The effects of the free bile acids (FBAs) cholic acid (CA), deoxycholic acid (DCA), and chenodeoxycholic acid on the bioenergetics and growth of lactobacilli and bifidobacteria were investigated. It was found that these FBAs reduced the internal pH levels of these bacteria with rapid and stepwise kinetics and, at certain concentrations, dissipated ⌬pH. The bile acid concentrations that dissipated ⌬pH corresponded with the MICs for the selected bacteria. Unlike acetate, propionate, and butyrate, FBAs dissipated the transmembrane electrical potential (⌬⌿). In Bifidobacterium breve JCM 1192, the synthetic proton conductor pentachlorophenol (PCP) dissipated ⌬pH with a slow and continuous kinetics at a much lower concentration than FBAs did, suggesting the difference in mode of action between FBAs and true proton conductors. Membrane damage assessed by the fluorescence method and a viability decrease were also observed upon exposure to CA or DCA at the MIC but not to PCP or a short-chain fatty acid mixture. Loss of potassium ion was observed at CA concentrations more than 2 mM (0.4؋ MIC), while leakage of other cellular components increased at CA concentrations more than 4 mM (0.8؋ MIC). Additionally, in experiments with membrane phospholipid vesicles extracted from Lactobacillus salivarius subsp. salicinius JCM 1044, CA and DCA at the MIC collapsed the ⌬pH with concomitant leakage of intravesicular fluorescent pH probe, while they did not show proton conductance at a lower concentration range (e.g., 0.2؋ MIC). Taking these observations together, we conclude that FBAs at the MIC disturb membrane integrity and that this effect can lead to leakage of proton (membrane ⌬pH and ⌬⌿ dissipation), potassium ion, and other cellular components and eventually cell death.

The effects of ageing on the oral (gastrointestinal) absorption of D-xylose were investigated by analysing the gastrointestinal disposition after oral administration to young (9 weeks) and old (53 weeks) rats. A linear model assuming first-order gastric emptying followed by first-order intestinal absorption was fitted to remaining fraction vs time profiles for the stomach and small intestine to estimate the gastric emptying rate constant (kg) and the intestinal absorption rate constant (ka). In young and old rats, Kg values were 0.087 +/- 0.008 and 0.070 +/- 0.007 min-1, respectively, and ka values were 0.020 +/- 0.002 and 0.018 +/- 0.002 min-1, suggesting an insignificant effect on ageing on the rate of oral absorption. The average intestinal lumen volume (Vav) was unchanged with ageing, and so was the apparent intestinal membrane permeability clearance (CLapp) as the product of Ka and Vav. However, the small intestinal transit time (Tsi) was suggested to be twice that in older rats (171 min) than in young rats (78 min) by the analysis of gastrointestinal disposition of inulin, a non-absorbable marker. It was also shown that our preceding finding of an increase in the fraction absorbed of D-xylose with ageing can be solely ascribable to the delay in intestinal transit. Thus, among various determinants of oral absorption, only Tsi was found to be altered with ageing. The CLa,app and ka of passively absorbed drugs such as D-xylose may be generally unchanged, and the fraction absorbed may increase with ageing by the delay in intestinal transit.

The effects of ageing on the oral absorption of D-xylose were investigated in rats. The pharmacokinetic analysis of D-xylose concentration in plasma after oral administration showed that the fraction absorbed was increased to 0.998 +/- 0.002 and 0.950 +/- 0.049, respectively, in old (52 weeks) and very old (102 weeks) rats, compared with 0.768 +/- 0.052 in young (9 weeks) rats, while the absorption rate constant was not significantly changed: 0.944 +/- 0.233, 0.844 +/- 0.143 and 0.725 +/- 0.004 h-1, respectively, in young, old and very old rats. The absorbed fractions estimated from faecal and urinary excretion were in agreement with those by the pharmacokinetic analysis. Thus, the present study demonstrated an increase in the extent of the oral absorption of D-xylose with ageing. The increase in the extent of absorption might be caused by a delay in the intestinal transit, because the absorption rate constant was unchanged. These results suggest potential increases with ageing in the fractions absorbed of hydrophilic drugs such as D-xylose where oral absorption is incomplete.

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