Regulation of the Ca2+ dependence of smooth muscle contraction

Role and regulation of Abelson tyrosine kinase in Crk-associated substrate/profilin-1 interaction and airway smooth muscle contraction

BACKGROUND

Airway smooth muscle contraction is critical for maintenance of appropriate airway tone, and has been implicated in asthma pathogenesis. Smooth muscle contraction requires an "engine" (myosin activation) and a "transmission system" (actin cytoskeletal remodeling). However, the mechanisms that control actin remodeling in smooth muscle are not fully elucidated. The adapter protein Crk-associated substrate (CAS) regulates actin dynamics and the contraction in smooth muscle. In addition, profilin-1 (Pfn-1) and Abelson tyrosine kinase (c-Abl) are also involved in smooth muscle contraction. The interplays among CAS, Pfn-1 and c-Abl in smooth muscle have not been previously investigated.

METHODS

The association of CAS with Pfn-1 in mouse tracheal rings was evaluated by co-immunoprecipitation. Tracheal rings from c-Abl conditional knockout mice were used to assess the roles of c-Abl in the protein-protein interaction and smooth muscle contraction. Decoy peptides were utilized to evaluate the importance of CAS/Pfn-1 coupling in smooth muscle contraction.

RESULTS

Stimulation with acetylcholine (ACh) increased the interaction of CAS with Pfn-1 in smooth muscle, which was regulated by CAS tyrosine phosphorylation and c-Abl. The CAS/Pfn-1 coupling was also modified by the phosphorylation of cortactin (a protein implicated in Pfn-1 activation). In addition, ACh activation promoted the spatial redistribution of CAS and Pfn-1 in smooth muscle cells, which was reduced by c-Abl knockdown. Inhibition of CAS/Pfn-1 interaction by a decoy peptide attenuated the ACh-induced actin polymerization and contraction without affecting myosin light chain phosphorylation. Furthermore, treatment with the Src inhibitor PP2 and the actin polymerization inhibitor latrunculin A attenuated the ACh-induced c-Abl tyrosine phosphorylation (an indication of c-Abl activation).

CONCLUSIONS

Our results suggest a novel activation loop in airway smooth muscle: c-Abl promotes the CAS/Pfn-1 coupling and actin polymerization, which conversely facilitates c-Abl activation. The positive feedback may render c-Abl in active state after contractile stimulation.

The Dynamic Actin Cytoskeleton in Smooth Muscle

Smooth muscle contraction requires both myosin activation and actin cytoskeletal remodeling. Actin cytoskeletal reorganization facilitates smooth muscle contraction by promoting force transmission between the contractile unit and the extracellular matrix (ECM), and by enhancing intercellular mechanical transduction. Myosin may be viewed to serve as an "engine" for smooth muscle contraction whereas the actin cytoskeleton may function as a "transmission system" in smooth muscle. The actin cytoskeleton in smooth muscle also undergoes restructuring upon activation with growth factors or the ECM, which controls smooth muscle cell proliferation and migration. Abnormal smooth muscle contraction, cell proliferation, and motility contribute to the development of vascular and pulmonary diseases. A number of actin-regulatory proteins including protein kinases have been discovered to orchestrate actin dynamics in smooth muscle. In particular, Abelson tyrosine kinase (c-Abl) is an important molecule that controls actin dynamics, contraction, growth, and motility in smooth muscle. Moreover, c-Abl coordinates the regulation of blood pressure and contributes to the pathogenesis of airway hyperresponsiveness and vascular/airway remodeling in vivo. Thus, c-Abl may be a novel pharmacological target for the development of new therapy to treat smooth muscle diseases such as hypertension and asthma.

Physiological vs. pharmacological signalling to myosin phosphorylation in airway smooth muscle

KEY POINTS

Smooth muscle myosin regulatory light chain (RLC) is phosphorylated by Ca²⁺ /calmodulin-dependent myosin light chain kinase and dephosphorylated by myosin light chain phosphatase (MLCP). Tracheal smooth muscle contains significant amounts of myosin binding subunit 85 (MBS85), another myosin phosphatase targeting subunit (MYPT) family member, in addition to MLCP regulatory subunit MYPT1. Concentration/temporal responses to carbachol demonstrated similar sensitivities for bovine tracheal force development and phosphorylation of RLC, MYPT1, MBS85 and paxillin. Electrical field stimulation releases ACh from nerves to increase RLC phosphorylation but not MYPT1 or MBS85 phosphorylation. Thus, nerve-mediated muscarinic responses in signalling modules acting on RLC phosphorylation are different from pharmacological responses with bath added agonist. The conditional knockout of MYPT1 or the knock-in mutation T853A in mice had no effect on muscarinic force responses in isolated tracheal tissues. MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

ABSTRACT

Ca²⁺ /calmodulin activation of myosin light chain kinase (MLCK) initiates myosin regulatory light chain (RLC) phosphorylation for smooth muscle contraction with subsequent dephosphorylation for relaxation by myosin light chain phosphatase (MLCP) containing regulatory (MYPT1) and catalytic (PP1cδ) subunits. RLC phosphorylation-dependent force development is regulated by distinct signalling modules involving protein phosphorylations. We investigated responses to cholinergic agonist treatment vs. neurostimulation by electric field stimulation (EFS) in bovine tracheal smooth muscle. Concentration/temporal responses to carbachol demonstrated tight coupling between force development and RLC phosphorylation but sensitivity differences in MLCK, MYPT1 T853, MYPT1 T696, myosin binding subunit 85 (MBS85), paxillin and CPI-17 (PKC-potentiated protein phosphatase 1 inhibitor protein of 17 kDa) phosphorylations. EFS increased force and phosphorylation of RLC, CPI-17 and MLCK. In the presence of the cholinesterase inhibitor neostigmine, EFS led to an additional increase in phosphorylation of MYPT1 T853, MYPT1 T696, MBS85 and paxillin. Thus, there were distinct pharmacological vs. physiological responses in signalling modules acting on RLC phosphorylation and force responses, probably related to degenerate G protein signalling networks. Studies with genetically modified mice were performed. Expression of another MYPT1 family member, MBS85, was enriched in mouse, as well as bovine tracheal smooth muscle. Carbachol concentration/temporal-force responses were similar in trachea from MYPT1^SM+/+ , MYPT1^SM-/- and the knock-in mutant mice containing nonphosphorylatable MYPT1 T853A with no differences in RLC phosphorylation. Thus, MYPT1 T853 phosphorylation was not necessary for regulation of RLC phosphorylation in tonic airway smooth muscle. Furthermore, MLCP activity may arise from functionally shared roles between MYPT1 and MBS85, resulting in minimal effects of MYPT1 knockout on contraction.

