Different pathways of calcium sensitization activated by receptor agonists and phorbol esters in vascular smooth muscle

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Endothelium-independent vasorelaxant effect of Asphodelus tenuifolius Cav. via inhibition of myosin light chain kinase activity in the porcine coronary artery

ETHNOPHARMACOLOGICAL RELEVANCE

Asphodelus tenuifolius Cav. (Asphodelaceae), a wild, terrestrial, annual stemless herb, is widely used in traditional medicine for the treatment of hypertension, diabetes, atherosclerosis and circulatory problems. A previous research study from our laboratory revealed that A. tenuifolius has beneficial effects in reducing blood pressure and improves aortic endothelial dysfunction in chronically glucose fed rats. Despite the fact that A. tenuifolius reduces blood pressure and improves endothelial function in vivo, there are no detailed studies about its possible mechanism of action.

AIM OF THE STUDY

This study was designed to provide pharmacological basis and mechanism of action for the traditional use of A. tenuifolius in hypertension and circulatory problems. We explored the vasorelaxant effect of A. tenuifolius and its underlying vasorelaxation mechanism in porcine coronary artery rings.

MATERIALS AND METHODS

Aqueous methanolic crude extract of A. tenuifolius was prepared by maceration process and then activity guided fractionation was carried out by using different polarity based solvents. Phytochemical studies were carried out using LC-DAD-MS. Segments of porcine distal coronary artery were set up in a wire myograph for isometric force measurements. Extract/fractions of A. tenuifolius seeds were tested for vasodilator activity by measurement of changes in tone after pre-contraction with the thromboxane mimetic U46619 in the presence or absence of inhibitors of intracellular signaling cascades.

RESULTS

Crude extract/fractions of A. tenuifolius produced dose dependent endothelium independent vasorelaxant response in coronary rings, whereas, the butanol fraction of A. tenuifolius (BS-AT) produced the largest relaxation response with 100% relaxation at 1 mg/ml, therefore the mechanism of relaxation of this fraction was determined. The relaxation to BS-AT was unaffected by removal of the endothelium, pre-contraction with KCl, or the presence of the non-selective potassium channel blocker tetraethylammonium, indicating that the relaxation was endothelium-independent, and does not involve activation of potassium channels. BS-AT (1 mg/ml) inhibited the contractile response to calcium,the L-type calcium channel activator BAY K8664,and ionomycin, indicating that it inhibits calcium-induced contractions. The relaxation response to BS-AT was attenuated in the absence of extracellular calcium. However, relaxations to BS-AT were also reduced after deletion of calcium from intracellular stores with cyclopiazonic acid. Incubation with 1 mg/ml BS-AT also inhibited phosphorylation of myosin light chains in homogenates of coronary artery.

CONCLUSION

The butanol extract of Asphodelus tenuifolius produces a large endothelium-independent relaxation of the porcine coronary artery through inhibition of calcium-induced contractions. The effect appears to be downstream of calcium influx, possibly through inhibition of myosin light chain kinase. This study supports previous studies demonstrating that A. tenuifolius reduces blood pressure. Future studies will aim to determine the active compounds underlying this response.

Ethyl rosmarinate relaxes rat aorta by an endothelium-independent pathway

Ethyl rosmarinate is an ester derivative of rosmarinic acid, a major constituent of Hyptis suaveolens. The present study investigated the vasorelaxant mechanism of ethyl rosmarinate in isolated rat aortic rings using an organ bath system. Ethyl rosmarinate (0.1 µM-3mM) produced concentration-dependent relaxation in aortic rings pre-contracted with phenylephrine (10 µM), exhibiting a pD2 value of 4.56 ± 0.08 and an Emax value of 93.82 ± 5.00% (in endothelium-intact rings), as well as a pD2 value of 4.42 ± 0.05 and an Emax value of 92.10 ± 3.78% (in endothelium-denuded rings). In the endothelium-denuded rings, the vasorelaxant effect of ethyl rosmarinate was reduced by only 4-aminopyridine (1mM); however, this was not the case with tetraethylammonium (5mM), glibenclamide (10 µM), barium chloride (1mM), and 1H-[1,2,4] oxadiazolo [4,3-a]quinoxalin-1-one (ODQ, 1 µM). Ethyl rosmarinate also reduced the contraction induced by phenylephrine (10 µM) and caffeine (20mM) in a Ca(2+)-free solution, and inhibited the contraction induced by increasing extracellular Ca(2+) influx, which was induced by KCl (80 mM). Ethyl rosmarinate (10 µM) inhibits concentration-response curves for phenylephrine, while in the same concentration of ethyl rosmarinate has no effect on contractions induced by increasing concentrations of calcium in the presence of high extracellular potassium. Our results suggests that ethyl rosmarinate induces relaxation in aortic rings via an endothelium-independent pathway, which involves the opening of voltage-gated potassium (Kv) channels and the blockade of both Ca(2+)release from intracellular stores and extracellular Ca(2+) influx. Moreover, ethyl-rosmarinate acts on the extracellular Ca(2+) influx inhibition by interacting with voltage-operated calcium channels (VOCCs) and receptor-operated calcium channels (ROCCs).

Time-dependent phenotypic and contractile changes of pulmonary artery in chronic hypoxia-induced pulmonary hypertension

Phenotypic and contractile changes in pulmonary arterial smooth muscle cells (PASMCs) were examined in rats with pulmonary hypertension induced by hypoxia. Exposure to hypoxia induced pulmonary hypertension within 1-4 weeks. Staining with BrdU revealed that proliferative activities of PASMCs peaked at 1 week of hypoxic exposure, and then moderate proliferative activity was maintained for the next 2-4 weeks. The beta-actin/alpha-actin ratio also increased at 1-2 weeks of exposure to hypoxia. Absolute contractility of the pulmonary arterial ring continuously decreased during hypoxia, whereas the basal active tonus of the pulmonary artery increased at 1-3 weeks. Nicardipine, the ETA-receptor antagonis, CI-1034 and the rho-kinase inhibitor Y27632 partially inhibited the elevated active tonus. Endothelin-1 content in the pulmonary hypertensive lung was continuously increased during exposure to hypoxia. In conclusion, the hypoxia-induced proliferative activity of PASMCs comprised a transient phase followed by a sustained phase. The change in PASMCs from a contractile to a synthetic phenotype also correlated with proliferative activity, which subsequently decreased PASMC contractility. The continuous production of endothelin-1 upon hypoxic exposure might contribute to the increased basal tonus of the pulmonary arterial wall, which might subsequently increase pulmonic arterial pressure, resulting in accelerated pulmonary hypertension.

Attenuation of contractility in rat epididymal vas deferens by Rho kinase inhibitors

The involvement of Ca(2+) sensitization mediated through Rho kinase in the contractility of rat epididymal vas deferens was investigated using Rho kinase inhibitors, trans-4-[(1R)-1-aminoethyl]-N-4-pyridinilcyclohexanecarboxamide dihydrochloride (Y-27632) and 1-(5-isoquinolinesulphonyl)homopiperazine (HA 1077), in comparison with myosin light chain kinase (MLCK) inhibitors, wortmannin and 1-(5-chloronaphthalenesulphonyl)homopiperazine (ML-9) and agents that affect protein kinase C (PKC) and non-receptor tyrosine kinase intracellular signalling. 2 In Ca(2+)-free/ethyleneglycol-bis-(beta-aminoethylether)N,N,N('),N(')-tetraacetic acid (EGTA) (1 mM) medium, noradrenaline evoked sustained contractions. Y-27632 and HA 1077 caused a concentration-dependent inhibition and complete relaxation (IC(50), 1.08 and 1.75 microM respectively). The Ca(2+)-free contraction was reduced by wortmannin (10 microM) or ML-9 (10 microM) but not by inhibitors of diacylglycerol metabolism, 3-[2-[4[bis(4-Fluoropheny)methylene]-1-piperidinyl]-2,3-dihydro-2-thioxi-4(H)-quinazolinone (R59949) (10 microm) or 1,6-bis(cyclohexyloximinocarbonylamino)hexane (RHC-80267) (10 microM) or by the phospholipase A(2) (PLA(2)) inhibitor, quinacrine (up to 100 microM) or tyrosine kinase inhibitor, genistein (30 microM). 3 In the presence of Ca(2+) (2.5 mM), noradrenaline (100 microM) evoked rhythmic activity and biphasic tonic contractions. Y-27632 (1-10 microM) or HA 1077 (1-10 microM) reduced the amplitude of rhythmic activity and tonic contractions. ML-9 (10 microM) attenuated the occurrence of rhythmic activity and modestly reduced the tonic contractions. ML-9 (10 microM) combined with Y-27632 (10 microM) significantly reduced the tonic contractions. ML-9 (30 microM) alone (or combined with Y-27632 10 microM) suppressed the rhythmic activity and substantially reduced (or abolished) the tonic contractions. 4 Contractions evoked by high [K(+)](o) (120 mM) or alpha,beta-methylene ATP (10 microM) were reduced significantly by Y-27632 (1-3 microM) indicating that the Rho kinase signalling pathway is activated by direct tissue depolarization or by stimulation of ligand-gated P(2X) purinoceptors. 5 Collectively, these results indicate that Ca(2+)-sensitization mediated by Rho kinase is involved in agonist- or depolarization-induced contraction of rat epididymal vas deferens. It is the major contractile mechanism underlying noradrenaline-induced Ca(2+)-free responses. It contributes to Ca(2+)-dependent rhythmic contractility and optimizes the development of full contractile tension triggered through calmodulin/MLCK activation by stimulated influx of Ca(2+).

