Ultrastructural immunolocalization of IGF-1 and insulin receptors in rat pituitary culture: evidence of a functional interaction between gonadotroph and lactotroph cells.

We have investigated the expression of receptors for insulin and insulin-like growth factor 1 (IGF-1) in rat pituitary cells in vitro and examined the morphological and proliferative changes induced in adenohypophyseal cells by insulin and IGF-1. The proliferation of lactotrophs was determined by double-immunostaining for bromodeoxyuridine and prolactin. Incubation with insulin (10, 100 or 1000 ng/ml) or IGF-1 (5, 30 or 100 ng/ml) for 48 or 72 h significantly increased the number of lactotrophs undergoing mitosis. Co-incubation of insulin or IGF-1 with genistein (25 microM), an inhibitor of the tyrosine kinase receptor, reduced the proliferation of lactotrophs elicited by the hormone and the growth factor. The receptors for insulin and IGF-1 were localized in intact pituitary cells by ultrastructural immunocytochemistry with the colloidal gold-protein A technique. Gonadotrophs expressed both receptors, specific labelling being restricted to this cell type. Electron-microscopical observations of pituitary cell cultures incubated with insulin or IGF-1 revealed gonadotroph cells exhibiting the fine-structural features of enhanced protein synthetic activity. These findings suggest that both insulin and IGF-1 are able to induce the proliferation of lactotrophs through an indirect mechanism mediated by a factor synthesized by gonadotroph cells, in addition to stimulating the biosynthetic activity of the gonadotroph in a direct manner.