Solubilization of human platelet alpha-adrenergic receptors: evidence that agonist occupancy of the receptor stabilizes receptor--effector interactions.

The alpha-adrenergic receptors of human platelet membranes can be directly identified by both a radiolabeled agonist, [3H]epinephrine, and a radiolabeled antagonist, [3H]yohimbine. Digitonin solubilizes a binding component from the membrane that is indistinguishable from the alpha-receptor identified in the native platelet membrane as assessed by (i) order of potency of agonists and antagonists and (ii) affinity of the receptor for [3H]-yohimbine and competing antagonists. However, the solubilized receptor demonstrates a reduced affinity for agonists and a loss of the ability of guanine nucleotides to modulate receptor affinity for agonists. Prelabeling of human platelet membranes with [3H]-epinephrine results in a guanine nucleotide-sensitive agonist-receptor complex that sediments more rapidly in sucrose gradients than do unoccupied or antagonist-occupied receptors. Thus, agonist occupancy of the alpha-receptor prior to membrane solubilization may promote or stabilize receptor interaction with effector components in the membrane, one of which may be the GTP regulatory protein responsible for modulation of receptor affinity.