Simultaneous measurement of intracellular Na+ and Ca2+ during K(+)-free perfusion in isolated myocytes

To study the relationship between intracellular Na+ concentration ([Na+]i) and intracellular Ca2+ concentration ([Ca2+]i), guinea pig ventricular myocytes were loaded with both the Na(+)-sensitive probe, sodium-binding benzofuran isophthalate (SBFI), and the Ca(2+)-sensitive probe, fluo 3. [Na+]i was measured from the ratio image at 510 nm when excited at 340/380 nm. [Ca2+]i, expressed as the percent change of corrected fluo 3 fluorescence, was measured at 540 nm when excited at 500 nm. The fluorescent spectra of these probes were sufficiently different to allow for simultaneous measurement. After 30 min perfusion of K(+)-free solution, [Na+]i of rod-shaped cells increased from 6.4 +/- 0.5 to 20.6 +/- 2.6 mM, and [Ca2+]i increased to 256 +/- 36% of the control. [Ca2+]i was higher in spontaneously contracting cells and shortened cells than in rod-shaped cells at similar levels of [Na+]i. When Ca(2+)-free solution or Ni2+ (5 mM) was applied, [Ca2+]i was lower than in cells perfused with K(+)-free solution alone. It was suggested that extracellular Ca2+ and the Na(+)-Ca2+ exchange were involved in the increase in [Ca2+]i. In conclusion, we have developed a new method for the simultaneous measurement of [Na+]i and [Ca2+]i in isolated myocytes, which should be useful to study the relation between [Na+]i and [Ca2+]i.