Chronic administration of a new potent agonist of growth hormone-releasing hormone induces compensatory linear growth in growth hormone-deficient rats: Mechanism of action

To assess the efficacy of a potent agonist analog of GH-releasing hormone (GH-RH), [Dat¹, Gln⁸, Orn^12,21, Abu¹⁵, Nle²⁷, Asp²⁸, Agm²⁹] hGH-RH (1-29) (JI-38), we investigated the effects of its chronic administration on growth responses in monosodium glutamate (MSG)-lesioned and normal young rats. Body weight (BW), body length (BL), tibia length (TIL), and tail length (TAL) were monitored. Basal serum GH concentrations, GH responses to bolus injections of GH-RH, pituitary GH and serum IGF-I concentrations were measured by RIA. Pituitary GH-RH receptor concentration and binding affinity was also evaluated after the treatment. Neonatal treatment with MSG resulted, as expected, in blunted growth and a decrease in serum and pituitary GH concentration and serum IGF-I levels. A reduction in GH-RH receptor concentration, associated with increased binding affinity of the GH-RH receptor was also found. Chronic administration of GH-RH agonist JI-38 in doses of 2 μg at 12-hour intervals for 2 weeks markedly increased the GH responsiveness to GH-RH and stimulated growth, with MSG-treated animals achieving the growth rate of normal controls. Acceleration of growth was associated with stimulated GH synthesis and IGF-I secretion, although basal serum GH levels did not change. Pituitary GH-RH receptor concentration and binding affinity were not significantly modified by the treatment. Treatment of normal young growing rats with agonist JI-38 did not further increase the normal growth acceleration in these rats, but stimulated the GH synthesis and augmented the GH secretory responsiveness. The treatment of MSG-lesioned rats with GH-RH agonist was generally more effective in female than in male animals, and in some cases masked the sex differences in growth rate. Our findings provide the first evidence that the blunted growth rate of the MSG-lesioned rats is associated with a decreased pituitary GH-RH receptor concentration. Our work demonstrates that administration of GH-RH agonist JI-38 is able to restore the normal growth rate of the GH-deficient rats by stimulating GH synthesis and IGF-I secretion.