Factors involved in the sensitivity of Stentor to colchicine, lumicolchicine, and vinblastine sulfate.

Oral regeneration by the ciliate Stentor coeruleus is inhibited by colchicine (Cc), but only at a relatively high concentration (0.9 mM); moreover, regeneration is inhibited by an even lower concentration of lumicolchicine (LCc) (0.2 mM). Together these results suggest that Cc may not be acting via tubulin binding. To evaluate this possibility we: (1) tested the effect of both drugs, and vinblastine sulfate (Vb) for comparison, on a population of labile cytoplasmic microtubules; and (2) measured the kinetics of association of all three drugs with regenerating cells. We found that Cc and Vb reduced the number of microtubules only at concentrations that blocked regeneration, whereas LCc blocked regeneration without reducing microtubule number. In addition, LCc associated with the cells much more readily than Cc, such that the cell-associated concentration of Cc that blocked regeneration was actually several fold lower than the effective concentration of LCc. We propose that common effects of Cc and LCc unrelated to tubulin binding play no more than a minor role in Cc effects on regeneration and conclude that Cc acts primarily if not exclusively via its antimicrotubule activity.