Prolactin-secreting pituitary adenomas: serum and tissue prolactin levels with ultrastructural correlation

Adenomatous tissue was obtained from 10 women with evidence of prolactin-secreting pituitary adenoma at the time of transsphenoidal removal of the tumor. Tissue levels of prolactin (hPRL) were estimated after extraction and were compared to preoperative peripheral serum levels of hPRL. No correlation was found between the tissue and serum concentrations of hPRL, although high levels of hPRL were found in the tissue extracts. Electron microscopy confirmed active secretion of the individual cells, and the ultrastructural findings were compatible with prolactin-secreting adenomas. Associated with the active secretion was extensive exocytosis of the secretory granules away from the capillary wall. These findings suggest that, although active secretion of hPRL is occurring, large amounts of secreted hPRL are not available for immediate uptake into the capillary plexus; thus a slow absorption of hPRL could be taking place from the adenomatous tissue. These data could explain the failure of provocative tests of hPRL stimulation and inhibition to unequivocally characterize an adenoma in an individual with elevated serum levels of hPRL.