Combined effects of human growth hormone (GH)-releasing factor-44 (GRF) and somatostatin (SRIF) on post-SRIF rebound release of GH and prolactin: a model for GRF-SRIF modulation of secretion.

Somatostatin (SRIF) and GRFs play key roles in regulating GH secretion. We previously presented a model of SRIF-cAMP interaction; SRIF blocks rat (r) GH release without preventing its accumulation in a potentially releasable pool. This phenomenon may represent a mechanism whereby tonic SRIF inhibition and its subsequent reduction or withdrawal can modulate the magnitude if not the initiation of rGH pulses. Herein we test that model using human GRF-44 (hGRF-44). Tritium-prelabeled rat anterior pituitary fragments were perifused until stored [3H]rGH and [3H]rPRL release rates were stable. SRIF (10 or 25 nM), with and without hGRF-44 (3 or 10 nM), was added in short (1-h hGRF-44) and long (3-h hGRF-44) protocols; SRIF was then withdrawn while hGRF-44 was continued. Release of stored prelabeled [3H]rGH and [3H]rPRL was assessed by immunoprecipitation. Effects on PRL release were followed for comparison. SRIF-induced inhibition of release was only partially reversed by hGRF-44. At these concentrations and so long as SRIF was present, hGRF-44 could not stimulate the rate of hormone release to values above pre-SRIF basal rates. On the other hand, the amplitude of post-SRIF rebound release was increased by prolonging exposure to SRIF alone, by including hGRF-44 with SRIF, by increasing the amount of hGRF-44 included with SRIF, by prolonging exposure to hGRF-44 plus SRIF, and by using a smaller concentration of SRIF during exposure to hGRF-44. Interaction of hGRF-44-SRIF effects generated peak rates of hormone release after SRIF withdrawal which exceeded the maximum rates achieved using hGRF-44 alone in this system. Lactotroph responses were much smaller, but qualitatively resembled somatotroph responses. We conclude that the interplay of simultaneous hGRF-44 and SRIF effects can regulate the amplitude of rGH pulses. Although GRF can initiate physiological GH release, and GRF antisera can block GH pulses, we suggest that the surge of release that follows reduction of SRIF-induced inhibitory tone in vitro represents a potential mechanism that could contribute to the initiation of some pulses of release. Finally, we also present a theoretical model of secretagogue interactions at the cellular level to explain our results. The model is compatible with either a homogeneous cell population in which each secretory cell has multiple capabilities or a heterogeneous cell population composed of cell subgroups with complementary secretory abilities.