188 



J. C. Stoclet, Y. Cohen: Calcium Exchanges In the Aorta of the Rat 



it fits all other experimental determinations. The corresponding rates of exchange are 

 2.8//M/min/g for the rapid exchange and 0.012 //M/min/g for the slow exchange. 



We found a ^-Br space of 650 ± 15 //I and a 2-»Na space of 830 ±350 //I per gram 

 of aorta. 



P.S.i 



02 to 30m\T) 12 3 hours V ' V8 



Time 



Fig. 2. Variations with time of the specific activity of aorta calcium (R. S. a), rapidly exchangeable ("inter- 

 stitial") calcium (R. S.) and slowly exchangeable calcium (R. S. 4) after intravenous injection of '•^CaClj to 

 the male rat of 200 g: calculated curves and mean of experimental measures (fiducial limits at a probability 



of 0.95) 



Discussion 



The wall of the aorta is made up mainly of elastic membranes which are known 

 to be very sensitive to calcification in pathological or extraphysiological conditions. 

 It seems reasonable to admit that the high calcium content of the aorta in the 

 physiological state is characteristic of non-degraded elastic tissue. It could represent 

 the first of the three steps in calcification of elastic tissue recently described by Urist 

 et al. (1964). This high calcium content masks the cellular calcium of the smooth 

 muscle fibers of the aorta. We did not find any diffusible or bound calcium. 



The kinetics of aorta calcium are very similar to those of bone calcium since we 

 found the same three compartments in the aorta as are usually found in bone. We 

 should like to emphasize two points: 1. The aorta is the only soft tissue of the rat 

 (amongst several striated and smooth muscles) where slowly exchangeable and non- 

 exchangeable calcium have been found in vivo. 2. Rapidly exchangeable calcium is 

 usually located in extracellular fluids where calcium concentration is about the same 

 as in a plasma ultrafiltrate. The situation is different in the aorta but reminds the one 

 of bone. Although its extracellular space (^^Br) is high (65 per cent of the wetweight), 

 the aorta also contains "extra" sodium and rapidly exchangeable calcium. 