Mechanisms of endothelin-1-induced contraction in pulmonary arteries from chronically hypoxic rats

Endothelin-1 (ET-1), a potent vasoconstrictor, is believed to contribute to the pathogenesis of hypoxic pulmonary hypertension. Previously we demonstrated that contraction induced by ET-1 in intrapulmonary arteries (IPA) from chronically hypoxic (CH) rats occurred independently of changes in intracellular Ca2+concentration ([Ca2+]i), suggesting that ET-1 increased Ca2+sensitivity. The mechanisms underlying this effect are unclear but could involve the activation of myosin light chain kinase, Rho kinase, PKC, or tyrosine kinases (TKs), including those from the Src family. In this study, we examined the effect of pharmacological inhibitors of these kinases on maximum tension generated by IPA from CH rats (10% O2for 21 days) in response to ET-1. Experiments were conducted in the presence of nifedipine, an L-type Ca2+channel blocker, to isolate the component of contraction that occurred without a change in [Ca2+]i. The mean change in tension caused by ET-1 (10−8M) expressed as a percent of the maximum response to KCl was 184.0 ± 39.0%. This response was markedly inhibited by the Rho kinase inhibitors Y-27632 and HA-1077 and the TK inhibitors genistein, tyrphostin A23, and PP2. In contrast, staurosporine and GF-109203X, inhibitors of PKC, had no significant inhibitory effect on the tension generated in response to ET-1. We conclude that the component of ET-1-induced contraction that occurs without a change in [Ca2+]iin IPA from CH rats requires activation of Rho kinase and TKs, but not PKC.

Involvement of RhoA-mediated Ca2+ sensitization in antigen-induced bronchial smooth muscle hyperresponsiveness in mice

Background

It has recently been suggested that RhoA plays an important role in the enhancement of the Ca²⁺ sensitization of smooth muscle contraction. In the present study, a participation of RhoA-mediated Ca²⁺ sensitization in the augmented bronchial smooth muscle (BSM) contraction in a murine model of allergic asthma was examined.

Methods

Ovalbumin (OA)-sensitized BALB/c mice were repeatedly challenged with aerosolized OA and sacrificed 24 hours after the last antigen challenge. The contractility and RhoA protein expression of BSMs were measured by organ-bath technique and immunoblotting, respectively.

Results

Repeated OA challenge to sensitized mice caused a BSM hyperresponsiveness to acetylcholine (ACh), but not to high K⁺-depolarization. In α-toxin-permeabilized BSMs, ACh induced a Ca²⁺ sensitization of contraction, which is sensitive to Clostridium botulinum C3 exoenzyme, indicating that RhoA is implicated in this Ca²⁺ sensitization. Interestingly, the ACh-induced, RhoA-mediated Ca²⁺ sensitization was significantly augmented in permeabilized BSMs of OA-challenged mice. Moreover, protein expression of RhoA was significantly increased in the hyperresponsive BSMs.

Conclusion

These findings suggest that the augmentation of Ca²⁺ sensitizing effect, probably via an up-regulation of RhoA protein, might be involved in the enhanced BSM contraction in antigen-induced airway hyperresponsiveness.

Direct block by bisindolylmaleimide of the voltage-dependent K+ currents of rat mesenteric arterial smooth muscle

We investigated the effect of bisindolylmaleimide (I), a widely used protein kinase C (PKC) inhibitor, on the voltage-dependent K(+) (Kv) currents of rat mesenteric arterial smooth muscle cells using the whole-cell patch-clamp technique. Bisindolylmaleimide (I) reversibly and dose-dependently inhibited the Kv currents with an apparent K(d) value of 0.23+/-0.001 microM. The blockade was apparently through the acceleration of the decay rate of the Kv currents. The apparent rate constants of association and dissociation for bisindolylmaleimide (I) were 17.9+/-1.6 microM(-1) s(-1) and 4.1+/-1.5 s(-1), respectively. The inhibition of Kv current by bisindolylmaleimide (I) was steeply voltage-dependent between -30 and 0 mV (voltage range of channel activation). Bisindolylmaleimide (I) had no effect on the steady-state activation and inactivation of the Kv currents. Applications of trains of pulses at 1 or 2 Hz lead to a progressive increase in the bisindolylmaleimide (I)-blockade, and the recovery from bisindolylmaleimide (I)-block at -80 mV exhibited a time constant of 577.2+/-52.7 ms. Bisindolylmaleimide (V), an inactive analogue of bisindolylmaleimide (I), similarly inhibited the Kv currents with an apparent K(d) value of 1.48+/-0.004 microM, but other PKC inhibitor chelerythrine little affected the Kv currents. These results suggest that bisindolylmaleimide (I) directly inhibits the Kv currents of rat mesenteric arterial smooth muscle cells independently of PKC inhibition, in a state-, voltage-, time- and use-dependent manner.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

Chronic treatment with interleukin-1beta attenuates contractions by decreasing the activities of CPI-17 and MYPT-1 in intestinal smooth muscle

Interleukin-1beta (IL-1beta) is a proinflammatory cytokine that plays a central role in inflammatory bowel disease (IBD). In order to elucidate the mechanism of motility disorders frequently observed in IBD, we investigated the long term effects of IL-1beta on rat ileal smooth muscle contractility by using an organ culture system. When ileal smooth muscle strips were cultured with IL-1beta (10 ng/ml), contractions elicited by high K+ and carbachol were inhibited in a time-dependent manner. IL-1beta more strongly inhibited the carbachol-induced contractions than high K+ with decreasing myosin light chain phosphorylation. In the alpha-toxin-permeabilized ileal muscle, carbachol with GTP or guanosine 5'-3-O-(thio)triphosphate increased the Ca2+ sensitivity of contractile elements, and this G protein-coupled Ca2+ sensitization was significantly reduced in the IL-1beta-treated ileum. Among the functional proteins involved in the smooth muscle Ca2+ sensitization, CPI-17 expression was significantly reduced after the culture with IL-1beta, whereas the expressions of RhoA, ROCK-I, ROCK-II, MYPT-1, myosin light chain kinase, and myosin phosphatase (PP1) were unchanged. The phosphorylation level of CPI-17 by carbachol was low in accordance with the decrease in CPI-17 expression due to IL-1beta treatment. In contrast, constitutively phosphorylated MYPT-1 was also decreased in the IL-1beta-treated muscles. These results suggest that long term treatment with IL-1beta decreases either CPI-17 expression or MYPT-1 phosphorylation, which may result in an increase in myosin phosphatase activity to reduce force generation. Based on these findings, we consider IL-1beta to be an important mediator of gastrointestinal motility disorders in IBD, and CPI-17 and MYPT-1 are key molecules in the decreased smooth muscle contractility due to IL-1beta.

Involvement of Rho-kinase in contraction of guinea-pig aorta induced by prostanoid EP3 receptor agonists

1. The mechanism of contraction of guinea-pig isolated aorta induced by the prostanoid EP(3) receptor agonist sulprostone (0.1-300 nM) has been investigated. In 60% of the experiments, the sulprostone log concentration-response curve (maximum=15-40% of 100 nM U-46619 response; low-responders) was unaffected by the removal of extracellular Ca(2+), blockade of L-type Ca(2+) channels with nifedipine and depletion of internal Ca(2+) stores. In the remaining preparations (35-65% of 100 nM U-46619 response; high-responders), contractions to higher sulprostone concentrations showed a nifedipine-sensitive component, which was enhanced by charybdotoxin. 2. In Ca(2+)-free Krebs solution, established contractions to 300 nM sulprostone were abolished by the Rho-kinase inhibitors H-1152, Y-27632 and HA-1077 (IC(50) values=190, 770 and 2030 nM). The PKA/Rho-kinase inhibitor H-89 (10 nM-10 micro M) caused enhancement progressing to inhibition. The selective PKC inhibitor Ro 32-0432 (3 micro M) had no effect, while staurosporine, recently shown to be a potent Rho-kinase inhibitor, abolished sulprostone responses (IC(50) approximately 47 nM), but its action was slow. The MAP kinase inhibitors SB 202190, SB 203580 and PD 80958 produced little inhibition. 3. In normal Krebs solution, H-1152 and Y-27632 abolished established contractions to 300 nM sulprostone and 1 micro M phenylephrine, and partially inhibited 10 micro M phenylephrine and 50 mM K(+) responses. 4. The results are discussed in relation to the reported potencies of the protein kinase inhibitors in enzyme assays. Activation of the Rho-kinase pathway appears to be a primary mechanism of contraction induced by EP(3) receptor agonists in guinea-pig aorta.

Effects of prostaglandin F2alpha on membrane currents in rabbit middle cerebral arterial smooth muscle cells

Although PGF(2alpha) affects contractility of vascular smooth muscles, no studies to date have addressed the electrophysiological mechanism of this effect. The purpose of our investigation was to examine the direct effects of PGF(2alpha) on membrane potentials, Ca(2+)-activated K(+) (K(Ca)) channels, delayed rectifier K(+) (K(V)) channels, and L-type Ca(2+) channels with the patch-clamp technique in single rabbit middle cerebral arterial smooth muscle cells (SMCs). PGF(2alpha) significantly hyperpolarized membrane potentials and increased the amplitudes of total K(+) currents. PGF(2alpha) increased open-state probability but had little effect on the open and closed kinetics of K(Ca) channels. PGF(2alpha) increased the amplitudes of K(V) currents with a leftward shift of the activation and inactivation curves and a decrease in the activation time constant. PGF(2alpha) decreased the amplitudes of L-type Ca(2+) currents without any significant change in threshold or apparent reversal potentials. This study provides the first finding that the direct effects of PGF(2alpha) on middle cerebral arterial SMCs, at least in part, could attenuate vasoconstriction.

Essential role of rho kinase in the Ca2+ sensitization of prostaglandin F(2alpha)-induced contraction of rabbit aortae

Inhibition of dephosphorylation of the 20 kDa myosin light chain (MLC(20)) is an important mechanism for the Ca(2+)-induced sensitization of vascular smooth muscle contraction. We investigated whether this mechanism operates in prostaglandin F(2alpha) (PGF(2alpha))-induced contraction of rabbit aortic smooth muscle and, if so, whether protein kinase C (PKC) or rho-associated kinase (rho kinase) contribute to the inhibition of dephosphorylation. In normal medium, PGF(2alpha) (10 microM) increased the phosphorylation of MLC(20) and developed tension. The rho-kinase inhibitors fasudil and hydroxyfasudil inhibited these changes, despite having no effect on a phorbol-ester-induced MLC(20) phosphorylation. After treatment with verapamil or chelation of external Ca(2+) with EGTA, PGF(2alpha) increased the MLC(20) phosphorylation and tension without an increase in [Ca(2+)](i), all of which were sensitive to fasudil and hydroxyfasudil. ML-9, a MLC kinase inhibitor, quickly reversed the KCl-induced MLC(20) phosphorylation and contraction to the resting level. However, fractions of PGF(2alpha)-induced contraction and MLC(20) phosphorylation were resistant to ML-9 but were sensitive to fasudil. Ro31-8220 (10 microM), a PKC inhibitor, did not affect the phosphorylation of MLC(20) and the tension caused by PGF(2alpha), thus excluding the possibility of the involvement of PKC in the PGF(2alpha)-induced MLC(20) phosphorylation. PGF(2alpha) increased phosphorylation at Thr654 of the myosin binding subunit (MBS) of myosin phosphatase, which is a target of rho kinase, and fasudil decreased the phosphorylation. These data suggest that the PGF(2alpha)-induced contraction is accompanied by the inhibition of MLC(20) dephosphorylation through rho kinase-induced MBS phosphorylation, leading to Ca(2+) sensitization of contraction. An actin-associated mechanism may also be involved in the PGF(2alpha)-induced sensitization.

Mechanism of enhanced calcium sensitivity and alpha 2-AR vasoreactivity in chronic NOS inhibition hypertension

PKC augments calcium sensitivity in spontaneously hypertensive rats and contributes to alpha(2)-adrenergic receptor (AR) contraction in rabbit saphenous vein. We showed previously that denuded aortic rings from N(omega)-nitro-l-arginine-treated hypertensive rats (LHR) contract more to CaCl(2) and to the alpha(2)-AR agonist UK-14304 than do rings from normotensive rats (NR). We hypothesized that enhanced PKC activity or a change in PKC isoform contributes to augmented calcium sensitivity and enhanced alpha(2)-AR contraction in LHR aorta. Current studies demonstrate that non-isoform-specific PKC inhibitors reduced UK-14304 contraction in both NR and LHR aorta. However, the calcium-dependent PKC inhibitor Gö-6976 only attenuated contraction in LHR aorta. Additionally, UK-14304 translocated PKC-delta to the membrane in NR aorta, whereas PKC-alpha was translocated to the membrane in LHR aorta. Finally, in ionomycin-permeabilized aorta Gö-6976 eliminated enhanced basal and augmented alpha(2)-AR-stimulated calcium sensitivity in LHR aorta but did not affect NR contraction. Together, these data suggest that PKC-alpha contributes to augmented calcium sensitivity and alpha(2)-AR reactivity after chronic nitric oxide synthase inhibition hypertension.

Sphingosylphosphorylcholine is a novel messenger for Rho-kinase-mediated Ca2+ sensitization in the bovine cerebral artery: unimportant role for protein kinase C

Although recent investigations have suggested that a Rho-kinase-mediated Ca2+ sensitization of vascular smooth muscle contraction plays a critical role in the pathogenesis of cerebral and coronary vasospasm, the upstream of this signal transduction has not been elucidated. In addition, the involvement of protein kinase C (PKC) may also be related to cerebral vasospasm. We recently reported that sphingosylphosphorylcholine (SPC), a sphingolipid, induces Rho-kinase-mediated Ca2+ sensitization in pig coronary arteries. The purpose of this present study was to examine the possible mediation of SPC in Ca2+ sensitization of the bovine middle cerebral artery (MCA) and the relation to signal transduction pathways mediated by Rho-kinase and PKC. In intact MCA, SPC induced a concentration-dependent (EC50=3.0 micromol/L) contraction, without [Ca2+]i elevation. In membrane-permeabilized MCA, SPC induced Ca2+ sensitization even in the absence of added GTP, which is required for activation of G-proteins coupled to membrane receptors. The SPC-induced Ca2+ sensitization was blocked by a Rho-kinase inhibitor (Y-27632) and a dominant-negative Rho-kinase, but not by a pseudosubstrate peptide for conventional PKC, which abolished the Ca2+-independent contraction induced by phorbol ester. In contrast, phorbol ester-induced Ca2+ sensitization was resistant to a Rho-kinase inhibitor and a dominant-negative Rho-kinase. In primary cultured vascular smooth muscle cells, SPC induced the translocation of cytosolic Rho-kinase to the cell membrane. We propose that SPC is a novel messenger for Rho-kinase-mediated Ca2+ sensitization of cerebral arterial smooth muscle and, therefore, may play a pivotal role in the pathogenesis of abnormal contraction of the cerebral artery such as vasospasm. The SPC/Rho-kinase pathway functions independently of the PKC pathway.

Effects of Y-27632 on acetylcholine-induced contraction of intact and permeabilized intrapulmonary bronchial smooth muscles in rats

In the present study, the effects of a selective Rho-associated coiled-coil forming protein kinase (ROCK) inhibitor, Y-27632 [(+)-(R)-trans-4-(1-aminoethyl)-(4-pyridyl)cyclohexanecarboxamide dihydrochloride] on acetylcholine-induced contraction and Ca(2+) sensitization of rat bronchial smooth muscle were examined. Intact and beta-escin-permeabilized muscles of the third branch of intrapulmonary bronchi were used. In intact muscles, Y-27632 (10(-6)-10(-4) M) concentration-dependently inhibited acetylcholine-induced contractile responses. In acetylcholine (10(-3) M)-precontracted intact muscles, the maximal relaxation (about 50% inhibition of contraction) was obtained by a concentration of 10(-4) M Y-27632, which had no effect on the resting tone. In beta-escin-permeabilized muscles, addition of acetylcholine (10(-5)-10(-3) M) plus GTP (100 microM) induced a further contraction, i.e., Ca(2+) sensitization at a constant Ca(2+) concentration of pCa=6.0. The acetylcholine-induced Ca(2+) sensitization was completely blocked in the presence of 10(-4) M Y-27632, whereas the Ca(2+)-induced contraction itself was not affected by Y-27632. Immunoblot study revealed the expression of ROCK-I and ROCK-II proteins in the intrapulmonary bronchi of rats. These findings suggest that Y-27632 dilates acetylcholine-mediated contraction of rat bronchial smooth muscle by inhibiting RhoA/ROCK-mediated Ca(2+) sensitization.

