Effects of temperature and membrane phase transitions on ligand binding to alpha 2-receptors of human platelets

MJ Lohse, KN Klotz and U Schwabe

The binding of agonists and antagonists to alpha 2-adrenergic receptors of human platelets was studied. The receptors showed homogeneous affinities for antagonists but two affinity states for the agonist (-)- epinephrine, which were modulated by guanine nucleotides. Van't Hoff plots of antagonist binding had a break point at about 18 degrees and considerable diversity between 18 degrees and 0 degree. Agonist binding to both affinity states showed a similar break point; agonist binding to the high affinity state was characterized by a large entropy component compared to the low affinity state. This entropy component was reduced at higher concentrations of sodium, indicating that it may be due to liberation of sodium ions. Measurements of the fluorescence of 1-anilin-8-naphthalenesulfonate showed thermotropic phase transitions of the platelet membranes at about 17 degrees. The transition temperature was decreased to about 12 degrees by addition of 10 mM octanoic acid. Octanoic acid also shifted the break points of the van't Hoff plot of antagonist and low affinity agonist binding from 18 degrees to 12 degrees. High affinity agonist binding, however, remained unchanged. It is concluded that agonist-specific thermodynamic characteristics of ligand binding to alpha 2-receptors of human platelets can only be investigated by regarding differences between high and low affinity agonist binding. These differences include an entropy increase upon ligand binding, which is in part due to enhanced liberation of sodium ions, and a loss of sensitivity to fluidity changes in the outer layer of the plasma membrane.

Volume 29, Issue 3, pp. 228-234, 03/01/1986
Copyright © 1986 by American Society for Pharmacology and Experimental Therapeutics