Feedback pressure-flow responses in normal and angiotensin-prostaglandin-blocked rats

We have examined the response of directly and indirectly (stop-flow) measured glomerular capillary hydrostatic pressure (PGC) and single nephron glomerular filtration rate (SNGFR) to increases in late proximal tubular flow rate in hydropenic rats and rats in which angiotensin II (ANG II) and prostaglandin generation was reduced by 3- to 5-day pretreatment with converting enzyme inhibitor (MK-421) and meclofenamate. In control rats, PGC (48 +/- 2 mmHg) decreased 9 +/- 1 mmHg when 25 nl/min was added to late proximal flow in unobstructed tubules, and PGC decreased 9 +/- 1 mmHg when late proximal perfusion rate was increased from 0 to 40 nl/min, incrementally, in wax-blocked tubules. The turning point or half-maximal response for PGC was at perfusion rates of 23 +/- 2 nl/min. Stop-flow estimated PGC (47 +/- 1 mmHg = control) responses were nearly identical. SNGFR decreased from 30 +/- 1 to 21 +/- 1 nl/min with increased perfusion in control rats. In ANG II-prostaglandin-blocked rats, PGC and stop-flow pressure responses were completely eliminated, yet SNGFR response persisted (36.2 to 28.0 nl/min) but to a somewhat lesser extent. Both direct and indirect PGC decrease with increases in late proximal flow rate in untreated rats. Studies in ANG II-prostaglandin-blocked rats suggest that tubuloglomerular feedback SNGFR responses can occur without changes in PGC, possibly via parallel changes in afferent and efferent arteriolar resistances.