Biphasic effect of sodium fluoride and guanyl nucleotides on binding to prostaglandin E2 receptors in rat epididymal adipocyte membranes

Both NaCl and NaF promoted PGE₂ binding to epididymal adipocyte membranes by apparent increase in the binding affinity. In order to distinguish between the effect of fluoride and the ‘salt effect’ of sodium on PGE₂ binding, the effects of Mg²⁺ and guanyl nucleotides on PGE₂ binding in the presence of NaCl or NaF were compared. Mg²⁺ decreased PGE₂ binding; high NaF concentration abolished this inhibition, while increased NaCl concentratipns did not affect the Mg²⁺ inhibition. In the presence of Mg²⁺ the effects of NaCl and NaF were additive. The enhancement of PGE₂ binding by fluoride, unlike sodium, was dependent on the presence of Mg²⁺. Induction of the membranes with GDPβS, Gpp(NH)p, GTP or GTPγS increased PGE, binding. Gradual increase in NaF concentrations in the presence of guanyl nucleotides resulted in stimulation of PGE₂ binding at low NaF concentrations and inhibition of PGE₂ binding at higjh NaF concentrations. No changes in the stimulatory action of NaCl on PGE₂ binding were observed in the simulatenous presence of NaCl and guanyl nucleotides. A biphasic effect on PGE₂ binding was observed with a wide concentration range of guanyl nucleotides. Treatment of the isolated membranes with cholera or pertussis toxins stimulated the adenylyl cyclase activity of the membranes, but failed to influence PGE₂ binding. The implications of these findings are discussed.