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- Title
Mechanisms of Na uptake, ammonia excretion, and their potential linkage in native Rio Negro tetras ( Paracheirodon axelrodi, Hemigrammus rhodostomus, and Moenkhausia diktyota).
- Authors
Wood, Chris; Robertson, Lisa; Johannsson, Ora; Val, Adalberto
- Abstract
Mechanisms of Na uptake, ammonia excretion, and their potential linkage were investigated in three characids (cardinal, hemigrammus, moenkhausia tetras), using radiotracer flux techniques to study the unidirectional influx ( J), efflux ( J), and net flux rates ( J) of Na and Cl, and the net excretion rate of ammonia ( J). The fish were collected directly from the Rio Negro, and studied in their native 'blackwater' which is acidic (pH 4.5), ion-poor (Na, Cl ~20 µM), and rich in dissolved organic matter (DOM 11.5 mg C l). J, J, and J were higher than in previous reports on tetras obtained from the North America aquarium trade and/or studied in low DOM water. In all three species, J was unaffected by amiloride (10 M, NHE and Na channel blocker), but both J and J were virtually eliminated (85-99 % blockade) by AgNO (10 M). A time course study on cardinal tetras demonstrated that J blockade by AgNO was very rapid ( <5 min), suggesting inhibition of branchial carbonic anhydrase (CA), and exposure to the CA-blocker acetazolamide (10 M) caused a 50 % reduction in J. Additionally, J was unaffected by phenamil (10 M, Na channel blocker), bumetanide (10 M, NKCC blocker), hydrochlorothiazide (5 × 10 M, NCC blocker), and exposure to an acute 3 unit increase in water pH. None of these treatments, including partial or complete elimination of J (by acetazolamide and AgNO respectively), had any inhibitory effect on J. Therefore, Na uptake in Rio Negro tetras depends on an internal supply of H from CA, but does not fit any of the currently accepted H-dependent models (NHE, Na channel/V-type H-ATPase), or co-transport schemes (NCC, NKCC), and ammonia excretion does not fit the current 'Na/NH exchange metabolon' paradigm. Na, K-ATPase and V-type H-ATPase activities were present at similar levels in gill homogenates, Acute exposure to high environmental ammonia (NHCl, 10 M) significantly increased J, and NH was equally or more effective than K in activating branchial Na,(K) ATPase activity in vitro. We propose that ammonia excretion does not depend on Na uptake, but that Na uptake (by an as yet unknown H-dependent apical mechanism) depends on ammonia excretion, driven by active NH entry via basolateral Na,(K)-ATPase.
- Subjects
RIO Negro (Argentina); SODIUM; AMMONIA; PARACHEIRODON; TETRAS; HEMIGRAMMUS; CHARACIDAE; RADIOACTIVE tracers
- Publication
Journal of Comparative Physiology B: Biochemical, Systemic & Environmental Physiology, 2014, Vol 184, Issue 7, p877
- ISSN
0174-1578
- Publication type
Article
- DOI
10.1007/s00360-014-0847-7