Injury is sustained by isolated hearts on repletion with calcium after a short 
period of perfusion with calcium-free medium at 37 degrees. A possible mechanism 
for the 'calcium paradox' is that exposure to a calcium-fre medium removes 
extracellular calcium rendering the sarcolemma more permeable to calcium. On 
calcium repletion, cell injury is triggered by calcium influx. Since lanthanum 
is known to displace calcium from extracellular pools in heart, it was used in 
an attempt to modulate the injury of the calcium paradox. The presence of 10 
microM lanthanum in the calcium-free perfusion fluid was found to inhibit 
totally protein release normally produced by subsequent calcium exposure. When 
lanthanum was added after the calcium-free period and before calcium repletion, 
protein release was only partly prevented. This shows that a change in membrane 
properties occurred during the calcium-free perfusion period which could be 
prevented, but not reversed, by the addition of lanthanum.