Frequency response characteristics of sympathetic and autoregulatory vasomotor responses in the kidney and hindlimb

Abstract

The literature suggests vasomotor responses might be faster in the kidney than in other vascular beds. We compared vasoconstrictor response times to sympathetic nerve stimulation (SNS) as well as autoregulatory responses (AR) to a 20 mmHg step increase of perfusion pressure in the kidney and hindlimb of anesthetized rats. SNS of renal and lumbar nerves (1–8 Hz for 60 s) led to similar reductions in renal (RBF, −59% at 8 Hz) and iliac blood flow (IBF, −64% at 8 Hz). RBF fell biphasically with a rapid component within 3–5 s (τ =3.0 s) and a slower one over 30–50 s (τ =11 s), both contributing equally. In contrast, IBF fell monophasically within 3–5 s (τ =1.5 s). Dynamic modulation of SNS (~5 Hz, Delta;t on, Delta;t off, Delta;t=1, 2, 3, 4, 8, 15, 60 s) showed similar corner frequencies for RBF and IBF at ~0.15 Hz and an additional one for RBF at ~0.05 Hz. AR responses displayed a bimodal adaptation of RBF with a fast initial response within 9 s (τ =3.3 s) providing 38% AR efficiency, and a subsequent slower response within 30–120 s bringing total AR to 100%. Previous studies support the fast response of AR reflecting the myogenic response and the secondary tubuloglomerular feedback (TGF). AR in IBF displayed only one component (τ =2.5 s) providing 34% AR efficiency. It is concluded that the fast components of SNS and AR vasoconstrictor responses are of similar speed in kidney and skeletal muscle. In addition, secondary slower mechanisms operate in the kidney reducing the speed of overall RBF adaptation. Whether the secondary component in the SNS response is the same as TGF in AR requires further study. Supported by NIH (HL02334) and Guyton Award for Excell. in Integr. Physiol..