A micropuncture study of water transport by dog jejunal villi in vitro

Role of villus microcirculation in intestinal absorption of glucose: coupling of epithelial with endothelial transport

Capillaries in jejunal villi can absorb nutrients at rates several hundred times greater (per gram tissue) than capillaries in other tissues, including contracting skeletal muscle and brain. We here present an integrative hypothesis to account for these exceptionally large trans-endothelial fluxes and their relation to epithelial transport. Equations are developed for estimating concentration gradients of glucose across villus capillary walls, along paracellular channels and across subjunctional lateral membranes of absorptive cells. High concentrations of glucose discharged across lateral membranes to subjunctional intercellular spaces are delivered to abluminal surfaces of villus capillaries by convection-diffusion in intercellular channels without significant loss of concentration. Post-junctional paracellular transport thus provides the series link between epithelial and endothelial transport and makes possible the large trans-endothelial concentration gradients required for absorption to blood. Our analysis demonstrates that increases of villus capillary blood flow and permeability-surface area product (PS) are essential components of absorptive mechanisms: epithelial transport of normal digestive loads could not be sustained without concomitant increases in capillary blood flow and PS. The low rates of intestinal absorption found in anaesthetised animals may be attributed to inhibition of normal villus microvascular responses to epithelial transport.

Scaling of dimensions of small intestines in non-ruminant eutherian mammals and its significance for absorptive mechanisms

The mucosal surface area of small intestines in non-ruminant eutherian mammals increases approximately in proportion to the 0.6 power of body mass, whereas resting metabolic rate (RMR) increases approximately with the 0.74 power of body mass; the mass exponent for field metabolic rates (FMR) may exceed 0.8. These relationships imply that the average rate of absorption of metabolic substrates, expressed per unit area of mucosal surface, is greater in large animals than in small. In the present paper I collate data from the literature relating mucosal surface area, fluid absorption and glucose transport rates to body size. Glucose-stimulated fluid absorption per unit area of mucosal surface increases with body size, whereas transcellular, carrier-mediated glucose transport per unit area decreases with body size. In perfused jejunal segments of normal human subjects the rates of fluid absorption per unit area of mucosa are five to ten times greater than in laboratory rats. The absorbed fluid contains glucose in amounts that may greatly exceed the maximum transport capacity of the apical glucose transporter. It follows that the paracellular component of glucose absorption increases with body size. Scaling of intestinal dimensions and transport therefore provides new information about the relative contributions of transcellular and paracellular pathways to absorption of nutrients.

Villous motility: relationship to lymph flow and blood flow in the dog jejunum

Villous contraction frequency, lymph flow, blood flow, and arteriovenous oxygen difference were measured in dog jejunum. Venous pressure elevation and plasma dilution were used to increase capillary fluid filtration. Both perturbations produced concomitant increases in villous contraction frequency and lymph flow. A highly significant correlation (r = 0.83, p less than 0.001) was obtained between villous contraction frequency and lymph flow. This finding, coupled with the observation that stimulation of net fluid absorption increases villous contraction frequency, suggests that villous motility is increased by a myogenic response elicited by increments in interstitial fluid pressure. In another series of experiments local arterial pressure was reduced in 20-mmHg steps from 120 to 20 mmHg. Although blood flow fell proportionately to arterial pressure, villous contraction frequency and oxygen uptake were maintained at a normal level when arterial pressure was between 120 and 60 mmHg. Villous motility and oxygen consumption fell progressively as arterial pressure was reduced below 60 mmHg. This observation indicates that ischemia does not alter villous contraction frequency unless blood flow is reduced below the level necessary to maintain normal tissue oxygenation.

