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FIGURE 17 Effects of testosterone in a boy with short stature and delayed puberty: (A) before testosterone, (B) during therapy with long acting testosterone. Note the increase in frequency and amplitude of growth hormone secretory episodes in the treated subjects. (From Link K, Blizzard RM, Evans WS, Kaiser DL, Parker MW, Rogol AD, J Clin Endocrinol Metab 1986; 62:159-164. With permission.)

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FIGURE 17 Effects of testosterone in a boy with short stature and delayed puberty: (A) before testosterone, (B) during therapy with long acting testosterone. Note the increase in frequency and amplitude of growth hormone secretory episodes in the treated subjects. (From Link K, Blizzard RM, Evans WS, Kaiser DL, Parker MW, Rogol AD, J Clin Endocrinol Metab 1986; 62:159-164. With permission.)

amplitude of secretory pulses of GH (Fig. 17), and GHRH concentrations are increased in peripheral blood of boys and girls during puberty. The concentration of IGF-I in blood also increases during the pubertal growth spurt or after androgens are given to prepubertal children. This increase is probably a consequence of increased secretion of GH.

We still do not understand the basis for the either stimulatory or inhibitory effects of estrogen on linear growth. At the same concentrations that inhibit growth, estrogens increase GH secretion, and, as we have seen in Fig. 7, plasma concentrations of GH are higher in women than in men. In addition, the GH secretory apparatus tends to be more sensitive to environmental influences in women than in men, and the circulating concentrations of GH tend to rise more readily in women in response to provocative stimuli. Inhibitory effects on growth appear to result from interference with the action of GH at the level of its target cells. Estrogens act directly on the epiphyseal plates, causing them to lose their capacity to replenish cartilage progenitor cells. Estrogens, which are not catabolic, also antagonize effects of GH on nitrogen retention and minimize the increase in IGF-I in blood of hypophy-sectomized or hypopituitary individuals treated with GH. Neither estrogens nor androgens affect the ability of IGF-I to act.

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