EFFECTS OF ENDURANCE EXERCISE ON FREE TESTOSTERONE CONCENTRATION AND THE BINDING-AFFINITY OF SEX-HORMONE BINDING GLOBULIN (SHBG)

Our purpose was to examine the changes in free testesterone concentrat ion following moderately prolonged endurance exercise to determine whe ther such changes were due to alterations in the binding affinity of s ex hormone binding globulin (SHBG). Ten trained men completed control (45 min rest) and exercise (45 min @ 70% Vover dotO(2)max) experimenta l sessions. Blood samples were collected before (pre-) and immediately after (post-) each of the sessions and analyzed for total testesteron e, free testosterone, LH, FSH, cortisol, estradiol, SHBG, epinephrine and norepinephrine. The association constant (K-a) of SHBG for testost erone was assessed to evaluate binding affinity. No significant differ ence was seen between the pre-control and the pre-exercise session val ues for any of the measures. However, within the exercise session sign ificant (p < 0.01) increases in post samples were evident for total te stosterone (+ 32.0%), free-testosterone (+ 39.6%), cortisol (+ 38.0%), norepinephrine (+ 365.2%) and epinephrine (+ 225.8%). All other hormo nal changes and the responses for K-a were non-significant. The result s of the correlation analysis indicated that only norepinephrine was s ignificantly (p < 0.05) related to the changes observed in free testos terone (r = + 0.92). We conclude that the free testosterone increase w ith moderately, prolonged endurance exercise was not associated with t he change in the binding affinity of SHBG. Furthermore, the data sugge st that the exercise induced increase in testosterone involves increas ed production which may be mediated by sympathetic stimulation of the testicles.