Airways smooth muscle plasticity correlation of structure and function.

摘要

Smooth muscle is widely distributed in the body and controls different physiological functions in animals primarily by generating force and changing length. In the airways, excessive shortening of airway smooth muscle (ASM) has been implicated in pulmonary diseases like asthma. This dissertation is focused on understanding the subcellular ultra-structure and molecular mechanism conferring the ability of ASM to shorten.; Three related groups of experiments (projects) formed the foundation of this thesis. The first project examined various conditions under which myosin filaments assembled (or disassembled) in intact ASM, under the hypothesis that muscle cell length and intracellular calcium levels regulated the number and length of the myosin filaments in vivo. The experiments were designed to investigate the changes in the ASM thick filament content before and after the muscle had been activated and after the muscle had been adapted at a longer length, as well as the effect of resting calcium level on thick filament stability. The results showed that in ASM, muscle activation and muscle adaptation at a longer length favor filament formation, and that the resting calcium level is crucial for partial preservation of the filaments in the relaxed state.; The second project examined the role of actin polymerization as part of the normal ASM response to various stimuli. It was postulated that the same responses observed in myosin thick filaments (first project) could be elicited from actins under the corresponding conditions. ASM bundles were fixed for electron microscopic analysis in the relaxed and activated states at two lengths; the muscle preparations were also fixed after a period of oscillatory strain, a condition known to cause depolymerization of myosin filaments. The results indicated that contractile activation, increased cell length and oscillatory strain enhanced actin polymerization. It was also shown that contractile activation did not preferentially enhance actin polymerization in areas near dense bodies. It was demonstrated then, that actin thin filaments in vivo are dynamic structures whose length and number are regulated by the cell in response to changes in extracellular environment and that polymerization/depolymerization of the thin filaments occurs uniformly across the whole cell cross-section.; The third and final project of this thesis investigated the changes in mechanical properties and ultra-structure of ASM immediately after a quick stretch and after the muscle had been fully adapted at the stretched length, to elucidate the effects of contractile filament overlap, isotonic load, myosin evanescence and other intrinsic factors influencing the amount of shortening. (Abstract shortened by UMI.)