Vinblastine paracrystals from cultured cells are calcium-stable

Vinblastine has two distinct tubulin-related effects in 3T6 fibroblasts and J774.2 macrophages. It depolymerizes microtubules and it induces the formation of paracrystals in the cytoplasm. These paracrystals are retained in cytoskeletons prepared by Triton extraction and are stable to treatment with calcium. The direct addition of vinblastine to cytoskeletons does not alter the organization of microtubules. The two effects of vinblastine are concentration-dependent, as assayed by binding of [³H]taxol and tubulin immunofluorescence. At low concentrations, vinblastine depolymerizes cellular microtubules; at high concentrations the drug induces the formation of paracrystals.