Changes in Serum Testosterone Levels Following Acute LH Treatment in Immature and Mature Rats1

Serum testosterone levels were found to be significantly higher in male rats aged 83 days compared to rats aged 37 days, whereas serum luteinizing hormone (LH) levels in the same samples were not different. Serum testosterone levels in 90-day-old male rats were elevated by 30 min after the ip administration of 10 μg LH/100 g body wt and remained elevated through 150 min. In a similar experiment with 31-day-old males, testosterone levels were elevated at 90 min but not at 30 or 180 min, and with 37-day-old rats testosterone levels were elevated by 60 min and remained elevated through 150 min. One hundred minutes after receiving 1 μg LH/100 g body wt testosterone levels in 36-day-old rats were slightly elevated, 10 μg raised their testosterone levels to the normal adult male range, but 100 μg caused no further increase. Seventy-six-day-old males did not respond to 1 μg LH/100 g body wt but testosterone levels were greatly elevated by 10 μg/l00 g body wt. The serum testosterone level attained by the adults in response to 10 μg LH/100 g body wt (31.7 ± 7.34 ng/ml) was significantly higher than the level attained by the immature rats (5.85 ± 0.89 ng/ml) in response to the same LH dose. The significantly higher serum testosterone levels found in the adult rats receiving 10 μg LH/100 g body wt indicate that changes in the responsiveness of the testis to LH may be an important part of puberty in the male rat.