| Abstract: | In the duck red blood cell, Na-K-2Cl cotransportexhibits two modes of ion movement: net cotransport and obligate cationexchange. In high-K cells, the predominant exchange is K/K (or K/Rb).In high-Na cells, it becomes Na/Na (or Na/Li). Both represent partial reactions in which a fully loaded carrier releases part of its cargo,rebinds fresh ions, and returns back across the membrane fully loaded.Net cotransport occurs when the carrier unloads completely and returnsempty. This mode has a fixed stoichiometry of 1Na:1K:2Cl under allconditions tested. The ion requirements of the two exchanges differ:K/K exchange requires only K and Cl outside but all three ions inside.Na/Na exchange requires all three ions outside but only Na inside. Wepropose a simple model in which the carrier can only move when eitherfully loaded or completely empty and in which the ions bind in astrictly ordered sequence. For example, externally, a Na binds firstand then a Cl, followed by a K and a second Cl. Internally, the firston is the first off (glide symmetry), so the Na is released first andthen the first Cl, followed by the K and finally by the second Cl. Onlythen can the empty form return to the outside to start a new cycle. |
