Activity-dependent extracellular K+ fluctuations in canine Purkinje fibres.

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 pacemaker. Changes in extracellular K+ activity (Ko) affect the conduction velocity and the natural automatic rate of the Purkinje fibre. Both anatomical and electrophysiological studies of the Purkinje fibres have demonstrated the possibility of Ko fluctuations in the narrow intercellular clefts. We investigated using K+ selective electrodes, the effect of the Ko fluctuations observed during activity in canine Purkinje fibres. Summation of Ko fluctuations during single action potentials leads to elevation of baseline Ko during rapid trains. However, unlike in ventricular muscle where depolarization is seen, membrane potential is 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.