Further evidence for an inverse relationship between macula densa NaCl concentration and filtration rate

It has been concluded that tubulo-glomerular feedback mechanism is triggered by changes in NaCl concentration ([NaCl]) at the macula densa. This conclusion is based on the demonstration that changes in filtration rate produced during retrograde perfusion of the loop of Henle depend upon the perfusate [NaCl]. Experiments were performed to evaluate whether the effect on glomerular function of orthograde perfusion of the loop of Henle is consistent with this conclusion. Early proximal flow rate (VEP), stop-flow pressure (PSF), early distal chloride concentration ([C]), and flow rate were measured during perfusion of the loop of Henle with mannitol solution (300 mosm kg-1), 30mM NaCl + mannitol (300 mosm kg-1), 140 mM Na isethionate and artificial tubular fluid. When distal flow exceeded 10 nl min-1, the magnitude of the glomerular response was predictable from the [Cl]. The linear regression line, delta VEP = -0.27 [Cl] + 4.3, did not differ from that obtained previously with the retrograde technique. Retrograde perfusion with 140 mM Na isethionate was without effect on VEP. We conclude that the effect on glomerular function of perfusion of the loop of Henle in either an orthograde or a retrograde direction with these solutions depends upon the chloride concentration at the macula densa.