The mechanism of antifertility effect of testosterone undecanoate on male rat was investigated. Eight 12-week old rats were injected with 20 mg/kg of testosterone undecanoate at bi--week intervals for 3 months. As compared to that in the 10 control rafs, the sperm density in testis rete fluid of the treatment rats declined by 7%, the motility of sperm from epididymis cauda reduced to 6%. While the testosterone level in serum increased to 255 %, the testosterone level in testis rete fluid decreased to 55%. All of these differences were significant. The androgen receptor gene expression in the testis and epididymis was suppressed in the treatment group. The decrease in output of the sperm and sperm motility of epididymis cauda may be due to the reduced testosterone production by Leydig cells and suppression of androgen receptor gene expression in testis and epididymis.
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