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1 Department of Cell & Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7545, USA
This rat renal blood flow (RBF) study quantified the impact of nitric oxide synthase (NOS) inhibition on the myogenic response and the balance of autoregulatory mechanisms in the time domain following a 20 mmHg-step increase or decrease in renal arterial pressure (RAP). When RAP was increased, the myogenic component of renal vascular resistance (RVR) rapidly rose within the initial 7–10 s, exhibiting an 
5 s time constant and providing 36% of perfect autoregulation. A secondary rise between 10 and 40 s brought RVR to 95% total autoregulatory efficiency, reflecting tubuloglomerular feedback (TGF) and possibly one or two additional mechanisms. The kinetics were similar after the RAP decrease. Inhibition of NOS (by L-NAME) increased RAP, enhanced the strength (79% autoregulation) and doubled the speed of the myogenic response, and promoted the emergence of RVR oscillations (0.2 Hz); the strength (52%) was lower at control RAP. An equi-pressor dose of angiotensin II had no effect on myogenic or total autoregulation. Inhibition of TGF (by furosemide) abolished the L-NAME effect on the myogenic response. RVR responses during furosemide treatment, assuming complete inhibition of TGF, suggest a third mechanism that contributes 10–20% and is independent of TGF, slower than the myogenic response, and abolished by NOS inhibition. The hindlimb circulation displayed a solitary myogenic response similar to the kidney (35% autoregulation) that was not enhanced by L-NAME. We conclude that NO normally restrains the strength and speed of the myogenic response in RBF but not hindlimb autoregulation, an action dependent on TGF, thereby allowing more and slow RAP fluctuations to reach glomerular capillaries.
(Received 18 July 2005;
accepted after revision 11 October 2005;
first published online 13 October 2005)
Corresponding author A. Just: Department of Cell and Molecular Physiology, 6341 Medical Biomolecular Research Bldg, CB 7545, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7545, USA. Email: just{at}med.unc.edu
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