Contraction in Spirostomum is apparently directed by 2 antagonistic systems which respond to changes in free calcium concentration. The system responsible for imparting contractile tension consists of bundles of 4-5 nm microfilaments which can be calcium-activated. The second system consists of pellicular microtubules which may have the capacity to impart tensile force and react with each other through cross-bridging. Cells subjected to electrical stimulation to induce contraction in the presence of 0.1 mM 2,4-dinitrophenol and KCN failed to recover their original resting lengths. Similar treatment with colchicine produced the same effect, implying an interruption in the activity or structural integrity of the microtubular system. Treatment with 0.5 µg/ml cytochalasin B inhibited all contractile responses, presumably by blocking the activity of microfilaments.

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