Recovery from High pH Exposure in the Rainbow Trout: White Muscle Ammonia Storage, Ammonia Washout, and the Restoration of Blood Chemistry

Abstract

The physiological responses of rainbow trout were followed during 48 h of high pH (pH 9.5) exposure and a further 48 h of recovery at pH 8.0. High pH exposure temporarily inhibited ammonia excretion (JAmm) and led to a sixfold increase in plasma total ammonia (TAmm) concentration. By 24 h at pH 9.5, JAmm had returned to preexposure (control) rates but plasma TAmm concentration remained elevated. The fish also developed a transient metabolic alkalosis (increased metabolic base) and a sustained respiratory alkalosis (decreased plasma CO₂ tension [PaCO₂]). Plasma Na⁺ and Cl⁻ concentrations were reduced by 5% after 48 h at pH 9.5. An "ammonia washout, " of about 5,000 μmol ․ kg⁻¹ occurred during the first 12 h of the recovery period, with fivefold elevations in JAmm during the first few hours. This ammonia washout was accompanied by a return of plasma TAmm concentration to preexposure levels after 3 h. The amount of excess TAmm excreted by the fish during the washout was about 50-fold greater than extracellular fluid (ECF) TAmm stores. Subsequent experiments indicated the white muscle (WM) intracellular fluid (ICF) compartment had stored at least 40% of the excess TAmm. The TAmm concentrations in the WM ICF were 2.5-fold greater in fish held at pH 9.5 for 48 h than in those held at pH 8.0. Estimates of the ECF and WM ICF pH, NH₃ partial pressures (PNH₃), and NH⁺₄ concentrations indicated the development of favorable ECF:ICF electrochemical gradients for NH⁺₄ uptake, and PNH₃ gradients for NH₃ uptake, by the WM during the initial period of high pH exposure. There was a partial return toward steady state by 48 h. Thus, the WM serves as an "ammonia reservoir" for rainbow trout when plasma TAmm increases owing to temporary reductions in branchial ammonia excretion. The rapid return of other physiological indices such as arterial pH (pHa), PaCO₂, and plasma Na⁺ and Cl⁻ to control levels during recovery (in 3-8 h), as well as constant arterial O₂, tension (Pao₂), suggested that high-pH-induced physiological disturbances are reversible and that there is no high-pH-induced gill histopathology during such short-term exposures.