Summary A detergent-extracted cell model of the heterotrichous ciliate Spirostomum ambiguum showed ATP-independent contraction on addition of > 10−7 M Ca2+, which is caused by contraction of the myoneme. To further understand the mechanism of cell contraction, the effects of Mg2+ and other cations were examined. Among various kinds of cations tested, Ba2+, Mn2+, Sr2+ and Cd2+ induced slight contraction, but they required higher concentrations (> 10−3 M). Contraction of the cell model was not induced by Mg2+. However, it shifted the threshold of Ca2+ concentration for inducing contraction to higher levels. The Hill constant, denoted as the number of calcium ions cooperatively bound to a contractile element in the myoneme, was 2–3 at 0–1 mM Mg2+, while it decreased with increasing Mg2+ concentration. The Ca2+-induced contraction was not influenced by changing ionic strength of the reactivation medium. These results suggest that Mg2+ ions interact with the Ca2+-binding sites which induce contraction of the myoneme.
