Selective impairment of excitatory pressor responses after prolonged simulated microgravity in humans

Prolonged head down bed rest-induced inactivity impairs tonic autonomic regulation while sparing oscillatory cardiovascular rhythms in healthy humans

“…Prolonged HDBR projects are technically unique because maintaining participants strictly in bed requires highly organized personnel and professional skills. Regarding HDBR-related cardiovascular studies, the analysis of cardiovascular variability signals has been used as a tool to investigate autonomic regulation [25,28,29] with conflicting results [14,17]. With HDBR, a clear-cut reduction of RR variance and BRS was observed ( Table 1), suggesting that this intervention impairs tonic cardiac vagal regulation, in line with most prior investigations [12,30].…”

“…2) at REC þ 5 (1.6 AE 1.1 mmHg) as compared with BDC (14.4 AE 3.1 mmHg, P < 0.05), supporting further the notion of an inhibitory influence of HDBR on vasoconstriction. In addition, the MSNA response showed a minimum during REC þ 5, suggesting that the previous observations [14] of a reduced pressor responses obtained with a shorter HDBR exposure might be extended to tonic MSNA response during longer HDBR conditions. Clearly, our findings relate to small masses' light static exercise and should not be generalized to other kinds of exercise, eventually involving larger muscle masses or greater exercise intensity [35,36].…”

“…An accompanying impairment of sympathetic vascular regulation after inactivity would be suggested by a reduced pressor responsiveness to simple excitatory stimuli, such as handgrip [13,14] and by a reduced norepinephrine turnover [15] following HDBR.…”

“…Data regarding the oscillatory properties of neural cardiac regulation, as assessed indirectly by spectral analysis of RR variability, are also conflicting, as the low-frequency component has been described as increased [17] or unchanged [14] after HDBR. Virtually no data are available on oscillatory properties of MSNA after HDBR.…”

Altered profile of baroreflex and autonomic responses to lower body negative pressure in chronic orthostatic intolerance

“…Other conditions in which an elevated sympathetic drive is likely, such as arterial hypertension [35], are also characterized by signs of blunted cardiac autonomic and enhanced vascular sympathetic responses to gravitational challenges. Increased vascular sympathetic responsiveness to active standing can be induced in normal subjects by prolonged simulated microgravity [38], also producing a reversible impairment of baroreflex gain. Altered norepinephrine transporter (NET) dynamics might also be implicated, as suggested by studies in conscious animals with induced NET blockade [39], showing a reduction in baroreflex gain and an increase of pressure overshoot to nitroprusside infusion, which produces both a reduction in arterial pressure and in cardiac volumes [19].…”