Prolactin and Estrogen Binding Sites in the Mammary Gland of the Lactating and Non-Lactating Rat

Abstract

Mammary glands from lactating and non-lactating postpartum rats were assayed for prolactin (PRL) and estradiol (E2) binding sites. In lactating animals serum PRL and tissue E2 and PRL binding increased rapidly in the first 2 days, postpartum and thereafter declined until day 15 to 20 postpartum. In non-lactating rats in the postpartum period, no comparable changes in the 3 parameters were observed. Hysterectomy of pregnant rats on day 21 was followed by an increase in serum PRL concentrations but no increase in mammary tissue binding of PRL or E2 was found. Similar results were observed after hysterectomy and oophorectomy except no increase in serum PRL was noted. The administration of estradiol to the mother immediately after birth decreased mammary tissue PRL binding sites, while the administration of hydroxyprogesterone decreased binding of E2 but left PRL binding unchanged. Postpartum suppression of PRL secretion with bromocriptine decreased PRL but not E2 binding and, in addition, decreased weight gain of the pups. The changes caused by bromocriptine were partially reversed by the simultaneous administration of o[ovine]PRL during the treatment period. The data indicate that mammary gland PRL receptors are increased by PRL and thus exhibit the same autoregulation noted previously in the liver. Estrogens, which increase hepatic PRL binding, exert an opposite effect on mammary gland PRL receptors. The increase in serum PRL at the end of gestation is in part dependent on the presence of ovaries and the suckling stimulus causes a further increase.