Flagellar quiescence response in sea urchin sperm induced by electric stimulation

To investigate the mechanism of the flagellar quiescence in sperm, we examined the effect of electric stimulation of individual spermatozoa of the sea urchin, Hemicentrotus pulcherrimus. Stimulation with a suction electrode attached to the sperm head elicited a flagellar quiescence response, in which the sperm showed a typical cane-shaped bend in the proximal region of the flagellum when the electrode was used as anode. Cathodic stimulation also induced quiescence, but was much less effective than anodic stimulation. During the quiescence response, which lasted for 1-3 s, no new bend was initiated, and subsequently the flagellum resumed normal beating. The quiescence response required the presence of Ca2+ (> 2 mM) in sea water, and was inhibited by Co2+ and La3+. At low Ca2+ concentrations (2-5 mM), the angle of the cane-shaped bend was smaller than that at 10 mM Ca2+; thus the angle of the cane-shaped bend, characteristic of the quiescence response is dependent on Ca2+ concentration. These results suggest that the quiescence response is triggered by a depolarization of the flagellar membrane, followed by an influx of Ca2+ into the flagellum through Ca2+ channels. The increase in Ca2+ concentration within the flagellum affects the amount of sliding and thus produces a cane-shaped proximal bend of various angles, while inhibiting both the propagation of the proximal bend (principal bend) and the formation of a new reverse bend.