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Based on our previous findings on the modifying effect of calmodulin (CaM) on the physicochemical properties of biomembrane, we have investigated the possible relationship between intracellular CaM content and endoplasmic reticulum (ER) membrane fluidity and function during liver regeneration. The degree of ER membrane fluidity was estimated by fluorescence polarization analysis with the 1,6-diphenyl-1,3,5-hexatriene probe. Microsomal guanylate cyclase (GC) was used as a functional parameter. The kinetics of the increase in the ER membrane fluidity during liver regeneration was strictly parallel to the CaM surge and was matched by an increase in GC activity. The stimulative effect of splenectomy on liver regeneration and its inhibition by Walker-256 tumor, inferred from the corresponding alterations of CaM levels, were mirrored by the modulation in GC activity. The fluidizing effect of CaM on ER membrane was concluded from the drop in thermotropic transition temperature from 28.3 ± 1.6°C in control membranes to 17.8 ± 1.1°C in membranes from regenerating livers and to 19.8 ± 1.2°C in control membranes treated with CaM. Arrhenius plots of GC activity exhibited a transition temperature of 25.5 ± 1.25°C in controls, which shifted to 20.5 ± 0.9°C in ER membranes from regenerating livers and to 21.7 ± 1.1°C in control membranes treated with CaM. The Hill coefficient for the allosteric activation of the GC by Mn·GTP decreased from 1.49 ± 0.16 in controls to 0.93 ± 0.085 in membranes from regenerating cells and to 0.86 ± 0.073 in CaM-treated membranes. Both effects of CaM were consistent with a fluidity increase in the enzyme's lipid microenvironment. The results of the present study suggest that an early key event in liver regeneration may be the CaM-induced modulation of ER membrane fluidity and function.
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© 1997 Academic Press. Published by Elsevier Inc. All rights reserved.