A Model of the Potassium Effect

The protoplasm of certain cells is able to distinguish electrically between K(+) and Na(+). This has been called the potassium effect. This is illustrated by experiments with Nitella. When 0.01 M KCl which has stood in contact with Nitella is replaced by 0.01 M NaCl the P.D. changes in a positive direction by an amount which varies between 30 and 95 mv. This ability to distinguish between K(+) and Na(+) disappears with the removal of an organic substance from the cell. The amount of this substance is doubtless too small to make it possible to obtain enough for analysis. An attempt has therefore been made to find an organic compound which can produce similar effects. It is found that when M/1 KCl in contact with nitrobenzene (previously shaken with M/1 KCl) is replaced by M/1 NaCl the potential changes in a positive direction to the extent of 67 mv. which compares favorably with the values found in Nitella. This is not due to a greater mobility in nitrobenzene of K(+) as compared with Na(+): this is evident from measurements of concentration effects with nitrobenzene (M/1 KCl vs. M/10 KCl and M/1 NaCl vs. M/10 NaCl). It might be brought about if KCl produced in nitrobenzene a sufficient preponderance of ions (simple or complex) as compared with NaCl. Whether this occurs could not be determined but it was found that nitrobenzene shaken with M/1 KCl has a higher conductivity than when shaken with M/1 NaCl. Measurements with salicylates showed that K-salicylate has a partition coefficient about 11.7 times as great as that of Na-salicylate. It was also found that when M/1 K-salicylate in contact with nitrobenzene (previously shaken with M/1 K-salicylate) is replaced by M/1 Na-salicylate there is a change of potential in a positive direction amounting to 56 mv. To what extent phase boundary potentials may enter into the observed values cannot be determined at present. The model resembles the Nitella cell in that RbCl and KCl are negative to NH(4)Cl which in turn is negative to NaCl and still more so to LiCl (in the model CsCl is negative to KCl but in Nitella it is positive). It likewise resembles Nitella in that the potassium effect is lessened by the addition of guaiacol.