Effects of bromocriptine and occlusion of nipples on prolactin receptor and lactose synthetase activity in the mammary gland of the lactating rat.

The response of prolactin receptor and lactose synthetase to suppression of plasma concentrations of prolactin was examined in normal and occluded (teat-sealed) mammary glands of Sprague-Dawley rats, Rats, with mammary glands unilaterally occluded, were given bromocriptine (2.5 mg/kg per 12 h) between 5 and 8 post partum. Bromocriptine reduced plasma prolactin concentrations from 460.4 +/- 120.8 (mean +/- s.e.m.) to 2.56 +/- 0.89 ng/ml within 12h whilst concentrations in control rats were 553.4 +/- 110.25 ng/ml. Lactose synthetase activity declined rapidly, within 24h, in occluded glands of both groups but was maintained for 24 h in normal glands of bromocriptine-treated rats and decreased thereafter. Prolactin receptors also declined significantly within 24h in occluded glands. Desaturation of the prolactin receptor by bromocriptine treatment in vivo was compared with desaturation by exposure of membranes to MgCl2 in vitro. Both treatments enhanced prolactin binding but the increase after treatment with MgCl2 may have been partly artefactual since there was a selective loss of protein from the membranes. These results indicate that the prolactin receptor in rat mammary gland may be maintained after acute suppression of prolactin secretion.