Tissue fluid pressure, lymph pressure, and fluid transport in rat intestinal villi

Tissue fluid pressure (Pt) and lymph pressure (Pl) as well as fluid transport between initial lymphatics and tissue matrix were determined in the villi of the jejunum in vivo and in vitro. When the intestine was absorbing fluid, free tissue fluid as a micro pool appeared in the villi. Pt and Pl were 2.4 +/- 0.7 and 2.9 +/- 0.8 mm Hg (mean +/- SE), respectively, in the villi in vivo (26 villi of 5 rats) or 0.8 +/- 0.4 and 1.2 +/- 0.5 mm Hg, respectively, in the villi in vitro (30 villi of 5 rats). Pl was significantly (P less than 0.01) higher than Pt. When Evans blue in saline was continuously infused into an initial lymphatic, dye rapidly leaked out into the tissue matrix, but when it was infused into the free tissue fluid, dye did not enter the lymphatics but leaked out of the epithelial layer, apparently due to low hydraulic conductance of the tissue matrix to fluid transport in this direction. Furthermore, in the villi with fat absorption, a retrograde flow of chylous lymph out of the villous tips always occurred, indicating that there are large pores at the villous tips to allow free passage of fluid and chylomicrons. From these findings and other evidences, it is inferred that during fluid absorption a fraction of the lymph may be formed by the transport of the luminal fluid directly into the terminal end of the initial lymphatics via large pores at the villous tips presumably by inhibition of the tissue matrix or by lymphatic suction or both.

Effects of stretching and stirring on water and glucose absorption by canine mucosal membrane

A 'mini' canine mucosal membrane preparation permitting simultaneous determination of water (Jv) and glucose (Jg) absorption rates, microscopic examination or micropuncture of the villi was used in this study. The small membranes were more stretched than the large ones, with more than a one-fold increase in both Jv and Jg, apparently due to a change in architectural orientation between the villi and subvillous supporting tissue so as to facilitate water transport via the lymphatic system. During stirring of the bathing solution, the villi in the small membranes were widely separated from each other with more to-and-fro swaying movements than in the large ones. Stirring was seen to cause up-and-down movements of the loosely suspended large membranes but not the small ones. In the small membranes stirring caused no change in Jv but an increase in Jg due to the increase in glucose concentration in the absorbate, while in the large membranes both Jv and Jg were greatly increased. It is thus considered that the increase in absorption in the large membranes caused by stirring is mainly due to the increased membrane movements promoting lymph flow.

Epithelial cell extrusion during fluid transport in canine small intestine

Epithelial cell extrusion during fluid transport was studied under both in vitro and in vivo conditions. The rate of cell extrusion from the villus tips in vitro increased by about onefold in the villi with obstruction of lymph flow associated with the increase of lymph and tissue fluid pressure. When lymph pressure in the jejunal and ileal villi was increased to 6.4 +/- .2 and 12.3 +/- .5 mmHg, respectively, by injection of Ringer solution into the central lacteals, fluid leaked out of the villi and a shedding of epithelium occurred. Vigorous villus spasmodic contraction induced by cocaine or atropine also caused a shedding of epithelium. Cells always appeared in the lumen of intestine in vivo either during fluid absorption or secretion. A copious secretion of fluid, increase of cell loss, and congestion of blood in the villi occurred by the action of cholera toxin, MgSO4, and choline chloride. The rate of cell loss was highest during fluid secretion induced by an elevation of tissue fluid pressure such as at high venous pressure or during intra-arterial histamine infusion. It is thus concluded that elevated tissue fluid pressure is involved in epithelial cell extrusion during fluid transport.

Ionic dependence of protein transport across the new-born pig intestine

1. The everted small intestine of the new-born pig transports albumin to its serosal surface at rates which depend on the ionic composition of the bathing medium.2. Albumin transport takes place most rapidly at sodium and potassium concentrations of 120 and 6 mM respectively. Increasing the concentration of sodium or potassium, or decreasing the concentration of sodium, reduces the net transfer of albumin.3. Albumin can increase the serosal transfer of sodium, potassium and water. The calculated ionic composition of fluid transported in the presence of albumin closely resembles that presented to the mucosal surface over a wide range of sodium and potassium concentrations.4. Lowering the concentration of sodium reduces water transfer but only if albumin is present in the mucosal medium. Albumin-stimulated water transfer is only seen when the external concentration of sodium is high.5. Raising the concentration of potassium reduces water transport, whether or not albumin is present, but this reduction is too small to account for the inhibitory effect of potassium on albumin transport.6. The mutual interaction that exists between the transport of albumin and sodium is probably responsible for the increased potassium transport seen at low external concentrations of potassium. Higher concentrations of potassium appear directly to inhibit the interaction between the transport of sodium and albumin.