Androgen distribution in male rats.

Testosterone and 5 alpha-dihydrotestosterone (DHT) concentrations in tissue and body fluid were determined in adult male rats to investigate the mode of androgen distribution. The highest concentration of androgen (testosterone + DHT) and the smallest ratio of DHT/testosterone were observed in the intratesticular tissue fluid. The androgen concentration in testicular vein blood was much higher than in any other sample of body fluid. Among the body fluid samples, except to that in the intrascrotal fluid, DHT/testosterone ratio remained almost equal to that in the intratesticular tissue fluid. The intrascrotal fluid and blood samples from the testicular and epididymal arteries had significantly higher androgen concentrations than thoracic duct lymph or aortic blood. Among the tissues, the accessory sex organs and kidney fat pad had higher androgen concentrations and larger DHT/testosterone ratio than muscle, including the levator ani muscle. The DHT/testosterone ratios in muscle specimens were not different from those in body fluid. The caput epididymis and surrounding adipose tissue (the epididymal fat pad) had the highest androgen concentration, and the largest DHT/testosterone ratio was observed in the caput epididymis. These observations support the theory of the local transport from the testis to the caput epididymis and from the testicular vein to the testicular artery. Thus, the intrascrotal androgen targets, especially the caput epididymis, receive androgen effectively, and it was suggested that the epididymal fat pad plays an important role in maintaining a high androgen concentration in the intrascrotal androgen targets.