The effect of cytochalasin B and D on the fertilization of sea urchins

Two electrical events occur across the plasma membrane of the sea urchin egg during the interaction with the fertilizing spermatozoon; the first, a step-like depolarization pecedes the fertilization potential (FP) by 11 sec at 20°C. Cytochalasins B and D alter these events in several ways depending upon dose and time of exposure of the gametes. When both the egg and the spermatozoon are exposed to these microfilament-active drugs, the former for minutes whilst the latter in the order of seconds, the delay between the two electrical events is increased twofold. We suggest that a microfilament-involved process is occurring during this delay period which, although itself not altering the electrical properties of the plasma membrane, determines the time of initiation of the FP. Pretreating spermatozoa with higher doses of either CB or CD, inhibits fertilization completely or lowers the successful collision rate (α); we suggest that the two drugs lower α by affecting the acrosomal region of the spermatozoon. Although both drugs increase the delay period to the same degree CD has fewer secondary effects than CB.