Tributyltin impedes early sperm-egg interactions at the egg coat level in the ascidian Phallusia mammillata but does not prevent sperm-egg fusion in naked eggs

摘要

The impact of tributyltin (TBT), was investigated on the occurrence of early gamete contacts and the functioning of the ionic channels of unfertilized or fertilized egg of Phallusia mammillata. TBT significantly inhibited the Na~+ current of unfertilized oocytes up to 60%, in both a dose (1-10 #mu#M) and time (30 min to 8 h) dependent manner, while it moderately affected the Ca~(2+) and K~+ currents. Also TBT induced a statistically significant dose-dependent decrease in the percentage of eggs that responded electrically to fertilization (from 100 to 33%, respectively, for 0.1-3 #mu#M TBT). As the sperm concentration increased, the dose-response curve of the percentage of eggs that responded electrically to fertilization shifted towards higher TBT concentrations which did not affect the capability for sperm to fertilize eggs. The effects of TBT were weakly reversible, depending on both TBT concentrations and the duration of incubation in TBT solutions. TBT (5 #mu#M) prevented early sperm egg contacts (5 X 10~7 sp ml~(-1)) at the vestment level of intact eggs, but did not lower the percentage of electrical responses in naked eggs. Nevertheless, it had a deleterious effect on the transduction mechanism of the sperm signal into the egg or, more directly, on the sperm-activated channels of the egg membrane. This was evidenced by naked eggs displaying strongly truncated electrical responses, although they were penetrated by sperm-nuclei. Finally, N-acetylglucosamine. an intrinsic vitelline coat (VC) monosaccharide involved in the sperm binding to VC, added (100 mM) to 5 #mu#M TBT in natural seawater, in order to compete with the pollutant, did not reduce the deleterious effect of TBT on the early gamete contacts at VC level.