Non-adrenergic binding sites for the "alpha 2-antagonist" [3H] idazoxan in rabbit urethral smooth muscle

In the present study, we have provided evidence that [3H] rauwolscine and [3H] idazoxan bind to different sites in rabbit urethra. The [3H] idazoxan capacity and affinity was 215 +/- 14 fmol/mg protein and 1.59 +/- 0.16 nM while [3H] rauwolscine binding parameters were 45.9 +/- 3.4 fmol/mg protein and 2.39 +/- 0.27 nM. [3H] idazoxan specific binding was inhibited only by compounds possessing an imidazoli(di)ne or a guanidinium moiety, while [3H] rauwolscine specific binding was inhibited by phenylethanolamines and classical alpha 2-antagonists. [3H] idazoxan was inhibited by KCl in a competitive and by MnCl2 in a non-competitive way, while other cations such as Na+, Li+ and Mg2+ did not inhibit [3H] idazoxan binding. Moreover, we investigated the regional distribution of [3H] idazoxan and [3H] rauwolscine along the rabbit urethra using quantitative autoradiography. Analysis of the films revealed a different distribution of these two binding sites on the urethral sections.