Properties of the Na +/K + pump current in small neurons from adult rat dorsal root ganglia

Abstract

1. The present investigation was undertaken to characterize the Na +/K + pump current in small (≤25 μm in soma diameter) dorsal root ganglion (DRG) neurons isolated from lumbar L4-6 segments of adult rats. 2. The Na +/K + pump current was identified as an ouabain-sensitive current during square voltage steps to membrane potentials between +40 and -120 mV, using the whole-cell patch-clamp technique in which Ca 2+ and K + channel currents and Na +/C a2+ exchange currents were minimized. The Na +/K + pump current was practically time-independent over the entire voltage range examined and exhibited a voltage-dependence; its current - voltage (I-D relationship displayed a positive slope at potentials between -120 and 0 mV but nearly plateau levels at positive membrane potentials. 3. The concentration-dependent block of Na +/K + pump current (activated by 30 mM pipette Na +) by ouabain at concentrations between 0.1 μM and 5 mM was biphasic and was well described using a two-binding site model with dissociation constants for high- and low-affinity binding sites of 0.20 and 140.1 μM, respectively. The relative amplitude of the Na +/K + pump current produced by low- and high-affinity sites (probably α1β1 and α3β1 isozymes, respectively) was estimated to be 13:1 in the presence of 30 mm Na + in the pipette solution. 4. Additionally, the activation of Na +/K + pump current by pipette Na + at concentrations ranging from 5 to 100 mM also exhibited a biphasic concentration dependence which can be reasonably well fitted by assuming the existence of two isozymes having high and low affinities for Na + (6.7 and 67.6 mM, respectively). 5. Thus, the present investigation provides functional evidence to suggest that the Na +/K + ATPase comprises two functionally distinct isozymes as expected for α1β1 and α3β1 in rat small DRG neurons.