A23187 increases calcium permeability of store sites more than of surface membranes in the rabbit mesenteric artery

Reduced Function of Endothelial Nitric Oxide and Hyperpolarization in Artery Grafts with Poor Runoff

BACKGROUND

The endothelium regulates vascular tonus by releasing nitric oxide (endothelium-derived nitric oxide, EDNO) and hyperpolarizing factor (endothelium-derived hyperpolarizing factor, EDHF). In vein grafts with poor runoff, lack of function of these factors causes severe intimal hyperplasia. This study evaluated how the functions of EDNO and EDHF are altered in artery grafts under poor runoff conditions.

MATERIALS AND METHODS

The right common carotid arteries of rabbits were excised and implanted in their original positions as autogenous grafts under normal runoff conditions ("nonoccluded grafts") or poor runoff conditions ("poor runoff grafts"). Histochemical changes, acetylcholine (ACh)-induced effects on endothelium-dependent relaxation and smooth muscle cell (SMC) hyperpolarization were examined.

RESULTS

Both artery graft types displayed negligible intimal hyperplasia. In the absence and presence of an EDNO synthase inhibitor, ACh-induced relaxation was lower in grafts with poor runoff than in nonoccluded grafts. Furthermore, ACh-induced but not nonreceptor agonist A23187-induced SMC hyperpolarization was lower in the poor runoff graft group than in the nonoccluded graft group.

CONCLUSIONS

Unlike in those in vein grafts, the functions of EDNO and EDHF in autogenous carotid artery grafts under poor runoff conditions were reduced but partly maintained. In such artery grafts, intimal hyperplasia caused by surgical operation was not present. These results may explain some of the mechanisms underlying the improved patency of artery grafts compared with vein grafts.

Staphylococcus aureus Panton-Valentine Leukocidin triggers an alternative NETosis process targeting mitochondria

Panton-Valentine Leukocidin (PVL) is a bicomponent leukotoxin produced by 3%-10% of clinical Staphylococcus aureus (SA) strains involved in the severity of hospital and community-acquired infections. Although PVL was long known as a pore-forming toxin, recent studies have challenged the formation of a pore at the plasma membrane, while its endocytosis and the exact mode of action remain to be defined. In vitro immunolabeling of human neutrophils shows that Neutrophil Extracellular Traps (NETosis) is triggered by the action of purified PVL, but not by Gamma hemolysin CB (HlgCB), a structurally similar SA leukotoxin. PVL causes the ejection of chromatin fibers (NETs) decorated with antibacterial peptides independently of the NADPH oxidase oxidative burst. Leukotoxin partially colocalizes with mitochondria and enhances the production of reactive oxygen species from these organelles, while showing an increased autophagy, which results unnecessary for NETs ejection. PVL NETosis is elicited through Ca²⁺ -activated SK channels and Myeloperoxidase activity but is abolished by Allopurinol pretreatment of neutrophils. Moreover, massive citrullination of the histone H3 is performed by peptidyl arginine deiminases. Inhibition of this latter enzymes fails to abolish NET extrusion. Unexpectedly, PVL NETosis does not seem to involve Src kinases, which is the main kinase family activated downstream the binding of PVL F subunit to CD45 receptor, while the specific kinase pathway differs from the NADPH oxidase-dependent NETosis. PVL alone causes a different and specific form of NETosis that may rather represent a bacterial strategy conceived to disarm and disrupt the immune response, eventually allowing SA to spread.

Enhancement of Nitric Oxide Production Is Responsible for Minimal Intimal Hyperplasia of Autogenous Rabbit Arterial Grafts

BACKGROUND

Vascular endothelium induces smooth muscle cell (SMC) relaxation mainly mediated by endothelium-derived nitric oxide (EDNO) and endothelium-derived hyperpolarizing factor (EDHF). It has previously been reported that functions of these endothelium factors have been greatly impaired in vein grafts. The present study was undertaken to determine whether the functions of EDNO and EDHF might be altered in artery graft.Methods and Results:In rabbits, the right carotid artery was excised and implanted in its original position as an autogenous graft ("artery graft") and the non-operated left carotid artery served as the "control artery". Histochemical changes, acetylcholine (ACh)-induced effects on the intracellular concentration of Ca²⁺([Ca²⁺]_i) in endothelial cells, endothelium-dependent SMC hyperpolarization and relaxation, and tissue cGMP content were examined on post-operative day 28. "Artery graft" displayed a minimal amount of intimal hyperplasia. When compared with the "control artery", it exhibited greater ACh-induced, endothelium-dependent relaxation, but the reverse was true when EDNO production was blocked. In the "artery graft" (vs. the "control artery"), basal cGMP content was greater, whereas the [Ca²⁺]_iincrease in endothelial cells and the endothelium-dependent SMC-hyperpolarization induced by ACh were less.

CONCLUSIONS

It is suggested that the [Ca²⁺]_i-independent EDNO production covers the loss of function of endothelium-dependent SMC hyperpolarization and minimizes intimal hyperplasia caused by surgical operation in autogenous carotid artery graft.

Increase in intracellular Ca(2+) level by phenylsulfamide fungicides, tolylfluanid and dichlofluanid, in rat thymic lymphocytes

Tolylfluanid, a phenylsulfamide fungicide, is one of the many pesticides that are frequently detected in crops. Therefore, its health risk is a concern. Micromolar concentrations of tolylfluanid induce chromosomal aberrations and micronuclei in mammalian lymphocytes. The findings prompted us to study the cellular actions of tolylfluanid and another frequently detected pesticide, dichlofluanid, at submicromolar and micromolar concentrations. Of the cellular actions of chemicals, the action on cellular Ca(2+) homeostasis is important since Ca(2+) is involved in cell signaling and death. Consequently, in this study, the effects of phenylsulfamide fungicides were examined on rat thymocytes by using fluorescent probes in order to further characterize the cellular actions of phenylsulfamide fungicides. Both phenylsulfamide fungicides exhibited biphasic, early and late, increase in intracellular Ca(2+) levels. The early phase was dependent on intracellular Ca(2+) release and increased membrane Ca(2+) permeability. The late phase was owing to Ca(2+) influx via activation of store-operated Ca(2+) channels and the further increase of membrane ionic permeability. Voltage-dependent Ca(2+) channels were not involved. The increases in intracellular Ca(2+) levels by phenylsulfamide fungicides were observed at drug concentrations of 0.1 μM or more (up to 10 μM). Thus, it is plausible that micromolar concentrations of phenylsulfamide fungicides deregulate intracellular Ca(2+) homeostasis in rat thymocytes. Both phenylsulfamide fungicides at 10 μM promoted the transition from intact living cells to living cells with phosphatidylserine-exposed membranes. This was not the case for phenylsulfamide fungicides at 3 μM. The potency of tolylfluanid was similar to that of dichlofluanid. Although the information on residual concentrations of tolylfluanid and dichlofluanid is very limited, their residual concentrations do not reach micromolar levels. It is unlikely that humans will develop adverse effects on exposure to phenylsulfamide fungicides under present environmental conditions.

SK3 channel and mitochondrial ROS mediate NADPH oxidase-independent NETosis induced by calcium influx

Neutrophils cast neutrophil extracellular traps (NETs) to defend the host against invading pathogens. Although effective against microbial pathogens, a growing body of literature now suggests that NETs have negative impacts on many inflammatory and autoimmune diseases. Identifying mechanisms that regulate the process termed "NETosis" is important for treating these diseases. Although two major types of NETosis have been described to date, mechanisms regulating these forms of cell death are not clearly established. NADPH oxidase 2 (NOX2) generates large amounts of reactive oxygen species (ROS), which is essential for NOX-dependent NETosis. However, major regulators of NOX-independent NETosis are largely unknown. Here we show that calcium activated NOX-independent NETosis is fast and mediated by a calcium-activated small conductance potassium (SK) channel member SK3 and mitochondrial ROS. Although mitochondrial ROS is needed for NOX-independent NETosis, it is not important for NOX-dependent NETosis. We further demonstrate that the activation of the calcium-activated potassium channel is sufficient to induce NOX-independent NETosis. Unlike NOX-dependent NETosis, NOX-independent NETosis is accompanied by a substantially lower level of activation of ERK and moderate level of activation of Akt, whereas the activation of p38 is similar in both pathways. ERK activation is essential for the NOX-dependent pathway, whereas its activation is not essential for the NOX-independent pathway. Despite the differential activation, both NOX-dependent and -independent NETosis require Akt activity. Collectively, this study highlights key differences in these two major NETosis pathways and provides an insight into previously unknown mechanisms for NOX-independent NETosis.

Triclocarban-induced change in intracellular Ca²⁺ level in rat thymocytes: cytometric analysis with Fluo-3 under Zn²⁺-free conditions

Triclocarban (TCC) is an antimicrobial used in personal hygiene products. Recent health concerns arose after TCC was detected in the blood of human subjects who showered with soap containing TCC. In this study, the effect of TCC on intracellular Ca(2+) concentration in rat thymocytes was examined using Fluo-3, an indicator of intracellular Ca(2+). TCC at concentrations ranging from 0.1 μM to 3 μM increased intracellular Ca(2+) concentration biphasically: first by releasing Ca(2+) from intracellular Ca(2+) stores and then inducing Ca(2+) influx through store-operated Ca(2+) channels. The threshold TCC concentration to increase intracellular Ca(2+) concentration in this study was lower than the maximum TCC concentrations reported in human blood samples. Therefore, we anticipate that TCC at concentrations reported in human blood samples might disturb intracellular Ca(2+) signaling in human lymphocytes.

