A model of Na-K-2Cl cotransport based on ordered ion binding and glide symmetry

Abstract

In the duck red blood cell, Na-K-2Cl cotransport exhibits two modes of ion movement: net cotransport and obligate cation exchange. 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 returns empty. This mode has a fixed stoichiometry of 1Na:1K:2Cl under all conditions 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. We propose a simple model in which the carrier can only move when either fully loaded or completely empty and in which the ions bind in a strictly ordered sequence. For example, externally, a Na binds first and then a Cl, followed by a K and a second Cl. Internally, the first on is the first off (glide symmetry), so the Na is released first and then the first Cl, followed by the K and finally by the second Cl. Only then can the empty form return to the outside to start a new cycle.

Kinetics of hyperosmotically stimulated Na-K-2Cl cotransporter in Xenopus laevis oocytes

“…Furthermore, we show that a partial transport of K ϩ and Cl Ϫ without transport of Na ϩ necessitates internal release of K ϩ before Na ϩ . Therefore, the order of external ion binding is: Cl Ϫ , followed by Na ϩ , the second Cl Ϫ , and then K ϩ , and the order of release inside is K ϩ , the second Cl Ϫ , Na ϩ , and then the first Cl Ϫ , differentiating our model from the glide symmetry model proposed by McManus and colleagues (32).…”

“…In the previous section, we show that the model proposed by Lytle et al (32) could not be supported under rapid equilibrium assumptions (Fig. 1); therefore, we proposed an alternative order of ion binding for the Na-K-2Cl cotransporter (Fig.…”

The Residues Determining Differences in Ion Affinities among the Alternative Splice Variants F, A, and B of the Mammalian Renal Na-K-Cl Cotransporter (NKCC2)

“…The Na-K-Cl cotransporter is thought to translocate four ions across the membrane in a single file mechanism in the order Na ϩ , Cl Ϫ , K ϩ , Cl Ϫ (19). It is thus intriguing to propose that the TM2 site might bind one ion pair, and ICL1 the other ion pair.…”

The permeability of red blood cells to chloride, urea, and water

“…The duck RBC is an oval nucleated cell with axes 13.1×7.4 μm and a thickness of 1 μm in the non-nucleated part of the cell. Duck RBC V was calculated to be ~175 fl, as 1 kg of cell dried solids equals 14.7×10 12 cells (Lytle et al, 1998), and normal cells contain 1.55-1.57 l cell water per 1 kg cell dried solids (Lytle and McManus, 2002;present study). Amphiuma RBC V and V w /A were calculated from V w =0.682 (w/w) (Siebens and Kregenow, 1985; present study), A=5000 μm…”