The cells of the macula densa are sensitive to the ionic content and water volume of the fluid in the DCT, producing molecular signals that promote renin secretion by other cells of the juxtaglomerular apparatus. The release of renin is an essential component of the renin-angiotensin-aldosterone system (RAAS), which regulates blood pressure and volume.
The close proximity and prominence of the nuclei cause this segment of the DCT wall to appear darker in microscopic preparations, hence the name macula densa.
A decrease in blood pressure results in a decreased concentration of sodium and chloride ions at the macula densa. (This is due to reduced filtration by the glomerulus: less filtrate is expelled into Bowman's space and the proximal convoluted tubule; the resulting fluid reaching the macula will have a lower sodium chloride concentration after the sodium chloride is removed along the thick ascending limb of Henle.)
In response, the macula densa cells release prostaglandins, which triggers granular juxtaglomerular cells lining the afferent arterioles to release renin into the bloodstream. (The juxtaglomerular cells can also release renin independently of the macula densa, as they are also triggered by baroreceptors lining the arterioles, and release renin if a fall in blood pressure in the arterioles is detected.) Furthermore, activation of the sympathetic nervous system stimulates renin release through activation of beta-1 receptors.
Study Findings from University of California, Department of Environmental Toxicology Broaden Understanding of Biochemistry.(Report)
Feb 14, 2011; New research, 'TCDD-induced cyclooxygenase-2 expression is mediated by the nongenomic pathway in mouse MMDD1 macula densa cells...