Evidence for a carotid body homolog in the lizardTupinambis merianae

Abstract

The homolog to the mammalian carotid body has not yet been identified in lizards. Observational studies and evolutionary history provide indirect evidence for the existence of a chemoreceptor population at the first major bifurcation of the common carotid artery in lizards, but a chemoreceptive role for this area has not yet been definitively demonstrated. We explored this possibility by measuring changes in cardiorespiratory variables in response to focal arterial injections of the hypoxia mimic sodium cyanide (NaCN) into the carotid artery of 12 unanesthetized specimens of Tupinambis merianae. These injections elicited increases in heart rate (f H ; 101±35% increase) and respiratory rate (f R ; 620±119% increase), but not mean arterial blood pressure (MAP). These responses were eliminated by vagal denervation. Similar responses were elicited by injections of the neurotransmitters acetylcholine (ACh) and serotonin (5-HT) but not norepinephrine. Heart rate and respiratory rate increases in response to NaCN could be blocked or reduced by antagonists to ACh (atropine) and/or 5-HT (methysergide). Finally, using immunohistochemistry, we demonstrate the presence of putative chemoreceptive cells immunopositive for the cholinergic cell marker vesicular ACh transporter (VAChT) and 5-HT on internal lattice-like structures at the carotid bifurcation. These results provide evidence in lizards for the existence of dispersed chemoreceptor cells at the first carotid bifurcation in the central cardiovascular area that have similar properties to known carotid body homologs, adding to the picture of chemoreceptor evolution in vertebrates.

Regulation of heart rate in vertebrates during hypoxia: A comparative overview

“…The altered responses may stem from oxygen-sensitive chemoreceptors in a variety of sites, including the carotid artery, aorta, and pulmonary artery 81,[92][93][94][95][96] that may overshadow the potential contribution from pulmonary stretch receptors. In the tegu lizard, direct stimulation of carotid body chemoreceptors with cyanide results in an increase in heart rate, 97 but because ventilation also increased, an indirect contribution from pulmonary stretch receptors remains possible.…”