A synthetic form of human pancreatic growth hormone releasing factor (GRF-44-NH2) was shown to be a potent stimulator of growth hormone (GH) secretion and cellular cyclic AMP levels in cultured sheep pituitary cells. A small dose-dependent stimulation of prolactin secretion was also observed. Somatostatin (0.5 μM) completely blocked the maximal GRF (1 nM)-stimulated secretion without a significant effect on cyclic AMP levels. Dopamine (0.1 μM) inhibited the GRF-elevated GH secretion by 50% and lowered cyclic AMP levels by 30%. Dopamine (0.1 μM) inhibition of basal prolactin secretion was not affected by GRF (1 nM). The data support the hypothesis that cyclic AMP is involved in the action of GRF but suggest that somatostatin can inhibit GRF-induced secretion of GH independently of cyclic AMP.
