Mg2+-dependent stabilization during triton X-100 extraction of Spirostomum followed by calcium reactivation

The contractility in the ciliate Spirostomum is regulated by the action of Ca2+ on organized arrays of 60 A microfilaments and microtubules. Contractile cell models of the Protozoa were prepared by treatment with a calcium-free EGTA buffered medium with 0.02% triton X-100 and 20 mMMgCl2. Extracted cells were made to contract when calcium was buffered to greater than 10-5M. Ca2+ removal in the presence of ATP resulted in partial re-extension. The results suggest that the high concentration of Mg2+ functionally stabilizes the contractile elements during extraction. Ca2+ activation can overcome the Mg2+ effect only when the ratio of Mg2+:Ca2+ is less than 10(3).