Diurnal variations of serum and testicular testosterone and dihydrotestosterone (DHT) in Djungarian hamsters (Phodopus sungorus): testes are the main source for circulating DHT.

Testes and sera of adult Djungarian hamsters kept in a light:dark cycle of 16L:8D were collected every 1.5-3 hr for 24 hr. The concentrations of testosterone (T) and its metabolite 5 alpha-dihydrotestosterone (DHT) were estimated by HPLC separation of the steroids followed by radioimmunoassay. Both testicular and serum concentrations of T and DHT exhibited strong diurnal variations (P < 0.001) with highest levels at the end of the light phase, while both hormones declined drastically during the dark period. The ratios of DHT/T, as an indicator of the activity of the enzyme 5 alpha-reductase, were on average 0.39 in the testes and 0.29 in serum. Comparisons of individual data revealed highly significant correlations between testicular and serum concentrations of the hormones (T:r = 0.87; P < 0.0001; DHT:r = 0.69, P < 0.0001). Likewise, the individual values of testicular and serum DHT/T ratios correlated highly significantly (r = 0.60; P < 0.001). The results indicate that in Djungarian hamsters the main source of DHT is the testes, while the peripheral conversion of T to DHT seems to be negligible.