Guanosine Nucleotides Regulate B2 Kinin Receptor Affinity of Agonists But Not of Antagonists: Discussion of a Model Proposing Receptor Precoupling to G Protein

Abstract

AbstractThe effect of nucleotides on binding of the B2 kinin (BK) receptor agonist [[3]H]BK and the antagonist [[3]H]NPC17731 to particulate fractions of human foreskin fibroblasts was studied. At 0C, particulate fractions exhibited a single class of binding sites with a K of 2.3n for [[3]H]BK and a K of 3.8n for the antagonist [[3]H]NPC17731. Incubation with radioligands at 37C for 5 min gave a reduction of agonist, as well as antagonist, binding that was between 0 40% depending on the preparation, even in the absence of guanosine nucleotides. As shown by Scatchard analysis, this reduction in specific binding was due to a shift in the affinity of at least a fraction of the receptors. The presence at 37C of the guanine nucleotides GTP, GDP and their poorly hydrolyzable analogs left [[3]H] NPC17731 binding unaffected, but reduced the receptor affinity for [[3]H]BK to a K of about 15 n. The maximal number of receptors, however, was unchanged. This affinity change was strongly dependent on the presence of bivalent cations, in particular Mg[2+]. It was reversed by incubation at 0C. The rank order of the guanosine nucleotides for [[3]H]BK binding reduction was GTP[γS] = Gppp [greater than] GTP = GDP [greater than] GDP[βS]. GMP, ATP, ADP and AMP showed no influence on agonist binding. A model for the interaction of the B2 kinin receptor with G proteins is discussed.