Albumin and GTP modulate the affinity of prostaglandin E2 receptors in rat epididymal adipocyte membranes.

Detailed studies of PGE2 binding to isolated rat adipocyte membranes revealed two different classes of binding sites, namely high affinity-low capacity binding sites and low affinity-high capacity binding sites. Addition of albumin or GTP to the incubation medium enhanced the specific binding of PGE2 by decreasing the dissociation constant of the low affinity-high capacity binding sites. Albumin also increased the affinity of PGE2 binding to native canine renal medullary membranes and enhanced the binding of PGE2 to prostaglandin receptors solubilized from these membranes. Pretreatment of the adipocyte membranes with the alkylating agent NEM completely abolished the enhancement of PGE2 binding by GTP, while the enhancement of PGE2 binding by albumin was only partially inhibited. The enhancement of PGE2 binding by GTP was shown to be dependent on the presence of Mg+2, while the albumin effect was independent of Mg+2. These results suggest that the affinity of the prostaglandin receptors is modulated by more than one mechanism.