Prostaglandin E2 Receptor Binding and Action in Human Fat Cells*

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Abstract
The prostaglandin E2 (PGE2) receptor in human adipocytes was identified by the use of [3H]PGE2. The receptor binding at physiological temperaure and pH was specific, saturable, and slowly reversible. Half-maximal displacement for [3H]PGE2 binding occurred with 2.5 nmol/liter. Half-maximal inhibition of isoproterenol-induced lipolysis was achieved at a concentration of PGE2 of 3.8 nmol/liter and half-maximal inhibition of basal lipolysis was achieved at a concentration of PGE2 of 0.9 nmol/liter. The order of potency for prostaglandin inhibition of receptor binding and antilipolytic effect was the same, with PGE2 ≫ PGF2α ≫ arachidonic acid. Scatchard analysis of the binding data revealed a nonlinear plot indicating the existence of two or more binding sites with different affinities. The binding sites of high affinity had an equilibrium constant (Ka) of 2 nmol/liter and a total binding capacity of 58 fmol/106 adipocytes which corresponds to about 33,000 binding sites per adipocyte. The binding sites of low affinity had a Kd of 56 nmol/liter and a total binding capacity of 700 fmol/106 adipocytes. We conclude that [3H]PGE2 binds to receptors in isolated human adipocytes and that their antilipolytic effects are mediated by this binding.