Central blood volume influences sympathetic sudomotor nerve traffic in warm humans

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Abstract
The objective of this study was to test whether changes in central blood volume can induce reflex effects on sweating. Multi‐unit skin sympathetic nerve activity (SSA) was recorded from the posterior cutaneous nerve of the forearm or radial nerve branches in 11 healthy volunteers. Skin electrical resistance and skin blood flow were recorded in the area innervated by the impaled nerve fascicle. Sudomotor nerve traffic and sweating was induced by whole body heating. Lower body negative pressure (LBNP) and tilting (3d̀ head up) was used for blood volume displacement from the chest to the lower body. Low levels of LBNP (5 and 10 mmHg) had no effect on blood pressure, heart rate or skin blood flow but induced a prompt inhibition of SSA and a reduced number of transient skin resistance changes (n= 9), both rapidly returning to control levels after cessation of LBNP. Quantitatively, the effect was similar at both levels of LBNP. Head up tilting also reduced SSA (n= 3, tilt manoeuvres). A capacity for mental stress‐induced SSA increase remained during LBNP. Spontaneous flucturations in blood pressure did not affect SSA, arguing against arterial (high‐pressure) baroreceptors modulating SSA. Consequently, the present results indicate that unloading of cardiopulmonary (low‐pressure) volume receptors reduces sympathetic sudomotor nerve traffic and sweating in warm subjects. It is suggested that the reflex contributes to counteracting hypovolaemia.