Physiological role of growth hormone (GH)-releasing factor and somatostatin in the dynamics of GH secretion in adult male rat.

To investigate the physiological role of GRF and somatostatin (SRIF) in GH secretion in adult male rats, we prepared in vitro models using the perifusion system of cultured rat anterior pituitary cells exposed to various combinations of human GRF-(1-44)NH2 (hGRF) and SRIF. We studied the following three models on GRF secretion: 1) pulsatile GRF secreted at 1-h intervals, 2) pulsatile GRF secreted at 3-h intervals, and 3) GRF continuously secreted. When 5-min pulses of 20 nM GRF were delivered at 1-h intervals, the responses to GRF gradually declined. The addition of continuous 20 nM SRIF with short pauses prevented this attenuated response and produced high peaks of GH at 3-h intervals. When 5-min pulses of 20 nM GRF were delivered at 3-h intervals, three high peaks of GH were observed regardless of the addition of SRIF. Pretreatment with GRF pulses enhanced the peaks of GH during SRIF pauses. When 20 nM GRF was continuously delivered, a rapid attenuated response to GRF was observed. Although the addition of continuous 20 nM SRIF with short pauses produced three small peaks of GH, these results were caused by the post-SRIF rebound release. These observations suggest that preexposure to prolonged SRIF can prevent an attenuated response to repeated GRF pulses, and that pretreatment with GRF pulses enhances post-SRIF rebound release.