Cation Activation of the Basolateral Sodium-Potassium Pump in Turtle Colon

The current generated by electrogenic sodium-potassium exchangeat the basolateral membrane of the turtle colon can be measured directlyin tissues that have been treated with serosal barium (to block the basolateralpotassium conductance) and mucosal amphotericin B (to reduce the cationselectivity of the apical membrane). We studied the activation of this pumpcurrent by mucosal sodium and serosal potassium, rubidium, cesium, andammonium. The kinetics of sodium activation were consistent with binding tothree independent sites on the cytoplasmic side of the pump. The pump wasnot activated by cellular lithium ions. The kinetics of serosal cation activationwere consistent with binding to two independent sites with the selectivity Rb >K > Cs > NH₄. The properties and kinetics of the basolateral Na/K pump inthe turtle colon are at least qualitatively similar to those ofthe well-characterizedNa/K-ATPase of the human red blood cell .