Alpha-adrenoceptor agonists produce Ca2+ oscillations in isolated rat aorta: role of protein kinase C

We investigated the relationship between tension development and the cytosolic free Ca2+ level ([Ca2+]i) in responses to norepinephrine (NE) and selective alpha2-adrenoceptor agonist, UK14,304 of the endothelium-denuded rat aorta loaded with fura PE-3. NE (3 x 10(-8) M) evoked a rapid increase in [Ca2+]i followed by slight decreasing to a steady state level and produced a contraction. After the NE-induced increase in [Ca2+]i had reached a maximum, the [Ca2+]i showed persistent oscillations. The Ca2+ oscillations were superimposed on the sustained increase in [Ca2+]i. UK14,304 (3 x 10(-6) M) also evoked an increase in [Ca2+]i and produced a contraction. However, the UK14,304-induced effect on [Ca2+]i was characterized by pronounced oscillations, and the amplitude of the sustained increase in [Ca2+]i was less than that seen with NE. Protein kinase C inhibitor, Ro31-8220 (3 x 10(-6) M) and verapamil (10(-5) M) abolished both NE and UK14,304-evoked Ca2+ oscillations. UK14,304-induced contractions were also strongly inhibited by Ro31-8220 and verapamil. However, NE induced contractions were partly inhibited by these inhibitors. The sustained increases in [Ca2+]i evoked NE and UK14,304 were not significantly inhibited by Ro31-8220 and verapamil. These results suggest that NE and UK14,304 produce Ca2+ oscillations during sustained contractions in rat aorta. The alpha2 adrenoceptor agonist, UK14,304-induced sustained contraction and Ca2+ oscillations may be due to PKC activation and opening of voltage-dependent L type Ca2+ channels.

Acetylcholine-induced smooth muscle contraction of intrapulmonary small bronchi is augmented in antigen-induced airway hyperresponsive rats

Smooth muscle responsiveness of intrapulmonary small bronchi obtained from repeatedly antigen-challenged rats was compared with that from control animals to determine whether smooth muscle contractility of peripheral airways is augmented by such repeated challenge. In intact (non-permeabilized) smooth muscles of intrapulmonary bronchi, the acetylcholine (ACh)-induced contractile response was significantly augmented in the repeated challenge group, although 60-mM K+-induced contraction was within the normal level. In beta-escin-permeabilized muscles, no significant difference between groups was observed in the Ca2+-induced contractile responses. Thus, augmented ACh-induced contraction of intact intrapulmonary small bronchial smooth muscle might be, at least in part, due to an enhanced ACh-mediated Ca2+-sensitizing signal.

Pharmacomechanical coupling in rat vas deferens: effects of agents that modulate intracellular release of calcium and protein kinase C activation

The effects of agents that modulate intracellular release of calcium and protein kinase C (PKC) activation on noradrenaline (NA)-induced contractions of epididymal vas deferens in calcium-free/EGTA (1 mM) medium were investigated. NA (100 microM) or methoxamine (100 microM) evoked repeatable contractions. Clonidine (100-300 microM) was ineffective. The contractions to NA were reduced by procaine (1-10 mM) but not by thapsigargin (0.1-30 microM), ryanodine (1-30 microM) or TMB-8 (1-30 microM). Contractions to cumulative additions of NA (1-100 microM) were enhanced in the presence of cyclopiazonic acid (10 & 30 microM) but not ryanodine (10 & 30 microM). Sequential contractions to NA were not blocked by PKC inhibitors, calphostin C (1 microM) or Ro 31-8220 (1-30 microM) but were reduced by H-7 (1-30 microM), a broad spectrum protein kinase inhibitor. Although RT-PCR experiments detected mRNA for some Ca2+-dependent/DAG-activated and Ca2+-independent/DAG-activated PKC isoforms in epididymal vas deferens, the PKC activators, phorbol 12, 13-dibutyrate (100 microM) or phorbol 12-myristate 13-acetate (100 microM) failed to activate the tissues in calcium-free medium but enhanced subsequent contractions to NA. These results indicate a limited role for intracellular calcium stores and phorbol ester/DAG-sensitive PKC isoforms in NA-induced contraction of epididymal rat vas deferens in calcium-free medium. The results suggest that pharmacomechanical coupling triggered by NA may involve the sensitization of contractile myofilaments to Ca2+ or a Ca2+-independent mechanism. The possible involvement of Ca2+-independent/DAG-insensitive PKC isoforms and agonist-dependent but PKC-independent sensitization pathway is discussed.

Augmented acetylcholine-induced, Rho-mediated Ca2+ sensitization of bronchial smooth muscle contraction in antigen-induced airway hyperresponsive rats

Treatment with acetylcholine (ACh) of a beta-escin-permeabilized intrapulmonary bronchial smooth muscle of the rat induced force when the Ca2+ concentration was clamped at 1 microM. The ACh-induced Ca2+ sensitization of myofilaments was significantly greater in antigen-induced airway hyperresponsive rats than in control rats. The ACh-induced Ca2+ sensitization was completely blocked by treatment with Clostridium botulinum C3 exoenzyme, an inactivator of Rho family of proteins. Moreover, the protein level of RhoA in the intrapulmonary bronchi was significantly increased in the airway hyperresponsive rats. Thus, increased airway smooth muscle contractility observed in asthmatics may be related to augmented agonist-induced, Rho-mediated Ca2+ sensitization of myofilaments.

Examination of adenosine receptor-mediated relaxation of the pig coronary artery

1. The adenosine receptors mediating relaxation of porcine isolated left anterior descending coronary arteries (LAD) and the effects of the level and type of preconstriction on the responses to adenosine analogues were examined in the present study. 2. Relaxation responses to the non-selective adenosine receptor agonist N-ethylcarboxamidoadenosine (NECA) were endothelium independent. N-Ethylcarboxamidoadenosine, GR 79236 (A1 receptor selective) and 8-cyclopentyl-1,3-dipropylxanthine (CGS 21680) (A2A receptor selective) produced full relaxation in LAD precontracted to 50% of the response to potassium depolarization with the thromboxane receptor agonist U46619. The order of potency was CGS 21680 = NECA > GR 79236, consistent with that defining the A2A receptor subtype. 3. 3,7-Dimethyl-1-propargylxanthine (DMPX; A2 receptor selective) competitively antagonized NECA and CGS 21680 with pKB values of 4.95 +/- 0.09 and 5.06 +/- 0.22, respectively. The A1 receptor selective antagonist 1,3-[3H]-dipropyl-8-cyclopentylxanthine (DPCPX) had no effect on NECA relaxation, even in the presence of DMPX. 4. The sensitivity to relaxation by NECA was dependent on the precontracting agent. Arteries precontracted with endothelin (ET)-1 were most sensitive to NECA, U46619-precontracted arteries were intermediate and KCl-precontracted arteries were least sensitive. 5. The potency of NECA was reduced when the preconstriction level was increased from 50 to 90% of maximum in U46619-precontracted arteries (pEC50 7.94 +/- 0.12 and 7.35 +/- 0.04, respectively) and, in KCl-precontracted arteries, both the potency and maximum effect of NECA were reduced when the preconstriction level increased from 50 to 80% of maximum (pEC50 7.52 +/- 0.13 and 6.91 +/- 0.26, respectively; maximum responses 82.5 +/- 10.2 and 23.9 +/- 3.6%, respectively, of the preconstricted tone). Relaxation responses to NECA were independent of the level of precontraction in ET-1-precontracted arteries. 6. In porcine LAD, relaxation responses to adenosine analogues were endothelium independent and were mediated via A2A adenosine receptors. Responses to NECA were dependent on both the level and type of preconstriction.

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.

Selective activation of excitation-contraction coupling pathways by ET(A) and ET(B) receptors in guinea-pig tracheal smooth muscle

1. Signalling events responsible for endothelin(A) (ET(A)) and ET(B) receptor-induced contraction were examined in epithelium-denuded guinea-pig tracheal smooth muscle strips. Selective stimulation of each subtype was achieved by a combination of ET-1 (100 nM) and ET(A) and ET(B) receptor-selective antagonists, BQ-123 (10 microM) and BQ-788 (3 microM), respectively. 2. Both ET(A) and ET(B) receptors induced long-lasting contraction that was totally dependent on Ca2+ influx. Stimulation of ET(A) receptor induced both transient and sustained (Ca2+)i increases whereas that of ET(B) receptor induced only a sustained increase. Suppression of the transient (Ca2+)i increase by U73122 (3 microM) did not affect the ET(A)-induced sustained (Ca2+)i increase and tension development. Stimulation of ET(A) receptor, but not ET(B), induced phosphoinositide breakdown and protein kinase C (PKC). The activated PKC contributed to the contraction by increasing the Ca2+ sensitivity of the contractile apparatus. 3. Thus, ET(A) receptor is coupled both with phospholipase C/Ca2+/PKC signalling and Ca2+ influx pathways whereas ET(B) receptor was coupled only with the latter. 4. Stimulation of ET(B) receptor, but not ET(A), caused membrane depolarization measured with a fluorescent indicator, bis-(1,3 dibutylbarbituric acid)-trimethine oxonol. Both nifedipine (1 microM) and verapamil (10 microM) abolished ET(B)-induced Ca2+ influx and contraction, while they barely affected ET(A)-induced responses. 5. Therefore, the Ca2+ influx pathways activated by each subtype appeared to be completely different; ET(A) and ET(B) receptors opens voltage-independent Ca2+ channels and L-type voltage-dependent Ca2+ channels, respectively.

