Sympathetic nerve activity and peripheral vasodilator capacity in young and older men

Interindividual variability in sympathetic nerve activity (SNA) has provided insight into integrative mechanisms contributing to blood pressure (BP) regulation in humans. In young people, the influence of high SNA on BP is balanced by lower cardiac output and less adrenergic vasoconstrictor responsiveness. Older people have higher SNA and higher BP. We hypothesized that SNA has a restraining effect on peripheral vasodilator responsiveness in young and older men, such that individuals with higher tonic SNA would show less forearm vasodilatation to exogenous vasodilators. We measured muscle SNA (MSNA; microneurography) and forearm vasodilator responses to intra-arterial infusions of acetylcholine (ACh; endothelium dependent) and sodium nitroprusside (SNP; endothelium independent) in 13 young (age; 27 ± 1 yr) and 16 older (61 ± 2 yr) men. Forearm vascular conductance (FVC) responses to ACh were lower in the older men at the two highest doses (2 and 4 microg·100 ml(-1)·min(-1); Δ395 ± 81 vs. 592 ± 87% and 412 ± 87 vs. 616 ± 132%, P < 0.05), and MSNA was higher (64 ± 4 vs. 41 ± 2 bursts/100 hb; P < 0.05). There was no difference in the FVC response to SNP between young and older men (P > 0.05). In young men, there was an inverse relationship between resting MSNA and FVC responses (%change) to both ACh and SNP (r = -0.83 and r = -0.83, respectively; P < 0.05). In older men, however, this relationship was not observed. Tonic SNA may act to restrain vasodilator responses in young men, whereas in older men a lack of such restraint may be protective against the pressor effects of higher SNA.