SIMULTANEOUS MEASUREMENT OF [NA-3 - EFFECTS OF STROPHANTHIDIN IN VENTRICULAR MYOCYTES(](I) AND CA2+ TRANSIENTS USING SODIUM BINDING BENZOFURAN ISOPHTHALATE AND FLUO)

We have developed a new method of measuring [Na+](i) and Ca2+ transien ts from a beating myocyte using sodium-binding benzofuran isophthalate (SBFI) and fluo-3. There was no interaction between the two probes, a nd the artifact due to contraction could be eliminated in the measurem ent of [Na+](i). [Na+](i) in guinea pig ventricular myocytes stimulate d at 1 Hz was 8.0 +/- 0.7 mM. Strophanthidin increased the base and th e amplitude of Ca2+ transients in association with an increase in [Nat ]i. When triggered activities were induced, the amplitude of Ca2+ tran sients decreased while [Na+](i) and the base of Ca2+ transients contin ued to increase. These results suggested that the diastolic [Ca2+](i) was closely related to [Na+](i). However, since systolic [Na+](i) coul d be influenced by other factors, the systolic [Ca2+](i) was dissociat ed from [Na+](i) in the condition of Ca2+ overload.