REGULATORY MECHANISMS OF GROWTH-HORMONE SECRETION ARE SEXUALLY DIMORPHIC

Sexually dimorphic growth hormone (GH) secretory pattern is important in the determination of gender-specific patterns of growth and metabol ism in rats. Whether GH secretion in humans is also sexually dimorphic and the neuroendocrine mechanisms governing this potential difference are not fully established. We have compared pulsatile GH secretion pr ofiles in young men and women in the baseline state and during a conti nuous intravenous infusion of recombinant human insulin-like growth fa ctor I (rhIGF-I), During the baseline study, men had large nocturnal G H pulses and relatively small pulses during the rest of the day, In co ntrast, women had more continuous GH secretion and more frequent GH pu lses that were of more uniform size. The infusion of rhIGF-I (10 mu g/ kg/h) potently suppressed both spontaneous and growth hormone-releasin g hormone (GHRH)-induced GH secretion in men. In women, however, rhIGF -I had less effect on pulsatile GH secretion and did not suppress the GH response to GHRH. These data demonstrate the existence of sexual di morphism in the regulatory mechanisms involved in GH secretion in huma ns. The persistence of GH responses to GHRH in women suggests that neg ative feedback by IGF-I might be expressed, in part, through suppressi on of hypothalamic GHRH.