Electrogenic transport of glucose in the normal upper duodenum. II. Unstirred water layer and estimation of real transport constants.

The thickness of the unstirred water layer in the normal upper duodenum was measured in 11 healthy volunteers. We used the technique of simultaneous intestinal perfusion and measurement of the corresponding transmucosal potential differences (pd). The unstirred layer was calculated from the half time required to establish an Na+ diffusion potential across this layer. At a flow rate of 17 ml/min the thickness was 351 +/- 13 microns (mean +/- 1 SEM). The level increased significantly as the flow rate was reduced. The results were used to calculate estimates of the real transport constants, which describe the carrier-mediated electrogenic transport of glucose in the normal upper duodenum. Pdrealmax was calculated to be 1.1 +/- 0.15 mV, Jrealmax 6.6 nmol/cm2 X sec, and Krealm 25.8 mM. The origin of the transport-related potential change during glucose absorption was examined. It is suggested that about 60% of the total measured potential response may be attributed to a Na+ diffusion potential across the intestinal unstirred water layer.