Tubuloglomerular feedback responses with native and artificial tubular fluid

Experiments were performed to compare the tubuloglomerular feedback response to native and artificial tubular fluid. The change in early proximal flow rate produced by changes in loop of Henle flow rate was measured in anesthetized rats using micropuncture techniques. Loop perfusion fluid was either an artificial solution with an electrolyte composition similar to that of proximal fluid (ATF) or native tubular fluid (NTF) collected from the late proximal tubule. In control rats, in rats on a low NaCl diet, in rats on restricted food intake, and in acutely saline-expanded rats no differences were detected between ATF- and NTF-perfused nephrons. In rats receiving 10 g NaCl/100 g, diet, responses with ATF and NTF to a flow change from 0 to 15 nl/min did not differ significantly; maximum feedback responses (flow change from 0 to 40 nl/min) were, however, significantly greater with ATF (-25.4%) than NTF (-12.9%). Chloride absorption was not different with the two perfusates. With both ATF and NTF a significant negative correlation was found between maximum responses and NaCl intake, with the slope being steeper with NTF. We conclude that some unidentified constituent of tubular fluid affects maximum feedback responses during very high NaCl intake, but feedback responses to physiological flow rate changes appear to be independent of luminal factors.