Sequential focal and global elevations of sperm intracellular Ca2+ are initiated by the zona pellucida during acrosomal exocytosis.
The acrosome reaction is a Cai(2+)-dependent secretory event that is initiated in mammalian sperm by ZP3, the stimulatory agonist in the egg's zona pellucida. Video image processing-enhanced fluorescence microscopy was used to monitor sperm Cai2+ responses to agonist during exocytosis. Two types of agonist-dependent Cai2+ transport pathways were defined. The first mediates small, transient Cai2+ elevations that are restricted to the sperm head. Dye emission and quenching studies indicate that this focal channel is not voltage-regulated, conducts several divalent metal cations (Co2+, Mn2+, and Ni2+) in addition to Ca2+, and has the properties of a poorly selective cation channel. The second transporter mediates sustained Cai2+ elevations throughout the cell and is pharmacologically identical to the L-type of voltage-sensitive Ca2+ channel. These channels are distinguished by inhibitor sensitivity and by their regulation during sperm maturation. A model is proposed in which ZP3 initiates a G protein-independent opening of the cation channel, producing depolarization of sperm membrane potential and consequent opening of L channels. The coordinate regulation of sperm Ca2+ channels by ZP3 during gamete adhesion promotes acrosomal exocytosis.
Florman
H M
HM
Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545.
D000177:Acrosome / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002413:Cations, Divalent; D002417:Cattle; D005089:Exocytosis; D016257:Fura-2; D008297:Male; D051379:Mice; D013094:Spermatozoa / Q000254:growth & development / Q000378:metabolism*; D015044:Zona Pellucida / Q000187:drug effects / Q000502:physiology*
D015220:Calcium Channels; D002413:Cations, Divalent; D002118:Calcium; D016257:Fura-2