Saturation of the internal sodium site of the sodium pump can distort estimates of potassium affinity.

The Na+/K+ exchange pump in cardiac Purkinje strands has been well studied with the voltage clamp and Na+-selective microelectrodes. Models describing the observed results suggest that the pump rate can be considered proportional to [Na+]i over the range examined and depends on external [K+] in accordance with Michaelis-Menten kinetics. Estimates of the external [K+] that achieves a half-maximal pump rate (Km) range from 0.9 to 6.3 mM depending on the preparation and method of estimation. Here we show that much of the variability in the estimates of the Km can be eliminated when saturation of the internal Na+ pump site is taken into account. If the half-activation concentration for saturation of this Na+ site is sufficiently high (greater than 20 mM), removal of intracellular Na+ in response to a Na+ load will approximate first-order kinetics. Under these conditions however, Na+ saturation will nevertheless cause large systematic errors in estimates of the K+ dependence of pump activity.