Factors affecting intracellular sodium during repetitive activity in isolated sheep Purkinje fibres.

Abstract

SUMMARY1. Intracellular Na+ activity (a'a) 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 aka 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 aka was usually found.2. Recovery of aka after a pulse sequence followed a roughly exponential time course. The half-time of decline after a rise in aNa 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 aka during a sequence of 300 ms pulses.3. Application of 2 mM-Cs+ to block the pace-maker current (it) resulted in a decrease in resting aia by 0-85 + 0-45 mm (n = 6) and an outward current shift. Na+ loading during a depolarizing pulse train was greaterjn 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 2S5 x 10-5 M-tetrodotoxin (TTX) produced a fall in resting aNa of 0-55 + 0-2 mm (n = 6) and an outward current shift, suggesting that a TTXsensitive component of steady-state Na+ current exists at potentials in the region -65 to -80 mV.5. TTX greatly reduced the rise in a'a during a depolarizing pulse train at all pulse durations tested. A fall in aia 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 aka after a train of 50 ms pulses was 0-25 + 0-2 mM (n= 8).6. The component of Na+ loading remaining at short pulse durations in the presence of TTX was abolished by 9-6 x 10-6 M-gallopamil (D600). The fall in aNa * Correspondence to Dr G. Hart, Cardiac Department, John Radcliffe Hospital, Oxford OX3 9DU.M. R. BOYETT, G. HART AND A. J. LEVI at the end of a pulse train in the presence of TTX and gallopamil increases at longer pulse durations.7. Na+ loading in TTX was unchanged or only slightly reduced by 5 mm-Mn2+. 8. The increment in aka during a pulse train was greater if the membrane was hyperpolarized during the interpulse interval.9. These results show that during repetitive activity mechanisms operate which tend to reduce aNa, namely a diminished leak current and deactivation of if, together with mechanisms which increase aka, i.e. the fast Na+ current and a second route for Na+ entry which is resistant to TTX but sensitive to gallopamil. We suggest that the TTX-insensitive component of Na+...

Effects of repetitive activity on developed force and intracellular sodium in isolated sheep and dog Purkinje fibres.

“…We have previously shown that the change of aka during a train of voltage-clamp pulses is variable and aka can rise or fall according to the experimental conditions and the preparation (Boyett et al 1987). One of our earliest observations was that if aka rose during the train there was invariably a slow rise in force (which we describe as a ' positive staircase') whereas if aia fell there was frequently a progressive decline of force throughout much of the train (a 'negative staircase').…”

“…aNa AND THE FORCE STAIRCASE Na+-sensitive micro-electrodes. For reasons discussed elsewhere (Boyett et al 1987), at the start and end of a voltage-clamp train there was a transient deflection of the aia signal, and during the train the ala signal could be shifted up or down. Both of these features are artifacts.…”

“…The methods employed in this study have been fully described elsewhere (Boyett, Hart & Levi, 1986;Boyett et al 1987). To summarize, Purkinje fibres were dissected from sheep hearts obtained from a local abattoir or from dog hearts obtained at the end of experiments carried out by colleagues.…”

“…We have manipulated the changes of aka during the train and observed the effect on the force staircase. Our previous study of the rate-dependent changes of aka (Boyett, Hart & Levi, 1987) presented a number of ways of manipulating the changes of aka during a pulse train. Aspects of this work have been presented at meetings of the Physiological Society (Boyett, Hart & Levi, 1985; .…”

Time course of changes in intracellular K+, Na+, and pH of subendocardial Purkinje cells during the first 24 hours after coronary occlusion.

“…33 If the hormonal environment leads to increased cAMP levels, the activation range of the pacemaker current would be shifted in a positive direction,34 contributing to a Na+ influx even for depolarized Vm.…”