Carbachol-induced protein phosphorylation changes in bovine tracheal smooth muscle

Loss of desmin triggers mechanosensitivity and up-regulation of Ankrd1 expression through Akt-NF-κB signaling pathway in smooth muscle cells

Muscle cells, including human airway smooth muscle cells (HASMCs) express ankyrin repeat protein 1 (Ankrd1), a member of ankyrin repeat protein family. Ankrd1 efficiently interacts with the type III intermediate filament desmin. Our earlier study showed that desmin is an intracellular load-bearing protein that influences airway compliance, lung recoil, and airway contractile responsiveness. These results suggest that Ankrd1 and desmin may play important roles on ASMC homeostasis. Here we show that small interfering (si)RNA-mediated knockdown of the desmin gene in HASMCs, recombinant HASMCs (reHASMCs), up-regulates Ankrd1 expression. Moreover, loss of desmin in HASMCs increases the phosphorylation of Akt, inhibitor of κB kinase (IKK)-α, and inhibitor of κB (IκB)-α proteins, leading to NF-κB activation. Treatment of reHASMCs with Akt, IKKα, IκBα, or NF-κB inhibitor inhibits the loss of desmin-induced Ankrd1 up-regulation, suggesting Akt/NF-κB-mediated Ankrd1 regulation. Transfection of reHASMCs with siRNA specific for p50 or p65 corroborates the NF-κB-mediated Ankrd1 regulation. Luciferase reporter assays show that NF-κB directly binds on Ankrd1 promoter and up-regulates Ankrd1 levels. Overall, our data provide a new link between desmin and Ankrd1 regulation, which may be important for ASMC homeostasis.

Desmin regulates airway smooth muscle hypertrophy through early growth-responsive protein-1 and microRNA-26a

Bronchial biopsies of asthmatic patients show a negative correlation desmin expression in airway smooth muscle cell (ASMC) and airway hyperresponsiveness. We previously showed that desmin is an intracellular load-bearing protein, which influences airway compliance, lung recoil, and airway contractile responsiveness (Shardonofsky, F. R., Capetanaki, Y., and Boriek, A. M. (2006) Am. J. Physiol. Lung Cell. Mol. Physiol. 290, L890-L896). These results suggest that desmin may play an important role in ASMC homeostasis. Here, we report that ASMCs of desmin null mice (ASMCs(Des-/-)) show hypertrophy and up-regulation microRNA-26a (miR-26a). Knockdown of miR-26a in ASMCs(Des-/-) inhibits hypertrophy, whereas enforced expression of miR-26a in ASMCs(Des+/+) induces hypertrophy. We identify that Egr1 (early growth responsive protein-1) activates miR-26a promoter via enhanced phosphorylation of Erk1/2 in ASMCs(Des-/-). We show glycogen synthase kinase-3β (GSK-3β) as a target gene of miR-26a. Moreover, induction of ASMCs(Des-/-) hypertrophy by the Erk-1/2/Egr-1/miR-26a/GSK-3β pathway is consistent in human recombinant ASMCs, which stably suppresses 90% endogenous desmin expression. Overall, our data demonstrate a novel role for desmin as an anti-hypertrophic protein necessary for ASMC homeostasis and identifies desmin as a novel regulator of microRNA.

Intermediate filaments in smooth muscle

The intermediate filament (IF) network is one of the three cytoskeletal systems in smooth muscle. The type III IF proteins vimentin and desmin are major constituents of the network in smooth muscle cells and tissues. Lack of vimentin or desmin impairs contractile ability of various smooth muscle preparations, implying their important role for smooth muscle force development. The IF framework has long been viewed as a fixed cytostructure that solely provides mechanical integrity for the cell. However, recent studies suggest that the IF cytoskeleton is dynamic in mammalian cells in response to various external stimulation. In this review, the structure and biological properties of IF proteins in smooth muscle are summarized. The role of IF proteins in the modulation of smooth muscle force development and redistribution/translocation of signaling partners (such as p130 Crk-associated substrate, CAS) is depicted. This review also summarizes our latest understanding on how the IF network may be regulated in smooth muscle.

Desmin modulates lung elastic recoil and airway responsiveness

Desmin is a structural protein that is expressed in smooth muscle cells of both airways and alveolar ducts. Therefore, desmin could be well situated to participate in passive and contractile force transmission in the lung. We hypothesized that desmin modulates lung compliance, lung recoil pressure, and airway contractile response. To test this hypothesis, respiratory system complex impedance (Zin,rs) at different positive end-expiratory pressure (PEEP) levels and quasi-static pressure-volume data were obtained in desmin-null and wild-type mice at baseline and during methacholine administration. Airways and lung tissue properties were partitioned by fitting Zin,rs to a constant-phase model. Relative to controls, desmin-null mice showed 1) lower values for lung stiffness and recoil pressure at baseline and induced airway constriction, 2) greater negative PEEP dependence of H and airway resistance under baseline conditions and cholinergic stimulation, and 3) airway hyporesponsiveness. These results demonstrate that desmin is a load-bearing protein that stiffens the airways and consequently the lung and modulates airway contractile response.

The paradox of smooth muscle physiology

Vascular smooth muscle tone is controlled by a balance between the cellular signaling pathways that mediate the generation of force (contraction) and the release of force (relaxation). The signaling events that activate contraction include Ca(2+)-dependent myosin light chain phosphorylation. The signaling events that mediate relaxation include the removal of a contractile agonist (passive relaxation) and activation of cyclic nucleotide-dependent signaling pathways in the continued presence of a contractile agonist (active relaxation). The major questions that remain in contractile physiology include (1) how is tonic force maintained when intracellular Ca(2+) levels and myosin light chain phosphorylation have returned to basal levels; and (2) what is the mechanism of cyclic nucleotide-dependent relaxation? This review focuses on these specific controversies surrounding the molecular mechanisms of contraction and relaxation of vascular smooth muscle.

