INHIBITION OF NA-PUMP EXPRESSION BY IMPAIRMENT OF PROTEIN GLYCOSYLATION IS INDEPENDENT OF THE REDUCED SODIUM ENTRY INTO THE CELL()

Previous studies indicate that inhibition of protein N-glycosylation r educes Na+-pump activity. Since this effect is preceded by an inhibiti on of the entry of sodium into the cell, it is unclear whether the red uced Na+-pump is produced by the inactivation of protein glycosylation per se or by the lower intracellular sodium concentration. We compare d the effects of tunicamycin, which inhibits protein glycosylation, an d amiloride, which inhibits the entry of sodium into the cell, on the expression of the Na(+)pump activity in A6 cells. The short-circuit cu rrent across A6 epithelia, which corresponds to sodium ions transporte d through the Na+ channel and the Na+-pump, was almost totally inhibit ed after 24-hr treatment with 1 mu g/ml tunicamycin. The maximal Na+-p ump activity, measured after permeabilizing the apical cell membrane w ith amphotericin B, was only 30% inhibited. This inhibition increased to 80% after 72-hr treatment with tunicamycin. Thus, tunicamycin inhib its the activities of both the apical Na+ channel and the basolateral Na+-pump. However, the reduced number of Na+-pump molecules, as well a s the inhibition of the Na+-pump activity, were not observed when the Na+ channel was inhibited for 72-hr with amiloride. Thus, the reduced Na+-pump expression produced by inactivation of protein glycosylation is not secondary to reduced entry of sodium into the cell.