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1 Department of Kinesiology, University of Colorado, Boulder, Colorado 80309; and 2 Baker Medical Research Institute, Prahran 3181, Victoria, Australia
Received 26 November 1996; accepted in final form 20 February 1997.
Mazzeo, Robert S., Chakravarthi Rajkumar, Garry Jennings,
and Murray Esler. Norepinephrine spillover at rest and during submaximal exercise in young and old subjects. J. Appl. Physiol. 82(6): 1869-1874, 1997.
Aging is
associated with elevations in plasma norepinephrine
concentrations. The purpose of this investigation was to
examine total body and regional norepinephrine spillover as an
indicator of sympathetic nerve activity. Eight young (26 ± 3 yr)
and seven old (69 ± 5 yr) male subjects were studied at rest and
during 20 min of submaximal cycling exercise at 50% of peak work
capacity. Norepinephrine spillover was determined by continuous
intravenous infusion of
[3H]norepinephrine.
Arterial norepinephrine concentrations were significantly greater at
rest for old vs. young subjects (280 ± 36 vs. 196 ± 27 ng/ml,
respectively). Whereas total norepinephrine spillover did not differ
between groups at rest, hepatomesenteric norepinephrine spillover was
50% greater in old subjects compared with their young counterparts (51 ± 7 vs. 34 ± 5 ng/min, respectively). Additionally,
norepinephrine clearance rates at rest were significantly lower for the
old subjects (
23%). During exercise, plasma
norepinephrine concentrations increased compared with rest, with old
subjects again demonstrating greater values than the young group.
Hepatomesenteric norepinephrine spillover was significantly greater
(+36%) during exercise for old subjects compared with young; however,
no difference was found for whole body spillover rates between age
groups. Norepinephrine clearance rates remained depressed
(
30%) in the old subjects during exercise. Clearance of
epinephrine mirrored that for norepinephrine both at rest and during
exercise across age groups. It was concluded that in old subjects, a
reduction in norepinephrine clearance and an increase in regional
norepinephrine spillover can account for the higher plasma
norepinephrine concentrations observed at rest. This relationship is
not exacerbated by the stress imposed during an acute bout of exercise.
sympathetic nerve activity; epinephrine; hepatomesenteric; aging
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