Sodium-free contractures were studied in myocardial strips from R. pipiens with extracellular sodium (Na+o) replaced by choline chloride and extracellular calcium (Ca2+o) varied with EGTA-buffer. Normal myocardium was compared with that damaged by adrenaline (ADR) or isoproterenol (ISO). 2. Frog myocardium, damaged by in vivo injections of catecholamines, remained relaxed when exposed to Na+/Ca2+-free solutions. Only in 2 out of 18 experiments were small contractures observed after several hours. 3. Addition of KCN to the Na+/Ca2+-free solution caused small contractures after several hours in 7 out of 10 experiments. 4. The time to maximum Na+-free contractures was correlated to Ca2+o in a dose-dependent manner, but not influenced by catecholamine-induced myocardial damage. 5. Cell injury in the frog heart after in vivo injections of catecholamines does not affect the sarcolemmal Na+/Ca2+-exchange and is not associated with passive leakage of Ca2+ from the extracellular to the intracellular space.
