Compartmental Distributions of Carbon Dioxide and Ammonia in Rainbow Trout at Rest and Following Exercise, and the Effect of Bicarbonate Infusion

The carbon dioxide content of the intracellular compartment of fish muscle was determined by direct measurements of CO₂ and pH in tissue homogenates of rainbow trout, Oncorhynchus mykiss. The results agree with the concept that compartmental distribution of CO₂ is pH-dependent and that muscle membranes are not very permeable to bicarbonate. The interaction between CO₂ and ammonia excreted from fish muscle was also investigated by altering plasma CO₂ content using bicarbonate infusion following exhaustive exercise. Removal of the acid boundary layer in white muscle by bicarbonate infusion resulted in retention of ammonia in the muscle, indicating that ammonia excretion across the muscle membrane might be enhanced by the hydration of excreted CO₂ in the extracellular fluid. Passive diffusion of NH₃, rather than NH₄⁺ transfer, is probably the dominant pathway of ammonia excretion through fish muscle membranes.