Vimentin dephosphorylation at ser-56 is regulated by type 1 protein phosphatase in smooth muscle

Background

The intermediate filament protein vimentin undergoes reversible phosphorylation and dephosphorylation at Ser-56, which plays an important role in regulating the contraction-relaxation cycles of smooth muscle. The protein phosphatases that mediate vimentin dephosphorylation in smooth muscle have not been previously investigated.

Methods

The associations of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) with vimentin in mouse tracheal rings was evaluated by co-immunoprecipitation. Lentivirus-mediated shRNA against PP1 was used to assess the role of PP1 in vimentin dephosphorylation and the vimentin-associated process in smooth muscle.

Results

Co-immunoprecipitation analysis showed that vimentin interacted with PP1, but barely with PP2A, in airway smooth muscle. Knockdown of PP1 by lentivirus-mediated shRNA increased the acetylcholine-induced vimentin phosphorylation and smooth muscle contraction. Because vimentin phosphorylation is able to modulate p130 Crk-associated substrate (p130CAS) and actin polymerization, we also evaluated the role of PP1 in the biological processes. Silencing of PP1 also enhanced the agonist-induced the dissociation of p130CAS from vimentin and F/G-actin ratios (an index of actin polymerization). However, PP1 knockdown did not affect c-Abl tyrosine phosphorylation, an important molecule that controls actin dynamics.

Conclusions

Taken together, these findings suggest that PP1 is a key protein serine/threonine phosphatase that controls vimentin Ser-56 dephosphorylation in smooth muscle. PP1 regulates actin polymerization by modulating the dissociation of p130CAS from vimentin, but not by affecting c-Abl tyrosine kinase.

On a phenomenological model for active smooth muscle contraction

This paper presents a three-dimensional phenomenological model for the description of smooth muscle activation. A strain energy function is proposed as sum of the strain energy stored in the passive tissue, consisting of elastin and collagen, and an active calcium-driven energy related to the chemical contraction of the smooth muscle cells. Further, the proposed model includes the dispersions of the orientations of smooth muscle cells and collagen. These dispersions, measured in experiments, can be directly inserted into the model. The approach is implemented into the framework of the finite element method. Consequently, beside a validation with experiments the modelling concept is used for a three-dimensional numerical study.

Influence of dispersion in myosin filament orientation and anisotropic filament contractions in smooth muscle

A new constitutive model for the biomechanical behaviour of smooth muscle tissue is proposed. The active muscle contraction is accomplished by the relative sliding between actin and myosin filaments, comprising contractile units in the smooth muscle cells. The orientation of the myosin filaments, and thereby the contractile units, are taken to exhibit a statistical dispersion around a preferred direction. The number of activated cross-bridges between the actin and myosin filaments governs the contractile force generated by the muscle and also the contraction speed. A strain-energy function is used to describe the mechanical behaviour of the smooth muscle tissue. Besides the active contractile apparatus, the mechanical model also incorporates a passive elastic part. The constitutive model was compared to histological and isometric tensile test results for smooth muscle tissue from swine carotid artery. In order to be able to predict the active stress at different muscle lengths, a filament dispersion significantly larger than the one observed experimentally was required. Furthermore, a comparison of the predicted active stress for a case of uniaxially oriented myosin filaments and a case of filaments with a dispersion based on the experimental histological data shows that the difference in generated stress is noticeable but limited. Thus, the results suggest that myosin filament dispersion alone cannot explain the increase in active muscle stress with increasing muscle stretch.

Abl knockout differentially affects p130 Crk-associated substrate, vinculin, and paxillin in blood vessels of mice

Actin polymerization has recently emerged as an important cellular process that regulates smooth muscle contraction. Abelson tyrosine kinase (Abl) has been implicated in the regulation of actin dynamics and force development in vascular smooth muscle. In the present study, the systolic blood pressure was lower in Abl(-/-) knockout mice compared with wild-type mice. The knockout of Abl diminished the tyrosine phosphorylation of p130 Crk-associated substrate (CAS, an adapter protein associated with smooth muscle contraction) in resistance arteries upon stimulation with phenylephrine or angiotensin II. The agonist-elicited enhancement of F-actin-to-G-actin ratios in arteries assessed by fluorescent microscopy was also reduced in Abl(-/-) mice. It has been known that vinculin is a structural protein that links actin filaments to extracellular matrix via transmembrane integrins, whereas paxillin is a signaling protein associated with focal contacts mediating actin cytoskeleton remodeling. The expression of vinculin and paxillin at protein and messenger levels was lower in arterial vessels from Abl knockout mice. However, the agonist-induced increase in myosin phosphorylation was not attenuated in arteries from Abl knockout mice. These results indicate that Abl differentially regulates Crk-associated substrate, vinculin, and paxillin in arterial vessels. The Abl-regulated cellular process and blood pressure are independent of myosin activation in vascular smooth muscle.

The small heat shock-related protein, HSP20, is a cAMP-dependent protein kinase substrate that is involved in airway smooth muscle relaxation

Activation of the cAMP/cAMP-dependent PKA pathway leads to relaxation of airway smooth muscle (ASM). The purpose of this study was to examine the role of the small heat shock-related protein HSP20 in mediating PKA-dependent ASM relaxation. Human ASM cells were engineered to constitutively express a green fluorescent protein-PKA inhibitory fusion protein (PKI-GFP) or GFP alone. Activation of the cAMP-dependent signaling pathways by isoproterenol (ISO) or forskolin led to increases in the phosphorylation of HSP20 in GFP but not PKI-GFP cells. Forskolin treatment in GFP but not PKI-GFP cells led to a loss of central actin stress fibers and decreases in the number of focal adhesion complexes. This loss of stress fibers was associated with dephosphorylation of the actin-depolymerizing protein cofilin in GFP but not PKI-GFP cells. To confirm that phosphorylated HSP20 plays a role in PKA-induced ASM relaxation, intact strips of bovine ASM were precontracted with serotonin followed by ISO. Activation of the PKA pathway led to relaxation of bovine ASM, which was associated with phosphorylation of HSP20 and dephosphorylation of cofilin. Finally, treatment with phosphopeptide mimetics of HSP20 possessing a protein transduction domain partially relaxed precontracted bovine ASM strips. In summary, ISO-induced phosphorylation of HSP20 or synthetic phosphopeptide analogs of HSP20 decreases phosphorylation of cofilin and disrupts actin in ASM, suggesting that one possible mechanism by which HSP20 mediates ASM relaxation is via regulation of actin filament dynamics.

