Mechanisms of vasoconstriction induced by endothelin-1 in smooth muscle of rabbit mesenteric artery

Calcium handling coupled to the endothelin ETA and ETB receptor-mediated vasoconstriction in resistance arteries: Differential regulation by PI3K, PKC and RhoK

Abnormal endothelin-1 (ET-1) activity is involved in the pathogenesis of vascular diseases such as essential and pulmonary arterial hypertension, coronary artery disease, and cerebrovascular disease, blockade of ET receptors having shown efficacy in clinical assays and experimental models of hypertension. Augmented Ca2+ influx and changes in Ca2+ sensitization associated with arterial vasoconstriction underlie increased systemic vascular resistance in hypertension. Since peripheral resistance arteries play a key role in blood pressure regulation, we aimed to determine here the specific Ca2+ signaling mechanisms linked to the ET receptor-mediated vasoconstriction in resistance arteries and their selective regulation by protein kinase C (PKC), Rho kinase (RhoK), the phosphatidylinositol 3-kinase (PI3K) and the mitogen-activated protein kinase (MAPK). ET-1-induced contraction was mediated by the endothelin ETA receptor with a minor contribution of vascular smooth muscle (VSM) endothelin ETB receptors. ET receptor activation elicited Ca2+ mobilization from intracellular stores, extracellular Ca2+ influx and Ca2+ sensitization associated with contraction in resistance arteries. Vasoconstriction induced by ET-1 was largely dependent on activation of canonical transient receptor potential channel 3 (TRPC3) and extracellular Ca2+ influx through nifedipine-sensitive voltage-dependent Ca2+ channels. PI3K inhibition reduced intracellular Ca2+ mobilization and Ca2+ entry without altering vasoconstriction elicited by ET-1, while PKC has dual opposite actions by enhancing Ca2+ influx associated with contraction, and by inhibiting Ca2+ release from intracellular stores. RhoK was a major determinant of the enhanced sensitivity of the contractile filaments underlying ET-1 vasoconstriction, with also a modulatory positive action on Ca2+ influx and intracellular Ca2+ release. Augmented RhoK and PKC activities are involved in vascular dysfunction in hypertension and vascular complications of insulin-resistant states, and these kinases are thus potential pharmacological targets in vascular diseases in which the ET pathway is impaired.

Endothelin-1 in the pathophysiology of obesity and insulin resistance

The association between plasma endothelin-1 (ET-1) and obesity has been documented for decades, yet the contribution of ET-1 to risk factors associated with obesity is not fully understood. In 1994, one of first papers to document this association also noted a positive correlation between plasma insulin and ET-1, suggesting a potential contribution of ET-1 to the development of insulin resistance. Both endogenous receptors for ET-1, ETA and ETB are present in all insulin-sensitive tissues including adipose, liver and muscle, and ET-1 actions within these tissues suggest that ET-1 may be playing a role in the pathogenesis of insulin resistance. Further, antagonists for ET-1 receptors are clinically approved making these sites attractive therapeutic targets. This review focuses on known mechanisms through which ET-1 affects plasma lipid profiles and insulin signalling in these metabolically important tissues and also identifies gaps in our understanding of ET-1 in obesity-related pathophysiology.

G protein-coupled receptor kinases as therapeutic targets in the heart

G protein-coupled receptors (GPCRs) are critical cellular sensors that mediate numerous physiological processes. In the heart, multiple GPCRs are expressed on various cell types, where they coordinate to regulate cardiac function by modulating critical processes such as contractility and blood flow. Under pathological settings, these receptors undergo aberrant changes in expression levels, localization and capacity to couple to downstream signalling pathways. Conventional therapies for heart failure work by targeting GPCRs, such as β-adrenergic receptor and angiotensin II receptor antagonists. Although these treatments have improved patient survival, heart failure remains one of the leading causes of mortality worldwide. GPCR kinases (GRKs) are responsible for GPCR phosphorylation and, therefore, desensitization and downregulation of GPCRs. In this Review, we discuss the GPCR signalling pathways and the GRKs involved in the pathophysiology of heart disease. Given that increased expression and activity of GRK2 and GRK5 contribute to the loss of contractile reserve in the stressed and failing heart, inhibition of overactive GRKs has been proposed as a novel therapeutic approach to treat heart failure.

Diversity and plasticity in signaling pathways that regulate smooth muscle responsiveness: Paradigms and paradoxes for the myosin phosphatase, the master regulator of smooth muscle contraction

A hallmark of smooth muscle cells is their ability to adapt their functions to meet temporal and chronic fluctuations in their demands. These functions include force development and growth. Understanding the mechanisms underlying the functional plasticity of smooth muscles, the major constituent of organ walls, is fundamental to elucidating pathophysiological rationales of failures of organ functions. Also, the knowledge is expected to facilitate devising innovative strategies that more precisely monitor and normalize organ functions by targeting individual smooth muscles. Evidence has established a current paradigm that the myosin light chain phosphatase (MLCP) is a master regulator of smooth muscle responsiveness to stimuli. Cellular MLCP activity is negatively and positively regulated in response to G-protein activation and cAMP/cGMP production, respectively, through the MYPT1 regulatory subunit and an endogenous inhibitor protein named CPI-17. In this article we review the outcomes from two decade of research on the CPI-17 signaling and discuss emerging paradoxes in the view of signaling pathways regulating smooth muscle functions through MLCP.

Inhibitory effect of rhynchophylline on contraction of cerebral arterioles to endothelin 1: role of rho kinase

ETHNOPHARMACOLOGICAL RELEVANCE

Rhynchophylline (Rhy) is a major ingredient of Uncaria rhynchophylla (UR) used to reduce blood pressure and ameliorate brain ailments. This study was to examine the role of Rho kinase (ROCK) in the inhibition of Rhy on contraction of cerebral arterioles caused by endothelin 1 (ET-1).

MATERIALS AND METHODS

Cerebral arterioles of male Wistar rats were constricted with ET-1 for 10 min followed by perfusion of Rhy for 20 min. Changes in the diameters of the arterioles were recorded. The effects of Rhy on contraction of middle cerebral arteries (MCAs) were determined by a Multi-Myograph. Western blotting and immunofluorescent staining were used to examine the effects of Rhy on RhoA translocation and myosin phosphatase target subunit 1 (MYPT1) phosphorylation.

RESULTS

In vivo, Rhy (30-300 µM) relaxed cerebral arterioles constricted with ET-1 dose-dependently. In vitro, Rhy at lower concentrations (1-100 µM) caused relaxation of rat MCAs constricted with KCl and Bay-K8644 (an agonist of L-type voltage-dependent calcium channels (L-VDCCs)). Rhy at higher concentrations (>100 µM) caused relaxation of rat MCAs constricted with ET-1, which was inhibited by Y27632, a ROCK׳s inhibitor. Western blotting of rat aortas showed that Rhy inhibited RhoA translocation and MYPT1 phosphorylation. Immunofluorescent staining of MCAs confirmed that phosphorylation of MYPT1 caused by ET-1 was inhibited by Rhy.

