Extensive skinning of cell membrane diminishes the force-inhibiting effect of okadaic acid on smooth muscles of Guinea pig hepatic portal vein

Effects of cytochalasin D on relaxation process of skinned taenia cecum and carotid artery from guinea pig

Actin linked regulatory mechanisms are known to contribute contraction/relaxation in smooth muscle. In order to clarify whether modulation of polymerization/depolymerization of actin filaments affects relaxation process, we examined the effects of cytochalasin D on relaxation process by Ca2+ removal after Ca2+-induced contraction of β-escin skinned (cell membrane permeabilized) taenia cecum and carotid artery preparations from guinea pigs. Cytochalasin D, an inhibitor of actin polymerization, significantly suppressed the force during relaxation both in skinned taenia cecum and carotid artery. The data fitting analysis of the relaxation processes indicates that cytochalasin D accelerates slow (latch-like) bridge dissociation. Cytochalasin D seems to directly disrupts actin filament organization or its length, resulting in modulation of actin filament structure that prevents myosin binding.

Inhibitory effects of rubratoxin A, a potent inhibitor of protein phosphatase 2, on the Ca2+-dependent contraction of skinned carotid artery from guinea pig

Rubratoxin A, a potent inhibitor of PP2A, is known to suppress smooth muscle contraction. The inhibitory role of PP2A in smooth muscle contraction is still unclear. In order to clarify the regulatory mechanisms of PP2A on vascular smooth muscle contractility, we examined the effects of rubratoxin A on the Ca²⁺-induced contraction of β-escin skinned carotid artery preparations from guinea pigs. Rubratoxin A at 1 µM and 10 µM significantly inhibited skinned carotid artery contraction at any Ca²⁺ concentration. The data fitting to the Hill equation in [Ca²⁺]-contraction relationship indicated that rubratoxin A decreased F_max-Ca2+ and increased [Ca²⁺]₅₀, indices of Ca²⁺ sensitivity for the force and myosin-actin interaction, respectively. These results suggest that PP2A inhibition causes downregulation of the myosin light chain phosphorylation and direct interference with myosin-actin interaction.

Accelerating effects of blebbistatin on relaxation process of cell membrane permeabilized trachea and taenia cecum from guinea pig

Blebbistatin, a potent inhibitor of myosin II, is known to suppress smooth muscle contraction without affecting myosin light chain phosphorylation level. In order to clarify the regulatory mechanisms of blebbistatin on phasic and tonic smooth muscles in detail, we examined the effects of blebbistatin on relaxation process by Ca²⁺ removal after Ca²⁺-induced contraction of β-escin skinned (cell membrane permeabilized) trachea and taenia cecum preparations from guinea pigs. Blebbistatin significantly suppressed the force during relaxation both in skinned trachea and taenia cecum. The data fitting analysis of the relaxation processes indicates that blebbistatin accelerates slow (latch-like) bridge dissociation.

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Force-inhibiting effect of Ser/Thr protein phosphatase 2A inhibitors on bovine ciliary muscle

Ciliary muscle is a smooth muscle characterized by a rapid response to muscarinic receptor stimulation and sustained contraction. Although it is evident that these contractions are Ca(2+)-dependent, detailed molecular mechanisms are still unknown. In order to elucidate the role of Ser/Thr protein phosphatase 2A (PP2A) in ciliary muscle contraction, we examined the effects of okadaic acid and other PP2A inhibitors on contractions induced by carbachol (CCh) and ionomycin in bovine ciliary muscle strips (BCM). Okadaic acid inhibited ionomycin-induced contraction, while it did not cause significant changes in CCh-induced contraction. Fostriecin showed similar inhibitory effects on the contraction of BCM. On the other hand, rubratoxin A inhibited both ionomycin- and CCh-induced contractions. These results indicated that PP2A was involved at least in ionomycin-induced Ca(2+)-dependent contraction, and that BCM had a unique regulatory mechanism in CCh-induced contraction.

