca catechu, on the secretion of catecholamines (CA) evoked by cholinergic agonists and the membrane depolarizer from isolated perfused rat adrenal gland.

METHODS

Adrenal glands were isolated from male Sprague-Dawley rats. The adrenal glands were perfused with Krebs bicarbonate solution by means of a peristaltic pump. The CA content of the perfusate was measured directly using the fluorometric method.

RESULTS

Arecoline (0.1-1.0 mmol/L) perfused into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretory responses evoked by acetylcholine (ACh) (5.32 mmol/L), 1.1-dimethyl-4-phenyl piperazinium iodide (DMPP) (100 micromol/L for 2 min) and 3-(m-choloro-phenyl-carbamoyl-oxy)-2-butynyl trimethyl ammonium chloride (McN-A-343) (100 micromol/L for 2 min). However, lower doses of arecoline did not affect CA secretion of high K(+) (56 mmol/L); higher doses greatly reduced CA secretion of high K(+). Arecoline also failed to affect basal catecholamine output. Furthermore, in adrenal glands loaded with arecoline (0.3 mmol/L), CA secretory response evoked by Bay-K-8644 (10 micromol/L), an activator of L-type Ca(2+) channels, was markedly inhibited, whereas CA secretion by cyclopiazonic acid (10 micromol/L), an inhibitor of cytoplasmic Ca(2+)-ATPase, was not affected. Nicotine (30 micromol/L), which was perfused into the adrenal gland for 60 min, however, initially enhanced ACh-evoked CA secretory responses. As time elapsed, these responses became more inhibited, whereas the initially enhanced high K(+)-evoked CA release diminished. CA secretion evoked by DMPP and McN-A-343 was significantly depressed in the presence of nicotine.

CONCLUSION

Arecoline dose-dependently inhibits CA secretion from isolated perfused rat adrenal gland evoked by activation of cholinergic receptors. At lower doses arecoline does not inhibit CA secretion through membrane depolarization, but at larger doses it does. This inhibitory effect of arecoline may be mediated by blocking the calcium influx into the rat adrenal medullary chromaffin cells without the inhibition of Ca(2+) release from the cytoplasmic calcium store. There seems to be a difference in the mode of action of nicotine and arecoline in rat adrenomedullary CA secretion.

NMDA receptor-mediated epileptiform persistent activity requires calcium release from intracellular stores in prefrontal neurons

Various normal and pathological forms of synchronized population activity are generated by recurrent excitation among pyramidal neurons in the neocortex. However, the intracellular signaling mechanisms underlying this activity remain poorly understood. In this study, we have examined the cellular properties of synchronized epileptiform activity in the prefrontal cortex with particular emphasis on a potential role of intracellular calcium stores. We find that the zero-magnesium-induced synchronized activity is blocked by inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPases, phospholipase C (PLC), the inositol 1,4,5-trisphosphate (IP3) receptor, and the ryanodine receptor. This same activity is, however, not affected by application of metabotropic glutamatergic receptor (mGluR) agonists, nor by introduction of an mGluR antagonist. These results suggest that persistent synchronized activity in vitro is dependent upon calcium release from internal calcium stores through the activation of PLC-IP3 receptor pathway. Our findings also raise the possibility that intracellular calcium release may be involved in the generation of pathologic synchronized activity in epilepsy in vivo and in physiological forms of synchronized cortical activity.