Roles of cytosolic Ca2+ concentration and myofilament Ca2+ sensitization in age-dependent cerebrovascular myogenic tone

In light of evidence that immature arteries contain a higher proportion of noncontractile smooth muscle cells than found in fully differentiated mature arteries, the present study explored the hypothesis that age-related differences in the smooth muscle phenotype contribute to age-related differences in contractility. Because Ca(2+) handling differs markedly between contractile and noncontractile smooth muscle, the present study specifically tested the hypothesis that the relative contributions of Ca(2+) influx and myofilament sensitization to myogenic tone are upregulated, whereas Ca(2+) release is downregulated, in immature [14 days postnatal (P14)] compared with mature (6 mo old) rat middle cerebral arteries (MCAs). Myofilament Ca(2+) sensitivity measured in β-escin-permeabilized arteries increased with pressure in P14 but not adult MCAs. Cyclopiazonic acid (an inhibitor of Ca(2+) release from the sarcoplasmic reticulum) increased diameter and reduced Ca(2+) in adult MCAs but increased diameter with no apparent change in Ca(2+) in P14 MCAs. La(3+) (Ca(2+) influx inhibitor) increased diameter and decreased Ca(2+) in adult MCAs, but in P14 MCAs, La(3+) increased diameter with no apparent change in Ca(2+). After treatment with both La(3+) and CPA, diameters were passive in both adult and P14 MCAs, but Ca(2+) was decreased only in adult MCAs. To quantify the fraction of smooth muscle cells in the fully differentiated contractile phenotype, extents of colocalization between smooth muscle α-actin and SM2 myosin heavy chain were determined and found to be at least twofold greater in adult than pup MCAs. These data suggest that compared with adult MCAs, pup MCAs contain a greater proportion of noncontractile smooth muscle and, as a consequence, rely more on myofilament Ca(2+) sensitization and Ca(2+) influx to maintain myogenic reactivity. The inability of La(3+) to reduce cytosolic Ca(2+) in the pup MCA appears due to La(3+)-insensitive noncontractile smooth muscle cells, which contribute to the spatially averaged measurements of Ca(2+) but not contraction.

Antispasmodic activity of licochalcone A, a species-specific ingredient of Glycyrrhiza inflata roots

Licochalcone A, a species-specific and characteristic retrochalcone ingredient of Glycyrrhiza inflata root, has been shown to possess multiple bioactive properties. However, its muscle relaxant activity has not been reported previously. Licochalcone A showed a concentration-dependent relaxant effect on the contraction induced by carbachol (50% effective concentration (EC50) = 5.64 ± 1.61 μm). KCl (EC50 5.12 ± 1.68 μm), BaCl2 (EC50 1.97 ± 0.48 μm) and A23187 (EC50 2.63 ± 2.05 μm). Pretreatment with licochalcone A enhanced the relaxant effect of forskolin, an adenylyl cyclase activator, on the contraction in a similar manner to 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase (PDE) inhibitor. Furthermore, the IC50 (22.1 ± 10.9 μm) of licochalcone A against cAMP PDE was similar to that of IBMX (26.2 ± 7.4 μm). These results indicated that licochalcone A may have been responsible for the relaxant activity of G. inflata root and acted through the inhibition of cAMP PDE.

Muscarinic receptor subtypes mediating Ca2+ sensitization of intestinal smooth muscle contraction: studies with receptor knockout mice

In the present study, we have characterized muscarinic receptor subtypes that mediate carbachol-induced Ca2+ sensitization of contraction in intestinal smooth muscle, using mutant mice lacking M(2) or M(3) muscarinic receptors or both receptor subtypes. In alpha-toxin-permeabilized muscle strips from wild-type (WT) mice, isometric tension responses to Ca2+ applied cumulatively (pCa 7.0-5.0) were increased when the muscarinic agonist carbachol (100 microM) was added to the medium, as judged from shifts of pCa-tension curves in both 50% effective concentration (EC(50)) and maximum response (E(max)) of pCa-tension curve. In preparations from M(2)-knockout (KO) mice, pCa-tension curves were also shifted by carbachol (100 microM), and the extents of the EC(50) and E(max) changes resembled those observed in preparations from WT mice. In preparations from M(3)-KO or M(2)/M(3)-double KO mice, however, no significant changes in pCa-tension curves were obtained after carbachol application. The G(q/11)-type G-protein inhibitor YM-254890 (1 microM) completely blocked the Ca2+ sensitization of contraction induced by carbachol in M(2)-KO or WT preparations. The results strongly support the idea that the muscarinic activation of Ca2+ sensitization in intestinal smooth muscles is mediated by the M(3) muscarinic receptor coupled to G(q/11)-type G-proteins, without any significant involvement of the other muscarinic receptor subtypes including M(2).

Role of Ca2+ stores in acetylcholine-induced all-or-none shortening of smooth muscle cells from guinea-pig taenia caecum

1. We have reported previously that isolated single smooth muscle cells from guinea-pig taenia caecum respond to acetylcholine (ACh) in an all-or-none manner. 2. To clarify the roles of intracellular Ca(2+) stores in the all-or-none response of isolated smooth muscle cells from guinea-pig taenia caecum to ACh, we examined the inositol 1,4,5-trisphosphate (IP(3))-induced contractile response in Staphylococcus aureus alpha-toxin-permeabilized smooth muscle cells and the effect of depletion of intracellular Ca(2+) stores on the all-or-none response to ACh in intact smooth muscle cells. 3. alpha-Toxin-permeabilized smooth muscle cells responded to 3-30 nmol/L or 0.3-3 nmol/L IP(3) in the presence of 0.2 micromol/L Ca(2+) with 1 mmol/L EGTA or 0.1 mmol/L EGTA, respectively, in an all-or-none manner. These results suggest that Ca(2+) release induced by IP(3) is Ca(2+) dependent and is evoked in an all-or-none manner. 4. In the presence of the Ca(2+) ionophore A23187 (0.1 micromol/L) or the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (1 micromol/L), the shortening of intact smooth muscle cells induced by increasing concentrations of ACh showed a graded response, but not an all-or-none response. 5. In conclusion, the results suggest that Ca(2+) release from Ca(2+) stores induced by IP(3) plays an important role in the all-or-none response of intact smooth muscle cells to ACh.

Mechanisms of the inhibitory effects of a phenoxazine compound, 2-amino-4,4.ALPHA.-dihydro-4 .ALPHA.-7-dimethyl-3H-phenoxazine-3-one, on the contraction of the smooth muscle of the guinea pig taenia cecum

To elucidate the mechanisms involved in the relaxing effect of 2-amino-4, 4alpha-dihydro-4alpha-7-dimethyl-3H-phenoxazine-3-one (Phx-1) on smooth muscle, we investigated its effects on the contraction of both intact and skinned (cell membrane permeabilized) preparations from the guinea pig taenia cecum. In intact preparations, Phx-1 concentration-dependently suppressed the contraction induced by either acetylcholine (ACh) or high-K(+) with an IC(50) value estimated at around 100 muM. Similar inhibitory actions of Phx-1 on force were observed in intracellular Ca store depleted preparations. In cell membrane depolarized preparations in the absence of extracellular Ca, however, Phx-1 had little effect on either caffeine- or ACh-induced contractions. In skinned preparations, Phx-1 suppressed Ca(2+)-induced contractions at concentrations higher than 100 muM. These results suggest that inhibition of smooth muscle contraction by Phx-1 is due mainly to inhibition of Ca(2+)-influx, although Phx-1 also seems to have direct inhibitory effects on the activities of the contractile apparatus.

Na+-K+-ATPase is involved in the sustained ACh-induced hyperpolarization of endothelial cells from rat aorta

BACKGROUND AND PURPOSE

Inhibition of Na(+)-K(+)-ATPase is known to attenuate endothelium-dependent relaxation in many arteries. The purpose of this study was to evaluate the role of Na(+)-K(+)-ATPase in the regulation of endothelial membrane potential at rest and during stimulation by ACh.

EXPERIMENTAL APPROACH

Membrane potential was recorded from the endothelium of rat aorta using the perforated patch-clamp technique.

KEY RESULTS

Superfusion with K(+)-free solution produced a depolarization of about 11 mV from the resting value of -42.9+/-0.9 mV. Reintroduction of 4.7 mM K(+) transiently hyperpolarized endothelial cells to -52.4+/-1.8 mV and the membrane potential recovered within 10 min. Ouabain 500 microM depolarized endothelium by about 11 mV and inhibited the hyperpolarization induced by K(+) reintroduction into the K(+)-free solution. However, 500 nM ouabain did not affect the resting membrane potential or the hyperpolarization induced by K(+) reintroduction. Pre-exposure to ouabain 500 microM, but not 500 nM, attenuated the sustained component of hyperpolarization to ACh without affecting the amplitude of the transient peak hyperpolarization. In K(+)-free solution, the amplitude of peak hyperpolarization to ACh was increased, while the sustained component of hyperpolarization was attenuated.

