Abstract
Attached, spread, chick heart fibroblasts were induced to shorten by treatment with the ionophore, A23187. The shortening resulted from the retraction of the leading lamellae and other major cell processes. The response was dependent on external calcium with a threshold close to, and a maximal effect at, physiological concentrations. The shortening was also induced in Colcemid-treated cells and therefore did not involve a depolymerization of microtubules. Indirect evidence indicates that the shortening was preceded by an increase in tension in the spread cell. The response is consistent with the effect of an increase in the cytoplasmic concentration of free calcium on a calcium-sensitive actomyosin system in the spread fibroblast. Although the retraction of non-spreading processes mimicked the intermittent retraction of similar trailing processes during normal movement of fibroblasts, the response to the ionophore differed in that the leading lamellae were also induced to retract. This difference implies that a general increase in the cytoplasmic concentration of free calcium alone cannot account for the intermittent shortening that occurs in normal movement.
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