Testosterone production at by the testis is under control of the hypothalamic-pituitary unit (22). The operation and components of this axis are discussed in detail elsewhere in this volume (see Chapters 1-3) and are not repeated here. The hypothalamic-pituitary-testicular axis hormones addressed in this discussion are luteinizing hormone (LH), follicle-stimulating hormone (FSH), inhibin, and prolactin (PRL).
Testosterone is freely diffusible through tissues, and, thus, its secretion from the testis is directly affected by testicular blood flow. Testicular blood flow is a function of cardiac output, vascular vasoconstriction, and vasodilatation. Therefore, factors that influence vascular tone can also affect testosterone secretion (e.g., increased sympathetic nervous system activity) (23).
The metabolic clearance rate (MCR) of testosterone varies in normal men but is approx 100 L/d (16,17). MCR involves target-tissue uptake, as well as degradation by the liver, and is influenced by the portion of the hormone bound to carrier proteins (principally sex hormone-binding globulin [SHBG]). The degradation process involves the conversion of testosterone into functional metabolites, such as estradiol and dihy-drotestosterone (DHT) and degradation products, such as 17-ketosteroids and glu-curonides, which are excreted into the urine (16,17,24). Hepatic clearance is primarily a function of hepatic blood flow, so that changes in the hepatic blood flow influence testosterone's removal rate (16). This latter phenomenon is influenced by exercise, because hepatic blood flow is reduced during exercise when blood is shunted toward the active muscles (3,25-27).
During exercise, there is the movement of plasma from the vascular space. Because testosterone is bound to carrier proteins, an increase or decrease in the plasma volume leads to dilution or concentration of the circulating testosterone level. These changes are not indicative of changes in the normal hormonal turnover rate (25). For example, it is not uncommon during prolonged exercise (>1 h) for transient plasma volume decreases of 10 to 20% to occur (25). Whether these highly transient changes in the concentration of testosterone result from changes in plasma volume have a physiological effect is a point of debate and remains to be determined.
In addressing factors affecting blood levels of testosterone, it is important to consider the other physiological and nonphysiological factors that could account for variations in testosterone response in research studies. Examples include the blood-sampling method, diurnal variations in hormone concentrations, age, hormone detection methodology, emotional stress, diet, sleep patterns, and experimental research protocol or design. These factors are discussed in detail elsewhere (6,24,25). Collectively, all of the aforementioned considerations must be carefully examined when comparing the hormonal results of research studies if a valid interpretation of the endocrine system's responses to exercise is to be made. Figure 1 illustrates some of these physiological and nonphysiological factors that affect circulating testosterone levels in exercising men.
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