α‐ADRENOCEPTORS IN THE MOUSE VAS DEFERENS AND THEIR EFFECTS ON ITS RESPONSE TO ELECTRICAL STIMULATION

Abstract

Noradrenaline (ID50, 0.75 μM) 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. Phenylephrine (1.0‐3.0 μM) produced a dose‐related contraction of the mouse isolated vas deferens and potentiated the responses to field stimulation. 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. Thymoxamine (0.3 μM) antagonized the effects of phenylephrine, but not those of clonidine. From a consideration of the known characteristics of pre‐ and postsynaptic α‐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. 1978 British Pharmacological Society