Effects of vascular perfusion on the accumulation, distribution and transfer of 3-O-methyl-D-glucose within and across the small intestine.

1. Factors affecting the transfer of the non-metabolized, ;actively transported' sugar, 3-O-methyl-D-glucose (3MG) across the small intestinal epithelium have been examined in vascularly perfused anuran intestine. Transfer has been studied during absorption in the steady state, and also during the period of transition from one steady state to another.2. During the steady state, the rate of absorption of 3MG from the intestinal lumen is equal to the rate of appearance in the portal venous effluent; this rate of transfer is to a small, but significant, extent directly related to the rate of arterial perfusion. With phlorizin in the intestinal lumen, transfer across the epithelium is reduced to very low rates which are independent of the rate of vascular perfusion.3. The apparent size of the tissue pool(s) of 3MG that have to be loaded to achieve the steady state rate of transfer are less than those that unload into the vascular bed after 3MG is removed from the intestinal lumen. This ;up-down asymmetry' is abolished when phlorizin is present in the intestinal lumen during the unloading phase.4. When 3MG is abruptly removed from the intestinal lumen after the tissue has been previously loaded with the sugar, the rate of washout into the vascular bed can be described by the sum of two exponential terms. The two terms differ in that the rate constant of the earlier ;fast' term is sensitive to the rate of vascular perfusion, while the later, ;slow', rate constant is insensitive to flow rate. The total quantity of 3MG that can be unloaded from the tissue into the portal venous effluent is decreased when phlorizin is present in the intestinal lumen during the unloading phase.5. Absorption from the lumen of the anuran intestine continues while the mesenteric circulation is interrupted. An estimate of the concentration of 3MG during the period of vascular stoppage can be made from the quantity recovered in the portal venous effluent when vascular perfusion is reinstituted (;vascular stop-flow'). The extent of the accumulation depends upon the duration of the vascular stoppage and the presence of Na ions in the intestinal lumen is essential for accumulation to occur.6. The findings are discussed in relation to the transfer of 3MG between various possible compartments in the tissue during absorption. Evidence is presented that a re-uptake of previously absorbed 3MG may occur across the brush border membrane. Such a recycling of 3MG across the epithelium implies that the apparent unidirectional fluxes measured across the epithelium between the bulk phase of the lumen and the blood may underestimate the size of fluxes across the epithelium at the cellular level.