PROTON EJECTION AS A MAJOR FEATURE OF THE CA2-PUMP()

H+ ejection and Ca2+ uptake promoted by the sarcoplasmic reticulum (SR ) Ca2+-pump are similarly stimulated by millimolar Mg2+. This cannot b e assigned to enhanced Ca2+ uptake and H+ displacement from internal m etal binding sites since: (1) loading SR vesicles with high Mg2+ conce ntrations does not impair H+ ejection; (2) loading SR vesicles with Mn 2+ does not depress H+ ejection occurring during Mn2+ uptake; (3) H+ e jection occurs even when Ca2+ accumulation inside the vesicles is prev ented with Ca2+ ionophores. It is concluded that the Ca2+-pump promote s an active Ca2+/H+ countertransport stimulated by Mg2+. Finally, a me chanism for Ca2+ translocation is proposed in basic physico-chemical t erms.