Controls had no evidence of thyroid disease or vitamin B12 deficiency. They had no ... of less than the fifth centile for controls of the same age plus a moderate or ...
EARLY REPORTS
Early report
Abnormal function of potassium channels in platelets of patients with Alzheimer’s disease H Asita de Silva, Jeffrey K Aronson, David G Grahame-Smith, Kim A Jobst, A David Smith
Summary Background Reports of abnormalities of potassium-channel function in various cultured cells of Alzheimer’s disease patients led us to attempt to characterise the pharmacological characteristics of the abnormal channel. Methods We studied platelets from 14 patients with Alzheimer-type dementia and 14 non-demented controls matched for age and sex. The effects of specific inhibitors of K+ channels on the efflux of rubidium-86 ions, a radioactive analogue of K+, from the platelets were measured. Findings Normal platelets contain three types of K+ channel, sensitive to the inhibitory actions of apamin (smallconductance calcium-dependent potassium channels), charybdotoxin (of less specificity, but probably intermediateconductance calcium-dependent K+ channels), and ␣-dendrotoxin (voltage-sensitive K+ channels). However, 86 Rb+ efflux from the platelets of patients with Alzheimer-type dementia was not inhibited by either apamin or charybdotoxin. By contrast, inhibition by ␣-dendrotoxin did occur. Interpretation Our results suggest that calcium-dependent K+ channels in platelets are selectively impaired in Alzheimer’s disease. A similar abnormality in neurons could contribute to the pathophysiology of the disorder.
Lancet 1998; 352: 1590–93
Introduction Abnormalities of potassium (K+) channel function have been shown in cultured cells from patients with Alzheimer’s disease.1 In particular, a 113 pS K+ channel sensitive to tetraethylammonium is absent or not functional in cultured fibroblasts from patients with Alzheimer’s disease,2 and this defect was mimicked in normal fibroblasts by the addition of amyloid -protein,3 which is also plentiful in platelets.4,5 However, tetraethylammonium is not a selective inhibitor of K+ channels, so the pharmacological identity of the abnormal channel in cultured fibroblasts is not clear. We studied abnormalities of K+ channels, specifically calcium-dependent K+ channels, in platelets from patients with dementia of the Alzheimer type compared with nondemented controls.
Patients and methods Patients We studied 28 people recruited as part of the Oxford Project to Investigate Memory and Ageing (OPTIMA); 14 of the patients had dementia of the Alzheimer type and 14 were non-demented controls matched for sex and age. The patients had an annual assessment,6,7 at which time a full range of blood tests was done to exclude metabolic or other causes of dementia, and cognitive University Department of Clinical Pharmacology, Radcliffe Infirmary, Oxford OX2 6HE, UK (H A de Silva M B, J K Aronson D G Grahame-Smith FRCP) and Oxford Project to Investigate Memory and Ageing (OPTIMA), Universty Department of Pharmacology, Oxford, UK (K A Jobst MRCP, A D Smith DPhil) Correspondence to: Dr Jeffrey K Aronson
THE LANCET • Vol 352 • November 14, 1998
FRCP,
function was assessed with the Cambridge examination for mental disorders of the elderly (CAMDEX).8 Diagnoses were made according to standard criteria.9,10 Additional information from the annual assessments included computed tomography data from both axial sections and temporal-lobe-oriented sections7,11,12 and single photon emission computed tomography (SPECT) regional cerebral blood-flow data.6,7,11 Controls had no evidence of thyroid disease or vitamin B12 deficiency. They had no evidence of cognitive dysfunction (CAMDEX scores13 were more than 79/108), they had no memory problems, and they did not have the combination of a minimum medial temporal lobe width on computed tomography of less than the fifth centile for controls of the same age plus a moderate or greater parietotemporal perfusion deficit on SPECT. The investigator who did the in-vitro studies (H A de S), did not have access to the diagnoses or to the clinical and scan information. The study was approved by the Central Oxford Research Ethics Committee.
