Preferential localized uptake of K+ and Cs+ over Na+ in the A-band of freeze-dried embedded muscle section: detection by x-ray microanalysis and laser microprobe mass analysis.

Sections of freeze-dried embedded frog muscle were exposed to aqueous solutions containing various combinations of the salts LiCl, NaCl, KCl, and CsCl. Energy dispersive X-ray microanalysis and laser microprobe mass analysis of these sections showed selective and preferential accumulation of K+ (and Cs+) over Na+ on specific protein sites in the A-bands. The selectivity coefficient exceeded 10 and involved a total K+ accumulation of about 40 mmoles/kg (in comparison with a value of about 80 mmoles/kg in normal living frog muscle). These findings support the view that selective K+ adsorption on intracellular proteins is the primary cause of selective accumulation of K+ in living cells.