Evidence for Adrenergic Transmission of the Carotid Chemoreceptor Impulses in the Toad

Abstract

RECENTLY when recording afferent impulse activity in the nerve supplying the carotid labyrinth in the toad, we have found chemoreceptor fibres which show increased discharge in response to hypoxia just as do those of the mammalian carotid body1. Although acetylcholine has been proposed as the normal chemical transmitter of the mammalian glomus nerve-endings2–8 the evidence is not as yet wholly conclusive. Indeed, electron microscopy has revealed that the mammalian glomus epithelioid cells show numerous catecholamine-containing granules and the opinion has been advanced that the catecholamine may be the transmitter substance3. There is little physiological evidence for this, however, and even noradrenaline itself does not stimulate chemoreceptor discharge6. Based on the standpoint that the adrenaline-like substance produced in the body is not necessarily solely identifiable with noradrenaline, we have tried to stimulate carotid chemoreceptor discharge by electrical excitation of the cardiac sympathetic fibres. Our results indicate that an adrenergic mechanism is responsible for chemosensory transmission.