Insulin and insulin antagonists evoke phosphorylation of P20 at serine 157 and serine 16 respectively in rat skeletal muscle

We show here that insulin and insulin antagonists differentially modify phosphorylation of three phospho-isoforms of P20 (termed S1, S2 and S3) in rat skeletal muscle. Precise phosphorylation sites of S1 and S2 were mapped to serine 157 and serine 16 respectively. Insulin evoked phosphorylation of P20 at serine 157 through the phosphatidylinositol (PI) 3-kinase pathway. Epinephrine and calcitonin gene-related peptide decreased phosphorylation at serine 157 and increased phosphorylation at serine 16 and other unidentified sites. These results demonstrate that the PI-3-kinase pathway of anabolic insulin and the cAMP pathway of catabolic hormones converge on P20 and suggest a potential role of this protein in regulating energy metabolism of skeletal muscle.

Amylin evokes phosphorylation of P20 in rat skeletal muscle

To investigate the signal transduction events underlying amylin's actions, the amylin-evoked protein phosphorylation cascade was analysed using two-dimensional gel electrophoresis. We found that phosphorylation of three isoelectric variants of P20 (termed ARPP1, ARPP2 and ARPP3) was associated with amylin's actions in rat skeletal muscle. Amylin decreased phosphorylation of ARPP1 and increased phosphorylation of ARPP2 and ARPP3 in a dose-dependent manner. Insulin inhibited amylin-evoked phosphorylation of ARPP2 and ARPP3. The amylin-selective antagonist rat amylin-(8-37) completely reversed amylin's action on ARPP3 and partially decreased phosphorylation of ARPP2. By contrast, the CGRP-selective antagonist, human CGRP-(8-37) blocked phosphorylation of ARPP2 but had little effect on ARPP3. These results suggest that amylin modifies phosphorylation of P20 via two independent mechanisms, and that P20 might be a molecule mediating amylin's biological functions.

Protein kinase C promotes spontaneous relaxation of streptolysin-O-permeabilized smooth muscle cells from the guinea-pig stomach

Isolated single smooth muscle cells from the fundus of a guinea-pig stomach were permeabilized by use of streptolysin-O (0.5 U/ml). Most of the permeabilized cells responded to 0.6 microM Ca2+, but not to 0.2 microM Ca2+, with a resulting maximal cell shortening to approximately 71% of the resting cell length. These cells were relaxed again by washing with the Ca2+-free solution (2.5 nM free Ca2+) for 3-5 min. Addition of 10 microM acetylcholine (ACh) resulted in both a marked decrease in the concentration of Ca2+ required to trigger a threshold response and an increase in the maximal cell shortening, indicating that the cells retained the muscarinic receptor function. When the cell treated with a protein kinase C (PKC) inhibitor, K-252b (1 microM), for 3 min was exposed to 10 microM ACh in the presence of K-252b, the cell shortened within 2 min with a maximal cell shortening. When the cell shortening was induced by 10 microM ACh plus 1 microM Ca2+ in the presence of K-252b (1 microM) or more selective PKC inhibitors, such as calphostin C (1 microM) or PKC pseudosubstrate peptide (100 microM), the extension of the shortened cells, by washing with the Ca2+-free solution, was significantly inhibited. In contrast, K-252b (1 microM) did not inhibit the relaxation of Ca2+-induced shortened cells. These results suggest that the receptor-mediated activation of PKC in the process of ACh-induced cell shortening plays a role in the subsequent relaxation of the shortened cells.

Cyclosporine-induced renal artery smooth muscle contraction is associated with increases in the phosphorylation of specific contractile regulatory proteins

Cyclosporine A (CSA) is a type 2B phosphatase inhibitor which can induce contraction of renal artery smooth muscle. In this investigation, we examined the phosphorylation events associated with CSA-induced contraction of bovine renal artery smooth muscle. Contractile responses were determined in a muscle bath and the corresponding phosphorylation events were determined with whole cell phosphorylation and two-dimensional gel electrophoresis. CSA-induced contractions were associated with increases in the phosphorylation of the 20 kDa myosin light chains (MLC20) and different isoforms of the small heat shock protein, HSP27. Cyclic nucleotide-dependent relaxation of CSA-induced contractions was associated with increases in the phosphorylation of another small heat shock protein, HSP20, and decreases in the phosphorylation of the MLC20, and some isoforms of HSP27. These data suggest that CSA-induced contraction and relaxation of vascular smooth muscle is associated with increases in the phosphorylation of specific contractile regulatory proteins.

The small heat shock-related protein-20 is an actin-associated protein

PURPOSE

The activation of cyclic nucleotide-dependent signaling pathways in vascular smooth muscle is important for the prevention of vein graft spasm and neointimal hyperplasia. Cyclic nucleotide-dependent relaxation is associated with an increase in the phosphorylation of a small heat shock-related protein (HSP20). In this investigation, we examined the mechanisms by which HSP20 may modulate relaxation.

METHODS

The relaxation responses of the bovine carotid artery smooth muscles were determined in a muscle bath. HSP20 phosphorylation was quantitated with isoelectric-focusing immunoblots. The association with actin was determined with coimmunoprecipitation and cosedimentation. Molecular sieving columns were used to examine the macromolecular associations of HSP20.

RESULTS

The activation of cyclic nucleotide signaling pathways leads to the complete relaxation of carotid smooth muscle. This relaxation response is associated with an increase in the phosphorylation of HSP20. Actin coimmunoprecipitated with HSP20, and the association of actin with recombinant HSP20 in vitro was phosphorylation-state dependent. Finally, HSP20 exists in large (>100 kDa) aggregates, which dissociate with the activation of cyclic nucleotide signaling pathways.

