Quantitative analysis of hypothalamic-hypophyseal-testicular system: why testosterone can act under negative feedback control.

Quantitative analysis of the positive and negative feedback actions of testosterone (T) was carried out using intact rats, and orchidectomized rats implanted with T-filled Silastic capsules. Target organ weights and serum levels of LH and T were examined 7 days later, and response ratios of positive and negative actions were plotted against logarithm of serum T. From x-intercepts and slopes of regression lines, thresholds and responsiveness of reactions were calculated, respectively. Maintenance T levels necessary to maintain onset weights of the organ were also calculated from the regression lines. We compared 5 parameters at various ages, 1) thresholds for lowering serum LH, 2) mean T concentration of intact animals, 3) upper limit of serum T (mean + 2SD), 4) maintenance T levels, and 5) lower 95% limit of the thresholds for target organ response in orchidectomized animals. Though T can act between thresholds for target organ response and upper limit of serum T, the action range of T to induce organ growth over the onset organ weight (growth-inducing range) should be the area between the maintenance T level and the upper limit for serum T. At 3 weeks of age, the threshold for serum LH was very low, which makes the upper limit of serum T lower than maintenance T, allowing no growth-inducing range. From 5 weeks of age, the threshold for serum LH increased, and the serum T level with its upper limit also increased over the maintenance T level, allowing the presence of a growth-inducing range of T to make the growth of target organs possible.