Blebbistatin, a myosin II inhibitor, suppresses Ca(2+)-induced and "sensitized"-contraction of skinned tracheal muscles from guinea pig

Blebbistatin, a potent inhibitor of myosin II, has inhibiting effects on Ca(2+)-induced contraction and contractile filament organization without affecting the Ca(2+)-sensitivity to the force and phosphorylation level of myosin regulatory light chain (MLC20) in skinned (cell membrane permeabilized) taenia cecum from the guinea pig (Watanabe et al., Am J Physiol Cell Physiol. 2010; 298: C1118-26). In the present study, we investigated blebbistatin effects on the contractile force of skinned tracheal muscle, in which myosin filaments organization is more labile than that in the taenia cecum. Blebbistatin at 10 μM or higher suppressed Ca(2+)-induced tension development at any given Ca(2+) concentration, but had little effects on the Ca(2+)- induced myosin light chain phosphorylation. Also blebbistatin at 10 μM and higher significantly suppressed GTP-γS-induced "sensitized" force development. Since the force inhibiting effects of blebbistatin on the skinned trachea were much stronger than those in skinned taenia cecum, blebbistatin might directly affect myosin filaments organization.

Rho-kinase mediated cytoskeletal stiffness in skinned smooth muscle

The structurally dynamic cytoskeleton is important in many cell functions. Large gaps still exist in our knowledge regarding what regulates cytoskeletal dynamics and what underlies the structural plasticity. Because Rho-kinase is an upstream regulator of signaling events leading to phosphorylation of many cytoskeletal proteins in many cell types, we have chosen this kinase as the focus of the present study. In detergent skinned tracheal smooth muscle preparations, we quantified the proteins eluted from the muscle cells over time and monitored the muscle's ability to respond to acetylcholine (ACh) stimulation to produce force and stiffness. In a partially skinned preparation not able to generate active force but could still stiffen upon ACh stimulation, we found that the ACh-induced stiffness was independent of calcium and myosin light chain phosphorylation. This indicates that the myosin light chain-dependent actively cycling crossbridges are not likely the source of the stiffness. The results also indicate that Rho-kinase is central to the ACh-induced stiffness, because inhibition of the kinase by H1152 (1 μM) abolished the stiffening. Furthermore, the rate of relaxation of calcium-induced stiffness in the skinned preparation was faster than that of ACh-induced stiffness, with or without calcium, suggesting that different signaling pathways lead to different means of maintenance of stiffness in the skinned preparation.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Blebbistatin, a myosin II inhibitor, suppresses contraction and disrupts contractile filaments organization of skinned taenia cecum from guinea pig

To explore the precise mechanisms of the inhibitory effects of blebbistatin, a potent inhibitor of myosin II, on smooth muscle contraction, we studied the blebbistatin effects on the mechanical properties and the structure of contractile filaments of skinned (cell membrane permeabilized) preparations from guinea pig taenia cecum. Blebbistatin at 10 microM or higher suppressed Ca(2+)-induced tension development at any given Ca(2+) concentration but had little effects on the Ca(2+)-induced myosin light chain phosphorylation. Blebbistatin also suppressed the 10 and 2.75 mM Mg(2+)-induced, "myosin light chain phosphorylation-independent" tension development at more than 10 microM. Furthermore, blebbistatin induced conformational change of smooth muscle myosin (SMM) and disrupted arrangement of SMM and thin filaments, resulting in inhibition of actin-SMM interaction irrespective of activation with Ca(2+). In addition, blebbistatin partially inhibited Mg(2+)-ATPase activity of native actomyosin from guinea pig taenia cecum at around 10 microM. These results suggested that blebbistatin suppressed skinned smooth muscle contraction through disruption of structure of SMM by the agent.

