Intracellular Na+ activity (aiNa) was measured using neutral-carrier Na+-sensitive micro-electrodes in voltage-clamped sheep Purkinje fibres during and after 4 min sequences of depolarizing pulses applied to around 0 mV, at a rate of 2.5 Hz. After trains of pulse duration 50 ms the mean increase in aiNa was 0.65 +/- 0.3 mM (mean +/- S.D., n = 18) whereas with longer pulse durations this rise became progressively smaller. At pulse durations of 300 ms a fall in aiNa was usually found. 2. Recovery of aiNa after a pulse sequence followed a roughly exponential time course. The half-time of decline after a rise in aiNa using 50 ms pulses was 111 +/- 52 s (n = 10), compared with a half-time of 318 +/- 116 s (n = 6) for recovery from a fall in aiNa during a sequence of 300 ms pulses. 3. Application of 2 mM-Cs+ to block the pace-maker current (if) resulted in a decrease in resting aiNa by 0.85 +/- 0.45 mM (n = 6) and an outward current shift. Na+ loading during a depolarizing pulse train was greater in 2 mM-Cs+ than in control solution. The rise in aiNa produced by a train of 50 ms pulses in Cs+ was 1.15 +/- 0.4 mM (n = 10). At short pulse durations in the presence of Cs+, Na+ loading at the end of a pulse train increased as a function of pulse duration, becoming maximal at a duration of approximately 50 ms and then diminishing at longer pulse durations. 4. Application of 2.5 X 10(-5) M-tetrodotoxin (TTX) produced a fall in resting aiNa of 0.55 +/- 0.2 mM (n = 6) and an outward current shift, suggesting that a TTX-sensitive component of steady-state Na+ current exists at potentials in the region -65 to -80 mV. 5. TTX greatly reduced the rise in aiNa during a depolarizing pulse train at all pulse durations tested. A fall in aiNa was now found after trains of shorter pulse duration than in control solution. Similar results were obtained in the absence of TTX if the pulse train was initiated from a holding potential which was positive to the Na+ current (iNa) threshold. When iNa had been blocked, using either TTX or a low holding potential, the mean rise in aiNa after a train of 50 ms pulses was 0.25 +/- 0.2 mM (n = 8).(ABSTRACT TRUNCATED AT 400 WORDS)
