Chang, M.H*; Chediack, J.G.; Caviedes-Vidal, E.; Karasov, W.H.: Enhanced passive absorption in the presence of luminal nutrients in house sparrows

We previously demonstrated in house sparrows substantial absorption in vivo of L-glucose, the stereoisomer of D-glucose that does not interact with the intestine’s D-glucose transporter. The mechanism, as first proposed by Pappenheimer and coworkers in 1987, might be solvent drag through paracellular spaces, activated by Na+-coupled nutrient transport. This hypothesis predicts that absorption of L-glucose might be highest when it is administered orally in the presence of D-glucose and/or amino acids. We tested this in two experiments with house sparrows (Passer domesticus) using pharmacokinetic methods to measure absorption. In Exp. 1, fractional absorption of L-glucose (administered by oral gavage and measured by enzymatic method in plasma) in fed sparrows was higher (0.46 +- 0.02, n = 7) than in sparrows fasted 12 h (0.40 +- 0.01, n = 6) (simple t-test T = -2.27, P = 0.044, DF= 11). The greater absorption was apparently not due simply to longer retention time of digesta, because no significant difference was found for retention times. In Exp. 2, fractional absorption of radiolabeled L-glucose was significantly higher when administered to fasted birds in solution with 200 mM D-glucose (+ 80 mM NaCl) than when administered with 200 mM mannitol (+ 80 mM NaCl) (respectively, 0.79 + 0.04 vs. 0.66 + 0.03; T= -2.47, P=0.035, DF= 9). In the former group, the absorption rateof L-glucose, determined by pharmacokinetic methods, was 67% faster (P = 0.032). Our results are consistent with the solvent-drag hypothesis, and indicate that small, hydrophilic compounds are absorbed faster and to a greater extent when there are nutrients (e.g., sugar, amino acids) in the gut. Supported by FONCYT (01-03101) to ECV, and NSF IBN-9723793 toWHK.