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

The purpose of this investigation was to test the hypothesis that periphera l vasoconstriction and orthostatic tolerance are associated with increased circulating plasma concentrations of noradrenaline, vasopressin and renin-a ngiotensin. Sixteen men were categorized as having high (HT, n=9) or low (L T, n=7) tolerance to lower body negative pressure (LBNP) based on whether t he endpoint of their pre-syncopal-limited LBNP (peak LBNP) exposure exceede d -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 lega rterial pulse volume (measured by impedance rheography), an index of periph eral vascular constriction, from baseline to peak LBNP was greater (P < 0.0 5) in the HT group (-0.041 +/- 0.005 ml 100 ml(-1)) compared to the reducti on in the LT group (-0.025 +/- 0.003 ml 100 ml(-1)). Greater peak LBNP in t he HT group was associated with higher (P < 0.05) average elevations in pla sma concentrations of vasopressin (pVP, Delta=+7.2 +/- 2.0 pg ml(-1)) and p lasma renin-angiotensin (PRA, Delta=+2.9 +/- 1.3 ng Ang II ml(-1) h(-1)) co mpared 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 conce ntrations were increased (P < 0.05) from baseline to peak LBNP in both HT a nd LT groups, with no statistically distinguishable difference between grou ps. These data suggest that the renin-angiotensin and vasopressin systems m ay contribute to sustaining arterial pressure and orthostatic tolerance by their vasoconstrictive actions.