FERTILIZATION AND THIMEROSAL STIMULATE SIMILAR CALCIUM SPIKING PATTERNS IN MOUSE OOCYTES BUT BY SEPARATE MECHANISMS

Exposure of freshly ovulated mouse oocytes to a fertilising spermatozo on, thimerosal, Sr2+ or acetylcholine induced similar Ca2+ spiking res ponses. We propose that each of the four agents reduces the threshold for Ca2+ release from internal stores, but by different mechanisms. Al l agents except thimerosal stimulated oocyte activation, but thimerosa l caused dissassembly of the meiotic spindle and thus prevented progre ss into interphase. Dithiothreitol (DTT) completely blocked and revers ed the spiking responses induced by thimerosal, but facilitated and ac celerated those induced by spermatozoa, Sr2+ and acetylcholine. The st imulatory 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.