Fig. 4. Effect of pH on the total uptake of Pi in pyloric caeca of rainbow trout. Values are means ± S.E.M. (N=5). *At the same pH, Pi uptake in Na-containing (Na+) medium was significantly greater than that in Na-free (Na–) medium (two-tailed t-test). The incubation medium contained 0.1 mmol l^–1 Pi, and estimated active Pi uptake was 75% of the total uptake. In Na-free medium, choline chloride replaced NaCl of the normal medium iso-osmotically. The pH of the medium was adjusted using HCl and KOH. A single buffer system (Tris-citrate) was used for all the incubation media. Pi uptake increased at high pH in Na+ medium, but not in Na-free medium. Significant Pi uptake was seen in Na-free medium at pH 4–9.