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Departments of 1 Human Services, 2 Medicine, and 4 Health Evaluation Sciences, University of Virginia, Charlottesville, Virginia 22903; and 3 Department of Pediatrics and Pharmacology, Rainbow Babies and Children's Hospital, Case Western Reserve University, Cleveland Ohio 44106
To test the hypothesis
that heightened sympathetic outflow precedes and predicts the magnitude
of the growth hormone (GH) response to acute exercise (Ex), we studied
10 men [age 26.1 ± 1.7 (SE) yr] six times in randomly assigned
order (control and 5 Ex intensities). During exercise, subjects
exercised for 30 min (0900-0930) on each occasion at
a single intensity: 25 and 75% of the difference between lactate
threshold (LT) and rest (0.25LT, 0.75LT), at LT, and at 25 and 75% of
the difference between LT and peak (1.25LT, 1.75LT). Mean
values for peak plasma epinephrine (Epi), plasma norepinephrine (NE),
and serum GH concentrations were determined [Epi: 328 ± 93 (SE),
513 ± 76, 584 ± 109, 660 ± 72, and 2,614 ± 579 pmol/l; NE: 2.3 ± 0.2, 3.9 ± 0.4, 6.9 ± 1.0, 10.7 ± 1.6, and 23.9 ± 3.9 nmol/l; GH: 3.6 ± 1.5, 6.6 ± 2.0, 7.0 ± 2.0, 10.7 ± 2.4, and 13.7 ± 2.2 µg/l for 0.25, 0.75, 1.0, 1.25, and 1.75LT, respectively]. In
all instances, the time of peak plasma Epi and NE preceded peak GH
release. Plasma concentrations of Epi and NE always peaked at 20 min
after the onset of Ex, whereas times to peak for GH were 54 ± 6 (SE), 44 ± 5, 38 ± 4, 38 ± 4, and 37 ± 2 min
after the onset of Ex for 0.25-1.75LT, respectively. ANOVA
revealed that intensity of exercise did not affect the foregoing time
delay between peak NE or Epi and peak GH (range 17-24 min), with
the exception of 0.25LT (P < 0.05). Within-subject
linear regression analysis disclosed that, with increasing exercise
intensity, change in (
) GH was proportionate to both NE
(P = 0.002) and Epi (P = 0.014). Furthermore, within-subject multiple-regression analysis
indicated that the significant GH increment associated with an
antecedent rise in NE (P = 0.02) could not be explained by changes in Epi alone (P = 0.77). Our results suggest
that exercise intensity and GH release in the human may be coupled
mechanistically by central adrenergic activation.
catecholamines; epinephrine; norepinephrine
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