The direction of sperm tail movement (red to black to blue) changes with a sharp pulse of Ca2+ (top).

Some free-swimming sperm detect the presence of an egg through chemotactic signals and respond by altering their swimming trajectory. On page 725, Wood et al. show that intact sea urchin sperm respond not to overall increases in intracellular Ca2+ levels, as previously thought, but to rapid changes in Ca2+ concentration. Ca2+ entry into the flagella is biphasic, with a fast and slow phase, but only the initial fast influx affects sperm trajectory.

Studies on demembranated sperm indicated that the cells change direction by asymmetrical bending of their flagella in response to overall Ca2+ increases and that cGMP signaling was involved. But until now scientists didn't have the tools to watch the process in real time in intact cells.

Using a novel fast strobe lighting system for fluorescence microscopy, which they devised, Wood et al. attempted to stimulate chemotaxis by releasing caged cGMP in Strongylocentrotus purpuratus sperm cells loaded with a fluorescent Ca2+ indicator. As predicted, Ca2+ levels immediately increased in both the head and the flagella. However, the flagella showed a short rapid burst of Ca2+ influx followed by a longer slower uptake. Ca2+ increases induced transient bending in the flagella. Repeated stimuli induced repeated direction changes, even if the overall Ca2+ concentration was already above baseline levels.

Nimodipine, which is used to block voltage-gated Ca2+ channels, blocked the initial fast influx into the flagella, suggesting that such channels mediate the fast uptake, but did not affect the slow portion of the Ca2+ uptake. In the presence of nimodipine, the sperm did not alter their swimming direction, even though the intracellular Ca2+ increased.

The fact that a slow rise in Ca2+ was not sufficient to induce flagellar bending leads the team to suggest two possibilities. Either there is a Ca2+ sensor that detects only a rapid flux in concentration, or the sensor is located so close to the nimodipine-sensitive channel that it only responds to Ca2+ coming in via this port.