The potential impact of novel investigational compounds on human fertility.

There is considerable concern that the incidence of infertility in humans may be increasing, in some instances due to the action of bioactive xenobiotic compounds found in our environment; for example, high concentrations of xenobiotics with estrogenic activity can interfere with normal testicular function and fertility. However, recent studies have shown that very low concentrations of several estrogenic xenobiotics can have subtle, unexpected effects on sperm function. When tested in vitro, these compounds stimulate spermatozoa to become fertile very quickly, but continued stimulation causes them to burn out and lose fertilising ability; similar responses occurring in vivo could reduce fertility. In contrast, several other compounds, structurally related to amfetamine, have been shown to act on spermatozoa in vitro in a positive manner, stimulating cells to 'switch on' quickly and then preventing burnout so that they maintain fertilising potential; similar responses occurring in vivo could enhance fertility. These results could have implications for either reducing or enhancing natural fertility.