Effects of some organophosphates on fertilization of mouse gametes in vitro.

Abstract

Infertility is a common reproductive disorder observed in farm animals and humans. Some environmental chemicals are known to cause reproductive failure. This study was designed to examine the potential of some organophosphate insecticides to inhibit fertilization of B6D2 mouse gametes in vitro and to explore the possible mechanism of any inhibition. The presence of malathion or parathion in the capacitation and the insemination media had little effect on the number of eggs fertilized. However, a metabolite of parathion, paraoxon, reduced fertilization to 40% of control values. Further experiments showed that paraoxon inhibits fertilization by reducing the fertilizing ability of sperm without causing sperm quiescence or death. The longer the sperm were exposed to paraoxon during capacitation, the greater the inhibitory effect. Once the sperm were capacitated, paraoxon did not inhibit fertilization. The inhibition of sperm fertility, which was reversible after one hour of exposure to paraoxon, became irreversible after three hours of exposure. Addition of acetylcholine (ACh) to the media dramatically decreased the inhibitory effect of paraoxon on fertilization. Using the CTC fluorescence assay it was demonstrated that paraoxon inhibits fertilization by preventing sperm capacitation. In the presence of both paraoxon and ACh, sperm capacitated as in controls. The inhibitory effect of paraoxon on sperm capacitation appears to be mediated by inhibition of acetylcholinesterase (AChE) or an AChE-like enzyme on the sperm membrane. It is hypothesized that this enzyme, under physiological conditions, is activated by ACh and functions to initiate membrane permeability changes, which results in cation migration in the direction necessary for capacitation to take place. In the presence of both paraoxon and ACh, sufficient enzyme can be activated for initiating capacitation before the enzyme is irreversibly inhibited. The inhibition of sperm capacitation by metabolites of organophosphate insecticides may cause infertility in livestock. Consequently, these studies suggest that acetylcholine might be used in artificial insemination to improve reproductive efficiency.