Characteristics of feedback control of glomerular filtration rate

Increases in the flow rate of fluid through the loop of Henle are associated with decreases in the rate at which fluid is filtered by the glomerulus in the same nephron. The anatomical basis for this negative feedback phenomenon is very likely the close relationship between the macula densa segment of the distal tubule and the vascular components of the juxtaglomerular apparatus. Convincing evidence indicating the existence of a feedback control mechanism has come largely from micropunture experiments in which flow through single Henle loops is controlled while either filtration rate by, or capillary pressure in, the glomerulus of the same nephron is measured. Aspects of the negative feedback response that have been characterized quantitatively include the magnitude of the changes in glomerular function, the rates of tubule fluid flow that elicit the changes, and the sensitivity of the response in different physiological circumstances. Some information about the sensing step in the feedback pathway in rats has come from microperfusion experiments employing changes in the composition of fluids used to perfuse the loop of Henle or changes in the electrical driving forces across the wall of the distal tubule. Taken together, results of experiments using these different approaches indicate that changes in loop of Henle flow rate may be sensed by a process that requires transport of some constituent of distal tubule fluid, possibly the chloride ion, from the distal tubule lumen into some compartment within the juxtaglomerular apparatus.