Modulation by acetylcholine of adrenergic transmission in the rabbit ear artery

1. Low concentrations of acetylcholine (4 times 10(-11) and 1 times 10(-10) M) increase the vasoconstrictor response of the isolated ear artery of the rabbit to stimulation of the periarterial sympathetic nerves. Higher concentrations (4 times 10(-8) M and greater) decrease the response. 2. Low concentrations of acetylcholine (1 times 10(-11) and 1 times 10(-10) M) increase the stimulation-induced efflux of radioactivity from artery segments previously incubated with [3H]-noradrenaline. Higher ocncentrations (3 times 10(-8) M and greater) decrease the efflux. 3. Neither atropine nor hexamethonium affects the facilitatory action of low concentrations of acetylcholine on adrenergic transmission in the rabbit ear artery. 4. Atropine antagonizes the inhibitory effect of higher concentrations of acetylcholine on adrenergic transmission.