UREA INHIBITS THE NA-K PUMP IN HUMAN ERYTHROCYTES

Recent studies have established that urea alters the activity of sever al volume-sensitive cation transport pathways. However, it has remaine d unclear whether urea has any effect on transport pathways that are n ot volume-sensitive. We examined the effect of urea on Na-K pump in th e human erythrocytes. Ln cells from nine subjects, 500 mM urea inhibit ed 52 +/- 10% of the pump activity measured as the ouabain-sensitive ( OS) K influx. Urea inhibited the OS K influx reversibly, in a concentr ation-dependent manner. [H-3] oubain binding, a measure of the number of Na-K pump sites remained unchanged with urea. Urea decreased the V- max for ouabain-sensitive K influx, but did not alter the apparent K-m , for external K, Furthermore, urea did not alter the apparent K-m for intracellular Na. The ion turnover per pump site was decreased in the presence of urea. Thus, physiologically relevant urea concentration i nhibit the Na-K pump in human erythrocyte. The inhibition of the Na-K pump by urea suggests that the effects of urea may not be limited to v olume-sensitive transporters, but may be more widespread.