Potassium adaptation: 39K-NMR evidence for intracellular compartmentalization of K+

To investigate the effect of K{sup +} uptake on the intracellular environment, both {sup 39}K-nuclear magnetic resonance (NMR) and K{sup +}-selective electrodes were used to measure K{sup +} activity with acute K{sup +} loading in control and K{sup +}-adapted rats. These results were then compared with tissue K{sup +}, measured by flame photometry. There was a lower NMR K{sup +} visibility (ratio of NMR signal to tissue content) in muscle and liver in K{sup +}-adapted rats, compared with controls before and after an acute K{sup +} load. This lower K{sup +} visibility in K{sup +}-adapted rats was confirmed in liver homogenate with the K{sup +}-specific electrode. In liver homogenates from control and K{sup +}-adapted rats, addition of RbCl increased the NMR K{sup +} signal more in K{sup +}-adapted rats than controls. This is consistent with the displacement of K{sup +}, by Rb{sup +}, from NMR-undetected sites. These results suggest that some 10-15% of intracellular K{sup +} may be within a compartment not detectable by NMR or electrodes and that chronic K{sup +} loading leads to an increased capacity of this compartment.