Release of Free Ca²⁺ from the Nematocysts of Aiptasia mutabilis during the Discharge

Abstract

Although the mechanism of discharge of nematocysts is still controversial, most authors agree that bound calcium plays an definite role in stabilizing the resting state of capsules. Therefore, the kinetics of the release of Ca²⁺ during the discharge is expected to provide useful information about the mechanism of discharge itself. The investigation was performed on microbasic mastigophores extruded in 1 M citrate from the acontia of Aiptasia mutabilis. The release of free Ca²⁺ during the discharge of a population of nematocysts induced by treatment with hypoosmotic medium was revealed by the light emission of aequorin. The time course of light emission was related to the discharge curve. The recorded light signal had two different components: a short-latency one preceding the discharge and a longer one paralleling the discharge. The effectiveness of the hypoosmotic medium in inducing the discharge was not constant in various samples. Unresponsive nematocysts gave rise to a different light signal. These data suggest that in microbasic mastigophores of Aiptasia some change in the outer layer of the capsule wall is associated with the discharge. In conclusion, in the discharge of the microbasic mastigophores of Aiptasia mutabilis, the unbinding of calcium from protein molecules of the capsule fluid could be involved. However, also in these nematocysts, the osmotically generated energy for discharge is not effective unless the "stopper" is opened.