K+ channel fluxes We prepared platelets and studied their K+ channel fluxes14 by loading fresh platelets with 86Rb+ (a radioactive analogue of K+)15,16 then stimulating 86Rb+ efflux with thrombin or ionomycin. We tested platelets from 12 patients in each group with thrombin and platelets from 11 patients in each group with ionomycin. One sample from the control group was lost because of a technical problem in the laboratory. We have shown17 that thrombin and ionomycin stimulate 86Rb+ efflux from platelets via three types of K+ channels. These are: calcium-dependent K+ channels sensitive to the highly selective inhibitor apamin (small-conductance calcium-dependent SKCa channels); calcium-dependent K+ channels sensitive to charybdotoxin (KCh channels, which probably have intermediate conductance in the platelet, IKCa channels, since platelets do not have large-conductance, BKCa channels17); and voltage-gated Kv channels sensitive to ␣-dendrotoxin. We ensured that there were no biological factors that could have altered ion transport in the patients, such as differences in serum ion concentrations and the presence of hypertension, renal failure, or diabetes mellitus.
Statistical analysis Characteristic Demography Age (years) M/F
Patients with Alzheimer’s disease (n=14)
Controls (n=14)
67·1 (7·5) 9/5
67·9 (7·7) 9/5
Serum biochemistry Sodium (mmol/L) Potassium (mmol/L) Calcium (mmol/L) Urea (mmol/L) Creatinine (µmol/L) Blood glucose (mmol/L) Total serum cobalamins (ng/L)
139 (3) 3·9 (0·4) 2·43 (0·10) 5·7 (1·3) 104 (12) 5·9 (2·1) 270 (90)
139 (2) 4·0 (0·3) 2·32 (0·13) 6·3 (2·7) 103 (24) 5·6 (1·1) 315 (125)
Blood pressure (mm Hg) Systolic Diastolic
154 (16) 90 (14)
140 (15) 81 (14)
Haematology Platelet count (⫻109/L) Haemoglobin (g/L) Mean cell volume (fL) White-cell count (⫻109/L)
233 (58) 139 (8) 89 (4) 6·9 (1·5)
233 (62) 142 (11) 90 (5) 6·5 (1·5)
Data are mean (SD).
Table 1: Baseline characteristics
1
EARLY REPORTS
Controls 200
150
150
Thrombin
50
0
200
Alzheimer's disease patients
150
Thrombin
100
50
0
200
Alzheimer's disease patients
150
Thrombin
100
50
50 Thrombin Thrombin + ␣-dendrotox in Non-stimulated
Thrombin Thrombin + charybdotoxin Thrombin + apamin Non-stimulated
0
0 0
2
4
6 8 Time (min)
10
12
14
Figure 1: Effects of apamin and charybdotoxin on thrombinstimulated 86Rb+ efflux in controls and patients with Alzheimer’s disease Cumulative efflux curves were analysed by ANOVA with repeated measures. The percentage inhibitions of efflux were not normally distributed and were compared by rank-sum test. Normal and abnormal responses to inhibitors were analysed by Fisher’s exact test.
Results Controls and patients had similar baseline characteristics (table 1). Non-stimulated cumulative 86Rb+ efflux was linear with time and did not differ between patients with Alzheimer’s disease and controls (figures 1 and 2). This efflux is partly mediated by the sodium/potassium/chloride cotransport system;14 thus, the findings suggest that this system functions normally in Alzheimer’s disease. In controls, thrombin stimulated an increase in cumulative 86Rb+ efflux from platelets (top panels in figures 1 and 2). Overall, both apamin and charybdotoxin 2
Thrombin
100 Cumulative 86Rb+ efflux (pmol per 2·5x107 platelets)
100 Cumulative 86Rb+ efflux (pmol per 2·5x107 platelets)
Controls
200
0
2
4
6 8 Time (min)
10
12
14
Figure 2: Effect of ␣-dendrotoxin on thrombin-stimulated 86Rb+ efflux in controls and patients with Alzheimer’s disease
(figure 1) caused significant inhibition of cumulative 86Rb+ efflux (each p
