The release of acetylcholine from carotid body tissues. Further study on the effects of acetylcholine and cholinergic blocking agents on the chemosensory discharge.

1. Both carotid bodies were removed from cats and placed in a small Perspex channel through which Locke solution was allowed to flow under a layer of paraffin oil. Stimulation of the upstream (;donor') organ elicited an increased sensory discharge in the downstream (;detector') preparation (Loewi effect).2. This effect was enhanced by eserine, depressed by hexamethonium and blocked by either mecamylamine or acetylcholinesterase. The Loewi effect did not disappear when chronically denervated (4 days) ;donor' carotid bodies were stimulated.3. These experiments led to the conclusion that ACh is released by the carotid body tissues (probably from the glomus cells) during stimulation and that this substance is responsible for the initiation of the chemosensory discharges.4. In single preparations the chemosensory endings proved to be very sensitive to ACh especially in the presence of eserine which, in all probability, inactivated the tissue cholinesterase. Curarizing agents such as mecamylamine, hexamethonium, (+)-tubocurarine and atropine blocked the response of the sensory endings to applied ACh in what appeared to be ;surmountable' antagonism.