Vinblastine-induced microtubular paracrystals in prolactin cells of anterior pituitary gland of lactating rats.

Vinblastine sulfate in physiological saline was injected directly into the pituitary glands of lactating rats. Injections were made through the ear canal using a syringe equipped with a 24-gauge needle. The animals were killed at 2, 4, or 6 hours after the injections. When the anterior pituitary glands were examined by electron microscopy, many microtubular paracrystalline deposits were seen in the prolactin and growth hormone cells. The usual cytoplasmic microtubules and microfilaments were not seen in the cytoplasm of these cells. Granular extrusion (exocytosis) was markedly depressed, and an accumulation of secretory granules was definitely observed in the prolactin cells after the administration of vinblastine.