Characteristics of sarcoplasmic reticulum membrane preparations isolated from skeletal muscles of active and hibernating ground squirrel Spermophilusundulatus

The total Ca-ATPase activity in the sarcoplasmic reticulum (SR) membrane fr action isolated from Skeletal muscles of winter hibernating ground squirrel Spermophilus undulatus is, similar to2.2-fold lower than in preparations o btained from summer active animals, This is connected in part with similar to 10% decrease of the content of Ca-ATPase protein in SR membranes, Howeve r, the enzyme specific activity calculated with correction for its content in SR preparations is still similar to2-fold lower in hibernating animals. Analysis of the protein composition of SR membranes has shown that in addit ion to the decrease in Ca-ATPase content in hibernating animals, the amount of SR Ca-release channel (ryanodine receptor) is decreased similar to2-fol d, content of Ca-binding proteins calsequestrin, sarcalumenin, and histidin e-rich Ca-binding protein is decreased similar to3-4-fold, and the amount o f proteins with molecular masses 55, 30, and 22 kD is significantly increas ed. Using the cross-linking agent cupric-phenanthroline, it was shown that in SR membranes of hibernating ground squirrels Ca-ATPase is present in a m ore aggregated state, The affinity of SR membranes to the hydrophilic fluor escent probe ANS is higher and the degree of excimerization of the hydropho bic probe pyrene is lower (especially for annular lipids) in preparations f rom hibernating than from summer active animals. The latter indicates an in crease in the microviscosity of the lipid environment of Ca-ATPase during h ibernation. We suggest that protein aggregation as well as the changes in p rotein composition and/or in properties of lipid bilayer SR membranes can r esult in the decrease of enzyme activity during hibernation.