From pharmacomechanical coupling to G-proteins and myosin phosphatase

A brief summary of recent studies of pharmacomechanical coupling is presented, with emphasis on the role of GTP-binding proteins and Ca(2+)-independent regulation of contraction (Ca(2+)-sensitization/desensitization) through regulatory myosin light chain (MLC20) phosphorylation and dephosphorylation. Pharmacomechanical regulation of cytosolic [Ca2+] is largely, though not solely, controlled by the phosphatidylinositol cascade and Ca(2+)-pumps of the plasma membrane and the sarcoplasmic reticulum. The monomeric GTPase, RhoA, is a major upstream component of Ca(2+)-sensitization. Its crystal structure and apparently obligatory translocation to the plasma membrane for activation of its downstream effectors are described. Inhibition of RhoA activity by a membrane-permeant ADP-ribosylating bacterial exoenzyme, DC3B, causes severe depression of the tonic component of agonist-induced contraction, suggesting that this component is largely due to Ca(2+)-sensitization. A relatively specific inhibitor (Y27632) of Rho-kinase, a downstream effector of Ca(2+)-sensitization (Uehata et al 1997), also inhibits oxytoxin-induced Ca(2+)-sensitization of myometrium. The major mechanism of physiological, G-protein-coupled Ca(2+)-sensitization is through inhibition of smooth muscle myosin phosphatase (SMPP-1M), whereas conventional or novel protein kinase Cs play very little or no role in this process. Mechanisms of Ca(2+)-desensitization include inhibition of myosin light chain kinase and activation of SMPP-1M. Activation of SMPP-1M in phasic smooth muscle can be attributed, at least in part, to the synergistic phosphatase activating activities of a cyclic nucleotide-dependent kinase and its major substrate, telokin.

Tyrosine phosphorylation as a convergent pathway of heterotrimeric G protein- and rho protein-mediated Ca2+ sensitization of smooth muscle of rabbit mesenteric artery

1. The aim of this study was to determine whether different signal transduction mechanisms underlie the Ca2+ sensitizing effects of guanosine 5'-O-(3-thiotriphosphate) (GTP(gamma)S) and receptor agonists on beta-escin-skinned smooth muscle of rabbit mesenteric artery. 2. In the homogenate of the beta-escin-skinned arterial strip, C3 exoenzyme of Clostridium botulinum catalyzed the [32P]-ADP-ribosylation of only one protein that had the same molecular mass as the protein detected in Western blots with anti-rho p21 antibody. Pretreatment of preparations with C3 resulted in great inhibition of GTP(gamma)S-induced Ca2+ sensitization, although the effect of GTP(gamma)S at higher concentrations (> or = 30 microM) was not completely blocked by this treatment. In contrast, the enhancement by phenylephrine and histamine, in the presence of guanosine 5'-triphosphate, of the Ca2+-induced contraction was not affected by C3 pretreatment. 3. The protein kinase C (PKC) inhibitors calphostin C and staurosporine completely eliminated the enhancement by phorbol ester 12,13-dibutyrate of the Ca2+-induced contraction. However, these PKC inhibitors had no effect on GTP(gamma)S- and receptor agonist-induced Ca2+ sensitization. 4. The tyrosine kinase inhibitors genistein and tyrphostin 25 caused an irreversible and complete block of the enhancement by GTP(gamma)S of the Ca2+-induced contraction without affecting this Ca2+ contraction. The inactive genistein analogue daidzein did not modify the effect of GTP(gamma)S. The Ca2+ sensitizing effects of phenylephrine and histamine were also blocked by these tyrosine kinase inhibitors. 5. These results suggest that rho p21 predominantly mediates GTP(gamma)S-induced Ca2+ sensitization of beta-escin-skinned smooth muscle of rabbit mesenteric artery, while the Ca2+ sensitizing actions of heterotrimeric G protein-coupled receptor agonists do not involve this small G protein. However, it seems that tyrosine phosphorylation, but not PKC activation, plays an important role in both of the rho p21 protein- and heterotrimeric G protein-mediated Ca2+ sensitization mechanisms.

The effects of the Rho-kinase inhibitor Y-27632 on arachidonic acid-, GTPgammaS-, and phorbol ester-induced Ca2+-sensitization of smooth muscle

The effects of the Rho-kinase inhibitor, Y-27632 [1] on Ca2+-sensitization of force induced by arachidonic acid (AA), phorbol 12,13-dibutyrate (PDBu), GTPgammaS, and by the stable thromboxane analog, 9,11-dideoxy-9alpha,11alpha-methanoepoxy-PGF2alpha (U-46619), were determined in alpha-toxin-permeabilized smooth muscles. Y-27632 relaxed (up to 99%) Ca2+-sensitization by GTPgammaS (10 microM) and U46619 (1 microM), but not by PDBu (20 microM), and reduced GTPgammaS-induced myosin light chain (MLC20) phosphorylation from 28% to 17% (P=0.002). GTPgammaS-induced force sensitization was inhibited by Y-27632 more potently when the inhibitor was added during the plateau of force than prior to stimulation. In alpha-toxin-permeabilized smooth muscle, Y-27632 inhibited AA (50 microM)-induced Ca2+-sensitization of force (by 66 +/- 1.3%) and reduced MLC20 phosphorylation. In contrast, Y-27632 did not relax force Ca2+-sensitized by AA in smooth muscle permeabilized with Triton X-100. We conclude that (i) AA induces Ca2+-sensitization through dual mechanisms, one mediated by Rho-kinase (or a related kinase), and (ii) Rho-kinase is not required for phorbol ester-induced Ca2+-sensitization.

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.

The unimportance of being (protein kinase C) epsilon

The purpose of our study was to determine the mechanism through which phorbol esters and smooth muscle myosin phosphatase inhibitors can induce contraction of smooth muscle in the absence of Ca2+. Protein kinase C-epsilon (PKC-epsilon) was previously implicated in this process based largely on its supposed absence in the ferret portal vein, and a correlation was drawn between the presence of this isoform and the ability of smooth muscle to contract independently of Ca2+ and phosphorylation of the 20 kDa regulatory light chains of myosin (MLC20). We demonstrate here, with two antibodies, one to the NH2 terminus and the other to the COOH terminus of PKC-epsilon, that epsilon is present in both ferret portal vein and rabbit portal vein smooth muscle, neither of which exhibits phorbol ester-induced contraction in the absence of Ca2+. However, in the presence of clamped submaximal Ca2+, phorbol es ter increased MLC20 phosphorylation from 17.7+/-1.7% to 46.4+/-3.6% in ferret portal vein smooth muscle and evoked an increase in force. Prolonged (48 h) incubation of ferret portal vein with phorbol esters completely down-regulated PKC-epsilon, as shown by Western blots, and abolished the phorbol ester-evoked contraction at submaximal Ca2+, but not Ca2+-independent, contractions induced by the phosphatase inhibitor microcystin. Contractions induced by microcystin in Ca2+-free solution were associated with increased phosphorylation of myosin light chain kinase (MLCK). Activation of MLCK by autophosphorylation in the absence of Ca2+ occurs in vitro (1). We conclude that PKC-epsilon is neither necessary nor sufficient for Ca2+-independent regulation of myosin II in smooth muscle, but contractions induced by agents that inhibit smooth muscle myosin phosphatase in the absence of Ca2+ may be mediated by MLCK autophosphorylated or activated by another Ca2+-independent kinase.

Regulatory mechanisms of calcium sensitization of contractile elements in smooth muscle

It is evident that smooth muscle contraction is regulated not only by the Ca2+/calmodulin/myosin light chain kinase system but also by modulation of Ca2+ sensitivity. Changes in free calmodulin concentrations, myosin light chain phosphorylation elicited by rho/rho-kinase, regulation of myosin phosphatase activity and thin filament-linked mechanisms are the possible mechanisms for regulation of Ca2+ sensitivity.

Effect of phorbol esters on Ca2+ sensitivity and myosin light-chain phosphorylation in airway smooth muscle

We studied in beta-escin-permeabilized canine tracheal smooth muscle (CTSM) the effect of the protein kinase C (PKC) agonist phorbol 12,13-dibutyrate (PDBu) on isometric force at a constant submaximal Ca2+ concentration (i.e., the effect on Ca2+ sensitivity) and regulatory myosin light-chain (rMLC) phosphorylation. PDBu increased Ca2+ sensitivity, an increase associated with a concentration-dependent, sustained increase in rMLC phosphorylation. PDBu altered the relationship between rMLC phosphorylation and isometric force such that the increase in isometric force was less than that expected for the increase in rMLC phosphorylation observed. The effect of four PKC inhibitors [calphostin C, chelerythrine chloride, a pseudosubstrate inhibitor for PKC, PKC peptide-(19-31) (PSSI), and staurosporine] on PDBu-induced Ca2+ sensitization as well as the effect of calphostin C and PSSI on rMLC phosphorylation were determined. Whereas none of these compounds prevented or reversed the PDBu-induced increase in Ca2+ sensitivity, the PDBu-induced increase in rMLC phosphorylation was inhibited. We conclude that PDBu increases rMLC phosphorylation by activation of PKC but that the associated PDBu-induced increases in Ca2+ sensitivity are mediated by mechanisms other than activation of PKC in permeabilized airway smooth muscle.

