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

Effects of EMD57033, an activator of actomyosin ATPase activity, on the relaxation process of cell membrane-permeabilized carotid artery and taenia cecum from guinea pigs

Smooth muscle relaxation after contraction is thought to reflect "latch-like" slow cycling bridge formation and deformation. However, how actin-myosin interaction contributes to the transfer from fast-cycling cross bridges to slow-cycling bridges is still unclear. The thiadiazinone compound EMD57033 is known to bind to an allosteric pocket in the myosin motor domain and to increase basal and actin-activated myosin ATPase activity and contractile force in striated muscles. Therefore, we investigated whether EMD57033 affected the relaxation process after Ca2+ removal by affecting slow cycling bridge formation and/or deformation in β-escin skinned (cell membrane-permeabilized) carotid artery and taenia cecum from guinea pigs. EMD57033 at ≥30 µM decreased the force decay during relaxation in both the skinned carotid artery and taenia cecum, irrespective of the presence of ATP. A kinetic analysis in the present study indicated that EMD57033 significantly prolonged τslow-detach, a time constant of detachment of the slow cycling bridge, in both the skinned carotid artery and taenia cecum, irrespective of the presence of nucleoside triphosphates (ATP or ITP). Further studies are necessary to elucidate how EMD57033 modulates the smooth muscle myosin (SMM) structure, SMM activity, and thick filament organization, affecting slow cycling bridge formation and deformation, although EMD57033 might change slow cycling bridge formation, resulting in both cycling rate modulation and an increase in the affinity of SMM to actin.

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.