Frequency response of autoregulation.

To estimate the frequency response of autoregulation and to test whether macula densa feedback alone mediates renal blood flow (RBF) autoregulation in rats, we imposed sudden increases (10 to 20 mm Hg) in arterial pressure (AP) and compared the time courses of renal vascular resistance (RVR) changes and of flow changes at the macula densa. RVR decreased abruptly after the AP increase, but returned to control levels in less than 1 sec, continued to increase, reaching 50% completion within 3 sec, and achieved final values 15 to 20 sec after AP change. To predict transient changes in tubule flow, we derived a model of pressure and flow and combined it with measurements of tubule pressure made during AP steps. With these measured pressures as inputs, the model predicted a prompt rise in GFR followed by a decline that returned GFR halfway to control values within 3 to 4 sec and more slowly thereafter. Predicted early distal flow increased perceptibly only after 1 sec and rose slowly thenceforth. The disturbance in early distal flow appears to arrive too late to signal the rapid changes in RVR. Some other mechanism, possibly myogenic, must have intervened. The results suggest that two mechanisms mediate autoregulation, each with a different response time. The putative myogenic mechanism is about one order of magnitude faster than the macula densa.