Real-time analysis of the role of Ca²⁺ in flagellar movement and motility in single sea urchin sperm

Eggs of many marine and mammalian species attract sperm by releasing chemoattractants that modify the bending properties of flagella to redirect sperm paths toward the egg. This process, called chemotaxis, is dependent on extracellular Ca²⁺. We used stroboscopic fluorescence imaging to measure intracellular Ca²⁺ concentration ([Ca²⁺]i) in the flagella of swimming sea urchin sperm. Uncaging of cyclic GMP induced Ca²⁺ entry via at least two distinct pathways, and we identified a nimodipine-sensitive pathway, compartmentalized in the flagella, as a key regulator of flagellar bending and directed motility changes. We found that, contrary to current models, the degree of flagellar bending does not vary in proportion to the overall [Ca²⁺]i. Instead we propose a new model whereby flagella bending is increased by Ca²⁺ flux through the nimodipine-sensitive pathway, and is unaffected by [Ca²⁺]i increases through alternative pathways.