RESUMO
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. Km values for Na+ transport in fish held in 100⯵molâ¯L-1 Na+ water ranged from 49 to 143⯵molâ¯L-1, and Jmax values spanned 450 to 1205â¯nmolâ¯g-1â¯h-1. After one month in 1000⯵molâ¯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 Km and lower Jmax values. Low pH water inhibited Na+ uptake (JinNa) in all, and stimulated Na+ efflux (JoutNa) 2.5 to 3.5-fold in three of the four species. Oscars alone had had a measurable JinNa at pHâ¯3.5 and no stimulation of JoutNa. We measured JinNa in oscars and keyhole cichlids during exposure to 100⯵molâ¯L-1 Ethoxzolemide (EZ), an inhibitor of carbonic anhydrase, and 1â¯mmolâ¯L-1 NH4Cl (HEA). EZ inhibited JinNa by about 50% and HEA reduced JinNa by 80-90%. These results suggest that Na+ uptake involves H+ extrusion. Acute exposure to 1⯵molâ¯L-1 CuSO4 and 60â¯nmolâ¯L-1 AgNO3 inhibited JinNa in both species by 30-85%. Exposure of oscars to 5-fold higher concentrations of both metals did not further inhibit JinNa, but it did stimulate JoutNa 50-150%. The response to metals of both species are similar to other species that have been examined.