Effect of volume and pH on surface receptor number in macrophages

Incubation of rabbit alveolar macrophages in hypo-osmotic solutions transiently increases cell volume and inhibits membrane internalization, resulting in an increase in surface receptor number. Since recent reports suggest that hypo-osmotic treatment decreases intracellular pH, and that reduced pH inhibits receptor internalization, pH was measured in hypo-osmotically treated macrophages. We found that cells incubated in iso-osmotic solutions of pH less than 7.2 exhibited a decrease in intracellular pH upon exposure to hypo-osmotic solutions, while cells in iso-osmotic solutions of pH greater than 7.2 had an increase in pH upon exposure to hypo-osmotic solutions. The relative increase in surface receptor number was unaffected by the initial pH or by the direction of change in pH. Incubation of cells in high K+/low Na+ hypotonic buffers induced a persistent increase in cell volume and surface receptor number. Cell volume and surface receptor number fell to baseline values after restoration of isotonicity by the addition of hypertonic sucrose. These manipulations had little effect on intracellular pH. We conclude that the inhibition of membrane internalization observed in cells exposed to hypo-osmotic solutions is independent of changes in intracellular pH. The inhibition of internalization observed in this system may be due directly to forces produced as a consequence of cell swelling.