OBJECTIVE: This study is to explore the effects on specific bindings between [ 3H]RX821002, alpha-2 adrenergic receptor antagonist and alpha-2 adrenergic receptor in rat brain by G-protein modulation. METHODS: The radioligand binding receptor study was conducted with [ 3H]RX821002, a new alpha-2 adrenergic receptor antagonist, in the presence or absence of Gpp(NH)p and pertussis toxin. RESULTS: The alpha-2 adrenergic receptors were saturated with [ 3H]RX821002 in the fashion of the single binding site. The dissociation constant (Kd) was 0.70+/-0.30 nM, and maximum binding (Bmax) was 599.9+/-283.4 fmol/mg protein. The saturation study showed that the maximum binding (B max ; 668.0+/-50.1 fmol/mg protein) was increased and the dissociation constant (Kd ; 0.61+/-0.14 nM) was decreased significantly in the presence of Gpp (NH)p compared to those (B max ; 559.8+/-81.9 fmol/mg protein, Kd ; 0.87+/-0.14 nM) in the absence of Gpp (NH)p (by paired t-test ; B max, p=0.023, Kd, p=0.005). In the presence of pertussis toxin, the maximum binding (B max ; 617.0+/-58.5 fmol/mg protein) was increased significantly (by paired t-test ; B max, p=0.001) but the issociation constant (Kd ; 0.92+/-0.24 nM) was not decreased compared to those (B max ; 554.1+/-66.1 fmol/mg protein, Kd ; 0.89+/-0.24 nM) in the absence of pertussis toxin. CONCLUSION: These results confirm that the binding profiles between [ 3H]RX821002 and alpha-2 adrenergic receptors be modified by G-protein modulation. This suggests that the drug effects on receptors be influenced by various conditions such as G-protein modulation.
Animals ; Binding Sites ; Brain* ; GTP-Binding Proteins* ; Guanylyl Imidodiphosphate ; Pertussis Toxin ; Rats* ; Receptors, Adrenergic, alpha-2