Antitumor effect of bromocriptine on estrogen-induced rat prolactinomas: ultrastructural alterations of secretory granules.

A quantitative analysis of ultrastructural alterations of secretory granules in correlation with prolactin (PRL) concentrations in estrogen-induced rat prolactinomas was performed to investigate the mechanism of the cytosuppressive action of bromocriptine (CB). The secretory granules of CB-treated adenomas increased in number but not in volume. The degree of exocytosis of the granules showed no significant decrease as reported previously, but rather an increase in adenomas treated with CB for 3 days. Radioimmunoassay revealed a decrease in PRL concentrations of adenoma tissues. and a marked reduction in serum PRL levels. These results suggest a low level of PRL in the secretory granules of the CB-treated pituitary adenomas.