Myogenic activation and calcium sensitivity of cannulated rat mesenteric small arteries

Pressure-induced activation of vascular smooth muscle may involve electromechanical as well as nonelectromechanical coupling mechanisms. We compared calcium-tone relations of cannulated rat mesenteric small arteries during pressure-induced activation, depolarization (16 to 46 mmol/L K+), and alpha1-adrenergic stimulation (1 micromol/L phenylephrine). The intracellular calcium concentration was expressed as the fura-2 ratio, normalized to the maximal and minimal ratios. In order to compare activation levels at various pressures, tone was expressed as the ratio of active wall tension to the maximal active tension. The passive and maximal active pressure-diameter relations needed for the calculation of tone were determined in a separate set of experiments, using isometric loading of cannulated vessels. Pressure steps from 20 to 60 and then to 100 mm Hg caused a modest rise of calcium. Nifedipine (1 micromol/L) blocked both the calcium rise and the resulting myogenic responses. Electromechanical coupling could not fully account for the myogenic response: the calcium sensitivity, defined as the slope of the calcium-tone relation, was five times higher during pressure-induced activation compared with potassium stimulation and twice as high as the sensitivity during alpha1-adrenergic stimulation. We therefore conclude that the myogenic response involves a small but necessary rise in calcium due to influx through L-type calcium channels, as well as a nonelectromechanical coupling mechanism that greatly enhances the calcium sensitivity of the contractile machinery.

Intracellular Ca2+ elevation and contraction due to prostaglandin F2alpha in rat aorta

Prostaglandin F2alpha was tested to determine (a) whether its effect on intracellular Ca2+ levels ([Ca2+]i) and force in vascular smooth muscle was mediated through activation of the thromboxane A2 and/or prostaglandin receptor, and (b) the relative roles of Ca2+ influx via L-type and non-L-type Ca2+ channels in prostaglandin receptor-mediated contraction. [Ca2+]i and force were measured simultaneously in fura-2-loaded rat aortic strips. The thromboxane A2 receptor antagonist, SQ29548 ([1S]-1a,2b(5Z),3b,4a-7-(3-[2-[(phenylamino)carbonyl] hydrazinomethyl)-7-oxobicyclo-[2.2.1]hept-2-yl-5-heptenoic acid), prevented the prostaglandin F2alpha-induced plateau [Ca2+]i elevation and force by 80-90%, while abolishing these responses due to the thromboxane A2 receptor agonist, U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F2alpha). Prostaglandin F2alpha (+ SQ29548)-induced plateau [Ca2+]i elevation and force were not inhibited by verapamil. Ni2+, a non-selective cation channel blocker, in the presence of verapamil, abolished the prostaglandin F2alpha (+ SQ29548)-elevated [Ca2+]i, while the contraction was only partially inhibited. These results suggest that, in rat aorta, (1) elevated [Ca2+]i and force due to high prostaglandin F2alpha concentrations largely results from thromboxane A2 receptor activation, and (2) the prostaglandin component of the prostaglandin F2alpha-induced contraction is dependent on Ca2+ influx via non-L-type channels.

Role of protein kinase C in calcium sensitization during muscarinic stimulation in airway smooth muscle

Muscarinic receptor stimulation increases Ca2+ sensitivity, i.e., the amount of force produced at a constant submaximal cytosolic Ca2+ concentration ([Ca2+]i), in permeabilized smooth muscle preparations. It is controversial whether this increase in Ca2+ sensitivity is in part mediated by protein kinase C (PKC). With the use of a beta-escin permeabilized canine tracheal smooth muscle (CTSM) preparation, the effect of four putative PKC inhibitors [calphostin C, chelerythrine chloride, a pseudosubstrate inhibitor for PKC [PKC peptide-(19-31)], and staurosporine] on Ca2+ sensitization induced by acetylcholine (ACh) plus GTP was determined. Preincubation with each of the inhibitors did not affect subsequent Ca2+ sensitization induced by muscarinic receptor stimulation in the presence of a constant submaximal [Ca2+]i, neither did any of these compounds reverse the increase in Ca2+ sensitivity induced by ACh plus GTP. Administration of a 1,2-diacylglycerol analog, 1-oleoyl-2-acetyl-sn-glycerol, did not induce Ca2+ sensitization at a constant submaximal [Ca2+]i. Thus we found no evidence that PKC mediates increases in Ca2+ sensitivity produced by muscarinic receptor stimulation in permeabilized CTSM.

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.

Different regulation of myofilament Ca2+ sensitivity in beta-escin-skinned cardiac and vascular smooth muscles

We compared the effects of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS, an activator of G-protein), phorbol 12,13-dibutylate (PDB, an activator of protein kinase C) and pimobendan (an inotropic agent with Ca2+-sensitizing action) on the Ca2+ sensitivity of the contractile proteins in beta-escin-skinned muscle preparations obtained from rabbit left ventricles and mesenteric arteries. After the skinning procedure, when GTPgammaS (100 microM) or PDB (1 microM) was added to the Ca2+ solutions, pCa50 were significantly increased in preparations obtained from vascular smooth muscle, but not from cardiac muscle, indicating that G-protein- and protein kinase C-mediated direct Ca2+ sensitization may occur only in smooth muscle, but not in cardiac muscle. In contrast, pimobendan (50 microM) increased the Ca2+ responsiveness only in cardiac muscle. Therefore, we conclude that, in addition to the common regulatory factors affecting Ca2+ sensitivity such as intracellular pH and phosphorylation by protein kinase A, there are other means of regulation of Ca2+ sensitivity working differently in cardiac and in vascular smooth muscles.

Possible role of atypical protein kinase C activated by arachidonic acid in Ca2+ sensitization of rabbit smooth muscle

1. Diacylglycerol (DAG; 10 microM), an activator of conventional and novel protein kinases C (cPKCs and nPKCs), induced Ca2+ sensitization of force in isolated intact and alpha-toxin-permeabilized femoral artery (FA) and portal vein (PV), and increased the phosphorylation of myosin light chain (MLC20) at the same peptides phosphorylated by myosin light chain kinase. 2. Ca2+ sensitization by DAG was specifically inhibited by a pseudosubstrate peptide inhibitor of cPKCs (PKC alpha(22-30) peptide; 50 microM). Similarly, GF 109203X (600 nM), an inhibitor of cPKCs and nPKCs, completely abolished Ca2+ sensitization by phorbol 12,13-dibutyrate (PDBu; 1 microM). In contrast, Ca2+ sensitization induced by the alpha1-adrenergic agonist phenylephrine (100 microM) was not inhibited by these inhibitors of cPKCs and nPKCs. 3. A pseudosubstrate peptide inhibitor of the atypical PKCs (aPKCs) PKC zeta(116-124) (50 microM) significantly (about 50%) inhibited the Ca2+ sensitization of force and MLC20 phosphorylation induced by 100 microM phenylephrine and by 300 microM arachidonic acid, but not that by DAG (10 microM) or PDBu (1 microM). 4. A phospholipase A2 (PLA2) inhibitor, ONO-RS-082 (10 microM), abolished the release of arachidonic acid and partially (by 40%) inhibited the Ca2+ sensitization induced by phenylephrine in FA smooth muscle. This effect was not additive to the inhibition observed with the aPKC inhibitor peptide, suggesting that arachidonic acid and aPKCs exert their effects via the same pathway, probably through activation of aPKC(s) by arachidonic acid. 5. Western blot analysis with antibodies to aPKCs revealed aPKCs zeta, lambda (or iota) and an unidentified 64 kDa protein. The distribution (cytosolic and particulate) of these proteins was not affected by PDBu (1 microM). 6. Our results are consistent with a significant role for atypical (or related) PKCs through a PLA2-arachidonic acid-aPKC pathway in agonist-induced Ca2+ sensitization, in parallel with a similar, but minor role of the DAG-cPKC cascade. The inability of the combination of the two (aPKC and cPKC) inhibitors to completely eliminate Ca2+ sensitization also suggests the presence of a third, still unidentified, pathway of this mechanism.

