Effect of fitness on reflex sympathetic neurovascular transduction in middle-age men.
PURPOSE
Muscle sympathetic nerve activity (MSNA) is increased in older endurance-trained men, yet the reflex sympathetic forearm vasoconstrictor response to graded lower body negative pressure (LBNP) diminishes with age. The aim of this study was to assess the influence of aerobic exercise capacity on this altered neurovascular coupling. We hypothesized that during graded LBNP, the forearm vascular resistance (FVR)-MSNA relationship would be steeper in sedentary versus fit men.


METHODS
We therefore studied 20 healthy middle-age men (age = 52 ± 2 yr, mean ± SE), 10 physically active (FIT) and 10 sedentary (SED) (129% ± 4% vs 85% ± 3% of predicted peak oxygen uptake) during 4 min each of LBNP at -5, -10, -20, and -40 mm Hg, applied in a random order. We determined HR, plasma norepinephrine, and MSNA (microneurography) and derived FVR from blood pressure and forearm blood flow (plethysmography). The FVR-MSNA relationship was determined by linear regression in each group separately, and groups were compared using multiple linear regression.


RESULTS
MSNA burst frequency and FVR at rest and during LBNP (P < 0.003) were similar in the two groups, whereas HR was significantly lower (P < 0.002) both at rest and during LBNP in FIT men (P < 0.05). FVR during LBNP correlated positively with MSNA in the SED group (r = 0.44, P < 0.001) but not in the FIT group (r = 0.19, P = 0.10). Multiple linear regression confirmed that both MSNA (P < 0.001) and fitness level (P = 0.04) contribute to the forearm vascular response.


CONCLUSIONS
Thus, during simulated orthostasis, middle-age SED men exhibit a significant FVR-MSNA relationship, which is not evident in age-matched FIT men. This alteration in neurovascular coupling may potentially affect cardiovascular risk in middle-age men.