Certain cytochrome P450 enzymes such as CYP2C9 and CYP2C19 can also oxidize testosterone at the C17 position to form androstenedione. In addition to 6β- and 16β-hydroxytestosterone, 1β-, 2α/β-, 11β-, and 15β-hydroxytestosterone are also formed as minor metabolites. The 6β-hydroxylation of testosterone is catalyzed mainly by CYP3A4 and to a lesser extent CYP3A5 and is responsible for 75 to 80% of cytochrome P450-mediated testosterone metabolism.
When a situation is viewed as threatening/stressful cortisol reactivity is dominant, whereas a testosterone response will be seen when a situation is viewed as challenging (Wobber et al., 2010). Given that prior studies have viewed skydiving as a stressor, it may also be fruitful to examine testosterone reactivity in concert with stress biomarkers such as cortisol and autonomic activation. The produce hormones that regulate the immune system, blood pressure, metabolism, and the stress response.
Certainly, there is substantial evidence to suggest that there is a complex interplay between the brain and both neuroendocrine systems (including testosterone) and the ANS (Hastings & Miller, 2014; Kreibig, 2010; Thayer, Ahs, Fredrikson, Sollers, & Wager, 2012). It is possible that both T and sympathetic response work together in exciting contexts to enhance the positive valence of a context over time. Sympathetic response serves to increase somatic arousal, which is typically described as the "fight or flight" response (Kemeny, 2003), although in positively valenced situations might also be described as the "excite and delight" response (Allison et al., 2012). Moreover, the HR response persisted even when controlling for parasympathetic activation (as indicated by RMSSD) suggesting that sympathetic activation was driving the HR response. These findings are consistent with the idea that individual differences when examining neuroendocrine responses as the same context can exert a very different physiological impact from one individual to another (e.g., Josephs, Mehta, & Carré, 2011). Thus, a subset of participants in that study may have responded to the task as a negative stressor rather than a thrilling or exciting experience. Moreover, Chatterton and colleagues did not assess the degree to which participants enjoyed the experience of skydiving.
Studies conducted in rats have indicated that their degree of sexual arousal is sensitive to reductions in testosterone. Sexual arousal and masturbation in women produce small increases in testosterone concentrations. Testosterone levels follow a circadian rhythm that peaks early each day, regardless of sexual activity. Regular monitoring during treatment typically includes hematocrit levels every 3-6 months to prevent polycythemia, along with PSA monitoring in men over 40. Decline of testosterone production with age has led to interest in androgen replacement therapy.
This body of evidence indicates that EPHB6 and male sex hormones are acting in concert to regulate catecholamine secretion and blood pressure. This explains the reduced resting-state blood catecholamine levels, and hence the blood pressure, in male but not female EPHB6 knock mice. Regular exercise can also boost testosterone levels, and since testosterone is a slower-acting hormone than adrenaline, this effect is a bit longer-lasting than that of adrenaline release. While short-term exposures to adrenaline are unlikely to cause effects, long-term stress and routine adrenaline exposure may downregulate testosterone production. Finally, there has been some suggestion in the literature that enzyme-linked immunoassay techniques, like the ones used in the current study, may under-estimate testosterone levels in women (see, for example, Welker et al., 2016). However, during within team practice competitions (Kivlighan et al., 2005), video game competitions (Carré et al., 2013; Mazur, Susman, & Edelbrock, 1997), and response to election results (Stanton, Beehner, Saini, Kuhn, & Labar, 2009), gender differences in testosterone reactivity have been observed. Third, testosterone reactivity may play a similar role in both males and females, but context, including experimental design, may largely determine whether testosterone reactivity will or will not manifest gender differences.
Testosterone may prove to be an effective treatment in female sexual arousal disorders, and is available as a dermal patch. There is a time lag effect when testosterone is administered, on genital arousal in women. Women's level of testosterone is higher when measured pre-intercourse vs. pre-cuddling, as well as post-intercourse vs. post-cuddling. Androgens may modulate the physiology of vaginal tissue and contribute to female genital sexual arousal. Men who watch a sexually explicit movie have an average increase of 35% in testosterone, peaking at 60–90 minutes after the end of the film, but no increase is seen in men who watch sexually neutral films. This reaction engages penile reflexes (such as erection and ejaculation) that aid in sperm competition when more than one male is present in mating encounters, allowing for more production of successful sperm and a higher chance of reproduction.