Flow dependence of nonelectrolyte absorption in the nephron.

The axial flow dependence of nonelectrolyte absorption was examined in terms of a model incorporating interactions between net volume absorption and both saturable and nonsaturable solute absorption. The model solutions demonstrated that changes in transepithelial solute transport are produced by changes in the average luminal solute concentration. Even passive non-saturable solute absorption was shown to exhibit dependence on the perfusion rate, and, therefore, on the solute delivery rate, which could be incorrectly interpreted as demonstrating the presence of a saturable absorptive mechanism. For a unidirectional lumen-to-bath solute flux mediated in part by a saturable mechanism, the observed flux is dependent on the permeability of any parallel nonsaturable permeation pathway. This permeability also sets a lower bound on the luminal solute concentration which may be achieved during active net solute absorption by determining the rate of passive solute backleak. Extension of the model to incorporate dependence of net volume absorption on the delivery of nonelectrolytes predicted a relationship between perfusion rate and net volume absorption equivalent to approximately one-third of complete glomerulotubular balance.