Effect of perfusion rate on absorption, surface area, unstirred water layer thickness, permeability, and intraluminal pressure in the rat ileum in vivo
Lewis, L.D.; Fordtran, J.S.
Gastroenterology 68(6): 1509-1516
1975
ISSN/ISBN: 0016-5085 PMID: 1132632 Document Number: 92468
Intestinal absorption and intraluminal pressures were measured at perfusion rates between 0.3 and 200 ml/min in the rat ileum in vivo. Glucose absorption from a 72 mmoles/litre glucose solution and tritiated water diffusion rate were used to reflect changes in mucosal surface area. Glucose absorption from a 4 mmoles/litre solution was used to indicate changes in unstirred water layer thickness, and mannitol and urea absorption was used as markers of passive mucosal permeability. In a partly obstructed intestinal segment, designed to keep the gut partially filled even at low perfusion rates and to minimize surface area change as perfusion rate increased, glucose absorption from a 4 mmoles/litre solution increased by 150% as perfusion rate was increased from 1 to 100 ml/min; 40% increase was due to increased surface area (estimated from change in water-3H absorption) and 110% was due to thinning of the unstirred water layer. Because mannitol was not absorbed, none of the enhanced glucose absorption rate need be due to enhanced mucosal permeability, even though intraluminal pressure was increased at higher perfusion rates. Urea absorption was slightly influenced by surface area and by permeability changes, but not by the thickness of the unstirred water layer. This model was used to explore the effect of unstirred water layer thickness on the inhibitory effect of Na replacement by Mg on glucose absorption from a 4 mmolar glucose solution. Inhibition of Na removal was equal at 1, 10, 100 and 200 ml/min perfusion rates, suggesting that unstirred water layer thickness does not play an important part in the interaction of glucose and Na absorption when intraluminal Na concentration is reduced. Additional experiments in an unobstructed ileal segment showed that the major effect of enhanced perfusion rate was to increase mucosal surface area; relatively high rates of perfusion were required to thin significantly the unstirred water layer when intestinal outflow was not obstructed.