The effect of membrane cholesterol on the sodium pump in red blood cells.

Abstract

Human red blood cells (RBC) were incubated with phosphatidylcholine vesicles to obtain partial depletion of their membrane cholesterol. The kinetic parameters of the Na pump (affinities and maximal fluxes) were determined in these cells and compared with controls. In the presence of internal K, progressive cholesterol depletion gradually induces an increase of the maximal flux and a reduction of the apparent affinity for internal Na. In the absence of internal K, cholesterol depletion has no apparent effect, indicating that the observed changes were mediated by the internal K. Cholesterol depletion acts by increasing the following 2 K effects: activation of the maximal fluxes and reduction of the internal Na affinity. The cholesterol effect (on the K-pump interaction) is specific: it occurs without any change in the affinities for internal Na and inorganic phosphate and for external Na and K, and without change in the number of pumping sites. Under physiological conditions, cholesterol reduces the cation translocation rate and increases the selectivity of the pump for internal Na.