QUANTITATIVE CHARACTERIZATION OF THE TUBULOGLOMERULAR FEEDBACK RESPONSE - EFFECT OF GROWTH

Studies were performed to characterize quantitatively the effect of changing loop of Henle flow rate on single nephron glomerular filtration rate (SNGFR) in male Sprague-Dawley rats of varying body weight. Rats weighing 100, 220 and 350 g were studied using standard renal micropuncture techniques. The relationships between loop of Henle flow rate (.ovrhdot.VLP) and SNGFR was characterized for individual nephrons by multiple determinations of SNGFR during loop perfusion. An inverse sigmoidal relationship was observed that could be described as .DELTA.SNGFR = a/(1 + ek(b - .ovrhdot.VLP)), where .DELTA.SNGFR is the change in SNGFR from the value measured at zero loop flow, a is .DELTA.SNGFRmax, the maximum change, b is .ovrhdot.V1/2, the flow rate at which the response is half maximum, and k is [4f'' (.ovrhdot.V1/2)]/a with f'' (.ovrhdot.V1/2) the slope at .ovrhdot.V1/2. .DELTA.SNGFRmax increased with increasing body size (7.9 .+-. 1.16, 18.9 .+-. 0.90, and 25.2 .+-. 2.73 nl/min, respectively, in the 3 groups), and the curve shifted to the right (.ovrhdot.V1/2 = 10.3 .+-. 0.8, 15.4 .+-. 0.83, and 22.3 .+-. 1.22 nl/min). The maximum slope increased (f'' (.ovrhdot.V1/2) = 0.9 .+-. 0.19, 1.7 .+-. 0.16, and 3.2 .+-. 0.70), but the exponential constant k was uninfluenced by growth. Independent of rat size, a 10% increase in loop flow at the midpoint produced a 5-10% decrease in SNGFR. Free-flow values of SNGFR and .ovrhdot.VLP were found to lie in the most sensitive range of the feedback curve.