Ca~(2+) Bursts Occur Around A Local Minimal Concentration Of Attractant And Trigger Sperm Chemotactic Response

摘要

Ca~(2+) is known to have important roles in sperm chemotaxis, although the relationship between intracellular Ca~(2+) concentration ([Ca~(2+)]_i) and modulation of the swimming and chemotactic behavior of spermatozoa has not been elucidated. Using a highspeed Ca~(2+) imaging system, we examined the chemotactic behavior and [Ca~(2+)]_i in individual ascidian sperm cells exhibiting chemotactic responses toward sperm activating and attracting factor (SAAF), a chemoattractant released by eggs. In this study, we found that transient [Ca~(2+)]_i increased in the flagellum (Ca~(2+) bursts) concomitantly with a change in the swimming direction in an SAAF gradient field. During the initial phase of the Ca~(2+) bursts, the f lagellum of the spermatozoon exhibited highly asymmetric waveforms enabling the quick turning of the swimming path. However, the f lagellum subsequently changed to symmetric beating, causing the spermatozoon to swim straight. Interestingly, during such responses, [Ca~(2+)]j remained higher than the basal level, indicating that the series of responses was not simply determined by Ca~(2+) concentrations. Also, we found that Ca~(2+) bursts were consistently evoked at points at which the spermatozoon attained around a temporally minimal value for a given SAAF concentration. We concluded that Ca~(2+) bursts induced around a local minimal SAAF concentration trigger a sequence of flagellar responses comprising quick turning followed by straight swimming to direct spermatozoa efficiently toward eggs.