Tubuloglomerular feedback and SNGFR autoregulation in the rat

Experimental and simulation studies were undertaken to examine what influence the state of filtration dynamics has on single nephron glomerular filtration rate (SNGFR) autoregulation, to verify the presence of partial autoregulation during tubuloglomerular feedback (TGF) inhibition, and to determine whether small changes in tubular reabsorption could enhance the ability of TGF to regulate SNGFR during autoregulation. The experimental studies in hydropenic rats revealed partial autoregulation of glomerular capillary pressure (using the stop-flow method) and SNGFR during TGF inhibition and complete SNGFR autoregulation with functional TGF. A small significant decrease in fractional volume reabsorption was observed in the distal measurements with increased arterial pressure. Results from a mathematical model of glomerular filtration suggest that SNGFR is inherently more sensitive to changes in arterial pressure in animals exhibiting filtration pressure equilibrium rather than filtration pressure disequilibrium. These data and the simulation results provide additional support for the existence of a TGF-independent autoregulatory mechanism that appears to be an intrinsic property of the preglomerular vasculature. Finally, analysis of autoregulation with a simple control-system model suggests that small changes in proximal tubule or loop of Henle volume reabsorption can provide some or all of the stimulus for TGF activation, thereby significantly increasing the ability of TGF to stabilize SNGFR in response to a rise in arterial pressure.