Abnormal cardiac function associated with sympathetic nervous system hyperactivity in mice

α_2A-Adrenergic receptors (ARs) in the midbrain regulate sympathetic nervous system activity, and both α_2A-ARs and α_2C-ARs regulate catecholamine release from sympathetic nerve terminals in cardiac tissue. Disruption of both α_2A- and α_2C-ARs in mice leads to chronically elevated sympathetic tone and decreased cardiac function by 4 mo of age. These knockout mice have increased mortality, reduced exercise capacity, decreased peak oxygen uptake, and decreased cardiac contractility relative to wild-type controls. Moreover, we observed significant abnormalities in the ultrastructure of cardiac myocytes from α_2A/α_2C-AR knock-out mice by electron microscopy. Our results demonstrate that chronic elevation of sympathetic tone can lead to abnormal cardiac function in the absence of prior myocardial injury or genetically induced alterations in myocardial structural or functional proteins. These mice provide a physiologically relevant animal model for investigating the role of the sympathetic nervous system in the development and progression of heart failure.