Tentacle contraction inDiscophrya collini: The effects of ionophore A23187 and ruthenium red on Ca2+-induced contraction and uptake of extracellular calcium

Summary The tentacles of the suctorian protozoonDiscophrya collini are stimulated to contract by externally applied Ca²⁺. The role of extracellular Ca²⁺ in tentacle contraction was studied by monitoring⁴⁵Ca²⁺ uptake, using ionophore A23187 to facilitate membrane transport of calcium and ruthenium red (RR) as an inhibitor of transport. The degree of tentacle retraction was dependent upon external Ca²⁺ concentration and studies with⁴⁵Ca²⁺ using scintillation counting indicated a linear relationship between external Ca²⁺ concentration and Ca²⁺ uptake. Uptake of Ca²⁺ was enhanced in the presence of the ionophore while RR caused little inhibition.⁴⁵Ca²⁺ uptake was only partially inhibited by RR when cells were subjected to a Ca²⁺, ionophore and RR mixture. Grain counts from light microscope autoradiographs after treatment of cells with⁴⁵Ca²⁺/ionophore,⁴⁵Ca²⁺/RR or⁴⁵Ca²⁺ alone showed heavy, light and intermediate labelling respectively. In all instances the grains were evenly distributed within the cell.These observations are interpreted as supporting the suggestion that the ionophore enhances both the uptake of extracellular Ca²⁺ and release of Ca²⁺from an internal source, while the RR could only partially prevent movement of Ca²⁺ through the plasma mebrane. A model is presented suggesting that tentacle retraction is mediated by cytosolic Ca²⁺ levels which are determined by the fluxing of Ca²⁺ across the plasma membrane and the membrane of elongate dense bodies which act as internal Ca²⁺ reservoirs.