Diffusion of Ca2+ and the quiescent response of sea urchin sperm flagella

The starting and stopping transients observed in sea urchin sperm flagella in the presence of high Ca2+ are believed to begin with an influx of Ca2+ into the axoneme and to end, as indicated by resumption of normal beating, when the Ca2+ has been reduced to very low levels by an active extrusion process. If the influx and efflux processes were uniformly distributed along the length of the flagellum, it is not likely that the starting and stopping transients would occur as a well defined sequence of events that always proceeds from the proximal to the distal end. Theoretical analysis of the concentration profiles of Ca2+ expected if Ca2+ influx occurred along the length of the flagellum but efflux was restricted to the proximal end shows that the time required to reduce [Ca2+] in the distal portion of the flagellum would generally be longer than the observed recovery times. Localization of both the influx and efflux processes near the proximal end, however, yields concentration profiles consistent with observations on the duration of starting and stopping transients.