Frequency domain analysis of renal autoregulation in the rat

The frequency response of renal blood flow was estimated to test our suggestion that autoregulation had one fast and one slow component. Arterial pressure was varied periodically by a pump connected to the distal aorta or aperiodically by stimulating atrial fibrillation. There was only one regulatory mechanism, corresponding to the slow component seen earlier. Two apparent regulators were seen when pressure was measured far from the renal artery (right common carotid), rather than near (superior mesenteric). Simultaneous measurements of pressure in both arteries showed interference of the pressure wave from the heart with pressure waves from the pump in the distal aorta at frequencies near that of the heart rate. Measurements made in the superior mesenteric artery were likely to be correct. The earlier results were in error, and frequency response methods detect only one regulator. Animals made acutely hypertensive behaved similarly. The effect of the regulator appears as a phase shift at frequencies of 0.25 Hz and below and an attenuated flow magnitude at frequencies below 0.1 Hz. This frequency response is consistent with the flow propagation time to the macula densa. We conclude that macula densa feedback was the only flow-regulating mechanism detected.