CONCLUSION

These data support a role of HSP20 phosphorylation in mediating smooth muscle relaxation, possibly via a direct interaction of large aggregates of HSP20 with the contractile elements.

Protein kinase C activation during Ca2+-independent vascular smooth muscle contraction

The cellular signaling mechanisms that modulate the sustained vascular smooth muscle contractions that occur in vasospasm are not known. We and others have hypothesized that a kinase cascade involving protein kinase C (PKC) modulates sustained vascular smooth muscle contraction. The purpose of this investigation was to develop a model in which the traditional contractile pathways involving myosin light chain phosphorylation are not activated and determine if the PKC pathway is activated under these conditions. The phosphorylation of caldesmon, myosin light chain (MLC20), and the specific PKC substrate, MARCKS (myristoylated, alanine-rich C-kinase substrate) was measured in bovine carotid arterial smoothmuscle (BCASM) stimulated with phorbol 12,13-dibutyrate (PDBu) under Ca2+-containing and Ca2+-free conditions. PDBu stimulation led to increases in caldesmon and MARCKS phosphorylation to the same degree in the presence or absence of Ca2+. PDBu stimulation but did not lead to increases in MLC20 phosphorylation over basal levels in Ca2+-free conditions. Immunoblot analysis of BCASM using PKC isoform-specific antibodies demonstrated the presence of one "Ca2+- dependent" PKC isoform: alpha, and two of the "Ca2+-independent" isoforms: epsilon and zeta. These data suggest that Ca2+-independent isoforms of PKC may play a role in the sustained phase of BCASM contractions through a kinase cascade that involves caldesmon and MARCKS phosphorylation but not MLC20 phosphorylation.

Identification of the protein kinase C isoenzymes in human lung and airways smooth muscle at the protein and mRNA level

The protein kinase C (PKC) isoenzymes expressed by human peripheral lung and tracheal smooth muscle resected from individuals undergoing heart-lung transplantation were identified at the protein and mRNA level. Western immunoblot analyses of human lung identified multiple PKC isoenzymes that were differentially distributed between the soluble and particulate fraction. Thus, PKC alpha, PKC betaII, PKC epsilon, and PKC zeta were recovered predominantly in the soluble fraction whereas the eta isoform was membrane-associated together with trace amounts of PKC alpha and PKC epsilon. PKC beta1-like immunoreactivity was occasionally seen although the intensity of the band was uniformly weak. Immunoreactive bands corresponding to PKCs gamma, delta, or theta were never detected. Reverse transcription-polymerase chain reaction (RT-PCR) of RNA extracted from human lung using oligonucleotide primer pairs that recognise unique sequences in each of the PKC genes amplified cDNA fragments that corresponded to the predicted sizes of PKC alpha, PKC betaI, PKC betaII, PKC epsilon, PKC zeta, and PKC eta (consistent with the expression of PKC isoenzyme protein) and, in addition, mRNA for PKC delta; PCR fragments of the expected size for the supposedly muscle-specific isoform, PKC theta, or the atypical isoenzyme, PKC lambda, were never obtained. The complement and distribution of PKC isoforms in human trachealis were similar, but not identical, to human lung. Thus, immunoreactive bands corresponding to the alpha, betaI, betaII, epsilon, and zeta isoenzymes of PKC were routinely labelled in the cytosolic fraction. In the particulate material PKC alpha, PKC epsilon, PKC alpha, PKC eta, and PKC mu were detected by immunoblotting. With the exception of PKC zeta, RT-PCR analyses confirmed the expression of the PKC isoforms detected at the protein level and, in addition, identified mRNA for PKC delta. Collectively, these data clearly demonstrate the expression of multiple PKC isoenzymes in human lung and tracheal smooth muscle, suggesting that they subserve diverse multifunctional roles in these tissues.

Ca2+ transient, cell volume, and microviscosity of the plasma membrane in smooth muscle

Despite pronounced differences by which membrane-depolarizing or phospholipase C-activating stimuli initiate contractile responses, a rise in [Ca2+]i is considered the primary mechanism for induction of smooth muscle contractions. Subsequent to the formation of the well-characterized Ca(2+)4-calmodulin complex, interaction with the catalytic subunit of myosin light chain kinase triggers phosphorylation of 20 kDa myosin light chain and activates actin-dependent Mg2+-ATPase activity, which ultimately leads to the development of tension. The present article reviews the fundamental mechanisms leading to an increase in [Ca2+]i and discusses the biochemical processes involved in the transient and sustained phases of contraction. Moreover, the commentary summarizes current knowledge on the modulatory effect of changes in the microviscosity of the plasma membrane on the Ca2+ transient as well as the contractile response of smooth muscle. Evidence has accumulated that these changes in microviscosity alter the activity of membrane-bound enzymes and affect the generation of endogenous mediators responsible for the regulation of cytosolic Ca2+ concentrations and for the [Ca2+]i-sensitivity of myosin light chain phosphorylation.

