Heterogeneity of fractionated alveolar macrophages in starved rats

Abstract

Phagocytosis in alveolar macrophages (AM) was enhanced in rats starved for 2 days, but was suppressed by more than 4 day starvation. AM from these rats were separated into four subfractions designated I, II, III and IV by discontinuous Percoll gradient centrifugations. Phagocytosis was greater in higher density fractions (III and IV) than lower density ones (I and II). At 2 days of starvation, the phagocytic activity of higher density fractions was elevated compared to the same fractions from fed rats. Lower density fractions showed high levels of lysosomal enzymes (cathepsin B plus L and β-glucuronidase) compared to higher density ones. These enzymes decreased in parallel with days of starvation. Ultrastructurally, AM of lower density fractions were large and contained more cytoplasmic vacuoles associated with lysosomes than those of higher density, which had numerous microvilli on the cell surface. The fractions of intermediary density overlapped in most of these characteristics. AM fractions in starved rats were heterogeneous with regard to phagocytic activity and lysosomal enzymes. When unfractionated AM of fed rats were incubated with supernatant from alveolar lavage fluid of rats starved for 2 days, their phagocytosis was remarkably enhanced. This result indicated that the lavage fluid of starved rats contained factors enhancing phagocytosis of macrophage and differences of phagocytic activity may reflect the presence of subpopulations among AM from starved rats that undergo different processes of maturation or aging.