In vitro perfused, non-isolated small intestine: ontogeny of transmural hydraulic permeability.

Net fluid flux and transmural hydrostatic hydraulic conductance (Lp) were measured in the rat ileum during postnatal ontogeny, utilizing a refined in vitro, capillary-perfused (non-isolated) preparation for free-flow studies. Volume absorption at low transmural pressure gradient (1-2 cmH2O) was 0.99 +/- 0.22 ml/g wet weight.hr in the mature ileum and 1.55 +/- 0.42 ml/g wwt.hr in the differentiating intestine. Hydrostatic hydraulic conductance of the mature ileum was 0.76 microliter/cm2.cm H2O.hr, indicating that normal hydrostatic pressure gradients are not important for modulation of intestinal absorption. In contrast, Lp was 3.82 microliter/cm2.cmH2O.hr in the differentiating ileu, suggesting a significant contribution of pressure-dependent solvent drag to isotonic solute absorption.