The effect of bromocriptine on the mammary glands of hyperprolactinemic rats. A histologic, ultrastructural and...

Old female Long-Evans rats with elevated serum prolactin levels were used to study the effect of bromocriptine on the stimulated mammary glands by histology, electron microscopy, and morphometry. Untreated rats were divided into normoprolactinemic (below 30 ng prolactin/ml serum) and hyperprolactinemic control groups (above 30 ng/ml). Treated groups were injected subcutaneously with 1 mg/kg of bromocriptine once daily for 1 day, 30 days and 44 days. One group was treated with bromocriptine for 30 days, and then the drug was withdrawn for an additional 14 days. The light and electron microscopic features of breasts of hyperprolactinemic control, 1 day treatment and withdrawal groups resembled those of postpartum lactating breasts. A unique feature was the presence of large membrane-bound bodies, possibly lysosomes. The breasts of normoprolactinemic control, 30 and 44-day treatment groups were similar by histology and ultrastructure to involuted breasts after cessation of lactation. However, many large lysosome-like bodies and microvilli were found in the alveolar cells which remained tightly apposed. Hyperprolactinemic control rats showed breast stimulation which could be correlated with serum prolactin levels. Serum prolactin concentrations fell within 1 day of bromocriptine treatment, whereas breast tissue responded only after a longer interval. Pituitary weights could be correlated with serum prolactin levels. Withdrawal of bromocriptine resulted in an elevation of serum prolactin levels and breasts returned to the stimulated stage. It can be concluded that bromocriptine reversibly suppresses the stimulation of mammary glands in hyperprolactinemic rats.