Examination of ionoregulatory characteristics of South American cichlids

We examined ionoregulatory traits of four cichlid species from South America, oscars (Astronotus ocellatus), Tapajos cichlids (Geophagus sp.), Macmaster's dwarf cichlids (Apistogramma macmasteri), and keyhole cichlids (Cleithracara maronii), all inhabitants of ion-poor waters. K_m values for Na⁺ transport in fish held in 100 μmol L^-1 Na⁺ water ranged from 49 to 143 μmol L^-1, and J_max values spanned 450 to 1205 nmol g^-1 h^-1. After one month in 1000 μmol L^-1 Na⁺ water, kinetic parameters for Na⁺ uptake in three of the four species acclimated, but only oscars displayed the "typical" pattern of higher K_m and lower J_max values. Low pH water inhibited Na⁺ uptake (J_in^Na) in all, and stimulated Na⁺ efflux (J_out^Na) 2.5 to 3.5-fold in three of the four species. Oscars alone had had a measurable J_in^Na at pH 3.5 and no stimulation of J_out^Na. We measured J_in^Na in oscars and keyhole cichlids during exposure to 100 μmol L^-1 Ethoxzolemide (EZ), an inhibitor of carbonic anhydrase, and 1 mmol L^-1 NH₄Cl (HEA). EZ inhibited J_in^Na by about 50% and HEA reduced J_in^Na by 80-90%. These results suggest that Na⁺ uptake involves H⁺ extrusion. Acute exposure to 1 μmol L^-1 CuSO₄ and 60 nmol L^-1 AgNO₃ inhibited J_in^Na in both species by 30-85%. Exposure of oscars to 5-fold higher concentrations of both metals did not further inhibit J_in^Na, but it did stimulate J_out^Na 50-150%. The response to metals of both species are similar to other species that have been examined.