[Experimental study of the rebound phenomenon after testosterone administration--changes in enzyme activities in...

To examine the endocrinological and enzymatic aspects of the rebound phenomenon induced after cessation of testosterone (T) administration, adult male Wistar rats were treated with 0.3 mg of testosterone propionate daily for 39 days. Serum LH and T levels were both at the control level when the rebound phenomenon was recognized. Testicular T concentration, however, remained at a higher level. At the same time, the changes in 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), 17 alpha-hydroxylase, C17-C20 lyase and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activities were investigated using the whole homogenate of rat testis. 3 beta-HSD, 17 alpha-hydroxylase and C17-C20 lyase activities began to elevate at the time when serum LH began to elevate. 3 beta-HSD and C17-C20 lyase activities returned to the control level after about 30 and 50 days of cessation of T administration, respectively. 17 alpha-hydroxylase activity and testicular T level continued higher although serum LH concentration was at the control level. 17 beta-HSD activity was strongly suppressed and returned to the control level after about 30 days of cessation of T administration. It was assumed that the rebound phenomenon was induced by the higher level of testicular T which was produced by the elevated enzyme activity of 17 alpha-hydroxylase under the characteristic conditions in rat testis, perhaps where the sensitivity of 17 alpha-hydroxylase for LH was increased following cessation of T administration.