Effects of porcine stalk median eminence and prostaglandin E2 on rat growth hormone secretion in vivo and their inhibition by somatostatin.

The studies described were performed to develop a technically simple, yet sensitive in vivo model for growth hormone (GH) releasing activity in porcine stalk median eminence (pSME) extracts, to compare the GH releasing effects of pSME with those of prostaglandin E2 (PGE2), and to study the effect of somatostatin (SRIF) on the above stimuli. The use of the one day estrogen-primed male rat in conjunction with intracarotid injection of test materials provided a model sensitive to the injection of one pSME. Neither increasing the duration of estrogen pre-treatment nor reserpine resulted in a greater response. The GH releasing effects of pSME were directly related to the preinjection GH level. Two successive injections of pSME at 30 minute intervals evoked similar responses. In contrast, PGE2 effects were not potentiated by estrogen pre-treatment and were independent of the preinjection GH level. The GH releasing effect of pSME was not related to its content of TRH or K+. Extracts of porcine cerebral cortex also contained GH releasing activity, although at a lower concentration than in pSME. Somatostatin inhibited the GH releasing effects of pSME, PGE2, and cerebral cortex extract. These results provide evidence for the direct inhibitory effect of SRIF on GH secretion in vivo and suggest that SRIF is capable of blocking a variety of different stimuli to GH release.