CONCLUSIONS AND IMPLICATIONS

These results indicate that electrogenic Na(+)-K(+)-ATPase partially contributes to the sustained hyperpolarization of endothelial cells from rat aorta in response to ACh. They also suggest that the alpha1, but not alpha2 or alpha3 isoforms, is involved in ACh-mediated hyperpolarization.

A Supraclinical Dose of Tramadol Stereoselectively Attenuates Endothelium-Dependent Relaxation in Isolated Rat Aorta

Tramadol, a combination of R(-) and S(+) enantiomers, inhibits both the acetylcholine-mediated response of muscarinic receptors and the muscarine-induced accumulation of cyclic guanosine monophosphate. Our goals in this in vitro study were to investigate the effects of tramadol on endothelium-dependent relaxation induced by acetylcholine, to determine whether this effect of tramadol is stereoselective, and to elucidate the associated cellular mechanism in rat aorta. In endothelium-intact rings precontracted with phenylephrine with or without naloxone, dose-response curves for acetylcholine, histamine, and calcium ionophore A23187 were generated in the presence and absence of tramadol (racemic, R(-) and S(+)). Sodium nitroprusside dose-response curves were generated in the presence and absence of racemic tramadol. Racemic tramadol (5 x 10(-5) 10(-4) M) attenuated acetylcholine-induced relaxation in the rings with or without naloxone. R(-) tramadol, 5 x 10(-5) M, attenuated acetylcholine-induced relaxation, whereas S(+) tramadol, 5 x 10(-5) M, did not. Racemic tramadol (10(-4) M) had no effect on dose-response curves for calcium ionophore A23187 or sodium nitroprusside. Taken together, these results indicate that tramadol, at a supraclinical dose (5 x 10(-5) M), stereoselectively attenuates endothelium-dependent relaxation via an inhibitory effect at levels proximal to nitric oxide synthase activation on a pathway involving nonspecific endothelial receptor activation.

Increase in number of annexin V-positive living cells of rat thymocytes by intracellular Pb(2+)

Lead is ubiquitous in our environment and lead poisoning is a major public health problem worldwide. In this study, to see if intracellular Pb(2+) induces the exposure of phosphatidylserine in rat thymocyte membranes, we have examined the effect of PbCl(2) on rat thymocytes treated with A23187 using a flow cytometer with appropriate fluorescent indicators under nominally-Ca(2+)-free condition. PbCl(2) at 1-30 μM dose-dependently induced the exposure of phosphatidylserine on outer membranes, associated with increasing the concentration of intracellular Pb(2+). The potency of intracellular Pb(2+) to induce the apoptotic change in thymocyte membranes seems to be greater than those of intracellular Ca(2+) and Cd(2+). Results suggest that intracellular Pb(2+) triggers apoptosis of rat thymocytes. This action of Pb(2+) may be one of mechanisms for the lead-induced changes in immunity.

PbCl2-induced hyperpolarization of rat thymocytes: involvement of charybdotoxin-sensitive K+ channels

The effect of PbCl2 on membrane potential and intracellular divalent metal cation concentrations of rat thymocytes was examined by flow cytometry. PbCl2 at concentrations of 0.3 microM or higher (up to 10 microM) produced persistent, dose-dependent hyperpolarization (decrease in the intensity of di-BA-C4 fluorescence). Removal of external Ca2+ did not significantly affect the PbCl2-induced hyperpolarization. Charybdotoxin, a specific antagonist of Ca(2+)-dependent K+ conductance, greatly attenuated the PbCl2-induced hyperpolarization. PbCl2 increased the intensity of fluo-3 fluorescence under both normal Ca2+ and nominally Ca(2+)-free conditions. These results suggest that Pb2+ enters thymocytes, causing an increase in fluo-3 fluorescence, and activates Ca(2+)-dependent K+ channels, resulting in hyperpolarization. The persistent activation of K+ channels by Pb2+, leading to persistent hyperpolarization, may be one mechanism whereby Pb2+ alters immune function, as membrane potential changes influence physiological functions of lymphocytes.

Sevoflurane and bradykinin-induced calcium mobilization in pulmonary arterial valvular endothelial cells in situ: sevoflurane stimulates plasmalemmal calcium influx into endothelial cells

Kinins locally synthesized in the cardiovascular tissue are believed to contribute to the regulation of cardiovascular homeostasis by stimulating the endothelial cells to release nitric oxide, prostacyclin, or a hyperpolarizing factor via autocrine-paracrine mechanisms. This study was designed to investigate the action of sevoflurane on bradykinin-induced Ca2+ mobilization in endothelial cells in situ. Utilizing fura-2-loaded rat pulmonary arterial valve leaflets, the effects of sevoflurane were examined on bradykinin-induced increases in intracellular Ca2+ concentration ([Ca2+]i) in endothelial cells in situ. In the presence of extracellular Ca2+ (1.5 mM), bradykinin (3-30 microM) produced an initial phasic and a subsequent tonic increase in [Ca2+]i in a concentration-dependent manner. However, it produced only the phasic increase in [Ca2+]i in the absence of extracellular Ca2+. Sevoflurane (5%, 0.67 mM) inhibited both the phasic and tonic responses to bradykinin. In these experiments, sevoflurane (3-5%) generated sustained increases (approximately 20-40% of the bradykinin-induced maximal increase in [Ca2+]i) in the resting [Ca2+]i level. Sevoflurane still increased [Ca2+]i after depletion of the intracellular Ca stores with ionomycin (0.1 microM ). However, the sevoflurane-induced increase in [Ca2+]i was eliminated by removal of the extracellular Ca and attenuated by NiCl (1-3 mM). In conclusion, in the pulmonary arterial valvular endothelial cells, sevoflurane inhibits both bradykinin-induced Ca2+ release from the intracellular stores and bradykinin-induced plasmalemmal Ca2+ influx. In addition, sevoflurane appears to stimulate the plasmalemmal Ca2+ influx and thereby increase the endothelial [Ca2+]i level. Sevoflurane might influence the pulmonary vascular tone through its direct action on the pulmonary arterial valvular endothelial cells.

Developmental changes in ryanodine- and IP(3)-sensitive Ca(2+) pools in ovine basilar artery

To explore the hypothesis that cerebrovascular maturation alters ryanodine- and inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) pool sizes, we measured total intracellular Ca(2+) with (45)Ca and the fractions of intracellular Ca(2+) released by IP(3) and/or caffeine in furaptra-loaded permeabilized basilar arteries from nonpregnant adult and term fetal (139-141 days) sheep. Ca(2+) mass (nmol/mg dry weight) was similar in adult (1.60 +/- 0.18) and fetal (1.71 +/- 0.16) arteries in the pool sensitive to IP(3) alone but was significantly lower for adult (0.11 +/- 0.01) than for fetal (1.22 +/- 0.11) arteries in the pool sensitive to ryanodine alone. The pool sensitive to both ryanodine and IP(3) was also smaller in adult (0.14 +/- 0.01) than in fetal (0.85 +/- 0.08) arteries. Because the Ca(2+) fraction in the ryanodine-IP(3) pool was small in both adult (5 +/- 1%) and fetal (7 +/- 4%) arteries, the IP(3) and ryanodine pools appear to be separate in these arteries. However, the pool sensitive to neither IP(3) nor ryanodine was 10-fold smaller in adult (0.87 +/- 0.10) than in fetal (8.78 +/- 0.81) arteries, where it accounted for 72% of total intracellular membrane-bound Ca(2+). Thus, during basilar artery maturation, intracellular Ca(2+) mass plummets in noncontractile pools, decreases modestly in ryanodine-sensitive pools, and remains constant in IP(3)-sensitive pools. In addition, age-related increases in IP(3) efficacy must involve factors other than IP(3) pool size alone.

Characterization of changes in mechanical responses to histamine in omental resistance arteries in pre-eclampsia

Changes in the effect of histamine on the smooth muscle of resistance arteries in pre-eclampsia were investigated by measuring isometric contractions in endothelium-denuded strips of omental resistance arteries from pre-eclamptic and normotensive pregnant women (pregnancy-term matched). Histamine (0.03 -1 microM) caused concentration-dependent relaxation of the contraction induced by 9, 11-epithio-11,12-methano-thromboxane A(2) (STA(2)) in strips from both groups. Sensitivity (for pre-eclampsia: pD(2)=6.66+/-0.04, n=5 and for normotensive pregnant women: pD(2)=7.07+/-0.03, n=10, P<0.001) was lower and the maximum response (90.6+/-0.6% vs 95.5+/-1.1%, P<0.05) was smaller in strips from pre-eclamptic women. Although 8-bromoadenosine-3', 5'-cyclic monophosphorothioate (Sp-isomer: Sp-8-Br-cAMPS, 0.1 - 0.3 mM), a phosphodiesterase (PDE)-resistant activator of adenosine-3',5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase, concentration-dependently attenuated the contraction induced by STA(2) in strips from both groups, the sensitivity (for pre-eclampsia: pD(2)=3.68+/-0.04, n=5 and for normotensive pregnant women: 3.94+/-0.09, n=7, P:=0.02) was lower and the maximum response (64.2+/-2.4% vs 74.9+/-4.4%, P:<0.05) was smaller in pre-eclampsia. In beta-escin-skinned strips, the pD(2) value for the contraction-inducing effect of Ca(2+) did not differ significantly between the two groups (for pre-eclampsia, n=6; for normotensive pregnant women, n=6). Thus, omental resistance arteries from human subjects with pre-eclampsia showed (i) a weaker H(2)-receptor-mediated relaxation to histamine and (ii) a weaker cyclic AMP-analogue-induced relaxation, suggesting that the reduced action of histamine may be partly due to a decreased effect of cyclic AMP.

