Intracellular localization of relaxin in membrane-bound granules in the pregnant rat luteal cell

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Abstract
The cellular organelle responsible for the storage and subsequent release of relaxin from the pregnant rat luteal cells is described. Relaxin was localized in 100- to 200-nm diameter membrane-bound granules found in the corpus luteum of pregnant rat ovaries. Antiserum to porcine relaxin and goat anti-rabbit IgG colloidal gold were used to label the hormone in luteal tissue from Days 17 and 20 of pregnancy. The Au particles were limited to the small granules with none found in mitochondria of larger (300-500 nm diameter) membrane-bound organelles resembling lysosomes. Au particles were not deposited on the granules of control tissue in which the porcine relaxin antiserum was substitued with normal rabbit serum or porcine relaxin antiserum absorbed with purified porcine relaxin. Exocytosis involving the incorporation of the granule membrane into the cell membrane followed by release of product was only once observed. In all tissue sections studied, the membrane sac surrounding the granule was seen to attach to the cell membrane by a filamentous structure. Empty vesicles (100-200 nm diameter) which appear to have discharged the granular product were also noted around the cell periphery.