The Calcium response of Rat Sperm: Evidence for a dynein-based mechanism.

 

Kathleen A. Lesich and Charles B. Lindemann

 

Calcium is known to regulate the waveform of the beat in most eukaryotic flagella. In rat sperm, the calcium response takes the form of a change in the symmetry of the beat, which at its most extreme produces an arrest of the flagellum in the shape of a fishhook. We used Triton X-100 demembranated rat sperm (sperm models) to measure the stiffness of the flagellum. Force-calibrated glass microprobes were used to straighten flagella that were in the calcium fishhook conformation (Moritz et al., Cell Motility and the Cytoskeleton, 49:33-40, 2001). The torque required to straighten the flagellum was calculated from the force measurement. In this study, sperm models were poisoned with 50μM sodium vanadate to disable the dyneins and then bent into a fishhook conformation with a microprobe. We found that the torque required to straighten a fishhook, 1.8 10-7 dyne cm (n= 13), and the torque required to form a fishhook in vanadate treated cells, 2.0 (0.7) 10-7 cm (n=17), was nearly the same. This, combined with the observation that the calcium induced fishhook conformation is lost when treated with 50μM vanadate, suggests that dynein is the source of the torque that maintains the fishhook. The best interpretation of these findings is that a fishhook represents a Newtonian balance between active dynein torque and passive elastic torque. This means that at high calcium (pCa 4-5) the dyneins on one side of the axoneme remain permanently in the force producing state. Supported by N. S. F. grant #MCB-0110024.