Aging and the exercise pressor reflex in humans

Background - Blood flow limitation to exercising muscles engages the muscle reflex during exercise, evoking an increase in heart rate (HR), blood pressure (BP), and muscle sympathetic nerve activity (MSNA). Methods and Results - In the current study, we examined forearm flow and autonomic responses to ischemic handgrip in young and older subjects. We studied 6 younger subjects (mean age 23.5±2.2 years) and7 older subjects (mean age 65.0±2.4 years). Subjects performed rhythmic handgrip (thirty 1-sec contractions/min) at 30% maximal voluntary contraction during six 1-minute stages: freely perfused exercise (E1) and exercise with forearm pressure of + 10, +20, +30, +40, and +50 mm Hg (E2 through E6). We measured HR, BP, MSNA, forearm flow velocity, forearm venous oxygen saturation, H⁺, and lactate. Compared with E1, ischemic exercise (E2 through E6) increased HR, BP, and MSNA, reduced forearm velocity, lowered venous oxygen saturation, and raised venous lactate and H⁺. Compared with the younger subjects, the older subjects had attenuated BP at E6, attenuated MSNA indices (%Abursts, bursts/100 heart beats and signal averaged MSNA), attenuated H⁺ at E6, a trend toward higher levels of oxygen saturation, and similar forearm velocity and HR responses. Conclusions - Aging attenuates the muscle reflex.