AUTOREGULATION OF RENAL BLOOD-FLOW IN THE CONSCIOUS DOG AND THE CONTRIBUTION OF THE TUBULOGLOMERULAR FEEDBACK

1. The aim of this study was to investigate the autoregulation of rena l blood flow under physiological conditions, when challenged by the no rmal pressure fluctuations, and the contribution of the tubuloglomerul ar feedback (TGF). 2. The transfer function between 0.0018 and 0.5 Hz was calculated from the spontaneous fluctuations in renal arterial blo od pressure (RABP) and renal blood flow (RBF) in conscious resting dog s. The response of RBF to stepwise artificially induced reductions in RABP was also studied (stepwise autoregulation). 3. Under control cond itions (n = 12 dogs), the gain of the transfer function started to dec rease, indicating improving autoregulation, below 0.06-0.15 Hz (t = 7- 17 s). At 0.027 Hz a prominent peak of high gain was found. Below 0.01 Hz (t > 100 s), the gain reached a minimum (maximal autoregulation) o f -6.3 +/- 0.6 dB. The stepwise autoregulation (n = 4) was much strong er (-19.5 dB). The time delay of the transfer function was remarkably constant from 0.03 to 0.08 Hz (high frequency (HF) range) at 1.7 s and from 0.0034 to 0.01 Hz(low frequency (LF) range) at 14.3 s, respectiv ely. 4. Nifedipine, infused into the renal artery, abolished the stepw ise autoregulation (-2.0 +/- 1.1 dB, n = 3). The gain of the transfer function (n = 4) remained high down to 0.0034 Hz; in the LF range it w as higher than in the control (0.3 +/- 1.0 dB, P < 0.05). The time del ay in the HF range was reduced to 0.5 s (P < 0.05). 5. After ganglioni c blockade (it = 7) no major changes in the transfer function were obs erved. 6. Under furosemide (frusemide) (40 mg + 10 mg h(-1) or 300 mg + 300 mg h(-1) I.V.) the stepwise autoregulation was impaired to -7.8 +/- 0.3 or -6.7 +/- 1.9 dB, respectively (n = 4). In the transfer func tion (n = 7 or n = 4) the peak at 0.027 Hz was abolished. The delay in the LF range was reduced to -1.1 or -1.6 s, respectively. The transfe r gain in the LF range (-5.5 +/- 1.2 or -3.8 +/- 0.8 dB, respectively) did not differ from the control but was smaller than that under nifed ipine (P < 0.05). 7. It is concluded that the ample capacity for regul ation of RBF is only partially employed under physiological conditions . The abolition by nifedipine and the negligible effect of ganglionic blockade show that above 0.0034 Hz it is almost exclusively due to aut oregulation by the kidney itself. TGF contributes to the maximum autor egulatory capacity, but it is not required for the level of autoregula tion expended under physiological conditions. Around 0.027 Hz, TGP eve n reduces the degree of autoregulation.