Correlation between Serum Testosterone Circadian and Intra-Testicular Testosterone Rhythm in a Rat Model

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It is well known that in most of the mammals, serum testosterone (T) level has a circadian rhythm. There are few studies discussed whether intra-testicular Testosterone (IT-T) levels have circadian variation. We aimed to compare the serum luteinizing hormone (LH), T and IT-T levels in rats for a 24-h period to examine if IT-T levels have circadian rhythm in Sprague-Dawley rats. Totally 48 male Sprague-Dawley rats were randomly divided into 12 groups, and 4 rats were assigned to each time point from 8 AM to 6 AM in the next morning. After decapitation, the plasma was collected immediately and bilateral testes were retrieved thereafter for further processing for IT-T. We found that serum LH, serum T and IT-T all expressed circadian variations. The serum T and IT-T rhythms were comparable with the highest T level at 2 AM and the lowest level at 2 PM. These two rhythms matched compatibly in every time point. LH rhythm was slightly different from both T and IT-T rhythm with the highest level at 0 AM and the lowest level at 2 PM. Serum T and IT-T concentrations showed a positive correlation, while serum LH has no significant correlation with T or IT-T. We also noticed that in the awaken phase (from 8 PM to 6 AM), the serum T/IT-T ratio remains relatively constant (0.44 ± 0.21 ~ 0.26 ± 0.12%). On the contrary, the serum T/IT-T ratio fluctuated significantly (0.28 ± 0.09 ~ 1.15 ± 0.87%) in the sleeping phase (6 AM to 8 PM). The correlation at awaken phase is much stronger than that at sleeping phase. In summary, we found that IT-T has the same circadian variation with serum T which controlled by the hypothalamic -pituitary-testicular system. IT-T concentration is highly correlated with serum T.