Calcium sensitivity and cooperativity of permeabilized rat mesenteric lymphatics

Lymphatic muscle contraction is critical for the centripetal movement of lymph that regulates fluid balance, protein homeostasis, lipid absorption, and immune function. We have demonstrated that lymphatic muscle has both smooth and striated muscle contractile elements; however, the basic contractile properties of this tissue remain poorly defined. We hypothesized that contractile characteristics of lymphatic myofilaments would be different from vascular smooth muscle myofilaments. To test this hypothesis, -log[Ca(2+)] (pCa)-tension relationship was determined for alpha-toxin permeabilized mesenteric lymphatics, arteries, and veins. The Ca(2+) sensitivity (pCa(50)) of mesenteric lymphatics was significantly lower compared with arteries (6.16 +/- 0.05 vs. 6.44 +/- 0.02; P < 0.05), whereas there was no difference in pCa(50) between lymphatics and veins (6.16 +/- 0.05 vs. 6.00 +/- 0.10; not significant). The Hill coefficient for alpha-toxin-permeabilized lymphatics was not significantly different from arteries but was significantly greater than that of the veins (1.98 +/- 0.19 vs. 1.21 +/- 0.18; P < 0.05). In addition, the maximal tension and pCa(50) values were significantly greater in alpha-toxin-permeabilized lymphatics compared with beta-escin-permeabilized lymphatics (0.27 +/- 0.03 vs. 0.15 +/- 0.01 and 6.16 +/- 0.05 vs. 5.86 +/- 0.06 mN/mm, respectively; P < 0.05), whereas the Hill coefficient was significantly greater in beta-escin-permeabilized lymphatics. Western blot analyses revealed that CPI-17 levels were significantly decreased by about 50% in beta-escin-permeabilized lymphatics, compared with controls, whereas no change in the level of calmodulin was detected. Our data constitute the first description of the pCa-tension relationship in permeabilized lymphatic muscle. It suggests that differences in myofilament Ca(2+) sensitivity and cooperativity among lymphatic muscle and vascular smooth muscles contribute to the functional differences that exist between these tissues.

Mechanisms of the inhibitory effects of a phenoxazine compound, 2-amino-4,4.ALPHA.-dihydro-4 .ALPHA.-7-dimethyl-3H-phenoxazine-3-one, on the contraction of the smooth muscle of the guinea pig taenia cecum

To elucidate the mechanisms involved in the relaxing effect of 2-amino-4, 4alpha-dihydro-4alpha-7-dimethyl-3H-phenoxazine-3-one (Phx-1) on smooth muscle, we investigated its effects on the contraction of both intact and skinned (cell membrane permeabilized) preparations from the guinea pig taenia cecum. In intact preparations, Phx-1 concentration-dependently suppressed the contraction induced by either acetylcholine (ACh) or high-K(+) with an IC(50) value estimated at around 100 muM. Similar inhibitory actions of Phx-1 on force were observed in intracellular Ca store depleted preparations. In cell membrane depolarized preparations in the absence of extracellular Ca, however, Phx-1 had little effect on either caffeine- or ACh-induced contractions. In skinned preparations, Phx-1 suppressed Ca(2+)-induced contractions at concentrations higher than 100 muM. These results suggest that inhibition of smooth muscle contraction by Phx-1 is due mainly to inhibition of Ca(2+)-influx, although Phx-1 also seems to have direct inhibitory effects on the activities of the contractile apparatus.

Synthetic peptides of actin-tropomyosin binding region of troponin I and heat shock protein 20 modulate the relaxation process of skinned preparations of taenia caeci from guinea pig

To explore the possible role of the thin filament-linked regulation of cross-bridge cycling in living smooth muscle contraction, we studied the effects of TnIp and HSP20p, a synthetic peptide originating from an actin tropomyosin binding region of rabbit cardiac troponin I (residues 136-147; GKFKRPTLRRVR), and that of human heat shock protein 20 (residues 110-121; GFVAREFHRRYR) on the relaxation of skinned (cell membrane ilized) preparations from guinea pig taenia caeci. An active stress of the skinned preparations, resulting from actin-myosin interaction, rapidly decayed following Ca(2+) removal (relaxation). TnIp accelerated the initial rapid phase and slowed the following slow phase of the relaxation. On the other hand, HSP20p only slowed the whole process of the relaxation. The relaxation time courses were well fitted in a double exponential manner, and the double exponential decay of the stress could be explained as a portion of fast-detaching cross bridges not to dissociate rapidly by Ca(2+) removal, but to transfer to latch bridges dissociating very slowly. Our present results suggested that (i) TnIp and HSP20p accelerated transferring from fast-detaching cross bridges to slow-detaching (latch) bridges, and (ii) TnIp accelerated dissociation of the fast-detaching cross bridges and the latch bridges, while HSP20p slowed dissociation the fast-detaching cross bridges. Since TnIp and HSP20p are thought to bind to actin and tropomyosin, but not to myosin, we concluded that through thin-filament-dependent mechanisms these peptides regulated the formation and/or deformation of latch bridges in smooth muscle. The thin-filament-dependent regulation might physiologically control the stress maintenance and relaxation in smooth muscle cells.