Actions of Mn(2+) in the presence of ionophore A23187 in taenia coli of guinea-pig

In this work, we have investigated whether Mn(2+)enters the cytoplasm in the presence of ionophore for divalent cations, A23187 and induces contraction of taenia coli. In Ca(2+)-free, 60 m m K(+)medium, the application of 5 m m Mn(2+)evoked contraction and a concomitant increase in Mn(2+)influx into the cytoplasm. However, in Ca(2+)-free medium, the application of 5 m m Mn(2+)in the presence of 1x10(-5)m A23187 did not evoke both contraction and Mn(2+)influx. These results suggest that Mn(2+)penetrates through Ca(2+)channels in a state of membrane depolarization with K(+)and activates the contractile proteins in taenia coli. However, Mn(2+)does not enter the cytoplasm in the presence of ionophore A23187 and then does not induce contraction.

Molecular analysis of non-specific supersensitivity induced by AF64A in rat iris smooth muscle

Characteristics of supersensitivity induced by the pretreatment with AF64A, an inhibitor of choline uptake at parasympathetic nerve endings, were examined in rat iris sphincter. In preparations isolated and skinned by beta-escin after the micro injection of AF64A to eyes in vivo, the amplitude of maximum contraction in pCa 4.5 solution was increased by 180% of the control from the contralateral eyes. The Ca2+ sensitivity of the contractile system was slightly but significantly increased by AF64A injection; the half maximum contraction was obtained at pCa 5.87 and 6.05 in the control and AF64A-injected eyes, respectively. The increase in maximum contraction in AF64A injected ones was neither affected by the addition of calmodulin, GTPgammaS nor H-7. The increase in Ca2+ sensitivity by AF64A injection was not affected by calmodulin, enhanced by GTPgammaS and abolished by H-7. AF64A injection increased the total protein content only by 30% of the control. The contents of contractile proteins per iris were quantified using Western blotting with monoclonal antibodies. The contents of actin and calponin were increased by AF64A, whereas those of myosin, calmodulin and caldesmon were not affected. The results indicate that AF64A-induced enhancement of the maximum contraction is not mainly due to the increase in the contents of major contractile proteins and that the increase in Ca2+ sensitivity could be due to the mechanism in which changes in protein kinase C and/or GTP binding protein activity are involved.

PC12 cells transfected with a C-terminal fragment of the amyloid precursor protein (APP C-100), exhibit enhanced sensitivity to the calcium ionophore A23187, and diminished sensitivity to hydrogen peroxide

Extracellular neuritic plaques are a hallmark of Alzheimer's disease. The core protein of plaques is Abeta, a 39-43 amino acid peptide derived from the amyloid precursor protein (APP). APP C-100 is a C-terminal fragment of APP, 100 amino acids long, whose sequence includes Abeta. To determine whether APP C-100 expression alters cellular vulnerability to calcium and H(2)O(2), rat PC12 cells were modified to overexpress APP C-100. Cellular survival (as measured in the MTT assay) was determined as a function of concentration for the calcium ionophore, A23187, and for H(2)O(2) in APP C-100 transfectants and vector-transfected controls. APP C-100 expression significantly increased cellular vulnerability to A23187, and decreased vulnerability to H(2)O(2).

Acetylcholine-induced membrane potential changes in endothelial cells of rabbit aortic valve

1. Using a microelectrode technique, acetylcholine (ACh)-induced membrane potential changes were characterized using various types of inhibitors of K+ and Cl- channels in rabbit aortic valve endothelial cells (RAVEC). 2. ACh produced transient then sustained membrane hyperpolarizations. Withdrawal of ACh evoked a transient depolarization. 3. High K+ blocked and low K+ potentiated the two ACh-induced hyperpolarizations. Charybdotoxin (ChTX) attenuated the ACh-induced transient and sustained hyperpolarizations; apamin inhibited only the sustained hyperpolarization. In the combined presence of ChTX and apamin, ACh produced a depolarization. 4. In Ca2+-free solution or in the presence of Co2+ or Ni2+, ACh produced a transient hyperpolarization followed by a depolarization. In BAPTA-AM-treated cells, ACh produced only a depolarization. 5. A low concentration of A23187 attenuated the ACh-induced transient, but not the sustained, hyperpolarization. In the presence of cyclopiazonic acid, the hyperpolarization induced by ACh was maintained after ACh removal; this maintained hyperpolarization was blocked by Co2+. 6. Both NPPB and hypertonic solution inhibited the membrane depolarization seen after ACh washout. Bumetanide also attenuated this depolarization. 7. It is concluded that in RAVEC, ACh produces a two-component hyperpolarization followed by a depolarization. It is suggested that ACh-induced Ca2+ release from the storage sites causes a transient hyperpolarization due to activation of ChTX-sensitive K+ channels and that ACh-activated Ca2+ influx causes a sustained hyperpolarization by activating both ChTX- and apamin-sensitive K+ channels. Both volume-sensitive Cl- channels and the Na+-K+-Cl- cotransporter probably contribute to the ACh-induced depolarization.

Neuronal swelling and surface area regulation: elevated intracellular calcium is not a requirement

Neurons are mechanically robust. During prolonged swelling, molluscan neurons can triple their apparent membrane area. They gain surface area and capacitance independent of extracellular Ca concentration ([Ca]e), but it is unknown if an increase in intracellular Ca concentration ([Ca]i) is necessary. If Ca for stimulating exocytosis is unnecessary, it is possible that swelling-induced membrane tension changes directly trigger surface area readjustments. If, however, Ca-mediated but not tension-mediated membrane recruitment is responsible for surface area increases, swelling neurons should sustain elevated levels of [Ca]i. The purpose of this investigation is to determine if the [Ca]i in swelling neurons attains levels high enough to promote exocytosis and if any such increase is required. Lymnaea neurons were loaded with the Ca concentration indicator fura 2. Calibration was performed in situ using 4-bromo-A-23187 and Ca-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), with free Ca concentration ranging from 0 to 5 microM. Swelling perturbations (medium osmolarity reduced to 25% for 5 min) were done at either a standard [Ca]e or very low [Ca]e level (0.9 mM or 0.13 microM, respectively). In neither case did the [Ca]i increase to levels that drive exocytosis. We also monitored osmomechanically driven membrane dynamics [swelling, then formation and reversal of vacuole-like dilations (VLDs)] with the [Ca]i clamped below 40 nM via 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). [Ca]i did not change with swelling, and VLD behavior was unaffected, consistent with tension-driven, [Ca]i-independent surface area adjustments. In addition, neurons with [Ca]i clamped at 0.1 microM via an ionophore could produce VLDs. We conclude that, under mechanical stress, neuronal membranes are compliant by virtue of surface area regulatory adjustments that operate independent of [Ca]i. The findings support the hypothesis that plasma membrane area is regulated in part by membrane tension.

Basal intracellular free Mg2+ concentration in smooth muscle cells of guinea pig tenia cecum: intracellular calibration of the fluorescent indicator furaptra

Longitudinal muscle strips dissected from tenia cecum of guinea pig were loaded with the Mg2+ indicator, furaptra, and the relation between the fluorescent ratio signal (R) and cytoplasmic free Mg2+ concentration ([Mg2+]i) was studied in smooth muscle cells at 25 degrees C. After the application of ionophores (4-bromo-A23187, monensin, and nigericin), a small immediate offset of R (deltaRjump) was followed by a slow change in R (deltaRslow), which reached a steady level within 2-5 h. The deltaRjump was independent of Mg2+ concentration in solution ([Mg2+]o), and was thought to be unrelated to the change in [Mg2+]i. The direction of the deltaRslow depended on [Mg2+]o with a reversal at approximately 1 mM [Mg2+]o. The intracellular calibration curve was constructed from the steady levels of deltaRslow, and the dissociation constant was 5.4 mM. With the intracellular calibration curve and correction for the deltaRjump, basal [Mg2+], was estimated to be 0.98 +/- 0.05 mM (mean +/- SE, n = 12). When the same calibration was applied to A7r5 cells and rat ventricular myocytes, estimates of basal [Mg2+]i of these cells were 0.74 +/- 0.02 mM (n = 33) and 1.13 +/- 0.06 mM (n = 9), respectively. These results suggest that the basal [Mg2+] level is approximately 1 mM at least in some types of smooth muscle cells, as generally found in striated muscles.

