Exposure to pH 3.5 water has no effect on the gills of the Amazonian tambaqui (Colossoma macropomum)

Tambaqui (Colossoma macropomum) are a model species for tropical fish physiology, but details are lacking about their ionoregulatory response to acid waters. To provide specifics, we measured unidirectional Na+ fluxes in low pH waters. Sodium influx (JinNa) was uninhibited during acute exposure to pH 4.5 and 3.5, and Na efflux (JoutNa) rose only slightly at pH 3.5; net Na+ flux (JnetNa) remained positive at all pH. Similarly, during 24 h transfer to pH 3.5 JinNa, JoutNa, and JnetNa were unchanged at all times. Taking a closer look at the mechanism of Na+ transport in the gills of tambaqui we found that JinNa was uninhibited by HMA, a Na+/H+-exchanger blocker, and Benzamil, a Na+-channel inhibitor, casting doubt on their role in Na+ uptake in this fish. Measurement of Na+/K+-ATPase (NKA) and H+-ATPase (VHA) activity showed that neither changed at low pH compared to measurements at pH 6.5. Western blot analysis of ATPase expression saw no changes in amount of NKA and VHA at low pH, and immunohistochemistry showed expression of both NKA and VHA on lamellae and interlamellar region of tambaqui gills and that both proteins co-localized to the same gill cells. Location of expression also did not change in low pH water. Amazingly, tambaqui seem unaffected by pH 3.5 water, making them one of the most acid-tolerant fish species examined so far. In addition, they appear to share key ionoregulatory traits with other fish of the order Characiformes, which suggest a common origin for the ionoregulatory attributes.