Temperature shifts induce the selective loss of alveolar-macrophage plasma membrane components.

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Abstract
A shift in the incubation temperature of rabbit alveolar macrophages (0.degree. C .fwdarw. 37.degree. C .fwdarw. 0.degree. C) resulted in a 40-60% reduction in the ability of cells to bind .alpha.macroglobulin .cntdot. 125I-trypsin complexes (.alpha.M .cntdot. 125I-T). The reduction in binding activity did not reflect a disruption of cell integrity since the levels of intracellular components (lactate dehydrogenase, .beta.-N-acetylhexosaminidase) or other plasma membrane components (alkaline phosphodiesterase) were unaltered. Analysis of receptor-ligand interaction indicated that the temperature shift effected a decline in receptor number rather than an alteration in ligand-receptor affinity. A temperature shift evidently resulted in the loss of unoccupied receptors and ligand bound to receptors was not lost. After ligand internalization, receptors were removed by the temperature shift. The rate of receptor loss was maximal when cells were incubated at temperatures > 24.degree. C. Receptor loss was not prevented by treatment of cells with colchicine, cytochalasin B or N-ethylmaleimide, but was prevented by treatment with the cross-linking agent p-formaldehyde. The reduction in .alpha.M .cntdot. 125I-T binding activity evidently resulted from shedding of receptors into the media since media obtained from temperature-shifted cells contained material that competed with cell-bound receptors for .alpha.M .cntdot. 125I-T. Binding of .alpha.M .cntdot. 125I-T was diminished on membrane fragments obtained from temperature-shifted cells. Incubation with Triton X-100, of cells whose receptors were occupied with .alpha.M .cntdot. 125I-T, led to the extraction of 40% of cell-bound activity. No radioactivity was extracted from cells labeled with .alpha.M .cntdot. 125I-T after a temperature shift. Measurement of ligand accumulation by control and temperature-shifted cells incubated at 20.degree. C indicated that control cells exhibited a subpopulation of receptors capable of binding ligand but only slowly internalizing it. This subpopulation was not present on temperature-shifted cells. Surface receptors for .alpha.macroglobulin-protease complexes are evidently heterogeneous and the temperature shift resulted in the selective loss of membrane components.