Characterization of the electrogenic Na+-K+ pump in horizontal cells isolated from the carp retina.
The electrogenic Na+-K+ pump current in horizontal cells acutely dissociated from the carp retina was investigated using a nystatin-perforated patch recording configuration under voltage-clamp conditions. In the presence of suitable blockers for known voltage-dependent Na+, K+ and Ca2+ conductances, the pump current was activated in a concentration-dependent manner by adding K+ ions to external solution. The EC50 value and Hill coefficient for the external K+ concentration were 0.66 mM and 1.39, respectively. The pump current did not show any significant voltage dependency at the physiological potential range between -90 and 20 mV either with or without external Na+ ions. In the presence of 120 mM external Na+ concentration, the addition of 3 mM K+ to the external solution induced a steady outward pump current even when the patch-pipette (internal) solution did not contain Na+. A large outward shift of the holding current was observed by removing external Na+. The result thus suggests that continuous Na+ influxes exist across the plasma membrane in the presence of external Na+. When Na+ was removed from both external and internal solutions, a transient outward pump current was observed by adding K+ to the external solution, thus indicating that the transient pump current was activated by the residual intracellular Na+ ions. The pump current was suppressed by ouabain in a concentration-dependent manner, and the ouabain-sensitive inhibition curve was fitted by two components. The IC50 values of high- and low-sensitive pump currents for ouabain were 20 nM and 10.4 microM, respectively, indicating the existence of at least two isoforms of the pump in the horizontal cells.
Shimura
M
M
Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Tamai
M
M
Zushi
I
I
Akaike
N
N
D000818:Animals; D002347:Carps; D066298:In Vitro Techniques; D007700:Kinetics; D009474:Neurons / Q000166:cytology / Q000201:enzymology / Q000502:physiology*; D010042:Ouabain / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D012160:Retina / Q000166:cytology / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium