Interaction of Dibucaine and Calcium Ion on a Calcium Pump Reconstituted from Defined Components of Intestinal Brush Border

The uptake of Ca ++ by reconstituted proteoliposomes involves an ATP-independent as well as an ATP-dependent process. In the presence of 0.7 mM ATP, 1 mM dibucaine inhibited Ca ++ uptake and (Mg ++ + Ca ++ )-ATPase activity, but 0.2 mM dibucaine stimulated both. The latter reduced the velocity of ATP-independent Ca ++ uptake at low Ca ++ concentrations, but increased it at a high concentration (0.15 mM of Ca ++ . ATP-dependent Ca ++ uptake was facilitated by 0.2 mM dibucaine only in the presence of a high concentration of Ca ++ . Kinetic analyses of the data obtained with the lower dose of dibucaine suggest that this compound competes with Ca ++ for binding sites on the outer surface of membranes and that its interaction with Ca ++ induces a conformational change of membranes accompanied by an increase in the passive permeability of Ca ++ and activation of the ATP-mediated Ca ++ pump.