Postsynaptic IP3 receptor-mediated Ca2+ release modulates synaptic transmission in hippocampal neurons

Ca(2+)-dependent mechanisms are important in regulating synaptic transmission. The results herein indicate that whole-cell perfusion of inositol 1,4,5-trisphosphate receptor (IP(3)R) agonists greatly enhanced excitatory postsynaptic current (EPSC) amplitudes in postsynaptic hippocampal CA1 neurons. IP(3)R agonist-mediated increases in synaptic transmission changed during development and paralleled age-dependent increases in hippocampal type-1 IP(3)Rs. IP(3)R agonist-mediated increases in EPSC amplitudes were inhibited by postsynaptic perfusion of inhibitors of Ca(2+)/calmodulin, PKC and Ca(2+)/calmodulin-dependent protein kinase II. Postsynaptic perfusion of inhibitors of smooth endoplasmic reticulum (SER) Ca(2+)-ATPases, which deplete intracellular Ca(2+) stores, also enhanced EPSC amplitudes. Postsynaptic perfusion of the IP(3)R agonist adenophostin (AdA) during subthreshold stimulation appeared to convert silent to active synapses; synaptic transmission at these active synapses was completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Postsynaptic IP(3)R-mediated Ca(2+) release also produced a significant increase in spontaneous EPSC frequency. These results indicate that Ca(2+) release from intracellular stores play a key role in regulating the function of postsynaptic AMPARs.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Influence of naloxone on catecholamine release evoked by nicotinic receptor stimulation in the isolated rat adrenal gland

The present study was designed to investigate the effect of naloxone, a well known opioid antagonist, on the secretion of catecholamines (CA) evoked by cholinergic stimulation and membrane-depolarization in the isolated perfused rat adrenal glands, and to establish its mechanism of action. Naloxone (10(-6) approximately 10(-5) M), perfused into an adrenal vein for 60 min, produced dose- and time-dependent inhibition of CA secretory responses evoked by ACh (5.32 x 10(-3) M), high K+ (5.6 x 10(-2) M), DMPP (10(-4) M) and McN-A-343 (10(-4) M). Naloxone itself also failed to affect the basal CA output. In adrenal glands loaded with naloxone (3 x 10(-6) M), the CA secretory responses evoked by Bay-K-8644, an activator of L-type Ca2+ channels, and cyclopiazonic acid, an inhibitor of cytoplasmic Ca(2+)-ATPase, were also inhibited. In the presence of met-enkephalin (5 x 10(-6) M), a well known opioid agonist, the CA secretory responses evoked by ACh, high K+, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly inhibited. Taken together, these results suggest that naloxone greatly inhibits the CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors as well as that by membrane depolarization. It seems that these inhibitory effects of naloxone does not involve opioid receptors, but might be mediated by blocking both the calcium influx into the rat adrenal medullary chromaffin cells and the uptake of Ca2+ into the cytoplasmic calcium store, which are at least partly relevant to the direct interaction with the nicotinic receptor itself.

Vasorelaxant effects of the chloroformic crude extract of Bupleurum fruticosum L. (Umbelliferae) roots on rat thoracic aorta

The chloroformic crude extract of roots of Bupleurum fruticosum L. (Umbelliferae) showed a concentration-dependent vasorelaxing effect on aortic rings endothelium-deprived and pre-contracted by norepinephrine (NE). The pharmacological effect was not produced through the stimulation of cyclooxygenase, adenyl cyclase, or guanylyl cyclase, since selective inhibitors did not prevent the extract-induced responses. The incubation of the aortic rings with the chloroformic extract (10(-4) g/ml) produced a depression of the concentration-contractile response curve to NE, in normal conditions, and this effect was more evident in Ca2+-free Tyrode solution, suggesting an action on the intracellular mobilization of Ca2+ ions. Moreover, the vasodilator action of Bupleurum fruticosum extract was resistant to the pre-treatment with nifedipine and to the pre-treatment with cyclopiazonic acid (blocker of Ca2+/ATPase). Finally, the chloroformic extract of Bupleurum fruticosum produced a reduction of the contraction obtained by caffeine, an opener of ryanodine-sensitive receptors, suggesting that the plant could elicit the vasorelaxing response by the blockade of ryanodine-sensitive Ca2+ channels of the sarcoplasmic reticulum.