Effects of hydrogen peroxide on contraction of skinned aorta from guinea pigs

We examined the effects of hydrogen peroxide (H2O2) on the contractile force of cell membrane permeabilized descending thoracic aorta from guinea pigs. H2O2 enhanced generated force in the Ca2+-induced contraction at any given concentration of Ca2+, and maintained force level in the 30 mM Mg2+-induced Ca2+-independent force maintenance. H2O2 seems to directly enhance the cross-bridge interaction of aortic smooth muscles.

Regulation of human airway ciliary beat frequency by intracellular pH

pHi affects a number of cellular functions, but the influence of pHi on mammalian ciliary beat frequency (CBF) is not known. CBF and pHi of single human tracheobronchial epithelial cells in submerged culture were measured simultaneously using video microscopy (for CBF) and epifluorescence microscopy with the pH-sensitive dye BCECF. Baseline CBF and pHi values in bicarbonate-free medium were 7.2 +/- 0.2 Hz and 7.49 +/- 0.02, respectively (n = 63). Alkalization by ammonium pre-pulse to pHi 7.78 +/- 0.02 resulted in a 2.2 +/- 0.1 Hz CBF increase (P < 0.05). Following removal of NH4Cl, pHi decreased to 7.24 +/- 0.02 and CBF to 5.8 +/- 0.1 Hz (P < 0.05). Removal of extracellular CO2 to change pHi resulted in similar CBF changes. Pre-activation of cAMP-dependent protein kinase (10 microM forskolin), broad inhibition of protein kinases (100 microM H-7), inhibition of PKA (10 microM H-89), nor inhibition of phosphatases (10 microM cyclosporin + 1.5 microM okadaic acid) changed pHi-mediated changes in CBF, nor were they due to [Ca2+]i changes. CBF of basolaterally permeabilized human tracheobronchial cells, re-differentiated at the air-liquid interface, was 3.9 +/- 0.3, 5.7 +/- 0.4, 7.0 +/- 0.3 and 7.3 +/- 0.3 Hz at basolateral i.e., intracellular pH of 6.8, 7.2, 7.6 and 8.0, respectively (n = 18). Thus, intracellular alkalization stimulates, while intracellular acidification attenuates human airway CBF. Since phosphorylation and [Ca2+]i changes did not seem to mediate pHi-induced CBF changes, pHi may directly act on the ciliary motile machinery.

Troponin I inhibitory peptide suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation

To explore possible mechanisms involving the thin filament-linked regulation of contraction in living smooth muscles, we studied the effects of a synthetic peptide of rabbit cardiac troponin I [residues 136-147] (TnIp), which is a minimal sequence required to inhibit striated muscle acto-tropomyosin-myosin ATPase activity, on the mechanical properties of beta-escin skinned preparations of taenia caeci from guinea pig. TnIp reversibly suppressed the Ca(2+)-activated force without significant effects on the Ca(2+) sensitivity and on the phosphorylation level of myosin regulatory light chain (MLC(20)). TnIp also reversibly suppressed the Ca(2+)/calmodulin-independent contraction induced by 30mM Mg(2+). An analogue of TnIp, which lost inhibiting action on acto-tropomyosin-myosin ATPase activity, affected neither Ca(2+)-activated nor 30mM Mg(2+)-induced contraction. These results indicate that TnIp suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation rather than inhibiting the Ca(2+)/calmodulin-dependent thick and thin filament activating processes.