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 reduces Na(+)-pump activity. Since this effect is preceded by an inhibition of the entry of sodium into the cell, it is unclear whether the reduced Na(+)-pump is produced by the inactivation of protein glycosylation per se or by the lower intracellular sodium concentration. We compared the effects of tunicamycin, which inhibits protein glycosylation, and amiloride, which inhibits the entry of sodium into the cell, on the expression of the Na(+)-pump activity in A6 cells. The short-circuit current across A6 epithelia, which corresponds to sodium ions transported through the Na+ channel and the Na(+)-pump, was almost totally inhibited after 24-hr treatment with 1 microgram/ml tunicamycin. The maximal Na(+)-pump activity, measured after permeabilizing the apical cell membrane with amphotericin B, was only 30% inhibited. This inhibition increased to 80% after 72-hr treatment with tunicamycin. Thus, tunicamycin inhibits the activities of both the apical Na+ channel and the basolateral Na(+)-pump. However, the reduced number of Na(+)-pump molecules, as well as 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.