Activity-dependent extracellular K⁺ fluctuations in canine Purkinje fibres

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 electrophysiological^5,6 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 seen^7,8, 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.