Dissociation of Crk-associated substrate from the vimentin network is regulated by p21-activated kinase on ACh activation of airway smooth muscle

The intermediate filament protein vimentin has been shown to be required for smooth muscle contraction. The adapter protein p130 Crk-associated substrate (CAS) participates in the signaling processes that regulate force development in smooth muscle. However, the interaction of vimentin filaments with CAS has not been well elucidated. In the present study, ACh stimulation of tracheal smooth muscle strips increased the ratio of soluble to insoluble vimentin (an index of vimentin disassembly) in association with force development. ACh activation also induced vimentin phosphorylation at Ser(56) as assessed by immunoblot analysis. More importantly, CAS was found in the cytoskeletal vimentin fraction, and the amount of CAS in cytoskeletal vimentin was reduced in smooth muscle strips on contractile stimulation. CAS redistributed from the myoplasm to the periphery during ACh activation of smooth muscle cells. The ACh-elicited decrease in CAS distribution in cytoskeletal vimentin was attenuated by the downregulation of p21-activated kinase (PAK) 1 with antisense oligodeoxynucleotides. Vimentin phosphorylation at this residue, the ratio of soluble to insoluble vimentin, and active force in smooth muscle strips induced by ACh were also reduced in PAK-depleted tissues. These results suggest that PAK may regulate CAS release from the vimentin intermediate filaments by mediating vimentin phosphorylation at Ser(56) and the transition of cytoskeletal vimentin to soluble vimentin. The PAK-mediated dissociation of CAS from the vimentin network may participate in the cellular processes that affect active force development during ACh activation of tracheal smooth muscle tissues.

Role of vimentin in smooth muscle force development

Vimentin intermediate filaments undergo spatial reorganization in cultured smooth muscle cells in response to contractile activation; however, the role of vimentin in the physiological properties of smooth muscle has not been well elucidated. Tracheal smooth muscle strips were loaded with antisense oligonucleotides (ODNs) against vimentin and then cultured for 2 days to allow for protein degradation. Treatment with vimentin antisense, but not sense, ODNs suppressed vimentin protein expression; neither vimentin antisense nor sense ODNs affected protein levels of desmin and actin. Force development in response to ACh stimulation or KCl depolarization was lower in vimentin-deficient tissues than in vimentin sense ODN- or non-ODN-treated muscle strips. Passive tension was also depressed in vimentin-depleted muscle tissues. Vimentin downregulation did not attenuate increases in myosin light chain (MLC) phosphorylation in response to contractile stimulation or basal MLC phosphorylation. In vimentin sense ODN-treated or non-ODN-treated smooth muscle strips, the desmosomal protein plakoglobin was primarily localized in the cell periphery. The membrane-associated localization of plakoglobin was reduced in vimentin-depleted muscle tissues. These studies suggest that vimentin filaments play an important role in mediating active force development and passive tension, which are not regulated by MLC phosphorylation. Vimentin downregulation impairs the structural organization of desmosomes, which may be associated with the decrease in force development.

Theoretical model of the interactions between Ca2+, calmodulin and myosin light chain kinase

Active Ca2+/calmodulin (CaM)-dependent myosin light chain kinase (MLCK) plays an important role in the process of MLC phosphorylation and consecutive smooth muscle contraction. Here, we propose a mathematical model of a detailed kinetic scheme describing interactions among Ca2+, CaM and MLCK and taking into account eight different aggregates. The main model result is the prediction of the Ca2+ dependent active form of MLCK, which is in the model taken as proportional to the concentration of Ca4CaM.MLCK complex. Wegscheider's condition is additionally applied as a constraint enabling the prediction of some parameter values that have not yet been obtained by experiments.

Downregulation of profilin with antisense oligodeoxynucleotides inhibits force development during stimulation of smooth muscle

The actin-regulatory protein profilin has been shown to regulate the actin cytoskeleton and the motility of nonmuscle cells. To test the hypothesis that profilin plays a role in regulating smooth muscle contraction, profilin antisense or sense oligodeoxynucleotides were introduced into the canine carotid smooth muscle by a method of reversible permeabilization, and these strips were incubated for 2 days for protein downregulation. The treatment of smooth muscle strips with profilin antisense oligodeoxynucleotides inhibited the expression of profilin; it did not influence the expression of actin, myosin heavy chain, and metavinculin/vinculin. Profilin sense did not affect the expression of these proteins in smooth muscle tissues. Force generation in response to stimulation with norepinephrine or KCl was significantly lower in profilin antisense-treated muscle strips than in profilin sense-treated strips or in muscle strips not treated with oligodeoxynucleotides. The depletion of profilin did not attenuate increases in phosphorylation of the 20-kDa regulatory light chain of myosin (MLC20) in response to stimulation with norepinephrine or KCl. The increase in F-actin/G-actin ratio during contractile stimulation was significantly inhibited in profilin-deficient smooth muscle strips. These results suggest that profilin is a necessary molecule of signaling cascades that regulate carotid smooth muscle contraction, but that it does not modulate MLC20 phosphorylation during contractile stimulation. Profilin may play a role in the regulation of actin polymerization or organization in response to contractile stimulation of smooth muscle.

Role of Crk-associated substrate in the regulation of vascular smooth muscle contraction

A pool of actin monomers is induced to polymerize into actin filaments during contractile stimulation of smooth muscle. The inhibition of actin dynamics by actin polymerization inhibitors depresses active force generation in smooth muscle. In this study, we hypothesized that Crk-associated substrate plays a role in the regulation of contraction and actin dynamics in vascular smooth muscle. Antisense or sense oligodeoxynucleotides for Crk-associated substrate were introduced into carotid smooth muscle tissues by chemical loading. The treatment of smooth muscle strips with antisense oligodeoxynucleotides inhibited the expression of Crk-associated substrates; it did not influence the expression of actin, myosin heavy chain, and paxillin. Sense oligodeoxynucleotides did not affect the expression of these proteins in smooth muscle tissues. Force generation in response to stimulation with norepinephrine or KCl was significantly lower in antisense-treated muscle strips than in sense-treated strips or in muscle strips not treated with oligodeoxynucleotides. The downregulation of Crk-associated substrate did not attenuate increases in phosphorylation of the 20-kDa regulatory light chain of myosin in response to stimulation with norepinephrine. The increase in F-actin/G-actin ratio during contractile stimulation was significantly inhibited in antisense-treated smooth muscle strips. Contractile activation of smooth muscle increased the association of profilin with actin monomers; the depletion of Crk-associated substrate inhibited the increases in the profilin-actin complex in response to contractile stimulation. These results suggest that Crk-associated substrate is a necessary molecule of signaling cascades that regulate active force generation in smooth muscle. This molecule may regulate actin dynamics in smooth muscle in response to contractile stimulation.

