MINERAL METABOLISM 389 



ous fractures of the pelvis and back. Less drastic deficiencies are 

 hard to detect and may be common in highly producing dairy herds. 

 Eventually the weight declines significantly, milk production falls, 

 reproduction is impaired, and finally the more severe and obvious 

 symptoms appear. Response to extra phosphate is usually quite good. 

 Bran, cottonseed and linseed meals, milk, gluten, and soybeans are 

 the best feeds from the point of phosphate content. Bone meal and 

 phosphate rock (after reduction of fluoride to a safe level) are the 

 most useful supplements in fortifying corn stover, hay, silage, and 

 grains. Pastures can be made satisfactory by liberal application of 

 phosphate fertilizers, which increase the content of this element in 

 the plants and raise the bulk productivity of the pasture at the same 

 time. 



Large excesses of phosphate can be toxic in the sense that the ab- 

 sorption of most of the cations is impeded. This antagonism works 

 both ways and has been mentioned in connection with calcium, iron, 

 magnesium, and manganese. Herbivores excrete phosphate mainly in 

 the feces, even the phosphate eliminated from the tissues. The carni- 

 vores excrete phosphate primarily in the urine, and man is intermedi- 

 ate. In addition, man and perhaps horses lose appreciable amounts 

 in sweat. 



Potassium 



Some animals (rats, pigs, and chickens) have higher requirements 

 for potassiimi than they do for sodium. In others the daily needs of 

 K+ are relatively lower. All animals need this element and use it in a 

 variety of roles. 



As an important ion in physiological fluids, K+ enters into the 

 electrolyte and water balances. Not so abundant as Na+ in plasma, 

 interstitial fluid, and urine, potassium ion is less of a factor than its 

 close relative. However, K+ is the more plentiful of the two inside 

 cells and presimiably is instrumental in the osmotic concentrations of 

 most cells. For example, there is 6 times as much K+ as Na+ in 

 muscle and more than 20 times as much in human erythrocytes. 

 Clearly this concentration effect relative to sodium indicates that 

 potassium performs somewhat different functions, although acting 

 mostly in unknown ways. 



Along with Mg+ + , potassium is an activator for the metabolic 

 enzyme pyruvic kinase in the glycolytic step. During muscular con- 



CH2=C— COO- J,, 



1 + ADP • > CHaCOCOO- + .\TP 



OPO3H- P^""" '■"'"''' pyruvate 



phosphoenolpyruvate 



