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 degrees C were 7.1 x 10(-6) and 1.3 x 10(-6) M, respectively. The decay of Ip 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 degrees C were 4.0 x 10(2) s(-1) M-1 and 6.3 x 10(-3) s(-1), respectively. At 30 degrees C, k(+ 1) increased to 2.8 x 10(3) 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. (C) 1998 Elsevier Science B.V. All rights reserved.