Transmembrane Ca²⁺ gradient-mediated change of fluidity in the inner layer of phospholipids modulates Ca²⁺-ATPase of sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) vesicles with (1000 folds) or without transmembrane Ca²⁺ gradient have been prepared. Different fluorescence probes (DPH, TMA-DPH and n-AS), were used to determine the effect of transmembrane Ca²⁺ gradient on the lipid fluidity both in outer and inner layer of Ca²⁺-ATPase-containing SR vesicles. The results showed that transmembrane Ca²⁺ gradient could significantly decrease the fluidity of the inner layer of SR membrane, while no obvious change was monitored in the outer layer. This may be deduced that Ca²⁺-ATPase might be modulated mainly by the transmembrane Ca²⁺ gradient-mediated alteration of physical state of phospholipid in the inner layer of SR membrane.