Time-dependent autoregulation of renal blood flow in conscious rats.

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow is mediated by a rapid myogenic response and a slower tubuloglomerular feedback mechanism. In 25 male conscious rats, ramp-shaped changes in RPP were induced to quantify dynamic properties of autoregulation. Decremental RPP ramps immediately followed by incremental ramps were made for four different rates of change, ranging from 0.118 to 1.056 mmHg/s. Renal blood flow and cortical and medullary fluxes were assessed, and the corresponding relative conductance values were calculated continuously. During RPP decrements, conductance increased. With increasing rate of change of RPP decrements, maximum conductance increased from 10% to 80%, as compared with control. This response, which indicates the magnitude of autoregulation, was more pronounced in cortical versus medullary vasculature. Pressure at maximum conductance decreased with increasing rate of change of RPP decrements from 88 to 72 mmHg. During RPP increments, dependence of maximum conductance changes on the rate of change was enhanced (-20 to 110% of control). Thus, a hysteresis-like asymmetry between RPP decrements and increments, a resetting of autoregulation, was observed, which in direction and magnitude depended on the rate of change and duration of RPP changes. In conclusion, renal vascular responses to changes in RPP are highly dependent on the dynamics of the error signal. Furthermore, the method presented allows differentiated stimulation of various static and dynamic components of pressure-flow relationship and, thus, a direct assessment of the magnitudes and operating pressure range of active mechanisms of pressure-flow relationships.