Calcium Exchanges In the Aorta of the Rat 



187 



The decrease with time of the specific activity of plasma calcium in the same 

 rats as presently used has been described previously by one of us (Stoclet, 1964) 

 and has been shown to correspond to the scheme shown in Fig. 1. ■'•'Ca once intro- 



Male rats of 200 g 



Plasma Ca 

 0.6 mg 



56 mg/hour 



Absorption 



+ 



bone resorption 



• 



3 mfr/hour 



Interstitial fluid calcium 

 4 ms 



5 mg/hour 



Bound cell Ca 

 16 me 



Excretion 



+ 



bone formation 



Ditfusible cell Ca 

 2.9 mg 



and dvnamics in the 



duced into the blood plasma rapidly exchanges with "interstitial calcium" located in 

 interstitial fluid and connective tissue. Interstitial calcium is itself submitted to much 

 slower exchange reactions with tissue calcium and is continuously renewed by uni- 

 directional processes; resorption from bone and absorption supply calcium which is 

 lost by deposition in bone and excretion. Non-extracellular tissue calcium is dis- 

 tributed in three compartments in which the exchange rates are very different i.e. 

 diffusible calcium, bound calcium and slowly exchangeable bone calcium. Diffusible 

 and bound calcium have been identified in soft tissues (smooth and striated muscle) 

 but the corresponding compartments shown in Fig. 1 are larger than the calcium 

 content of the soft tissues of the whole rat. In growing rats some of the exchangeable 

 fractions of bone calcium are included in diffusible and bound compartments since 

 their exchange rates are respectively equal to the one of diffusible and bound cellular 

 calcium (Stoclet and Cohen, 1963). 



The specific activity of calcium contained in each compartment has been calcu- 

 lated at the different experimental times and compared with the specific activity of 

 aorta calcium at the same time. Fig. 2 shows the variations with time of the specific 

 activity of calcium in the aorta (R. S.a), in "interstitial" rapidly exchangeable com- 

 partment (R. S.) and in slowly exchangeable compartment (R. S. 4). The plain line 

 represents the calculated curve for the following distribution of aorta calcium: rapidly 

 exchangeable (extracellular) 3.5 //M per g, slowly exchangeable (like bone calcium) 

 6.7 //M per g and non-exchangeable calcium 9.0 tiM per g. This calculated curve has 

 been plotted from ■'^Ca radioactivity in the aorta at two minutes and four hours and 