Irsogladine inhibits ionomycin-induced decrease in intercellular communication in cultured rabbit gastric epithelial cells

Effects of irsogladine on ionomycin-induced decrease in intercellular communication and increase in intracellular concentration of Ca2+ ([Ca2+]i) were investigated in cultured rabbit gastric epithelial cells. Ionomycin (10(-7)-10(-6) M) transiently and concentration-dependently inhibited intercellular communication concomitantly with the elevation of [Ca2+]i in the presence and absence of extracellular Ca2+. Irsogladine (10(-5) M), which has been shown to facilitate intercellular communication, suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. The suppression of the ionomycin effects by irsogladine was independent of extracellular Ca2+. TMB-8 [8-(diethylamino)octyl-3,4,5-trimethoxy-benzoate hydrochloride] (10(-6) M) also suppressed the ionomycin-induced elevation of [Ca2+]i and decrease in intercellular communication. These results indicate that the ionomycin-induced decrease in intercellular communication may be due to Ca2+ mobilization from intracellular stores. Inhibitory effects of irsogladine and TMB-8 on the ionomycin-induced decrease in intercellular communication may be produced by suppressing Ca2+ mobilization.

Modification of endothelium-dependent relaxation by propofol, ketamine, and midazolam

Since volatile anesthetics, barbiturates, and local anesthetics have been reported to inhibit endothelium-dependent relaxation, we hypothesized that any drug with anesthetic action would suppress this relaxation. In the present study, using rat thoracic aortae, we attempted to determine whether nonbarbiturate intravenous anesthetics, including midazolam, propofol, and ketamine, suppress endothelium-dependent relaxation, and to clarify the mechanism(s) involved. Acetylcholine-induced, endothelium-dependent relaxation was significantly attenuated by propofol and ketamine, but was unaffected by midazolam. Sodium nitroprusside (SNP)-induced relaxation was attenuated by propofol, but not by midazolam or ketamine. The acetylcholine-stimulated 3',5'-cyclic guanosine monophosphate (cGMP) level was reduced by pretreatment with propofol and ketamine but not by midazolam, and that stimulated by SNP was reduced by propofol but not by ketamine or midazolam. We conclude that propofol and ketamine suppress endothelium-dependent relaxation, whereas midazolam has no influence. Moreover, the suppressive effect of ketamine on endothelium-dependent relaxation is mediated by suppression of nitrous oxide (NO) formation, whereas that of propofol may be mediated at least partly by suppression of NO function.

Methylmercury induces Ca(2+)-dependent hyperpolarization of mouse thymocytes: a flow cytometric study using fluorescent dyes

The effect of methylmercury on mouse thymocytes was examined using fluorescent dyes for membrane potential and intracellular Ca2+. Methylmercury at concentrations of 1 microM or higher (up to 30 microM) produced hyperpolarization in a dose-dependent fashion. Charybdotoxin and quinine, but not 4-aminopyridine and tetraethylammonium, greatly suppressed methylmercury-induced hyperpolarization. Removal of external Ca2+ reduced the degree of hyperpolarization. Pretreatment of thymocytes with A23187 under Ca(2+)-free conditions abolished the hyperpolarization induced by methylmercury. Under both normal and Ca(2+)-free conditions methylmercury increased the intracellular concentration of Ca2+. The results suggest that the increase in intracellular Ca2+ is mediated through a Ca2+ release from intracellular stores as well as through influx of external Ca2+. Therefore, it is likely that methylmercury increases the intracellular concentration of Ca2+, resulting in activation of Ca(2+)-dependent K+ conductance of mouse thymocytes.

All-or-none augmentation of Ca2+ sensitivity in alpha-toxin-permeabilized single smooth muscle cells from guinea-pig taenia caecum

1. Isolated smooth muscle cells from guinea-pig taenia caecum were permeabilized by use of Staphylococcus aureus alpha-toxin, and the sarcoplasmic reticulum Ca2+ store was depleted by exposure to 0.1 microM A23187. 2. Shortening of alpha-toxin-permeabilized single smooth muscle cells was induced by increasing free Ca2+ but was not induced by 0.2 microM free Ca2+. 3. Shortening of the permeabilized cells was caused by application of acetylcholine (ACh) with free Ca2+ concentration held at 0.2 microM. Permeabilized smooth muscle cells responded to 0.3 microM or 1 microM ACh with 0.2 microM Ca2+ with maximal shortening. The concentration-response relationship to ACh had a very steep slope and the cell shortening appeared to be an all-or-none response rather than a graded response, as was the shortening of intact cells to ACh. 4. The shortening of permeabilized cells was also induced by application of guanosine 5'-triphosphate (GTP) with 0.2 microM free Ca2+, showing an all-or-none response. The threshold concentration of GTP that induced an all-or-none response was between 10 microM and 30 microM. 5. These results suggest that Ca2+ sensitivity is augmented by stimulation of the muscarinic receptor or GTP-binding protein(s) in an all-or-none manner. It seems probable that this contributes to the all-or-none response to ACh in intact smooth muscle cells.

Action of heparin and ruthenium red on responses of reversibly-permeabilised rat mesenteric arteries

Heparin and ruthenium red were introduced intracellularly into rat mesenteric resistance arteries via reversible-permeabilisation. Heparin and ruthenium red inhibited contractile responses to noradrenaline, but not caffeine in Ca(2+)-free conditions. Neither heparin nor ruthenium red significantly inhibited peak contractile responses to K+, noradrenaline or caffeine in physiological saline, although heparin significantly increased the time taken for peak force to develop in response to noradrenaline. Noradrenaline and calcium concentration-response relationships were unaffected by heparin. Experiments with permeabilised, fura-2 loaded vessels indicated that heparin inhibited Ca2+ release induced by noradrenaline, but did not inhibit caffeine-induced Ca2+ release. The peak rise in intracellular Ca2+ following K+, or noradrenaline in physiological saline was unaffected by heparin. The use of reversible permeabilisation may prove a useful approach, allowing introduction of a variety of membrane-impermeant blockers of second messenger systems into intact resistance arteries.

Effect of tri-n-butyltin on intracellular Ca2+ concentration of mouse thymocytes under Ca(2+)-free condition

Effect of tri-n-butyltin at concentrations ranging from 100 nM to 1 microM on the intracellular Ca2+ concentration of mouse thymocytes was examined under Ca(2+)-free conditions in comparison with those of 50 nM A23187, 100 nM thapsigargin and 10 microM cyclopiazonic acid, using the fluorescent dye for intracellular Ca2+, fluo-3. Tri-n-butyltin persistently increased the intensity of fluo-3 fluorescence while A23187, thapsigargin and cyclopiazonic acid produced a transient augmentation of the fluorescence. Pretreatment with A23187 greatly decreased the fluorescence responses induced by 1 microM tri-n-butyltin. However, the effect of thapsigargin and cyclopiazonic acid on the tri-n-butyltin-induced response was much weaker than that of A23187. In the presence of tri-n-butyltin, the transient response produced by A23187 was greatly prolonged. Results may suggest that tri-n-butyltin increases the membrane Ca2+ permeability of the intracellular organelles (cellular calcium stores) and decreases the Ca2+ pump activity of thymocyte membrane, resulting in a sustained increase in the intracellular Ca2+ concentration under Ca(2+)-free concentration.

Tetrahexylammonium ions increase Ca2+ sensitivity of contraction of guinea-pig ileal smooth muscle

Effects of tetraalkylammonium ions, having tetraalkyl chains of increasing length from ethyl to octyl, on inositol-trisphosphate (InsP3)-induced Ca2+ release and contractile mechanics were examined in guinea-pig skinned ileal smooth muscle longitudinal strips. Although tetrahexylammonium ions (THexA) appeared to be the most potent inhibitor of Ca2+ release among the tetraalkylammonium ions examined, an additional and more prominent effect was found, i.e., the contraction induced by Ca2+ release showed a large sustained component in the presence of THexA. Potentiation of the contraction by THexA (above 30 microM) was also observed in skinned fibers in which the sarcoplasmic reticulum function was destroyed by treatment with A23187. The potentiating effect of THexA was the most potent by far among the tetraalkylammonium ions examined and was elicited by Ca(2+)-dependent and GTP-binding-protein-independent mechanisms. The potentiation was not due to activation of myosin light-chain kinase. The selective inhibitors of myosin light-chain kinase, protein kinase C and calmodulin reduced THexA-induced potentiation of contraction only at concentrations above 30 microM, at which non-specific effects are likely. Furthermore, relaxation induced by changing pCa from 4.5 to 8.5 was not affected by 1 mM THexA, suggesting that the potentiating effect is not mainly due to inhibition of myosin light-chain phosphatase. In conclusion, ThexA sensitizes guinea-pig skinned ileal smooth muscle to Ca2+ in a structure-selective manner. This sensitization appears not to be mediated mainly by a GTP-binding protein, by activation of myosin light-chain kinase or protein kinase C, by enhanced Ca2+ binding to calmodulin, or by inhibition of myosin light-chain phosphatase.

