Dissociated anterior pituitary cells derived from estrogen‐treated female rats were incubated with radioiodinated cationic ferritin (CFI*) for 2 min and subsequently in the absence of CFI* for varying periods of time up to 3 hr in order to quantitate, using electron microscopic autoradiography, the distribution of retrieved plasma membrane in these cells. Following a 2‐min incubation with CFI*, autoradiographic grains were found to be associated almost exclusively with the plasma membrane. With increasing periods of incubation in the absence of CFI*, grain‐density analysis revealed increasing levels of CFI* in multiple intracellular organelles. The levels of CFI* were greatest for the lysosomes, intermediate for the mature secretory granules, and least for the Golgi cisternae and immature secretory granules. These findings are consistent with the idea that a portion of the retrieved plasma membrane is degraded in lysosomes and that the remainder is recycled to organelles comprising the secretory pathway to be reutilized in successive waves of the secretory cycle.
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