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 (V(M)) was measured upstream from the perturbation (V(H)) by a noninvasive optical technique. The dependence of V(M) on V(H) was estimated by polynomial regression. By using fractional compensation (C = -dV(M)/dV(H)), as an index of homeostatic efficiency, we constructed efficiency profiles (C vs. V(H)). At V(H) = 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.