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 (fH; 101±35% increase) and respiratory rate (fR; 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 either the antagonist to ACh (atropine) and/or 5-HT (methysergide). Finally, using immunohistochemistry we demonstrated the presence of putative chemoreceptive cells immuno-positive 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 for the existence of dispersed chemoreceptor cells at the first carotid bifurcation in the central cardiovascular area in lizards with similar properties to known carotid body homologs, adding to the picture of chemoreceptor evolution in vertebrates.
