Multiple activation currents can be evoked in Xenopus laevis eggs when cortical granule exocytosis is inhibited by weak bases

The fertilization potential in Xenopus eggs under normal circumstances is considered to be a unique event. It is associated with a concomitantly occurring cortical granule exocytosis. If eggs were exposed to weak bases, exocytosis was inhibited but the fertilization potential could still be evoked. After recovery from this first transient increase in membrane conductance, a second could be elicited by a further stimulus. A fertilization potential could be triggered either before or after the egg had undergone an electrically induced activation potential. This suggests that sperm receptors and sperm activated ionic channels in the egg membrane remain functional following the conductance change, at least when the exocytotic event was prevented. A transient conductance increase could only be induced by NH4+ (pH 9.0) in unactivated eggs that had not undergone cortical granule exocytosis. Tremendous variation was noticed between successive activation currents elicited in the same egg. Under voltage-clamp at 0 mV holding potential, the current often changed from inward to outward. Although cortical granule exocytosis may only play a minor role in the transient conductance change triggered at fertilization, it may well be involved in subsequent modifications of membrane conductance.