Trout sperm swimming patterns and role of intracellular Ca++

We investigated the swimming patterns of trout sperm using computer‐assisted analyses of video microscopy. Under full activation conditions, in which 80–100% of sperm activate their motility, sperm swim in circular paths for 2–5 sec, followed by 30–60 sec of a more linear swimming, and, finally, cessation of movement, with a straightening of the flagella. Threshold activation, in which 50% of the sperm activate, is characterized by circular patterns of swimming for less than 20 sec, with straightened flagella upon cessation. Full activation and threshold activation are observed in low‐K+ solution or in an Mg++ ‐supplemented K+ solution. Similarities in swimming patterns in low‐K+ solution and in a Mg++ ‐supplemented K+ solution suggest a common underlying mechanism of activation. Initiation of movement in solutions with high Ca++ to K+ ratio is similar to activation in K+ ‐free solution. However, sperm in Ca++ ‐supplemented media resume circular swimming within 20–25 sec after activation, and, upon cessation of movement, the flagella are frequently cane shaped or bent. Differences in swimming patterns upon activation by high Ca++ concentration suggest additional effects of Ca++ on regulating swimming patterns. We used the fluorescent Ca++ indicator Fluo‐3 to measure changes in intracellular Ca++ concentration upon activation. Intracellular Ca++ concentration transiently increases upon activation, with peak Ca++ concentration coinciding with the period of circular swimming. This transient increase in Ca++ concentration is seen in the absence of external Ca++, providing strong evidence for the released of Ca++ from intracellular stores upon activation.