Vasomotor sympathetic nerve responses to static handgrip after simulated microgravity.

During the isometric exercise, the autonomic nervous system has a major role in the regulation of arterial blood pressure, blood flow, and perfusion to the active skeletal muscle in humans. An increase in cardiac sympathetic nerve activity and a withdrawal of vagal tone cause elevations of heart rate, cardiac contractility and cardiac output. An increase in vasomotor sympathetic nerve activity causes a vascular constriction to redirect the oxygen transport to the contracting muscle. It has been reported that the autonomic and cardiovascular systems are commonly affected by the exposure to the real (i.e. spaceflight) and simulated microgravitational environment( i.e. 6 degrees head-down bed rest (HDBR)). The alteration in the autonomic system includes a reduced vagal tone, and an attenuated carotid-cardiac baroreflex function. It may also include an altered control of peripheral vessels, a reduced sympathetic vasoconstriction, an attenuated release of norepinephrine from sympathetic nerve terminal, an impaired myogenic contractility, an up- or down regulation of alpha-adrenergic receptors, and an attenuated multiple vasodilatation. Therefore, there seems to be a possibility that HDBR alters the sympathetic and cardiovascular responses to isometric exercise in humans, however, this possibility has not been examined efficiently. We compared the vasomotor sympathetic and cardiovascular responses to sustained handgrip (HG) until fatigue followed by post-exercise muscle ischemia (PEMI) after 14 days of HDBR, to those before HDBR, in 16 healthy males.