Insulin-like growth factor I in the anterior pituitary of the clawed frog Xenopus laevis: Immunocytochemical and autoradiographic indication for a paracrine action and corelease with prolactin

Insulin-like growth factor I (IGF-I) and its receptor are present in human and rat anterior pituitary. However, few data exist on the potential presen ce of IGF-I or its receptor in the non-mammalian pituitary and the cellular sites of IGF-I production have not been identified in any species. Thus, w e investigated the anterior pituitary of the clawed frog Xenopus laevis whi ch is widely used to study growth and differentiation. The study was perfor med with antisera against mammalian insulin-like growth factor I (IGF-I), p rolactin (PRL) and growth hormone (GH) using immunohistochemical and immuno cytochemical techniques. IGF-I binding was determined by in-vitro receptor autoradiography. The PRL-and GH-immunoreactive cells exhibited distinct dis tribution patterns. Neither at the light nor the electron microscopical lev el any colocalization of PRL-and GH-immunoreactivities was apparent. The PR L-immunoreactive cells exhibited round granules of medium electron density (mean diameter: 312 nm) and the GH-immunoreactive cells spherical granules of medium electron density (mean diameter: 165 nm). By the use of serial se mithin sections IGF-I-immunoreactivity was exclusively located in PRL-immun oreactive cells. At the ultrastructural level, IGF-I-immunoreactivity was c onfined to the secretory granules in coexistence with PRL-immunoreactivity using the double labelling immunogold technique. Specific IGF-I binding sit es were localized throughout the pituitary. The results provide evidence fo r a concomitant release of PRL and IGF-I and suggest autocrine/paracrine ac tions of IGF-I in the anterior pituitary.