Down-regulation of G-protein-mediated Ca2+ sensitization in smooth muscle

Prolonged treatment with guanosine 5'-[gamma-thio]triphosphate (GTP gamma S; 5-16 h, 50 microM) of smooth muscle permeabilized with Staphylococcus aureus alpha-toxin down-regulated (abolished) the acute Ca2+ sensitization of force by GTP gamma S, AIF-4, phenylephrine, and endothelin, but not the response to phorbol dibutyrate or a phosphatase inhibitor, tautomycin. Down-regulation also abolished the GTP gamma S-induced increase in myosin light chain phosphorylation at constant [Ca2+] and was associated with extensive translocation of p21rhoA to the particulate fraction, prevented its immunoprecipitation, and inhibited its ADP ribosylation without affecting the immunodetectable content of G-proteins (p21rhoA, p21ras, G alpha q/11, G alpha i3, and G beta) or protein kinase C (types alpha, beta 1, beta 2, delta, epsilon, eta, theta, and zeta). We conclude that the loss of GTP gamma S- and agonist-induced Ca2+ sensitization through prolonged treatment with GTP gamma S is not due to a decrease in the total content of either trimeric (G alpha q/11, G alpha i3, and G beta) or monomeric (p21rhoA and p21ras) G-protein or protein kinase C but may be related to a structural change of p21rhoA and/or to down-regulation of its (yet to be identified) effector.

Mechanisms of altered contractile responses to vasopressin and endothelin in canine coronary collateral arteries

BACKGROUND

Mature coronary collateral arteries are hyperresponsive to vasopressin; in contrast, contractile responses of collaterals to endothelin are attenuated. Our goal was to determine the cellular mechanisms underlying these differences in reactivity using two sizes of canine collateral arteries isolated from hearts subjected to chronic coronary occlusion.

METHODS AND RESULTS

Contractile responses to vasopressin (100 mmol/L) were enhanced threefold to fourfold in near-resistance (approximately 200 microns lumen diameter) and conduit (approximately 500 microns lumen diameter) collateral arteries compared with similarly sized noncollateral coronary arteries (P < .01). In contrast, contractions of both sizes of collaterals in response to endothelin (0.01 to 30 nmol/L) were smaller than responses of size-matched noncollateral arteries (P < .05). Pretreatment with either indomethacin (5 mumol/L), a cyclooxygenase inhibitor, or NG-nitro-L-arginine methyl ester (100 mumol/L), a nitric oxide synthase inhibitor, did not alter the relative responsiveness of collateral arteries to vasopressin or endothelin compared with noncollateral arteries. Vasopressin produced greater increases of intracellular free Ca2+ (measured by use of fura-2 microfluorometry and Ca(2+)-dependent 42K+ efflux) in smooth muscle of collateral arteries than in smooth muscle of noncollateral arteries (P < .05). Surprisingly, endothelin-induced increases of Ca2+ were not different in smooth muscle of collateral and noncollateral arteries (P > .05).

CONCLUSIONS

We conclude that altered contractile responsiveness of collateral arteries to vasopressin and endothelin does not result from altered synthesis/release of nitric oxide or prostaglandins. Parallel enhancement of vasopressin-mediated Ca2+ and contractile responses suggests increases in vasopressin receptor number, affinity, and/or efficiency of coupling mechanisms in collateral smooth muscle. The dissociation between endothelin-induced contractile and Ca2+ responses of collaterals indicates that the mechanisms involved in increasing Ca2+ sensitivity of contractile proteins during endothelin stimulation may be altered in collateral arteries.

Inhibition by bosentan, an endothelin antagonist, of the hypersensitivity to Ca2+ channel activator evoked by salt-loading in basilar artery of stroke-prone spontaneously hypertensive rats

High salt diet dramatically decreases the life time of spontaneously hypertensive stroke-prone rats (SHRSP). This has been related to an increase in the incidence of stroke. We have investigated the influence of high salt diet on the reactivity to the Ca2+ channel activator Bay K 8644 of basilar artery isolated from SHRSP. The results show that the sensitivity of basilar artery to Bay K 8644 was increased by salt load and that this hypersensitivity was blunted by bosentan, an ETA/ETB antagonist.

Separate upstream and convergent downstream pathways of G-protein- and phorbol ester-mediated Ca2+ sensitization of myosin light chain phosphorylation in smooth muscle

The effect of phorbol ester-induced down-regulation of protein kinase C (PKC) on diacylglycerol (sn-1,2-dioctanoylglycerol, diC8)- and G-protein-coupled Ca2+ sensitization and on the relationship between phosphorylation of the regulatory myosin light chains (MLC20) and force during Ca2+ sensitization were investigated in rabbit portal vein (PV), femoral artery (FA) and ileum smooth muscle. The effects of phorbol dibutyrate (PDBu), guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and agonists on the membrane versus cytosolic distribution of PKC isoenzymes were also determined. Down-regulation of PKC abolished Ca2+ sensitization of force and the accompanying increases in MLC20 phosphorylation induced by PDBu, as well as Ca2+ sensitization of force by diC8, but not that by GTP[S], aluminum fluoride (AIF4-) or agonists (phenylephrine, endothelin or carbachol). Down-regulation also inhibited the PDBu-, but not the GTP[S]-induced increase in force under Ca(2+)-free conditions. In ileum, PDBu translocated PKCs alpha, beta 1, beta 2, epsilon and theta to the membrane fraction, and GTP[S] caused a small translocation of PKC-epsilon. Carbachol- and GTP[S]-induced Ca2+ sensitization remained unaffected in down-regulated ileum in which no cytosolic PKC-epsilon was detectable. We conclude that, although both phorbol ester-induced and G-protein-coupled Ca2+ sensitization of force are mediated by increased MLC20 phosphorylation, it is likely that PKCs alpha, beta 1, beta 2, epsilon and theta do not play an essential role in, although they may contribute to, the G-protein-coupled mechanism.

Calphostin C-sensitive enhancements of force by lysophosphatidylinositol and diacylglycerols in mesenteric arteries from the rat

1. A pharmacological characterization was made of the effects of lysophosphatidyl-inositol (lysoPI) and -ethanolamine (lysoPE) on the Ca(2+)-sensitivity of contraction in alpha-toxin permeabilized rat mesenteric arteries. The effect of GTP gamma S (G-protein activator), diacylglycerols (DAGs, dioctanoyl glycerol (diC8) and 1-stearoyl-2-arachidonoyl-sn-glycerol) and phorbol myristate acetate (PMA, protein kinase C (PKC) activator) on Ca(2+)-sensitivity was also assessed. 2. LysoPI increased the Ca(2+)-sensitivity, demonstrated by both an increase in tension induced by 1 microM [Ca2+]free and an increase in the Ca(2+)-sensitivity of Ca2+ concentration-tension curves. LysoPE did not enhance force or Ca(2+)-sensitivity. 3. GTP gamma S enhanced force at constant Ca2+, increased the Ca(2+)-sensitivity, and increased force under Ca(2+)-free conditions. PMA also increased force at constant Ca2+ and increased Ca(2+)-sensitivity, but caused no force development under Ca(2+)-free conditions. 4. DAGs, both diC8 and the more physiological relevant DAG, 1-stearoyl-2-arachidonoyl-sn-glycerol, enhanced force at constant Ca2+ and increased the Ca(2+)-sensitivity. DiC8, in contrast to 1-stearoyl-2-arachidonoyl-sn-glycerol, caused force development under Ca(2+)-free conditions and substantially enhanced force at maximal Ca(2+)-induced contraction. GDP-beta-S abolished the increased Ca(2+)-sensitization induced by noradrenaline, but not that by DAGs. 5. The PKC inhibitor calphostin C completely abolished Ca(2+)-sensitization induced by all of the Ca(2+)-sensitizing agents. 6. These results show that lysoPI can increase the Ca(2+)-sensitivity of smooth muscle contraction, and the Ca(2+)-sensitization induced by DAGs was not completely G-protein mediated, because it was not inhibited by GDP-beta-S. A central role for PKC in regulation of Ca(2+)-sensitization in rat mesenteric small arteries was indicated by the abolishment of Ca(2+)-sensitization by calphostin C.

Identification of protein kinase C isoforms in rat mesenteric small arteries and their possible role in agonist-induced contraction

We have identified immunologically the protein kinase C (PKC) isoforms present in rat mesenteric small arteries, defined their distribution between particulate and soluble fractions, and studied their involvement in phorbol ester-induced contraction. Our analysis revealed the presence of the CA(2+)-dependent PKCs (alpha and gamma), Ca(2+)-independent PKCs (delta and epsilon), and the atypical isoform (zeta). PKCbeta could not be detected, whereas PKCgamma is likely to be of neural origin. All isoforms exhibited different distributions. PKCalpha, PKCepsilon, and PKCzeta were found in both particulate and soluble fractions. In contrast, PKCdelta was mainly in the particulate fraction, and PKCgamma was in the soluble fraction. Phorbol esters, which activate PKC and cause smooth muscle contraction, downregulated only the alpha and delta isoforms. This was associated with a parallel loss of contractile response to phorbol ester. The force developed to submaximal concentrations of noradrenaline was decreased after phorbol dibutyrate pretreatment, although the sensitivity and maximal response were unchanged. Phorbol ester pretreatment did not affect the contractile response to vasopressin. The sensitivity to non-receptor-mediated contraction, caused by k+ in the presence of prazosin, was slightly reduced by 4 alpha- and 4 beta-phorbol ester pretreatment. Maximal tension in response to this agonist was not affected. We conclude that PKCalpha and/or PKCdelta is necessary for phorbol ester-mediated contraction but is not essential for noradrenaline-, vasopressin-, or k(+)-induced contraction, demonstrating differences in the mechanisms involved in the contractile response between these agents.