CONCLUSIONS

These results demonstrate that Rhy is a potent inhibitor of contraction of cerebral arteries caused by ET-1 in vivo and in vitro. The effect of Rhy was in part mediated by inhibiting RhoA-ROCK signaling.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Enhancement of myofilament calcium sensitivity by acute hypoxia in rat distal pulmonary arteries

Hypoxic contraction of pulmonary arterial smooth muscle is thought to require increases in both intracellular Ca(2+) concentration ([Ca(2+)](i)) and myofilament Ca(2+) sensitivity, which may or may not be endothelium-dependent. To examine the effects of hypoxia and endothelium on Ca(2+) sensitivity in pulmonary arterial smooth muscle, we measured the relation between [Ca(2+)](i) and isometric force at 37°C during normoxia (21% O(2)-5% CO(2)) and after 30 min of hypoxia (1% O(2)-5% CO(2)) in endothelium-intact (E+) and -denuded (E-) rat distal intrapulmonary arteries (IPA) permeabilized with staphylococcal α-toxin. Endothelial denudation enhanced Ca(2+) sensitivity during normoxia but did not alter the effects of hypoxia, which shifted the [Ca(2+)](i)-force relation to higher force in E+ and E- IPA. Neither hypoxia nor endothelial denudation altered Ca(2+) sensitivity in mesenteric arteries. In E+ and E- IPA, hypoxic enhancement of Ca(2+) sensitivity was abolished by the nitric oxide synthase inhibitor N(ω)-nitro-l-arginine methyl ester (30 μM), which shifted normoxic [Ca(2+)](i)-force relations to higher force. In E- IPA, the Rho kinase antagonist Y-27632 (10 μM) shifted the normoxic [Ca(2+)](i)-force relation to lower force but did not alter the effects of hypoxia. These results suggest that acute hypoxia enhanced myofilament Ca(2+) sensitivity in rat IPA by decreasing nitric oxide production and/or activity in smooth muscle, thereby revealing a high basal level of Ca(2+) sensitivity, due in part to Rho kinase, which otherwise did not contribute to Ca(2+) sensitization by hypoxia.

Cellular and molecular mechanisms regulating vascular tone. Part 2: regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells

Understanding the physiological mechanisms regulating vascular tone would lead to better circulatory management during general anesthesia. This two-part review provides an overview of current knowledge about the cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). The first part reviews basic mechanisms controlling the cytosolic Ca2+ concentration in vascular smooth muscle cells, and the Ca2+-dependent regulation of vascular tone. This second part reviews the regulatory mechanisms modulating Ca2+ mobilization and/or myofilament Ca2+ sensitivity in vascular smooth muscle cells-including Rho/Rho kinase, protein kinase C, arachidonic acid, Ca2+/calmodulin-dependent protein kinase II, caldesmon, calponin, mitogen-activated protein kinases, tyrosine kinases, cyclic nucleotides, Cl- channels, and K+ channels.

Mechanism underlying H2O2-induced inhibition of acetylcholine-induced contraction in rabbit tracheal smooth muscle

The mechanism underlying the inhibition by H2O2 of acetylcholine-induced contraction was investigated in epithelium-denuded strips of rabbit trachea. Acetylcholine (10 microM) generated a phasic, followed by a tonic increase in both the intracellular Ca2+ concentration ([Ca2+]i) and force. Although the acetylcholine-induced tonic contraction was around 9 times the high K+ (80 mM)-induced one, the two stimulants induced similar [Ca2+]i increases (around 0.2 microM), indicating that acetylcholine generates tonic contraction via increases in both [Ca2+]i and myofilament Ca2+-sensitivity. H2O2 (30 microM) (a) enhanced the acetylcholine-induced tonic (not phasic) increase in [Ca2+]i but attenuated both phases of the acetylcholine-induced contraction and (b) enhanced the high K+-induced increase in [Ca2+]i but did not modify the high K+-induced contraction. In beta-escin-skinned strips, application of acetylcholine in the presence of GTP enhanced the contraction induced by 0.3 microM Ca2+ so that its amplitude became similar to that induced by 1 microM Ca2+. H2O2 (30 microM) attenuated the contraction induced by 0.3 microM Ca2+ (alone or in the presence of acetylcholine) but not those induced by higher concentrations of Ca2+ alone (0.5 microM and 1 microM). These results indicate that H2O2 acts directly on contractile proteins in rabbit tracheal smooth muscle to inhibit the contraction induced by low concentrations of Ca2+ (<0.5 microM). An action of H2O2 that increases [Ca2+]i (and thereby masks this reactive-oxygen-induced inhibition of myofilament Ca2+-sensitivity) is apparent in the presence of high K+ but not of acetylcholine. Thus, in rabbit tracheal smooth muscle H2O2 downregulates myofilament Ca2+-sensitivity more potently during acetylcholine-induced contraction than during high-K+-induced contraction, leading to an effective inhibition of the former contraction.

Endothelin-1 Induces Contraction via a Syk-Mediated p38 Mitogen-Activated Protein Kinase Pathway in Rat Aortic Smooth Muscle

Although spleen tyrosine kinase (Syk) has crucial roles in various cells, its function on vascular smooth muscle contraction has not been determined. In the present study, we performed experiments to determine if Syk contributes to the endothelin-1 (ET-1)-mediated contraction in rat aortic smooth muscle. ET-1-induced contraction of aortic strips was inhibited by piceatannol, PD98059, and SB203580, inhibitors of Syk, extracellular signal-regulated kinase 1/2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK), respectively. Piceatannol also attenuated high K(+)-induced contraction. ET-1 dose-dependently enhanced the activity of Syk and this was inhibited by piceatannol in both rat aortic strip and rat aortic smooth muscle cells. The phosphorylation of p38 MAPK and heat shock protein 27 (HSP27), but not that of ERK1/2, in response to ET-1 was inhibited by both piceatannol and SB203580. These results suggest that Syk may play an important role in the regulation of aortic smooth muscle contraction induced by ET-1, which may be mediated by the p38 MAPK/HSP27 signaling pathway.

Coronary Vasomotor Reactivity to Endothelin-1 in the Prediabetic Metabolic Syndrome

OBJECTIVE

The purpose of the present investigation was to test the hypothesis that coronary vasoconstrictor responses to endothelin-1 are augmented in the prediabetic metabolic syndrome.

METHODS

ELISA was used to measure plasma endothelin-1 and intracoronary endothelin-1 dose-response experiments were conducted in vivo on normal control and high-fat-fed prediabetic dogs. Additionally, isolated left circumflex (LCX) coronary arteries and arterioles (< 160 microm) were used for in vitro functional studies and molecular analyses (quantitative real-time PCR and Western blotting).

RESULTS

Plasma endothelin-1 concentrations were not different between control and prediabetic dogs. Coronary vasoconstriction to endothelin-1 was similar in control and prediabetic dogs, both in vivo and in isolated arterioles. Nonetheless, real-time PCR analysis revealed significant decreases in ET(A) receptor transcript levels in LCX coronary arteries and arterioles. Also, Western blotting revealed a significant decrease in ET(A) receptor protein in LCX coronary arteries.

CONCLUSIONS

The findings of the present investigation indicate that although ET(A) receptor-signaling is sensitized by induction of the metabolic syndrome, endothelin-mediated coronary vasoconstriction does not significantly contribute to coronary dysfunction at this early stage of prediabetes.

Endothelin-1 increases intracellular Ca(2+) in rabbit pulmonary artery smooth muscle cells through phospholipase C

In freshly isolated rabbit pulmonary artery smooth muscle cells, endothelin (ET)-1 induced a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) followed by a return to the initial [Ca(2+)](i). This response was not abolished by the voltage-dependent Ca(2+) channel blocker nicardipine or removal of Ca(2+) from the bath solution but was inhibited by ryanodine and thapsigargin. This finding suggested that the increase in [Ca(2+)](i) induced by ET-1 was attributable to release of Ca(2+) from ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores. The transient increase in [Ca(2+)](i) induced by ET-1 was also inhibited by pretreatment with antagonists of ET type A and B (ET(A) and ET(B)) receptors (BQ-123 and BQ-788, respectively). Furthermore, the ET(B) receptor agonist IRL-1620 induced an increase in [Ca(2+)](i) that was followed by a sustained increase in [Ca(2+)](i); the sustained increase in [Ca(2+)](i) was blocked by nicardipine. Using the nystatin-perforated patch-clamp technique, we found that IRL-1620 caused an increase in Ca(2+) current that was inhibited by addition of ET-1. ET-1 did not inhibit Ca(2+) current when cells were pretreated with BQ-123. These results suggested that when both receptor types are activated, the opposing responses lead to abolition of the sustained [Ca(2+)](i) increases induced by ET(B) receptor activation. Western blot analysis confirmed expression of ET(A) and ET(B) receptors. Finally, U-73122 inhibited the ET-1-induced [Ca(2+)](i) increase, indicating that phospholipase C was involved in modulation of the ET-1-induced [Ca(2+)](i) increase in rabbit pulmonary artery smooth muscle cells.

