An in vitro and in vivo Study of the Distribution of Ammonia Between Plasma and Red Cells of Rainbow Trout (Salmo Gairdneri)

Fish are ammoniotelic animals and therefore produce ammonia as the end-product of protein metabolism. (The term ammonia or T_amm will be used to indicate the total ammonia concentration, while NH₄⁺ and NH₃ will refer to ammonium ion and ammonia, respectively.) Ammonia is a weak base that is produced as NH₃ or NH₄⁺ depending on the biochemical reaction and exists in solution as NH₃ and NH₄⁺. Owing to the relatively high pK of ammonia (pK = 9-7 at 10°C) and the physiological pH of body fluids, the predominant form of ammonia in tissue compartments is the ionic form, NH₄⁺. Biological membranes are highly permeable to NH₃ and much less permeable to NH₄⁺ (Klocke, Andersson, Rotman & Forster, 1972; Castell & Moore, 1971; Bown et al. 1975; Boron, 1980; Lockwood, Finn, Campbell & Richman, 1980), which requires ion carriers for transport. Thus, the extent of movement of NH₄⁺ between tissue compartments depends on the availability of these carriers and their affinity for NH₄⁺. Transfer of ammonia between tissue compartments is largely determined by NH₃ gradients (see Randall & Wright, 1987), but NH₄⁺ electrochemical gradients may also be important (Thomas, 1974; Boron & DeWeer, 1976). The purpose of this study was to determine whether ammonia was passively distributed between red cells and plasma at rest and during an extracellular acidosis. Protons are passively distributed across red cell membranes over a range of pH values in trout (Heming et al. 1986). Thus, if ammonia is passively distributed, the distribution will be determined by red cell-to-plasma pH gradients.