Final answer:
Macula densa cells in the tubuloglomerular feedback mechanism detect changes in sodium concentration in the distal tubule when GFR is high, prompting afferent arteriole contraction to rebalance GFR.
Step-by-step explanation:
When the glomerular filtration rate (GFR) is above normal, the tubuloglomerular feedback mechanism initiates a response from the macula densa cells. These cells, located in the distal convoluted tubule (DCT), are specialized for monitoring the sodium (Na+) concentration in the terminal portion of the ascending loop of Henle. When there is less time for NaCl to be reabsorbed in the proximal convoluted tubule (PCT) due to high GFR, it results in increased osmolarity in the filtrate.
The macula densa cells detect this elevation in sodium concentration and fluid flow rate, and they release ATP and adenosine. These molecules function as paracrine signals that prompt the juxtaglomerular cells in the afferent arteriole to contract, thus reducing blood flow and GFR to maintain a balanced filtration rate.