Homeostatic efficiency of tubuloglomerular feedback in hydropenia, euvolemia, and acute volume expansion.

We assessed the homeostatic efficiency of the tubuloglomerular feedback (TGF) system in Inactin-anesthetized Munich-Wistar rats by use of perturbation analysis in closed-loop micropuncture studies. Nephrons were studied in vivo under conditions of hydropenia (HYD, n = 17), euvolemia (EUV, n = 23), and acute isoncotic extracellular volume expansion (EXP, n = 15). Proximal tubular flow was perturbed in free-flowing nephrons with a microperfusion apparatus. Flow rate (VM) was measured upstream from the perturbation (VH) by a noninvasive optical technique. The dependence of VM on VH was estimated by polynomial regression. By using fractional compensation (C = -dVM/dVH), as an index of homeostatic efficiency, we constructed efficiency profiles (C vs. VH). At VH = 0, C tended toward higher values with decreasing volume status, although the effect did not achieve significance. The maximum value of C did not differ between groups. The efficiency profiles shifted leftward with each increment in volume (P < 0.03, HYD vs. EXP), suggesting that the TGF system adapts to acute increments in volume by shifting the efficiency profile in favor of a vasodilatory role.