Role of 100-kDa cytosolic PLA2 in ACh-induced contraction of cat esophageal circular muscle

Phosphorylated HSP20 modulates the association of thin-filament binding proteins: caldesmon with tropomyosin in colonic smooth muscle

Small heat shock proteins HSP27 and HSP20 have been implicated in regulation of contraction and relaxation in smooth muscle. Activation of PKC-α promotes contraction by phosphorylation of HSP27 whereas activation of PKA promotes relaxation by phosphorylation of HSP20 in colonic smooth muscle cells (CSMC). We propose that the balance between the phosphorylation states of HSP27 and HSP20 represents a molecular signaling switch for contraction and relaxation. This molecular signaling switch acts downstream on a molecular mechanical switch [tropomyosin (TM)] regulating thin-filament dynamics. We have examined the role of phosphorylation state(s) of HSP20 on HSP27-mediated thin-filament regulation in CSMC. CSMC were transfected with different HSP20 phosphomutants. These transfections had no effect on the integrity of actin cytoskeleton. Cells transfected with 16D-HSP20 (phosphomimic) exhibited inhibition of acetylcholine (ACh)-induced contraction whereas cells transfected with 16A-HSP20 (nonphosphorylatable) had no effect on ACh-induced contraction. CSMC transfected with 16D-HSP20 cDNA showed significant decreases in 1) phosphorylation of HSP27 (ser78); 2) phosphorylation of PKC-α (ser657); 3) phosphorylation of TM and CaD (ser789); 4) ACh-induced phosphorylation of myosin light chain; 5) ACh-induced association of TM with HSP27; and 6) ACh-induced dissociation of TM from caldesmon (CaD). We thus propose the crucial physiological relevance of molecular signaling switch (phosphorylation state of HSP27 and HSP20), which dictates 1) the phosphorylation states of TM and CaD and 2) their dissociations from each other.

Signal-transduction pathways that regulate smooth muscle function I. Signal transduction in phasic (esophageal) and tonic (gastroesophageal sphincter) smooth muscles

Contraction of esophageal (Eso) and lower esophageal sphincter (LES) circular muscle depends on distinct signal-transduction pathways. ACh-induced contraction of Eso muscle is linked to phosphatidylcholine metabolism, production of diacylglycerol and arachidonic acid (AA), and activation of the Ca(2+)-insensitive PKCepsilon. Although PKCepsilon does not require Ca(2+) for activation, either influx of extracellular Ca(2+) or release of Ca(2+) from stores is needed to activate the phospholipases responsible for hydrolysis of membrane phospholipids and production of second messengers, which activate PKCepsilon. In contrast, the LES uses two distinct intracellular pathways: 1) a PKC-dependent pathway activated by low doses of agonists or during maintenance of spontaneous tone, and 2) a Ca(2+)-calmodulin-myosin light chain kinase (MLCK)-dependent pathway activated in response to maximally effective doses of agonists during the initial phase of contraction. The Ca(2+) levels, released by agonist-induced activity of phospholipase C, determine which contractile pathway is activated in the LES. The Ca(2+)-calmodulin-MLCK-dependent contractile pathway has been well characterized in a variety of smooth muscles. The steps linking activation of PKC to myosin light chain (MLC20) phosphorylation and contraction, however, have not been clearly defined for LES, Eso, or other smooth muscles. In addition, in LES circular muscle, a low-molecular weight pancreatic-like phospholipase A2 (group I PLA2) causes production of AA, which is metabolized to prostaglandins and thromboxanes. These AA metabolites act on receptors linked to heterotrimeric G proteins to induce activation of phospholipases and production of second messengers to maintain contraction of LES circular muscle. We have examined the signal-transduction pathways activated by PGF(2alpha) and by thromboxane analogs during the initial contractile phase and found that these pathways are the same as those activated by other agonists. In response to low doses of agonists or during maintenance of tone, presumably due to low levels of calcium release, a PKC-dependent pathway is activated, whereas at high doses of PGF(2alpha) and thromboxane analogs, in the initial phase of contraction, calmodulin is activated, PKC activity is reduced, and contraction is mediated, in part, through a Ca(2+)-calmodulin-MLCK-dependent pathway. The PKC-dependent signaling pathways activated by PGF(2alpha) and by thromboxanes during sustained LES contraction, however, remain to be examined, but preliminary data indicate that a distinct PKC-dependent pathway may be activated during maintenance of tonic contraction, which is different from the one activated during the initial contractile response. The initial contractile response to low levels of agonists depends on activation of G(q). Sustained contraction in response to PGF(2alpha) may involve activation of the monomeric G protein RhoA, because the contraction is inhibited by the RhoA-kinase antagonist Y27632. This shift in signal-transduction pathways between initial and sustained contraction has been recently reported in intestinal smooth muscle.

