Effects of pituitary adenylate cyclase-activating polypeptide (PACAP), prostaglandin E2 (PGE2) and growth hormone releasing factor (GRF) on the release of growth hormone from cultured bovine anterior pituitary cells in vitro.

The effect of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone (GH) release was compared with that of prostaglandin E2 (PGE2) and growth hormone releasing factor (GRF) from cultured bovine anterior pituitary cells in vitro. Both PACAP and PGE2 stimulated GH release at concentrations as low as 10(-9) and 10(-8) M, respectively, (P < 0.01). However, GRF released GH at a concentration as low as 10(-13) M (P < 0.01). Percent increases of GH compared with controls were not significantly different among GRF, PACAP, and PGE2 at 10(-7) M; however, the increases of GH by the 10(-8) M GRF, PACAP and PGE2 were 196, 118, and 27%, respectively, (P < 0.01), and 124, 65, and 1% in the 10(-9) M media, respectively, (P < 0.01). When GRF and somatostatin (SS) were added together, the GH releasing effect of GRF was blunted (P < 0.01). Similar bluntness were observed in PACAP and PGE2, when SS was added. The stimulatory effects of GRF and PGE2 together were similar to that by either GRF or PGE2 alone. When GRF and PACAP were added together, the GH released by both secretagogues was greater than that by PACAP alone (P < 0.01); however, a synergistic effect was not clear when compared with GRF alone. These findings suggest that PACAP and PGE2 may modulate the release of GH in cattle.