Evidence against adrenergic motor transmission in the guinea-pig vas deferens

Abstract

1. Field stimulation of desheathed preparations of guinea-pig vas deferens, treated with a ganglion-blocking agent, has revealed the presence of two tetrodotoxin-susceptible components in the motor response, suggesting the existence of two sets of post-ganglionic motor nerve fibres of different excitability: one set responding maximally to pulses of 0.1-0.4 msec; the other, to pulses of 2 msec. No distinction could be made pharmacologically between the two components.2. Cooling potentiated that component in the twitch-responses which was due to stimulation of the more excitable fibres.3. The sensitivity of the longitudinal muscle to the motor action of noradrenaline was low and was subject to considerable animal variation. But normal responses to post-ganglionic field stimulation were elicited in noradrenaline-insensitive preparations, in which the twitches elicited by 5 pulses could not be matched with noradrenaline, even 100-125 mug/ml.4. In some forty experiments, small doses of noradrenaline inhibited the twitch-responses evoked by either set of motor fibres. This inhibition differed from that produced by isoprenaline in two respects. Firstly, propranolol did not antagonize the noradrenaline inhibition, thus excluding an action on beta-adrenoceptors; and secondly, noradrenaline did not depress contractions elicited by muscarine or by 5-methylfurmethide.5. Phenoxybenzamine, 10(-6) g/ml., produced a thousandfold reduction in the sensitivity of the muscle to the motor action of noradrenaline, without any decrease in the height of the twitches elicited by 0.1 or 1 msec pulses.6. The twitch-responses were not affected by combined alpha + beta adrenoceptor blockade with phentolamine and propranolol.7. Tyramine, amphetamine, tranylcypromine and prostaglandin E(2) inhibited the twitches but potentiated the contractile effect of noradrenaline.8. The twitch-responses and their inhibition by noradrenaline were present in preparations taken from reserpinized animals.9. Although the twitch-responses could be paralysed by bretylium or guanethidine, the foregoing results excluded adrenergic transmission at the motor endings. Cholinergic transmission was also excluded by negative findings with anticholinesterases, atropine, nicotine and (+)-tubocurarine.10. Motor transmission by histamine, 5-hydroxytryptamine, gamma-aminobutyric acid or ATP was also excluded.