Renal blood flow dynamics and arterial pressure lability in the conscious rat

It is not known whether renal blood flow (RBF) is still autoregulated when the kidney is exposed to large transient blood pressure (BP) fluctuations s uch as those occurring spontaneously in conscious sinoaortic baroreceptor-d enervated (SAD) rats. In this study, BP and RBF were simultaneously recorde d in 8 SAD rats (2 weeks before study) and 8 baroreceptor-intact rats durin g approximate to3 hours of spontaneous activity. The kidney used for RBF re cordings was denervated to prevent the interference of changes in renal sym pathetic tone with autoregulatory mechanisms. In intact rats, RBF variabili ty (coefficient of variation 9.1 +/-0.8%) was larger (P <0.02) than BP vari ability (5.9 +/-0.2%). This was mainly because of slow changes in RBF that were unrelated to BP and also to a prominent oscillation of RBF of approxim ate to0.25-Hz frequency. Autoregulatory patterns were identified at frequen cies <0.1 Hz and provided a modest attenuation of BP fluctuations. In SAD r ats, RBF variability (12.4 +/-1.6%) was lower (P<0.02) than BP variability (18.2<plus/minus>1.1%). Autoregulation powerfully attenuated BP changes <0. 1 Hz (normalized transfer gain 0.21 +/-0.02 in the 0.0015- to 0.01-Hz frequ ency range) but at the expense of an oscillation located at approximate to0 .05 Hz that possibly reflected the operation of the tubuloglomerular feedba ck. Large transient hypertensive episodes were not translated into RBF chan ges in SAD rats. We conclude that autoregulatory mechanisms have an ample c apacity to protect the kidney against spontaneous BP fluctuations in the co nscious rat. This capacity is not fully used under normal conditions of low BP variability.