[Early hypoxic contracture of the myocardium in adult and newborn rats].

The effect of 30 min substrate free hypoxia (H) on isometric tension was studied in isolated myocardium (M) of adult (A) and newborn (N) rats. The perfusion with 50% Na+ H solution caused in AM the development of H contracture which was more than 50% higher than control contracture. H perfusion with 0.1 mM Ca2+, 1.0 mM La3+, and 10.0 mM of caffeine provides the discrimination of control and hypoNa+ contractures. It is assumed that early H contracture in AM is a result of inability of Ca-sequestering system to accumulate intracellular Ca2+ and Ca2+ influxing through the sarcolemma. In myocardium of N rats Na-Ca exchange is proposed as a main source of Ca2+ for H contracture development.