Abstract: Raising the internal sodium ion activity (a_Naⁱ) by way of intermittent fast beating the sheep cardiac Purkinje fibers in different frequencies resulted in declining of a_Naⁱ at the very end of the train of beats.Both of a_Naⁱ increments and declining rates (d(a_Naⁱ/dt) paralleled with the stimulation frequencies.The values of d(a_Naⁱ)/dt were plotted against the corre-sponding a_Naⁱ to obtain a correlation curve (sodium pump-internal sodium relationship, SP-IS).Sodium pump activity can be evaluated by comparing the slopes of correlation curves.The intersects of the correlation curves and the ordinate (Vmax) was got with extrapola-tion.The sum of values of Vmax and the maximum d(a_Naⁱ)/dt represented the maximum ac-tivity of sodium pump.The pump activity can be calculated simply and repeatedly with this method.The effects of high/low pH, K⁺, Ca²⁺, and 4 mmol/L Mn²⁺, 5×10^-7mol/L iso-prenaline on pump activity were observed simultaneously.

Key words: internal sodium ion activity, sodium pump activity, ion-selective microelectrode, Purkinje fibre, isoprenaline