Cardiovascular and respiratory reflexes in the tropical fish, traira (Hoplias malabaricus): CO2/pH chemoresponses.

To examine the distribution and physiological role of CO2/pH-sensitive chemoreceptors in the gills of the tropical fish, traira (Hoplias malabaricus), fish were exposed to acute environmental hypercarbia (1.25, 2.5 and 5.0% CO2 in air) and subjected to injections of HCl into the ventral aorta and buccal cavity. This was done before and after selective denervation of branchial branches of the IXth and Xth cranial nerves to various gills arches. Hypercarbia produced a significant decrease in heart rate, a mild hypotension as well as increases in both ventilation rate and ventilation amplitude. The data suggest that the hypercarbic bradycardia and increase in ventilation frequency arise from receptors exclusively within the gills but present on more than the first gill arch, while extra-branchial receptors may also be involved in controlling the increase in ventilation amplitude. With the exception of a decrease in heart rate in response to HCl injected into the ventral aorta, the acid injections (internal and external) did not mimic the cardiorespiratory responses observed during hypercarbia suggesting that changes in CO2 are more important than changes in pH in producing cardiorespiratory responses. Finally, the data indicate that chemoreceptors sensitive to CO2/pH and to O2 in the gills of this species involved in producing ventilatory responses are distributed in a similar fashion, but that those involved in producing the bradycardia are not.