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. Autoregulatio n of renal blood flow is mediated by a rapid myogenic response and a slower tubuloglomerular feedback mechanism. In 25 male conscious rats, ramp-shape d changes in RPP were induced to quantify dynamic properties of autoregulat ion. Decremental RPP ramps immediately followed by incremental ramps were m ade 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. Dur ing RPP decrements, conductance increased. With increasing rate of change o f RPP decrements, maximum conductance increased from 10% to 80%, as compare d with control. This response, which indicates the magnitude of autoregulat ion, was more pronounced in cortical versus medullary vasculature. Pressure at maximum conductance decreased with increasing rate of change of RPP dec rements from 88 to 72 mmHg. During RPP increments, dependence of maximum co nductance changes on the rate of change was enhanced (-20 to 110% of contro l). Thus, a hysteresis-like asymmetry between RPP decrements and increments , a resetting of autoregulation, was observed, which in direction and magni tude depended on the rate of change and duration of RPP changes. In conclus ion, renal vascular responses to changes in RPP are highly dependent on the dynamics of the error signal. Furthermore, the method presented allows dif ferentiated stimulation of various static and dynamic components of pressur e-flow relationship and, thus, a direct assessment of the magnitudes and op erating pressure range of active mechanisms of pressure-flow relationships.