EFFECT OF GONADECTOMY ON GROWTH AND GH RESPONSIVENESS IN DWARF RATS

Normal rats are sexually dimorphic in their growth and GH secretion. G onadectomy (Gnx) changes the GH secretory pattern and this could expla in differential growth rates in male and female rats. Gonadal steroids may also affect tissue growth directly, or by changing their responsi veness to GH. The effects of Gnx on growth, GH responsiveness, and hep atic GH receptors have now been studied in young (4-7 weeks old) GH-de ficient dwarf rats in which the effects of steroid-induced alterations in residual endogenous GH secretion will be much less pronounced. Gro ups of intact and Gnx dwarf rats (n=5-7) were infused with recombinant human GH (144 mu g/day) either continuously or in a pulsatile pattern (every 3 h) for 7 days, whilst control groups received saline infusio ns. Gains in weight, length and tibial bone growth were measured. Fema le dwarf rats grew significantly more slowly than male dwarfs. Gnx in male dwarfs inhibited growth significantly, whereas ovariectomy had a lesser stimulatory effect in females. Hepatic lactogenic and somatogen ic receptors were higher in females and ovariectomy lowered their valu es towards male levels. Pulsatile GH infusions were more effective tha n continuous infusions of the same daily GH dose, but when the differe nt underlying growth rates (measured in saline-infused Gnx animals) we re taken into account, the responsiveness to pulses of hGH was not dif ferent between males and females or between intact and Gnx animals. We have concluded that Gnx in dwarf rats does affect growth, but the eff ects are small in comparison with those seen in normal rats. Since pul satile GH infusions stimulated growth more effectively than continuous GH, irrespective of the gonadal steroid status, and the responsivenes s to exogenous GH was not changed by Gnx, the results imply that chang es in the GH secretory pattern induced by gonadal steroids may have a larger impact on the growth rate than direct effects at the tissue lev el, at least in the rat.