Effects of Aging on the Responsiveness of the Human Cardiac Sympathetic Nerves to Stressors

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Abstract
Background Aging increases human sympathetic nervous activity at rest. Because of the probable importance of neural stress responses in the heart as triggers for clinical end points of coronary artery disease, it is pertinent to investigate whether sympathetic nervous responses to stresses are increased by aging. Methods and Results We applied kinetic methods for measuring the fluxes to plasma of neurochemicals relevant to sympathetic neurotransmission in younger (aged 20 to 30 years) and older (aged 60 to 75 years) healthy men during mental stress (difficult mental arithmetic), isometric exercise (sustained handgrip), and dynamic exercise (supine cycling). The increase in total norepinephrine spillover to plasma with mental stress was unaffected by age. In contrast, the increase in cardiac norepinephrine spillover was two to three times higher in the older subjects (P<.05). The probable mechanism of this higher cardiac norepinephrine spillover was reduced neuronal reuptake of the transmitter, because age...