Abstract
Prolactin dissociates more readily from rat liver than from rabbit mammary prolactin receptors. The rate of dissociation is dependent in the time of association. In rat liver, prolactin dissociates from receptors at the cell periphery (plasma membrane, PM) more rapidly than those of microsomes, a major component of which is the intracellular Golgi membranes. The dissociation curves following 1 hr of association can be resolved into a fast and slow component by logarithmic transformation, with a greater than twofold increase in the fast componment of the dissociation rate constant (k
2) in PM compared to microsomal membranes. With longer association times (10 hr), plasma membranes develop a more slowly dissociable component, with dissociation characteristics (rate constants) similar to those observed in microsomes following 1 hr of association. Another means of visualizing this difference is by a Scatchard plot of prolactin binding to PM, microsomal, and Golgi membranes. The affinity constants in the microsomal and Golgi fractions are identical whereas that for the PM fractions more than twofold lower. The decreased affinity in the PM would allow prolactin to more readily dissociate from its receptor than from receptors with higher affinity. Although the differences between PM and Golgi receptors observed are small, they are confirmed by direct measurements of affinity constants and dissociation rate constants. Therefore, it appears that receptors with lower affinity at the cell periphery are those involved in the initial mechanism of action of prolactin.
