Effect of hypo‐osmotic incubation on membrane recycling

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Abstract
Incubation of alveolar macrophage in hypo‐osmotic media causes a time‐and temperature‐dependent increase in the number of surface receptors for three different ligands. Exposure of cell to solutions of 210 mOsM or less, at 37°C but not at 0°C, resulted in an increase in the number of surface receptors for diferric transferrin, alpha‐macroglobulin‐protease complexes, and mannose‐terminated glycoproteins. Upon media dilution at 37°C, surface receptor number reached a maximum within 5 min and returned to near‐normal values by 30 min. The increase in surface receptor number was the result of a decrease in the rate of internalization of receptors, either occupied or unoccupied. The rate of receptor exteriorization was unaltered by hypo‐osmotic incubation of cells. The rate of fluid‐phase pinocytosis was also inhibited upon incubation in hypo‐osmotic solution. In experiments in which both receptor‐mediated endocytosis and fluid phase pinocytosis were measured on the same samples, inhibition of both processes occurred with the same kinetics and to a similar extent. The rate of receptor‐mediated endocytosis recovered to normal rates after 60 min in hypo‐osmotic solutions, whereas the rate of fluid phase pinocytosis did not recover to the same extent.