Electron microscopic study of actin polymerization in airway smooth muscle

Actin polymerization as part of the normal smooth muscle response to various stimuli has been reported. The actin dynamics are believed to be necessary for cytoskeletal remodeling in smooth muscle in its adaptation to external stress and strain and for maintenance of optimal contractility. We have shown in our previous studies in airway smooth muscle that myosins polymerized in response to contractile activation as well as to adaptation at longer cell lengths. We postulated that the same response could be elicited from actins under the same conditions. In the present study, actin filament formation was quantified electron microscopically in cell cross sections. Nanometer resolution allowed us to examine regional distribution of filaments in a cell cross section. Airway smooth muscle bundles were fixed in relaxed and activated states at two lengths; muscle preparations were also fixed after a period of oscillatory strain, a condition known to cause depolymerization of myosin filaments. The results indicate that contractile activation and increased cell length nonsynergistically enhanced actin polymerization; the extent of actin polymerization was substantially less than that of myosin polymerization. Oscillatory strain increased thin filament formation. Although thin filament density was found higher in cytoplasmic areas near dense bodies, contractile activation did not preferentially enhance actin polymerization in these areas. It is concluded that actin thin filaments are dynamic structures whose length and number are regulated by the cell in response to changes in extracellular environment and that polymerization and depolymerization of thin filaments occur uniformly across the whole cell cross section.