Charge Changes in Sarcoplasmic Reticulum and Ca²⁺ -ATPase Induced by Calcium Binding and Release: A Study Using Lipophilic Ions

Abstract

Changes in the charge of sarcoplasmic reticulum (SR) vesicles are studied using lipophilic ions, which are adsorbed by the membrane phase. Upon addition of MgATP, phenyldecarbaundecaborane (PCB⁻) and tetraphenylboron (TPB⁻) are taken up by the SR vesicles, while tetraphenylphosphonium (TPP⁺) is released into the water phase. The PCB⁻ uptake occurs as well under conditions when SR membrane is shunted by high Cl⁻ concentration. MgATP induces minor additional binding of PCB⁻ in the presence of oxalate and it is followed by release of the lipophilic anion from the vesicles. EGTA partly reverses the ATP effect, and calcium ionophore A23187 plus EGTA reverses it completely. Vesicles that were preliminarily loaded by Ca²⁺ demonstrate higher passive and lower ATP-dependent PCB⁻ binding. Activation of isolated Ca²⁺-ATPase in the presence of 0.1 mM EGTA results in PCB⁻ release into the medium and additional TPP⁺ binding to the enzyme. We suggest that the redistribution of the lipophilic ions between the water phase and SR membrane reflects charge changes in Ca²⁺ -binding sites inside both SR vesicles and Ca²⁺-ATPase molecules in the course of Ca²⁺ translocation.

Key Words:

• sarcoplasmic reticulum
• Ca²⁺-ATPase
• Ca²⁺ binding
• lipophilic ions
• charge redistribution