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Journal of Experimental Biology 144,215-233 (1989)
Published by Company of Biologists 1989

Ammonia, Urea and H⁺ Distribution and the Evolution of Ureotelism in Amphibians

CHRIS M. WOOD ¹, R. S. MUNGER ¹, and D. P. TOEWS ²

¹ Department of Biology, McMaster University Hamilton, Ontario, Canada L8S 4K1
² Department of Biology, Acadia University Wolfville, Nova Scotia, Canada BOP 1X0

In theory, the distribution of ammonia across cell membranes (Tamm_i/Tamm_e) between intracellular and extracellular fluids (ICF and ECF) may be determined by the transmembrane pH gradient (as in mammals), the transmembrane potential (as in teleost fish), or both, depending on the relative permeability of the membranes to NH₃ and NH₄⁺ (pNH₃/pNH₄⁺). The resting distributions of H⁺ (via [¹⁴C]DMO), ammonia and urea between plasma and skeletal muscle, and the relative excretion rates of ammonia-N and urea-N, were measured in five amphibian species (Bufo marinus, Ambystoma tigrinum, Rana catesbeiana, Necturus maculosus and Xenopus laevis). Although urea_i/urea_e ratios were uniformly close to 1.0, Tamm_i/Tamm_e. ratios correlated directly with the degree of ammoniotelism in each species, ranging from 9.1 (Bufo, 10% ammoniotelic) to 16.7 (Xenopus, 79% ammoniotelic). These values are intermediate between ratios of about 30 (low pNH₃/pNH₄⁺) in ammoniotelic teleost fish and about 3 (high pNH₃/pNH₄⁺) in ureotelic mammals. The results indicate that amphibians represent a transitional stage in which ammonia distribution is influenced by both the pHi-pHe gradient and the membrane potential, and that a reduction in cell membrane permeability to NH₄⁺ (i.e. increased pNH₃/pNH₄⁺) was associated with the evolution of ureotelism. Hyperosmotic saline exposure increased urea excretion 10-fold in Xenopus, while ammonia excretion remained unchanged. Tamm_i/Tamm_e fell, but this response was attributable to an abolition of the pHi-pHe gradient, rather than a physiological change in the cell membrane pNH₃/pNH₄⁺.

Key words: ammonia, urea, intracellular pH, amphibian, Bufo, Rana, Ambystoma, Necturus, Kenopus

Accepted on February 6, 1989

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