Intracellular sodium ion activity: reliable measurement and stimulation-induced change in cardiac Purkinje fibers

Recently Na + -selective microelectrodes (NaSM) have been used to measure quantitatively small changes in intracellular sodium ion activity [Formula: see text] and to determine a precise time course of comparatively rapid change in [Formula: see text]. In such studies, accurate measurement of [Formula: see text] requires the following criteria: (i) NaSM should have a fast response time and (ii) an NaSM and a conventional voltage microelectrode should measure the same membrane potential. These criteria were evaluated by measuring [Formula: see text] when membrane potential of cardiac Purkinje fibers was suddenly hyperpolarized and depolarized by changing stimulation rate. The NaSM coated with a conductive silver paint had fast response times so that rapid changes in [Formula: see text] could be reliably measured. The cardiac Purkinje fibers stimulated at a constant rate generated uniform membrane voltage and the NaSM and conventional microelectrode measured virtually the same membrane potential. This result is somewhat different from that reported under voltage-clamp condition by other investigators. The [Formula: see text] of the fibers increased as the stimulation rate was increased over the range of 0.5–3 Hz. In fibers stimulated at 1 Hz, cessation of stimulation was immediately followed by an exponential decline of [Formula: see text] with an average time constant of 53 ± 9 s (SD, n = 8), or rate constant of 0.020 ± 0.004/s. Restimulation of the fibers produced an exponential rise of [Formula: see text] with an average time constant of 65 ± 12 s (n = 8). Similar results were obtained in fibers stimulated at 2 Hz. The average rates of rise of [Formula: see text] after the onset of stimulations at 1 and 2 Hz were 1.0 and 1.5 mM/min, equivalent to increments in net sodium influx of 13.2 and 19.8 pmol∙cm −2 ∙s −1 , respectively. The average maximum rate of [Formula: see text] rise produced by the application of 10 −5 M strophanthidin to the fibers stimulated at 1 Hz was 1.3 ± 0.5 mM/min, equivalent to a net sodium influx of 17.2 pmol∙cm −2 ∙s −1 .