GROWTH-HORMONE RECEPTOR-BINDING PROTEIN IN RAT ANTERIOR-PITUITARY

GH is synthesized by the somatotrophs in the pituitary, where it may h ave paracrine actions. In order to identify the GH target cells in rat pituitary, the cellular distributions of rat GH receptor binding prot ein messenger ribonucleic acid (rGH-RBP mRNA) and protein were investi gated at the electronmicroscopic level, using in situ hybridization an d immunocytology, respectively, on ultrathin frozen sections of rat pi tuitary. Ultrastructural distribution of(125)I-bGH, 30 min after intra cardiac administration was also performed in order to determine the pi tuitary cell types that bind the labeled hormone. In situ hybridizatio n was performed using digoxigenin-labeled oligonucleotide probe reveal ed by indirect immunogold reaction. rGH-RBP mRNA was readily identifie d in the cytoplasmic matrix, associated with the endoplasmic reticulum and the nucleus of the somatotrophs, the lactotrophs and the gonadotr ophs. No significant signal was detected in the corticotrophs or thyro trophs. The number of gold particles in each pituitary cell type was e stimated by direct counting, and was compared to the results of hybrid ization performed on rat liver sections as a control. The results show ed that the level of rGH-RBP mRNA was higher in hepatocytes than in th e pituitary cells, and was higher in the somatotrophs and lactotrophs than in the gonadotrophs. Immunocytological dectection of rGH-RBP was performed using two monoclonal antibodies (mAbs 43 and 263) directed a gainst independent epitopes of the extracellular domain of the rGH-R. Indirect immunocytological detection showed regionalization of rGH-RBP ; it was present in the cytoplasmic matrix and the nucleus of the hepa tocytes and in discrete pituitary cells: somatotrophs, lactotrophs and gonadotrophs, but not in thyrotrophs or corticotrophs. Gold particle number was also higher in somatotrophs and lactotrophs than in gonadot rophs and higher in the nucleus compared to the cytoplasmic matrix. Ra dioiodinated GH was uptaken 30 min after injection by the same three p ituitary cell types, showing evidence for the functional role of the G H receptor. In conclusion, we find that the cellular localization of r GH-RBP mRNA and protein is similar in discrete cell subpopulations of the pituitary, suggesting a direct effect of GH on somatotrophs, lacto trophs and gonadotrophs, through paracrine, autocrine or intracrine me chanisms.