Activity-dependent extracellular K+ fluctuations in canine Purkinje fibres

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Abstract
Cardiac Purkinje fibres are important in cardiac electrophysiology and pathology due to their specialized role in the ventricular conducting system, and their properties as a second cardiac pacemaker1. Changes in extracellular K+ activity (K0) affect the conduction velocity2 and the natural automatic rate of the Purkinje fibre3. Both anatomical4 and electrophysiological5,6 studies of the Purkinje fibres have demonstrated the possibility of K0 fluctuations in the narrow intercellular clefts. We investigated, using K+ selective electrodes, the effect of the K0 fluctuations observed during activity in canine Purkinje fibres. Summation of K0 fluctuations during single action potentials leads to elevation of baseline K0 during rapid trains. However, unlike in ventricular muscle where depolarization is seen7,8, membrane potential in non-automatic Purkinje fibres hyperpolarizes in response to rapid beating. Prolonged depletions of extracellular K+ following long periods of overdrive are associated with slowly changing membrane currents, which markedly influence automaticity, action potential duration, and membrane potential.