Fig. 2. Total, passive and active Pi uptake in the pyloric caeca of rainbow trout in incubation media containing various [Pi]: (A) Nonlinear plot of total, passive and active components; inset shows enlarged section of first part of curve. (B) Eadie–Hofstee plot of active component. Values are means ± S.E.M. (N=6–10 tissues from 3–4 fish). Total Pi uptake was determined in the normal incubation medium, and passive Pi uptake was estimated (Est.) from the slope of normal incubation medium (>1 mmol l^–1). The active component of Pi uptake in the pyloric caeca was estimated by difference. Pi uptake in PFA-containing medium (broken line) was slightly higher than the estimated passive uptake at high Pi concentrations (>5 mmol l^–1). Using Equation 2, _max=64.6±7.6 (mean ± S.E.M.) nmol g tissue^–1 min^–1; K_m=0.474±0.199 mmol l^–1; K_d=0.039±0.002 min^–1. In A, percentages of the active component in total Pi transport were as follows (N=6–10): 99.8 (extrapolation) at 0.28 nmol l^–1 (tracer concentration); 74.5±2.6 at 0.1 mmol l^–1; 62.7±3.1 at 0.3 mmol l^–1; 51.2±6.2 at 0.6 mmol l^–1; 56.5±1.9 at 1 mmol l^–1; 45.9±3.0 at 2 mmol l^–1; 17.8±8.4 at 5 mmol l^–1; 12.5±2.5 at 10 mmol l^–1; and 7.7 (extrapolation) at 20 mmol l^–1.