Cesium Determination in Physiological Fluids and Tissues

The physiological medium of living cells mainly consists of alkali and alkaline earth ion solutions. The different concentrations of sodium-, potassium- and calcium-ions 

It has been shown that a concentration of 1 mmol/1 cesium in Tyrode solution as the outer medium of cardiac Purkinje fibers — part of the excitable system of the heart — is sufficient to suppress the potassium outward current (pacemaker current) nearly completely . When the concentration of cesium exceeds 20 mmol/1 all detectable potassium currents in cardiac Purkinje fibers are blocked. Since this effect occurred within one or two minutes it was believed that cesium ions are able to block the potassium channels in the membrane from the outside. One possible explanation would be based upon the fact that cesium ions are bigger than potassium ions (diameter [Cs] = 1.77 A [K] = 1.33 A). Therefore, if they eventually invade the potassium channels, they should be hampered in their motion or even immobilized because of their bigger size. If this was the case, cesium ions would invade potassium channels whenever they reach cell membranes and would irreversibly block them. Besides, cesium ions should not be detectable inside the cells of the excitable system, since they are imable to penetrate the cell membranes. 

From the standpoint of all classical microanalytical experiments on organic cell materials containing alkali ions this hypothesis seemed to be correct. However, the only reason opposite to this was that a concentration of cesium, which would be enough to block all potassium channels (about 100 nmol/1), was too small for such measurements. 

According to the calculations on channel numbers this is sufficient to block all potassium channels in the membranes. If cesium ions are able to block the potassium outward current in the excitable system of the heart at all, they do not do so by plugging the channels from the outside of the membrane. After the application of a Tyrode solution, containing 1 mmol/1 cesium, for only 1.5 min and a prolonged 

These relatively high concentrations measured by FDMS can also be detected by other methods with adequate accuracy, although the FD method has the advantage that tissues need no pretreatment. Parallel measurements of the same preparations by atomic absorption and emission spectroscopy have revealed no significant deviations from the authors results. 

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