Effect of perfusion rate on absorption, surface area, unstirred water layer thickness, permeability, and intraluminal pressure in the rat ileum in vivo.

Intestinal absorption and intraluminal pressures were measured at perfusion rates between 0.3 and 200 ml per min in the rat ileum in vivo. Glucose absorption from a 72 mM glucose solution and tritiated water ([3-H]water) diffusion rate were used to reflect changes in mucosal surface area. Glucose absorption from a 4 mM solution was used to indicate changes in unstirred water layer thickness, and mannitol and urea absorption were used as markers of passive mucosal permeability. In a partially obstructed intestinal segment, designed to keep the gut partially filled even at low perfusion rates and to minimize surface area change as perfusion rate was increased, glucose absorption from a 4 mM solution increased by 150% as perfusion rate was increased from 1 to 100 ml per min. Forty per cent of this increase was due to increased surface area (estimated from the change in [3-H]water absorption), and 110% of the increase is attributed to thinning of the unstirred water layer. Because mannitol absorption was zero at all perfusion rates, none of the enhanced glucose absorption rate need be attributed to enhanced mucosal permeability, even though intraluminal pressure was increased at higher perfusion rates. Urea absorption was apparently influenced by surface area and by permeability changes, but not by the thickness of the unstirred water layer. This model was also used to explore the effect of unstirred water layer thickness on the inhibitory effect of sodium replacement by magnesium on glucose absorption from a 4 mM glucose solution. Inhibition by sodium removal was equal at 1, 10, 100, and 200 ml per min perfusion rates, suggesting that unstirred water layer thickness does not play an important role in the interaction of glucose and sodium absorption when intraluminal sodium concentration is reduced. Additional experiments in an unobstructed ileal segment revealed that the major effect of enhanced perfusion rate is to increase mucosal surface area; relatively high rates of perfusion were required to thin significantly the unstirred water layer when intestinal outflow was not partially obstructed.