Endothelin-1 Receptor Antagonist BQ123 Prevents Pulmonary Artery Hypertension Induced by Low Ambient Temperature in Broilers

Evidence has indicated that endothelin-1 is related to the pathogenesis of hypertension. To characterize the role of endothelin-1 (ET-1) in the development of pulmonary hypertension syndrome in broilers, the blockade effect of ETA receptor (ET(A)) antagonist, BQ123, on blood pressure in experimental models of pulmonary hypertension was examined. Birds were locally anesthetized and instrumented with venous catheters for pulmonary arterial pressure (PAP) and right ventricular pressure (RVP), followed by packed cell volume (PCV) and Ascites heart index (AHI) measured, after exposed to low ambient temperature for 7 or 14 d. In treated groups, BQ123 (0.4 or 2.0 microg each time, 2 times a day), administered in abdominal cavities for 7 or 14 d during birds kept in low ambient temperature, prevented both PAP and RVP increasing, especially the high dose BQ123 lowered PAP and RVP to normotensive levels as that in control under normal temperature, whereas significant increases (p<0.05) were found in the two parameters of broilers in both untreated and saline treated group under low ambient temperature compared with those of birds in control. Furthermore, there was also a reduction in low ambient temperature-induced right ventricular hypertrophy in the groups administered BQ123. The preventive effect of BQ123 suggests that ET-1 is associated with the development of broilers' pulmonary hypertension, which leads to the development of ascites, and BQ123 can prevent the occurrence of pulmonary hypertension.

Mechanisms involved in carbachol-induced Ca(2+) sensitization of contractile elements in rat proximal and distal colon

1. Mechanisms involved in Ca(2+) sensitization of contractile elements induced by the activation of muscarinic receptors in membrane-permeabilized preparations of the rat proximal and distal colon were studied. 2. In alpha-toxin-permeabilized preparations from the rat proximal and distal colon, Ca(2+) induced a rapid phasic and subsequent tonic component. After Ca(2+)-induced contraction reached a plateau, guanosine 5'-triphosphate (GTP) and carbachol (CCh) in the presence of GTP further contracted preparations of both the proximal and distal colon (Ca(2+) sensitization). Y-27632, a rho-kinase inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization more significantly in the proximal colon than in the distal colon. 3. Y-27632 at 10 microm had no effect on Ca(2+)-induced contraction or slightly inhibited phorbol-12,13-dibutyrate-induced Ca(2+) sensitization in either proximal or distal colon. Chelerythrine, a protein kinase C inhibitor, inhibited GTP plus CCh-induced Ca(2+) sensitization in the distal colon, but not in the proximal colon. The component of Ca(2+) sensitization that persisted after the chelerythrine treatment was completely inhibited by Y-27632. 4. In beta-escin-permeabilized preparations of the proximal colon, C3 exoenzyme completely inhibited GTP plus CCh-induced Ca(2+) sensitization, but PKC(19-31) did not. In the distal colon, C3 exoenzyme abolished GTP-induced Ca(2+) sensitization. It inhibited CCh-induced sensitization by 50 % and the remaining component was inhibited by PKC(19-31). 5. These results suggest that both protein kinase C and rho pathways in parallel mediate the Ca(2+) sensitization coupled to activation of muscarinic receptors in the rat distal colon, whereas the rho pathway alone mediates this action in the proximal colon.

CPI-17-deficient smooth muscle of chicken

Ca(2+) sensitivity of arterial contractility is governed by regulating myosin phosphatase activity in response to agonist stimuli. CPI-17, a myosin phosphatase inhibitor phosphoprotein, is phosphorylated concomitantly with agonist-induced contractile Ca(2+) sensitization in mammalian artery. CPI-17 has not been detected in chicken artery, but is readily detectable in pigeon artery. To evaluate a role of CPI-17, we compared contractility of the arteries of 'CPI-17-deficient' chicken with those of CPI-17-rich rabbit and pigeon, and studied the effect of CPI-17-reconstitution in chicken artery. Other major regulatory/contractile proteins for Ca(2+) sensitization are expressed in both chicken and rabbit arteries. Agonists, such as an alpha(1)-agonist and endothelin-1, produced significant contraction in arteries of all species under physiological Ca(2+)-containing conditions. Depletion of Ca(2+) abolished these contractions in chicken but partially inhibited them in rabbit and pigeon arteries. Unlike CPI-17-rich tissues, chicken arteries exerted little Ca(2+) sensitization in response to alpha(1)-agonist or endothelin-1. GTPgammaS produced a slight Ca(2+) sensitizing effect in chicken artery, but this was significantly smaller compared with CPI-17-rich tissues. A PKC activator (PDBu) did not generate but rather reduced a contraction in both intact and alpha-toxin-permeabilized chicken artery in contrast to a large contraction in CPI-17-rich arteries. Myosin light chain phosphorylation was reduced by PDBu in chicken but elevated in rabbit artery. Addition of recombinant CPI-17 into beta-escin-permeabilized chicken artery restored PDBu-induced and enhanced GTPgammaS-induced Ca(2+) sensitization. Thus, CPI-17 is essential for G protein/PKC-mediated Ca(2+) sensitization in smooth muscle.

Ozone exposure may enhance airway smooth muscle contraction by increasing Ca(2+) refilling of sarcoplasmic reticulum in guinea pig

Ozone induces airway hyperresponsiveness, but there is controversy about effects of ozone on smooth muscle per se. We therefore investigated effects of in vivo ozone exposure on intracellular calcium mobilization in relation to tracheal smooth muscle contractility in the guinea pig in vitro. Guinea pigs underwent ozone exposure or sham exposure (3 ppm, 2 h). Then, a tracheal smooth muscle strip was mounted in an organ bath to record isometric tension. Effects of ozone exposure on acetylcholine-induced contraction of smooth muscle were as follows. Contraction was not altered in normal Krebs solution, but was increased in Ca(2+)-free solution in ozone-exposed animals. Decline of tension on repetitive application of acetylcholine in Ca(2+)-free solution was reduced, while the tension decline rate while acetylcholine was washed out with Ca(2+)-free solution was facilitated in ozone-exposed animals. Tension decline during the continuous administration of acetylcholine in Ca(2+)-free solution was slowed. Contraction occurred more quickly in Ca(2+)-free solution in ozone-exposed animals. Results suggest that ozone has a direct action on airway smooth muscle by changing Ca(2+) mobilization; Ca(2+) refilling via a Ca(2+) pump and Ca(2+) release via Ca(2+) channels in the sarcoplasmic reticulum were increased, while Ca(2+) extrusion via the plasma membrane Ca(2+) pump was unchanged.

