Dopamine Internalization by and Intracellular Distribution within Prolactin Cells and Somatotrophs of the Rat Anterior Pituitary as Determined by Quantitative Electron Microscopic Autoradiography*

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Abstract
The internalization and intracellular distribution of [3H]dopamine in PRL cells and somatotrophs of the female rat anterior pituitary were investigated. Dissociated pituitary cells were incubated with [3H]dopamine for 5 min and subsequently chased in medium lacking radioactive dopamine for various time periods up to 3 h. The localization of [3H]dopamine was monitored by high resolution quantitative electron microscopic autoradiography. Results obtained by grain density analysis revealed that dopamine is internalized by these cells and sequestered in a multiplicity of intracellular membranous organelles. These included the lysosomes and those organelles involved in the secretory pathway, chief among which were the Golgi cisternae and the immature (type I and II) and mature (type III and IV) secretory granules. The highest grain density values throughout the entire chase period were recorded for the lysosomal compartment. This is consistent with the finding that approximately 70% of the internalized dopamine was lost from the PRL cells within the time frame of the chase period, most likely due to its metabolism or release. Although the overall grain density of the nucleus was low at all chase periods, analysis of the grain density distribution of the various nuclear profiles of the post-pulse time point revealed that nuclear labeling was not uniform, suggesting that dopamine is taken up by the nucleus and localized in discrete areas of this compartment. The internalization event itself was found to be a receptor-dependent one since the simultaneous incubation of pituitary cells with [3H] dopamine and physiological concentrations (10-5 M and 10-7 M) of its unlabeled analog resulted in a 50–60% decrease in the uptake of [3H]dopamine by PRL cells, as indicated by grain density analysis. Somatotrophs showed a similar pattern of dopamine internalization and sequestration with the exception that labeling of the Golgi cisternae could not be established with certainty by the analytical methods employed. The data obtained provide a firm morphological basis for the intracellular distribution of dopamine in the two numerically most frequent (PRL cells and somatotrophs) cell types of the rat anterior pituitary gland.