Let me be a little more specific. Your muscles can take circulating hormonally inactive steroids (i.e. DHEA) and convert them to active hormones (testosterone, dihydrotestosterone, and estradiol) in the muscles themselves. That’s what a bunch of researchers in Japan have discovered and they have published some studies revealing their findings. What makes it more intriguing is that they have discovered that this intramuscular steroid conversion is stimulated by exercise.
The enzymes 3beta-hydroxysteroid dehydrogenase and 17beta-hydroxysteroid dehydrogenase are both expressed in muscle tissue and together they can convert DHEA to testosterone. The levels of these enzymes have been shown to increase acutely with exercise. Also, 5alpha-reductase (5AR) is the enzyme that converts testosterone to dihydrotestosterone (DHT). DHT is a much stronger male hormone (androgen) than testosterone. 5-AR enzyme is normally not present in substantial amounts in muscle tissue, however the researches have shown that 5-AR activity increases markedly in trained muscle – both acutely and chronically – and as a result DHT levels in muscle are increased.
What does all this mean? I am not entirely sure. It appears that muscle has the intrinsic ability to boost androgen concentration by scooping up inactive precursor steroid hormones and converting them “in house” to potent androgens. The fact that the muscle’s ability to do so increases in response to exercise makes one wonder if this is not a mechanism to boost the regenerative anabolic processes that take place after training induced muscular trauma.
What is also intriguing is that muscle seems to do all this by utilizing an adrenal steroid (DHEA) which is present in high amounts in both women and men (including castrates). Thus, it is possible that despite someone having very low testosterone concentrations (due to whatever reasons) that person may still be able to build substantial muscle mass in response to exercise due to the muscle’s ability to utilize the circulating DHEA pool in the blood. Keep in mind that DHEA manufacture in the body is regulated by entirely different mechanisms than testosterone production, and most people have high levels of this hormone even if they are a woman or are hypogonadal.
One more thought. We know that DHEA has very weak anabolic activity in animals because it has been assayed several times using the standard castrate rat model. However these rats were not trained. Might the apparent anabolic activity of DHEA been higher if the rats in the anabolic assays underwent intense exercise?
These are three of the studies on this subject that you can look at for yourself if you want to learn more.