Exactly When Do Potassium Ions Appear in Extracellular Fluid Bathing Sino-Atrial Cells

This study was an attempt to postulate a different mechanism for repolarisation of sino-atrial cells by re-study of published data of sinus node cell cycling. Potassium ion (K+) appearance in the extracellular compartment measured by K+-sensitive electrode during spontaneous cycling of sinus node cells was found in the historic manuscript (1). The extracellular K+ appearance had timing suggesting triggering by, rather than causing, repolarisation (determined from the newly calculated electron density trace). The K+ efflux continued after the cessation of repolarisation, suggesting that the K+ efflux was not the mechanism of repolarisation. Alternatively, it is suggested that K+ is released by the action potential from its diastolic fixation in the cell membrane, and then diffuses along its strong outwardly directed concentration gradient.