How do macula densa cells detect changes in GFR and what is their role in the tubuloglomerular feedback mechanism?

Macula densa cells detect changes in GFR by acting as chemoreceptors that monitor the NaCl concentration of filtrate entering the distal convoluted tubule. Because filtrate NaCl concentration varies with flow rate, a high GFR produces high NaCl levels, which triggers the macula densa to release vasoconstrictor chemicals that constrict the afferent arteriole and lower GFR. Conversely, a low GFR produces low NaCl, which inhibits those signals, causing afferent arteriole vasodilation and raising GFR. This tubuloglomerular feedback helps keep GFR relatively constant over a wide range of blood pressures.

Macula densa cells are tall, closely packed cells in the ascending limb of the nephron loop, forming part of the juxtaglomerular complex adjacent to the granular cells of the afferent arteriole. They function as chemoreceptors that monitor the NaCl content of the filtrate as it enters the distal convoluted tubule. In the tubuloglomerular feedback mechanism, the macula densa cells respond to the NaCl concentration, which varies directly with filtrate flow rate and therefore with GFR. When GFR increases, there is less time for NaCl reabsorption, so the NaCl concentration in the filtrate remains high. High NaCl stimulates the macula densa cells to release vasoconstrictor chemicals, including ATP and other substances, which cause intense constriction of the afferent arteriole. This constriction reduces blood flow into the glomerulus, lowering net filtration pressure and GFR back toward normal. In contrast, when GFR decreases, filtrate flow is slow and NaCl concentration is low because more NaCl has been reabsorbed. Low NaCl inhibits ATP release from the macula densa, leading to vasodilation of the afferent arteriole. This increases blood flow into the glomerulus, raising net filtration pressure and GFR. Through this flow-dependent feedback, the macula densa cells contribute to renal autoregulation, helping maintain a nearly constant GFR despite changes in systemic arterial blood pressure over the range of roughly 80 to 180 mm Hg.