MCI-154-induced relaxation in vascular smooth muscles of guinea pig

We investigated the vasorelaxant effects of MCI-154, a cardiotonic agent designed to target thin filaments in cardiac muscles in intact and skinned vessels from guinea pigs. In normal Krebs-Henseleit solution, MCI-154 (10(-7)-10(-4) M) inhibited the contractions induced by angiotensin II, (Ang II), endothelin-1 (ET-1), phenylephrine, and phorbol 12-myristate 13-acetate (PMA) in a concentration-dependent manner in guinea pig aorta. In Ca(2+)-free solutions, ET-1 and PMA caused slowly developing and sustained contractions in guinea pig aorta, whereas phenylephrine and caffeine induced transient contractions due to Ca2+ release from the sarcoplasmic reticulum (SR). MCI-154 (10(-7)-10(-4) M) inhibited the contractile responses to ET-1 and PMA. MCI-154 also reduced the contraction induced by Ca2+ release from phenylehrine- and caffeine-sensitive Ca2+ store sites. On the other hand, the relaxation response to MCI-154 was not affected by the presence of methylene blue, a guanylate cyclase inhibitor or by the removal of endothelial cells. MCI-154 decreased the Ca(2+)-activated tension development in saponin-treated skinned fibers from guinea pig femoral arteries. The effects of MCI-154 were not potentiated in the presence of protein kinase A (PKA), whereas those of cyclic AMP were potentiated, possibly because of lack of protein kinase A. The present experiments demonstrate that MCI-154 inhibits vascular contraction when the contractions are produced by any of three mechanisms: protein kinase C (PKC) activation, Ca2+ mobilization from store sites, or sensitization of contractile elements by Ca2+.

Phospholipase A2 and protein kinase C contribute to myofilament sensitization to 5-HT in the rabbit mesenteric artery

1. Calcium (Ca2+, 0.1-100 microM) stimulated concentration-dependent contractions in small strips from the rabbit mesenteric artery in which the smooth muscle cells had been permeabilized with Staphylococcus aureus alpha-toxin. 2. 5-Hydroxytryptamine (5-HT) and phenylephrine, each in the presence of 10 microM guanosine 5'-triphosphate (GTP), concentration-dependently stimulated additional contractions in strips sub-maximally contracted by the presence of a buffered concentration of calcium (0.3 microM). All the additional contraction was abolished with the selective inhibitor of protein kinase C, Ro 31-8220 (10 microM). 3. Quinacrine (10-50 microM), an inhibitor of phospholipase A2, selectively inhibited the sensitization to 5-HT, but did not alter the sensitization to either phenylephrine or GTP. 4. Myofilament sensitization to calcium was mimicked by exogenous arachidonic acid (300 microM, in the presence of indomethacin, miconazole and BW755c) and the stable analogue of arachidonic acid, 5,8,11,14-eicosatetrayonic acid (ETYA, 100 microM), and in both cases did not require the additional presence of GTP. Ro 31-8220, but not quinacrine, reduced the sensitization to arachidonic acid by around 30%. 5. These results indicate that G protein-linked myofilament sensitization to calcium in the mesenteric artery that follows the activation of 5-HT receptors, but not alpha 1-receptors, involves phospholipase A2. The sensitization stimulated by each of these different receptors, and a component of the response to arachidonic acid, also appears to involve the activation of protein kinase C.

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.

Effect of phorbol ester, 12-deoxyphorbol 13-isobutylate (DPB), on muscle tension and cytosolic Ca2+ in rat anococcygeus muscle

Effects of phorbol ester, 12-deoxyphorbol 13-isobutyrate (DPB), on muscle tension and cytosolic Ca2+ ([Ca2+]i) level was investigated in rat anococcygeus muscle in comparison with other smooth muscles. 1) DPB (10(-6) M) induced a large contraction and an elevation of [Ca2+]i level in rat aorta and small and rhythmic changes in tension and [Ca2+]i level in guinea pig ileum. However, DPB did not change either of the parameters in rat anococcygeus muscle. 2) DPB caused tension development without changing the [Ca2+]i level elevated by high K+, ionomycin or beta-escin in the anococcygeus muscle. 3) In the beta-escin permeabilized muscles of guinea pig ileum and urinary bladder, rabbit mesenteric artery and rat anococcygeus muscle, DPB enhanced the Ca(2+)-developed tension. Moreover, the enhancement was inhibited by H-7 (3 x 10(-5) M). 4) DPB did not cause muscle tension to develop in the muscle of rat aorta, guinea pig ileum and rat anococcygeus muscle, pretreated with phorbol 12-myristate 13-acetate for 24 hr. In conclusion, DPB showed different contractile effects on the aorta, ileum and anococcygeus muscle, respectively. The initiation of muscle tension by DPB probably requires [Ca2+]i and the DPB-induced enhancement may be due to a Ca2+ sensitization of contractile elements in the anococcygeus muscle. Therefore, the difference between the DPB-induced response of the anococcygeus muscle and those of the other muscles seems to be due to a different Ca2+ movement caused by DPB. Moreover, it is suggested that DPB develops muscle tension by increasing [Ca2+]i and enhances it through the mediation of protein kinase C in the anococcygeus muscle as well as the other smooth muscles.

Pharmacological investigation of signaling mechanisms contributing to phasic and tonic components of the contractile response of rat arteries to noradrenaline

The mechanisms contributing to the contractile responses to two different concentrations of noradrenaline (NA) in rat aorta and mesenteric artery were compared using nifedipine, which inhibits calcium influx through dihydropyridine-sensitive channels, ryanodine, which depletes intracellular calcium stores, and calphostin C, which inhibits protein kinase C (PKC). Both submaximal and maximal concentrations of NA induced a biphasic response in aorta and mesenteric artery, with an early fast phase and a later sustained tonic component. Calcium release from intracellular stores contributed to the phasic component of contraction to both concentrations of NA in aorta, although to a greater extent to the submaximal concentration. In aorta, inhibiting both intracellular calcium release and calcium influx through dihydropyridine-sensitive channels simultaneously or inhibiting PKC activity alone significantly reduced the tonic response to a maximal concentration of NA. However, the tonic response to a submaximal concentration of NA in aorta was significantly inhibited only when intracellular calcium stores were depleted with ryanodine. In mesenteric artery, the phasic response to a maximal concentration of NA was significantly depressed only when both calcium influx and intracellular calcium release were inhibited simultaneously. However, the phasic response to a submaximal concentration of NA was significantly inhibited by blocking calcium influx alone, but not by blocking intracellular calcium release alone. The tonic component of the contractile response to both concentrations of NA in mesenteric artery appeared to be mediated in part by calcium from both intracellular and extracellular sources.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of the endothelin ETA and ETB receptors to Ca2+ mobilization and Ca2+ sensitization in vascular smooth muscle

Effects of endothelins on cytosolic Ca2+ level ([Ca2+]i) and contraction were examined in the swine pulmonary artery and vein. In the artery, endothelin-1 and endothelin-3, but not sarafotoxin S6c and IRL 1620 (300 nM each), induced transient increase followed by sustained increase in [Ca2+]i and sustained contraction. These effects were inhibited by the ETA receptor antagonist, BQ-123. In the vein, endothelin-1 and endothelin-3 (300 nM each) induced sustained increase in [Ca2+]i and sustained contraction whereas sarafotoxin S6c and IRL 1620 (300 nM each) transiently increased both [Ca2+]i and contractile tension. The ETB receptor in the vein was desensitized by pretreatment with sarafotoxin S6c, abolishing the effects of sarafotoxin S6c and IRL 1620 without changing the effects of endothelin-1 and endothelin-3. In contrast, an ETB antagonist, RES-701-1, antagonized the effects of IRL 1620 without changing the effects of other stimulants. In both artery and vein, the maximum contraction induced by these stimulants was greater than that induced by KCl at a given [Ca2+]i. In the absence of external Ca2+, endothelin-1 and endothelin-3 induced transient increase in [Ca2+]i and slow sustained contraction in both artery and vein. In the vein, sarafotoxin S6c induced small sustained contraction without changing [Ca2+]i. In the permeabilized artery and vein, endothelin-1 augmented the contraction induced by Ca2+. These results suggest that the ETA receptors in the artery and vein are coupled to Ca2+ release (which does not seem to trigger contraction), Ca2+ influx and Ca2+ sensitization.(ABSTRACT TRUNCATED AT 250 WORDS)