Contractile responses to histamine and GTP gamma S in beta-escin-treated skinned smooth muscle of guinea pig ileum

To characterize the calcium (Ca2+)-releasing effects of histamine and GTP gamma S, the drug-induced tension developments were measured in beta-escin-treated skinned longitudinal smooth muscle of guinea pig ileum. Intracellular Ca2+ stores were loaded with Ca2+ by incubating the muscle for 10 min in a Ca(2+)-containing solution. Histamine (10-100 microM), applied after Ca(2+)-loading, produced a transient rise in tension. The effect of histamine was not preserved after treatment with 20 mM caffeine, a Ca(2+)-store releaser. The effect of histamine was potentiated by GTP; inhibited by GDP beta S, an antagonist of GTP for binding to G-proteins; or heparin, an antagonist of inositol 1,4,5-trisphosphate (IP3) for binding to its receptor; and mimicked by IP3. When GTP gamma S (20 microM) was applied and continued to be present for 15 min, a transient rise in tension followed by a small, sustained rise in tension was elicited. The effect of GTP gamma S was completely inhibited by GDP beta S. The initial, transient component of the biphasic GTP gamma S response was abolished or markedly inhibited after treatment with caffeine, heparin or the calcium ionophore A23187. The present results suggest that histamine and GTP gamma S cause a release of Ca2+ from caffeine-sensitive stores which is mediated by IP3 formed through a G-protein-coupled mechanism. The GTP gamma S-induced Ca2+ release is not considered to involve such an IP3-independent process as described in chemically-skinned arterial muscle.

Effects of dotarizine on 45Ca2+ movements and contractile responses in vascular smooth muscle

The inhibitory effects of dotarizine on 45Ca2+ movements and contractile responses were studied and compared, using the same parameters measured in rabbit aorta and basilar smooth muscle. Dotarizine 10(-9)-10(-5) M inhibited the contractile responses induced by high K+ (80 mM), noradrenaline (10(-6) M) or 5-hydroxytryptamine (5-HT, 10(-5) M). These effects were observed when dotarizine was added before or after the induced contractions and were more potent in basilar arteries than in aorta. Moreover, dotarizine at concentrations less than 10(-6) M did not modify the contractile response obtained in aorta rings. Contractile responses induced by the addition of Ca2+ to Ca(2+)-free high-K+ solution were also concentration dependently inhibited by dotarizine 10(-7)-10(-6) M in aorta and basilar arteries. Dotarizine also inhibited the contractile response induced by caffeine (20 mM) in aortic rings incubated in normal or in Ca(2+)-free medium. Dotarizine reduced the 45Ca(2+) uptake stimulated by high K+, noradrenaline or 5-HT even in the aorta or basilar artery, but the inhibition was greater in basilar arteries than in aorta. These results suggest that, in rabbit, dotarizine inhibits Ca2+ entry through Ca2+ channels, being more selective for the basilar artery, probably by acting on multiple sites to decrease the availability of intracellular free Ca2+ required for activation.

Functional roles of cyclic guanosine 3',5'-monophosphate analogue in cerebral vasodilation

1. Vasodilating effects of cyclic nucleotides in cerebral vasculature were examined using membrane permeable cyclic nucleotide analogues, 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP). 2. In isolated canine basilar artery (CBA), 8-Br-cGMP but not 8-Br-cAMP, significantly inhibited Ca(2+)-induced and agonist [serotonin(5-HT), prostaglandin(PG)F2 alpha or endothelin]-induced contraction, in a concentration-dependent manner. 3. When Ca2+ was depleted from intracellular store sites by pretreatment with A23187, 8-Br-cGMP but not 8-Br-cAMP strongly attenuated contractions induced by Ca(2+)-influx. 4. Neither 8-Br-cGMP nor 8-Br-cAMP modified contraction induced by caffeine which elicits Ca2+ release from intracellular Ca2+ store. 5. 8-Br-cGMP lowered the high K(+)-induced sustained [Ca2+]i elevation. 6. These results suggest that, at least in CBA, cGMP exerts its inhibitory effect on the contraction induced by influx of Ca2+, by reducing the level of [Ca2+]i and reducing [Ca2+]i sensitivity of the contractile machinery.

Characteristic features of noradrenaline-induced Ca2+ mobilization and tension in arterial smooth muscle of the rabbit

1. Effects of noradrenaline (NAd) on changes in cellular Ca2+ concentration ([Ca2+]i) and tension were investigated, and these effects were compared with those evoked by 128 mM K+ or caffeine in intact smooth muscle strips or by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) or caffeine in beta-escin-treated chemically skinned smooth muscle strips of the rabbit mesenteric artery. 2. In physiological solution containing 2.6 mM Ca2+, application of 128 mM K+ or 10 microM NAd produced a phasic, followed by a tonic increase in [Ca2+]i and tension. NAd (10 microM) produced a larger tonic tension than did 128 mM K+ but a smaller increase in [Ca2+]i. When the [Ca2+]i-tension relationship was observed in ionomycin- and 128 mM K(+)-treated muscle strips, 10 microM NAs shifted the relationship to the left and enhanced the maximum amplitude of contraction. These results suggest that NAd increases the sensitivity of contractile proteins to Ca2+ in smooth muscle of the rabbit mesenteric artery. 3. Noradrenaline (10 microM) or caffeine (10 mM), but not 128 mM K+, produced a phasic increase in both [Ca2+]i and tension in Ca(2+)-free solution containing 2 mM EGTA. When 10 mM caffeine had been applied in Ca(2+)-free solution, subsequent application of 10 microM NAd did not increase [Ca2+]i. By contrast, when 10 microM NAd had been applied in Ca(2+)-free solution, subsequent application of 10 mM caffeine still increased [Ca2+]i. Ryanodine (50 microM) abolished the increase in [Ca2+]i induced by 10 mM caffeine or 10 microM NAd in intact and in skinned smooth muscle strips. These results suggest that NAd releases Ca2+ from the ryanodine-sensitive Ca2+ storage sites. 4. Noradrenaline (10 microM) synthesized Ins(1,4,5)P3 in Ca(2+)-free solution in intact smooth muscle strips. Following application of 10 microM NAd, a relatively long time lag (around 1 s) was always observed before the initiation of the increase in [Ca2+]i whether in the presence or absence of Ca2+. The maximum rate of rise of [Ca2+]i induced by 10 mM caffeine was much larger than that induced by 10 microM NAd in Ca(2+)-containing or Ca(2+)-free solution (containing 2 mM EGTA). Both [Ca2+]i and tension reached their peak in a shorter time with caffeine (10 mM) than with 10 microM NAd. In Beta-escin-treated skinned smooth muscle strips, 20 microM Ins(1,4,5)P3 10 mM caffeine or 10 microM NAd increased Ca2+ in Ca(2+)-free solution following brief application of 0.3 microM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytosolic calcium increase in coronary endothelial cells after H2O2 exposure and the inhibitory effect of U78517F

1. Cytosolic calcium concentrations ([Ca2+]i) were determined with fura-2 on both resting (unstimulated) and A23187-stimulated coronary endothelial cells following injury by hydrogen peroxide (H2O2). 2. Treatment of cells with H2O2 (10(-4) M) caused an increase in the resting [Ca2+]i, which reached a maximum of five fold after 3 h. 3. The increase in resting [Ca2+]i was significantly attenuated by treatment with U78517F, a potent inhibitor of lipid peroxidation, at a concentration of 10(-6) M or greater. Catalase (50 u ml-1) also markedly inhibited the H2O2-induced rise in [Ca2+]i. Pretreatment with verapamil (10(-5) M), nifedipine (10(-6) M) or diltiazem (10(-5) M) had no effect on the increase in [Ca2+]i following addition of H2O2. 4. A23187 produced a transient increase in [Ca2+]i followed by a sustained plateau. The initial peak and plateau phase responses to A23187 were augmented by H2O2. This augmentation of [Ca2+]i was suppressed by U78517F or catalase but not by Ca-entry blockers. 5. Thus, it is likely that lipid peroxidation plays a critical role in the sustained increase in [Ca2+]i that occurs following treatment with H2O2 and that this continues in the presence of agonists which stimulate the endothelium. Voltage-gated Ca2+ channels do not seem to be involved in the genesis of cellular damage associated with sustained large increases in [Ca2+]i.

