Co-localization of Rab3B and oxytocin to electron dense granules of the sheep corpus luteum during the estrous cycle

Oxytocin and its carrier protein, neurophysin, are both associated with lut eal secretory granules which migrate from the paranuclear region to the cel l membrane where exocytosis takes place. Rab3 proteins are thought to be as sociated with membrane vesicles or granules undergoing exocytotic fusion wi th the plasma membrane. The objective of this study was to determine whethe r Rab3B is co-localized with oxytocin within the same secretory granules of large luteal cells obtained from corpora lutea of 16 Merino cross ewes at day 3, 7, 12 or 15 of the estrous cycle using immunocytochemistry. The mean granule density (granules/mu m(3)) was not significantly different (P > 0.05) between the days examined. Electron microscopic immunocytochemi stry showed that oxytocin and Rab3B were co-localized to the secretory gran ules on all days evaluated. Rab3B immunostaining was primarily located with in secretory granules scattered throughout the cytoplasm. The mean intensit y of labelling (number of gold particles) for oxytocin per mu m(2) cytoplas mic luteal tissue was significantly decreased on day 15 compared to those o bserved on days 3, 7 and 12 of estrous cycle. No significant changes were o bserved in the mean intensity of the Rab3B label at the different times of the cycle. The present study provides evidence that a member of the subfamily of Rab p roteins, Rab3B, is present and co-localized with oxytocin in the same secre tory granules of the ovine corpus luteum. These results implicate Rab3B pro tein directly or indirectly in the hormone secretory pathway of ovarian tis sue. Anat Rec 254.214-221, 1999. (C) 1999 Wiley-Liss. Inc.