Fig. 4. The effect of external Cu (Cu_o) on Cu accumulation by intestinal cells in the presence of normal (140 mmol l^–1, triangles) and low (11 mmol l^–1, squares) external [Na⁺] (Na⁺_o). (A) Total Cu accumulation by intestinal cells during 15 min incubation in Cu-containing saline and (B) net Cu accumulation revealed after rapid Cu accumulation in/on cells at time 0 (Fig. 3) had been deducted from the total Cu accumulation over 15 min. Values are means ± S.E.M. of N=5–6 separate experiments using fresh cells from different individual fish. ^*Significantly different from the control with no added Cu within treatment; Kruskal–Wallis test (P<0.05). Significantly different from normal Na⁺_o at the indicated Cu_o; Mann–Whitney U-test (P<0.05). Accumulation is normalised mg^–1 cell protein and h^–1 to allow comparison between plots, and with Fig. 3.