Simultaneous measurement of [Na+]i and Ca2+ transients in an isolated myocyte: effects of strophanthidin.

We have developed a new method to measure [Na+]i and Ca2+ transients of a beating cardiac myocyte using a Na(+)-sensitive fluorescent probe, sodium-binding benzofuran isophthalate (SBFI) and a Ca(2+)-sensitive fluorescent probe, fluo-3. There was no interaction between two probes, and the artifact due to contraction could be eliminated in the measurement of [Na+]i. [Na+]i in guinea pig ventricular myocytes stimulated at 1 Hz was 8.0 +/- 0.7mM. Strophanthidin (10 microM), initially, increased the amplitude and the basal level of Ca2+ transients in association with an increase in [Na+]i. When arrhythmias were induced, the amplitude of Ca2+ transients decreased while [Na+]i and the basal level of Ca2 transients continued to increase. These results suggested that the diastolic [Ca2+]i was closely related to [Na+]i. However, the systolic [Ca2+]i, which could be influenced by other factors, was dissociated from [Na+]i in the condition of Ca2+ overload.