Methodological aspects of receptor research: problems in the assessment of specific and non-specific binding of 3H-dihydroalprenolol to bovine lung membranes.

The binding of the radiolabelled beta-antagonist 3H-DHA to membranes prepared from bovine lung was examined. Three methods for determining the NSB of this ligand (200 microM of (-)-isoprenaline, 10 microM of (-)-propranolol and 10 microM of (-)-alprenolol) were investigated. In experiments designed to measure the Kd and Bmax of 3H-DHA to bovine lung membranes these three methods were not equivalent. (-)-Isoprenaline gave lower estimates of the Kd and Bmax than did (-)-propranolol, which in turn gave lower estimates than (-)-alprenolol. Critical examination of the characteristics of the binding of these three compounds using competition experiments showed that, at the concentrations used, an interaction of (-)-propranolol and (-)-alprenolol with both the receptor and the NSB site was responsible for the observed differences. This result was confirmed by a theoretical consideration of the binding reactions involved. Furthermore, these theoretical studies also revealed other potential problems arising from incorrect NSB measurement, which, if not appreciated, can cause serious misinterpretation of the nature of the receptor-ligand interactions.