Suspensions of cells dissociated from the anterior pituitary of the adult rat include many that contain intracellular PRL. After fixation, these cells can be identified and the distribution of their PRL determined by immunocytochemistry with anti-PRL antibodies. We have found that approximately 50% of the cells that contain intracellular PRL also have PRL or PRL-like immunoreactive material on the outer cell surface. Cell-surface PRL can be detected on unfixed anterior pituitary cells using anti-PRL antibodies and either fluorescence microscopy or fluorescence-activated cell sorting. Cell surface labeling by anti-PRL antibodies was restricted to PRL-containing cells; 85â€“97% of the labeled cells contained detectable intracellular PRL, while fewer than 2% contained detectable GH, ACTH, LH, or TSH. Subpopulations of live anterior pituitary cells could also be labeled on the cell surface by antibodies against GH, ACTH, LH, or TSH. This suggests that the presence of hormone at the cell surface may be a characteristic and identifying feature of different anterior pituitary cell types.
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