PHENYLETHANOLAMINOTETRALINES COMPETE WITH [H-3] DIHYDROALPRENOLOL BINDING TO RAT COLON MEMBRANES WITHOUT EVIDENCING ATYPICAL BETA-ADRENERGIC SITES

[H-3]Dihydroalprenolol ([H-3]DHA) specific binding (determined by the difference in the presence and absence of 20-mu-M (-)isoprenaline) to rat colon membranes was saturable (B(max) = 39.6 fmol/mg protein), of high affinity (K(d) = 0.87 nM) and stereospecific (IC50 330 and 3510 nM for (-)- and (+)isoprenaline, respectively); the Hill coefficient was close to one, indicating binding homogeneity. [H-3]DHA (0.6 nM) specific binding was potently inhibited (K(i) range 1.9-3.3 nM) by the non-selective beta-adrenoceptor antagonists pindolol, alprenolol and propranolol, but not by the non-adrenergic compounds 5-hydroxytryptamine, 8-hydroxydipropylaminotetraline, methysergide, dopamine and verapamil (K(i) > 10,000 nM). The selective beta-1- and beta-2-adrenoceptor antagonists CGP 20,712A and ICI 118,551 resulted in biphasic competition binding curves, whose low and high affinity components were compatible with two populations of binding sites accounting for about 75 (beta-2) and 25% (beta-1) of total sites. The relative competing potencies of reference adrenergic agonists also suggested a prevalence of beta-2-adrenergic sites. The new agonists phenylethanolaminotetralines (PEATs), highly selective for the atypical beta-adrenoceptors whose abundance in rat colon has been confirmed by comprehensive functional studies, had variable affinity for the [H-3]DHA-labelled sites depending on chirality, but with no substantial correlation with their pharmacological potency. Only 40% of [H-3]DHA binding, at a concentration about 10 times its K(d) for high affinity sites (beta-1 and beta-2), was prevented by saturating concentrations of isoprenaline. Under this condition, the representative PEAT, SR 58611A, highly potent and selective for atypical beta-adrenoceptors in functional tests, and its pharmacologically inactive enantiomer, both inhibited the residual binding equipotently. In conclusion, [H-3]DHA binding did not detect atypical beta-adrenoceptor sites in rat colon membranes, most probably because of its weaker affinity for them than. for the coexisting beta-1 and beta-2 sites. PEAT stereoisomers proved essential for assessing both the stereospecificity and the functional significance of this atypical binding and to compare their affinity for [H-3]DHA-labelled sites and pharmacological potency.