Role of Cl- current in endothelin-1-induced contraction in rabbit basilar artery

Cl- efflux induces depolarization and contraction of smooth muscle cells. This study was undertaken to explore the role of Cl- channels in endothelin-1 (ET-1)-induced contraction in rabbit basilar artery. Male New Zealand White rabbits (n = 26), weighing 1.8-2.5 kg, were euthanized by an overdose of pentobarbital. The basilar arteries were removed for isometric tension recording. ET-1 produced a concentration-dependent contraction of the rabbit basilar artery in the normal Cl- Krebs-Henseleit bicarbonate buffer (123 mM Cl-). The ET-1-induced contraction was reduced by the following manipulations: 1) inhibition of Na+-K+-2Cl- cotransporter with bumetanide (3 x 10(-5) and 10(-4) M), 2) bicarbonate-free solution to disable Cl-/HCO exchanger, and 3) preincubation of rings with the Cl- channel blockers niflumic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and indanyloxyacetic acid 94. The ET-1-induced contraction was enhanced by substitution of extracellular Cl- (10 mM) with methanesulfonic acid (113 mM). Cl- channels are involved in ET-1-induced contraction in the rabbit basilar artery.

Mechanisms of endothelin-1-induced potentiation of noradrenaline response in rat mesenteric artery

1. Subthreshold concentrations of endothelin (ET)-1 enhance the contractile responses to noradrenaline (NA). We investigated possible mechanisms underlying the ET-1-induced enhancement of vasoconstrictor responses to NA in rat perfused mesenteric arteries. 2. Perfusion of arteries with subpressor dose of ET-1 (3 x 10-10 mol/L) significantly potentiated the pressor responses to NA (10-6, 3 x 10-6 and 10-5 mol/L) and this action of ET-1 was endothelium independent. 3. The protein kinase C (PKC) inhibitors staurosporine (10-8 mol/L) and calphostin C (10-7 mol/L) markedly attenuated the ET-1-induced enhancement of NA responses. Vasoconstrictor responses to NA were potentiated when vessels were perfused with phorbol 12-myristate 13-acetate (10-8 mol/L). 4. The potentiating effect of ET-1 was efficiently suppressed by Y-27632 (10-6 mol/L), a selective Rho-kinase inhibitor. In the presence of both staurosporine and Y-27632, contractile responses to NA alone were decreased markedly and ET-1-induced potentiation was abolished. 5. Both staurosporine and Y-27632 decreased contractile responses to NA in arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats to levels observed in normotensive control animals. 6. These findings suggest that ET-1-mediated potentiation of responses to NA occurs through activation of either PKC or Rho-kinase. This mechanism seems to contribute to the enhanced vasoconstrictor responces to NA observed in DOCA-salt hypertensive rats, in which the responses to NA are enhanced tonically by endogenous vascular ET-1.

Endothelin-1 enhances eicosanoids-induced coronary smooth muscle contraction by activating specific protein kinase C isoforms

Endothelin-1 (ET-1), a potent vasoconstrictor, has been implicated in the pathogenesis of coronary vasospasm by enhancing coronary vasoconstriction to vasoactive eicosanoids; however, the cellular mechanisms involved are unclear. We investigated whether physiological concentrations of ET-1 enhance coronary smooth muscle contraction to vasoactive eicosanoids by activating specific protein kinase C (PKC) isoforms. Cell contraction was measured in single smooth muscle cells isolated from porcine coronary arteries, intracellular free Ca(2+) ([Ca(2+)](i)) was measured in fura-2-loaded cells, and the cytosolic and particulate fractions were examined for PKC activity and reactivity with isoform-specific anti-PKC antibodies using Western blots. In Hanks' solution (1 mmol/L Ca(2+)), ET-1 (10 pmol/L) did not increase basal [Ca(2+)](i) (81+/-2 nmol/L), but it did cause cell contraction (9%) that was inhibited by GF109203X (10(-6) mol/L), an inhibitor of Ca(2+)-dependent and Ca(2+)-indpendent PKC isoforms. The vasoactive eicosanoid prostaglandin F(2alpha) (PGF(2alpha), 10(-7) mol/L) caused increases in cell contraction (11%) and [Ca(2+)](i) (108+/-7 nmol/L) that were inhibited by the Ca(2+) channel blocker diltiazem (10(-6) mol/L). Pretreatment with ET-1 (10 pmol/L) for 10 minutes enhanced cell contraction to PGF(2alpha) (35%) with no additional increase in [Ca(2+)](i) (112+/-8 nmol/L). Direct activation of PKC by phorbol 12,13-dibutyrate (PDBu, 10(-7) mol/L) caused cell contraction (10%) and enhanced PGF(2alpha) contraction (33%) with no additional increase in [Ca(2+)](i) (115+/-7 nmol/L). The ET-1-induced enhancement of PGF(2alpha) contraction was inhibited by Gö6976 (10(-6) mol/L), an inhibitor of Ca(2+)-dependent PKC isoforms. Both ET-1 and PDBu caused an increase in PKC activity in the particulate fraction and a decrease in the cytosolic fraction and increased the particulate/cytosolic PKC activity ratio. Western blots revealed the Ca(2+)-dependent alpha-PKC and the Ca(2+)-independent delta-, epsilon-, and zeta-PKC isoforms. In resting tissues, alpha- and epsilon-PKC were mainly cytosolic, delta-PKC was mainly in the particulate fraction, and zeta-PKC was equally distributed in the cytosolic and particulate fraction. ET-1 (10 pmol/L) alone or PDBu (10(-7) mol/L) alone caused translocation of epsilon-PKC from the cytosolic to the particulate fraction, localized delta-PKC more in the particulate fraction, but did not change the distribution of zeta-PKC. PGF(2alpha) (10(-7) mol/L) alone did not change PKC activity. In tissues pretreated with ET-1 or PDBu, PGF(2alpha) caused additional increases in alpha-PKC activity. Thus, the enhancement of PGF(2alpha)-induced coronary smooth muscle contraction by physiological concentrations of ET-1 involves activation and translocation of alpha-PKC in addition to delta- and epsilon-PKC isoforms, and this may represent one possible cellular mechanism by which ET-1 could enhance coronary vasoconstriction to vasoactive eicosanoids in coronary vasospasm.

[Ca(2+)](i) signaling in renal arterial smooth muscle cells of pregnant rat is enhanced during inhibition of NOS

Vascular resistance and arterial pressure are reduced during normal pregnancy, but dangerously elevated during pregnancy-induced hypertension (PIH), and changes in nitric oxide (NO) synthesis have been hypothesized as one potential cause. In support of this hypothesis, chronic inhibition of NO synthesis in pregnant rats has been shown to cause significant increases in renal vascular resistance and hypertension; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the pregnancy-associated changes in renal vascular resistance reflect changes in contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) of renal arterial smooth muscle. Smooth muscle cells were isolated from renal interlobular arteries of virgin and pregnant Sprague-Dawley rats untreated or treated with the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 4 mg. kg(-1). day(-1) for 5 days), then loaded with fura 2. In cells of virgin rats incubated in Hanks' solution (1 mM Ca(2+)), the basal [Ca(2+)](i) was 86 +/- 6 nM. Phenylephrine (Phe, 10(-5) M) caused a transient increase in [Ca(2+)](i) to 417 +/- 11 nM and maintained an increase to 183 +/- 8 nM and 32 +/- 3% cell contraction. Membrane depolarization by 51 mM KCl, which stimulates Ca(2+) entry from the extracellular space, caused maintained increase in [Ca(2+)](i) to 292 +/- 12 nM and 31 +/- 2% contraction. The maintained Phe- and KCl-induced [Ca(2+)](i) and contractions were reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. Phe- and KCl-induced contraction and [Ca(2+)](i) were not significantly different between untreated and L-NAME-treated virgin rats or between untreated and L-NAME + L-arginine treated pregnant rats. In Ca(2+)-free Hanks', application of Phe or caffeine (10 mM), to stimulate Ca(2+) release from the intracellular stores, caused a transient increase in [Ca(2+)](i) and a small cell contraction that were not significantly different among the different groups. Thus renal interlobular smooth muscle of normal pregnant rats exhibits reduction in [Ca(2+)](i) signaling that involves Ca(2+) entry from the extracellular space but not Ca(2+) release from the intracellular stores. The reduced renal smooth muscle cell contraction and [Ca(2+)](i) in pregnant rats may explain the decreased renal vascular resistance associated with normal pregnancy, whereas the enhanced cell contraction and [Ca(2+)](i) during inhibition of NO synthesis in pregnant rats may, in part, explain the increased renal vascular resistance associated with PIH.

