Abstract: Measurements of the intracellular sodium activity(a_Naⁱ,as a indicator of sodium-pump activity)and the paracellular potassium activity(a_K⁰)were performed in sheep cardiac Purkinje fibres at rest and during stimulation using ion-selective microelectrodes.During a period of stimulation,paracellular potassium first increased and then declined below control values as long as pump activity was enhanced.The time course of the maximal diastolic potential was similar to pump activity,whereas that of the action potential duration deviated markedly. Further experiments on the effect of pump activation following challenges to zero potassium solution revealed that the rate dependence of the action potential duration was more sensitive to extracellular calcium than it was to the pump activity.Quantitative analysis of the data showed that in the physiological range of heart rate(60～150/min),the contribution of the electrogenic pump current to total repolarizing currents is between 12% and 25%.

Key words: Purkinje fibres, ion-selective microelectrode, sodium pump, K⁺current, action potential