RECOVERY FROM HIGH PH EXPOSURE IN THE RAINBOW-TROUT - WHITE MUSCLE AMMONIA STORAGE, AMMONIA WASHOUT, AND THE RESTORATION OF BLOOD-CHEMISTRY

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 (J(Amm)) an d led to a sixfold increase in plasma total ammonia (T-Amm) concentrat ion. By 24 h at pH 9.5, J(Amm) had returned to preexposure (control) r ates but plasma T-Amm concentration remained elevated The fish also de veloped a transient metabolic alkalosis (increased metabolic base) and a sustained respiratory alkalosis (decreased plasma CO2 tension [PacO (2)]). Plasma Na+ and Cl- concentrations were reduced by 5% after 48 h at pH 9.5. An ''ammonia washout,'' of about 5,000 mu mol . kg(-1) occ urred during the first 12 h of the recovery period, with fivefold elev ations in J(Amm) during the first few hours This ammonia washout was a ccompanied by a return of plasma T-Amm concentration to preexposure le vels after 3 h. The amount of excess T-Amm excreted by the fish during the washout was about 50-fold greater than extracellular fluid (ECF) T-Amm stores. Subsequent experiments indicated the white muscle (WM) i ntracellular fluid (ICF) compartment had stored at least 40% of the ex cess T-Amm. The T-Amm concentrations in the WM ICF were 2.5-fold great er in fish held at pH 9.5 for 48 h than in those held at pH 8.0. Estim ates of the ECF and WM ICF pH, NH3 partial pressures (PNH3), and NH4concentrations indicated the development of favorable ECF:ICF electroc hemical gradients for NH4+ uptake, and PNH3 gradients for NH3 uptake, by the WM during the intitial 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 T-Amm increases ow ing to temporary reductions in branchial ammonia excretion. The rapid return of other physiological indices such as arterial pH (pH(a)), Pac O(2), and plasma Na+ and Cl- to control levels during recovery (in 3-8 h) as well as constant arterial O-2 tension (PaO2) suggested that hig h-pH-induced physiological disturbances are reversible and that there is no high-pH-induced gill histopathology during such short-term expos ures.