Influence of age on respiratory modulation of muscle sympathetic nerve activity, blood pressure and baroreflex function in humans.

Alena Shantsila,James P Fisher,Anthony E Pickering,David B Mcintyre,Julian F R Paton,Gregory Y H Lip,Paul J Fadel

doi:10.1113/ep085071

Abstract

New Findings What is the central question of this study? Does ageing influence the respiratory‐related bursting of muscle sympathetic nerve activity (MSNA) and the association between the rhythmic fluctuations in MSNA and blood pressure (Traube–Hering waves) that occur with respiration? What is the main finding and its importance? Despite the age‐related elevation in MSNA, the cyclical inhibition of MSNA during respiration is similar between young and older individuals. Furthermore, central respiratory–sympathetic coupling plays a role in the generation of Traube–Hering waves in both young and older humans. Healthy ageing and alterations in respiratory–sympathetic coupling have been independently linked with heightened sympathetic neural vasoconstrictor activity. We investigated how age influences the respiratory‐related modulation of muscle sympathetic nerve activity (MSNA) and the association between the rhythmic fluctuations in MSNA and blood pressure that occur with respiration (Traube–Hering waves; THW). Ten young (22 ± 2 years; mean ± SD) and 10 older healthy men (58 ± 6 years) were studied while resting supine and breathing spontaneously. MSNA, blood pressure and respiration were recorded simultaneously. Resting values were ascertained and respiratory cycle‐triggered averaging of MSNA and blood pressure measurements performed. The MSNA burst incidence was higher in older individuals [22.7 ± 9.2 versus 42.2 ± 13.7 bursts (100 heart beats)−1, P < 0.05], and was reduced to a similar extent in the inspiratory to postinspiratory period in young and older subjects (by ∼25% compared with mid‐ to late expiration). A similar attenuation of MSNA burst frequency (in bursts per minute), amplitude and total activity (burst frequency × mean burst amplitude) was also observed in the inspiratory to postinspiratory period in both groups. A significant positive correlation between respiratory‐related MSNA and the magnitude of Traube–Hering waves was observed in all young (100%) and most older subjects (80%). These data suggest that the strength of the cyclical inhibition of MSNA during respiration is similar between young and older individuals; thus, alterations in respiratory–sympathetic coupling appear not to contribute to the age‐related elevation in MSNA. Furthermore, central respiratory–sympathetic coupling plays a role in the generation of Traube–Hering waves in both healthy young and older humans.

Highlights

We investigated how age influences the respiratory-related modulation of muscle sympathetic nerve activity (MSNA) and the association between the rhythmic fluctuations in MSNA and blood pressure that occur with respiration (Traube–Hering waves; THW)
Healthy ageing is associated with elevated plasma catecholamine concentrations, increased noradrenaline spillover from the heart, brain and kidneys and greater sympathetic neural activity directed to the skeletal muscle vasculature
During normal breathing in young healthy individuals, MSNA is inhibited during mid-inspiration, reaching a nadir when lung volume is at its highest and peaking when lung volume is at its lowest

Summary

Introduction

Healthy ageing is associated with elevated plasma catecholamine concentrations, increased noradrenaline spillover from the heart, brain and kidneys and greater sympathetic neural activity directed to the skeletal muscle vasculature (muscle sympathetic nerve activity, MSNA; Sundlof & Wallin, 1978; Seals & Esler, 2000). The mechanistic basis for the age-related elevation in sympathetic neural firing remains unclear It has been known since the earliest direct recordings that sympathetic nerve activity shows respiratory modulation (Adrian et al 1932). The respiratory modulation of MSNA was not assessed in terms of sympathetic burst occurrence (i.e. incidence) or strength (i.e. amplitude; Malpas & Ninomiya, 1992; Malpas et al 1996; Sverrisdottir et al 2000; Kienbaum et al 2001a); whether ageing affects the within-breath modulation of these distinct parameters of sympathetic nerve activity is unclear

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