Studies of the sensing mechanism in the tubuloglomerular feedback pathway

Two aspects of the single nephron feedback response previously observed by us have been reexamined. The first, the effect of modifying the composition of fluid used to perfuse the loop of Henle of a single nephron, was studied in rats by comparing flow-induced changes in stop-flow pressure (PSF) with three different perfusion solutions: artificial tubule fluid (ATF); 0.3 M mannitol (M); and mannitol plus 25 to 30 mM sodium chloride (M + E). The second, triggering of the feedback response by injection of ionic current into the distal tubule, was studied in similarly prepared rats by monitoring PSF while passing current. Increasing the rate of loop of Henle perfusion with either ATF or M + E resulted in similar decreases in PSF. In contrast, with M, changes in PSF were usually transient and if persistent were smaller than the changes observed with the other two solutions. When loops of Henle were perfused with ATF at a constant rate, injection of current into the early distal tubule making the lumen more negative resulted in decreases in PSF. Currents of opposite polarity caused no change in PSF if loop flow rate was low; these currents increased PSF if the loop flow rates had previously been high. Current-induced feedback responses were obtained with micropipette electrodes filled with either potassium chloride or lithium acetate. Addition of 10(-4) M furosemide blocked the current-induced feedback responses.