Quantification of calcium paradox in neonatal rat hearts

Many indexes of damage have been used to quantify the calcium (Ca2+) paradox. We report here how different conclusions may be reached concerning the severity of the paradox when different indexes of damage are used. Hearts from 3- to 24-day-old and adult rats were isolated, depleted of Ca2+ for 5 min, and then Ca2+ repleted. Myoglobin loss, Ca2+ gain, and ultrastructural changes were monitored. Our results indicate that with increasing age each of these parameters shows a progression toward the adult state. However, this progression does not proceed at the same rate for each parameter. There was very little release of myoglobin from hearts of rats of less than 11 days old, but the amount released increased with increasing age. There appeared to be a direct relationship between the amount of myoglobin released and the proportion of extremely contracted cells with disrupted sarcolemma. However, hearts from rats of all ages gained Ca2+, suggesting that Ca2+ entry occurs via contracture-dependent and contracture-independent routes. Thus we are able to dissociate Ca2+ gain from the development of contracted cells and myoglobin release at the youngest ages. Different conclusions could therefore be reached as to the severity of the Ca2+ paradox at these youngest ages depending on which indexes of damage are documented.