Microtubules are mobilized in the lactating rat anterior pituitary gland during suckling

Microtubules in the lactating rat anterior pituitary gland were depolymerized into their constituent tubulin dimers by exposure of the pituitary to 4 C. The tubulin then was quantified with the [3H]colchicine binding assay, which was adapted for use with individual rat pituitary glands. The binding of [3H]colchicine to the tubulin fraction contained in high speed supernatants of lactating rat pituitary glands proved to be specific and saturable, and was pH, temperature, and time dependent. The amount of [3H]colchicine bound was linear over the range of protein concentrations tested (2-22 micrograms). To determine whether suckling affected the levels of microtubules in the anterior pituitary, tubulin levels were measured in groups of lactating rats after they were suckled (six pups per litter) for 0, 5, 10, 15, and 30 min following 4-5 h of nonsuckling. The tubulin concentration in the anterior pituitary progressively increased from 4 to 22 mumol during the 30 min of suckling; the increase was statistically significant (P less than 0.05) by the 10th min. Plasma PRL levels analyzed from trunk blood rose from 5 to 75 ng/ml during the 30-min suckling period. These results indicate that a mobilization of microtubules occurs in the anterior pituitary at the same time that PRL is being transformed and released into the circulation.