The Modification of the Reconstituted Sarcoplasmic ATPase by Monovalent Cations

1The effects of monovalent cations on the calcium-dependent ATPase activity of lipid-depleted and reconstituted sarcoplasmic membranes have been investigated. 2The calcium-dependent ATPase activity of sarcoplasmic membranes whose lipid components have been modified are significantly more sensitive to activating and/or inhibitory effects of monovalent cations than that of native sarcoplasmic membranes. 3Activating and inhibitory effects of the applied monovalent ions depend on the concentration of ionized calcium in the medium. In dilute calcium media ([Caz+] = 0.8 μM) modified ATPase preparations are always inhibited by high concentrations (>0.2M) of monovalent salts. Conversely, in concentrated calcium media ([Ca2+] = 0.18 mM) modified ATPase preparations are in most cases activated by increasing (>0.2 M) concentrations of monovalent salts. 4Activating effects of the monovalent ions decrease and specific inhibitory effects increase in the order K+, Ria+, Rb+, Cs+, Li+. This order corresponds to that found if the ions are arranged according to the increasing distance between potassium and the respective ion on the hydration enthalpy scale. 5The results have been discussed in relation to a specific competition between calcium ions bound to the inhibitory side at the inner surface of the sarcoplasmic membranes and the applied monovalent ions