NA+/CA2+ EXCHANGE ACTIVITY IS INCREASED IN ALZHEIMERS-DISEASE BRAIN-TISSUES

These studies were performed to determine the changes that occur in Na+/Ca2+ exchange activity in Alzheimer's disease (AD) brain tissues. Cerebral plasma membrane vesicles were purified by sucrose density gradient centrifugation from frozen postmortem hippocampal/temporal cortex tissue slices derived from age matched brains of normal, AD and non-Alzheimer dementia (NAD) origin (autopsy confirmed). Membrane marker assays (Na/K ATPase, muscarinic receptor, cytochrome c oxidase) revealed no change in membrane purity across different preparations. Thin-section electron microscopy revealed predominantly intact unilamellar vesicles. Vesicles were preincubated for 15 min (37-degrees-C) in buffer containing 132 mM NaCl, 5 mM KCl, 1.3 mM MgCl2, 10 mM glucose and 10 mM HEPES (pH 7.4). Ca2+ uptake was initiated by diluting vesicles 20-fold with buffer containing either 132 mM NaCl or 132 mM choline chloride and (CaCl2)Ca-45 then terminated by addition of 200-mu-M LaCl3 and rapid filtration. Ca2+ content increased rapidly at first and then maintained a steady plateau for up to 5 min. When the Ca2+ ionophore A23187 (10-mu-M) with 100-mu-M EGTA was added after 4 min, Ca2+ content was reduced to 10% of its original value. Ruthenium red (10-mu-M) had no effect on Ca2+ content. Na+ -dependent Ca2+ uptake (Ca2+ content measured in choline chloride minus that measured in NaCl) was increased in AD brains as evidenced by both an increase in the initial rise in Ca2+ content and in elevated values of peak plateau Ca2+ content. The K(m) and V(max) for Na+/Ca2+ exchange was estimated from Lineweaver-Burk analysis of the effect of increasing extravesicular concentrations of Ca2+ on Na+ -dependent Ca2+ uptake after 30 s. The values obtained for the K(m) (mu-M) and V(max) (nmol/mg/min) were (respectively): normal (n = 6) 57.9 +/- 28.1 and 4.45 +/- 0.58; AD (n = 6) 71.2 +/- 29.3 and 6.68 +/- 1.58; NAD (n = 4) 66.1 +/- 3.3 and 3.76 +/- 1.50 (mean +/- S.D.). The V(max) for Na+/Ca2+ exchange in AD brain tissues was significantly (P < 0.05) elevated when compared with normal or NAD brains. There was no correlation evident in either normal, AD, NAD or the total sample population between Na+/Ca2+ exchange V(max) or K(m) and autolysis time or age at death. The results suggest that in brain regions suffering the greatest degeneration due to AD the surviving neurons have increased Na+/Ca2+ exchange activity. This increase in Na+/Ca2+ exchange activity may provide clues to a better understanding of the pathogenesis of nerve cell degeneration in AD.