Spontaneous blood pressure fluctuations and renal blood flow dynamics.

Two mechanisms operating at 0.03-0.05 and 0.1-0.2 Hz are involved in autoregulation of renal blood flow (RBF). To examine the behavior of the faster system, the response of RBF to spontaneous fluctuations of arterial pressure was assessed in Sprague-Dawley rats anesthetized by isoflurane or halothane. During halothane anesthesia, autonomous oscillation of total RBF was observed at 0.10-0.15 Hz, and normalized admittance gain became negative at 0.11 +/- 0.01 Hz. During isoflurane anesthesia, there was autonomous power in blood flow in a broad peak between 0.15 and 0.25 Hz, and gain became negative at 0.15 +/- 0.01 Hz. Increasing inspired isoflurane concentration from 1.4 +/- 0.1% to 2.2 +/- 0.1% reduced pressure by 22 +/- 2 mmHg but did not alter blood flow or the transfer function, indicating that the operating frequency was not changed. In another experiment, changing from isoflurane to halothane increased peak power in the autonomous blood flow oscillation fivefold and reduced its frequency from 0.18 +/- 0.01 to 0.14 +/- 0.01 Hz. Gain became negative at a higher frequency (0.16 +/- 0.01 Hz) during isoflurane than halothane anesthesia (0.12 +/- 0.01 Hz). The results show that the 0.1-0.2 Hz system is reliably detected under unforced conditions and provides modest attenuation of pressure fluctuations at < or = 0.1 Hz. Its operating frequency under isoflurane anesthesia is consistent with previous estimates from barbiturate-anesthetized rats, whereas it operates significantly slower under halothane anesthesia.