A simple steady-state model for feedback control of glomerular filtration rate.

Repeat measurements of SNGFR during different rates of microperfusion of the loop of Henle have demonstrated that the alterations of SNGFR were greatest between perfusion rates of 10 and 20 nl/min, but that changes also occurred below 10 nl/min and above 20 nl/min. By a simple curve-fitting procedure, yielding parameter estimates for single nephrons, we determined that the half-maximum response was achieved at a perfusion rate of 16.3 nl/min and that at that flow the slope of the relationship between VLP and SNGFR was 1.84. From the curve describing the perfusion response and from measurements of the proximal transport rate, we estimated that, at the operating point of the feedback loop, VLP and SNGFR should be approximately 18 and 35 nl/min, respectively. Measurements made without interruption of the feedback pathway confirmed this prediction, suggesting that perfusion adequately mimics the free-flow state. We conclude, that under the experimental conditions studied, the operating point of the feedback mechanism is located within the flow range in which the mechanism is most sensitive, so that both increases and decreases in tubular flow from the normal values should produce changes in filtration rate.