Physiological adaptations of rainbow trout to chronically elevated water pH (pH = 9.5)

Recent investigations have demonstrated that rainbow trout cope with acute high pH (pH > 9.0) exposure (lasting 3-8 days) through their ability to counteract high-pH-induced disturbances to ammonia excretion (J_Amm), acid-base homeostasis, and electrolyte balance. In the present investigation our goal was to establish how these physiological processes were modulated during chronic (28-day) high pH (pH = 9.5) exposure. Chronic high pH led to minimal mortality, and there were no long-term changes in stress indicators levels, such as cortisol or glucose. J_Amm was initially reduced by 40% at high pH but rapidly recovered and fluctuated around control rates, thereafter. Decreased J_Amm was associated with an initial 2.5-fold increase in plasma ammonia concentrations (T_Amm), followed by a return toward pre-exposure levels after 3 days. Overall, plasma T_Amm was slightly higher (40-80%) in the treatment fish, and this likely led to plasma P_NH3S that were sufficient to sustain J_Amm at high pH. White muscle T_Amm stores were also chronically elevated, by 50-100%. There was a transient, twofold elevation of J_Urea immediately following high-pH exposure, but by 3 days J_Urea had returned to control rates and stabilized thereafter. Plasma ion balance was well maintained at high pH, despite a chronic depression of Na⁺ influx. Even though there was a persistent respiratory alkalosis at alkaline pH, blood pH was effectively regulated by a simultaneous metabolic acid load, which was not associated with increased lactic acid production. White muscle intracellular pH (pH₁) was unaltered during high pH exposure. We conclude that the long-term survival of rainbow trout in alkaline environments is facilitated by higher steady-state internal ammonia concentrations, the development of a sustained, compensatory metabolic acidosis which offsets decreased plasma P_CO2, and the effective regulation of plasma electrolyte balance.