Driving Force Across a Leaky Epithelium
An investigator studies glucose uptake across an intestinal epithelial monolayer mounted in an Ussing chamber. The apical solution contains 5 mM glucose and 140 mM Na+; the basolateral solution contains 5 mM glucose and 140 mM Na+. The apical membrane carries SGLT1 (Na+/glucose cotransporter, 2 Na+ per glucose) and GLUT2. Ouabain is added to the basolateral side. Within minutes, apical glucose uptake falls by more than 90% even though the apical glucose gradient is unchanged and SGLT1 protein is still present at the surface. Separately, the investigator notes that a small paracellular leak of glucose persists after ouabain.
1. Explain why blocking the Na+/K+-ATPase collapses SGLT1-mediated glucose uptake despite an unchanged glucose gradient.
Read the worked answer
SGLT1 is a secondary active transporter; its driving force is the electrochemical Na+ gradient, not the glucose gradient. The Na+/K+-ATPase maintains low intracellular Na+ (and high K+), which is what makes the inward Na+ gradient steep. Ouabain inhibits the pump, intracellular Na+ rises, and the inward Na+ electrochemical potential collapses. With 2 Na+ coupled per glucose, the transport free energy is dominated by the Na+ term (roughly 2 x the Na+ electrochemical potential), so even a modest rise in internal Na+ can reverse or abolish net glucose influx. The glucose gradient being equal on both sides is irrelevant because glucose movement is energetically coupled to Na+, not driven by its own gradient here.
2. The residual glucose flux after ouabain is insensitive to SGLT1 inhibitors and is abolished by tightening tight junctions. Interpret this residual flux and explain what it tells you about interpreting the 90% drop.
Read the worked answer
The residual flux is paracellular, moving through the tight-junction pathway driven by the standing osmotic/electrochemical gradients rather than by a transporter. Its sensitivity to junction tightening (and insensitivity to SGLT1 inhibitors) identifies it as a leak, not carrier-mediated transport. This matters because the '90% drop' is a drop in total flux, part of which was never SGLT1-mediated. The correct conclusion is that the transcellular, SGLT1-dependent component fell essentially to zero, while the paracellular component was unaffected by ouabain. Reporting the drop without subtracting the leak overstates the residual transporter activity and confuses a junction property with a pump property.