Influence of age on intestinal bile acid transport in C57BL/6 mice

Abstract

Intestinal and hepatic bile acid transporters are important for enterohepatic bile acid circulation and pharmacokinetics. Based on previous literature, we hypothesized that the expression of bile acid transporters and intestinal bile acid absorption are lower in older individuals. Here, we measured active taurocholate absorption across the ileum of male C57BL/6 mice in two different age cohorts - young (age range of 89-224 days) and old (age range of 613-953 days). Also examined in these mice were mRNA expression of the major bile acid transporters - Asbt and Ostα/β in the ileum, and Ntcp, Oatp1b2 and Bsep in the liver. Mean intestinal taurocholate absorption was significantly lower (~50%) in mice in the older cohort compared to those in the younger cohort. In the ileum, the expression of Asbt was significantly lower in the older cohort, but expression of Ostα/β was not affected by age. The lower capacity for intestinal bile acid absorption in the older animals is consistent with their lower expression level of Asbt. Of the hepatic bile acid transporters examined, expression of Ntcp and Oatp1b2 were significantly lower in the older mice. This is the first study to directly measure intestinal bile acid absorption as a function of age. The data suggest a lower capacity for intestinal bile acid absorption in older animals. Also, lower expression of Asbt, Ntcp, and Oatp1b2 in older individuals could influence pharmacokinetics of drug substrates.

Keywords: Age; Ussing chambers; bile acid; bile acid transporters; intestine; liver.

Figure 1

Shown are representative Ussing chamber experiments examining the unidirectional secretory flux (blood‐to‐lumen), unidirectional absorptive flux (lumen‐to‐blood), and the net active flux (the difference between the unidirectional fluxes) of taurocholate across the ileum of a young mouse (A, 209 days old) and an old mouse (B, 613 days old). In both cases, net active taurocholate transport was in the direction of absorption and reached steady state at ~2 h. The steady‐state active taurocholate absorption rate was greater in the young mouse compared to the old mouse. Experiments were conducted under voltage‐clamped conditions with 5 μmol/L taurocholate bathing both sides of the tissue.

Figure 2

Shown is active taurocholate absorption at steady‐state by the ileum in Ussing chambers as a function of age. (A) Individual data points for active taurocholate (TC) absorption by the ileum from the mice in the young and old cohorts. (B) Mean active taurocholate absorption by the ileum of young versus old mice. Ileum from mice in the young cohort (age range of 89–224 days, n = 9) had significantly greater active taurocholate absorption than mice in the old cohort (age range of 613–953 days, n = 8), P < 0.001, two‐tailed unpaired Student's t‐test. ***, P < 0.001

Figure 3

Relative mRNA expression of intestinal (Asbt, Ostα, and Ostβ, Panel A) and hepatic (Ntcp, Bsep and Oatp1b2, Panel B) bile acid transporters in the young cohort compared to the old cohort. The mRNA expression of the genes was expressed relative to the expression level of a 223‐day old mouse, which was set to 1. In the ileum, there was a significant difference in the expression of Asbt (P < 0.05, Mann–Whitney U test), but not in the expression of Ostα or Ostβ. In the liver, there was a significant difference in the expression of Ntcp and Oatp1b2 (P < 0.05, Mann–Whitney U test), but not in Bsep. *, P < 0.05.