Ca2+-dependent activation of Rho and Rho kinase in membrane depolarization-induced and receptor stimulation-induced vascular smooth muscle contraction

Ca2+ sensitization of vascular smooth muscle (VSM) contraction involves Rho-dependent and Rho-kinase-dependent suppression of myosin phosphatase activity. We previously demonstrated that excitatory agonists in fact induce activation of RhoA in VSM. In this study, we demonstrate a novel Ca2+-dependent mechanism for activating RhoA in rabbit aortic VSM. High KCl-induced membrane depolarization as well as noradrenalin stimulation induced similar extents of sustained contraction in rabbit VSM. Both stimuli also induced similar extents of time-dependent, sustained increases in the amount of an active GTP-bound form of RhoA. Consistent with this, the Rho kinase inhibitors HA1077 and Y27632 inhibited both contraction and the 20-kDa myosin light chain phosphorylation induced by KCl as well as noradrenalin, with similar dose-response relations. Either removal of extracellular Ca2+ or the addition of a dihydropyridine Ca2+ channel antagonist totally abolished KCl-induced Rho stimulation and contraction. The calmodulin inhibitor W7 suppressed KCl-induced Rho activation and contraction. Ionomycin mimicked W7-sensitive Rho activation. The expression of dominant-negative N19RhoA suppressed Ca2+-induced Thr695 phosphorylation of the 110-kDa regulatory subunit of myosin phosphatase and phosphorylation of myosin light chain in VSM cells. Finally, either the combination of extracellular Ca2+ removal and depletion of the intracellular Ca2+ store or the addition of W7 greatly reduced noradrenalin-induced and the thromboxane A2 analogue-induced Rho stimulation and contraction. Taken together, these results indicate the existence of the thus-far unrecognized Ca2+-dependent Rho stimulation mechanism in VSM. Excitatory receptor agonists are suggested to use this pathway for simulating Rho.

Calcium-dependent and calcium-independent contractions in smooth muscles

This review examines signal transduction pathways mediating agonist-induced contraction of circular muscle in the body of the esophagus and in the lower esophageal sphincter (LES). In the LES, circular muscle agonists activate a well-defined contractile pathway, involving calcium (Ca(2+))-induced activation of calmodulin and myosin kinase, causing phosphorylation of 20-kDa myosin light chains (MLCs) and contraction. In this pathway, phosphorylation and contraction may be modulated by other factors, resulting, for instance, in inhibition of phosphatase activity, which may potentiate MLC phosphorylation. The agonist-activated contractile pathway of circular muscle from the esophageal body is not as well defined, and it is different from the LES contractile pathway, as it depends on activation of a Ca(2+)-independent protein kinase C (PKC), PKC-epsilon. In this pathway, agonist-induced Ca(2+) influx and/or release activate phospholipases to produce second messengers, such as diacylglycerol and arachidonic acid. The second messengers, however, activate a PKC-epsilon and a contractile pathway, which is Ca(2+) independent. This contractile pathway depends on activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 and of p38 MAP kinase. These kinases are, in turn, linked to the small heat-shock protein HSP27, to integrin-linked kinase, and perhaps to other Ca(2+)-independent kinases, such as zipper kinase capable of producing MLC phosphorylation and contraction.

Is myosin phosphatase regulated in vivo by inhibitor-1? Evidence from inhibitor-1 knockout mice

1. The Ca(2+) sensitivity of smooth muscle contractility is modulated via regulation of phosphatase activity. Protein phosphatase inhibitor-1 (I-1) is the classic type-1 phosphatase inhibitor, but its presence and role in cAMP-dependent protein kinase (PKA) modulation of smooth muscle is unclear. To address the relevance of I-1 in vivo, we investigated smooth muscle function in a mouse model lacking the I-1 protein (I-1((-/-)) mice). 2. Significant amounts of I-1 protein were detected in the wild-type (WT) mouse aorta and could be phosphorylated by PKA, as indicated by (32)P-labelled aortic extracts from WT mice. 3. Despite the significant presence of I-1 in WT aorta, phenylephrine and KCl concentration- isometric force relations in the presence or absence of the PKA pathway activator isoproterenol (isoprenaline) were unchanged compared to I-1((-/-)) aorta. cGMP-dependent protein kinase (PKG) relaxation pathways were also not different. Consistent with these findings, dephosphorylation rates of the 20 kDa myosin light chains (MLC(20)), measured in aortic extracts, were nearly identical between WT and I-1((-/-)) mice. 4. In the portal vein, I-1 protein ablation was associated with a significant (P < 0.05) rightward shift in the EC(50) of isoproterenol relaxation (EC(50) = 10.4 +/- 1.4 nM) compared to the WT value (EC(50) = 3.5 +/- 0.2 nM). Contraction in response to acetylcholine as well as Ca(2+) sensitivity were similar between WT and I-1((-/-)) aorta. 5. Despite the prevalence of I-1 and its activation by PKA in the aorta, I-1 does not appear to play a significant role in contractile or relaxant responses to any pharmacomechanical or electromechanical agonists used. I-1 may play a role as a fine-tuning mechanism involved in regulating portal vein responsiveness to beta-adrenergic agonists.

Ca2+-induced contraction of cat esophageal circular smooth muscle cells

ACh-induced contraction of esophageal circular muscle (ESO) depends on Ca2+ influx and activation of protein kinase Cepsilon (PKCepsilon). PKCepsilon, however, is known to be Ca2+ independent. To determine where Ca2+ is needed in this PKCepsilon-mediated contractile pathway, we examined successive steps in Ca2+-induced contraction of ESO muscle cells permeabilized by saponin. Ca2+ (0.2-1.0 microM) produced a concentration-dependent contraction that was antagonized by antibodies against PKCepsilon (but not by PKCbetaII or PKCgamma antibodies), by a calmodulin inhibitor, by MLCK inhibitors, or by GDPbetas. Addition of 1 microM Ca2+ to permeable cells caused myosin light chain (MLC) phosphorylation, which was inhibited by the PKC inhibitor chelerythrine, by D609 [phosphatidylcholine-specific phospholipase C inhibitor], and by propranolol (phosphatidic acid phosphohydrolase inhibitor). Ca2+-induced contraction and diacylglycerol (DAG) production were reduced by D609 and by propranolol, alone or in combination. In addition, contraction was reduced by AACOCF(3) (cytosolic phospholipase A(2) inhibitor). These data suggest that Ca2+ may directly activate phospholipases, producing DAG and arachidonic acid (AA), and PKCepsilon, which may indirectly cause phosphorylation of MLC. In addition, direct G protein activation by GTPgammaS augmented Ca2+-induced contraction and caused dose-dependent production of DAG, which was antagonized by D609 and propranolol. We conclude that agonist (ACh)-induced contraction may be mediated by activation of phospholipase through two distinct mechanisms (increased intracellular Ca2+ and G protein activation), producing DAG and AA, and activating PKCepsilon-dependent mechanisms to cause contraction.