Inhibitory effects of digoxin and digitoxin on corticosterone production in rat zona fasciculata-reticularis cells

The aim of the present study was to investigate the direct effects and action mechanisms of digitalis on the production of corticosterone in rat adrenocortical cells. Male rats were challenged with digoxin (1 microg ml(-1) kg(-1)) in the presence or absence of adrenocorticotropin (ACTH, 5 microg ml(-1) kg(-1)) administered by intravenous injection to the right jugular vein. Blood samples were collected at 0, 30, 60, and 120 min following the challenge. The concentration of corticosterone in the rat plasma samples was measured by radioimmunoassay. Zona fasciculata-reticularis (ZFR) cells in male rats were prepared and then incubated with or without digoxin or digitoxin in the presence or absence of ACTH (10(-9) m), forskolin (10(-7) m), 8-bromo-cyclic 3' : 5'-adenosine monophosphate (10(-4) m), cyclopiazonic acid (CPA, 10(-5) m), trilostane (10(-6) m), 25-OH-cholesterol (10(-5) m), pregnenolone (10(-5) m), progesterone (10(-5) m), or deoxycorticosterone (10(-5) m) at 37 degrees C for 1 h before collection of the media. Corticosterone or pregnenolone levels were measured by radioimmunoassay. A single injection of digoxin did not alter the basal level of plasma corticosterone, but did inhibit the level of plasma corticosterone released in response to ACTH in vivo. Administration of digoxin or digitoxin decreased both spontaneous and ACTH-stimulated release of corticosterone in vitro. Digoxin (10(-7)-10(-5) m) and digitoxin (10(-7)-10(-5) m), but not ouabain (10(-7)-10(-5) m), dose-dependently inhibited corticosterone production in response to forskolin and 8-Br-cyclic AMP in rat ZFR cells. Both digoxin (10(-6)-10(-5) m) and digitoxin (10(-6)-10(-5) m) attenuated corticosterone production in response to CPA. Digoxin (10(-5) m) or digitoxin (10(-5) m) inhibited cytochrome P450 side-chain cleavage enzyme (cytochrome P450scc) activity (catalyses conversion of cholesterol to pregnenolone in the presence of trilostane) in rat ZFR cells. The enzyme activity of 11 beta-hydroxylase (catalyses conversion of deoxycorticosterone to corticosterone) in ZFR cells was also inhibited by the administration of digoxin (10(-5) m) or digitoxin (10(-5) m).10 These results together suggest that digoxin and digitoxin decrease the release of corticosterone by acting directly on ZFR cells via a Na+, K+-ATPase-independent mechanism involving the inhibition of the activities of adenylyl cyclase, cytochrome P450scc and 11 beta-hydroxylase, as well as the functioning of cyclic AMP and intracellular calcium.

Cotinine inhibits catecholamine release evoked by cholinergic stimulation from the rat adrenal medulla

The aim of the present study was to clarify whether cotinine affects the release of catecholamines (CA) from the isolated perfused rat adrenal gland, and to establish the mechanism of its action, in comparison with the response of nicotine. Cotinine (0.3-3 mM), when perfused into an adrenal vein for 60 min, inhibited CA secretory responses evoked by ACh (5.32 mM), DMPP (a selective neuronal nicotinic agonist, 100 microM for 2 min) and McN-A-343 (a selective muscarinic M1-agonist, 100 microM for 2 min) in dose- and time-dependent manners. However, cotinine did not affect CA secretion by high K+ (56 mM). Cotinine itself also failed to affect basal CA output. Furthermore, in the presence of cotinine (1 mM), CA secretory responses evoked by Bay-K-8644 (an activator of L-type Ca2+ channels, 10 microM) and cyclopiazonic acid (an inhibitor of cytoplasmic Ca2+-ATPase, 10 microM) were relative time-dependently attenuated. However, nicotine (30 microM), given into the adrenal gland for 60 min, initially rather enhanced CA secretory responses evoked by ACh and high K+, followed by the inhibition later, while it time-dependently depressed the CA release evoked by McN-A-343 and DMPP. Taken together, these results suggest that cotinine inhibits greatly CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors, but does fail to affect that by the direct membrane-depolarization. It seems that this inhibitory effect of cotinine may be exerted by the cholinergic blockade, which is associated with blocking both the calcium influx into the rat adrenal medullary chromaffin cells and Ca2+ release from the cytoplasmic calcium store. It also seems that there is a big difference in the mode of action between cotinine and nicotine in the rat adrenomedullary CA secretion.

