Growth hormone-releasing factor does not activate protein kinase C in somatotrophs

The purpose of this study was to investigate the involvement of protein kinase C in growth hormone-releasing factor (GRF) action by directly measuring the effect of GRF on protein kinase C activity in purified male rat somatotrophs. Somatotrophs were incubated with GRF (10(-7) M) for 0.33, 1, 3, 10, 30 and 90 min. Protein kinase C present in soluble and particulate fractions was partially purified using DEAE-cellulose chromatography, and protein kinase C activity was assayed. In control experiments, to insure protein kinase C activity could be activated, two known protein kinase C activators, phorbol 12-myristate 13-acetate (PMA) and dioctanoyl-rac-glycerol (diC8) were added for 3 min. Protein kinase C activity is present in somatotrophs. Under basal conditions the majority of the enzyme activity is located in the cytosol (approximately 90%). The protein kinase C activators caused a significant translocation of protein kinase C activity from soluble to particulate fractions at 3 min. GRF did not cause a translocation of protein kinase C activity even though GH release was significantly increased by 3 min. GRF did not significantly alter the specific activity of protein kinase C in the soluble or particulate fractions, except for a small (approximately 10%) increase in soluble activity at 90 min. We conclude that protein kinase C is present in the somatotrophs of the anterior pituitary. Protein kinase C, however, does not mediate the action of GRF and its role in signal transduction in somatotrophs awaits elucidation.