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1 Penn State Heart and Vascular Institute, Hershey, PA 17033, USA
Renal circulatory adjustments to stress contribute to blood pressure and volume regulation. Both handgrip (HG) and disengagement of baroreflexes with lower body negative pressure (LBNP) can engage the sympathetic nervous system (SNS). However, the effect of simultaneous HG and LBNP on the renal circulation in humans is not known. Eighteen young healthy volunteers were studied. Beat-to-beat changes in renal blood flow velocity (RBV; Duplex Ultrasound), mean arterial pressure (MAP; Finapres) and heart rate (ECG) were monitored during (a) 15 s HG at 30% maximum voluntary contraction (MVC); (b) LBNP at –10 and –30 mmHg (each level for 5 min); and (c) 15 s HG (at 30% MVC) during LBNP at both levels. Renal vascular resistance index (RVR units) was calculated by dividing MAP by RBV. The increases in RVR during HG alone (12 ± 6%) were not different from the responses noted during combined HG and LBNP (17 ± 6% at –10 mmHg and 25 ± 8% at –30 mmHg). These results suggest occlusion occurs between a neural circuit engaged during 15 s of HG (central command and/or the muscle mechanoreflex) and a circuit activated by LBNP. In additional experiments (n = 6), similar non-algebraic summation of RVR was seen during 15 s involuntary biceps contractions (engages only muscle reflexes) and LBNP. With respect to RVR, neural occlusion occurs between baroreflexes and the muscle mechanoreflex. Muscle mechanoreflex mediated renal vasoconstriction during short bouts of HG is not influenced by baroreflex disengagement.
(Received 29 December 2005;
accepted after revision 20 March 2006;
first published online 23 March 2006)
Corresponding author L. I. Sinoway: Penn State Heart & Vascular Institute, Cardiology, H047, 500 University Drive, Hershey, PA 17033, USA. Email: lsinoway{at}psu.edu
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