Effects on the rabbit coronary artery of LP-805, a new type of releaser of endothelium-derived relaxing factor and a K+ channel opener

In the rabbit epicardial coronary artery, 8-tert-butyl-6,7-dihydropyrolo[3,2-e]5-methylpyrazolo [1,5-a]pyrimidine-3-carbonitrile (LP-805, greater than 0.1 microM) hyperpolarized the muscle membrane in both proximal (diameter, 1-1.2 mm) and distal (diameter, 0.1-0.2 mm) regions of intact (+E) tissue, in which endothelium is present, and endothelium-denuded (-E) tissue. LP-805-induced hyperpolarization was inhibited by glibenclamide. In -E tissues in both regions, acetylcholine (ACh, greater than 0.1 microM) depolarized the membrane, and LP-805 inhibited the depolarization. However, in +E tissues, ACh (greater than 0.1 microM) transiently hyperpolarized the membrane that was not modified by glibenclamide (10 microM), charybdotoxin (100 nM), and NG-nitro-L-arginine (L-NNA, 100 microM). In -E tissues of both regions, LP-805 consistently inhibited the 10 microM ACh-induced contraction (IC50, 2.8 microM), and 10 microM glibenclamide shifted this concentration-response curve to the right (IC50, 20 microM). In +E tissues, LP-805 more potently inhibited the ACh-induced contraction (IC50, 0.3 microM), and this inhibition was prevented by L-NNA (100 microM) but not by indomethacin or glibenclamide (10 microM). In -E and +E tissues of both regions, LP-805 repolarized the high K(+)-induced depolarization (less than 20 mM) and relaxed the tissues precontracted by high K+ (less than 30 mM); these electrical and mechanical effects of LP-805 were prevented by glibenclamide (10 microM) in +E tissues. In +E tissues, the K(+)-induced contraction (less than 30 mM) was more strongly inhibited than in -E tissues, but after treatment with L-NNA, LP-805 relaxed -E and +E tissues precontracted to the same extent in the presence of high K+. LP-805 (10 microM) did not inhibit the Ca(2+)-induced contraction in skinned muscle tissues but did slightly inhibit the ACh-induced contraction in Ca(2+)-free solution containing 2 mM EGTA. Thus, LP-805 has a potent releasing action on endothelium-derived relaxing factor and also the potential to open the glibenclamide-sensitive K+ channel. These events would account for the dilation of the rabbit coronary artery exposed to LP-805.

Mechanisms of vasodilation induced by NKH477, a water-soluble forskolin derivative, in smooth muscle of the porcine coronary artery

To study the mechanism of vasodilation induced by 6-(3-dimethylaminopropionyl) forskolin (NKH477), a water-soluble forskolin derivative, its effects on the acetylcholine (ACh)-induced contraction of muscle strips of porcine coronary artery were examined. [Ca2+]i, isometric force, and cellular concentrations of cAMP and inositol 1,4,5-trisphosphate were measured. NKH477 (0.1-1.0 microM), isoproterenol (0.01-0.1 microM), or forskolin (0.1-1.0 microM) increased cAMP and attenuated the contraction induced by 128 mM K+ or 10 microM ACh in a concentration-dependent manner. These agents, at concentrations up to 0.3 microM, did not change the amount of cGMP. NKH477 (0.1 microM) attenuated the contraction induced by 128 mM K+ without corresponding changes in the evoked [Ca2+]i responses. ACh (10 microM) produced a large phasic increase followed by a small tonic increase in [Ca2+]i and produced a sustained contraction. The ACh-induced phasic increase in [Ca2+]i, but not the tonic increase, disappeared after application of 0.1 microM ionomycin. NKH477 (0.1 microM) attenuated both the increase in [Ca2+]i and the force induced by 10 microM ACh in muscle strips that were not treated with ionomycin and inhibited the ACh-induced contraction without corresponding changes in [Ca2+]i in ionomycin-treated muscle strips. These results suggest that NKH477 inhibits ACh-induced Ca2+ mobilization through its action on ionomycin-sensitive storage sites. In ionomycin-treated and 128 mM K(+)-treated muscle strips, 0.1 microM NKH477 shifted the [Ca2+]i-force relation to the right in the presence or absence of 10 microM ACh. In beta-escin-skinned smooth muscle strips, 0.1 microM NKH477 shifted the pCa-force relation to the right but had no effects on Ca(2+)-independent contraction. We conclude that in smooth muscle of porcine coronary artery, NKH477 inhibits ACh-induced contraction by both attenuating ACh-induced Ca2+ mobilization and reducing the sensitivity of the contractile machinery to Ca2+, possibly by activating cAMP-dependent mechanisms.

Receptor-coupled shortening of alpha-toxin-permeabilized single smooth muscle cells from the guinea-pig stomach

1. Isolated single smooth muscle cells from the fundus of the guinea-pig stomach were permeabilized by use of Staphylococcus aureus alpha-toxin. Receptor-coupled shortening of individual cells was monitored under phase contrast microscopy. 2. Most of the isolated cells responded to 0.6 microM Ca2+, but not to 0.3 microM Ca2+, with a resulting maximal shortening to approximately 65% of the resting cell length. The contractile activity of these permeabilized cells lasted for several hours and repeated shortening was readily achieved after washing out. 3. Addition of acetylcholine (ACh) at a maximal concentration (10 microM) resulted in a marked decrease in the concentration of Ca2+ required to trigger a threshold response from 0.6 microM to 0.2 microM, and 1 mM guanosine 5'-diphosphate (GDP) blocked this decrease. Moreover, treatment with 100 microM guanosine 5'-triphosphate (GTP) mimicked the action of ACh. 4. Addition of 100 microM inositol 1,4,5-trisphosphate (InsP3) with 0.2 microM Ca2+ did not cause cell shortening, whereas 10 microM ACh with 0.2 microM Ca2+ did, suggesting that InsP3-induced Ca2+ release is not involved in ACh-operated cell shortening. 5. The present study demonstrates an alpha-toxin-permeabilized single smooth muscle cell preparation which retains its receptor function and also provides an insight into mechanisms leading to augmentation of Ca2+ sensitivity by stimulation of muscarinic receptors or GTP-binding proteins.

Effects of cyclopiazonic acid, a novel Ca(2+)-ATPase inhibitor, on contractile responses in skinned ileal smooth muscle

1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR) of skeletal and cardiac muscles, on contractile responses induced by Ca(2+)-release from intracellular storage sites were examined in the longitudinal smooth muscle strip of the guinea-pig ileum skinned with beta-escin. 2. Ca(2+)-loading of storage sites (Ca(2+)-uptake) was performed in pCa 6.3 solution. The amount of Ca2+ taken up was monitored by use of the amplitude of contraction following application of 25 mM caffeine or 25 microM inositol 1,4,5-trisphosphate (IP3). 3. Contractile responses to caffeine or IP3 were reduced or abolished when the preceding Ca(2+)-uptake was performed in the presence of 0.1-10 microM CPA. The dose of CPA required to inhibit the contraction induced by caffeine or IP3 by 50% was approximately 0.6 microM. The CPA-sensitive Ca(2+)-uptake completely depended upon the presence of ATP in the solution during Ca(2+)-uptake. 4. When 1 microM CPA was added after Ca(2+)-uptake, the subsequent caffeine- or IP3-induced contraction was not significantly affected by the presence of CPA. 5. Acetylcholine-induced contraction was also almost abolished when the preceding Ca(2+)-uptake was performed in the presence of 10 microM CPA. 6. The relationship between pCa and contraction was not affected by the presence of 10 microM CPA in skinned fibres where Ca2+ storage sites had been destroyed by treatment with A23187. The enhancement of contraction in pCa 6.0 solution by calmodulin was not affected by 10 microM CPA.7. These results suggest that CPA selectively inhibits ATP-dependent Ca2"-uptake into intracellular storage sites in skinned ileal smooth muscle strips. CPA appears to be a potent, reversible, and very specific inhibitor of the Ca2+-pump in the storage sites of smooth muscle, and is an extremely valuable pharmacological tool.

Membrane hyperpolarization inhibits agonist-induced synthesis of inositol 1,4,5-trisphosphate in rabbit mesenteric artery

1. Effects of membrane hyperpolarization induced by pinacidil on Ca2+ mobilization induced by noradrenaline (NA) were investigated by measuring intracellular Ca2+ concentration ([Ca2+]i), isometric tension, membrane potential and production of inositol 1,4,5-trisphosphate (IP3) in smooth muscle cells of the rabbit mesenteric artery. 2. Pinacidil (0.1-10 microM) concentration dependently hyperpolarized the smooth muscle membrane with a reduction in membrane resistance. Glibenclamide (1 microM) blocked the membrane hyperpolarization induced by 1 microM-pinacidil. NA (10 microM) depolarized the smooth muscle membrane with associated oscillations. Pinacidil (1 microM) inhibited this response and glibenclamide (1 microM) prevented the action of pinacidil on both the NA-induced events. 3. In thin smooth muscle strips, 10 microM-NA produced a large phasic and a subsequent small tonic increase in [Ca2+]i with associated oscillations. These changes in [Ca2+]i seemed to be coincident with phasic, tonic and oscillatory contractions, respectively. Pinacidil (0.1-1 microM) inhibited the increases in [Ca2+]i and in tension induced by NA, but not by 128 mM-K+. Glibenclamide inhibited these actions of pinacidil. Pinacidil (1 microM) also inhibited the contraction induced by 10 microM-NA in strips treated with A23187 (which functionally removes cellular Ca2+ storage sites), suggesting that membrane hyperpolarization inhibits Ca2+ influxes activated by NA. 4. In Ca2(+)-free solution containing 2 mM-EGTA, NA (10 microM) transiently increased [Ca2+]i, tension and synthesis of IP3. Pinacidil (over 0.1 microM) inhibited the increases in [Ca2+]i, tension and synthesis of IP3 induced by 10 microM-NA in Ca2(+)-free solution containing 5.9 mM-K+, but not in a similar solution containing 40 or 128 mM-K+. Glibenclamide (1 microM) inhibited these actions of pinacidil. These inhibitory actions of pinacidil were still observed in solutions containing low Na+ or low Cl-. These results suggest that pinacidil inhibits NA-induced Ca2+ release from storage sites through an inhibition of IP3 synthesis resulting from its membrane hyperpolarizing action. 5. In beta-escin-treated skinned strips, NA (10 microM) or IP3 (20 microM) increased Ca2+ in Ca2(+)-free solution containing 50 microM-EGTA and 3 microM-guanosine triphosphate (GTP) after brief application of 0.3 microM-Ca2+, suggesting Ca2+ is released from intracellular storage sites. Heparin (500 micrograms/ml, an inhibitor of the IP3 receptor), but not pinacidil (1 microM) or glibenclamide (1 microM), inhibited the Ca2+ release from storage sites induced by NA or IP3. These results suggest that membrane hyperpolarization is essential for the inhibitory action of pinacidil on the NA-induced Ca2(+)-releasing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

