Progesterone withdrawal stimulates mammary gland tubulin polymerization in pregnant rats

The size of the polymerized tubulin pool in mammary glands of late pregnant rats increased 2.4-fold 24 h after bilateral ovariectomy using a [3H]colchicine binding assay for tubulin. The total (free plus polymerized) tubulin concentration was not altered. Stimulation of tubulin polymerization as well as the induced rise in lactose content were both completely inhibited in rats that had been fitted with progesterone implants at the time of surgery. The increase in polymerized tubulin was not blocked by 17 beta-estradiol implants, although this treatment did partially inhibit the rise in tissue lactose following ovariectomy. The results indicate that increased polymerization during induced lactogenesis reflects a shift of free tubulin into the polymerized pool and that this shift is related to the withdrawal of progesterone.