RELATIVE POTENCIES OF DIFFERENT CYTOCHALASINS FOR THE INHIBITION OF PHAGOCYTOSIS IN CILIATES

Ciliate actins have been reported to exhibit an unusual degree of sequ ence divergence, within the phylum Ciliophora, and when compared to ac tins from other organisms. To determine whether these primary structur al differences are correlated with pharmacological differences, we inv estigated the effects of seven cytochalasins on phagocytosis, which ha s been shown to be actin dependent in eukaryotic cells. The relative p otencies of cytochalasin inhibition of phagocytosis in Spirostomun amb iguum and Paramecium multimicronucleatum were similar. Dihydrocytochal asin B and cytochalasin A were the most potent of the seven cytochalas ins in both ciliates, and strongly inhibited phagocytosis at 20-40 mu M. Dihydrocytochalasin B was considerably more potent than either cyto chalasins B or D, a result unexpected on the basis of reports utilizin g other organisms. However, even at concentrations up to 100 mu M, dih ydrocytochalasin B did not inhibit the rate of Spirostomum defecation. After long-term treatment of Spirostomum with 50 mu M dihydrocytochal asin B, the sensitivity of phagocytosis to the drug was reduced. These results indicate that the relative cytochalasin sensitivities of acti n-dependent functions in ciliates differ from those of other organisms , and are consistent with the hypothesis that sequence differences mig ht give rise to substantial differences in the pharmacological propert ies of ciliate actins.