Peroxide sensitivity of endothelin responses in coronary artery smooth muscle: ET(A) vs. ET(B) pathways

Endothelins (ETs) contract de-endothelialized rings from left descending coronary artery via ET(A) or ET(B) receptors. Here we test the hypothesis that the actions of EA(A) and ET(B) receptors are similar in their sensitivities to damage by hydrogen peroxide. In Ca2+-containing Krebs' solution, 100 nM of the ET(B) agonist IRL1620 produced contractions with significantly smaller force (17.6+/-1.7 mN) than 50 nM of the ET(A) + ET(B) agonist ET-1 (73.2+/-4.6 mN) (p < 0.05). In Ca2+-free solutions, the contractions due to both agents were significantly smaller (p < 0.05). Pretreating the tissues with peroxide inhibited the contractions produced by either agent. The IC50 values for peroxide were significantly higher (p < 0.05) using ET-1 (1.0+/-0.3 mM in Ca2+, 1.4+/-0.1 mM in Ca2+-free) than using IRL 1620 (0.32+/-0.08 in Ca2+, 0.25+/-0.01 mM in Ca2+-free). Pretreating microsomes isolated from the artery smooth muscle with up to 10 mM peroxide did not significantly affect 125I-ET-1 binding to ET(A) or ET(B) receptors (p > 0.05). In comparing the peroxide induced inactivation of the various processes in this artery and based on literature, we conclude that the actions of ET(A) may also involve a peroxide resistant Ca2+-independent pathway(s).

ET(B)-mediated contraction differs between left descending coronary artery and its next branch

Pig left descending coronary artery (main artery) and its next branch (branch arteries) differ in many properties. Here we report on the receptor types and the Ca2+ pools utilized for endothelin (ET) contraction in 3 mm long de-endothelialized rings of the main (weight 7.38 +/- 0.38 mg) and the branch (1.07 +/- 0.03 mg) arteries. KCl (60 mM) contracted the main and the branch arteries with force of 41.8 +/- 3.1 and 16.9 +/- 1.0 mN (millinewton), respectively. Force of contraction for all the other agents was normalized taking the KCl value as 100%. We determined the total ET-induced responses using ET-1 and those mediated by ET(B) using IRL1620. In Ca2+-containing solutions, ET-1 contracted the main arteries with pEC50 = 8.2 +/- 0.1 and a maximum force of 98 +/- 5%. The branch arteries also gave similar values of pEC50 (8.4 +/- 0.1) and maximum force (99 +/- 14%). IRL1620 contracted the main and the branch arteries with pEC50 = 7.9 +/- 0.1 but the maximum force was significantly higher in the branch arteries (44 +/- 3%) than in the main (15 +/- 2%). In Ca2+-free solutions, the pEC50 values for ET-1 or IRL-1620 did not change but the maximum force of contraction was diminished considerably in both main and branch arteries. Thus, the left coronary artery and its next branch differ in that the role of ET(B) receptors is greater in the latter.

ET(A) receptors are the primary mediators of myofilament calcium sensitization induced by ET-1 in rat pulmonary artery smooth muscle: a tyrosine kinase independent pathway

We have investigated the possibility that ET-1 can induce an increase in myofilament calcium sensitivity in pulmonary artery smooth muscle. Arterial rings were permeabilized using alpha-toxin (120 microg ml(-1)), in the presence of A23187 (10 microM) to 'knock out' Ca2+ stores, and pre-constricted with pCa 6.8 (buffered with 10 mM EGTA). In the presence of this fixed Ca2+ concentration, 1 microM ET-1 induced a sustained, reversible constriction of 0.15 mN. Pulmonary arterial rings were freeze-clamped at the peak of the induced constriction (time matched). Subsequent densitometric analysis revealed that ET-1 (1 microM) increased the level of phosphorylated myosin light chains by 34% compared to an 11% increase in the presence of pCa 6.8 alone. In contrast to ET-1, the selective ET(B) receptor agonist Sarafotoxin S6C (100 nM) failed to induce a significant constriction. The constriction induced by 1 microM ET-1 was reversibly inhibited when the preparation was preincubated (15 min) with the ETA receptor antagonist BQ 123 (100 microM). The constriction measured 0.13 mN in the absence and 0.07 mN in the presence of 100 microM BQ 123. In contrast, the constriction induced by 1 microM ET-1 measured 0.19 mN in the absence and 0.175 mN following a 15 min pre-incubation with the ET(B) antagonist BQ 788 (100 microM). The constriction induced by 1 microM ET-1 measured 0.14 mN in the presence and 0.13 mN following pre-incubation with the tyrosine kinase inhibitor Tyrphostin A23 (100 microM). We conclude that ET-1 induced an increase in myofilament calcium sensitivity in rat pulmonary arteries via the activation of ET(A) receptors and by a mechanism(s) independent of tyrosine kinase.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Regulation of Ca2+ sensitization by PKC and rho proteins in ovine cerebral arteries: effects of artery size and age

G protein-regulated Ca2+ sensitivity of vascular contractile proteins plays an important role in cerebrovascular reactivity. The present study examines the intracellular mechanisms that govern G protein-regulated Ca2+ sensitivity in cerebral arteries of different size and age. We studied beta-escin-permeabilized segments of common carotid, basilar, and middle cerebral arteries from nonpregnant adult and near-term fetal sheep. Activation of protein kinase C (PKC) by (-)-indolactam V or a phorbol ester produced receptor-independent increases in Ca2+ sensitivity. Such increases were more marked in immature arteries and were inversely correlated with artery size in both mature and immature arteries. However, inhibitors of PKC did not significantly affect increases in Ca2+ sensitivity in responses to either serotonin (5-hydroxytryptamine, 5-HT) or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Alternatively, deactivation of rho p21, a small G protein associated with Rho kinase, by exotoxin C3 fully prevented increases in Ca2+ sensitivity in responses to 5-HT or GTPgammaS in both adult and fetal arteries of all types. Neither inhibitors of PKC nor exotoxin C3 altered baseline Ca2+ sensitivity. We conclude that patterns of receptor- and/or G protein-mediated modulation of Ca2+ sensitivity are dependent on an intracellular pathway that involves activation of small G proteins and Rho kinase. In contrast, PKC has little, if any, role in agonist-induced Ca2+ sensitization under the present experimental conditions.

