Comparison of Bile Salt Perturbation of Duodenal and Jejunal Isolated Brush-Border Membranes

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Abstract
Rabbit duodenal and jejunal brush-border membrane vesicle integrity, fluidity and passive proton permeability were studied after in vitro exposure to deoxycholate, glycodeoxycholate or taurodeoxycholate. Duodenal and jejunal membrane mean vesicle volume [0.54 (0.05) and 0.54 (0.03) μl/mg, mean values with SE in parentheses], and proton permeability [6.30 (0.20) × 10-4 and 5.59 (0.17) × 10-4 cm•S-1] were similar, while membrane fluidity was slightly, but significantly, greater in jejunal membranes [diphenylhexatriene fluorescence anisotropy, (r₀/r)-1 = 0.499 (0.013) and 0.572 (0.012)]. The three bile salts (0.1–5.0 mM) caused concentration-related decreases in vesicle integrity as assessed by [14C]glucose retention at equilibrium. Jejunal membranes were more sensitive at low bile salt concentrations, 0.1 and 0.5 mM. With high concentrations of bile salts, 5 mM, above their critical micelle concentration, less than 10% of jejunal or duodenal vesicles remained. The bile salts caused concentration-related increases in jejunal membrane fluidity over the range 0.1–1.0 mM bile salt, but duodenal membrane fluidity was only increased at the highest concentration of bile salt. Proton permeability (Pnet) of the membranes was increased by the bile salts (0.1–1.0 mM), and again the jejunal membranes were more sensitive; Pnet increased by approximately 120, 150 and 170 in duodenal, and 150, 220 and 380% in jejunal membranes, with deoxycholate at 0.1, 0.5 and 1.0 mM, respectively. There were no consistent differences in the potency of the unconjugated and conjugated bile salts. The three variables were only significantly correlated for duodenal membranes. The data demonstrate that physiological concentrations of bile salts have a direct effect on intestinal apical membrane properties, and that the duodenal membranes are more resistant. The cause of the relative resistance of duodenal membranes cannot be simply explained by their lower fluidity, and its physiological advantage abstruse.

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