[Evaluation of free and bound fractions resulting from the interaction of prolactin with its specific receptors].

Binding of either "cold" or 125I-PRL to their specific receptors (fraction after centrifugation at 15,000 and 100,000 X g) obtained from late pregnant rat liver, pre- and post-dissociation with MgCl2, has been studied. Binding was higher with cold hormone (delta 21.63%) than with 125I-PRL. Similarly, binding to the 100,000 X g fraction was also higher than to the 15,000 X g one. Dissociation by MgCl2 improved binding to the 100,000 X g fraction (delta 17.27%), while reduced the 15,000 X g fraction binding (delta 11.71%), underlying the impurity of the latter fraction. Control studies with rLH, rFSH, hACTH, insulin, glucagon and hGH evidenced the specificity of the preparation to bind lactogenic hormones. Binding increases with PRL and receptor concentration, reaching equilibrium between bound PRL/unbound PRL. An amount of PRL unable to bind to the receptor is always present. Even with high receptor concentrations (3,500 micrograms/0.1 ml) there is still about 25% of unbound PRL. When reincubating this previously unbound PRL with a fresh receptor preparation identical to the one used in the first incubation, a similar proportion of bound PRL/unbound PRL is obtained. These results suggest the existence of a heterogeneity in the receptor preparation.