Maturation of the Regulation of Growth Hormone Secretion in Young Males with Hypogonadotropic Hypogonadism Pharmacologically Exposed to Progressive Increments in Serum Testosterone
To study the onset of the action of gonadal sex steroids on the GH axis in spontaneous puberty, which is prolonged and sparingly predictable, we present a clinical investigative paradigm in which six previously untreated boys with isolated hypogonadotropic hypogonadism were exposed to progressively higher testosterone levels designed to mimic the androgen environment recognized during the early stages of puberty. We administered three incremental doses of testosterone (25-, 50-, and 100-mg im injections), each over a period of 4 weeks. Studies of overnight pulsatile GH secretion and GH responses to GHRH alone or combined with l-arginine (a functional somatostatin antagonist) were performed before testosterone administration and after each dose of testosterone. Serum testosterone, but not estrogen, levels increased progressively in all subjects during therapy. Deconvolution analysis of GH release profiles disclosed that GH secretory burst mass was stimulated significantly even by 25 mg testosterone. This parameter was not altered further by higher doses of testosterone. Spontaneous GH secretory burst number and amplitude increased significantly only after the 50- and 100-mg testosterone treatments, after which the serum GH response to GHRH and arginine also rose significantly. In contrast, the GH response to GHRH alone was not significantly affected by any dose of testosterone. Serum testosterone levels correlated significantly with the primary parameters of nocturnal GH secretion. In summary, our experimental model suggests that in males even very small increases in circulating testosterone occurring during the earliest stages of puberty are able to amplify pulsatile GH secretion. Our concomitant secretagogue data further suggest that testosterone exerts its action at different sites in the hypothalamo-somatotropic axis, i.e. directly at the pituitary level, and also at hypothalamic loci, possibly increasing both GHRH and somatostatin release.