A quantitative study of the effects of cholinergic drugs on carotid chemoreceptors in the cat.

1. Conflicting qualitative evidence exists concerning the effects on chemoreceptor activity of some drugs which influence the cholinergic system. Quantitative evidence has been obtained in the present study which should resolve the conflict. 2. Experiments were performed in pentobarbitone-anaesthetized cats in which the activity of chemoreceptor units in the sinus nerve was used to assess chemoreceptor responses. The effects of drugs on responses to I.A. ACH and NaCN were determined from dose-response data obtained from several animals and expressed as mean dose ratios 3. The chemoreceptor response to ACh was slightly inhibited by atropine, alpha- and beta-bungarotoxin and HC-3, almost completely suppressed by mecamylamine, and markedly potentiated by physostigmine. 4. Concomitant responses to NaCN were unaffected by atropine, beta-bungarotoxin, mecamylamine or physostigmine. There was a slight inhibition following alpha-bungarotoxin and a potentiation after HC-3. 5. The results do not support the theory that ACh is an excitatory sensory transmitter in the carotid body.