CO₂ DETECTION, TAMBAQUI STYLE

Regulating your breathing can be tricky if you're a tambaqui swimming in the Amazon basin, where water CO₂ levels change dramatically with the seasons. Wondering how the fish track these changes, Katie Gilmour and Steve Perry at the University of Ottawa decided to take a closer look at the location of tambaqui's CO₂ receptors(p. 1095).

Gilmour and Perry travelled to Tadeu Rantin's lab in Brazil, where tambaqui experts Bill Milsom and Steve Reid joined the team. Collecting some tambaqui from a local fish farm, they set out to discover the orientation of tambaqui's CO₂ receptors. The team reasoned that if tambaqui have internally oriented receptors, the fish should detect rising blood CO₂ levels,but if they have externally oriented receptors, tambaqui should respond to CO₂ levels in the surrounding water instead.

First, the team needed to prepare the fish so that they could measure tambaqui's cardiorespiratory responses to changing blood and water CO₂ levels. They created an external blood flow loop, pulling blood from the artery that runs along the fish's back and returning it to the body,and inserted electrodes into the shunted bloodstream to take continuous blood gas measurements. But tambaqui have bony projections associated with their vertebral column, tucked away under the back muscles, and Gilmour remembers that it took `blood, sweat and tears to work cannula between these bones to set up the blood shunt'. Finally, they were ready to see how the fish handle fluctuating CO₂ levels.

They began by testing tambaqui's responses to internal CO₂changes. The team injected tambaqui with acetazolamide, which causes CO₂ levels to build up in the bloodstream. If tambaqui have internal receptors that detect blood CO₂ levels, the team expected to see ventilatory and cardiovascular responses in the acetazolamide-treated fish. But the treated fish didn't respond to their elevated blood CO₂ levels at all; they didn't seem to have internal receptors.

To find out if the fish detect high water CO₂ levels with external receptors instead, the team boosted water CO₂ levels in the tanks of the acetazolamide-treated fish and then rapidly dropped the CO₂ level back to normal again. If the fish have external receptors, the tambaqui's ventilatory and cardiovascular responses should track changes in water CO₂ levels. Sure enough, as the team increased water CO₂, they saw that the fish began hyperventilating and decreased their heart rate. And as water CO₂ plummeted back to normal levels, they saw that the tambaqui's ventilation and heart rate returned to normal very quickly, despite the fact that these fish still had high levels of CO₂ in their blood. Clearly, the fish were tracking changes in CO₂ levels in the water but not in their blood. `We were amazed to see how beautifully ventilation and heart rate tracked water CO₂ levels as they dropped, but didn't track blood CO₂levels at all,' Gilmour recalls.

As a final demonstration of the orientation of tambaqui's receptors, the team injected CO₂-enriched saline into the fish. They saw no response, bolstering their conclusion that tambaqui do not have internal receptors. But when they injected CO₂-enriched water into the tambaqui's mouth, where it joined the flow of ventilatory water over the gills, they were pleased to see cardiorespiratory responses in the fish,confirming that tambaqui have external receptors located on their gills. So,when CO₂ levels escalate in their Amazonian home, tambaqui's external receptors kick in straight away to help the fish cope with the respiratory challenge.