Feeding through your gills and turning a toxicant into a resource: how the dogfish shark scavenges ammonia from its environment.
Nitrogen (N) appears to be a limiting dietary resource for elasmobranchs, required not only for protein growth but also for urea-based osmoregulation. Building on recent evidence that the toxicant ammonia can be taken up actively at the gills of the shark and made into the valuable osmolyte urea, we demonstrate that the uptake exhibits classic Michaelis-Menten saturation kinetics with an affinity constant (Km) of 379 µmol l-1, resulting in net N retention at environmentally realistic ammonia concentrations (100-400 µmol l-1) and net N loss through stimulated urea-N excretion at higher levels. Ammonia-N uptake rate increased or decreased with alterations in seawater pH, but the changes were much less than predicted by the associated changes in seawater PNH3 , and more closely paralleled changes in seawater NH4+ concentration. Ammonia-N uptake rate was insensitive to amiloride (0.1 mmol l-1) or to a 10-fold elevation in seawater K+ concentration (to 100 mmol l-1), suggesting that the mechanism does not directly involve Na+ or K+ transporters, but was inhibited by blockade of glutamine synthetase, the enzyme that traps ammonia-N to fuel the ornithine-urea cycle. High seawater ammonia inhibited uptake of the ammonia analogue [14C]methylamine. The results suggest that branchial ammonia-N uptake may significantly supplement dietary N intake, amounting to about 31% of the nitrogen acquired from the diet. They further indicate the involvement of Rh glycoproteins (ammonia channels), which are expressed in dogfish gills, in normal ammonia-N uptake and retention.