Effects of optical isomers of ephedrine and methylephedrine on the twitch response in the isolated rat vas deferens and the involvement of alpha 2-adrenoceptors.

The effects of optical isomers of ephedrine (EPH) and methylephedrine (MEP) on the twitch response to electrical stimulation (1msec, 1Hz) in the isolated ratwas deferens were investigated to clarify the action on α2-adrenoceptors. l-EPH (10-7-3×10-5M) and d-EPH (10-6-10-4M) markedly inhibited the twitch response in the presence of prazosin (10-6M). l-MEP also inhibited the twitch response at high concentrations (3×10-5-10-3M). The rank order of inhibitory potency was l-EPH>d-EPHH>H>l-MEP. Yohimbine (3×10-7M), a selective α2-adrenoceptor antagonist, attenuated the twitch-inhibitory effects of EPH isomers and l-MEP. Furthermore, the twitch-inhibitory effects of EPH isomers and l-MEP were attenuated by reserpine treatment (8mg/kg, s.c.). On the other hand, d-MEP showed the potentiation of twitch response, and competitively antagonized the twitch-inhibitory effect of clonidine (10-9-10-6M) with the pA2 value of 4.3 in the presence of prazosin. These results suggest that EPH isomers and l-MEP have stimulating activity for presynaptic α2-adrenoceptors. In addition, the twitch-inhibitory effect of EPH isomers and l-MEP may be at least partly mediated through the release of noradrenaline. It is also suggested that d-MEP has competitive α2-adrenoceptor antagonist activity.