title: Effect of calcium, insulin and growth hormone on membrane fluidity. A spin label study of rat adipocyte and human erythrocyte ghosts.
abstract: ESR spectra were recorded from rat epididymal adipocyte ghosts labeled with the 5-nitroxide stearic acid spin probe, I(12,3). Polarity-corrected and approximate order parameters, that are sensitive to the flexibility of the incorporated label, were used to evaluate the membrane lipid fluidity. Addition of CaCl2 a 37 degrees C decreased the fluidity, as indicated by positive increases in the order parameters. The ordering effect of Ca2+ was concentration-dependent, reached saturation at approx. 3--4 mM, and was completely reversed by excess EGTA. Previous studies indicated that low- and high-affinity sites on adipocyte plasma membranes are able to bind 45Ca2+, and our results suggest that Ca2+-induced alterations in the lipid fluidity involve cation binding to low-affinity sites. The cellular movements of Ca2+ and, in particular, the binding of Ca2+ to the plasma membrane may play important roles in insulin's action on fat cell function. The possibility that insulin directly alters the membrane fluidity was tested by adding hormone to freshly-prepared I(12,3)-labeled adipocyte ghosts. Insulin, at concentrations (10(-6) M) that enhance glucose uptake into intact adipocytes, did not affect the fluidity of ghosts suspended in buffers with or without Ca2+. The fluidities of I(12,3)-labeled rat adipocyte ghosts or human erythrocyte ghosts were also unaffected by various forms of human growth hormone.
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000378:metabolism*; D004912:Erythrocytes / Q000378:metabolism*; D013006:Growth Hormone / Q000494:pharmacology*; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D008560:Membrane Fluidity / Q000187:drug effects*; D051381:Rats
chemical_list: D007328:Insulin; D004533:Egtazic Acid; D013006:Growth Hormone; D002118:Calcium