Investigation into the Mechanism of Ion Transport in Rio Negro Characiformes

Four fish species within the Characiform Order [(Rosy tetras (Hyphessobrycon rosaceus), Serpae tetras (Hyphessobrycon eques), Emperor tetras (Nematobrycon palmeri) and Penguin tetras (Thayeria obliqua)] are native to the ion-poor, acidic waters of the Rio Negro. In this paper, we focus on Wood’s revised Na+/NH4+ exchange model to gain better insight into the mechanisms of ion transport within the Characids. We exposed each species to two experimental treatments: (i) 1mM ammonia (high external ammonia [HEA]), to test coupling of Na+ uptake and ammonia excretion, and (ii) 100μM Acetazolamide (carbonic anhydrase inhibitor), to test coupling of Na+ uptake and H+ excretion. With the HEA treatment, three out of the four species resulted in neither a stimulation nor inhibition of Na+ uptake, indicating no loose coupling involved between NH4+ excretion and Na+ uptake. As for the Acetazolamide exposure, three out of the four species showed no significant difference in Na+ uptake, showing that Na+ uptake is not dependent on the protons provided from carbonic anhydrase. Thus, Wood’s Na+/ NH4+ exchange model is unsupported in these Characiformes