The alpha-subunit of glycoprotein hormones exists in the prolactin secretory granules of the bullfrog (Rana catesbeiana) pituitary gland.

Our recent finding that the number of immunoreactive alpha-subunit cells was invariably greater than the total number of immunoreactive gonadotropin (GTH) and thyrotropin (TSH) cells in the bullfrog (Rana catesbeiana) pituitary gland raises the possibility that the alpha-subunit also exists in pituitary cells other than GTH and TSH cells. The present study demonstrates that there are a considerable number of immunoreactive prolactin (PRL) cells that are also stained with antibody against the alpha-subunit when adjacent sections are immunocytochemically examined. Neither immunoreactive growth hormone nor adrenocorticotropin cells are stained with the antibody against the alpha-subunit. The specificity of the antibody against the alpha-subunit and of that against PRL was demonstrated by preabsorption test, non-competitive binding test, and immunoblot analysis. Double-immunolabeling with gold particles of different sizes for the alpha-subunit and PRL revealed that most of the immunolabeled PRL-secretory granules are also labeled with the alpha-subunit antibody. The gold particles indicating the presence of the alpha-subunit were mostly found in the peripheral zone of the secretory granules.