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 solublized 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.