Assessment of Perfusion and Oxygenation of the Human Renal Cortex and Medulla by Quantitative MRI during Handgrip Exercise

Abstract

Background: Renal flow abnormalities are believed to play a central role in the pathogenesis of nephropathy and in primary and secondary hypertension, but are difficult to measure in humans. Handgrip exercise is known to reduce renal arterial flow (RAF) by means of increased renal sympathetic nerve activity.

Methods: To monitor medullary and cortical oxygenation under handgrip exercise-reduced perfusion, we used contrast- and radiation-free magnetic resonance imaging (MRI) to measure regional changes in renal perfusion and blood oxygenation in ten healthy normotensive individuals during handgrip exercise. We used phase-contrast MRI to measure RAF, arterial spin labeling to measure perfusion, and both changes in transverse relaxation time (T₂*) and dynamic blood oxygenation level-dependent imaging to measure blood oxygenation.

Results: Handgrip exercise induced a significant decrease in RAF. In the renal medulla, this was accompanied by an increase of oxygenation (reflected by an increase in T₂*) despite a significant drop in medullary perfusion; the renal cortex showed a significant decrease in both perfusion and oxygenation. We also found a significant correlation (R²=0.8) between resting systolic BP and the decrease in RAF during handgrip exercise.

Conclusions: Renal MRI measurements in response to handgrip exercise were consistent with a sympathetically mediated decrease in RAF. In the renal medulla, oxygenation increased despite a reduction in perfusion, which we interpreted as the result of decreased GFR and a subsequently reduced reabsorptive workload. Our results further indicate that the renal flow response's sensitivity to sympathetic activation is correlated with resting BP, even within a normotensive range.

Keywords: ASL; BOLD; Hemodynamics and Vascular Regulation; MRI; blood pressure; chronic hypoxia.

Figure 1.

Oxygenation in the renal medulla increases during handgrip exercise despite falling perfusion. Perfusion (Perf) in the renal cortex and medulla decreased significantly along with renal arterial flow during the handgrip exercise. T₂*, an indicator of blood oxygenation, decreased significantly in cortex during the first 2 minutes, but increased significantly in the medulla. All parameters returned to pre-exercise values within 3 minutes of rest after stopping the handgrip exercise, with the exception of medullary T₂*, which remained significantly higher.

Figure 2.

BOLD signal intensity increases in the medulla and decreases in the cortex during the handgrip exercise. The BOLD signal intensity increases in the medulla (indicating a reduction in deoxyhemoglobin concentration) and decreases in the cortex (indicating increased deoxyhemoglobin) during the handgrip exercise.

Figure 3.

Reduction in renal artery flow correlates significantly with the resting systolic BP. The reduction in renal artery flow correlates significantly (P<0.01) with the resting systolic BP. (A) The 2-day average for each individual. (B) Day 1 and day 2 results from each individual, connected with a dotted line.