Growth hormone and prolactin secretion in cultured somatomammotroph cells

A somatomammotropic cell line (P0) derived from adult rat pituitaries has been maintained in culture for 2 yr. Secretion of GH and PRL by this cell line has been studied in response to hypophysiotropic peptides known to affect the release of both hormones as well as agents that affect second messenger systems in an attempt to characterize the stimulus-secretion mechanisms used by these cells. GH and PRL release during short term (4 h) incubations of P0 cells and primary cultures of dispersed rat pituitary cells was initially measured in response to GRF, TRH, vasoactive intestinal peptide (VIP), and SRIF. In P0 cells, the minimal effective dose of each of the hypophysiotropic peptides was comparable with respect to GH and PRL secretion. The effects of TRH and VIP were similar to those in freshly dispersed cells with respect to PRL release, whereas those of GRF and SRIF were less potent with respect to GH release. The stimulation of GH and PRL release in P0 cells by adenylate cyclase-related agents ((Bu)2 cAMP and forskolin) was comparable to that for GH secretion in mature somatotrophs but much greater than that of PRL release in mature lactotrophs. Stimulation of GH and PRL release in P0 cells by protein kinase C-related agents (diacylglycerol and phorbol ester) was also similar to that observed for GH release from mature pituitary cells, whereas minimal or undetectable effects were observed on PRL release from mature cells. The results indicate that the P0 somatomammotropic cell line possesses receptors, second messenger systems, and secretory characteristics of both somatotrophs and lactotrophs, although where differences exist, there is more resemblance to somatotrophs. They also demonstrate that the responses to each of the agents studied are bihormonal and appear to be regulated by a common mechanism.