ABSTRACT
1. The metamorphosis of the planulae ofHydractinia echinata (Hydrozoa) can be induced by a pulse-type (2-3 hrs) exposure to the ions of caesium, rubidium, lithium, and potassium. With mixtures of seawater with isoosmolar solutions of the chlorides of these ions the dose-response curves for Rb+-, Li+-, and K+-stimulation display optima peaks, the smallest effective range being shown by K+. The Cs+-curve is sigmoid shaped. The half-maximal doses are 6 mval Cs+, 50 mval Rb+, 80 mval Li+, and 130 mval K+ respectively. 2. Potassium-induced stimulation is an ouabain-insensitive event governed by the Gibbs-Donnan principle. With solutions containing only K+, Ca++, and Sucrose the optimal efficacy is always related to an invariable ratio of[Formula: see text], regardless of the actual concentrations employed. 3. The Cs+-induction is based upon active events which indicate the involvement of a carrier-like system. Indirect reaction kinetics taking the velocity of the larval response versus the dose as measure of cation uptake yield Michaelis-like saturation curves. Reduction of external calcium and sodium as well as application of the glutathione oxidizing agent diamide enhance the efficacy of Cs+. The shape of the response curve becomes closer to a rectangular hyperbola and the half-maximal concentration is shifted towards lower values. The inducing power of Cs+, Rb+, and Li+ (but not of K+) is abolished by ouabain (1 mM). These findings give evidence that the relevant ions act by stimulation of the Na+/K+-transport ATPase. 4. Thus, induction of metamorphosis by monovalent cations is due to alterations of membrane-bound functions. This applies to bacteria-induced metamorphosis as well.
