Activation of the capacity for sodium-induced pinocytosis in Amoeba proteus

Amoebae treated with cycloheximide or starved for 8-10 days lose their pinocytotic response to Na+. Their capacity for Na+-induced pinocytosis was activated after application of various physical or chemical stimuli (electrical stimulation, mechanical shearing forces, osmotic pressure, UV-light, alkali metal ions, capsaicin, and indole). The degree of activation was related to the intensity and duration of the stimulus and lasted several hours after the stimulus had been withdrawn. The dose-response curves of activating stimuli were always biphasic. Strong activating agents reduced the sensitivity of the amoeba to the inducer. At concentrations lower than those which induced pinocytosis, but in the same order of efficacy, inorganic cations were potent activating agents. Like induction of pinocytosis, activation by cations required minute amounts of Ca2+ and was inhibited by high concentrations of this ion. Activation may therefore be an early event during the induction of pinocytosis. Capsaicin and indole were potent activators, indicating that specific chemical stimuli may increase the capacity for pinocytosis. The activation may be the result of a secretory process adding area and structures to the old membrane which are necessary for the induction of pinocytosis.