Key Points
- Determine the structural presence and stability of thick myosin filaments in vertebrate smooth muscle under varying mechanical and functional conditions.
- Examined longitudinal muscle sections from guinea-pig ileum and comparative insect striated muscle using electron microscopy.
- Prepared tissue specimens under fixed mechanical tension, atropine-induced relaxation, and potassium-induced contracture in a slack state.
- Thick myosin filaments were regularly present in both longitudinal and cross-sections of smooth muscle fixed under mechanical tension and during atropine relaxation.
- Thick filaments were absent in slack muscles during potassium contracture and in slack bee-wing striated muscle, showing that filament disappearance is a preparation artefact prevented by tension.
- Ultrastructural evidence confirms that the sliding filament model of contraction applies directly to vertebrate smooth muscle without substantial modification.
Structured PICO
PPopulationGuinea-pig ileum (longitudinal layer of intestinal smooth muscle) and bee-wing muscle
IInterventionFixation at constant length (under mechanical tension) or relaxed by atropine
CComparatorSlack muscles in K+ contracture
OOutcomePresence of thick filaments (myosin) in electronmicrographssurrogate
Myosin filaments are regular constituents of vertebrate smooth muscle regardless of functional state, supporting the validity of the sliding filament model.