AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 1 278-R285, Copyright © 1995 by American Physiological Society
Aging effects on human sympathetic neuronal function
M. D. Esler, A. G. Turner, D. M. Kaye, J. M. Thompson, B. A. Kingwell, M. Morris, G. W. Lambert, G. L. Jennings, H. S. Cox and D. R. Seals
Baker Medical Research Institute, Prahran, Melbourne, Australia.
To study the effect of aging on human sympathetic nervous function, we
applied kinetic methods for measuring the fluxes to plasma of
neurochemicals relevant to sympathetic neurotransmission in younger (aged
20-30 yr) and older (aged 60-75 yr) healthy men. Mean plasma norepinephrine
concentration was 66% higher in older men, attributable to 22% lower
norepinephrine plasma clearance (P < 0.05) and 29% higher norepinephrine
spillover to plasma (difference not statistically significant). Regional
venous sampling disclosed that sympathetic outflow to all organs was not
activated by aging. Renal norepinephrine spillover was normal in older men.
Although spillover of norepinephrine from the heart was increased in older
men, 21.1 +/- 11.4 ng/min compared with 11.4 +/- 8.6 ng/min (P < 0.05),
diminished norepinephrine reuptake rather than increased cardiac
sympathetic nerve firing was the most likely cause, although somewhat
reduced intracardiac methylation of norepinephrine with aging also possibly
contributed. The extraction of tritiated norepinephrine from plasma during
transit through the heart was reduced, suggesting neuronal norepinephrine
reuptake was lowered and overflow of the norepinephrine precursor
dihydroxyphenylalanine and metabolites dihydroxyphenylglycol and
3-methoxy-4-hydroxy phenylglycol was normal, indicating that norepinephrine
synthesis and release were not increased.
