Tubuloglomerular feedback response in the prenatal and postnatal ovine kidney.

The tubuloglomerular feedback mechanism (TGF) plays an important role in regulating single-nephron glomerular filtration rate (GFR) by coupling distal tubular flow to arteriolar tone. It is not known whether TGF is active in the developing kidney or whether it can regulate renal vascular tone and thus GFR during intrauterine life. TGF characteristics were examined in late-gestation ovine fetuses and lambs under normovolemic and volume-expanded (VE) conditions. Lambs and pregnant ewes were anesthetized and the fetuses were delivered via a caesarean incision into a heated water bath, with the umbilical cord intact. Under normovolemic conditions, mean arterial pressure of the fetuses was lower than lambs (51 ± 1 vs. 64 ± 3 mmHg). The maximum TGF response (ΔP(SFmax)) was found to be lower in fetuses than lambs when tubular perfusion was increased from 0 to 40 nl/min (5.4 ± 0.7 vs. 10.6 ± 0.4 mmHg). Furthermore, the flow rate eliciting half-maximal response [turning point (TP)] was 15.7 ± 0.9 nl/min in fetuses compared with 19.3 ± 1.0 nl/min in lambs, indicating a greater TGF sensitivity of the prenatal kidney. VE decreased ΔP(SFmax) (4.2 ± 0.4 mmHg) and increased TP to 23.7 ± 1.3 nl/min in lambs. In fetuses, VE increased stop-flow pressure from 26.6 ± 1.5 to 30.3 ± 0.8 mmHg, and reset TGF sensitivity so that TP increased to 21.3 ± 0.7 nl/min, but it had no effect on ΔP(SFmax). This study provides direct evidence that the TGF mechanism is active during fetal life and responds to physiological stimuli. Moreover, reductions in TGF sensitivity may contribute to the increase in GFR at birth.