Sodium-free contractures were studied in myocardial strips from R. pipiens when extracellular sodium (Na+o) was replaced by choline chloride and extracellular free calcium (Ca2+o) was defined with EGTA-buffer. 2. Resting membrane potentials (RMP) were normal in sodium-free solutions with Ca2+o calculated below 1.0 x 10(-9) mol/l. 3. When Ca2+o was subsequently increased from zero to 1.0 x 10(-3) mol/l Na+-free contractures developed slowly with unchanged RMP even at maximum contracture, at which the intracellular ultrastructure is grossly altered. 4. The contractures developed significantly faster in the presence of 3 x 10(-6) mol/l ouabain. 5. In sodium-free solutions La3+ did not influence Ca2+-dependent contractures, apart from causing an increase in time to maximum contracture. 6. It is concluded that sarcolemmal integrity is maintained in frog myocardium treated initially with Na+/Ca2+-free solutions and then with Na+-free medium containing 1 mmol/l Ca2+. 7. Our experiments indicate that sodium-free, Ca2+o-dependent contractures are mediated by the Na+/Ca2+-exchange, operation at higher rates when Na+i is increased. La3+ (1 mmol/l) probably does not compete with Ca2+ at extracellular binding sites of the exchanger. 8. The Na+/Ca2+-exchange may under certain experimental conditions be able to increase Ca2+i to cytotoxic concentrations.
