Effect of ovine prolactin on tubulin function in the anterior pituitary lobe of the lactating rat

Ovine PRL (oPRL) was employed to investigate the role of anterior pituitary lobe microtubules in suckling-induced PRL secretion in the lactating rat. Groups of primiparous rats on days 12-14 postpartum were isolated from their pups for 4-5 h, then suckled for 10 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. After 10 min of suckling the pituitary soluble tubulin levels were reduced by about 25% (P less than 0.05), and polymerized tubulin levels increased by about 40% (P less than 0.05). When 3 mg oPRL were injected 4 h before suckling, the suckling-induced rise in the polymerized tubulin levels in the anterior pituitary lobe as well as plasma PRL levels were significantly inhibited (P less than 0.05). In the suckled rats injected with oPRL a 25% reduction in the total tubulin levels (soluble and polymerized) was observed. In a second experiment, each anterior pituitary lobe obtained from groups of rats suckled for 10 min was processed to obtain the total tubulin fraction. Suckling for 10 min stimulated the in vitro assembly of total tubulin fraction into microtubules by about 150% (P less than 0.05); 3 mg oPRL injected 4 h before suckling inhibited the suckling-induced rise in tubulin assembly (P less than 0.05). In a third experiment, suckling for 10 min stimulated GTPase activity in the total tubulin fraction by about 60% (P less than 0.05). Administration of oPRL 4 h before suckling caused about an 80% increase in GTPase activity. At a 1:2000 dilution, antitubulin antibodies maximally inhibited GTPase activity by about 40%, suggesting that a significant proportion of the enzyme activity can be due to tubulin present in the pituitary. These results suggest that PRL secretion is coupled to pituitary microtubules and that in addition to tubulin, other GTP-binding proteins might be involved in PRL secretion.