Regional differences in signalling transduction pathways among smooth muscle cells from rabbit colon

Smooth muscle cells (SMC) from the circular muscle layer of rabbit colon, taken from the proximal and distal regions that are known to have different physiological and motor activities, were used to highlight distinct regional intrinsic myogenic properties and to investigate the correlations between receptor and signalling transduction pathways. Contractile agonists were shown to be more potent on proximal than on distal SMC in inducing contraction and intracellular Ca(2+) increase. Concentration-response curves of agonists-induced Ca(2+) increase were constantly shifted to the right, though remaining parallel, with respect to contraction curves, independently of the region analysed. Using agents activating different steps of cAMP-or cGMP-mediated intracellular cascades, main regional differences were revealed as far as relaxation was concerned. Relaxation of proximal SMC was found to be essentially cGMP mediated, while that of distal SMC was cAMP mediated. In conclusion, the motor patterns of the two regions appear to be influenced by distinct regional biochemical characteristics that are intrinsic to colonic SMC.

Effect of Ca(2+)-independent mechanisms on the hypoxic relaxation of guinea-pig tracheal rings

Hypoxia induces bronchodilation in vivo and in vitro, but the mechanisms are still unclear. To evaluate whether an extra- or intracellular free Ca(2+) ion is involved in the mechanisms of hypoxic relaxation, we simultaneously measured cytosolic Ca(2+)levels and tensions in both intact and denuded guinea-pig tracheal strips precontracted with histamine (100 microM), and assessed the effect of hypoxia on guinea-pig tracheal rings precontracted with okadaic acid (10 microM) and calyculin-A (0.1 approximately 10 microM) under an extracellular Ca(2+)-free state. The exposure of tracheal rings to hypoxia induced an immediate decrease of tracheal tension without decrease in intracellular free Ca(2+)levels. In the presence of okadaic acid but not calyculin-A, hypoxic air exposure caused significant transient reductions in tracheal tone. Further, thapsigargin (5 microM or 10 microM) did not affect hypoxic bronchodilation, suggesting that the release of intracellular Ca(2+) does not take a role in hypoxic bronchodilation. Hypoxic dilation decreased ATP content in epithelium-intact rings but not epithelium-denuded rings, indicating a relationship between hypoxic dilation and change of adenine nucleotide in epithelium-intact rings. Our findings indicate that the epithelium dependent mechanisms of hypoxic relaxation of guinea pig tracheal rings preconstricted with histamine may not be related to the mobilization of extra and intra-cellular Ca(2+).

Preterm labour

Spontaneous preterm labour remains a major obstetric problem because of the high incidence of neonatal mortality or long-term handicap associated with it. The drugs available for the management of preterm labour are poorly effective and have potentially serious side-effects for the mother or fetus. In recent years, there has been a remarkable increase in the knowledge of the biochemical mechanism underlying uterine quiescence and contractility. Many of the G protein-coupled receptors that participate in the regulation of myometrial activity have been cloned and characterized, and their intracellular signalling pathways have been elucidated. The role of G protein receptor kinases in uterine tachyphylaxis is better understood. New developments in our understanding of the cellular mechanisms involved in uterine contractions in idiopathic and infection-associated preterm labour are expected, which will lead to better, more selective therapy for this problem. However, much research remains to be done before the mechanism of human parturition is fully understood.

Increased myosin light chain phosphorylation is not required for growth factor stimulation of collagen matrix contraction

Previous research suggested the possibility that contraction of floating collagen matrices by human fibroblasts required increased myosin light chain (MLC) phosphorylation. In the current studies, we show that increased MLC phosphorylation was neither necessary for platelet-derived growth factor (PDGF)-dependent matrix contraction nor sufficient for lysophosphatidic acid (LPA)-dependent contraction. In contrast, increased MLC phosphorylation did appear to be coupled to the formation of stress fibers by cells spreading in monolayer culture. Signal transduction pathways required for PDGF- and LPA-dependent matrix contraction involved phosphatidylinositol 3-kinase and the G(i) class of heterotrimeric G proteins, respectively. Our results indicate that PDGF- and LPA-dependent contraction of floating collagen matrices can be uncoupled from an increase in MLC phosphorylation.

Pharmacological characterization of the contractile effects of galanin (1-29)-NH2, galantide and galanin (1-14)-(alpha-aminobutyric acid8)scyliorhinin-I in the rat gastric fundus

Porcine galanin (1-29)-NH2, galantide (M15) and galanin (1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I used in concentrations of 300, 1,000 and 3,000 nM respectively caused contractions of rat fundus strips. The contractile responses to galanin(1-29)-NH2 were not modified by atropine (10 microM), guanethidine (10 microM), naloxone (1 microM), a mixture of propranolol (10 microM) and phentolamine (10 microM), indomethacin (10 microM), a mixture of mepyramine (10 microM) and cimetidine (10 microM), saralasin (10 microM), and spantide (100 microM). The effects of M15 and galanin(1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I were significantly decreased by atropine for 36 and 18% and by spantide for 37 and 26% respectively. Indomethacin inhibited the muscle response to M15 without influence on the galanin (1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I-induced action. These results support findings that galanin (1-29)-NH2 contracts rat gastric fundus strips by stimulating specific receptors localized on the surface of smooth muscle cells. M15 and galanin(1-14)-(alpha-aminobutyric acid8)-scyliorhinin-I seem to contract smooth muscles not only by acting at galanin receptors, but by interacting with muscarinic or tachykinin receptors or modulating the release of acetylcholine and substance P. Diltiazem (EC50 825 nM), dantrolene (EC50 30.2 microM) and the phospholipase C inhibitors U-73122 (EC50 549 microM) and U-73343 (EC50 751 microM) lowered the contraction to galanin(1-29)-NH2 in a concentration-dependent manner. These observations imply that though the extracellular Ca2+ influx plays a major role in the action of galanin(1-29)-NH2, the release of Ca2+ ions from the intracellular stores contributes to the response of smooth muscles of galanin(1-29) NH2. Norepinephrine (30, 60, 100 and 300 nM) concentration-dependently reduced the Emax to galanin (1-29)-NH2 and reduced the slopes of the concentration-contraction curves, without a notable change in EC50. Pertussis toxin pre-treatment (10 and 30 mg/kg intravenous [i.v.]), 120 h before the experiment, notably increased the maximal response of the rat gastric fundus to galanin(1-29)-NH2, without a significant change in the properties of the concentration-contraction curves (EC50, slopes). The observations may suggest that pertussis toxin-sensitive GTP-binding proteins are involved in the modulation of the excitatory effects of galanin(1-29)-NH2 in the rat gastric fundus.