Cyclic nucleotide-dependent vasorelaxation is associated with the phosphorylation of a small heat shock-related protein

Activation of cyclic nucleotide-dependent signaling pathways leads to the relaxation of various smooth muscles. One of the major phosphorylation events associated with cyclic nucleotide-dependent vasorelaxation in bovine trachealis and carotid artery smooth muscle is the phosphorylation of two 20-kDa phosphoproteins with pI values of 5.7 and 5.9 (previously designated pp8 and pp3, respectively). The present studies sought to determine the identities of pp3 and pp8 in vascular smooth muscle. The phosphopeptide maps for the pp8 and pp3 proteins were similar. Preparative two-dimensional gel electrophoresis and amino acid sequencing of a peptide fragment of the pp3 protein revealed a sequence identical to a 20-kDa heat shock-related protein (HSP20) previously purified from skeletal muscle. Western blot and immunoprecipitation analysis with anti-HSP20 antibodies demonstrated that the pp3 and pp8 proteins are phosphorylated forms of HSP20. In addition, HSP20 could be phosphorylated in vitro by both cAMP-dependent protein kinase and cGMP-dependent protein kinase. These data suggest that the phosphorylation of the heat shock-related protein HSP20 is associated with cyclic nucleotide-dependent relaxation of vascular smooth muscle.

Involvement of protein kinase C in muscarinic agonist-induced contractions of guinea pig ileal longitudinal muscle

1. Carbachol (10(-5) M) caused an initial transient contraction (phasic contraction) and a subsequent late contraction (tonic contraction) in the guinea pig ileum. The phasic contraction was markedly suppressed by calmodulin antagonist W-7, but not by protein kinase C inhibitor H-7. 2. The tonic contraction was suppressed by H-7, but not by W-7. 3. The carbachol-induced phasic contraction in the absence of extracellular Ca2+ was suppressed by both W-7 and H-7. 4. Carbachol (10(-5) M) stimulated the formation of inositol phosphates (IPs) in the guinea pig ileum. The carbachol-induced IPs formation was dependent on the extracellular Ca2+ concentration.

Anticholinesterase drugs stimulate phosphatidylinositol response in rat tracheal slices

Some anticholinesterase (anti-ChE) drugs induce airway smooth muscle contraction. Whether anti-ChE drugs stimulate muscarinic receptors in airway smooth muscle as well as nicotinic receptors in neuromuscular junction is unknown. Since there is a direct relationship between phosphatidylinositol (PI) response and airway smooth muscle contraction induced by muscarinic agonists, we examined the effects of neostigmine, physostigmine, pyridostigmine, and edrophonium on PI response in the airway smooth muscle. The rat tracheal slices were incubated in Krebs-Henseleit solution containing LiCl and [3H]myo-inositol in the presence of carbachol, anti-ChE, or none of them. [3H]inositol monophosphate (IP1), which is a degradation product of PI response, was counted with a liquid scintillation counter. Inositol monophosphate accumulation was stimulated by neostigmine, physostigmine, and pyridostigmine in a dose-dependent manner, but was not affected by edrophonium. These increases were completely inhibited by atropine. The results suggest that neostigmine, physostigmine, and pyridostigmine stimulate PI response in the airway smooth muscle, which would cause bronchoconstriction, while edrophonium does not affect PI response.

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.

Nitroglycerin inhibits the phosphorylation of intermediate filament proteins rather than myosin light chain on porcine coronary artery sustained contraction

The smooth muscle relaxation induced by nitroglycerin is hypothesized to be mediated by an increase in the cytoplasmic concentration of guanosine 3',5'-monophosphate (cGMP) and subsequent dephosphorylation of the 20-kilodalton myosin light chain (MLC). We investigated this hypothesis in procine coronary arterial smooth muscle stimulated with histamine (3 microM) or K+ (30 mM). Stimulation of [32P]Pi-labeled muscle with histamine or K+ for 2 min resulted in a four- or 6.2-fold increase, respectively, in the incorporation of 32P into MLC. After 48 min of exposure to histamine, MLC phosphorylation decreased to the basal level and the phosphorylation of desmin, synemin, and of three unidentified cytosolic proteins was increased. K+ stimulation resulted in a sustained increase of MLC phosphorylation but had no effect on the phosphorylation of desmin, synemin, or the three unidentified cytosolic proteins. Application of nitroglycerin (1 microM) 48 min after histamine stimulation inhibited the phosphorylation of desmin, synemin, and the three cytosolic proteins. The sustained phase of histamine-induced contraction was also inhibited to a greater extent then the acute phase of histamine-induced contraction and both the acute and sustained phases of K(+)-induced contraction. These results suggest that MLC phosphorylation is required for both phases of K(+)-induced contraction, whereas phosphorylation of intermediate filament proteins is required for the sustained phase of histamine-induced contraction. Intermediate filament proteins, rather than MLC, may also be the target for the relaxant action of nitroglycerin during histamine-induced sustained contraction.

Polyacrylamide gel electrophoretic methods in the separation of structural muscle proteins

Polyacrylamide gel electrophoresis plays a major role in analyzing the function of muscle structural proteins. This review describes one- and two-dimensional gel electrophoretic methods for qualitative and quantitative investigation of the muscle proteins, with special emphasis on determination of protein phosphorylation. The electrophoretic studies established the subunit structures of the muscle proteins, characterized their multiple forms, revealed changes in subunit composition or shifts in isoform distribution of specific proteins during development, upon stimulation or denervation of the muscle. Protein phosphorylation during muscle contraction is preferentially studied by two-dimensional gel electrophoresis. The same method demonstrated protein alterations in human neuromuscular diseases.

The involvement of protein kinase C in the contraction of human airway smooth muscle

The involvement of protein kinase C in the contraction of airway smooth muscle has been investigated in human isolated bronchus. Phorbol 12,13-dibutyrate (PDB) (10 microM) produced aw biphasic response--relaxation followed by contraction. The protein kinase C inhibitor, staurosporine (0.1 microM) reduced the contractile response to PDB from 89 +/- 2.9% to 53 +/- 4.5% of the response to 1 mM acetylcholine (P < 0.05, n = 6) but increased the relaxation response from 12 +/- 6.1% to 29 +/- 5% (P < 0.05, n = 6). Staurosporine also reduced the maximal contractile response to a single dose of histamine (10 microM) from 121 +/- 13% to 91 +/- 10% (P < 0.05, n = 4) and the sustained phase tension from 94 +/- 4% to 85 +/- 5% at 30 min (P < 0.05, n = 4). However, GF 109203X, a more selective inhibitor of protein kinase C at 0.1 microM, 1 microM and 10 microM had no effect on the maximal contractile response and reduced only the sustained phase of the contraction to histamine. These results suggest that protein kinase C plays a role in maintenance of contraction in human airway smooth muscle.

