Are Ba2+ and Sr2+ ions transported by the Na+-Ca2+ exchanger in frog atrial cells?

Ba2+ and Sr2+ ions are widely used to replace Ca2+ ions for the study of Ca2+ channel currents in electrophysiological experiments. Using the double sucrose gap technique, we investigated the effects of Sr2+ and Ba2+ ions on the Na+ Ca2+ exchange activity in frog atrial fibres where it is the major relaxation mechanism. With either Sr2+ or Ba2+ ions instead of Ca2+ in the extracellular bath, Na-free contractures reversibly developed but with different kinetics. Voltage clamp experiments showed that the tonic tension recorded in the presence of Sr2+ or Ba2+ was markedly increased following the addition of monensin, a Na+ ionophore known to increase the intracellular Na+ activity. In Na-free solutions (Li-substituted), it was possible to induce contractures by substituting Sr2+ or Ba2+ ions for extracellular Ca2+. These contractures could be relaxed by reintroducing Na+ or Ca2+ ions in the extracellular medium. Taken together, these results suggest that Sr2+ and Ba2+ ions can interact with the Na+-Ca2+ exchange mechanism and potentially participate not only in Na+-cation but also in Ca2+-cation exchanges on either side of the sarcolemmal membrane.