Vasodilator activity of Michelia figo Spreng. (Magnoliaceae) by in vitro functional study

The methanolic extract of leaves of Michelia figo Spreng. (Magnoliaceae), as well as several purified fractions, showed a concentration-dependent vasorelaxing effect on aortic rings endothelium-deprived and pre-contracted by norepinephrine (NE). For further pharmacological investigation on the mechanism of action, the fraction S4 was selected, since it showed the best vasodilator properties. The pharmacological effect was not produced through the stimulation of cyclooxygenase, adenyl cyclase, or guanylyl cyclase, since selective inhibitors did not prevent the fraction S4-induced effects. Moreover, the vasorelaxing effect of the fraction was resistant to the block of nifedipine-sensitive Ca(2+) channels. The fraction S4 (10(-4) g/ml) produced a shift towards the right of the concentration-contractile response curve to NE, in normal conditions, and the shift was more evident in Ca(2+)-free Tyrode solution, suggesting an action on intracellular Ca(2+)-channels. The vasodilator action of fraction S4 on NE pre-contracted rings was not prevented by cyclopiazonic acid (blocker of Ca(2+)/ATPase), which excludes a role for mechanisms involving the storage of Ca(2+) in the sarcoplasmic reticulum. The reduction of the contraction elicited by caffeine, an opener of ryanodine-sensitive receptors, suggests that the fraction S4 of Michelia figo leaves could produce the vasorelaxing response by the blockade of ryanodine-sensitive Ca(2+) channels of the sarcoplasmic reticulum.

Influence of lobeline on catecholamine release from the isolated perfused rat adrenal gland

It has been shown that lobeline (alpha-lobeline) is a lipophilic, nonpyridine, naturally occurring alkaloid obtained from Indian tobacco, Lobelia inflata. The present study was attempted to investigate the effect of lobeline on secretion of catecholamines (CA) evoked by ACh, high K(+), 1.1-dimethyl-4-phenyl piperazinium iodide (DMPP) and (3-(m-chloro-phenyl-carbamoyl-oxy)-2-butynyl trimethyl ammonium chloride (McN-A-343) from the isolated perfused rat adrenal gland and to establish the mechanism of its action. l-Lobeline (30-300 microM) perfused into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretory responses evoked by ACh (5.32 x 10(-3) M), DMPP (10(-4) M for 2 min) and McN-A-343 (10(-4) M for 2 min). However, lower dose of lobeline did not affect CA secretion by high K(+) (5.6 x 10(-2) M), higher dose of it reduced greatly CA secretion of high K(+). l-Lobeline itself did also fail to affect basal catecholamine output. Furthermore, in adrenal glands loaded with lobeline (100 microM), CA secretory response evoked by methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5-carboxylate (Bay-K-8644), an activator of L-type Ca(2+) channels was markedly inhibited while CA secretion by cyclopiazonic acid, an inhibitor of cytoplasmic Ca(2+)-ATPase was not affected. However, nicotine (30 microM), given into the adrenal gland for 60 min, initially rather enhanced CA secretory responses evoked by ACh (5.32 x 10(-3) M) and high K(+) (5.6 x 10(-2) M) followed by great inhibition later, while responses evoked by DMPP (10(-4) M for 2 min) and McN-A-343 (10(-4) M for 2 min) were greatly inhibited. Taken together, these results suggest that lobeline inhibits greatly CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors. Lobeline at lower dose does not affect that by membrane depolarization, but at larger dose inhibits that. It is thought that this inhibitory effect of lobeline may be mediated by blocking the calcium influx into the rat adrenal medullary chromaffin cells without the inhibition of Ca(2+) release from the cytoplasmic calcium store, which is relevant to its nicotinic antagonistic activity. It also seems that there is a difference in the mode of action between nicotine and lobeline in rat adrenomedullary CA secretion.

