Ammonia excretion in the seawater blue crab (Callinectes sapidus) occurs by diffusion, and not Na+/NH4+ exchange

Summary A series of experiments was conducted to investigate whether ammonia is excreted across the seawater-acclimated blue crab's gills as ionized NH₄⁺ or as the free base, NH₃. The net excretion rate of ammonia was not changed by transfer of the crabs to reduced (150 mM) Na⁺ solutions, by transfer to Na⁺- and K⁺-free artificial sea water, or by the sodium transport inhibitor amiloride. Ammonia excretion, therefore, does not appear to be linked to Na⁺ uptake in these animals, and appears to take place by passive diffusion. Since ammonia could diffuse either as NH₄⁺ or NH₃, we examined two other kinds of evidence. The trans-epithelial potential was measured in sea water and the various artificial media. In spite of a 10 mV more negative potential in Na⁺-, K⁺-free medium, the ammonia excretion was not reduced. Also, in alkalinized seawater in which the partial pressure gradient of NH₃ was reduced, but the concentration gradient of NH₄⁺ increased, ammonia excretion was reduced by about 70%. These results are consistent with the conclusion that ammonia excretion takes place by diffusion of the free base, NH₃.Abbreviations SW sea water - ASW artificial sea water - t.e.p. transepithelial potentialThe University of Texas Marine Science Institute Contribution No. 461Supported by NSF Grant PCM77-24358