title: The structure of the contractile apparatus in ultrarapidly frozen smooth muscle: freeze-fracture, deep-etch, and freeze-substitution studies.
abstract: The structure of the smooth muscle contractile apparatus was studied using ultrarapid freezing followed by freeze substitution or by longitudinal freeze-fracture, deep-etch, and platinum-carbon replication. Freeze substitution minimises the detrimental effects of chemical fixation and freeze fracture eliminates them entirely whilst revealing the ultrastructure in three dimensions. Unidirectionally shadowed freeze-fracture replicas of ultrarapidly frozen, relaxed, intact smooth muscle showed a well-preserved actin filament structure the 5.5-nm repeat of the actin subunits was clearly observed. In transversely fractured tissue the thick filaments were revealed, with a distribution comparable to that seen in transverse sections of freeze-substituted muscle. Relaxed muscle permeabilised using Triton X-100 showed a similar structure to that of intact tissue after ultrarapid freezing and examination both by freeze fracture and by freeze examination both by freeze fracture and by freeze substitution; the ratios of actin to myosin were also comparable. In permeabilised, rigorised tissue the structure of the actomyosin complex was revealed in detail; this was especially clear in freeze-substituted muscle. A cross-bridge spacing of 38 nm was measured in freeze-fractured, deep-etched tissue. The structural detail revealed is compatible with a side polar model of the actomyosin interaction and with the sliding filament mechanism of muscle contraction.
authors.affiliation: Department of Cardiac Medicine, National Heart and Lung Institute, London, United Kingdom.
authors.forename: J L
authors.initials: JL
authors.lastname: Hodgkinson
authors.forename: T M
authors.initials: TM
authors.lastname: Newman
authors.forename: S B
authors.initials: SB
authors.lastname: Marston
authors.forename: N J
authors.initials: NJ
authors.lastname: Severs
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry / Q000648:ultrastructure*; D000199:Actins / Q000648:ultrastructure; D000205:Actomyosin / Q000648:ultrastructure; D000818:Animals; D003106:Colon; D003285:Contractile Proteins / Q000648:ultrastructure*; D005613:Freeze Etching; D005614:Freeze Fracturing; D017110:Freeze Substitution; D006168:Guinea Pigs; D008854:Microscopy, Electron; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure*; D009206:Myocardium / Q000648:ultrastructure; D009218:Myosins / Q000648:ultrastructure; D017830:Octoxynol; D010539:Permeability / Q000187:drug effects
chemical_list: D000199:Actins; D003285:Contractile Proteins; D017830:Octoxynol; D000205:Actomyosin; D009218:Myosins