title: Guanine nucleotide and cation regulation of the binding of [3H]cyclohexyladenosine and [3H]diethylphenylxanthine to adenosine A1 receptors in brain membranes.
abstract: Guanine nucleotides, divalent cations, and sodium differentially regulate agonist and antagonist binding to adenosine A1 receptors in brain membranes. Guanine nucleotides decrease the binding of the adenosine A1 receptor agonist [3H]N6-cyclohexyladenosine ([3H]CHA) to guinea pig and bovine brain membranes by about 50% at 1--3 microM, while not affecting binding of the antagonist [3H]1,3-diethyl-8-phenylxanthine ([3H]DPX) to A1 receptors in bovine brain. GTP decreases the potency of agonists competing for [3H]DPX binding by 3--6 times, without altering the potency of antagonists. This effect can be used to grade experimental substances along an adenosine agonist-antagonist continuum. The 66% inhibition of [3H]CHA binding by 1 mM EDTA, with no change in [3H]DPX binding, suggests that endogenous divalent cations may regulate adenosine receptor interactions. Removal of endogenous divalent cations by EDTA treatment greatly increases the enhancement of [3H]CHA binding by divalent cations. Specific binding of [3H]CHA to guinea pig brain is increased 150--170% by 0.3--1.0 mM Mn2+, Mg2+, and Ca2+ following EDTA preincubation, secondary to an increase in apparent affinity and receptor number. Sodium ions also selectively regulate the binding of [3H]CHA. Sodium decreases [3H]CHA binding 40%, whereas lithium and potassium are ineffective. Sodium does not affect [3H]DPX binding.
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives* / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000378:metabolism*; D002412:Cations / Q000502:physiology*; D006150:Guanine Nucleotides / Q000502:physiology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D011983:Receptors, Purinergic; D012964:Sodium / Q000502:physiology; D014970:Xanthines / Q000378:metabolism*
chemical_list: D002412:Cations; D006150:Guanine Nucleotides; D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D014970:Xanthines; C027512:1,3-diethyl-8-phenylxanthine; C027513:N(6)-cyclohexyladenosine; D012964:Sodium; D000241:Adenosine