Phosphorylation of caldesmon by smooth-muscle casein kinase II

A caldesmon kinase activity was partially purified from an extract of chicken gizzard smooth muscle by sequential chromatography on columns of DEAE-Sephacel, MonoQ and Superose 12. This kinase was identified as casein kinase II by Western blotting using peptide-directed antibodies raised against the alpha, alpha' and beta subunits of human casein kinase II; the smooth muscle enzyme consisted of similar subunits of M(r) 43,000 (alpha), 39,000 (alpha'), and 27,000 (beta). Phosphorylation of caldesmon and casein by smooth muscle casein kinase II was optimal at approximately 0.1 M NaCl, did not require second messengers, and was inhibited by heparin. The kinase utilized either GTP or ATP as a substrate. Caldesmon was phosphorylated to approximately 1 mol Pi mol-1 caldesmon by smooth muscle casein kinase II with a Km for caldesmon of 4.9 microM. Two-dimensional thin-layer electrophoresis indicated phosphate incorporation into both serine and threonine. All the incorporated phosphate was recovered in the N-terminal peptide (residues 1-152) generated by cleavage at cysteine 153 with 2-nitro-5-thiocyanobenzoic acid. Purification of tryptic phosphopeptides and N-terminal sequencing revealed two principal sites of phosphorylation: serine 73 and threonine 83. The following four synthetic peptides corresponding to this domain of caldesmon were examined as substrates of casein kinase II: A = RRREVNAQNSVAEEE; B = AQNSVAEEE; C = RSTDDEAA; D = SVAEEETKRSTDDE. Interestingly, only peptides C and D were phosphorylated and both only at threonine. Phosphorylation of intact caldesmon did not affect the pattern of chymotryptic digestion suggesting that it does not induce a significant conformational change in the protein substrate. Phosphorylation also had no effect on the binding of caldesmon to actin or on the caldesmon-mediated inhibition of actomyosin MgATPase activity. However, phosphorylation completely abolished the interaction of caldesmon with immobilized smooth muscle myosin. These results are consistent with the localization of the myosin-binding domain near the N-terminus of caldesmon and of the actin-binding domain near the opposite end of the elongated molecule. Casein kinase II may therefore play a role in regulating caldesmon-myosin interaction and the ability of caldesmon to cross-link actin and myosin filaments in smooth muscle.

Intracellular calcium in canine cultured tracheal smooth muscle cells is regulated by M3 muscarinic receptors

1. The regulation of cytosolic Ca2+ concentrations ([Ca2+]i) during exposure to carbachol was measured directly in canine cultured tracheal smooth muscle cells (TSMCs) loaded with fura-2. Stimulation of muscarinic cholinoceptors (muscarinic AChRs) by carbachol produced a dose-dependent rise in [Ca2+]i which was followed by a stable plateau phase. The EC50 values of carbachol for the peak and sustained plateau responses were 0.34 and 0.33 microM, respectively. 2. Atropine (10 microM) prevented all the responses to carbachol, and when added during a response to carbachol, significantly, but not completely decreased [Ca2+]i within 5 s. Therefore, the changes in [Ca2+]i by carbachol were mediated through the muscarinic AChRs. 3. AF-DX 116 (a selective M2 antagonist) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, a selective M3 antagonist) inhibited the carbachol-stimulated increase in [Ca2+]i with pKB values of 6.4 and 9.4, respectively, corresponding to low affinity for AF-DX 119 and high affinity for 4-DAMP in antagonizing this response. 4. The plateau elevation of [Ca2+]i was dependent on the presence of external Ca2+. Removal of Ca2+ by the addition of 2 mM EGTA caused the [Ca2+]i to decline rapidly to the resting level. In the absence of external Ca2+, only an initial transient peak of [Ca2+]i was seen which then declined to the resting level; the sustained elevation of [Ca2+]i could then be evoked by the addition of Ca2+ (1.8 mM) in the continued presence of carbachol. 5.Ca2+ influx was required for the changes of [Ca2+]i, since the Ca2+-channel blockers, diltiazem(10 microM), nifedipine (10 microM), verapamil (10 microM) and Ni2+ (5 mM), decreased both the initial and sustained elevation of [Ca2+], in response to carbachol. These Ca2+-channel blockers also decreased the sustained elevation of [Ca2+], when applied during the plateau phase.6. In conclusion, we have demonstrated that the initial detectable increase in carbachol-stimulated[Ca2+]J is due to the release of Ca2+ from internal stores, followed by the flux of external Ca2+ into the cells. This influx of extracellular Ca2+ partially involves an L-type Ca2+-channel. M3 muscarinic receptors appear to mediate the Ca2+ mobilization in canine TSMCs.

