Mechanisms of Na+ uptake, ammonia excretion, and their potential linkage in native Rio Negro tetras (Paracheirodon axelrodi, Hemigrammus rhodostomus, and Moenkhausia diktyota)

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 (Jin), efflux (Jout), and net flux rates (Jnet) of Na+ and Cl−, and the net excretion rate of ammonia (JAmm). 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−1). Jin Na, Jin Cl, and JAmm 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, Jin Na was unaffected by amiloride (10−4 M, NHE and Na+ channel blocker), but both Jin Na and Jin Cl were virtually eliminated (85–99 % blockade) by AgNO3 (10−7 M). A time course study on cardinal tetras demonstrated that Jin Na blockade by AgNO3 was very rapid (<5 min), suggesting inhibition of branchial carbonic anhydrase (CA), and exposure to the CA-blocker acetazolamide (10−4 M) caused a 50 % reduction in Jin Na .. Additionally, Jin Na was unaffected by phenamil (10−5 M, Na+ channel blocker), bumetanide (10−4 M, NKCC blocker), hydrochlorothiazide (5 × 10−3 M, NCC blocker), and exposure to an acute 3 unit increase in water pH. None of these treatments, including partial or complete elimination of Jin Na (by acetazolamide and AgNO3 respectively), had any inhibitory effect on JAmm. 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+/NH4 + 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 (NH4Cl, 10−3 M) significantly increased Jin Na, and NH4 + 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 NH4 + entry via basolateral Na+,(K+)-ATPase. © 2014, Springer-Verlag Berlin Heidelberg.