Microprobe measurements of calcium binding in the contractile spasmoneme of a vorticellid

By means of an electron microprobe, the calcium content of isolated contractile organelles (spasmonemes) from the ciliate Zoothamnium was compared in extension and contraction. By using calcium buffers of different total concentrations, it was shown that the calcium in extended organelles was almost entirely due to concentration of solute during the drying of the specimen. Contracted organelles contained 1-7 g more calcium per kg of dry mass than extended ones, and this extra calcium was bound strongly. The quantity of calcium bound in this way is consistent with the theory that the energy for contraction is derived from the chemical potential of calcium ions. Stoichiometry suggests that between 1-4 and 2-1 calcium ions become bound to each molecule of a spasmonemal calcium-binding protein.