Sympathetic activation by lower body negative pressure decreases kidney perfusion without inducing hypoxia in healthy humans

Purpose There is ample evidence that systemic sympathetic neural activity contributes to the progression of chronic kidney disease, possibly by limiting renal blood flow and thereby inducing renal hypoxia. Up to now there have been no direct observations of this mechanism in humans. We studied the effects of systemic sympathetic activation elicited by a lower body negative pressure (LBNP) on renal blood flow (RBF) and renal oxygenation in healthy humans. Methods Eight healthy volunteers (age 19-31 years) were subjected to progressive LBNP at - 15 and - 30 mmHg, 15 min per level. Brachial artery blood pressure was monitored intermittently. RBF was measured by phase-contrast MRI in the proximal renal artery. Renal vascular resistance was calculated as the MAP divided by the RBF. Renal oxygenation (R2*) was measured for the cortex and medulla by blood oxygen level dependent (BOLD) MRI, using a monoexponential fit. Results With a LBNP of - 30 mmHg, pulse pressure decreased from 50 +/- 10 to 43 +/- 7 mmHg; MAP did not change. RBF decreased from 1152 +/- 80 to 1038 +/- 83 mL/min to 950 +/- 67 mL/min at - 30 mmHg LBNP (p = 0.013). Heart rate and renal vascular resistance increased by 38 +/- 15% and 23 +/- 8% (p = 0.04) at - 30 mmHg LBNP, respectively. There was no change in cortical or medullary R2* (20.3 +/- 1.2 s(-1) vs 19.8 +/- 0.43 s(-1); 28.6 +/- 1.1 s(-1) vs 28.0 +/- 1.3 s(-1)). Conclusion The results suggest that an increase in sympathetic vasoconstrictor drive decreases kidney perfusion without a parallel reduction in oxygenation in healthy humans. This in turn indicates that sympathetic activation suppresses renal oxygen demand and supply equally, thus allowing adequate tissue oxygenation to be maintained.Cardiovascular Aspects of Radiolog