Characterization of the electrogenic Na +–K + pump in bipolar cells isolated from carp retina

The electrogenic Na +–K + pump current (Ip) in carp bipolar cells was investigated under voltage-clamp conditions. The Ip was activated in a concentration-dependent manner by adding external K + (K + o), and was completely suppressed with 10 −4 M ouabain (EC 50=1.23 mM; Hill coefficient=1.36). The Ip was suppressed in a concentration-dependent manner by ouabain (IC 50=1.90 mM; Hill coefficient=0.93). The Ip did not show a distinct voltage dependency either with or without Na + o. A large outward shift of the holding current was observed by completely removing Na + o. In the presence of Na + o, a steady Ip was observed even in the absence of internal Na + (Na + i). These results suggest that continuous Na + influxes exist across the membrane. When external and internal Na + was removed, a transient Ip was observed (half decay time ( t 1/2) was 5.0±0.6 s), thus indicating that the transient Ip was activated by the residual Na + i. In the absence of Na + o, the transient Ip was also observed with lower than 8 mM Na + i. The t 1/2 depended on Na + i. However, a steady Ip was observed with 10 mM Na + i or more. The functional properties of the Ip are discussed.