The subcellular compartmentalization of endogenous dopamine in the anterior pituitary gland of the rat was investigated using continuous sucrose density gradient centrifugation. When anterior pituitary homogenates were layered on continuous sucrose density gradients (1.0-2.0 M) and centrifuged for 60 min at 40, 000 × g, dopamine recovered from the gradients was associated with two sets of subcellular particles. The particles in one set were recovered near the top of the gradient, whereas those in the other set were recovered near the bottom of the gradient in the region where particles containing PRL were also found. In fact, these dense dopamine-containing particles could not be separated from those particles which contained PRL. These findings were suggestive that dopamine and PRL were present in the same particle, viz. the PRL secretory granule. This interpretation was further strengthened when it was established that the PRL-containing granules were separable on the gradient from granules which contained GH, LH, FSH, ACTH, and TSH. When [!H]dopamine was added to the solution in which the anterior lobes were homogenized, no radioactivity was found to be associated with the dense dopaminecontaining particles. Also, the addition of a large excess of nonradiolabeled dopamine at the time of homogenization did not influence the amount of dopamine associated with the dense particles. Thus, the apparent association of dopamine with PRL secretory granules was not an artifact of the homogenization process per se. Therefore, it is concluded that an association exists between intracellular dopamine and the PRL secretory granule.
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