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.
Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
M
M
Shimura
M
M
Tamai
I
I
Zushi
N
N
Akaike
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