Effect of cholinergic antagonists on sympathetic ganglionic transmission of vasomotor reflexes from the carotid baroreceptors and chemoreceptors of the dog.

1. In anaesthetized dogs the reflex vascular resistance changes in a perfused hind limb were studied following carotid baroreceptor or chemoreceptor stimulation. 2. The observed rises in resistance were sympathetically mediated and thus provided a means of studying the action of the cholinergic antagonists on the sympathetic ganglion transmission. 3. The reflex response to carotid baroreceptor stimulation produced by lowering the pressure in the carotid sinuses was abolished by hexamethonium bromide but not reduced by hyoscine methyl bromide. 4. The reflex response to carotid body chemoreceptor stimulation, by hypoxia, was not altered by hexamethonium bromide but was greatly reduced by the hyoscine methyl bromide. No reflex response was seen when both antagonists were present. 5. These results indicate that sympathetic ganglion synaptic transmission during the baroreceptor reflex is mediated by nicotinic receptor activation. The transmission evoked by chemoreceptor stimulation involves muscarinic receptors with a subsidiary nicotinic pathway. High doses of an antagonist were necessary to block the muscarinic component of transmission and this is discussed in relation to previous work. 6. No non-cholinergic transmission of the reflex responses was observed.