Soluble and polymerized tubulin levels in the anterior pituitary lobe of the lactating rat during suckling.

A [3H]colchicine-binding assay was employed for estimating the relative amounts of soluble and polymerized tubulin present in an individual anterior pituitary lobe. Colchicine binding to these two tubulin pools was time dependent and dose responsive. Scatchard analysis of the binding revealed that [3H]colchicine bound to both fractions with similar affinity. After establishing the optimal conditions for the binding assay, the effect of suckling on the relative levels of the two tubulin pools was studied. Groups of primiparous rats on days 12-14 postpartum were isolated from their pups for 4-5 h, then suckled for 30, 60, and 90 min, and killed, and the anterior pituitary lobes were dissected out. Each pituitary lobe was processed to obtain the two tubulin pools, viz. soluble and polymerized tubulin fractions. Suckling for 30 min resulted in an increase in soluble tubulin levels and a concomitant decrease in the polymerized pools. At 60 and 90 min, the soluble tubulin levels gradually decreased to presuckled levels, whereas the polymerized pools increased from the 30-min levels to those observed at 0 min. In another experiment, suckling for 10 min resulted in a decrease in the soluble tubulin levels and a corresponding increase in the polymerized tubulin pool. Treatment of nonsuckled rats with domperidone for 10 min also resulted in a shift of the equilibrium between the two tubulin pools similar to that observed at 10 min of suckling. Administration of bromocriptine 30 min beforehand to nonsuckled rats and then suckling them for 10 min blocked the suckling-induced rise in the polymerized tubulin levels. These results suggest a regulatory role for dopamine in the suckling-induced shift in the equilibrium between the soluble and polymerized tubulin pools in the anterior pituitary lobe.