Internalization and Cellular Processing of Somatostatin in Primary Culture of Rat Anterior Pituitary Cells*

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Abstract
Somatostatin (SRIF) binding, internalization, and intracellular processing in primary culture of anterior pituitary cells have been studied using SRIF coupled to an electron-opaque marker, colloidal Au. After 2 min of incubation (37.degree. C), Au conjugated SRIF is localized on the cell surface, with 38% of the marker being found around microvilli, 10% at the junction of secretion vesicles with the plasma membrane, and 51% distributed over the remaining areas of the cell membrane. There was no internalization of SRIF at this time. After 20 min of incubation, distribution of the cell-surface bound hormone was similar to that at 2 min (40.6% at microvilli, 12% at the junction with the secretion vesicle, and 47.4% over the rest of the plasma membrane). However, 12% of the electron-opaque markers were found intracellularly in association with coated vesicles, intermediate-sized vesicles, lysosomes and Golgi structures. SRIF did not enter pituitary cells at 4.degree. C. To study the role of coated vesicles in internalization of SRIF, SRIF binding to isolated coated vesicles before and after various treatments and sonication was measured. SRIF binding to sonicated vesicles (3.46 .+-. 0.36 fmol/.mu.g protein) was much greater than to intact ones (0.75 .+-. 0.16 fmol/.mu.g protein), suggesting intraluminal localization of SRIF receptors in the coated vesicles. Approximately 80% of SRIF-binding sites were recovered on the intraluminal surface of the coated vesicles. Internalization of SRIF is a time- and temperature-dependent process. Within the cell, SRIF is routed to either lysosomes or the Golgi apparatus. Coated vesicles participate in intracellular translocation of SRIF-receptor complexes. The receptor for SRIF being internalized may be located on the intraluminal surface of the coated vesicle.