MCI-826 is a potent and selective antagonist of peptide leukotrienes (p-LTs) and has characteristics distinctive from those of FPL 55712

Antagonistic effects of a newly synthesized compound, (E)-2,2-diethyl-3'-[2-[2-(4-isopropyl)thiazolyl]ethenyl]succinanilic+ ++ acid sodium salt (MCI-826) on the contraction of the isolated guinea pig trachea and human bronchus induced by various agonists including peptide leukotrienes (p-LTs), histamine, acetylcholine (ACh), prostaglandin (PG) D2 and others were investigated and compared with the effects of a p-LT antagonist, FPL 55712, in some experiments. MCI-826 potently antagonized LTD4- and LTE4-induced contractions at extremely low concentrations in the isolated guinea pig trachea with pA2 values of 8.3 and 8.9, respectively, on a molar basis. These values indicated that MCI-826 is over 100 times stronger than FPL 55712. Similarly, MCI-826 at 10(-8) g/ml (2.4 x 10(-8) M) markedly antagonized LTD4-induced contractions of the isolated human bronchus. Although FPL 55712 fairly inhibited the 10(-9) g/ml LTC4-induced contraction of the isolated guinea pig trachea, MCI-826 had little effect on the contraction at high concentrations like 3 x 10(-6) g/ml (7.1 x 10(-6) M). MCI-826 modestly affected the other agonist-induced contractions and the resting tonus of the isolated guinea pig trachea at 10(-6) g/ml (2.4 x 10(-6) M) or higher concentrations, but FPL 55712 caused fair inhibition of some of those contractions and gradually lowered the resting tonus with time. These results indicate that MCI-826 is a highly potent and selective antagonist of LTD4 and LTE4 and can be a useful tool for biological and pharmacological experiments on p-LTs.

Phosphorylation of vascular smooth muscle caldesmon by endogenous kinase

Caldesmon was phosphorylated up to 1.2 molPi/mol using a partially purified endogenous kinase fraction. The phosphorylation site was within the C-terminal 99 amino acids. We were also able to phosphorylate caldesmon incorporated into native and synthetic smooth muscle thin filaments. Phosphorylation did not alter caldesmon binding to actin or inhibition of actomyosin ATPase. It also did not change Ca2+ sensitivity in native thin filaments. Phosphorylated caldesmon bound to myosin less than unphosphorylated caldesmon, especially when the myosin was also not phosphorylated. This work did not support the hypothesis that caldesmon function is modulated by phosphorylation.

[Cellular mechanism of muscle contraction of bronchial smooth muscle]

Airway smooth muscle is one of the main effector of bronchial reactivity. The understanding of the cellular mechanisms involved in the contraction of this muscle has advanced in the recent past since isolated cells in culture can now be studied. Extracellular messengers (neurotransmitters and mediators) as well as their specific membrane receptors have been analyzed in some details. The membrane transduction of extracellular messengers brings about the formation (or the increase in the concentration) of the intracellular second messenger which, in airway smooth muscle, is the cytosolic calcium (Ca2+i) via activation of calcium channels which depend on surface membrane potential changes (electromechanical coupling) on the one hand and mainly via mechanisms independent of surface membrane potential changes-so-called the pharmacomechanical coupling--which involves membrane phosphoinositides metabolism. Changes in Ca2+i activate contractile proteins leading the muscle to shorten and to develop force via several controlled steps such as phosphorylation of myosin or changes in the sensitivity to Ca2+ of the contractile elements. Experimental techniques that enable to simultaneously study different aspects of the cellular response are being developed in airway smooth muscle and are likely to provide complementary information about the cellular physiology and pathophysiology of this muscle.

Calponin and SM 22 isoforms in avian and mammalian smooth muscle. Absence of phosphorylation in vivo

Calponin is a basic smooth-muscle-specific protein capable of binding to F-actin, tropomyosin and calmodulin in vitro. Using two-dimensional gel electrophoresis, we show that calponin exists as multiple isoelectric variants in avian and mammalian tissues. During chick embryogenesis, one isoform is expressed in gizzard that shows a pI identical to the most basic adult alpha variant; around 10 d after hatching multiple isoforms then appear. SM 22 [Pearlstone, J. R., Weber, M., Lees-Miller, J. P., Carpenter, M. R. & Smillie, L. B. (1987) J. Biol. Chem. 262, 5985-5991], which has sequence-motifs related to calponin, displays a similar isoform pattern during development; one isoform (alpha) is present in the embryo and three in the adult. In living smooth-muscle strips from chicken gizzard and guinea pig taenia coli, labelled with 32PO4, no phosphate incorporation could be detected in any of the calponin or SM 22 isoforms during either contraction or relaxation. From the additional observation that antibodies against phosphoserine also failed to label calponin and SM 22 in two-dimensional gel immunoblots, we conclude that the multiple isoforms do not arise via differential phosphorylation. These results support the claim [Barany, M., Rokolya, A. & Barany, K. (1991) FEBS Lett. 279, 65-68] that calponin phosphorylation is not involved in smooth muscle regulation in vivo, as has been suggested from in vitro studies [Winder, S. J. & Walsh, M. J. (1990) J. Biol. Chem. 265, 10148-10155]. In vitro translation of porcine and chicken smooth-muscle mRNA produced only a single (alpha) isoform of calponin, suggesting that the adult isoforms do not derive from multiple gene products; in the same assay two polypeptides appeared in the position of SM 22, one corresponding to the alpha isoform and a second more basic spot, not observed in tissue samples. Whereas calponin and SM 22 appear synchronously during smooth muscle differentiation in vivo, SM 22 is not fully down-regulated like calponin, metavinculin and heavy-caldesmon in smooth muscle cells in culture, pointing to a differential regulation of expression of the alpha SM 22 isoform during smooth-muscle phenotype modulation in vitro.

