390 ANIMAL BIOCHEMISTRY 



traction, potassium is released extensively from the cells into the 

 extracellular fluid. Apparently K+ participates in changes in the 

 actomyosin system. There is also displacement of potassium in the 

 transmission of nerve impulses with rapid diffusion out of the cell 

 during stimulation and return when the nerve is at rest. Some K+ 

 is needed with Mg++ to activate choline acetylase in the resynthesis 

 of acetylcholine (page 308). Perhaps through its activity in the nerv- 

 ous system, potassium plays a part as a regulator in controlling the 

 heartbeat, where K+ increases and Ca++ decreases the rate. Both 

 must be present and in the proper ratio for suitable performance of 

 this organ. The individual effects are readily observed in tissue cul- 

 tures of heart cells which show pulsations corresponding to beats. 



Since plants contain far more K+ than Na+, daily allowances are 

 of little practical concern. Rats, pigs, and chickens apparently need 

 diets containing 0.2 to 0.3 per cent K+ on a dry basis. Any ordinary 

 diets adequate in other respects contain more than this proportion. 

 Human beings ingest 2 to 5 g. of K+ daily with perhaps half excreted 

 as unnecessary. 



Deficiencies are iniknown in ordinary individuals on ordinary diets 

 but can be produced experimentally. Poor growth and coat, kidney 

 and heart damage, paralysis of the legs, and death have been observed 

 in the laboratory. Potassium deficiency is observed clinically in con- 

 nection with a variety of special pathological states and is automati- 

 cally alleviated when the causative conditions are corrected. 



Potassium toxicity occurs only in the terminal phases of uremia, 

 Addison's disease, improper parenteral administration, and shock, or 

 after severe burns. Otherwise, an excess of K+ is quickly excreted, 

 mainly in the urine. Lesser quantities are eliminated via the feces 

 or perspiration. 



The antagonism of K+ for other ions involves Na+, Ca++, and 

 Rb+. Sodium and potassium levels are important and one ion ex- 

 changes for the other in nerve and muscle activity. During quiescent 

 periods the reverse exchange occurs. An example of the antagonism 

 between K+ and Ca++ was mentioned above. Rubidium is rather 

 toxic, but an increase in the K+ counteracts the presence of the 

 heavier ion. 



Selenium 



This element has been found essential for the formation of the 

 enzyme formic dehydrogenase in Escherichia coli. Also, since it is 

 accumulated in certain plants which grow only on soils of high 

 selenium content, it may turn out to be essential for these plants. 