The intracellular pathway of the acetylcholine-induced contraction in cat detrusor muscle cells

1. The present study was aimed to investigate intracellular pathways involved in acetylcholine (ACh)-induced contraction in cat detrusor muscle cells 2. Contraction was expressed as per cent shortening of length of individually isolated smooth muscle cells obtained by enzymatic digestion. Dispersed intact and permeabilized cells were prepared for the treatment of drugs and antibody to enzymes, respectively. Using Western blot, we confirmed the presence of related proteins. 3. The maximal contraction to ACh was generated at 10(-11) M. This response was preferentially antagonized by M3 muscarinic receptor antagonist rho-fluoro-hexahydrosiladifenidol (rhoF-HSD) but not by the M1 antagonist pirenzepine and the M2 muscarinic receptor antagonist methoctramine. We identified G-proteins (Gq/11), (Gs), (G0), (Gi1), (Gi2) and (Gi3) in the bladder detrusor muscle. ACh-induced contraction was selectively inhibited by (Gq/11) antibody but not to other G subunit. 4. The phosphatidylinositol-specific phospholipase C (PI-PLC) inhibitor neomycin reduced ACh-induced contraction. However, the inhibitors of the phospholipase D, the phospholipase A2 and protein kinase C did not attenuate the ACh-induced contraction. ACh-induced contraction was inhibited by antibody to PLC-beta1 but not PLC-beta3 and PLC-gamma. Thapsigargin or strontium, which depletes or blocks intracellular calcium release, inhibited ACh-induced contraction. Inositol 1,4,5-triphosphate IP3 receptor inhibitor heparin reduced ACh-induced contraction. 5. These results suggest that in cat detrusor muscle contraction induced by ACh is mediated via M3 muscarinic receptor-dependent activation of Gq/11 and PLC-beta1 and IP3-dependent Ca(2+) release.

Myosin light chain kinase- and PKC-dependent contraction of LES and esophageal smooth muscle

In smooth muscle cells enzymatically isolated from circular muscle of the esophagus (ESO) and lower esophageal sphincter (LES), ACh-induced contraction and myosin light chain (MLC) phosphorylation were similar. Contraction and phosphorylation induced by purified MLC kinase (MLCK) were significantly greater in LES than ESO. ACh-induced contraction and MLC phosphorylation were inhibited by calmodulin and MLCK inhibitors in LES and by protein kinase C (PKC) inhibitors in ESO. Contraction of LES and ESO induced by the PKC agonist 1,2-dioctanoylglycerol (DG) was unaffected by MLCK inhibitors. Caldesmon and calponin concentration-dependently inhibited ACh-induced contraction of ESO and not LES. In ESO, caldesmon antagonist GS17C reversed caldesmon- but not calponin-induced ACh inhibition. GS17C caused contraction of permeabilized ESO but had much less effect on LES. GS17C-induced contraction was not affected by MLCK inhibitors, suggesting that MLCK may not regulate caldesmon-mediated contraction. DG-induced contraction of ESO and LES was inhibited by caldesmon and calponinin, suggesting that these proteins may regulate PKC-dependent contraction. We conclude that calmodulin and MLCK play a role in ACh-induced LES contraction, whereas the classical MLCK may not be the major kinase responsible for contraction and phosphorylation of MLC in ESO. ESO contraction is PKC dependent. Caldesmon and/or calponin may play a role in PKC-dependent contraction.

Opposing roles of K(+) and Cl(-) channels in maintenance of opossum lower esophageal sphincter tone

The ionic basis underlying the maintenance of myogenic tone of lower esophageal sphincter circular muscle (LES) was investigated in opossum with the use of standard isometric tension and conventional intracellular microelectrode recordings in vitro. In tension recording studies, nifedipine (1 microM) reduced basal tone to 27.7 +/- 3.8% of control. The K(+) channel blockers tetraethylammonium (TEA, 2 mM), charybdotoxin (100 nM), and 4-aminopyridine (4-AP, 2 mM) enhanced resting tone, whereas apamin and glibenclamide were without affect. Cl(-) channel blockers DIDS (500 microM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (500 microM), as well as niflumic acid (0.1-300 microM), decreased basal tone, but tamoxifen was without effect. Intracellular microelectrode recordings revealed ongoing, spontaneous, spike-like action potentials (APs). Nifedipine abolished APs and depolarized resting membrane potential (RMP). Both TEA and 4-AP significantly depolarized RMP and augmented APs, whereas niflumic acid dose-dependently hyperpolarized RMP and abolished APs. These data suggest that, in the opossum, basal tone is associated with continuous APs and that K(+) and Ca(2+)-activated Cl(-) channels have important opposing roles in the genesis of LES tone.