.....We observed that Aβ peptides exert a chronotropic effect in neonatal cardiomyocytes, similar to α1-adrenergic receptor autoantibodies that we described earlier. Recently, it was shown that α1-adrenergic receptor could impair blood-brain flow. We hypothesized that Aβ peptides might elicit a signal transduction pathway in vascular cells, induced by α1-adrenergic receptor activation. Aβ (25-35) and Aβ (10-35) induced a positive chronotropic effect in the cardiac contraction assay (28.75±1.15 and 29.40±0.98 bpm), which was attenuated by α1-adrenergic receptor blockers (urapidil, 1.53±1.17 bpm; prazosin, 0.30±0.96 bpm). Both Aβ peptides induced an intracellular calcium release in vascular smooth muscle cells. Chronotropic activity and calcium response elicited by Aβ (25-35) were blocked with peptides corresponding to the first extracellular loop of the α1-adrenergic receptor. We observed an induction of extracellular-regulated kinase 1/2 phosphorylation by Aβ (25-35) in Chinese hamster ovary cells overexpressing α1-adrenergic receptor, vascular smooth muscle cells, and cardiomyocytes. We generated an activation-state-sensitive α1-adrenergic receptor antibody and visualized activation of the α1-adrenergic receptor by Aβ peptide. Aβ (25-35) induced vasoconstriction of mouse aortic rings and in coronary arteries in Langendorff-perfused rat hearts that resulted in decreased coronary flow. Both effects could be reversed by α1-adrenergic receptor blockade. Our data are relevant to the association between Alzheimer disease and hypertension. They may explain impairment of vascular responses by Aβ and could have therapeutic implications.