Fluid recirculation in Necturus intestine and the effect of alanine.
Fluid absorption by Necturus small intestine has been studied using radiolabeled dextrans as molecular probes of the paracellular pathway under voltage-clamped conditions. Fluxes of H3-dextrans of MW up to 20K were followed in both directions between mucosal (M) and serosal (S) baths by fractionating those that passed the epithelium as a function of molecular radius. Consideration of the unstirred layers in the baths and the surface geometry rules out any contribution made by solute polarization. The geometry of the paracellular system was measured by light microscopy, TEM and SEM, and values were used in conjunction with a program that calculates convective-diffusive coupling in the tight junctions, intercellular spaces and subepithelium in series. The results indicate that the net fluxes are due to the convection of fluid through two opposing paracellular fluid circuits with different size selectivity, resulting in net absorption at small radii. Alanine at 20 mM stimulates fluid and salt uptake by a factor of 2. Its effect on the two convective components is to increase the M to S flux and decrease the S to M. The selectivities are not significantly different from those before alanine treatment. The volume absorption predicted from the net probe fluxes is very close to that measured gravimetrically across the epithelium.
Hill
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AE
Physiological Laboratory, Downing St., Cambridge CB2 3EG, UK.
Shachar-Hill
B
B
D000409:Alanine / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D001826:Body Fluids / Q000187:drug effects / Q000378:metabolism; D001834:Body Water / Q000187:drug effects / Q000378:metabolism; D003911:Dextrans / Q000493:pharmacokinetics; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D007408:Intestinal Absorption / Q000187:drug effects; D007421:Intestine, Small / Q000187:drug effects* / Q000378:metabolism*; D009337:Necturus / Q000378:metabolism*; D009995:Osmosis; D012965:Sodium Chloride / Q000378:metabolism
D003911:Dextrans; D012965:Sodium Chloride; D000409:Alanine