Controversy regarding the secondary active water transport hypothesis

Historically, water transport across biological membranes has always been considered a passive process, i.e., the net water transport is proportional to the gradients of hydrostatic and osmotic pressure. More recently, this dogma was challenged by the suggestion that secondary active transporters such as the Na/glucose cotransporter (SGLT1) could perform secondary active water transport with a fixed stoichiometry. In the case of SGLT1, the stoichiometry would consist of one glucose molecule to two Na+ ions to 220-400 water molecules. In the present minireview, we summarize and criticize the evidence supporting and opposing this water cotransport hypothesis. Published and unpublished observations from our own laboratory are also presented in support of the idea that transport-dependent osmotic gradients begin to build up immediately after cotransport commences and are fully responsible for the cell swelling observed.