Evidence for adrenergic transmission of the carotid chemoreceptor impulses in the toad

Functional characteristics of arterial chemoreceptors in an amphibian (Bufo marinus)

In order to further describe the functional characteristics of arterial chemoreceptors of anuran amphibians, multi-unit chemoreceptor discharge frequency (MCDF) was recorded from the carotid (N = 23) or aortic nerve (N = 2) of pithed, unidirectionally ventilated toads (Bufo marinus). MCDF increased with decreasing PaO2: typically, the threshold PaO2 lay between 40 and 60 mmHg. The MCDF-PaO2 relationship was right-shifted along the PaO2 axis by increasing PaCO2 (N = 5). In three toads, the MCDF-PaO2 relationship was unaffected when CaO2 was reduced 35-89% by hemorrhage. MCDF was also unaffected by occlusion of the outflow from the heart, though it increased upon release of the occlusion. MCDF was stimulated by epinephrine, and inhibited by dopamine. Our results demonstrate that the MCDF responds to the range of PaO2 and PaCO2 values encountered in vivo, suggesting that arterial chemoreceptors may participate in ventilatory control in toads. The receptors do not respond to the rate of oxygen delivery per se, and may be influenced by catecholamines known to exist in the carotid labyrinth.

Ultrastructure of the human carotid body. A perspective on the mode of chemoreception

Surgically excised human carotid bodies have been analyzed by electron microscopy. Several ultrastructural features suggest that they are closely related to sympathetic neuroendocrine glands and autonomic ganglia: the chief cells synthesize intensely osmiophilic cytoplasmic granules, occasionally develop neuroid processes, and are intimately associated with neurilemmal sustentacular cells. Other lines of evidence indicate that secretion of the chief cell is norepinephrine or a closely related bioamine-neurotransmitter. The role of chief cells in mediating chemosensation may be explained by modulated release of the bioamine in response to local metabolic conditions. Human chief cells are richly innervated. Recognizable zones of synapse display an efferent type of polarity and sometimes appear adrenergic. Thus, reflexive sympathetic excitation of chief cells may be physiologically significant in man. The anatomic compartmentalization characteristic of carotid body chief cells is discussed in relation to a possible functional significance.

The content and localization of catecholamines in the carotid labyrinths and aortic arches of Rana temporaria

1. The catecholamine content of the carotid labyrinth of Rana temporaria was estimated by two different methods, and compared with that of the aortic arch. In both tissues adrenaline was found as the dominant amine, with traces of dopamine and noradrenaline detectable in the labyrinth only.2. Per gram of fresh tissue, the labyrinth usually contained more adrenaline than the aortic arch.3. On microscopic examination, the aortic arch was found to contain nerve fibres which fluoresced after treatment with formaldehyde, whereas within the carotid labyrinth the formaldehyde-induced fluorescence was localized in the cytoplasm of clusters of small cells. This fluorescence resembled that exhibited by glomus cells in the mammalian carotid body.