Abstract

A method for determining the relative concentrations and properties of β-1 and β-2-adrenergic receptors in tissues containing both receptor subtypes has been developed. Since (¹²⁵I)-iodohydroxybenzylpindolol has similar affinities for β-1 and β-2-adrenergic receptors, it is possible to determine the total concentration of β-adrenergic receptors in a tissue by Scatchard analysis of specific (¹²⁵I)-iodohydroxybenzylpindolol binding. In the presence of GTP, inhibition of specific (¹²⁵I)-iodohydroxybenzylpindolol binding in rat heart, lung and five regions of rat brain by agonists and antagonists that are not specific for β-1 or β-2-adrenergic receptors yields linear Hofstee plots with Hill coefficients of approximately 1.0. On the other hand, the inhibition of specific (¹²⁵I)-iodohydroxybenzylpindolol binding by drugs which have been shown to have different affinities for heart (β-1) and lung (β-2) receptors in vitro results in nonlinear Hofstee plots in each of these tissues. Two of these drugs (practolol and metoprolol) are more potent on β-1 than β-2 receptors and two of these drugs (zinterol and salmefamol) are more potent on β-2 than on β-1 receptors. The nonlinear Hofstee plots are consistent with there being two types of binding sites in each of the tissues with different affinities for the drugs. The relative number of each type of binding site and the affinity of each drug for each of the two types of site has been calculated using a computer based iterative procedure. Using this method, the relative percentages of the two receptor subtypes in rat heart, lung, cerebral cortex, caudate, cerebellum, hippocampus and diencephalon were determined. In each tissue, the use of four different drugs with different in vitro selectivity (two β-1 selective and two β-2 selective) resulted in approximately the same calculated β-1/β-2 ratio. This suggests that the assumption that the nonlinear Hofstee plots are composed of only two components is correct. In addition, the calculated affinity of each drug for β-1 and β-2 receptors was quantitatively similar in each of the seven tissues examined. The calculated ratios of β-l:β-2-adrenergic receptors are: heart 83:17; lung 15:85; cortex 81:19; caudate 76:24; cerebellum 15:85; hippocampus 81:19; and diencephalon 71:29. The absolute concentrations of β-1-adrenergic receptors in the brain regions examined varied by almost 20-fold. However, the absolute concentration of β-2-adrenergic receptors varied less than 3-fold. This suggests that β-2-adrenergic receptors in rat brain are associated with a more homogeneously distributed cellular element than are β-1-adrenergic receptors.