Tubuloglomerular feedback in animals with unilateral, partial ureteral occlusion

Previous experiments have shown that the sensitivity of the tubuloglomerular feedback system (TGF) is reduced by volume expansion in normal rats. This reduction in sensitivity is probably mediated by changes in the renal interstitial pressure. The present study was designed to investigate the TGF control system during volume expansion in rats with chronic, partial ureteral occlusion--hydronephrosis. Hydronephrosis was induced on the left or right side according to the method described by Ulm and Miller, in weanling Sprague-Dawley rats three weeks old. Three to six weeks later the rats were prepared for whole kidney and micropuncture experiments. Sham-operated animals were used as controls. Net interstitial pressure (that is, interstitial hydrostatic pressure minus interstitial oncotic pressure) was higher in the hydronephrotic, volume expanded animals than in the volume expanded controls. From findings in earlier investigations this increase in interstitial pressure would have been expected to reduce TGF sensitivity but this sensitivity was increased in the hydronephrotic kidneys, as indicated by a reduction in the turning point, the tubular flow rate at which 50% of the maximal stop-flow pressure response was observed (14.4 nl/min, sham-operated control 33.4 nl/min). Strong activity of the TGF mechanism was also indicated by a large proximal-distal difference in the single-nephron glomerular filtration rate (11.9 nl/min versus 3.3 nl/min in sham-operated controls) in the hydronephrotic kidney during volume expansion. Thus, in hydronephrotic kidneys in the latter condition the TGF mechanism was highly sensitive and activated to reduce the glomerular filtration rate. This mechanism may protect the diseased kidney from high intrapelvic pressures which otherwise could damage the kidney during saline volume expansion.