Contraction of Stress Fibers Extracted from Smooth Muscle Cells: Effects of Varying Ionic Strength

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Actin stress fibers (SFs) play a key role in regulation of cell adhesion, but the biochemical and biophysical properties intrinsic to SFs remain unclear. Here we extracted SFs from rat embryonic smooth muscle cells by deroofing, and evaluated the effects of varying ionic strength and temperature on their intactness. Wash buffers with ionic strength ranging from 90 to 490 mM were prepared, and the extracted SFs were incubated in a buffer with a particular ionic strength for 10 min or 24 h. Light and electron microscopy revealed that the extracted SFs comprised tightly packed straight bundles at low ionic strengths that became looser and exhibited a ragged pattern at high ionic strengths. The expression of α-actinin associated with the extracted SFs decreased with the increase in ionic strength. Unexpectedly, non-muscle myosin II and smooth muscle myosin in the extracted SFs displayed comparable expression levels over the different ionic strengths. ATP-induced contractility was better preserved at low ionic strengths, including the physiological ionic strength of 170 mM. The rate of ATP-induced enzymatic activity increased with increase in temperature, but the difference was not statistically significant. These results demonstrate that low ionic strength produces extracted SFs that are more intact with regard to structure and function.