Characterization of electrogenic Na/K pump in rat neostriatal neurons

The electrogenic Na/K pump current (Ip) was studied in the dissociated neostriatal neurons of the rat by using the nystatin-perforated patch recording mode. The Ip was activated by external K⁺ in a concentration-dependent manner with an EC₅₀ of 0.7 mM at a holding potential (V_H) of ?40 mV. Other monovalent cations also caused Ip and the order of potency was Tl⁺>K⁺, Rb⁺>NH₄⁺, Cs⁺>>>Li⁺. The Ip decreased with membrane hyperpolarization in an external solution containing 150 mM Na⁺, while the Ip did not show such voltage dependency without external Na⁺. Ouabain showed a steady-state inhibition of Ip in a concentration- and temperature-dependent manner at a V_H of ?40 mV. The IC₅₀ values at 20 and 30°C were 7.1×10^?6 and 1.3×10^?6 M, respectively. The decay of Ip after adding ouabain well fitted with a single exponential function. At a V_H of ?40 Mv, the association (k₊₁) and dissociation (k_?1) rate constants estimated from the time constant of the current decay at 20°C were 4.0×10² s^?1 M^?1 and 6.3×10^?3 s^?1, respectively. At 30°C, k₊₁ increased to 2.8×10³ s^?1 M^?1 while k_?1 showed no such change with a value of 1.8×10^?3 s^?1. A continuous Na⁺ influx was demonstrated by both the Na⁺-dependent leakage current and tetrodotoxin-sensitive Na⁺ current, which resulted in the continuous activation of the Na/K pump. It was thus concluded that the Na/K pump activity was well-maintained in the dissociated rat neostriatal neurons with distinct functional properties and that the activity of the pump was tightly connected with Na⁺ influxes.