Hemodynamic effect of amrinone depends on pretreatment vascular resistance in patients with evolving congestive heart failure: correlation between vascular resistance and neurohormonal activity

We investigated the hemodynamic effects of amrinone and assessed its effects on neurohormonal factors in 15 patients with evolving congestive heart failure with various origins. We serially determined the pulmonary and systemic vascular-resistance indices after amrinone infusion and examined the relation between changes in hemodynamic parameters and changes in concentrations of norepinephrine, atrial natriuretic peptide, angiotensin II, and endothelin-1 in the pulmonary capillary wedge region (PCWR) and in the peripheral veins. Amrinone significantly reduced pulmonary vascular-resistance index (PVRI; Wood x m2) in patients with high PVRI (> or =15) before the infusion, significantly reduced systemic vascular-resistance index (SVRI; Wood x m2) in patients with high SVRI (> or =50) before the infusion, and had little effect on vascular resistances in patients with low PVRI (<15) and low SVRI (<50). The reduction in PVRI was correlated with the reduction in the endothelin-1 level (r = 0.75) in the PCWR, and the reduction in SVRI with norepinephrine level (r = 0.70) in the peripheral veins. The angiotensin II level did not change throughout the study. These findings suggest that amrinone had selective hemodynamic effects on pulmonary and systemic circulations with neurohormonal effects, according to PVRI and SVRI before infusion.

Signal transduction in gastrointestinal smooth muscle

Signal transduction in gastric and intestinal smooth muscle is mediated by receptors coupled via distinct G proteins to various effector enzymes, including PI-specific PLC-beta 1 and PLC-beta 3, and phosphatidylcholine (PC)-specific PLC, PLD and PLA2. Activation of these enzymes is different in circular and longitudinal muscle cells, generating Ca(2+)-mobilizing (IP3, AA, cADPR) and other (DAG) messengers responsible for the initial and sustained phases of contraction, respectively. IP3-dependent Ca2+ release occurs only in circular muscle. Ca2+ mobilization in longitudinal muscle involves a cascade initiated by agonist-induced transient activation of PLA2 and formation of AA, AA-dependent depolarization of the plasma membrane and opening of voltage-sensitive Ca2+ channels. The influx of Ca2+ induces Ca2+ release by activating sarcoplasmic ryanodine receptor/Ca2+ channel and stimulates cADPR formation which enhances Ca(2+)-induced Ca2+ release. The initial [Ca2+]i transient in both muscle cell types results in Ca2+/calmodulin-dependent activation of MLC kinase, phosphorylation of MLC20 and interaction of actin and myosin. The sustained phase is mediated by a Ca(2+)-independent isoform of PKC, PKC-epsilon DAG for this process is generated by PLC- and PLD-mediated hydrolysis of PC. Relaxation is mediated by cAMP-and/or cGMP-dependent protein kinase which inhibit the initial [Ca2+]i transient and reduce the sensitivity of MLC kinase to [Ca2+]i. Relaxation induced by the main neurotransmitters, VIP and PACAP, involves two cascades, one of which reflects activation of adenylyl cyclase. A distinct cascade involves G-protein-dependent stimulation of Ca2+ influx leading to Ca2+/calmodulin-dependent activation of a constitutive eNOS in muscle cells; the generation of NO activates soluble guanylyl cyclase. The resultant activation of PKA and PKG is jointly responsible for muscle relaxation.

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.

The study on the relationship between the expression of calponin and gallstone formation

In order to understand the molecular mechanisms of gallstone formation, the expression of calponin in animal model of gallstone disease was studied. High-cholesterol diet was given to the guinea pigs to induce gallstone formation. RT-PCR and Western-blotting were used to evaluate expression level of calponin gene. Down-regulation of calponin gene expression was observed in animals with gallstone, while myosin expression was relatively stable. Our results indicated that the decrease of calponin could increase the pressure of Oddi's sphincter, aggravate the stasis of bile and promote the gallstone formation.

Actions of galanin and some of its analogues on rat isolated gastric fundus

The study was undertaken to characterize the effects of porcine galanin (pGal) and some of its analogues on rat gastric fundus muscle strips. pGal, galantide (M15) and pGal(1-14)-[Abu8]SCY-I evoked reproducible concentration-dependent contractions in concentrations of 1-300, 3-1,000 and 100-3,000 nM, respectively, with EC50 values of 13, 70 and 187 nM. Hill's coefficient for pGal is 1.03, indicating an interaction of one pGal molecule with one receptor, fulfilling criteria of classical receptor theory. For M15 and pGal(1-14)-[Abu8]SCY-I, Hill's coefficients are significantly different from 1, namely 0.73 and 1.56, so that one drug molecule may not interact with one receptor. The stimulatory effects of pGal were not modified by dibenamine 10 microM or glybenclamide 1 or 10 microM. Diltiazem 0.1, 1 and 10 microM, papaverine 0.1, 10 microM or dibutyryl cAMP (dib cAMP) 100 and 300 microM, blocked the contraction to pGal in a concentration-dependent manner, indicating an important role for the influx of extracellular calcium ions and regulation by cAMP the pGal-evoked contraction. Diltiazem, dibutyryl cAMP and papaverine were not competitive antagonists of pGal in the stomach smooth muscle.

Phosphorylation and partial sequence of pregnant sheep myometrium myosin light chain kinase

The function of the uterine smooth muscle in gestation and parturition is affected by a variety of hormones and biomolecules, some of which alter the intracellular levels of cAMP and Ca2+. Since the activity of smooth muscle MLCK has been shown to be modulated by phosphorylation, the effect of this modification of pregnant sheep myometrium (psm) MLCK by the catalytic subunit of cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) was studied. In contrast to other smooth muscle MLCK reported, PKA incorporates 2.0-2.2 moles phosphate into a mole of psm MLCK both in the presence and absence of Ca(2+)-calmodulin. Modification of serine residues inhibited the activity of the enzyme. PKC also incorporated 2.0-2.1 moles of phosphate per mole psmMLCK under both conditions but had no effect on the MLCK activity. Sequential phosphorylation by PKC and PKA incorporated 3.8-4.1 moles phosphate suggesting that the amino acid residues modified by the two kinases are different. Phosphoamino acid analysis of the MLCK revealed that PKC phosphorylated serine and threonine residues. The double reciprocal plots of the enzyme activity and calmodulin concentrations showed that the Vmax of the reaction is not altered by phosphorylation by PKA but the calmodulin concentration require for half-maximal activation is increased about 4-fold. Only 10 out of 17 monoclonal antibodies to various regions of the turkey gizzard MLCK cross-reacted with psmMLCK suggesting structural differences between these enzymes. Comparison of the deduced amino acid sequence of the cDNA encoding the C-terminal half of the psmMLCK molecule showed that while cgMLCK and psmMLCK are highly homologous, a number of nonconservative substitutions are present, particularly near the PKA phosphorylation site B (S828).

