AIM To study the antagonistic effect of 1-(2,6-dimethylphenox)-2-(3,4-dimethylphenyl ethylamino) propane hydrochloride (DDPH) on α 1-adrenoceptor (AR). METHODS Radioligand binding assay was used. Specific 125 I-BE2254 (2-β(4-hydroxyphenyl)-ethyl aminomethyl-tetralone) binding was measured by incubating membrane of rat cerebral cortex, spleen and the three cloned α 1-AR subtypes (α 1A , α 1B , α 1D ) stably expressed in human embryonic kidney 293 cell preparation with a single concentration of 125 I-BE2254 in the presence of 14 concentrations of DDPH. Equilibrium binding constant ( K I) and Hill coefficients ( n H) were determined from Hill plots. The -log value of the K I was expressed as p K I. Contractile responses of isolated rat aorta, renal artery ring and spleen were determined. The pA 2 values for DDPH in competitively inhibiting NE-stimulated contraction of tissues were measured with the method of Ainlakshana and Schild. RESULTS DDPH competitively inhibited binding of 125 I-BE2254 to α 1-AR in a concentration-dependent manner. The p K I values for DDPH in rat cerebral cortex and spleen were 7 17±0 06 and 7 41±0 11, respectively, and the Hill efficiency values were not significantly different from unit.
The p K I values for cloned α 1A , α 1B and α 1D -AR were 7 21±0 12, 6 88±0 04 and 7 26±0 06, respectively, and the Hill efficiency values were not significantly different from unit. Contractile studies showed that DDPH competitively antagonized the NE concentration-response curve with a pA 2 values of 7 40±0 23 in aorta, 7 41±0 04 in renal artery and 7 63±0 07 in spleen and the slopes of schild plot were not significantly different from unit. The p K I values for DDPH in tissues and the cloned α 1A or α 1D -AR were shown to fit well in with the pA 2 values in antagonizing NE-induced constriction in rat isolated aorta, renal artery and spleen. CONCLUSION These results suggest that DDPH appear to be a non-subtype selective competitive antagonist for α 1-AR.