Fertilisation and thimerosal stimulate similar calcium spiking patterns in mouse oocytes but by separate mechanisms.

Exposure of freshly ovulated mouse oocytes to a fertilising spermatozoon, thimerosal, Sr2+ or acetylcholine induced similar Ca2+ spiking responses. We propose that each of the four agents reduces the threshold for Ca2+ release from internal stores, but by different mechanisms. All agents except thimerosal stimulated oocyte activation, but thimerosal caused dissassembly of the meiotic spindle and thus prevented progress into interphase. Dithiothreitol (DTT) completely blocked and reversed the spiking responses induced by thimerosal, but facilitated and accelerated those induced by spermatozoa, Sr2+ and acetylcholine. The stimulatory effect of DTT was not simply a consequence of progress into interphase, but was attributable, at least in part, to an enhancement of divalent cation entry, as measured by Mn2+ quench analysis of fura-2 in both fertilised and unfertilised oocytes. Possible mechanisms by which DTT might achieve its effects are discussed.