Chemical evidence for associated TRF with subcellular fractions after incubation of intact rat prolactin cells (GH3) with 3 H-labelled TRF

Synthesis and internalization of atrial natriuretic factor in anterior pituitary cells

Atrial natriuretic factors (ANF) are a family of peptides originally identified in atrial cardiocytes and having natriuretic, diuretic and vasorelaxatory properties. ANF was recently reported to be synthesized in anterior pituitary cells. In the current study, the cell-specific sites of binding and internalization of ANF in the adenohypophysis, a site of ANF action, were investigated. By in vitro autoradiographic techniques, [125I]ANF was found to bind specifically to both anterior and posterior lobes of the pituitary, but not to the intermediate lobe. 5 min following intravenous injection of [125I]ANF, radiolabel was detected by ultrastructural autoradiography on gonadotrophs, corticotrophs and lactotrophs, but not thyrotrophs or somatotrophs. Label was found at both the plasma membrane level and cytoplasmic matrix, but not in the nuclei, of all three cell types. The cellular and subcellular localization of endogenous ANF-like immunoreactivity was also investigated. Specific immunoreactivity was also detected in gonadotrophs, corticotrophs and some lactotrophs. At the subcellular level in all three cell types, immunoreactivity was localized in the cytoplasmic matrix, on secretory granule membranes, and in mitochondria. Less dense staining was observed in nuclear euchromatin; endoplasmic reticulum and Golgi apparatus were not labelled in any of the cells. The data suggest that notwithstanding their reported ability to synthesize ANF, exogenous ANF can also bind and be internalized by adenohypophyseal gonadotrophs; the binding and presence of endogenous ANF-like immunoreactivity in corticotrophs and lactotrophs suggest that these cells may be sites of action for the atrial peptide.

Subcellular localization of thyrotropin-releasing hormone (TRH)- and neuropeptide Y (NPY)-like immunoreactivity in the neurointermediate lobe of the frog pituitary

The presence of thyrotropin-releasing hormone (TRH) and neuropeptide Y (NPY) has been demonstrated in the neural lobe and in the intermediate lobe of the frog pituitary by immunocytochemistry on ultrathin sections of neurointermediate lobes obtained by cryoultramicrotomy. In the neural lobe, separate populations of TRH- or NPY-immunoreactive nerve fibers were observed. Both neuropeptides were contained in dense-core secretory vesicles about 200 nm in diameter. In intermediate lobe cells, TRH- and NPY-like immunoreactivities were observed in the cytoplasmic matrix and more sparsely in secretory granules. Occasionally, immunoreactive TRH could be visualized at the plasma membrane level. In the nucleus, both peptides were detected in the euchromatin, in the vicinity of the heterochromatin and in the nucleolus. Conversely, gonadotropin-releasing hormone-like immunoreactivity could not be detected. These results provide immunocytological evidence for the presence of endogenous TRH and NPY in frog melanotrophs indicating that these peptides may participate in the regulation of intermediate lobe secretion.

Purification and characterization of plasma membrane fractions from cultured pituitary glands

Highly purified plasma membrane fractions have been prepared from GH3 pituitary cells grown in suspension cultures. These membrane fractions have been obtained by differential and sucrose gradient centrifugation and were characterized in terms of their lipid content, marker enzyme analysis and the binding of 3H-labelled thyrotropin-releasing hormone (TRH) to its receptor. Alkaline phosphatase and 5'-nucleotidase activities were enriched 12-to 15-fold in the plasma membrane fraction with somewhat greater enrichment (28-fold) of the specific binding component for [3H]TRH, with a specific activity of 2286 fmol [3H]TRH bound per mg protein. A single class of binding sites for TRH was observed with an apparent dissociation constant of 18 nM, a value similar to that observed for intact cells. No detectable TRH binding to the nuclear fraction was observed that could not be ascribed to residual plasma membrane contamination. By electron microscopy, these fragments appeared to be sealed vesicles with an average diameter of approximately 1800 A. The binding of 125I-labelled wheat germ agglutinin was used as a marker for plasma membrane purification. In addition to specific binding to this membrane fraction, specific binding was also observed in the nuclear fraction. Studies with fluorescein-labelled wheat germ agglutinin revealed that, in fixed cells, fluorescence was restricted to the plasma membrane. However, if the cells were treated with Triton before labelling, most of the fluorescence was then associated with the cell nucleus. Hence, the use of wheat germ agglutinin binding as a specific membrane marker must be reevaluated.

Secretion and crinophagy in prolactin cells

The mammotroph or prolactin-secreting cell, due to its distinctive fine structure, large granules, and sustained survival in vitro, has been a favorite object for studying mechanisms of pituitary secretion and its control. In this review the structural basis of prolactin secretion is summarized and the proposed sequence of intracellular events involved in prolactin secretion together with their site of loaction is outlined. The available evidence validating the various steps in prolactin secretion and their regulation is summarized. Finally, some of the unresolved problems remaining for the future are pinpointed.