A DIRECT MEASUREMENT OF-INCREASED DIVALENT-CATION INFLUX IN FERTILIZED MOUSE OOCYTES

On fertilisation of mouse oocytes, the fusing spermatozoon triggers a series of repetitive calcium (Ca2+) spikes. These Ca2+ spikes seem to be necessary for successful progression through the cell cycle and are regulated in a cell-cycle-dependent manner. The spikes appear to requ ire the linkage of continuous Ca2+ influx to the periodic release of C a2+ from intracellular stores by a process of Ca2+-induced Ca2+ releas e. The precise role of Ca2+ influx was explored using the manganese (M n2+)-quench technique to monitor unidirectional cation influx into sin gle mouse oocytes. There was a marked stimulation of cation influx ass ociated closely with the upsweep of the first and subsequent fertilisa tion Ca2+ spikes. A smaller but significant increase in the rate of ca tion influx persisted in the interspike period in fertilised oocytes. Spike-associated entry was not as apparent in oocytes stimulated to sp ike repetitively by thimerosal or acetylcholine application. Instead, there was a continuous increase in cation influx underlying Ca2+ spiki ng which commenced with the onset of the first spike. Using the specif ic microsomal inhibitor thapsigargin and the Ca2+ ionophore ionomycin, we found evidence for a capacitative entry mechanism in mouse oocytes . We propose that the persistent influx of Ca2+ observed in response t o all stimuli examined is controlled by a capacitative mechanism and s ets the frequency of spiking by determining the time taken to refill t he internal stores to a point where they are again sensitive enough to initiate the next spike.