Parallel observation of the occupancy of the alpha 2-adrenergic receptor in intact platelets and its ability to inhibit the adenylate cyclase.

The binding of the selective alpha2-adrenergic receptor antagonist [methyl-3H]yohimbine to intact human blood platelets was studied in parallel with an assessment of the ability of epinephrine to inhibit their accumulation of cyclic AMP in response to prostaglandin E1 and a phosphodiesterase inhibitor. Specifically bound [3H]yohimbine dissociated in a monoexponential fashion, and equilibrium binding showed 200-400 sites/platelet with a dissociation constant of 2-7 nM. The concentration of epinephrine which inhibited cyclic AMP accumulation was one-quarter of that required for an equal degree of inhibition of [3H]yohimbine binding. Conversely, the concentration of yohimbine required to inhibit the action of epinephrine on the adenylate cyclase was 10 times higher than that required for an equal degree of saturation of the alpha2-adrenoreceptor. The ability of epinephrine and the partial agonist p-aminoclonidine to compete with [3H]yohimbine for binding was not influenced by agonist-occupation of the ADP receptor, which also inhibits the adenylate cyclase. These results are not compatible with the concept that each adenylate cyclase unit is coupled to an alpha 2-receptor, nor do they indicate that the agonist-occupied alpha 2-receptor forms a complex with the enzyme which persists for the duration of the inhibitory effect. However, it is suggested that these results are compatible with a persistent inhibition of the cyclase being induced by a brief interaction between the agonist-occupied receptor and the adenylate cyclase.