Involvement of rho in GTP gamma S-induced enhancement of phosphorylation of 20 kDa myosin light chain in vascular smooth muscle cells: inhibition of phosphatase activity

In beta-escin-permeabilized cultured pig aortic smooth muscle cells GTP gamma S dose-dependently enhances Ca(2+)-induced, wortmannin-sensitive phosphorylation of 20 kDa myosin light chain (MLC20). GTP gamma S does not potentiate thiophosphorylation of MLC20, but does inhibit its dephosphorylation. Pretreatment with C. botulinum exotoxin C3, which specifically ADP-ribosylates and inactivates the rho family of the small molecular weight G proteins, completely abolishes the effects of GTP gamma S. These results indicate that rho is involved in the GTP gamma S-induced enhancement of Ca(2+)-dependent MLC20 phosphorylation in aortic smooth muscle cells, and strongly suggest that this effect of rho is due to inhibition of protein phosphatase activity toward MLC20.

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

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

pHi, [Ca2+]i, and myosin phosphorylation in histamine- and NH4(+)-induced swine carotid artery contraction

We examined the interaction among changes in pHi, [Ca2+]i, myosin light-chain phosphorylation, and contraction in arterial smooth muscle stimulated by histamine, NH4+, Tris+, and/or changes in extracellular pH (pHo). We loaded swine carotid medial tissues with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein to measure pHi or aequorin to measure [Ca2+]i. Incubation of tissues in NH4+ increased pHi, [Ca2+]i, myosin phosphorylation, and force. Washout of NH4+ decreased pHi and transiently further increased in [Ca2+]i and force. Incubation of tissues in a similar concentration of Tris+ or increasing pHo also increased pHi; however, there were only modest changes in [Ca2+]i and force. Increasing extracellular pH coincidentally with washout of NH4+ prevented the decrease in pHi but did not affect the NH4+ washout-induced contraction. These data suggest that NH4+ altered [Ca2+]i and contraction by mechanisms other than its effects on pHi. The type of pH buffer did not affect the [Ca2+]i, myosin phosphorylation, or stress response to histamine stimulation. The time course of changes in pHi was much slower than the time course of histamine-induced changes in [Ca2+]i, myosin phosphorylation, and stress. Addition of 10 mmol/L NH4+ concurrently with histamine aborted the histamine-induced decrease in pHi and significantly slowed the histamine-induced increase in [Ca2+]i, myosin phosphorylation, and stress. There was little effect on histamine-induced increases in [Ca2+]i, myosin phosphorylation, or contraction when three other protocols aborted the histamine-induced decrease in pHi. These data show that incubation in NH4+ can alter [Ca2+]i and contraction in both unstimulated and histamine-stimulated smooth muscle. However, these effects were not caused by NH4(+)-dependent changes in pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Preterm labour: a pharmacological challenge

Preterm labour is a major cause of perinatal mortality and morbidity, but its prevention is difficult because most of the available drugs lack uterine selectivity and have potentially serious side-effects for the mother or the foetus. In this article, Andrés López Bernal and colleagues discuss new evidence that shows pregnancy is associated with changes in G protein signalling and second messenger formation in human myometrium. During gestation uterine relaxation is favoured by a pronounced increase in G alpha s levels, thereby facilitating the effect of agonists that increase cAMP formation. The change in G alpha s is reversed in spontaneous labour enabling the uterus to become responsive to contractile agents. Although it is not established that these changes in G protein function are causally related to the spontaneous onset of labour, nevertheless they provide a novel viewpoint towards increased understanding of the cellular mechanisms of uterine contractility, which may result in better drugs for the management of preterm labour.

Isoproterenol increases peripheral [Ca2+]i and decreases inner [Ca2+]i in single airway smooth muscle cells

The effects of isoproterenol (Iso) or photolysis of caged adenosine 3',5'-cyclic monophosphate (cAMP) on intracellular Ca2+ concentration ([Ca2+]i) were studied in single airway smooth muscle cells. Changes in [Ca2+]i were measured ratiometrically. In cells loaded with 10 microM fura 2-acetoxymethyl ester (AM), superfusion of Iso (10 microM) increased [Ca2+]i in 20 of 22 cells from 153.2 +/- 21.3 to 252.8 +/- 38.3 nM, and this increase depended on extracellular Ca2+ and was blocked by ryanodine (50 microM). Photolysis of intracellular caged cAMP increased [Ca2+]i by 104.0 +/- 17.3 nM in 20 cells and decreased [Ca2+]i by 49.3 +/- 9.4 nM in 10 cells. With modified confocal microscopy, peripheral [Ca2+]i was increased and inner cytosolic [Ca2+]i was decreased during stimulation with Iso. Iso-induced decreases in [Ca2+]i were also observed with conventional optics in cells loaded with 0.5 microM fura 2-AM. However, in these same cells, Iso increased [Ca2+]i in the presence of low concentrations of ryanodine (1-20 microM). We concluded that Iso decreased [Ca2+]i in the inner cytosol but increased [Ca2+]i in the peripheral cytosol by mechanisms that depended on both extracellular and sarcoplasmic reticulum Ca2+ release channels. Our study suggests that fluorescence from a peripheral cytosol with high [Ca2+]i can sometimes confound the measurement of "cytosolic" [Ca2+]i with conventional optics.

