Mechanism of biphasic action of mono- and bivalent salts on the binding of prolactin to the receptor in the rabbits mammary gland.

The influence of NaCl, KCl, CaCl2, and MgCl2 on the binding of prolactin (PRL) to its receptor was investigated. The salts were dissolved in a metallic ion-free binding buffer and had biphasic effects on changes in the association rate constant (k+1) of PRL binding, depending on their concentrations: there was an increase in the k+1 at lower concentrations and a decrease at higher concentrations. The dissociation rate of bound PRL was unaffected. NaCl at any concentration did not change the binding capacity. Bivalent salts, at higher than 25 mM, increased the capacity about 1.6-fold as compared to the 0 mM control. By cross-linking the PRL-receptor complex, the band of a molecular weight (Mr) 34,500 receptor could always be detected on the autoradiogram. An Mr 78,000 receptor appeared only after incubation with bivalent salts. Data indicate that the binding of PRL to an Mr 78,000 receptor is directly regulated by bivalent cation.