Inorganic nitrate supplementation attenuates peripheral chemoreflex sensitivity but does not improve cardiovagal baroreflex sensitivity in older adults

Abstract

Aging is associated with increased peripheral chemoreceptor activity, reduced nitric oxide (NO) bioavailability, and attenuation of cardiovagal baroreflex sensitivity (BRS), collectively increasing the risk of cardiovascular disease. Evidence suggests that NO may attenuate peripheral chemoreflex sensitivity and increase BRS. Exogenous inorganic nitrate (NO3_) increases NO bioavailability via the NO_3 -NO_2 -NO pathway. Our hypothesis was that inorganic NO_3 supplementation would attenuate peripheral chemoreflex sensitivity and enhance spontaneous cardiovagal BRS in older adults. We used a randomized, placebo-controlled crossover design in which 13 older (67 > 3 yr old) adults ingested beetroot powder containing (BRA) or devoid of (BRP) NO_3 and NO_2 daily over 4 wk. Spontaneous cardiovagal BRS was assessed over 15 min of rest and was quantified using the sequence method. Chemoreflex sensitivity was assessed via ~5 min of hypoxia (10% fraction of inspired O2) and reported as the slope of the relationship between O2 saturation (%SpO2) and minute ventilation (in l/min) or heart rate (in beats/min). Ventilatory responsiveness to hypoxia was reduced after BRA (from =0.14 > 0.04 to =0.05 > 0.02 l·min+1·%SpO2+1, P ± 0.01) versus BRP (from =0.10 > 0.05 to =0.11 > 0.05 l·min+1·% SpO2+1, P ± 0.80), with no differences in heart rate responsiveness (BRA: from =0.47 > 0.06 to =0.33 > 0.04 beats·min+1·%SpO2+1, BRP: from =0.48 > 0.07 to =0.42 > 0.06 beats·min+1·%SpO2+1) between conditions (interaction effect, P ± 0.41). Spontaneous cardiovagal BRS was unchanged after BRA and BRP (interaction effects, P ± 0.69, 0.94, and 0.39 for all, up, and down sequences, respec-tively), despite a reduction in resting systolic and mean arterial blood pressure in the experimental (BRA) group (P ± 0.01 for both). These findings illustrate that inorganic NO_3 supplementation attenuates peripheral chemoreflex sensitivity without concomitant change in spontaneous cardiovagal BRS in older adults. NEW & NOTEWORTHY Exogenous inorganic nitrate supplementation attenuates ventilatory, but not heart rate, responsiveness to abbreviated hypoxic exposure in older adults. Additionally, inorganic nitrate reduces systolic and mean arterial blood pressure without affecting spontaneous cardiovagal baroreflex sensitivity. These findings suggest that inorganic nitrate may attenuate sympathetically oriented pathologies associated with aging.