The change in shape of 3T3 and L929 cells due to Bt2cAMP treatment is accompanied by altered intracellular distribution of microfilaments and microtubules. Bt2cAMP added to cells in low density culture causes (a) microfilaments to accumulate in bundles near the plasma membrane, mainly at the cell periphery, and (b) microtubules to accumulate beneath these microfilament bundles. In narrow cell processes that form characteristically in Bt2cAMP-treated L cells, microtubules accumulate in parallel arrays near the center of these processes. A new simple method for evaluating the relative distance of the cell from its underlying substratum is desribed. In normal medium, 3T3 cells attach to their substratum near the nucleus and at the tips of cell processes, bridging irregularities in the plastic surface. With Bt2cAMP treatment, attachment occurs at the cell edge and at many isolated points under the cytoplasm, and the cells conform more closely to irregularities of the underlying substratum. A model of the mechanism by which cAMP modulates cell shape is presented.

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