DISTINCTIONS IN BETA-ADRENERGIC-RECEPTOR INTERACTIONS WITH THE MAGNESIUM-GUANINE NUCLEOTIDE COUPLING PROTEINS IN TURKEY ERYTHROCYTE AND S49-LYMPHOMA MEMBRANES

  • 1 January 1982
    • journal article
    • research article
    • Vol. 8  (3) , 149-162
Abstract
Several homogeneous cell systems contain distinct subpopulations of .beta.-adrenergic receptors, distinguished by their relative sensitivity to N-ethylmaleimide (NEM) in the presence of agonist but not antagonist [isoproterenol, dihydroalprenolol and [125I]-iodohydroxybenzylpindolol]. The sensitivity to agonist/NEM inactivation requires receptor interaction with the Mg guanine nucleotide coupling proteins (G/F). The effects of agonist/NEM treatment were investigated on Mg2+ and GTP modulation of receptor affinity in 2 such systems, turkey erythrocytes and murine S49 lymphoma cells. In each system, the agonist/NEM-sensitive .beta.-receptor subpopulation exhibits both Mg2+ and GTP modulation of .beta.-receptor affinity for agonist. Mg2+ and GTP are not competitive with regard to alteration of receptor affinity; GTP can block the effect of Mg2+, but not vice versa. The agonist/NEM-resistant .beta.-receptor subpopulation shows distinct differences in Mg2+ and GTP effects when the turkey and S49 systems are compared. The agonist/NEM-resistant population in S49 shows no effect of Mg2+ or GTP on .beta.-receptor affinity for agonist whereas the resistant .beta.-receptors of turkey erythrocytes still exhibit modulation by both GTP and Mg2+. In this receptor population the actions of GTP and Mg2+ are apparently competitive, with increasing Mg2+ concentrations able to overcome the decrease in affinity induced by GTP. .beta.-Receptor interaction with the metal/nucleotide coupling proteins may differ significantly in the 2 systems examined. For S49 .beta.-receptors, free, unchelated GTP or GDP rather than MgGTP or MgGDP modulates receptor affinity for agonist.

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