J. F. LOUTIT 



one hundredfold, as would be expected if it were not strontium levels which 

 were important but combined calcium and strontium levels. 



Inertia of the System 

 For what are probably good physiological reasons the mechanism of control 

 for calcium metabolism has considerable inertia. Calcium is an essential 

 element for neuromuscular co-ordination and for the structure of the skeleton. 

 Calcium in the extracellular water amounts to just over 1 g and in the 

 skeleton to just over 1000 g in the 70 kg man. While far from all the skeletal 

 calcium is available it provides a very substantial bank on which the body 

 can draw in time of need to maintain a constant level of plasma calcium. 

 Minor changes in calcium balance evoke no rapid response. Substantial 

 changes in the intake of calcium are reflected in the total economy of 

 calcium only after long latent intervals. Thus, Walker et al.'^^ found that it 

 took many weeks for subjects to come into calcium balance after being 

 transferred from adequate to low calcium diets. On the other hand, and 

 understandably, an animal made chronically deficient of calcium will adapt 

 and take all available supplies with avidity and will continue to do so for a 

 long time. 



The growing animal is more responsive and labile than the adult. 

 Nevertheless, even under these circumstances, there is a steady pattern of 

 accumulation which is adhered to in spite of environmental irregularities, 

 unless these are extreme. 



Interpretation ofSrjCa Ratios 

 Analyses are available of human bones and various articles of diet for ^"Sr 

 and stable strontium (^Sr) in relation to calcium. The problem is how to 

 evaluate the data and what to use for purposes of prediction. In this exercise 

 it is important to bear in mind the physiological considerations discussed 

 above and particularly to be wary of the use of the equation derived by 

 Comar et al.^-\ 



. Sr/Ca bone 



Observed Ratio = ^ ,,-, 



Sr/Ca precursor 



unless it can be established that a state of equilibrium exists. No one is more 

 emphatic on this than Comar himself. 



Stillbirths 



Assays of the bones of stillbirths have been made for ^"Sr and ^Sr in 

 relation to calcium. There is a much greater variation in the ratio ^"Sr/Ca 

 than in the ratio ^Sr/Ca. If a state of equihbrium is assumed between foetal 

 bone and maternal plasma the data for ^Sr/Ca can be compared with other 

 observations on human plasma. Representative figures are 200 ii.g *Sr/g Ca 

 for bone of stillbirths (Bryant et al.^'^^'^*, Sowden and Stitch^^) and 400 [ig 

 ^Sr/g Ca for human plasma (personal communication from G. E. Harrison), 

 which would suggest a placental discrimination factor of two for man as well 

 as rat. 



Whereas it has been possible to assay the bones of individual stillbirths 

 for ^°Sr no figure has been obtained for individual human plasmas. On an 

 isotopic basis one could suggest that, if a median figure of about 0-5 S.U. 



121 



