Ca²⁺ bursts occur around a local minimal concentration of attractant and trigger sperm chemotactic response

Ca²⁺ is known to have important roles in sperm chemotaxis, although the relationship between intracellular Ca²⁺ concentration ([Ca²⁺]_i) and modulation of the swimming and chemotactic behavior of spermatozoa has not been elucidated. Using a high-speed 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 [Ca²⁺]_i increased in the flagellum (Ca²⁺ bursts) concomitantly with a change in the swimming direction in an SAAF gradient field. During the initial phase of the Ca²⁺ bursts, the flagellum of the spermatozoon exhibited highly asymmetric waveforms enabling the quick turning of the swimming path. However, the flagellum subsequently changed to symmetric beating, causing the spermatozoon to swim straight. Interestingly, during such responses, [Ca²⁺]_i remained higher than the basal level, indicating that the series of responses was not simply determined by Ca²⁺ concentrations. Also, we found that Ca²⁺ bursts were consistently evoked at points at which the spermatozoon attained around a temporally minimal value for a given SAAF concentration. We concluded that Ca²⁺ 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.