Competition between Na+ and Li+ for Unsealed and Cytoskeleton-Depleted Human Re...

• Author(s): Srinivasan, Chandra ; Minadeo, Nicole ; Toon, Jason ; Graham, Daniel ; Freitas, Duarte Mota de ; Geraldes, Carlos F. G. C.
• Date: 1999
• Persistent ID: http://hdl.handle.net/10316/3897
• Origin: Estudo Geral - Universidade de Coimbra
• Subject(s): lithium; human red blood cell membranes; cytoskeleton; multiple-quantum-filtered 23Na NMR; 7Li relaxation times
• Document Type: Article
• Language: English

Description

Evidence for competition between Li+ and Na+ for binding sites of human unsealed and cytoskeleton-depleted human red blood cell (csdRBC) membranes was obtained from the effect of added Li+ upon the 23Na double quantum filtered (DQF) and triple quantum filtered (TQF) NMR signals of Na+-containing red blood cell (RBC) membrane suspensions. We found that, at low ionic strength, the observed quenching effect of Li+ on the 23Na TQF and DQF signal intensity probed Li+/Na+ competition for isotropic binding sites only. Membrane cytoskeleton depletion significantly decreased the isotropic signal intensity, strongly affecting the binding of Na+ to isotropic membrane sites, but had no effect on Li+/Na+ competition for those sites. Through the observed 23Na DQF NMR spectra, which allow probing of both isotropic and anisotropic Na+ motion, we found anisotropic membrane binding sites for Na+ when the total ionic strength was higher than 40 mM. This is a consequence of ionic strength effects on the conformation of the cytoskeleton, in particular on the dimer-tetramer equilibrium of spectrin. The determinant involvement of the cytoskeleton in the anisotropy of Na+ motion at the membrane surface was demonstrated by the isotropy of the DQF spectra of csdRBC membranes even at high ionic strength. Li+ addition initially quenched the isotropic signal the most, indicating preferential Li+/Na+ competition for the isotropic membrane sites. High ionic strength also increased the intensity of the anisotropic signal, due to its effect on the restructuring of the membrane cytoskeleton. Further Li+ addition competed with Na+ for those sites, quenching the anisotropic signal. http://www.sciencedirect.com/science/article/B6WJX-45FKRY3-38/1/bc487aa2e53ceb4bdc5b795e1fbc7ee4