Efferent innervationt the chemoreceptor of the carotid labyrinth of 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.