{'fraction_non_alphanumeric': 0.049883449883449886, 'fraction_numerical': 0.010256410256410256, 'mean_word_length': 4.63254593175853, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Undershoots of intracellular sodium and the strength of contraction when the sodium‐potassium pump of isolated sheep Purkinje fibres is reactivated by potassium

We have recorded changes of membrane current, intracellular Na+ activity (aiNa), intracellular pH (pHi) and the strength of contraction in voltage‐clamped sheep Purkinje fibres during and after inhibition of the Na(+)‐K+ pump. The pump was inhibited by the removal of K+ from the bathing solution and this resulted in a rise in aiNa. On return of K+ to the bathing medium to reactivate the Na(+)‐K+ pump there was a fall of aiNa to below its control value after which aiNa slowly returned to its control value over the next 20 min. This ‘undershoot‘ of aiNa was seen in 69% of contracting fibres and 78% of quiescent fibres, and the mean (+/‐ S.D.) values of the undershoot in contracting and quiescent fibres were 0.66 +/‐ 0.15 and 0.9 +/‐ 0.35 mM, respectively. The undershoot of aiNa was observed regardless of whether the Na(+)‐K+ pump was reactivated with Rb+ or K+. It was not voltage dependent over the potential range studied (‐95 to ‐45 mV) and was not accompanied by a change of intracellular pH. The undershoot of aiNa could be the result of a long‐lasting increase in Na+ efflux or a long‐lasting decrease in Na+ influx. Zero [K+]o resulted in the loss of one Na+ current, the pacemaker current i(f), but when K+ was returned to the bathing medium i(f) recovered rapidly and is therefore unlikely to be responsible for the long‐lasting undershoot of aiNa. This conclusion was confirmed by the use of Cs+: although Cs+ blocked i(f), it did not block the undershoot of aiNa. The undershoot of aiNa was accompanied by (and, via Na(+)‐Ca2+ exchange, was presumably the cause of) an undershoot of the force of contraction. Undershoots are not only seen after reactivation of the Na(+)‐K+ pump: in a variety of different preparations, similar undershoots in aiNa and twitch force have been reported after a decrease in the frequency of stimulation. The undershoot of aiNa may be the result of novel feedback mechanism for the control of aiNa: the control of Na+ influx by aiNa.