The tubulo-glomerular feedback mechanism-a determinant for the autoregulation of the glomerular filtration rate in superficial and juxtamedullary nephrons.

The intrinsic myogenic hypothesis and the tubuloglomerular feedback mechanism (TGF) give the presently most cherished explanation to the autoregulation of renal blood flow and glomerular filtration rate. A series of experiments was performed on young, normohydrated rats in order to evaluate the importance of TGF as an autoregulatory factor of the single nephron glomerular filtration rate (SNGFR) in superficial and juxtamedullary nephron populations. Micropuncture techniques were applied to tubular structures of the renal surface and on the papilla for the measurement of hydrostatic pressures and SNGFR. The SNGFR was also measured with a modified Hanssen technique. A TV-technique was used to record the urine free flow rate in the loop of Henle. The net driving forces for glomerular filtration at the afferent end of the glomerular capillaries were estimated to be 19 and 47 mm Hg for superficial and juxtamedullary nephrons respectively, when the urine flow at the macula densa was zero. The SNGFR of the two nephron populations amounted to 29.6 and 84.1 nl . min-1 . g-1 K.W., as measured with the micropuncture technique. With a modified Hanssen technique the corresponding values were 25.8 and 27.7 nl . min-1 . g-1 K.W. (kidney weight). The SNGFR was found to be well autoregulated when the urine flow at the macula densa was intact, but not when the urine flow was interrupted. The flow rate in the loop of Henle was in free flow conditions 7.3 nl . min-1 . g-1 K.W. which shall be compared with 19.2 nl . min-1 . g-1 K.W. when the urine flow to the macula densa was zero. We conclude that SNGFR is mainly autoregulated by the TGF-mechanism in young, normohydrated rats at lower arterial pressures. In normal conditions TGF is highly activated for juxtamedullary nephrons, but not for the superficial ones. The high urine flow rate in the loop of Henle at reduced flow rates at the macula densa may invalidate the use of loop blockade in studies of water and solute transfer across the loop walls.