Regulation of histamine- and UTP-induced increases in Ins(1,4,5)P3, Ins (1,3,4,5)P4 and Ca2+ by cyclic AMP in DDT1 MF-2 cells

1. Stimulation of P2U-purinoceptors with UTP or histamine H1-receptors with histamine gave rise to the formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) in DDT1 MF-2 smooth muscle cells. 2. Stimulation of P2U-purinoceptors or histamine H1-receptors caused an increase in cytoplasmic Ca2+, consisting of an initial peak, representing the release of Ca2+ from internal stores and a sustained phase representing Ca2+ influx. 3. The P2U-purinoceptor-mediated Ca(2+)-entry mechanism was more sensitive to UTP than Ca(2+)-mobilization (EC50: 3.3 microM +/- 0.4 microM vs 55.1 microM +/- 9.2 microM), in contrast to these processes activated by histamine H1-receptors (EC50: 5.8 microM +/- 0.6 microM vs 3.1 microM +/- 0.5 microM). 4. Pre-stimulation of cells with several adenosine 3':5'-cyclic monophosphate (cyclic AMP) elevating agents, reduced the histamine H1-receptor-mediated formation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin completely inhibited Ins(1,4,5)P3 formation (IC50: 158 +/- 24 nM) whereas Ins(1,3,4,5)P4 formation was inhibited by only 45% (IC50: 173 +/- 16 nM). The P2U-purinoceptor-mediated production of these inositol phosphates was not affected by cyclic AMP. 5. Forskolin and isoprenaline reduced the histamine-induced increase in cytoplasmic Ca2+, as measured in Ca2+ containing medium and in nominally Ca(2+)-free medium but did not change the UTP-induced increase in cytoplasmic Ca2+. 6. These results clearly demonstrate that cyclic AMP differentially regulates components of the histamine induced phospholipase C signal transduction pathway. Furthermore, cyclic AMP does not affect the phospholipase C pathway activated by stimulation of P2U-purinoceptors in DDT1 MF-2 cells.

Mechanosensitive modulation of myosin phosphorylation and phosphatidylinositol turnover in smooth muscle

Myosin light chain phosphorylation and phosphatidylinositol turnover were measured at different muscle lengths in bovine tracheal smooth muscle. The relationship between myosin phosphorylation and muscle length was linear between optimal length (Lo) and 0.1 Lo in both unstimulated and carbachol-activated tissues. However, myosin phosphorylation in carbachol-activated tissues was more sensitive to changes in muscle length. As a result, suprabasal myosin phosphorylation induced by carbachol was significant at Lo but became insignificant at 0.1 Lo. Phosphatidylinositol turnover was assayed by measuring the formation of myo-[3H]inositol phosphates in unstimulated and carbachol-activated tissues using the Li+ method. Pairs of time-matched and length-matched muscle strips were used for control (unstimulated) and activation by carbachol. Phosphatidylinositol turnover in carbachol-activated tissues was more sensitive than that in unstimulated tissues to changing length. As a result, suprabasal phosphatidylinositol turnover induced by carbachol was significant at Lo but became insignificant at 0.1 Lo. These results indicated that myosin phosphorylation and phosphatidylinositol turnover mediated by muscarinic receptor activation were modulated by the mechanical state of smooth muscle.

Beta-adrenoceptors, cAMP and airway smooth muscle relaxation: challenges to the dogma

beta-Adrenoceptor agonists are assumed to induce airway smooth muscle relaxation through the cAMP-protein kinase A (PKA) phosphorylation cascade system. This traditional second messenger paradigm of beta-adrenoceptor agonist action is deeply engrained, but in this article Theodore Torphy reviews recent observations that force a re-examination of the dogma. For example, cAMP can activate protein kinase G as well as PKA, and this unanticipated dual action may contribute to the relaxant activity of cAMP. Other studies suggest that beta-adrenoceptor agonists can induce relaxation by a cAMP-independent mechanism involving a direct coupling of the beta-adrenoceptor to Ca(2+)-dependent K+ channels. Consequently, it is possible that multiple cAMP-dependent pathways act in concert with cAMP-independent pathways to mediate bronchodilation in response to beta-adrenoceptor agonists.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Phosphorylation of myosin light chain kinase: a cellular mechanism for Ca2+ desensitization

Phosphorylation of the regulatory light chain of myosin by the Ca2+/calmodulin-dependent myosin light chain kinase plays an important role in smooth muscle contraction, nonmuscle cell shape changes, platelet contraction, secretion, and other cellular processes. Smooth muscle myosin light chain kinase is also phosphorylated, and recent results from experiments designed to satisfy the criteria of Krebs and Beavo for establishing the physiological significance of enzyme phosphorylation have provided insights into the cellular regulation and function of this phosphorylation in smooth muscle. The multifunctional Ca2+/calmodulin-dependent protein kinase II phosphorylates myosin light chain kinase at a regulatory site near the calmodulin-binding domain. This phosphorylation increases the concentration of Ca2+/calmodulin required for activation and hence increases the Ca2+ concentrations required for myosin light chain kinase activity in cells. However, the concentration of cytosolic Ca2+ required to effect myosin light chain kinase phosphorylation is greater than that required for myosin light chain phosphorylation. Phosphorylation of myosin light chain kinase is only one of a number of mechanisms used by the cell to down regulate the Ca2+ signal in smooth muscle. Since both smooth and nonmuscle cells express the same form of myosin light chain kinase, this phosphorylation may play a regulatory role in cellular processes that are dependent on myosin light chain phosphorylation.

GTP gamma S-induced phosphorylation of myosin light chain kinase in smooth muscle

Phosphorylation of myosin light chain kinase by a Ca(2+)-dependent protein kinase increases the concentration of Ca2+/calmodulin required for half-maximal activation. The Ca2+ concentrations required for myosin light chain kinase phosphorylation in permeable smooth muscle are similar to those required for myosin light chain phosphorylation. Both GTP gamma S and carbachol increase the Ca2+ sensitivity of myosin light chain kinase phosphorylation as well as light chain phosphorylation. It is proposed that a similar G-protein mediated mechanism regulates the Ca(2+)-dependent phosphorylation of these two contractile proteins in smooth muscle.

Synergistic effects of cyclic AMP-related vasodilators and the phosphatase inhibitor okadaic acid

The phosphatase inhibitor okadaic acid at 100 nM slowly but completely inhibited high K(+)-induced contraction in the rat aorta (t1/2 = 118.9 min). High K(+)-induced contraction was partially inhibited (to 37-65%) by 1 microM forskolin, 100 microM dibutyryl cyclic AMP, 100 nM atrial natriuretic peptide, 1 microM nitroglycerin, 10 nM sodium nitroprusside, 300 pM nicardipine or 100 nM verapamil. The rate of relaxation due to okadaic acid became faster when the contraction was partially inhibited by these compounds. Augmentation of the relaxation was greater with forskolin and dibutyryl cyclic AMP than with the other inhibitors. These results support the suggestion that okadaic acid inhibits phosphatase to augment the phosphorylation due to cyclic AMP-dependent kinase, resulting in smooth muscle relaxation.