The relaxing effects of barbiturates in vascular smooth muscle of rat aorta

The effects of thiamylal and pentobarbital on contractions mediated through the influx of extracellular Ca(++) and the release of intracellularly stored Ca(++) were compared in rat aortic strips. Thiamylal (3 x 10(-5) M to 10(-3) M) and pentobarbital (10(-4) to 10(-3) M) significantly attenuated the contraction induced by KCl (20 mM), and shifted the dose-response curve for Ca(++) of KCl (20 mM)-treated strips downwards and to the right. Caffeine (10(-2) M)-induced contraction was significantly attenuated by thiamylal at concentrations greater than 10(-4) M and by pentobarbital at 3 x 10(-4) M. Only a high concentration (10(-3) M) of these barbiturates significantly inhibited the contractions induced by norepinephrine (NE, 10(-6) M) in Ca(++)-free medium. Contraction of strips without endothelium by a Ca(++) ionophore, A23187 (5 x 10(-6) M), in the presence of a Ca channel blocker, was relaxed by high concentrations of thiamylal (3 x 10(-4) M to 10(-3) M) and pentobarbital (10(-3) M). It is concluded that thiamylal inhibits contraction through an intracellular action as well as a Ca channel-blocking action in vascular smooth muscle of rat aorta. However, the intracellular action of pentobarbital is less potent than that of thiamylal.

Paradoxical effects of thioridazine on electromechanical coupling in the human and rat vas deferens

The effects of thioridazine on the responses of isolated human and rat vas deferens to high [K+]0, A23187 and caffeine were examined. In the presence of Ca2+ (2.5 mM), thioridazine (1-10 microM) induced spontaneous contractions but caused a dose-related inhibition of the phasic and secondary parts of the response to high [K+]0 (136 mM). The relaxation phase of the high [K+]0 response of the human vas deferens was unaffected by thioridazine (up to 10 microM). In Ca2(+)-free/EGTA (0.5 mM) media, thioridazine caused a dose-related potentiation, shortened the latency and prolonged the duration of high [K+]0 responses. Contractions to caffeine (20 mM) and A23187 (20-50 microM) were relatively unchanged by thioridazine (10 microM). The spontaneous activity caused by thioridazine (10 microM) was sensitive to the Ca2(+)-channel blockers nifedipine (10 microM) or verapamil (10 microM). These results indicate that the action of thioridazine during electromechanical coupling in the human and rat vas deferens may involve more than its blockade of voltage gated Ca2+ channels.

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

ATP activates cationic currents and modulates the calcium current through GTP-binding protein in rabbit portal vein

1. Effects of adenosine 5'-triphosphate (ATP) on ionic currents of dispersed smooth muscle cells of the rabbit portal vein were investigated using the voltage-clamp procedure. 2. ATP (greater than or equal to 300 microM) produced transient and maintained inward currents. The former was inactivated within a few seconds, but the latter lasted more than several minutes. The transient but not the maintained current was blocked by pre-treatment with alpha,beta-methylene adenosine 5'-triphosphate (AMP-CPP). The amplitude of the latter was increased by ATP in a concentration-dependent manner. The following investigations were made on the ionic mechanism of the ATP-induced maintained inward current. 3. In 2.5 mM-Ca(2+)-containing tetraethylammonium chloride (TEA-Cl) solution (2.5 mM-Ca(2+)-TEA+ solution), the reversal potential for the ATP-induced inward current was close to the Cl- equilibrium potential, and in 140 mM-Na+ (nominally Ca(2+)-free or 0.3 mM-EGTA-containing) solution, the reversal potential was coincident with the Na+ equilibrium potential. 4. In 2.5 mM-Ca(2+)-TEA+ solution but not in 140 mM-Na+ solution and in physiological salt solution (PSS), niflumic acid (10 microM), a Cl- channel blocker, and Cl(-)-deficient perfusate in the pipette markedly inhibited the ATP-induced inward current. These results imply that in 2.5 mM-Ca(2+)-TEA+ solution the ATP-activated ion channel may admit Ca2+ which then accelerates the Ca(2+)-dependent Cl- current, but in 140 mM-Na+ solution and in PSS this channel may admit only Na+. 5. Intracellular perfusion of guanosine 5'-O-(3-thio triphosphate (GTP gamma S) did not provoke the current, but significantly increased the amplitude of the ATP-induced inward current in 2.5 mM-Ca(2+)-TEA+, 140 mM-Na+ and 2.5 mM-Ba(2+)-containing TEA+ (2.5 mM-Ba(2+)-TEA+) solutions. On the other hand, intracellular perfusion of guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) reduced the amplitude of the ATP-induced inward current in the above solutions. 6. A low concentration of ATP (30 microM) transiently augmented the amplitude of the voltage-dependent Ca2+ current recorded in both 2.5 mM-Ca(2+)-TEA+ solution and PSS, but a high concentration of ATP (3 mM) consistently inhibited the voltage-dependent Ca2+ current in both solutions (4 mM-EGTA in the pipette). Such inhibition was partly prevented by application of 20 mM-EGTA in the pipette.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of the divalent cation ionophore ionomycin on the performance of isolated guinea-pig atria

In isolated electrically driven left and in spontaneously beating right guinea-pig atria, the calcium ionophore ionomycin produced a concentration-dependent positive inotropic and chronotropic effect with a threshold near 10(-7) mol/l and a pD2 of 6.31 +/- 0.09 and 5.94 +/- 0.07, respectively. At low [Ca2+]o (0.5 mmol/l), the positive inotropic effect of ionomycin (3 X 10(-6) mol/l) was strongly attenuated by ryanodine and nifedipine, and slightly attenuated by pindolol and mepyramine; atropine had no effect. The positive chronotropic effect of ionomycin was slightly reduced by cimetidine or pindolol, whereas atropine, nifedipine, and ryanodine showed no inhibitory activity. The oxygen consumption of resting left atria was significantly enhanced by addition of ionomycin. It is concluded that the action of ionomycin involves at least the following mechanisms: I) release of Ca2+ from sarcoplasmic reticulum, II) influx of Ca2+ from the extracellular space, and, having little significance, III) release of catecholamines and histamine from sympathetic nerve endings and tissue mast cells. However, additional mechanisms of action of ionomycin cannot be excluded.

Calcium dependency of the endothelium-dependent hyperpolarization in smooth muscle cells of the rabbit carotid artery

1. In smooth muscle cells of the rabbit carotid artery, ACh (greater than 10(-8) M) generated a hyperpolarization with two components (transient followed by sustained), only in the tissues with an intact endothelium. There were no detectable changes in the membrane potential, as elicited by ACh (up to 10(-5) M) in tissues with no endothelium or in the presence of atropine (10(-6) M). 2. Reduction of [Ca2+]o inhibited the sustained component which was not apparent in [Ca2+]o below 0.16 mM. In Ca2+-free (EGTA-containing) solution, the generation of the transient component of the hyperpolarization remained sustained but with a substantially reduced amplitude. 3. Procaine (greater than 10(-6) M) inhibited the ACh-induced hyperpolarization in a concentration-dependent manner, and at a concentration of procaine (10(-3) M) which caused substantial depolarization of the membrane, no detectable change was elicited by ACh. 4. Caffeine (10(-6)-10(-3) M) produced a transient hyperpolarization, independent of the presence or absence of the endothelium, and inhibited the sustained component of the ACh-induced hyperpolarization more so than the initial component. 5. A23187 (greater than 10(-8) M) hyperpolarized the smooth muscle membrane in a concentration-dependent manner, and this hyperpolarization was not generated in Ca2+-free solution or in the absence of endothelial cells. 6. In intact tissues, pre-treatment with A23187 resulted in a reduction of the subsequently generated ACh-induced hyperpolarization, in an irreversible manner. 7. It would thus appear that in the rabbit carotid artery, the endothelium-dependent hyperpolarization induced by ACh has Ca2+-dependent and Ca2+-independent components, and each may be related to the increase in endothelial [Ca2+]i by release from the intracellular store and by influx from the extracellular medium, respectively. The increased [Ca2+]i would trigger a release of an endothelium-derived hyperpolarizing factor (EDHF) from the endothelial cells.