Search

1 Noradrenaline (ID50, 0.75 micrometer) and clonidine (ID50, 2.8 nM) produced a dose-related inhibition of the twitch response of the isolated vas deferens of the mouse to electrical stimulation, their effectiveness decreasing as frequency of stimulation increased from 0.2 to 16 hertz. 2 Phenylephrine (1.0--2.0 micrometer) produced a dose-related contraction of the mouse isolated vas deferens and potentiated the responses to field stimulation. 3 Yohimbine (10 nM) antagonized the inhibitory effects of noradrenaline and clonidine, but had no effect on the motor activity of phenylephrine. At a concentration of 128 nM yohimbine potentiated the twitch response by 110% at 1 Hz, but its effectiveness decreased with increasing frequency of stimulation up to 16 hertz. 4 Thymoxamine (0.3 micrometer) antagonized the effects of phenylephrine, but not those of clonidine. 5 From a consideration of the known characteristics of pre- and postsynaptic alpha-adrenoceptors, it is concluded that the inhibitory effect of noradrenaline is produced by stimulation of the former and the effects of phenylephrine by stimulation of the latter.