Modulation of alfa-adrenoceptor activity by beta-adrenoceptor agonists and antagonists in rat brain cortex membranes

The influence of β-adrenoceptor activation and inhibition by isoprenaline and propranolol on the specific binding of nonselective α₁- and α₂-adrenoceptor antagonists [³H]prazosin and [³H]RX821002 in rat cerebral cortex subcellular membrane fractions was studied. It was established that for the α₁- and α₂-adrenoceptors the ligand–receptor interaction corresponds to the model of one affinity pool of receptors and binding of two ligand molecules by one dimer receptor. The parameters of [³H]prazosin binding to α₁-adrenoceptors were: K_d = 1.85 ± 0.16 nM, B_max = 31.14 ± 0.35 fmol/mg protein, n = 2. The parameters of [³H]RX821002 binding to α₂-adrenoceptors were: K_d = 1.57 ± 0.27 nM, B_max = 7.2 ± 1.6 fmol/mg protein, n = 2. When β-adrenoceptors were activated by isoprenaline, the binding of radiolabelled ligands with α₁- and α₂-adrenoceptors occurred according to the same model. The affinity to [³H]prazosin and the concentration of active α₁-adrenoceptors increased by 27% (K_d = 1.36 ± 0.03 nM) and 84% (B_max = 57.37 ± 0.28 fmol/mg protein), respectively. The affinity of α₂-adrenoceptors to [³H]RX821002 decreased by 56% (K_d = 3.55 ± 0.02 nM), and the concentration of active receptors increased by 69% (B_max = 12.24 ± 0.06 fmol/mg protein). Propranolol alters the binding character of both ligands. For [³H]prazosin and [³H]RX821002, two pools of receptors were detected with the following parameters: K_d1 = 1.13 ± 0.09, K_d2 = 6.07 ± 1.06 nM, B_m1 = 11.36 ± 1.77, Bm2 = 51.09 ± 0.41 fmol/mg protein, n = 2 and K_d1 = 0.61 ± 0.02, K_d2 = 3.41 ± 0.13 nM, B_m1 = 1.88 ± 0.028, B_m2 = 9.27 ± 0.08 fmol/mg protein, n = 2, respectively. The concentration of active receptors (B_max) increased twofold for both ligands. It was suggested that α₁- and α₂-adrenoceptors in rat cerebral cortex subcellular membrane fractions exist as dimers. A modulating influence of isoprenaline and propranolol on the specific binding of the antagonists to α₁- and α₂- adrenoceptors was revealed, which was manifested in the activating effect on the [³H]prazosin binding parameters, in the inhibitory effect on the [³H]RX821002 binding parameters, and in a change of the general character of binding for both ligands.