218 



CALCIUM 



Intake Levels. The approximate daily intake of calcium for the var- 

 ious species is as follows: man, 0.9 g; rat, 40 mg; laying hen, 3 g; sheep, 

 4 g; 100-lb pig, 12 g; and cattle, 17 g. These values are lower, of course, 

 for younger animals and increased under conditions of pregnancy or 

 lactation. In studies of oral administration there is usually no problem 

 in regard to the administration of too much mass because of the normal 

 high intake. The plasma level of calcium in most species ranges from 9 

 to 11 mg/100 ml, and toxicity symptoms may be produced if the concen- 

 tration gets above 15 mg/100 ml. For injection studies, therefore, the 

 specific activity of the preparation may be an important factor. It has 

 been reported that 60 to 90 mg/kg of the chloride or acetate injected into 

 cats or dogs will produce toxicity symptoms (Ca-1). Soft tissues of 

 animals contain 0.05 to 0.12 mg calcium per gram fresh weight, and bones 

 range from about 80 to 250 mg/g. A recommended nutrient solution for 

 plants contains about 160 ppm, and crop plants usually have from 0.1 to 

 3 per cent on a dry-matter basis. 



Radioassay. Preparations Ca-45-P-l and Ca-45-P-2 are most com- 

 monly used, and the choice between them is governed by the need for 

 increased specific activity vs. the increased cost. Likewise the use of the 

 other high-specific-activity preparations would have to be justified in 

 terms of cost. Ca"*^ should be counted with a thin-mica-window tube 

 (<4 mg/cm-) or an internal counter. Self-absorption considerations are 

 critical, so that a favorable Ca^V^fiass ratio must be used to avoid undue 

 loss of sensitivity. 



Chemistry. The chemical estimation of calcium in biological tissues 

 has been the subject of extensive investigation, and details are readily 

 available in the standard literature. In general, the sample is ashed, and 

 calcium oxalate precipitated from the ash solution and then measured 

 gravimetrically or by titration. No particular precautions are required 

 in the ashing process. Calcium oxalate represents an ideal compound for 



