Response to Zeuthen and Zeuthen's Comment to the Editor: Enough Local Hypertonicity Is Enough

In a Comment to the Editor, Zeuthen and Zeuthen criticize our treatment of the water cotransport hypothesis. In this response, we argue that we calculated water cotransport as if there were no significant local osmotic gradient generated in the first minute of Na/glucose cotransport. It is surprising to receive this type of criticism from Zeuthen and Zeuthen, as the same treatment was used in at least six studies from his laboratory where it is systematically assumed that “intracellular unstirred layers effects” are negligible. Zeuthen and Zeuthen also state that “the cotransport hypothesis predicts the measurements better than the osmotic hypothesis”. We present a quantitative comparison that challenges this contention. We would like to conclude by stating that our article was not about comparing different numerical models but about an experimental measurement of the local osmotic gradient generated after 20, 40, or 60 s of cotransport. Osmotic gradients were indeed detected, and were of appropriate amplitude to explain virtually all water transport observed.