SPECIFIC AND UNSPECIFIC BINDING OF [(-)DIHYDROALPRENOLOL-H-3 TO CARDIAC TISSUE

Identification of β-adrenergic receptors has been successfully performed by [3H](-)dihydroalprenolol binding to several tissues of a variety of species. Specific binding sites (receptors) have usually been defined by criteria as: velocity, reversibility, saturability and stereospecificity of binding. However. [3H](-)dihydroalprenolol does not only bind to specific sites but also to unspecific ones. The given definitions of unspecific binding sites are somewhat confusing. [3H](-)Dihydroalprenolol bound (2670 c.p.m./mg prot.) to cardiac membranes is inhibited in the presence ofunlabeled alprenolol in a concentration-dependent manner up to 103 M. From 109 to 105M this inhibition caused by (-) and (+) alprenolol was found to be stereospecific. In higher concentrations the measured inhibition of [3H](-)dihydroalprenolol binding to its receptor was no longer stereospecific. In heat denatured membranes [3H](-)dihydroalprenolol bound (1335 c.p.m./mg prot.) was identical with that of native membranes in the presence of 10-3 M (-) or (+) alprenolol. There was no inhibition of binding. however, in the presence of unlabeled (-)or (+) alprenolol in a concentration range below 103 M. At higher concentrations there was inhibition of binding, but it was not stereospecific. When an equilibrium of [3H](-)dihydroalprenolol binding to all cardiac membranes (native or heat denatured) had been established. addition of the unlabeled ligand led to a concentration-dependent and rapid displacement of the labeled ligand. Again, stereospecificity of the displacement was only observed in the native membranes. Inhibition of adenylate cyclase activity was measured at the same (-) and (+) alprenolol concentrations and showed the same stereospecific effects. Thus. the results indicate that the loss of stereospecificity in heat denatured membranes and in native membranes correlates with unspecific binding of [3H](-)dihydroalprenolol to cardiac membranes. It is quantitatively identical. It is concluded, that out of the criteria for specific binding mentioned, only stereospecificity seems to be valid for [3H](-)dihydroalprenolol binding to the β-adrenergic receptor. © 1979.