Ionic Events at Fertilization

The difference in ionic balance between the cytoplasm and the extracellular environment in all cells is maintained by ion channels and transporters located in the plasma membrane. In this chapter we review the voltage-gated channels and transporters common to both gametes and somatic cells and describe gamete specific ligand-gated channels in spermatozoa and oocytes. In lower deuterostome oocytes the fertilization potential is biphasic. The first event may be the result of gamete fusion, while the second larger depolarization is the result of the activation of several hundred non-specific channels in the oocyte plasma membrane. The fertilization channel is one of the largest channels known in biological membranes with a single channel conductance of upto 400pS and a reversal potential of +20mV. A soluble sperm factor appears to gate this channel via the ADPr/NO pathway. In higher deuterostomes the fertilization channel is a calcium-gated potassium channel. The type, number and topographical distribution of channels and transporters change continually during gameteogenesis, through fertilization to early embryonic cleavage stages indicating both the importance of ionic homeostasis and the role of second messengers in early development.