Cellular mechanisms and role of endothelin-1-induced calcium oscillations in pulmonary arterial myocytes

The effect of endothelin (ET)-1 on both cytosolic Ca2+ concentration ([Ca2+]i) and membrane current in freshly isolated myocytes, as well as on the contraction of arterial rings, was investigated in rat main pulmonary artery (RMPA) and intrapulmonary arteries (RIPA). ET-1 (5-100 nM, 30 s) induced a first [Ca2+]i peak followed by 3-5 oscillations of decreasing amplitude. In RMPA, the ET-1-induced [Ca2+]i response was fully abolished by BQ-123 (0.1 microM). In RIPA, the response was inhibited by BQ-123 in only 21% of the cells, whereas it was abolished by BQ-788 (1 microM) in 70% of the cells. In both types of arteries, the response was not modified in the presence of 100 microM La3+ or in the absence of external Ca2+ but disappeared after pretreatment of the cells with thapsigargin (1 microM) or neomycin (0.1 microM). In RPMA myocytes clamped at -60 mV, ET-1 induced an oscillatory inward current, the reversal potential of which was close to the equilibrium potential for Cl-. This current was unaltered by the removal of external Ca2+ but was abolished by niflumic acid (50 microM). In arterial rings, the ET-1 (100 nM)-induced contraction was decreased by 35% in the presence of either niflumic acid (50 microM) or nifedipine (1 microM). These results demonstrate that ET-1 via the ETA receptor only in RMPA and both ETA and ETB receptors in RIPA induce [Ca2+]i oscillations due to iterative Ca2+ release from an inositol trisphosphate-sensitive Ca2+ store. Ca2+ release secondarily activates an oscillatory membrane Cl- current that can depolarize the cell membrane, leading to an influx of Ca2+, this latter contributing to the ET-1-induced vasoconstrictor effect.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

PCA50941, a new 1,4-dihydropyridine, reverses endothelin-induced cardiogenic shock in the anesthetized goat

This study was designed to test the hypothesis that the properties of novel 1,4-dihydropyridine PCA50941 could favor the recovery of cardiogenic shock. Coronary blood flow (CBF), measured with an electromagnetic flow probe placed on the left circumflex coronary artery, systemic arterial pressure and heart rate were recorded in 24 anesthetized goats; left ventricular pressure and dP/dt were also recorded in 19 of these goats. Under control conditions, intracoronary injections in 5 goats of PCA50941 (10-120 microg) caused smaller reductions of CBF than those of Bay K 8644 (0.3-10 microg) (the reduction of CBF by 120 microg PCA50941 was 25% and that by 10 microg Bay K 8644 was 43%), and i.v. infusions in 4 goats of PCA50941 (10-300 microg/min) did not modify CBF nor the other hemodynamic variables recorded, whereas i.v infusion of Bay K 8644 (10-30 microg/min) reduced CBF by 20% and increased arterial pressure, left ventricular pressure and dP/dt. During control conditions and endothelin-induced cardiogenic shock, respectively, the values for 15 goats were: for CBF, 33+/-4 vs. 16+/-4 ml/min; for mean arterial pressure, 88+/-4 vs. 60+/-5 mm Hg; for left ventricular systolic pressure, 102+/-5 vs. 75+/-4 mm Hg; for dP/dt, 1453+/-147 vs. 925+/-101 mm Hg/s (all P<0.05), and for heart rate, 77+/-6 vs. 81+/-6 beats/min (P>0.05). Intravenous infusion of PCA50941 (100 microg/min) reversed the hemodynamic variables from the shock state to control values within 20 min in 5 of 6 animals, whereas i.v. administration of Bay K 8644 (10-30 microg/min) was not effective in 4 of 5 animals, and the vehicle (DMSO) was not effective in none of 4 animals in reversing the hemodynamic shock state. Therefore, it is suggested that PCA50941, a novel 1,4-dihydropyridine, has a cardiovascular profile that might be suitable for treating cardiogenic shock states.

Endothelin-1 and insulin activate the steady-state voltage dependent R-type Ca2+ channel in aortic smooth muscle cells via a pertussis toxin and cholera toxin sensitive G-protein

In single rabbit aortic smooth muscle cells, and at a concentration known to induce a maximum sustained increase of intracellular Ca2+ via activation of the steady-state voltage dependent R-type Ca2+ channels, endothelin-1 (10(-7) M) and insulin (80 microU/ml) were found to induce a sustained increase in cytosolic free Ca2+ ([Ca]i) levels that was significantly attenuated by pre-treatment with either pertussis toxin (PTX), cholera toxin (CTX) or removal of extracellular Ca2+. However, both PTX and CTX failed to inhibit the sustained depolarization-evoked sustained Ca2+ influx and [Ca]i elevation via activation of the R-type Ca2+ channels. Moreover, ET-1 and insulin-evoked sustained increases in Ca2+ influx were not attenuated by the selective PKC inhibitor, bisindolylmaleimide (BIS), or the specific L-type Ca2+ channel blocker, nifedipine, but were completely reversed by the R-type Ca2+ channel blocker, (-) PN 200-110 (isradipine). These data suggest that both insulin and ET-1 activate the nifedipine-insensitive but isradipine-sensitive steady state voltage dependent R-type Ca2+ channels present on rabbit VSMCs and these channels are directly coupled to PTX and CTX sensitive G protein(s).

Endothelin-1 activates phospholipases and channels at similar concentrations in porcine coronary arteries

Sensitivity of endothelin-1 (ET-1)-ion channel interactions has been proposed to exceed that of ET-1-phospholipase activation in vascular smooth muscle. We wanted to determine whether short-circuiting ion channels with staphylococcal alpha-toxin pores would shift the ET-1-force relation to the right as predicted from the above proposal. Medium size porcine coronary arteries (outer diameter 0.7-1.5 mm) were mounted on isometric force transducers. ET-1 concentration response curves were compared between intact rings and those subjected to alpha-toxin treatment with Ca buffered at 0.1 microM. The EC50 for treated rings (1.5 +/- 1.0 nM, n = 5 pigs) was similar to that for intact rings (1.9 +/- 0.4 nM). The Ca sensitivity of the alpha-toxin-treated rings (EC50 = 0.43 +/- 0.08 microM) was similar to that reported by other laboratories for intact and alpha-toxin-treated arteries and was shifted eightfold to the left by a high concentration of ET-1 (10 nM). Measurements of [32P]phosphatidic acid ([32P]PA) levels were used to evaluate phospholipase activity in intact arteries. The time courses for [32P]PA production and contraction were similar in response to high (100 nM) and to low (1 nM) ET-1. Significant increases in both steady-state contraction and [32P]PA occurred over a wide range of ET-1 concentrations tested (0.3-100 nM). Our findings support the concept that ET-1-phospholipase coupling is operative over the whole concentration range that induces contractile responses. It is suggested that both Ca entry and Ca sensitization processes are activated by ET-1 at low concentrations (<EC50) and that both processes contribute significantly to the integrated response.

Possible involvement of the novel CPI-17 protein in protein kinase C signal transduction of rabbit arterial smooth muscle

1. CPI-17 has recently been identified as a novel protein in vascular smooth muscle. In vitro , its phosphorylation and thiophosphorylation by protein kinase C (PKC) specifically inhibits the type 1 class of protein phosphatases, including myosin light chain (MLC) phosphatase. 2. Both of the phosphorylated CPI-17 states dose-dependently potentiated submaximal contractions at constant [Ca2+] in beta-escin-permeabilized and Triton X-100-demembranated arterial smooth muscle, but produced no effect in intact and less intensely permeabilized (alpha-toxin) tissue. Thiophosphorylated CPI-17 (tp-CPI) induced large contractions even under Ca2+-free conditions and decreased Ca2+ EC50 by more than an order of magnitude. Unphosphorylated CPI-17 produced minimal but significant effects. 3. tp-CPI substantially increased the steady-state MLC phosphorylation to Ca2+ ratios in beta-escin preparations. 4. tp-CPI affected the kinetics of contraction and relaxation and of MLC phosphorylation and dephosphorylation in such a manner that indicates its major physiological effect is to inhibit MLC phosphatase. 5. Results from use of specific inhibitors in concurrence with tp-CPI repudiate the involvement of general G proteins, rho A or PKC itself in the Ca2+ sensitization by tp-CPI. 6. Our results indicate that phosphorylation of CPI-17 by PKC stimulates binding of CPI-17 to and subsequent inhibition of MLC phosphatase. This implies that CPI-17 accounts largely for the heretofore unknown signalling pathway between PKC and inhibited MLC phosphatase.

