STUDIES OF THE MECHANISM UNDERLYING INCREASED NA+/CA2+ EXCHANGE ACTIVITY IN ALZHEIMERS-DISEASE BRAIN

The Na+/Ca2(+) exchanger was characterized in plasma membrane vesicles derived from frozen human postmortem tissues. The frontal cortex, temporal cortex and cerebellum of control and Alzheimer's disease (AD) tissues were compared. Na+/Ca2+ exchange activity was defined as the change in vesicular Ca2+ content seen after Na+ loaded vesicles were diluted into choline buffer. The time course of changes in Ca2+ content after dilution was similar in all three regions of control brain. In AD brain, both frontal and temporal cortex vesicles showed elevated Ca2+ content, most evident as an increased peak Ca2+ content at 2 min. The AD cerebellar cortex time course was similar to control and did not show an elevated peak at 2 min. No differences were seen in the passive permeability to Ca2+ when comparing plasma membrane vesicles prepared from control and AD brain. Vesicles from the frontal and temporal cortex of AD brain showed increases in the V-max, of the initial velocity of Ca2+ uptake when compared to control brain, whereas, the cerebellum did not. There were no significant effects of AD on the k(m), for Ca2+ activation of the initial velocity. Ca2+ influx measured during the rise in vesicular Ca2+ content was elevated in vesicles from AD temporal cortex when compared to control. Two known inhibitors (exchange inhibitory peptide and dichlorobenzamil) of the cardiac Na+/Ca2+ exchanger inhibited the human brain exchanger equally well in control and AD vesicles. Increased Na+/Ca2+ exchange activity was not due to astrocytic gliosis. An antibody to the cardiac exchanger was used to determine the molecular weight of the human brain Na+/Ca2+ exchanger. The molecular weight determinations from Western blots showed identical molecular weights of 100-110 kDa in both AD and control vesicle preparations. These data are entirely consistent with the proposal that increased Na+/Ca2+ exchange activity in AD reflects the properties of surviving neurons with elevated Na+/Ca2+ + exchange capacity and lend support to the Ca2+ hypothesis of AD'.