[Paracrine signals in the adenohypophysis]

We have demonstrated that the secretory activity of anterior pituitary cells is not only dependent on the hypothalamic releasing and inhibiting factors and peripheral hormones but that these cells are also capable to control their own function through locally produced paracrine factors. The latter substances are released by certain cell types in the gland and influence the secretory activity and proliferation of other cells in the neighbourhood. We found evidence that gonadotrophs release PRL releasing factors spontaneously as well as after stimulation with LHRH. A cholinergic system was discovered in ACTH cells. Acetylcholine released from these cells exerts a tonic inhibitory action on GH and PRL release. The folliculo-stellate cells, of which the function still remains undefined, induce an overall inhibition of excitability of most if not all hormone secreting cells. We also found new exogenous signals for secretion: a stimulatory action of angiotensin on PRL and GH secretion and of adrenaline and VIP on GH release, dual effects (stimulation and inhibition) of acetylcholine on PRL and GH release and of angiotensin on GH release. Several of the latter signals depend on intercellular communication. An important observation was that paracrine signals are under regulatory control of thyroid and glucocorticoid hormones. All these findings were obtained through the development of new technologies. A three-dimensional aggregate pituitary cell culture system with organotypic characteristics was developed. In order to study the influence of one cell type upon another, the different pituitary cell types had to be separated from each other.