<P> Regulation of renal blood flow is important to maintaining a stable glomerular filtration rate (GFR) despite changes in systemic blood pressure (within about 80 - 180 mmHg). In a mechanism called tubuloglomerular feedback, the kidney changes its own blood flow in response to changes in sodium concentration . The sodium chloride levels in the urinary filtrate are sensed by the macula densa cells at the end of the ascending limb . When sodium levels are moderately increased, the macula densa releases ATP and reduces prostaglandin E2 release to the juxtaglomerular cells nearby . The juxtaglomerular cells in the afferent arteriole constrict, and juxtaglomerular cells in both the afferent and efferent arteriole decrease their renin secretion . These actions function to lower GFR . Further increase in sodium concentration leads to the release of nitric oxide, a vasodilating substance, to prevent excessive vasoconstriction . In the opposite case, juxtaglomerular cells are stimulated to release more renin, which stimulates the renin--angiotensin system, producing angiotensin I which is converted by Angio - Tensin Converting Enzyme (ACE) to angiotensin II . Angiotensin II then causes preferential constriction of the efferent arteriole of the glomerulus and increases the GFR . </P> <P> This is so - called "steady - state system". An example is a system in which a protein P that is a product of gene G "positively regulates its own production by binding to a regulatory element of the gene coding for it," and the protein gets used or lost at a rate that increases as its concentration increases . This feedback loop creates two possible states "on" and "off". If an outside factor makes the concentration of P increase to some threshold level, the production of protein P is "on", i.e. P will maintain its own concentration at a certain level, until some other stimulus will lower it down below the threshold level, when concentration of P will be insufficient to make gene G express at the rate that would overcome the loss or use of the protein P . This state ("on" or "off") gets inherited after cell division, since the concentration of protein a usually remains the same after mitosis . However, the state can be easily disrupted by outside factors . </P> <P> Similarly, this phenomenon is not only restricted to genes but may also apply to other genetic units, including mRNA transcripts . Regulatory segments of mRNA called a Riboswitch can autoregulate its transcription by sequestering cis - regulatory elements (particularly the Shine - Dalgarno sequence) located on the same transcript as the Riboswitch . The Riboswitch stem - loop has a region complementary to the Shine - Dalgarno but is sequestered by complimentary base pairing in the loop . With sufficient ligand, the ligand may bind to the stem - loop and disrupt intermolecular bonding, resulting in the complimentary Shine - Dalgarno stem - loop segment binding to the complimentary Riboswitch segment, preventing Ribosome from binding, inhibiting translation . </P>

Which of the following is a description of autoregulation of renal blood flow