Inhibitory mechanism of bromocriptine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal medulla

The purpose of this study was to determine whether bromocriptine affects the catecholamines (CA) secretion evoked in isolated perfused rat adrenal glands, by cholinergic stimulation, membrane depolarization and calcium mobilization, and to establish the mechanism of its action. The perfusion of bromocriptine (1-10 microM) into an adrenal vein, for 60 min, produced relatively dose-dependent inhibition in the secretion of catecholamines (CA) evoked by acetylcholine (ACh, 5.32 mM), DMPP (100 microM for 2 min), McN-A-343 (100 microM for 2 min), cyclopiazonic acid (CPA, 10 microM for 4 min) and Bay-K-8644 (10 microM for 4 min). High K+ (56 mM)-evoked CA release was also inhibited, although not in a dose-dependent fashion. Also, in the presence of apomorphine (100 microM), which is also known to be a selective D2-agonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were also significantly depressed. However, in adrenal glands preloaded with bromocriptine (3 microM) in the presence of metoclopramide (15 microM), a selective D2-antagonist, the CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid considerably recovered as compared to that of bromocriptine only. Taken together, these results suggest that bromocriptine can inhibit the CA secretion evoked by stimulation of cholinergic receptors, as well as by membrane depolarization, in the perfused rat adrenal medulla. It is thought this inhibitory effect of bromocriptine may be mediated by inhibiting the influx of extracellular calcium and the release from intracellular calcium stores, through the activation of dopaminergic D2-receptors located in the rat adrenomedullary chromaffin cells. Furthermore, these findings also suggest that the dopaminergic D2-receptors may play an important role in regulating adrenomedullary CA secretion.

Comparison of catecholamine release in the isolated adrenal glands of SHR and WKY rats

1 The present study was designed to investigate the secretion of catecholamines (CA) evoked by stimulation of cholinergic receptors and membrane depolarization from the isolated perfused adrenal gland of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKYR) at adult age. 2 The wet weight of adrenal gland in SHR was greater than that in WKYR. The CA releasing responses evoked by acetylcholine (5.32 x 10-3 m), and high potassium (5.6 x 10-2 m), a membrane depolarizer, were significantly lower in WKYR than in SHR. 3 The secretory responses of CA evoked by DMPP (10-4 m for 2 min), a selective agonist of neuronal nicotinic receptors, and McN-A-343 (10-4 m for 2 min), a selective agonist of neuronal muscarinic receptors, were also significantly lower in WKYR than in SHR. 4 The CA release evoked by Bay-K-8644 (10-5 m), a dihydropyridine-sensitive Ca2+ channel activator, and cyclopiazonic acid (10-5 m), a selective inhibitor of Ca2+-ATPase in the endoplasmic reticulum, were also significantly greater in SHR than WKYR. 5 Taken together, these experimental results demonstrate that the CA secretion evoked by stimulation of cholinergic (nicotinic and muscarinic) receptors as well as membrane depolarization is enhanced more greatly in the perfused adrenal glands of SHR than in those of WKYR. It is suggested that the augmented CA release in SHR compared with WKYR was involved in essential hypertensive pathogenesis.

Effect of cyclopiazonic acid on membrane currents in isolated inner hair cells from guinea-pig cochlea

Cyclopiazonic acid (CPA) is a reticulum-like intracellular Ca(2+) store depletory, which raises intracellular Ca(2+) concentration. The effect of CPA on membrane currents in isolated inner hair cells (IHCs) from guinea-pig cochlea was investigated by the patch-clamp technique in the whole-cell configuration. Four out of eight IHCs showed an augmentation of the currents and the other four cells showed an inhibition of the currents by extracellular CPA application. The activation kinetics of outward currents were not changed by CPA. Three out of four IHCs obtained from the basal part of the cochlea demonstrated augmentation, whereas three out of four IHCs from the apical part demonstrated inhibition of the currents. This result suggests that Ca(2+)-activated currents were dominant in the basal IHCs of the cochlea.

