title: Ion regulation in ion-poor acidic water by the blackskirt tetra (Gymnocorymbus ternetzi), a fish native to the Amazon River.
abstract: We examined the ionoregulatory capabilities of the blackskirt tetra (Gymnocorymbus ternetzi), which is native to ion-poor acidic waters of the Amazon River. Examination of Na+ uptake, which was only slightly sensitive to the uptake blocker amiloride, revealed several specializations for uptake in these waters. Kinetic analysis of Na+ uptake (at pH 6.5) revealed a high maximum rate of uptake and a low Michaelis-Menten constant, which allows the tetras to take up Na+ at high rates even at very low water levels. At pH 4.5, a pH where they experience sizable ion disturbances, they displayed several mechanisms to restore balance. Kinetic analysis at pH 4.5 revealed that the maximum uptake rate rose 67% while the Michaelis-Menten constant remained unchanged. Further tests showed that the upregulation of Na+ uptake occurred within 12 h in response to a doubling of Na+ efflux. Despite these specializations of the Na+ uptake mechanism, blackskirt tetras were not especially tolerant of low pH. Upon exposure to pH 4.0, they experienced a massive loss of Na+ due to a fourfold increase of Na+ efflux (relative to pH 5.0) and an 80% inhibition of uptake. Measurement of Na+ efflux in waters with different Ca2+ levels and in the presence of LaCl, a strong Ca2+ competitor, correlated the stimulation of Na+ efflux at low pH with a low branchial affinity for Ca2+. These tests indicate that blackskirt tetras possess abilities to resist the disruptive effects of moderately low pH but cannot survive in waters with a pH of 4.0 or less because of leaching of Ca2+ from branchial tight junctions, which stimulates ion losses.
authors.affiliation: Department of Biology, University of San Diego, California 92110, USA. gonzalez@acusd.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: Gonzalez
authors.forename: V M
authors.initials: VM
authors.lastname: Dalton
authors.forename: M L
authors.initials: ML
authors.lastname: Patrick
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004357:Drug Synergism; D005399:Fishes / Q000378:metabolism* / Q000502:physiology*; D005618:Fresh Water / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D007811:Lanthanum / Q000494:pharmacology; D009994:Osmolar Concentration; D012964:Sodium / Q000494:pharmacology
chemical_list: C028521:lanthanum chloride; D007811:Lanthanum; D000584:Amiloride; D012964:Sodium; D002118:Calcium