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

Abstract

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 ($\dot V_{EP}$), stop-flow pressure (P _SF), early distal chloride concentration ([Cl]), and flow rate were measured during perfusion of the loop of Henle with mannitol solution (300 mosm kg⁻¹), 30 mM 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,$\Delta \dot V_{EP} = - 0.027{\text{ }}[Cl]{\text{ + 4}}{\text{.3}}$, did not differ from that obtained previously with the retrograde technique. Retrograde perfusion with 140 mM Na isethionate was without effect on$\dot V_{EP}$. 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.