Vasodilator activity of crude methanolic extract of Gentiana kokiana Perr. et Song. (Gentianaceae)

Gentiana kokiana Perr. et Song. is a plant employed in the traditional medicine of Tuscany (Italy) as antihypertensive remedy. The aim of this work was to evaluate a possible vascular action of the plant and to investigate its mechanism of action. The methanolic extract of roots showed an endothelium-independent vasodilator activity in aortic rings pre-contracted by norepinephrine (NE) 3 microM and a marked depression of the contracturant responses induced by KCl and caffeine, and by NE, both in Tyrode solution and Ca2+-free Tyrode solution. An action on the Ca2+-extracellular influx was discarded, while release or uptake mechanisms of the sarcoplasmic reticulum were hypothesized. However, the incapacity of cyclopiazonic acid (20 microM), a blocker of Ca2+/ATPase of the sarcoplasmic reticulum, to reduce the vasodilator action of the extract allowed to exclude the involvement of such a mechanism. A possible involvement of the ryanodine-sensitive Ca2+ channels is suggested.

Local Ca(2+) transients and distribution of BK channels and ryanodine receptors in smooth muscle cells of guinea-pig vas deferens and urinary bladder

1. The relationship between Ca(2+) sparks spontaneously occurring at rest and local Ca(2+) transients elicited by depolarization was analysed using two-dimensional confocal Ca(2+) images of single smooth muscle cells isolated from guinea-pig vas deferens and urinary bladder. The current activation by these Ca(2+) events was also recorded simultaneously under whole-cell voltage clamp. 2. Spontaneous transient outward currents (STOCs) and Ca(2+) sparks were simultaneously detected at -40 mV in approximately 50 % of myocytes of either type. Ca(2+) sparks and corresponding STOCs occurred repetitively in several discrete sites in the subplasmalemmal area. Large conductance Ca(2+)-dependent K(+) (BK) channel density in the plasmalemma near the Ca(2+) spark sites generating STOCs was calculated to be 21 channels microm(-2). 3. When myocytes were depolarized from -60 to 0 mV, several local Ca(2+) transients were elicited within 20 ms in exactly the same peripheral sites where sparks occurred at rest. The local Ca(2+) transients often lasted over 300 ms and spread into other areas. The appearance of local Ca(2+) transients occurred synchronously with the activation of Ca(2+)-dependent K(+) current (I(K,Ca)). 4. Immunofluorescence staining of the BK channel alpha-subunit (BKalpha) revealed a spot-like pattern on the plasmalemma, in contrast to the uniform staining of voltage-dependent Ca(2+) channel alpha1C subunits along the plasmalemma. Ryanodine receptor (RyR) immunostaining also suggested punctate localization predominantly in the periphery. Double staining of BKalpha and RyRs revealed spot-like co-localization on/beneath the plasmalemma. 5. Using pipettes of relatively low resistance, inside-out patches that included both clustered BK channels at a density of over 20 channels microm(-2) and functional Ca(2+) storage sites were obtained at a low probability of approximately 5%. The averaged BK channel density was 3-4 channels microm(-2) in both types of myocyte. 6. These results support the idea that a limited number of discrete sarcoplasmic reticulum (SR) fragments in the subplasmalemmal area play key roles in the control of BK channel activity in two ways: (i) by generating Ca(2+) sparks at rest to activate STOCs and (ii) by generating Ca(2+) transients presumably triggered by sparks during an action potential to activate a large I(K,Ca) and also induce a contraction. BK channels and RyRs may co-localize densely at the junctional areas of plasmalemma and SR fragments, where Ca(2+) sparks occur to elicit STOCs.

