Kinetics of glucose decarboxylation in the substrate-limited isolated perfused kidney

Rat kidneys were perfused with a cell-free perfusate containing substrate-free albumin, different glucose concentrations (0.20-5.0 mmol/l), and uniformly labeled 14C-glucose. The rate of glucose decarboxylation (Qox), as a function of [glucose]p, displayed saturation kinetics [Vmax = 0.35 mumol/(g.min); Km = 0.87 mmol/l]; saturation occurred at [glucose]p = 1.0-2.0 mmol/l. Although the presence of as low as 0.2 mmol/l of glucose significantly increased fractional sodium reabsorption (%TNa), there was no correlation between [glucose]p or Qox and % TNa. However, free water clearance (CH2O or CH2O/GFR) was directly proportional to [glucose]p and independent of Qox. We conclude that (1) in the absence of other substrates, renal glucose Qox saturates at hypoglycemic levels of glucose and (2) glucose plays an important role in the generation of solute-free water, a role that is unrelated to glucose Qox.