UBC Theses and Dissertations
Airways smooth muscle plasticity : correlation of structure and function Herrera, Ana Milena
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. ASM bundles were first adapted at a near-in-situ reference length (L[sub in situ]). Maximal isotonic shortenings under a load equivalent to 30% maximal isometric force (F[sub max]) were measured in preparations 1) contracting from the initial length of L[sub in situ], 2) contracting immediately after a 30% stretch from Lin situ and 3) contracting after adaptation at the stretched length. The muscle bundles were then fixed for electron microscopic examination at the end of the 3 protocols. The results indicated that under acute conditions (e.g., quick stretch) ASM behaves like striated muscle and that the sliding filament mechanism could adequately explain the observations. Under chronic conditions, i.e., after the muscle had been fully adapted to a length change, contractile filament reorganization that accompanied length adaptation altered the amount of maximal shortening without changing the contractile force. These results could be explained by a model where the maximally shortened length of the muscle was determined by the extent of contractile filament overlap and that a quick stretch resulted in a reduced overlap; adaptation at the stretched length led to polymerization and reorganization of the contractile filaments, and restoration of optimal overlap. Findings presented in this thesis suggest that the contractile apparatus of airways smooth muscle is malleable. The malleability likely reflects the cell's ability to alter the number and arrangement of the contractile units within a cell, through myosin and actin polymerization or depolymerization in a length-dependent manner. The new concept that smooth muscle, unlike striated muscle, is structurally plastic is strengthened by the data from this thesis and it likely will have a profound effect on our understanding of the tissue's behaviour in health and disease.
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