Our recent finding that the number of immunoreactive α-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 α-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 α-subunit when adjacent sections are immunocytochemically examined. Neither immunoreactive growth hormone nor adrenocorticotropin cells are stained with the antibody against the α-subunit. The specificity of the antibody against the α-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 α-subunit and PRL revealed that most of the immunolabeled PRL-secretory granules are also labeled with the α-subunit antibody. The gold particles indicating the presence of the α-subunit were mostly found in the peripheral zone of the secretory granules.
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