Taxol stabilizes or promotes the assembly of microtubules. In this report we characterize the rate, extent, and reversibility of taxol stabilization of calciumlabile microtubules in isolated mitotic spindles, principally from embryos of the sand dollar Echinarachnius parma. The intense depolymerizing action of 100 pM Ca2+ was used to assess the extent of stabilization by taxol. Changes in spindle microtubule assembly were evaluated and recorded by measuring changes in spindle birefringent retardation (BR). Membrane-free mitotic spindles, isolated with a calcium-chelating, nonionic detergent buffer, were stored in an EGTAgylcerol storage buffer to prevent microtubule depolymerization. When perfused with an EGTA-buffer without glycerol, microtubules in these isolated spindles depolymerized gradually over 60-120 min; but in isolated spindles perfused with buffer that contained 100 pM Ca2+, BR decreased by 90% within 2-5 sec. In contrast, spindles that were pretreated for 3 min with 1 pM taxol, or for about 30 sec with 10 pM taxol, lost less than 10% of their initial BR when perfused with buffer containing 100 pM Ca2+. The rate and extent of microtubule stabilization by taxol depended on both the concentration and the duration of exposure to taxol.
Taxol stabilization was reversible. After a 15 min preincubation with 1 pM or 10 pM taxol then washout, stability of spindle BR to 100 pM Ca2+ decreased exponentially with a time constant of 30-60 min. Thus taxol dissociates from spindle microtubules at significant rates; taxol-stabilized microtubules are not "fixed.