Summary

Ammonia (NH₃) is only slightly more soluble (+3.4%) in the plasma of rainbow trout than in water, and its pK' is only 0.14 units higher than in fresh water at 15 °C. Determination of these physico-chemical parameters together with measurements of blood and water pH and total ammonia concentrations allowed calculation of the mean resting partial pressure gradient across the gills (54 × 10⁻⁶Torr) and estimation of the gill permeability coefficient (D = 1.3 × 10⁻⁵cm⁻¹). Under normal resting conditions of low external NH₃ and pH, diffusive movement of NH₃ appears to account adequately for ammonia excretion in the rainbow trout; 90 ± 10% of the excreted ammonia appears to originate from the blood, rather than from de novo synthesis of ammonia in the gills. During the high external ammonia treatments, the fish reached a steady state with a net inward ammonia gradient, which could be accounted for by a counter-balancing Na⁺/NH₄⁺ exchange. Ammonium salt infusions or injections which stimulate Na⁺ influx are not sufficient demonstrations of Na⁺/NH₄⁺ exchange, since acidosis is produced, and alternate interpretations of the Na⁺ flux stimulation are possible, such as enhanced Na⁺/H⁺ exchange.