Tubuloglomerular feedback in juxtamedullary nephrons

Hitherto, no experimental evidence for the existence of tubuloglomerular feedback regulation (TGF) in juxtamedullary nephrons has been produced. We have developed two micropuncture techniques suitable for the papilla of young rats, which make it possible to investigate TGF in deep nephrons. The first consisted of measuring the tubular flow in the descending limb of Henle (VDLH) while the ascending limb (ALH) was perfused at various rates with modified Ringer solution. The single nephron filtration rate (SNGFR) was also measured in several juxtamedullary nephrons in these perfusion studies. A second micropuncture protocol was to measure the tubular stop-flow pressure (PSF) in the descending limb of Henle upstream to a wax block. Distal to the block, Ringer was perfused at various rates through ALH. The PSF, the maximal PSF decrease (delta PSFmax), and the flow rate at which 50% of the delta PSFmax was obtained (turning point, TP) were determined. The VDLH and the SNGFR decreased during Ringer perfusion (rate of more than 7.5 nl/min) to 39 +/- 4% (N = 8) and 44 +/- 4 (N = 8) of the value at zero perfusion, respectively. Even the stop-flow technique gave evidence for TGF in juxtamedullary nephrons: The PSF decreased from 40 +/- 2 mm Hg by a delta PSFmax of 14.1 +/- 1.4 mm Hg. The TP was 8.2 +/- 0.6 nl/min. Thus, similar to earlier reports on surface nephrons, the TGF is present also in the juxtamedullary nephrons, decreasing their glomerular capillary pressure, decreasing their SNGFR, and decreasing their flow rate in the descending limb of Henle when flow rate of the ascending limb is increased. Thus, TGF exerts its effect on GFR of the whole kidney not only in surface but also in juxtamedullary nephrons.