(-)-[3H]-dihydroalprenolol binding to beta-adrenergic receptors in porcine adipose tissue and skeletal muscle membrane preparations.

We have characterized binding of (-)-[3H]dihydroalprenolol (DHA) to the beta-adrenergic receptor population in membrane from longissimus and semitendinosus muscles and subcutaneous adipose tissue (middle and outer layers) of pigs weighing 100 to 120 kg at slaughter. Linearity of specific binding with increasing membrane protein concentration was confirmed for muscle and adipose tissue. Specific binding was saturable; the beta-receptor population showed a high affinity for the radioligand. Half-maximal binding was achieved at similar concentrations of the tracer (.48 to .73 nM) in all tissues. Maximal binding (B max) per milligram of protein for skeletal muscle was approximately half of that for adipose tissue (P < or = .05) and was greater (P < or = .05) in membrane from the middle than in membrane from the outer layer of adipose tissue. Adipocyte membrane preparations yielded Kd estimates similar to those obtained with adipose tissue, but Bmax values were greater (P < or = .05). Kinetic rate constants were estimated and used to calculate the Kd for comparison to that derived from nonlinear regression analysis of Scatchard plots. Association (k1) and dissociation (k2) estimates were similar between tissues and the resulting Kd values were comparable to those derived from equilibrium data. Stereospecificity of binding was verified using the (-) and (+) isomers of the beta-adrenergic agonist isoproterenol; selectivity for the (-) form ranged from 8- to 19-fold. Overall, the most notable differences among tissues were in receptor density expressed per milligram of protein. Kinetic rate constants, Kd estimates, and the degree of selectivity for (-)-isoproterenol were similar.