5-Hydroxytryptamine stimulates net Ca2+ flux in the ventricular muscle of a mollusc (Busycon canaliculatum) during cardioexcitation

Noninvasive, self-referencing calcium (Ca2+) electrodes were used to study the mechanisms by which 5-hydroxytryptamine (5-HT) affects net Ca2+ flux across the sarcolemma of myocytes from ventricular trabeculae (from a marine gastropod, Busycon canaliculatum). Treatment of isolated trabeculae with 5-HT causes a net Ca2+ efflux, which is 30% blocked by verapamil. These findings suggest that the efflux is in part the result of a previous Ca2+ influx through L-type Ca2+ channels and is due to a rapid Ca2+ extrusion mechanism inherent to the sarcolemma of these myocytes. 5-HT-induced net Ca2+ efflux is also reduced by about 40% by treatment with a sodium (Na+)-free, lithium (Li+)-substituted saline, which shuts down the Na-Ca exchanger during Ca2+ extrusion. Cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+ ATPase, almost completely abolishes the 5-HT-induced net Ca2+ efflux, suggesting that the SR rather than the extracellular pool is the primary Ca2+ reservoir serving 5-HT-induced excitation.

Influence of quinine on catecholamine release evoked by cholinergic stimulation and membrane depolarization from the rat adrenal gland

The present study was attempted to investigate the effect of quinine on secretion of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused rat adrenal gland. The perfusion of quinine (15-150 microM) into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretion evoked by ACh (5.32 x 10(-3) M), high K+ (5.6 x 10(-2) M), DMPP (10(-4) M for 2 min), McN-A-343 (10(-4) M for 2 min), cyclopiazonic acid (10(-5) M for 4 min) and Bay-K-8644 (10(-5) M for 4 min). Also, under the presence of pinacidil (10(-4) M), which is also known to be a selective potassium channel activator, CA secretory responses evoked by ACh, high potassium, DMPPF McN-A-343, Bay-K-8644 and cyclopiazonic acid were also greatly reduced. When preloaded along with quinine (5 x 10(-5) M) and glibenclamide (10(-6) M), a specific blocker of ATP-regulated potassium channels, CA secretory responses evoked by ACh, high potassium, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid were recovered as compared to those of quinine-treatment only. Taken together, these results demonstrate that quinine inhibits CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors as well as by membrane depolarization through inhibiting influx of extracellular calcium and release in intracellular calcium in the rat adrenomedullary chromaffin cells. These findings suggest that activation of potassium channels may be involved at least in inhibitory action of quinine on CA secretion from the rat adrenal gland.

Amperometric characterization of exocytotic events from single mast cells: dependence on external and internal Ca++ sources

Mast cells exocytotically release histamine/serotonin in response to different secretagogues. We have used substance P and compound 48/80 to study the Ca++ dependency of serotonin exocytosis from peritoneal mast cells using carbon fiber amperometric techniques. The exocytotic release pattern consists of a burst of events superimposed on a slow, transient, amperometric current baseline increase. Cellular re lease parameters (number, frequency and total charge of amperometric events) and individual event characteristics (charge integral, half width and peak amplitude) were similar for the two secretagogues used. Zero Ca++ conditions greatly reduced, without completely abolishing,cellular release parameters. Cyclopiazonic acid, an inhibitor of the endoplasmatic Ca++ ATPase, reduced the cellular exocytotic capacity and diminished the amplitude of individual exocytotic events more effectively than the 0 Ca++ condition. The cyclopiazonic acid effects occurred in the presence of external Ca++, indicating that this condition is not sufficient for maintaining full exocytotic capacity. The results confirm the importance of intracellular Ca++ for exocytotic activation. For the first time evidence is presented that the integrity of intracellular Ca++ pools determines the amplitude and frequency of individual exocytotic events. Saponin, a non-specific detergent, also induced quantal release similar to that obtained with substance P and compound 48/80. This release was not dependent on extracellular Ca++, but cyclopiazonic acid significantly reduced individual exocytotic release.