Branchial receptors and cardiorespiratory reflexes in a neotropical fish, the tambaqui (Colossoma macropomum)

This study examined the location and physiological roles of branchial chemo receptors involved in the cardiorespiratory responses to hypoxia and hyperc arbia in a neotropical fish that exhibits aquatic surface respiration, the tambaqui (Colossoma macropomum). Fish were exposed to abrupt progressive en vironmental hypoxia (18.6-1.3 kPa water P-O2) and hypercarbia (water equili brated with 5 % CO2 in air, which lowered the water pH from 7.0 to 5.0). Th ey were also subjected to injections of NaCN into the ventral aorta (to sti mulate receptors monitoring the blood) and buccal cavity (to stimulate rece ptors monitoring the respiratory water). All tests were performed before an d after selective denervation of branchial branches of cranial nerves IX an d X to the gill arches, The data suggest that the O-2 receptors eliciting r eflex bradycardia and increases in breathing frequency are situated on all gill arches and sense changes in both the blood and respiratory water and t hat the O-2 receptors triggering the elevation in systemic vascular resista nce, breathing amplitude, swelling of the inferior lip and that induce aqua tic surface respiration during hypoxia are extrabranchial, although branchi al receptors also contribute to the latter two responses. Hypercarbia also produced bradycardia and increases in breathing frequency, as well as hyper tension, and, while the data suggest that there may be receptors uniquely s ensitive to changes in CO2/pH involved in cardiorespiratory control, this i s based on quantitative rather than qualitative differences in receptor res ponses. These data reveal yet another novel combination for the distributio n of cardiorespiratory chemoreceptors in fish from which teleologically sat isfying trends have yet to emerge.