The relationship between O2 chemoreceptors, cardio-respiratory reflex and hypoxia tolerance in the neotropical fish Hoplias lacerdae

The localization, distribution and orientation of O2 chemoreceptors associated with the control of cardio-respiratory responses were investigated in the neotropical, Hoplias lacerdae. Selective denervation of the cranial nerves (IX and X) was combined with chemical stimulation (NaCN) to characterize the gill O2 chemoreceptors, and the fish were then exposed to gradual hypoxia to examine the extent of each cardio-respiratory response. Changes in heart rate (fH) and ventilation amplitude (Vamp) were allied with chemoreceptors distributed on both internal and external surfaces of all gill arches, while ventilation rate (fR) was allied to the O2 chemoreceptors located only in the internal surface of the first gill arch. H. lacerdae exposed to gradual hypoxia produced a marked bradycardia (45%) and 50% increase in Vamp, but only a relatively small change in fR (32%). Thus, the low fR response yet high Vamp were in accord with the characterization of the O2 chemoreceptors. Comparing these results from H. lacerdae with hypoxia-tolerant species revealed a relationship existent between general oxygenation of the individual species environment, its cardio-respiratory response to hypoxia and the characterization of O2 chemoreceptors.