12-O-tetradecanoyl phorbol-13-acetate stimulates rat growth hormone (GH) release through different pathways from that of human pancreatic GH-releasing factor.

The mechanism(s) of action of 12-O-tetradecanoyl phorbol-13-acetate (TPA) on rat (r) GH release was studied in primary rat pituitary cell cultures. TPA stimulated rGH release (3.2- to 4.1-fold above control value) and rTSH and rLH release (1.4- and 1.7-fold above control values, respectively), but not rPRL release. The ED50 of TPA on rGH secretion was 1.3 X 10(-9) M compared to 4.5 X 10(-11) M for human pancreatic GH-releasing factor [hpGRF-(1-44)]. If maximally effective doses of TPA or hpGRF-(1-44) were added to the cells, the magnitudes of the increase in rGH release were quite similar for both agents when the incubation period was less than 12 h. When (Bu)2cAMP was added simultaneously with various doses of TPA, (Bu)2cAMP increased rGH release beyond the maximal effect of TPA. There was an additive effect when hpGRF-(1-44) and TPA were used to stimulate rGH release. These results indicate that TPA enhances rGH release through a different pathway than hpGRF-(1-44). TPA failed to increase the formation of intra- and extracellular cAMP, whereas hpGRF-(1-44) increased both, suggesting that TPA stimulates rGH release through an cAMP-independent pathway(s). Protein kinase C has been postulated to be a receptor for TPA in human platelets. When phospholipase C, which activates protein kinase C via the formation of diacylglycerol, was added to the cells, rGH release was stimulated in a dose-dependent manner. This effect was not blocked by indomethacin. These results may suggest that activation of protein kinase C leads to rGH release. The observations are consistent with the hypothesis that TPA activates protein kinase C and causes the release of rGH in normal pituitary cells in culture. These findings indicate that the mechanism(s) of action of TPA on rGH release is different from that of hpGRF-(1-44).