PHARMACOLOGICAL ANALYSIS OF NEURALLY INDUCED INHIBITION OF CAROTID-BODY CHEMORECEPTOR ACTIVITY IN CATS

Abstract

Experiments were performed to determine the mechanism by which centrifugal impulses in the carotid sinus nerve (CSN) reduce the frequency of impulse traffic in afferent chemoreceptor fibers from the carotid body in cats. Recordings of chemoreceptor activity were made from single- or few-fiber preparations dissected off the CSN, while the remainder of the CSN was stimulated electrically to produce neurally induced inhibition of chemoreceptor activity. Various drugs were injected either i.v. or directly into the arterial blood supply to the carotid body. Catecholamines (dopamine, norepinephrine and epinephrine) inhibited spontaneous chemoreceptor activity, and .alpha. adrenergic antagonists abolished both this inhibition and that produced by electrical stimulation of the CSN in the same preparation. Atropine, but not nicotinic antagonists of acetylcholine, consistently blocked neurally induced inhibition but not that produced by catecholamines. Muscarinic agonists had no effect on spontaneous chemoreceptor activity. Centrifugal activity in the CSN causes release of endogenous catecholamines in the carotid body, and these catecholamines mediate neurally induced inhibition of chemoreceptor activity. These results are not compatible with the proposal that neurally induced inhibition of chemoreceptor activity is due to the vasomotor effects of acetylcholine.