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 CaCl 2 at 37°C decreased the fluidity, as indicated by positive increases in the order parameters. The ordering effect of Ca 2+ 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 45Ca 2+, and our results suggest that Ca 2+-induced alterations in the lipid fluidity involve cation binding to low-affinity sites. The cellular movements of Ca 2+ and, in particular, the binding of Ca 2+ 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 Ca 2+. The fluidities of I(12,3)-labeled rat adipocyte ghosts or human erythrocyte ghosts were also unaffected by various forms of human growth hormone.