title: Guanine nucleotide and cation regulation of radioligand binding to Ri adenosine receptors of rat fat cells.
abstract: 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.
mesh_terms: D000273:Adipose Tissue / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology*; D006160:Guanosine Triphosphate / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010660:Phenylisopropyladenosine / Q000378:metabolism; D011869:Radioligand Assay; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011983:Receptors, Purinergic; D012964:Sodium / Q000494:pharmacology; D014316:Tritium
chemical_list: D006150:Guanine Nucleotides; D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D014316:Tritium; D010660:Phenylisopropyladenosine; D006160:Guanosine Triphosphate; D012964:Sodium; D008274:Magnesium; D002118:Calcium