Guanine nucleotide and cation regulation of radioligand binding to Ri adenosine receptors of rat fat cells.

The modulation of radioligand binding at Ri adenosine receptors of rat fat cells by guanine nucleotides and cations was investigated. Guanine nucleotides (in the order of potency: GTP = GDP greater than Gpp(NH)p greater than 5'-GMP) decreased the binding of the Ri receptor agonist (-)N6-phenylisopropyl[3H]adenosine([3H]PIA), but did not affect binding of the antagonist 1,3-diethyl-8-[3H]phenylxanthine ([3H]DPX). Saturation of [3H]PIA binding revealed that GTP (100 mumol/l) converts the high affinity form of the Ri receptor into a low affinity form. This effect was confirmed in kinetic experiments. GTP decreased the potency of agonists in competing for [3H]DPX binding, as shown by a 50-fold shift of the Ki-value for (-)PIA, whereas antagonist-induced inhibition of binding remained unchanged. The divalent cations Mg2+ and Ca2+ produced a slight increase in [3H]PIA binding, but did not affect [3H]DPX binding. Mn2+ markedly decreased both agonist and antagonist binding at Ri adenosine receptors. Divalent cations reversed the guanine nucleotide-induced decrease of affinity of the Ri receptor. Na+ did not significantly affect agonist or antagonist binding but abolished the stimulatory effect of Mg2+ on agonist binding in the presence of GTP. Our data indicate that guanine nucleotides convert the Ri adenosine receptor of rat fat cells from a high to a low agonist affinity state and that the modulation of radioligand binding by mono- and divalent cations differs from that of Ri receptors of other tissues.