Evidence for Two Oxygen-Sensitive Chemoreceptor Loci in Channel Catfish,Ictalurus punctatus

Sodium cyanide (NaCN) was used as a chemical probe to help localize externally and internally oriented oxygen-Sensitive chemoreceptors and to identify the reflex effects they control in anesthetized, spontaneously breathing channel catfish (Ictalurus punctatus). Fish responded rapidly to NaCN (500μg) given in the inspired water flow (external NaCN) with transient bradycardia and a more prolonged stimulation of gill ventilation frequency (fg) and opercularpressure amplitude (POP). Internal injections of NaCN (50 μg), given via the dorsal aorta, stimulated fg and POP after about a 42-s latency but had no effect on heart rate (fb). Injections of NaCN (50 μg) into the ventral aorta significantly reduced the latency of response, stimulating POP and fg in 9–16s. Cardiovascular and ventilatory variables returned topreinjection levels within 30 min. These results suggest that cardiovascular and ventilatory reflex responses to hypoxia in channel catfish are mediated by both externally and internally oriented O₂-sensitive chemoreceptors. Externally oriented chemoreceptors appear to monitor the O2 tension of the inspired water and elicit bradycardia and hyperpnea when stimulated by aquatic hypoxia or external NaCN. Internal O2-sensitive chemoreceptors appear to respond to changes in blood O2 stimulus levels, in orjust downstream from the gills, and reflexively increase gill ventilation when stimulated by hypoxemia.