The effect of calcium on the aggregation of chicken gizzard thin filaments.

Electron microscopy demonstrates that thin filaments isolated from chicken gizzard smooth muscle in the absence of Ca2+ are aggregated into networks. In contrast, thin filaments isolated in the presence of Ca2+ are dissociated from each other. Electron microscopy also reveals that the respective state of aggregation in each type of preparation is reversible and dependent on Ca2+ concentration. Corresponding viscosity measurements indicate that network formation is associated with an increase in thin filament viscosity. We propose that thin filament aggregation in vivo may be responsible for the tension maintenance of smooth muscle during relaxation.