Potassium ion: is the bulk of intracellular K+ adsorbed?

When a major portion of the intracellular K(+) in frog muscle is reversibly replaced by Na(+), the extra Na(+) gained by the cells does not show the nuclear magnetic resonance signal that free Na(+) does. The data contradict the membrane theory but are in accord with the concept that the bulk of intracellular K(+) is adsorbed.