Coupling of excitation and cessation of cyclois in Nitella: role of divalent cations

The effect of external divalent cation salt solutions upon the association of an action potential and cessation of cytoplasmic streaming in Nitella was studied. Nitella cells remained excitable when immersed in solutions of CaCl₂, MgCl₂, BaCl₂, and SrCl₂. Cessation of streaming coincident with excitation occurred in solutions of CaCl₂ or SrCl₂ but not in solutions of MgCl₂ or BaCl₂. In cells exposed to solutions containing mixtures of MgCl₂ and CaCl₂, or MgCl₂ and SrCl₂, it was the [Ca]/[Mg] or [Sr]/[Mg] which determined the effect of an action potential upon the rate of streaming, rather than the absolute concentrations Ca⁺⁺ or Sr⁺⁺. The implications of these data are discussed with respect to the structure involved in the generation of cytoplasmic streaming and the relation of streaming to other types of biological motion.