Fig. 7. Effects of dietary P levels on NaPi cotransporter mRNA abundance (A,B) and Pi uptake (C,D) in the proximal intestine (Int.) and pyloric caeca (PC) of rainbow trout. Fish were fed either low-P (LP; open symbols) or high-P (HP; filled symbols) diet for 20 days. Serum Pi and bone P concentrations (x-axis) represent the P status of each fish. I-NaPi and PC-NaPi mRNA abundances were determined by RT-PCR-Southern blot (PC-NaPi in the intestine was negligible). The mRNA abundance was normalized by ß-actin mRNA abundance and expressed relative to the average of I-NaPi in PC of HP fish (=1.0). Pi uptake was determined in vitro in the incubation medium containing 0.1 mmol l^–1 Pi. At this Pi concentration, the active component represented 75% of total Pi uptake in PC (Fig. 3). (A) Significant correlations were found between serum Pi concentration and I-NaPi mRNA abundance in the intestine (slope P=0.002) and in PC (slope P=0.002), whereas correlation between serum Pi concentration and PC-NaPi mRNA abundance in PC (broken line) was weak (P=0.04). (B) Significant correlations were also found between bone P concentration and I-NaPi mRNA abundance in the intestine (slope P=0.005) and in PC (slope P<0.0001), whereas correlations between bone P concentration and PC-NaPi mRNA abundance in PC (broken line) were insignificant (P=0.2). In both A and B, PC-NaPi was dominant in PC in HP fish, whereas I-NaPi was dominant in PC in LP fish. I-NaPi abundance in the intestine was higher in both LP and HP fish than the total NaPi cotransporter abundance in PC. (C) Correlation between serum [Pi] and Pi uptake was significant in the intestine (slope P=0.001), but not in PC (broken line; slope P=0.4). (D) Correlation between bone [P] and Pi uptake was significant in the intestine (slope P=0.007), but not in PC (broken line; slope P=0.7). In both C and D, Pi uptake (g^–1 tissue) was higher in PC than in the intestine, especially in HP fish.