Passive water flows driven across the isolated rabbit ileum by osmotic, hydrostatic and electrical gradients.

Abstract

SUMMARY1. Water flows generated by osmotic and hydrostatic pressure and electrical currents were measured in sheets of isolated rabbit ileum at 20 'C. Flows across the mucosal and serosal surfaces were monitored continuously by simultaneous measurement of tissue volume change (with an optical lever) and net water flows across one surface of the tissue (with a capacitance transducer).2. Osmotic gradients were imposed across the mucosal and serosal surfaces of the tissue separately, using probe molecules of various sizes from ethanediol (68 Da) to dextrans (161000 Da). Flows across each surface were elicited with very short delay. The magnitudes of the flows were proportional to the osmotic gradient and related to the size of the probe molecule. Osmotic flow across the mucosal surface was associated with streaming potentials which were due to electro-osmotic water flow.3. The mucosal surface is a heteroporous barrier with narrow (0 7 nm radius, Lp (hydraulic conductivity) = (7-6±+ 16) x 10-9 cm s-1 cmH20-1) cation-selective channels in parallel with wide neutral pores (ca. 6-5 nm radius, Lp = (2-3 + 0 2) x 10-7 cm s-1 cmH20-1) which admit large pressure-driven backflows from the submucosa to the lumen. There is additional evidence for a further set of narrow electroneutral pores < 0 4 nm radius with Lp < 7 x 10-9 cm s-1 cmH20-1.The serosal surface has neutral pores of uniform radius (ca. 6-5 nm), Lp = (7-6± 16) x 10-8 cm s-1 cmH20-'.4. Hypertonic serosal solutions (100 mM-sucrose) cause osmotic transfer of fluid from isotonic mucosal solutions into the submucosa, expand it, and elevate the tissue pressure to 19-6 + 3-2 cmH20 (n = 4). Conversely, hypertonic mucosal solutions (100 mM-sucrose) draw fluid out of the submucosa in the presence of isotonic serosal solutions, collapse the submucosa, and lower the tissue pressure to -87-7 + 4-6 cmH20 (n= 5).5. Water flows coupled to cation movement could be generated across the mucosal surface in both directions by brief direct current pulses. The short latency of onset and cessation of flow (< 2 s), absence of polarization potentials, and high electroosmotic coefficients (range 50-520 mol water F-1), together with the presence of streaming potentials during osmotically generated water flows indicate electro-* Present address:

In Vitro Diffusion Barriers of the Mouse Jejunum in Ussing Chambers

“…The apparent extracellular space in the rat proximal small intestine has been shown to be larger when sucrose (Mr 362) was used as a marker than when PEG 900 (Mr 900) was used (16). In addition, the pore size of the subepithelial water-transport pathway has been suggested to be about 6 nm in diameter by Naftalin and Tripathi (17). Thus, the mucosasubmucosa layer may be able to limit the permeation of lucifer yellow over NaCl as we observed.…”

Effects of theophylline, choleragen and loperamide on rabbit ileal fluid and electrolyte transport in vitro

“…The hydraulic conductance of the mucosal shunt pathway in rabbit ileum in vitro is 2 x 10-cms-'cm H20-' (Naftalin & Tripathi, 1985;1986). Hence a theophylline-dependent decrease in distensibility from 2.5 x 10-3 to 1.5 x 10-3cmH20' would increase interstitial pressure without any immediate change in interstitial volume and so would increase fluid backflux via the wide mucosal shunt pathway from 25 to 50 Jl cm-2 h'.…”

“…The serosal borders of rabbit and rat intestine behave as a membrane with low conductance homogeneous porosity (mean pore radius of 6.5 nm) (Naftalin & Tripathi, 1985;Fromm et al, 1985). The hydraulic conductance of the serosal surface is in the range I -5 x 10-8 cmsr' cmH2O' (Table 6).…”

“…These chambers are described in detail elsewhere (Naftalin & Holman, 1974 Simultaneous bidirectional transepithelial fluxes of 36CI and "Br were measured by the methods previously described (Naftalin & Simmons, 1979 (Smulders & Wright, 1971;Biber et al, 1972). However, it does cross the mucosal surface of rabbit ileum via paracellular shunt channels (Naftalin & Tripathi, 1985;1986) The tissue was preincubated for a 30 min period at 37C in Krebs-Ringer solution, loperamide was added to the bathing solution when required during this period. 3H-labelled mannitol was then added to the serosal bathing solution.…”

“…The method of monitoring intestinal fluid movement has been fully described (Naftalin & Tripathi, 1985;1986 The hydraulic conductance, determined by the osmotic L, (cm s' cmH2O ') was determined as follows: addition of impermeant macromolecules to the serosal bathing solution induced osmotic flow across the submucosal layers (Naftalin & Tripathi, 1985). The Lp was estimated from the change in A J, mosmol', according to the following relationship:…”