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 TI⁺> 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 X 10^{- 6} and 1.3 x 10^-6 M, respectively. The decay trip after adding ouabain well fitted with a single exponential function. At a V(H) of -40 My, 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 X 10² s^-1 M^-1 and 6.3 X 10^-3 s^-1, respectively. At 30°C, k₊₁ increased to 2.8 X 10³ s^-1 M^-1 while k_-1 showed no such change with a value of 1.8 x 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.