Nitrogen metabolism, acid–base regulation, and molecular responses to ammonia and acid infusions in the spiny dogfish shark (Squalus acanthias)
Although they are ureotelic, marine elasmobranchs express Rh glycoproteins, putative ammonia channels. To address questions raised by a recent study on high environmental ammonia (HEA) exposure, dogfish were intravascularly infused for 24 h at 3 ml kg−1 h−1 with isosmotic NaCl (500 mmol l−1, control), NH4HCO3 (500 mmol l−1), NH4Cl (500 mmol l−1), or HCl (as 125 mmol l−1 HCl + 375 mmol l−1 NaCl). While NaCl had no effect on arterial acid–base status, NH4HCO3 caused mild alkalosis, NH4Cl caused strong acidosis, and HCl caused lesser acidosis, all predominantly metabolic in nature. Total plasma ammonia (TAmm) and excretion rates of ammonia (JAmm) and urea-N (JUrea-N) were unaffected by NaCl or HCl. However, despite equal loading rates, plasma TAmm increased to a greater extent with NH4Cl, while JAmm increased to a greater extent with NH4HCO3 due to much greater increases in blood-to-water PNH3 gradients. As with HEA, both treatments caused large (90 %) elevations of JUrea-N, indicating that urea-N synthesis by the ornithine-urea cycle (OUC) is driven primarily by ammonia rather than HCO3−. Branchial mRNA expressions of Rhbg and Rhp2 were unaffected by NH4HCO3 or NH4Cl, but v-typeH+-ATPase was down-regulated by both treatments, and Rhbg and Na+/H+ exchanger NHE2 were up-regulated by HCl. In the kidney, Rhbg was unresponsive to all treatments, but Rhp2 was up-regulated by HCl, and the urea transporter UT was up-regulated by HCl and NH4Cl. These responses are discussed in the context of current ideas about branchial, renal, and OUC function in this nitrogen-limited predator.