74 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



Although K+-uptake depends on the active metaboHsm of substrate, the 

 reverse is not true; metabolism is not dependent on K+-uptake. Actually there 

 is no fixed relationship between the rate of K-uptake and the rate of metabo- 

 lism. At a maximum i mol of K+ is moved for every 3 mols of glucose or for 

 every 12 mols of O2, but many factors will reduce this ratio (46). 



Because K+-uptake is dependent on an active metabolism, it is not surprising 

 that metabolic inhibitors have a depressing effect. However, certain of the 

 inhibitors can decrease or abolish the K-uptake without any concomitant 

 reduction in the rate of substrate utilization or of gas exchange. These include 

 NaF (42), NaNs, DNP (51, 4), and urethane (4). In this case the inhibitors 

 do not prevent the production of metabolic energy but prevent its utilization 

 for ion transport. 



When potassium ions move into the cell, they must, in order to maintain 

 electrical balance, either be accompanied by an appropriate number of anions, 

 or be exchanged for other cations from the cell. Rothstein (51) and Conway 

 (12) demonstrated that under most conditions the K+ was taken up in a i for i 

 exchange with H+. As a result the pn of the external medium drops to values 

 as low as 1.7. The average pH within the cell during fermentation is about 6 

 (8), indicating an H+ concentration of about i X 10""® m/1. The potassium 

 concentration is greater than o.i m/1. Yet a rapid K+-uptake and H+-excretion 

 occurs when the concentrations of K"*" and H+ in the medium are io~^ m/1. and 

 io~^ m/1., respectively. Each ion in this case moves against an apparent con- 

 centration gradient of about 1000 to i (46). 



There is a maximum amount of potassium which can be taken up which is 

 dictated by the extracellular concentrations of K+ and H+ and also by the 

 intracellular concentration of K+ in a manner which has not been completely 

 characterized. The maximum concentration of K+ in the cytoplasm approaches 

 0.3 m/1. under certain conditions (51). 



Nearly all of the data pertaining to K+-uptake have been obtained by meas- 

 uring the disappearance curve of K+ from the medium by chemical analysis. 

 If K+ moves only inward, with a neghgible simultaneous outward leakage, then 

 the measured rates of uptake represent the true rate of K+-transport. If, how- 

 ever, there is a simultaneous inward transport and outward leakage, then the 

 measurements represent the net effect of inflow and outflow. It has been pointed 

 out that in resting cells suspended in distilled water, there is only a very slow 

 outward leakage of K+ despite the high cellular concentration of this ion (51, 

 63). In contrast, the actively metabolizing cell leaks K+ at a higher rate despite 

 the fact that there is a net movement of K+ into the cell. For example, Hevesy 

 (24) in one of the earliest studies with radioactive potassium (K'*'-) found little 

 exchange of cellular and environmental K+ in slowly fermenting yeast, but 

 found a high rate of exchange in rapidly fermenting yeast, higher than could be 

 accounted for by the uptake of K+ measured chemically. The fermenting cells 



