THE EFFECTS OF (+/-)-IDAZOXAN AND ITS STEREOISOMERS ON MOUSE VAS-DEFERENS MOTILITY INVITRO - A COMPARISON WITH YOHIMBINE

The effects of idazoxan (IDZ) and its stereoisomers were compared to that of a classical alpha2-antagonist, yohimbine (YOH), on para-aminoclonidine (PAC)- and norepinephrine (NE)-mediated inhibition of the twitch response evoked in the mouse vas deferens by low-frequency (0.1 Hz) field stimulation. (+/-)-IDZ and (+)-IDZ antagonized the inhibitory effects of PAC, (+)-IDZ being twice as potent as (+/-)-IDZ; in contrast, antagonism by (-)-IDZ failed to meet Schild criteria for a competitive mechanism. YOH completely reversed the inhibition of twitch response induced by NE, but not that induced by PAC; in the latter case, residual inhibition was almost fully reversed by (+)-IDZ and to a lesser extent by (+/-)-IDZ, while (-)-IDZ proved ineffective. These results provide pharmacological evidence of alpha2-receptor heterogeneity at the vas deferens level. A possible additional mechanism involving imidazoline binding sites is discussed.