Changes in NaPi-IIb (A) mRNA or (B) protein expression level in rat upper intestine with age. Values are normalized by villin-1 protein or mRNA, respectively. The actual values are represented as dots and the columns represent mean ± SE. *P < 0.05, significant difference versus 4 weeks old; ^#P < 0.05, significant difference versus 9 weeks old (Tukey–Kramer test).