Modulation of sodium-potassium adenosine triphosphatase in cultured bovine retinal pigment epithelium by potassium.
OBJECTIVE
The aim of the study was to examine the extracellular potassium concentration [K+]o as a modulator of the retinal pigment epithelium (RPE) contribution to ion homeostasis in the subretinal space by measuring sodium-potassium adenosine triphosphatase (Na+/K+ ATPase) pump number and activity in cultured bovine RPE in the presence of different concentrations of extracellular potassium.


METHODS
Pump number was quantified by measuring specific binding of 3H-ouabain to bovine RPE exposed to low (0.5-2.5 mmol) or control (5.0 mmol) [K+]o over 72 hr. Na+/K+ ATPase activity in low and control [K+]o was quantified by measurement of 86Rb+ uptake.


RESULTS
Low [K+]o increased RPE pump number in a dose- and time-dependent manner, with lesser effects on denser than on sparser cultures. [K+]-modulation of pump number was reversible and did not occur in the presence of the protein synthesis inhibitor cycloheximide. In RPE exposed to low [K+]o for 72 hr, pump activity was decreased compared with the control but increased toward control levels when re-exposed to control [K+]o. The total potassium transported per cell was 60% of control in low [K+]o but increased to slightly above control when re-exposed to control [K+]o.


CONCLUSIONS
In cultured RPE cells, both Na+/K+ ATPase pump number and pump activity are affected by changes in [K+]o, with greater modulation in sparser cultures. If similar events occur in situ, the sodium pump in RPE may respond to changes in subretinal potassium ion concentration, especially if the RPE cell number is reduced as in aging or disease.
Suson
J D
JD
Department of Ophthalmology, Medical College of Wisconsin, Milwaukee 53226.
Burke
J M
JM
D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D010042:Ouabain / Q000494:pharmacology; D010857:Pigment Epithelium of Eye / Q000201:enzymology*; D011188:Potassium / Q000378:metabolism*; D012414:Rubidium Radioisotopes; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D012414:Rubidium Radioisotopes; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium