The capacity of cells of the anterior pituitary gland to internalize dopamine and incorporate it into subcellular particles was investigated in vivo and under in vitro conditions. Most (75-85%) of the dopamine in the anterior pituitary gland was found to be associated with dense subcellular particles that could not be distinguished from particles, i.e. secretory granules, containing PRL. The amount of dopamine associated with the dense particles was greater in anterior pituitary glands from rats which had been treated with the dopamine precursor, L-dihydroxyphenylalanine (L-dopa), than in anterior pituitary glands from untreated animals. The effect of L-dopa was attenuated when the rats were pretreated with the dopamine agonist, bromocriptine. It was found that incubation of anterior pituitary glands with dopamine resulted in a dose-related increase in the amount of dopamine associated with the dense subcellular particles and that the incorporation of dopamine into these particles occurred within 15 min. Preincubation of anterior pituitary glands with the dopamine antagonist, cis-flupenthixol, or the dopamine agonist, perebedil, prevented the incorporation of dopamine into dense particles, suggestive of the view that this incorporation involves a dopamine receptor. It is concluded that cells of the anterior pituitary gland have the capacity to internalize dopamine and incorporate it into dense subcellular particles. In addition, it is suggested that these dopamine-containing particles may be PRL secretory granules.
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