Splanchnic Extraction and Conversion of Testosteroneand Dihydrotestosterone inMan*

Abstract

The splanchnic extraction and interconversion of testosterone and dihydrotestosterone (DHT) were studied in 7 healthy men(ages 29–46 years) undergoing cardiac catheterization. During a constantinfusion of [l,2−³H]testosterone and[4−¹⁴C]DHT, the arterial and hepatic vein blood samples were taken and radioactive and non-radioactive testosterone and DHT were determined. Metabolic clearance rate (MCR), splanchnic extraction (SE), splanchnic clearance (SC), extrasplanchnic clearance (ESC), transfer constant in blood (_ρBB^T-DHT) and transfer constant across the liver (_ρBB^T-DHT) were calculated. The MCR^T was 952 ± 172(mean ± SD) I/day and MCR^DHT was764 ± 67 1/day in agreement with data from non-catheterized subjects. SE^T was 68.8 ± 7.1% (mean ± SD) and SE^DHT was 37.6 ± 5.9%. SE^T was significantly greater than SE^DHT (P < 0.001). The calculated SC^T and ESC^T were 638 ± 112(mean ± SD) 1/day and 314 ± 190 1/day, respectively. SC^DHTand ESC^DHT were 343 ± 95 (mean ± SD) 1/day and 421 ± 105 1/day, suggesting that a major fraction of testosterone is metabolized in the splanchnic organs and a higher fraction of DHT is metabolized in extrasplanchnic organs. In the interconversion study, overall conversion of testosterone to DHT in blood (_ρBB^T-DHT) was 4.0 ± 0.6% (mean ± SD). NO evidence for a net appearance of DHT by either mass or specific activity analysis in hepatic vein blood was observed in any infusion leading to the conclusion that the overall contribution of testosterone to circulating DHT from the liver (_ρBB^T-DHT) was undetectable. This work indicates that conversion of testosterone to DHT occurs entirely inextrasplanchnic tissue i n man.