Effects of inhibitors of protein kinase C and calpain in experimental delayed cerebral vasospasm

Vasospasm was produced in adult mongrel dogs by a two-hemorrhage method, and the spastic basilar arteries were exposed via the transclival route on Day 7. Tonic contraction was produced in the normal canine basilar arteries by a local application of KCl or serotonin after transclival exposure. The exposed spastic and tonic basilar arteries then received a topical application of the following: 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7), a potent inhibitor of protein kinase C acting at the catalytic domain; calphostin C, a specific inhibitor of protein kinase C acting at the regulatory domain; or calpeptin, a selective inhibitor of calpain. Both spastic and tonic basilar arteries were effectively dilated by H-7. Calphostin C caused only slight dilation of spastic basilar arteries but moderate dilation of tonic basilar arteries. Dilation in response to calpeptin was remarkable in the spastic basilar arteries but slight in the tonic basilar arteries. The doses of calphostin C and calpeptin required to obtain maximum effect were markedly lower in the tonic model than in the spastic model. The spastic and tonic models had a similar dose-dependent response to H-7 but quite a different response to calphostin C or calpeptin, suggesting a difference in the function of protein kinase C and calpain in the two models. Furthermore, the effect of calphostin C on the reversal of vasospasm was increased significantly after topical treatment with calpeptin. It is suggested that the majority of the catalytic domain of protein kinase C is dissociated from the regulatory domain, probably by a limited proteolysis with calpain, and is markedly activated in vasospasm.

Hsp27 is a mediator of sustained smooth muscle contraction in response to bombesin

We have identified the low MW 27 kD heat shock protein as a major phosphoprotein constituent of smooth muscle and have investigated its potential role in agonist induced smooth muscle contraction. The neuropeptides bombesin and substance P, which are present in neurons of the anorectal region, induce contraction of isolated smooth muscle cells from this region by activating different intracellular pathways. Substance P-induced contraction is 1,4,5-inositol trisphosphate (IP3)/calmodulin dependent, while contraction induced by bombesin is mediated by a protein kinase C (PKC)-dependent pathway. The sustained contraction induced by bombesin or exogenous PKC was blocked by preincubation of cells with monoclonal antibodies to hsp27, while the transient contraction induced by substance P or IP3 was unaffected by the antibodies. Preincubation with isotype matched control antibodies had no inhibitory effect on contraction induced in response to the agents used. These data support a novel role for hsp27 in the non calmodulin mediated sustained contraction induced by bombesin or PKC.

Pre- and postjunctional effects of phorbol 12-myristate-13-acetate in the mouse vas deferens

The protein kinase C stimulator phorbol 12-myristate-13-acetate (PMA) increased the release of noradrenaline from field-stimulated mouse vasa deferentia and antagonized the inhibitory effect of xylazine and FK 33-824. The mechanical response to field stimulation was not modified by PMA in unpretreated vasa, but the contractility was partly restored when the stimulation-response curve had been depressed by xylazine or FK 33-824. In contrast, PMA decreased the contractile response to exogenous noradrenaline and bethanechol. A similar but smaller reduction was obtained with 1,2-dioctanoyl-sn-glycerol. In vasa depolarized by KCl, PMA and verapamil reduced the amplitude of contractions elicited by CaCl2. Only the effect of verapamil could be reversed by the calcium ionophore A 23187, while the effect of PMA was greatly attenuated in muscles pretreated with ouabain. Phorbol 12-myristate-13-acetate-4-O-methylether was ineffective in all experiments. These results suggest that PMA increases noradrenaline release (prejunctional effect) and decreases vasal contractility (postjunctional effect) by activation of protein kinase C.

Oxytocin contracts rat uterine smooth muscle in Ca2(+)-free medium without any phosphorylation of myosin light chain

Contraction of rat uterine smooth muscle related to phosphorylation state of myosin light chain under various conditions was investigated. In the Ca2(+)-containing medium, both high K+ and oxytocin induced marked contraction of the muscle accompanied by pronounced phosphorylation of myosin light chain. In the Ca2(+)-free medium, although both vanadate and oxytocin induced slight contraction, phosphorylation of myosin light chain was only evident for vanadate but not for oxytocin. It was suggested that another mechanism distinct from myosin light chain phosphorylation might be involved in Ca2(+)-independent contraction of uterine smooth muscle elicited by oxytocin.

Calcium reversal, relaxation by calcium ion of various smooth muscles contracted by carbachol, norepinephrine or a phorbol ester in calcium-ion free medium

1. Ca reversal was reported on the longitudinal smooth muscle of rat uterus and the fundic part of the circular smooth muscle of guinea pig stomach; that is, addition of Ca ion to the Ca-free bathing solution in which the muscle is contracting to oxytocin or carbachol, respectively, relaxes the muscle. 2. We tested whether this inhibitory action of Ca ion is seen in other smooth muscles including vascular, air way, gastric and genital smooth muscles. 3. We found that Ca reversal was observed in the smooth muscle of rat thoracic aorta contracted by norepinephrine or by a phorbol ester, TPA, guinea pig tracheal smooth muscle contracted to carbachol, fundic smooth muscle of rat stomach to carbachol, corporal and antral smooth muscle of guinea pig stomach to carbachol and rat vas deferens to norepinephrine. 4. Ca reversal was observed not only when the muscle contracted by an agonist of the surface receptor such as oxytocin, norepinephrine or carbachol but also by a phorbol ester that is believed to cause contraction in the cell by activating phosphorylation. 5. Thus, we conclude that Ca reversal is a universal phenomenon in smooth muscles.

