Vasoactive neuroendocrine responses associated with tolerance to lower body negative pressure in humans.

The purpose of this investigation was to test the hypothesis that peripheral vasoconstriction and orthostatic tolerance are associated with increased circulating plasma concentrations of noradrenaline, vasopressin and renin-angiotensin. Sixteen men were categorized as having high (HT, n=9) or low (LT, n=7) tolerance to lower body negative pressure (LBNP) based on whether the endpoint of their pre-syncopal-limited LBNP (peak LBNP) exposure exceeded -60 mmHg. The two groups were matched for age, height, weight, leg volume, blood volume and maximal oxygen uptake, as well as baseline blood volume and plasma concentrations of vasoactive hormones. Peak LBNP induced similar reductions in mean arterial pressure in both groups. The reduction in leg arterial pulse volume (measured by impedance rheography), an index of peripheral vascular constriction, from baseline to peak LBNP was greater (P<0.05) in the HT group (-0.041 +/- 0.005 ml 100 ml-1) compared to the reduction in the LT group (-0. 025 +/- 0.003 ml 100 ml-1). Greater peak LBNP in the HT group was associated with higher (P<0.05) average elevations in plasma concentrations of vasopressin (pVP, Delta=+7.2 +/- 2.0 pg ml-1) and plasma renin-angiotensin (PRA, Delta=+2.9 +/- 1.3 ng Ang II ml-1 h-1) compared to average elevations of pVP (+2.2 +/- 1.0 pg ml-1) and PRA (+0.1 +/- 0.1 ng Ang II ml-1 h-1) in the LT group. Plasma noradrenaline concentrations were increased (P<0.05) from baseline to peak LBNP in both HT and LT groups, with no statistically distinguishable difference between groups. These data suggest that the renin-angiotensin and vasopressin systems may contribute to sustaining arterial pressure and orthostatic tolerance by their vasoconstrictive actions.