Alzheimer's amyloid-beta peptide inhibits sodium/calcium exchange measured in rat and human brain plasma membrane vesicles

Na+/Ca2+ exchange activity was measured by monitoring vesicular Ca2+ content after incubation in buffers containing Ca-45(2+). When Na+-loaded vesicles were placed into Na+-free buffer, vesicular Ca2+ content increased rapidly and reached a plateau after two to three minutes. Only preaggregated amyloid-beta(1-40) (A beta(1-40)) and A beta(25-35) reduced vesicular Ca2+ content. Both peptides produced a maximal reduction in Ca2+ content of approximately 50%. The peptides reduced Ca2+ content with similar potency and half maximal effects were seen at less than 10 mu M for A beta(25-35). Calcium-loaded vesicles mediate a rapid Ca2+/Ca2+ exchange, which also was inhibited by aggregated A beta(25-35). Aggregated A beta(25-35) did not affect the passive Ca2+ permeability of the vesicles. Aggregated A beta(25-35) reduced Ca2+ content in plasma membrane vesicles isolated from normal and Alzheimer's disease frontal cortex with less potency but the same efficacy as seen in rat brain. Aggregated A beta(25-35) did not produce nonspecific effects on vesicle morphology such as clumping or loss of intact vesicles. When placed in the buffer used to measure Ca2+ content, Congo Red at molar ratios of less than one blocked the inhibitory effect of preaggregated A beta(25-35). When added in equimolar concentrations to Freshly dissolved and unaggregated A beta(25-35), Congo Red also was effective at blocking the inhibitory effect on Ca2+ content. In contrast, vitamin E (antioxidant) and N-tert-butyl-a-phenylnitrone (spin trapping agent) failed to block the inhibitory action of aggregated A beta(25-35). The exact mechanisms of A beta-induced neurotoxicity in cell culture has yet to be solved. Accumulation of free radicals play a necessary role, but disruptions of Ca2+ homeostasis are also important. The data presented here are consistent with a proposed mechanism where aggregated A beta peptides directly interact with hydrophobic surfaces of the exchanger protein and/or lipid bilayer and interfere with plasma membrane Ca2+ transport. (C) 1997 IBRO. Published by Elsevier Science Ltd.