Phorbol 12,13-diacetate-induced contraction of the canine basilar artery: role of protein kinase C

The pharmacological and biochemical mechanisms of contractile responses to the protein kinase C (PKC) activator phorbol-12,13-diacetate (PDA) were investigated in canine basilar arteries. In the normal medium, PDA elicited a strong, dose-related, and slow-developing sustained contraction. Among the constrictors examined, including serotonin, prostaglandin F2 alpha, and endothelin, only PDA yielded contractions in a Ca2(+)-free medium. In both media, the PDA-induced contractions were virtually inhibited by either staurosporine, H-7, or quinacrine, while neither neurotransmitter blockades nor R24571 (calmidazolium) exerted significant effects. In addition, it was shown that 8-bromocyclic GMP, but not 8-bromocyclic AMP, markedly curtailed the PDA-induced contractions. Biochemical analysis, furthermore, showed that PDA induced increased phosphorylations of 27- and 96-kDa and proteins other than the myosin light chain (MLC) 20-kDa protein. Thus, the present results open up a novel mechanism of sustained cerebral artery contractions, where PKC activation rather than Ca2+/calmodulin/MLC system plays a key role that is regulated both by phospholipase A2 and by cyclic GMP.

1,2-diacylglycerol content in thoracic aorta of spontaneously hypertensive rats

Phosphoinositide metabolism participates in the control of cell calcium homeostasis. Because a notable neutral lipid (1,2-diacylglycerol) is generated from phosphoinositide hydrolysis and is assumed to be a secondary messenger, we determined 1,2-diacylglycerol content and its fatty acid profiles in the thoracic aorta of spontaneously hypertensive rats (SHR) and compared it with those of normotensive Wistar-Kyoto (WKY) rats. After the aorta was exposed to 10(-5) M norepinephrine as a stimulant, 1,2-diacylglycerol content in SHR was significantly higher by 33% than in WKY rats at 4 weeks of age, whereas there was no difference in 1,2-diacylglycerol content between the two strains at 20 weeks of age. Before norepinephrine stimulation, there was no significant difference in 1,2-diacylglycerol level between the two strains at 4 weeks of age. Analysis on a gas chromatograph showed that 1,2-diacylglycerol was composed of similar molecular species of fatty acids in aortas obtained from SHR and WKY rats. On the other hand, the cholesterol content of aortas was higher in SHR than in WKY rats at 20 weeks of age, whereas the difference at 4 weeks was not significant. Phosphatidylcholine, phosphatidylethanolamine, and triglyceride showed no significant difference between the two strains. It is concluded that norepinephrine-induced 1,2-diacylglycerol production increases in the thoracic aorta of SHR before the development of hypertension.

Inhibitory effects of protein kinase inhibitors and cytoskeletal inhibitors on Ca2(+)-free contraction of rat uterus

In rat uterine smooth muscle, sustained Ca2(+)-free contraction was observed by oxytocin in Ca2(+)-free solution. This Ca2(+)-free contraction was effectively inhibited by protein kinase inhibitors and cytoskeletal inhibitors but myosin-light chain kinase (MLCK) inhibitors were not so effective. Simultaneous addition of a protein kinase inhibitor and a cytoskeletal inhibitor caused synergistic inhibition. These results suggest that the mechanism for Ca2(+)-free contraction involves some protein kinase and cytoskeletal elements rather than MLCK.

Effects of H-7 (protein kinase inhibitor) and phorbol ester on aortic strips from spontaneously hypertensive rats

We investigated the vascular responsiveness to vasoactive agents and the inhibition by H-7 (1-(5-isoquinoline-sulfonyl)-2-methylpiperazine), which inhibits cyclic nucleotide-dependent protein kinases and protein kinase C(PKC) equally potently in helically cut strips of thoracic aortas from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The susceptibility of norepinephrine (NE)- and angiotensin II (Ang II)-induced contractions to H-7 was significantly higher in the aortas from SHR than in those from WKY. H-7 decreased the contractile responses to KCl to a similar extent in both strains without affecting the high K(+)-stimulated Ca2+ influx. H-7 produced a shift to the right of the dose-response curve for the PKC activator, 12-o-tetradecanoylphorbol-13-acetate (TPA) in the case of SHR aortas, while no such shift was noted in tissues from WKY. Functional alterations in the PKC branch of the Ca2+ messenger system in vascular smooth muscle may play an important role in SHR during the sustained contraction.

Aequorin luminescence, myosin phosphorylation, and active stress in tracheal smooth muscle

During muscarinic activation of canine tracheal smooth muscle with carbachol, myosin phosphorylation is significantly more sensitive than stress to the external Ca2+ concentration ([Ca2+]o) [W. T. Gerthoffer. Am. J. Physiol. 250 (Cell Physiol. 19): C597-C604, 1986]. To determine whether the intracellular Ca2+ concentration ([Ca2+]i) correlated more closely with changes in phosphorylation or force, we measured isometric force and light emitted by the luminescent intracellular Ca2+ indicator aequorin as [Ca2+]o was increased in the presence of 1 microM carbachol or 60 mM K+. Myosin phosphorylation was measured using an immunoblot assay in a second set of muscle strips treated identically. Stimulation with carbachol increased aequorin luminescence slightly in strips incubated in Ca2+-free solution. Active stress and aequorin luminescence subsequently increased in parallel as [Ca2+]o was increased. Myosin phosphorylation at 0.05 mM [Ca2+]o (0.30 +/- 0.04 mol Pi/mol light chain) was significantly higher than phosphorylation in Ca2+-free solution with no carbachol (0.12 +/- 0.048 mol Pi/mol light chain) and increased to a maximum of 0.56 +/- 0.03 mol Pi/mol light chain at 1.6 mM [Ca2+]o. In contrast, active stress and aequorin luminescence remained low at low [Ca2+]o and reached a maximum at 2.4 mM [Ca2+]o. Stimulation with carbachol produced greater increases in myosin phosphorylation and active stress for a given change in aequorin luminescence than did K+ depolarization. Stimulation with carbachol also produced a different phosphorylation-stress relationship than did K+ depolarization. These observations are consistent with the possibility that carbachol induces increases in the Ca2+ sensitivity of contractile proteins in tracheal smooth muscle.