Role of maxi-K+ channels in endothelin-induced vasoconstriction of mesenteric and submucosal arterioles

The action of endothelin in small intestinal resistance vessels of the guinea pig was studied by examining submucosal arteriole vasoactivity in vitro and electrical properties of mesenteric arteriole smooth muscle cells. Endothelin-1 (ET-1) constricted submucosal arterioles with a half-maximal effective concentration of 170 pM. ET-3 caused detectable constriction with a minimum of 20 nM. The ET-1 response was prolonged, with a time to 90% relaxation of 41 +/- 2.8 min after washout. The ETA antagonist BQ-123 (200 nM) decreased the sensitivity to ET-1 approximately 40-fold. Arterioles preconstricted with prostaglandin F2 alpha did not relax when superfused with ET-1, ET-3, or an ETB agonist, IRL-1620, and pretreatment with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine was ineffective in countering ET-1-induced constriction, indicating the absence of functional ETB receptors. Resting membrane potential in isolated cells was characterized by transient hyperpolarizing spikes (THs). ET-1 (20 nM) increased TH frequency and caused the emergence of a larger amplitude population. Under voltage clamp, spontaneous transient outward currents (STOCs) were seen that reversed at the K+ equilibrium potential. ET-1 increased STOC frequency and amplitude. Iberiotoxin (IBTX; 200 nM), a maxi-K+ channel antagonist, blocked the ET-1-induced THs and reduced STOC activity. IBTX or tetraethylammonium increased the rate and extent of ET-1-induced arteriole constriction. We suggest that ET-1-induced vasoactivity of ileal resistance arterioles involves ETA receptor-mediated early activation of maxi-K+ channels that serves to counter strong constriction.

Contraction and intracellular calcium-ion elevation of cultured human aortic smooth muscle cells by endothelin-1, vasoactive intestinal contractor (VIC) and the derivatives

Effects of endothelin (ET) family peptides and their derivatives on cellular contraction and calcium-ion level were examined by using cultured human vascular smooth muscle cells (VSM). Contraction of cultured human VSM, isolated from human fetal aortic segments, was induced within 1 min after the treatment with ET-1 (100 nM) as seen in the changes of cytosolic calcium-ion localization. In parallel with the cell contraction, cytosolic calcium-ion level in the human VSM increased very rapidly and then dropped with some oscillation as determined by Anchorage Cell Analyzing System. It was noted that transient calcium-ion mobilization rather than sustained calcium-ion influx was significant in the contraction of cultured human VSM. Vasoactive intestinal contractor (VIC), three amino acids different from ET-1, had less activity in increase of intracellular calcium-ion level and in percent of response cells than ET-1, ET-2, and VIC-S4L6 (one amino acid different from ET-1). EC50 of ET-1, VIC-S4L6, ET-2, and VIC were 0.5 nM, 0.6 nM, 2.0 nM, and 20 nM, respectively. VIC-like peptide (VIC-LP), 16 amino acids fragment of VIC precursor protein, had no effect with a single administration of up to 10 microM. However, the increase in calcium-ion level by VIC was suppressed with a prior treatment of cells with high concentration (10 microM) of VIC-LP. The establishment of cultured human VSM for the simultaneous examination of the contraction and calcium-ion level will provide a new system to study signal transduction of vasocontractor peptides.

Role of endothelin receptors, calcium and nitric oxide in the potentiation by endothelin-1 of the sympathetic contraction of rabbit ear artery during cooling

1. To examine further the potentiation by endothelin-1 on the vascular response to sympathetic stimulation, we studied the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz), under different conditions, at 37 degrees C and during cooling (30 degrees C). 2. Electrical stimulation produced frequency-dependent contraction, which was reduced (about 63% for 8 Hz) during cooling. At 30 degrees C, but not at 37 degrees C, endothelin-1 (1, 3 and 10 nM) potentiated the contraction to electrical stimulation in a dose-dependent way (from 43 +/- 7% to 190 +/- 25% for 8 Hz). 3. This potentiation by endothelin-1 was reduced by the antagonist for endothelin ETA receptors BQ-123 (10 microM) but not by the antagonist for endothelin ETB receptors BQ-788 (10 microM). The agonist for endothelin ETB receptors IRL-1620 (0.1 microM) did not modify the contraction to electrical stimulation. 4. The blocker of L-type Ca2+ channels verapamil (10 microM l-1) reduced (about 72% for 8 Hz) and the unspecific blocker of Ca(2+)-channels NiCl2 (1 mM) practically abolished (about 98%), the potentiating effects of endothelin-1 found at 30 degrees C. 5. Inhibition of nitric oxide synthesis with NG-nitro-L-arginine (L-NOARG, 0.1 mM) increased the contraction to electrical stimulation at 30 degrees C more than at 37 degrees C (for 8 Hz, this increment was 297 +/- 118% at 30 degrees C, and 66 +/- 15% at 37 degrees C). Endothelium removal increased the contraction to electrical stimulation at 30 degrees C (about 91% for 8 Hz) but not at 37 degrees C. Both L-NOARG and endothelium removal abolished the potentiating effects of endothelin-1 on the response to electrical stimulation found at 30 degrees C. 6. These results in the rabbit ear artery suggest that during cooling, endothelin-1 potentiates the contraction to sympathetic stimulation, which could be mediated at least in part by increasing Ca2+ entry after activation of endothelin ETA receptors. This potentiating effect of endothelin-1 may require the presence of an inhibitory tone due to endothelial nitric oxide.

Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 microM), histamine (100 microM), and the thromboxane A2 analogue U-46619 (100 microM) were negligible, carbachol (100 microM) and endothelin-1 (ET-1, 1 microM) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 microM) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely. GTP-gamma-S (30 microM) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 microM) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05). The effects of saturating concentrations of GTP-gamma-S and PDBu were additive. A synthetic peptide (T2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 microM, PDBu (3 microM) caused contraction of the T2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.

Role of endothelium in regulation of smooth muscle membrane potential and tone in the rabbit middle cerebral artery

1. The characteristic features of the endothelium-mediated regulation of the electrical and mechanical activity of the smooth muscle cells of cerebral arteries were studied by measuring membrane potential and isometric force in endothelium-intact and -denuded strips taken from the rabbit middle cerebral artery (MCA). 2. In endothelium-intact strips, histamine (His, 3-10 microM) and high K+ (20-80 mM) concentration-dependently produced a transient contraction followed by a sustained contraction. Noradrenaline (10 microM), 5-hydroxytryptamine (10 microM) and 9,11-epithio-11, 12-methano-thromboxane A2 (10 nM) each produced only a small contraction (less than 5% of the maximum K+-induced contraction). 3. N(G)-nitro-L-arginine (L-NOARG, 100 microM), but not indomethacin (10 microM), greatly enhanced the phasic and the tonic contractions induced by His (1-10 microM) in endothelium-intact, but not in endothelium-denuded strips, suggesting that spontaneous or basal release of nitric oxide (NO) from endothelial cells potently attenuates the His-induced contractions. Acetylcholine (ACh, 0.3-3 microM) caused concentration-dependent relaxation (maximum relaxation by 89.7 +/- 7.5%, n=4, P<0.05) when applied to endothelium-intact strips precontracted with His. L-NOARG had little effect on this ACh-induced relaxation (n=4; P<0.05). Apamin (0.1 microM), but not glibenclamide (3 microM), abolished the relaxation induced by ACh (0.3-3 microM) in L-NOARG-treated strips (n=4, P<0.05). 4. In endothelium-intact tissues, His (3 microM) depolarized the smooth muscle membrane potential (by 4.4 +/- 1.8 mV, n = 12, P < 0.05) whereas ACh (3 microM) caused membrane hyperpolarization (-20.9 +/- 3.0 mV, n = 25, P< 0.05). The ACh-induced membrane hypepolarization persisted after application of L-NOARG (-23.5 +/- 5.9 mV, n=8, P<0.05) or glibenclamide (-20.6 +/- 5.4 mV, n=5, P<0.05) but was greatly diminished by apamin (reduced to - 5.8 +/- 3.2 mV, n = 3, P< 0.05). 5. Sodium nitroprusside (0.1-10 microM) did not hyperpolarize the smooth muscle cell membrane potential (0.2 +/- 0.3 mV, n=4, P>0.05) but it greatly attenuated the His-induced contraction in endothelium-denuded strips (n-4, P<0.05). 6. These results suggest that, under the present experimental conditions: (i) spontaneous or basal release of NO from endothelial cells exerts a significant negative effect on agonist-induced contractions in rabbit MCA, and (ii) ACh primarily activates the release of endothelium-derived hyperpolarizing factor (EDHF) in rabbit MCA.

Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications

Smooth muscle contraction is the basis of the physiological reactivity of several systems (vascular, respiratory, gastrointestinal, urogenital ...). Hyperresponsiveness of smooth muscle may also contribute to a variety of problems such as arterial hypertension, asthma and spontaneous abortion. An increase in cytoplasmic calcium concentration ([Ca2+]i) is the key event in excitation-contraction coupling in smooth muscle and the relationship linking the [Ca2+]i value to the force of contraction represents the calcium sensitivity of the contractile apparatus (CaSCA). Recently, it has become evident that CaSCA can be modified upon the action of agonists or drugs as well as in some pathophysiological situations. Such modifications induce, at a fixed [Ca2+]i value, either an increase (referred to as sensitization) or a decrease (desensitization) of the contraction force. The molecular mechanisms underlying this modulation are not yet fully elucidated. Nevertheless, recent studies have identified sites of regulation of the actomyosin interaction in smooth muscle. Sensitization primarily results from the inhibition of myosin light chain phosphatase (MLCP) by intracellular messengers such as arachidonic acid or protein kinase C. In addition, phosphorylation of thin filament-associated proteins, caldesmon and calponin, increases CaSCA. Activation of small (monomeric) G-proteins such as rho or ras is also involved. Desensitization occurs as a consequence of phosphorylation of myosin light chain kinase (MLCK) by the calcium-calmodulin activated protein kinase II, or stimulation of MLCP by cyclic GMP-activated protein kinase. In the present review, examples of physiological modulation of CaCSA as well as pharmacological and pathophysiological implications are illustrated for some smooth muscles.

Endothelin peptides in brain diseases

Recently, scientists interested in diseases of the human brain have paid much attention to the endothelin group of peptides. Under normal conditions they are found in some types of neurons and in endothelial cells of microvessels but not in glial cells. This review focuses on the endothelin peptides and their involvement in various brain diseases. Particular attention is paid to their expression in reactive astrocytes seen in many pathological conditions of the human brain. Endothelin-1 is a very potent vasoconstrictor which may be involved in the vasospasm occurring in subarachnoid haemorrhage. Intracerebral injection or application to cerebral arteries in animals will cause a focal necrosis, apparently due to severe vasoconstriction. Reactive astrocytes occurring in cases with infarcts, lacunae, Alzheimer's disease, progressive multifocal leukoencephalopathy (PML) and subacute sclerosing panencephalitis (SSPE) express endothelin-like immunoreactivity. Astrocytes in vitro may produce, store and release endothelins. To some extent astrocytes grown in vitro mimic reactive astrocytes in vivo since in cultures astrocytes are removed from their natural environment which may trigger reactive responses. Therefore, in vivo reactive astrocytes may produce, store and release endothelins just as in vitro. If endothelins are released from reactive astrocytes they may act as mitogens and may influence microcirculation by inducing vasoconstriction of intracerebral arterioles. In such ways endothelins may contribute to the final lesions seen in cases with infarcts, lacunae, traumatic conditions, Alzheimer's disease and inflammatory diseases of the brain.

Role of protein kinase C in the endothelin-induced contraction in the rabbit saphenous vein

The role of protein kinase C in the endothelin-induced contraction was examined in the isolated rabbit saphenous vein in which endothelin-1, endothelin-3, sarafotoxin S6c and IRL 1620 (succinyl-[Glu9,Ala11,15]endothelin-1-(8-21))-induced contraction at the threshold concentrations of 0.1-1 pM. A selective inhibitor of protein kinase C, 500 nM calphostin C (2-[12-[2-(benzyloxy)propyl]-3, 10-dihydro-4,9-dihydroxy-2,6,7,11-tetramethoxy-3, 10-dioxo-1-perylenyl]-1-methylethyl carbonic acid 4-hydroxyphenyl ester), shifted the concentration-response curves for these agonists to the right 7.4- to 109-fold. In the vein in which the endothelin ETB receptor was desensitized, sarafotoxin S6c and IRL 1620 were ineffective whereas endothelin-1 and higher concentrations of endothelin-3 induced contractions by activating the endothelin ET(A) receptor. Calphostin C (500 nM) shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right more than 155-fold. Down-regulation of protein kinase C (by treatment with phorbol 12-myristate 13-acetate for 20 h) shifted the concentration-response curves for these agonists to the right before and after desensitization of the endothelin ETB receptor 3.7- to 59-fold. In the permeabilized smooth muscle, Ca(2+)-induced contraction was enhanced by endothelin-1, endothelin-3 and sarafotoxin S6c at concentrations much higher than those needed to induce contraction (threshold concentration was 3 nM). Calphostin C and down-regulation of protein kinase C shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right and downwards without changing the effect of sarafotoxin S6c. In the permeabilized muscle in which the endothelin ETB receptor was desensitized, endothelin-1 and endothelin-3 still augmented the Ca(2+)-induced contraction. Calphostin C and down-regulation of protein kinase C shifted the concentration-response curves for endothelin-1 and endothelin-3 to the right and downwards. These results suggest that protein kinase C is involved in the contraction mediated by the endothelin ET(A) and ETB receptors; and Ca2+ sensitization mediated by the endothelin ET(A) receptor is due to activation of protein kinase C whereas Ca2+ sensitization mediated by the endothelin ETB receptor may be due not only to the activation of protein kinase C but also to other mechanisms.

Receptor activation induces short-term modulation of arterial contractions: memory in vascular smooth muscle

This study examined the hypothesis that arteries retain a memory of receptor activation, resulting in temporary modulation of stimulus-contraction coupling. When pretreated for 30 min with 10(-5) M phenylephrine (PE), histamine, or prostaglandin F2 alpha (PGF2 alpha) and then relaxed fully for 10 min, steady-state increases in stress (S/So) and myosin light-chain phosphorylation (MLC20P/MLC20) produced by KCl in femoral arteries were weaker (0.33-0.57 S/So and 0.29-0.30 MLC20P/MLC20) than control responses (approximately 0.91 S/So and approximately 0.41 MLC20P/MLC20). The inhibitory effect lasted for at least 2 h and was not as strong in tissues pretreated for a 10-fold shorter duration or a 10-fold lower concentration of PE. When pretreated with low concentrations of PE (10(-7) M) and PGF2 alpha (10(-6) M), the early portion of subsequent KCl-induced contractile responses reached levels higher than control responses (0.79-0.86 S/So compared with approximately 0.70 S/So). These data support the hypothesis that receptor activation of arteries not only caused contractions but also stimulated another system, a response modulator that appeared to serve as memory of receptor activation.